Human-powered vehicles

Abstract

A drive apparatus 200 for the propulsion of an HPV comprises a drive-wheel connecting to a bottom-bracket assembly to revolve with it and delivering torque at a location outboard of a bearing supporti

Description

FIELD OF THE INVENTION This invention relates to human-powered vehicles (HPVs) whose drive can be augmented by an auxiliary source such as an electric motor. The HPV may be a bicycle. BACKGROUND HPVs are vehicles whose motion can be powered, at least in part, by an occupant or rider of the vehicle. An increasing number of HPVs are equipped with auxiliary drive sources. To provide assistance to HPVs, such as bicycles, an electric motor can be incorporated to provide additional power. This can, for example, be helpful when cycling up hills. Some systems can propel a bicycle even when the rider is not pedalling. Other systems provide power to a bicycle when the rider pedals. Bicycles of the latter type are sometimes called “pedal assist” electric bicycles. There are several ways in which power from an electric motor may be used to provide auxiliary drive to the bicycle, thus providing assistance to the rider. For example, drive may be provided to the wheels directly (e.g. by using specially designed wheels), or drive may be provided to the cranks of the bicycle. The latter systems tend to be positioned around the crank-set of the bicycle, sometimes called “mid-drive” assist units. Some mid-drive assist-units are integrally formed with the bicycle. For example, the bicycle may be manufactured containing the mid-drive assist unit, which is not removable in normal service. Other mid-drive assist units may be attached to a standard bicycle to upgrade the standard bicycle to an assisted bicycle. The process of attaching such mid-drive units is often complicated and requires skills and tools that the standard cyclist may not possess. Moreover, once the mid-drive assist unit has been fitted, it is usually not straightforward to remove the unit from the bicycle so that the bicycle returns to its original state. This is often because the bicycle had to be adapted in order to fit the mid-drive assist unit, for example, by replacing the chain set with an alternative chain set. This makes it harder to convert the bicycle from being an assisted bicycle to a non-assisted bicycle. Some mid-drive assist-units are fitted with a mechanism to allow the rider to pedal more slowly than the output that turns the wheels (e.g. the chain and chain rings). Such mechanisms, usually in the form of a one-way clutch or freewheel, see high stresses and are heavy bulky and expensive. There is a need for a mid-drive assist unit lacking the bulk and weight of such a mechanism. There is a need for a mid-drive assist unit that can be easily removed from the bicycle when not required. There is a need for a mid-drive assist unit in which the motor-assist function can be isolated, for example when the battery runs out or if the rider wishes to cycle without assistance, without experiencing drag from the motor. It is also desirable to measure the force applied to the pedals by the rider, which can be used to control an assist unit. It is further desirable to achieve the above without unduly increasing the stance width (the distance between the two pedals) of the bicycle. SUMMARY OF THE INVENTION According to one aspect there is provided a drive apparatus for providing assistance in the propulsion of an HPV, the HPV having: a frame; a bottom-bracket axle carried in a plurality of bearings which support the bottom-bracket axle in the frame of the HPV and allow it to revolve therein; a pair of diametrically opposite cranks, one at each end of the bottombracket axle, through which a rider can apply tangential force to deliver torque to the bottombracket axle and make it revolve; a mid-plane which is perpendicular to the axis of the bottom-bracket axle; a drive-side on one side of the mid-plane; a bottom-bracket output for onward delivery of propulsive drive which is attached to the bottom-bracket axle and is offset from the mid-plane of the HPV on the drive-side of the HPV; a bottom-bracket assembly comprising the bottom-bracket output, the bottom-bracket axle, and the pair of cranks, all of which are coupled so as to revolve as one relative to the frame of the HPV, wherein the drive apparatus comprises: a drive-wheel configured to connect to the bottom-bracket assembly so as to revolve with it, the drive-wheel being configured to deliver torque into the bottom-bracket axle through a connection which is axially positioned outboard of a bearing supporting the bottom-bracket axle and which, when connected, revolves in a plane which is offset from the mid-plane of the HPV; a motor comprising a rotor; a motor-output wheel which can, when the drive apparatus is connected to the bottom-bracket assembly, revolve about an axis parallel to and offset from the axis about which the bottom-bracket axle revolves; a drive-element which can couple the motor-output wheel to the drive-wheel so as to deliver torque from the motor to the drive-wheel; a one-way clutch in the drivetrain between the rotor and the bottom-bracket assembly, the one-way clutch being configured to engage when the rotor is revolving at an engagement speed, and to disengage when the rotor is revolving at a speed less than the engagement speed, the engagement speed being a predefined multiple of the speed at which the bottom-bracket assembly is revolving; and a controller configured to receive data from one or more sensors and to estimate from the data a force applied to the bottom-bracket axle by a rider through the cranks, wherein the controller is configured to use this information to decide a level of torque that the motor is to apply to the rotor. The controller may be configured to decide the level of torque that the motor is to apply to the rotor in response to determining that assistance is not required. The controller may comprise one or more processors configured to determine whether propulsive assistance is to be provided. The one or more processors may be configured to determine the torque to apply when propulsive assistance is to be provided. The one or more processors may be configured to determine the torque to apply when propulsive assistance is not to be provided. The processor(s) may make one or more of these determinations in response to a signal provided by the user to an input device in communication with the processor(s) and / or in response to sensing the speed of the HPV and / or the change in speed of the HPV and / or in response to sensing the rotational speed of the bottom-bracket assembly and / or in response to sensing the force applied to the pedals by the user. The drive apparatus may be provided with a bottom-bracket assembly. The bottom-bracket assembly may comprise: a bottom-bracket axle carried in a plurality of bearings for supporting the bottombracket axle in the frame of the HPV and allowing the bottom-bracket axle to revolve therein; a hollow cartridge having a bore and carrying in its bore the plurality of bearings, the cartridge being mountable to the frame of the HPV; a first mounting structure for the cartridge, the first mounting structure having an elastic resistance to rotation in yaw and roll of the cartridge relative to the frame of the HPV that is insufficient on its own to prevent said rotation when the bottom-bracket axle is subjected to transverse forces which are not in balance about the first mounting structure such that the first mounting structure provides a fulcrum for said rotation of the cartridge; an elastically compliant structure axially displaced from the first mounting structure when the cartridge is mounted to the frame of the HPV, the elastically compliant structure having a compliant resistance to the rotation in yaw and roll of the cartridge; one or more sensors configured to sense radial displacement of the cartridge arising from said rotation of the cartridge about the fulcrum; wherein the controller of the drive apparatus is configured to determine the torque to apply to the rotor in dependence on information from the one or more sensors relating to said sensed radial displacement of the cartridge. The one or more sensors may lie on the same side of the mid-plane as that where the drivewheel, the drive-element and the motor-output wheel lie. The controller may be configured to monitor the speed at which the bottom-bracket assembly revolves. The controller may be configured to monitor the speed at which the rotor revolves. During periods when assistance is not to be provided, the controller may be configured to decide from this information the torque to apply to the rotor. The bottom bracket axle may be mounted to revolve in a bottom-bracket shell whose minimum bore is less than 40mm. Torque from the rotor may be delivered through a speed-reduction mechanism to the motoroutput wheel. The one-way clutch may be positioned in the drive path from the rotor to the motor-output wheel. The drive-wheel, drive-element and motor-output wheel may lie on a non-drive-side of the HPV which is opposite to the drive-side. During periods when assist is not to be provided and the motor is revolving at the engagement-speed, the controller may be configured to apply a reverse torque to the rotor at a level which is a function of the level of reverse force that the controller calculates is due to a reverse torque applied by the rider through the cranks, based on the information it has from the one or more sensors. During periods when assistance is not to be provided and the motor is revolving at the engagement-speed, the controller may be configured to apply a reverse torque to the rotor at a level which is a function of the tension which the controller calculates is present in the drive-element coupling the motor-output wheel and the drive-wheel on the bottom-bracket assembly, based on the information it has from the one or more sensors. During periods when assistance is not to be provided and the motor is revolving at the engagement-speed, the controller may be configured to apply a reverse torque to the motor at a level which is a function of the difference between an actual measured speed of the motor with a calculated expected speed of the motor, the calculated expected speed of the motor being the speed that the controller would expect the motor to be revolving at if the clutch were disengaged. The controller may be configured to calculate a theoretical engagement speed as being the predefined multiple of the speed at which the bottom-bracket assembly is sensed to be revolving. The controller may further configured to apply a level of torque to the motor which is likely, given a known moment of inertia of the rotor, to maintain a pre-determined difference between the speed of the motor and the theoretical engagement speed. The drive apparatus may comprise a sensor for sensing the speed of the rotor. The drive apparatus may comprise a sensor for sensing the speed of the bottom-bracket assembly. During periods when assist is to be provided, the controller may apply forward torque to the motor at a level which is a function of the level of force that the controller calculates is due to rider torque applied to the cranks, based on the information it has from the one or more sensors. The drive-element may be a drive belt having a bias to adopt, when de-tensioned, a curve with a radius larger than that of the drive-wheel. The drive apparatus may further comprises: a tensioner being operable to apply tension to the belt whereby the belt can couple the motor to the drive-wheel, and being operable to release tension from the belt whereby the belt can follow a path which is free of the drive-wheel; and a guide for restraining the belt outboard of the drive-wheel along the path where the belt is free of the drive-wheel. The drive apparatus may comprise a housing attachable to the HPV, the motor and drivewheel each being carried by the housing, wherein the drive-wheel has an opening extending through its centre, the opening being accessible from opposing sides of the housing whereby a crank on the HPV can be passed through the opening for mounting to the drive-wheel. According to a second aspect there is provided a drive apparatus for providing assistance to an HPV having a bottom-bracket assembly, the bottom-bracket assembly comprising an axle, a pair of cranks for attachment to pedals, and a drive-side output, the axle and cranks and drive-side output being coupled so as to rotate as one, the drive apparatus comprising: a motor; an output from the motor which is couplable to the bottom-bracket assembly; a one-way clutch which, when engaged, couples the motor to the output to provide forwards propulsive torque to the bottom-bracket assembly, the clutch being configured to engage in response to the motor rotating at an engagement speed, and to disengage in response to the motor rotating at less than the engagement speed; and a controller which is able to determine the speed of the motor and a measure of pedalling effort and is arranged to use this information to control the output torque of the motor during periods when assistance is not to be provided. During periods when assistance is not to be provided, the controller may be configured to control the speed of the motor to be less than the engagement speed based on said information. Pedalling effort may be based on the speed of the bottom-bracket assembly. The controller may be configured to determine pedalling effort based on information received from one or more sensors configured to measure the speed of the bottom-bracket assembly. The controller may be configured to calculate a torque to apply to the motor during periods when assistance is not to be provided based on a determined difference between the measured speed of the motor with the expected speed of the motor. The controller may be configured to determine pedalling effort based on a detected coupling force arising from a coupling between the output of the motor and the bottom-bracket assembly. The controller may be configured to calculate a torque to apply to the motor during periods when assistance is not to be provided based on the detected coupling force. Pedalling effort may be based on a pedal force applied by a rider of the human powered vehicle through pedals of the HPV. The controller may be configured so that, when assistance is not to be provided and when the controller senses that the speed of the motor is below the engagement speed, the controller calculates a torque to apply to the motor as being that which is likely to maintain a pre-determined difference between the speed of the motor and the engagement speed. The predetermined difference may be based on a maximum gear ratio of the HPV. The drive apparatus may be provided with a bottom-bracket assembly for an HPV, the bottombracket assembly comprising: a hollow cartridge carrying in its bore a plurality of bearings to support a bottom-bracket axle which can revolve inside the bearings, the cartridge being mountable to a frame of the HPV; a first mounting structure for the cartridge, the first mounting structure having an elastic resistance to rotation in yaw and roll of the cartridge relative to the frame of the HPV that is insufficient on its own to prevent said rotation when the bottom-bracket axle is subjected to transverse forces which are not in balance about the first mounting structure such that the first mounting structure provides a fulcrum for said rotation of the cartridge; and a second mounting structure axially displaced from the first mounting structure when the cartridge is mounted to the frame of the HPV, the second mounting structure having a compliant resistance to the rotation in yaw and roll of the cartridge; and one or more sensors configured to sense radial displacement of the cartridge arising from said rotation of the cartridge about the fulcrum. The controller may be operable to interpret the pedal force from the one or more sensors. When assistance is not to be provided, the controller may be configured to calculate a torque to apply to the motor as a function of the pedal force. The controller may be operable to interpret the coupling force from the one or more sensors. When assistance is not to be provided, the controller may be configured to calculate a torque to apply to the motor as a function of the coupling force. The engagement speed may be proportional to the speed of the bottom-bracket assembly. According to a third aspect there is provided a drive apparatus for an HPV which comprises a crank-assembly and which is propelled by the crank-assembly, the drive apparatus comprising: a housing attachable to the HPV; a motor carried by the housing; and a drive-wheel in the housing and couplable to the motor to be driven thereby, the drivewheel being attachable to the crank-assembly to revolve therewith, the drive-wheel having an opening extending through its centre, the opening being accessible from opposing sides of the housing whereby a crank on the HPV can be passed through the opening for mounting to the drive-wheel. The drive-wheel may be captive in the housing in such a manner that it can float radially with respect to the housing. The housing may comprise internal formations for engaging the drive-wheel to hold the drivewheel captive in a position in which the opening of the drive-wheel is accessible from opposing sides of the housing. The internal formations may be configured to engage the periphery of the drive-wheel. The drive-wheel may have an axially-extending first ring structure. The internal formations may comprise a second ring structure located radially inboard or radially outboard of the first ring structure. The second ring structure may be located radially inboard of the first ring structure. The internal formations may comprise a third ring structure located radially outboard of the first ring structure. The internal formations may comprise only the second ring structure located radially inboard of the first ring structure. The housing may surround the drive-wheel circumferentially thereof. The housing may extend only partially around the circumference of the drive-wheel. The drive apparatus may further comprise a drive belt for coupling the drive-wheel to the motor. The drive-wheel may comprise a rim and a plurality of lobes extending radially inwardly from the rim. Each lobe may comprise a securing formation whereby the drive-wheel can be secured directly or indirectly to a crank on an HPV. The drive-wheel may comprise one or more through-holes whereby the drive-wheel can be secured to a crank on an HPV. The drive apparatus may be provided with an HPV comprising a pedal, wherein the opening of the drive-wheel is shaped so as to permit the pedal of the HPV to pass therethrough. The housing may comprise a plurality of first mounting structures whereby the housing can be mounted to an HPV and at least one of those mounting structures has an electrical conductor which comes into contact with an electrical conductor on the HPV when the drive apparatus is connected thereto for conveying electricity between the HPV and the drive apparatus. The drive apparatus may be provided with an HPV, the housing of the drive apparatus comprising a plurality of first mounting structures and the HPV comprising a plurality of second mounting structures. The first and second mounting structures may be disposed so that when the drive apparatus is mounted to the HPV with each of the first mounting structures engaged with a respective one of the second mounting structures the drive-wheel can be positioned so that a bottom-bracket axis of the HPV extends through the centre of the drive-wheel. The HPV may have a structural frame comprising the second mounting structures. The drive apparatus may be configured such that when each of the first mounting structures is engaged with a respective one of the second mounting structures and the drive-wheel is positioned so that the bottom-bracket axis of the HPV extends through the centre of the drive-wheel, the motor of the drive apparatus is located closer to a mid-plane of the frame than the drive-wheel. At least one of the first mounting structures may be located radially outboard of the periphery of the drive-wheel. At least one of the first mounting structures may be located radially inboard of the periphery of the drive-wheel. The drive-wheel may define an access port located such that when the centre of the drive-wheel is on the bottom-bracket axis of the HPV the said one of the first mounting structures can be accessed through the port in a direction parallel with the bottombracket axis for securing the drive apparatus to the HPV. The drive apparatus may be mounted to the HPV for driving motion of the HPV. The HPV may have a wheel for engaging a power transmission element, the wheel being mounted about the bottom-bracket axis on a drive-side of the HPV and the said power transmission element coupling the wheel to a road wheel of the HPV. The drive-wheel may be located on the opposite side of the HPV to the drive-side. The drive apparatus may further comprise a drive belt coupling the drive-wheel to the motor. The belt may have a bias to adopt when de-tensioned, a curve with a radius larger than that of the drive-wheel. The drive apparatus may further comprise: a tensioner being operable to apply tension to the belt and being operable to release tension from the belt; and a guide for restraining the belt outboard of the drive-wheel along a path where the belt is free of the drive-wheel. There is provided a method of attaching the drive apparatus to an HPV comprising a crank attached thereto, the method comprising: passing, whilst the crank remains attached to the HPV, the opening of the drive-wheel over the crank; and, subsequently, securing the drive-wheel to the crank. The crank may carry a pedal. The method may comprise performing the passing step whilst the pedal remains carried by the crank. The passing step may comprise tilting the drive apparatus with respect to the HPV and translating the drive apparatus along the crank. According to a fourth aspect there is provided a drive apparatus for an HPV, the drive apparatus comprising: a housing attachable to the HPV; a motor carried by the housing; a drive-wheel for providing drive from the motor to the HPV; a drive belt having a bias to adopt, when de-tensioned, a curve with a radius larger than that of the drive-wheel; a tensioner being operable to apply tension to the belt whereby the belt can couple the motor to the drive-wheel, and being operable to release tension from the belt whereby the belt can follow a path which is free of the drive-wheel; and a guide for restraining the belt outboard of the drive-wheel along the path where the belt is free of the drive-wheel. The drive belt may be a toothed belt. The tensioner may be an idler pulley moveable with respect to the housing for increasing the length of the path taken by a non-drive portion of the drive belt to apply tension to the drive belt. The tensioner may be operable to displace the motor with respect to the drive-wheel in a direction perpendicular to their axes of rotation to selectively apply tension to the belt. The tensioner may comprise a manually operable member moveable from a first position in which it causes the tensioner to apply tension to the belt to a second position in which it causes the tensioner to release tension from the belt, and a securing mechanism for securing the handle in the first position. The drive apparatus may be arranged to automatically turn off the motor in response to the tensioner being operated to release tension from the belt. A segment of the guide may be disposed at a constant radial offset outboard of the drivewheel. The guide may comprise a barrier that extends continuously for at least 90 degrees around the circumference of the drive-wheel. The guide may be discontinuous. The guide may be integral with the housing. The belt may be biased so as to bear outwardly against the guide when tension is released from the belt. The drive-wheel may have an opening extending through its centre, the opening being accessible from opposing sides of the housing whereby a crank on the HPV can be passed through the opening for mounting to the drive-wheel. The drive apparatus of the third or fourth aspect may be provided with a bottom-bracket assembly. The bottom-bracket assembly may comprise: a hollow cartridge carrying in its bore a plurality of bearings to support a bottom-bracket axle which can revolve inside the bearings, the cartridge being mountable to a frame of the HPV; a first mounting structure for the cartridge, the first mounting structure having an elastic resistance to rotation in yaw and roll of the cartridge relative to the frame of the HPV that is insufficient on its own to prevent said rotation when the bottom-bracket axle is subjected to transverse forces which are not in balance about the first mounting structure such that the first mounting structure provides a fulcrum for said rotation of the cartridge; a second mounting structure axially displaced from the first mounting structure when the cartridge is mounted to the frame of the HPV, the second mounting structure having a compliant resistance to the rotation in yaw and roll of the cartridge; and one or more sensors configured to sense radial displacement of the cartridge arising from said rotation of the cartridge about the fulcrum. The motor may have a rotor and a motor-output wheel. The drive belt may couple the motoroutput wheel to the drive-wheel so as to deliver torque from the motor to the drive-wheel. The drive apparatus may further comprise a one-way clutch configured to engage when the rotor of the motor is revolving at an engagement speed, and to disengage when the rotor is revolving at a speed less than the engagement speed. The engagement speed may be a predefined multiple of the speed at which the bottom-bracket assembly is revolving. The oneway clutch may be positioned in the drivetrain between the rotor and the drive-wheel. The drive apparatus may further comprise a controller configured to receive data from one or more sensors. The controller may be configured to use the data from the one or more sensors to decide a level of torque that the motor is to apply to the rotor. The controller may be configured to, in a drive state of the HPV, control the motor to cause it to apply the decided level of torque to the rotor. According to a fifth aspect there is provided a bottom-bracket assembly for a human-powered vehicle (HPV), the HPV comprising a drive-side and a non-drive-side and a power output means attached to the drive-side of the HPV for conveying power to a road wheel of the HPV, the bottom-bracket assembly comprising: a hollow cartridge carrying in its bore a plurality of bearings to support a bottom-bracket axle which can revolve inside the bearings, the cartridge being mountable to a frame of the HPV; a first mounting structure at a drive-side end of the cartridge, the first mounting structure having an elastic resistance to rotation in yaw and roll of the cartridge relative to the frame of the HPV that is insufficient on its own to prevent said rotation when the bottom-bracket axle is subjected to transverse forces which are not in balance about the first mounting structure such that the first mounting structure provides a fulcrum for said rotation of the cartridge; a second mounting structure positioned axially nearer to the non-drive-side of the HPV than the first mounting structure when the cartridge is mounted to the frame of the HPV, the second mounting structure having a compliant resistance to the rotation in yaw and roll of the cartridge; and one or more sensors configured to sense radial displacement of the cartridge arising from said rotation of the cartridge about the fulcrum. The first mounting structure may provide resistance to radial motion of the cartridge at the fulcrum. The imbalance of the transverse forces on the bottom-bracket axle may be due to tension in a final drive element of the power output means of the HPV, the final drive element being connected to the bottom-bracket axle. The imbalance of the transverse forces on the bottom-bracket axle may be due to differing forces applied by a rider of the HPV to each pedal of the HPV, each pedal being attached to a respective crank, and each crank being attached to the bottom-bracket axle. It may be arranged that the cartridge when it is fitted to an HPV is subjected to a tensile force which is imparted to the cartridge by an equal axial force at each of the first and the second mounting structures. It may be arranged that the cartridge when it is fitted to an HPV is subjected to a compressive force which is imparted to the cartridge by an equal axial force at each of the first and the second mounting structures. The position of the fulcrum provided by the first mounting structure at the drive-side end of the cartridge may be axially offset from the position of each of the plurality of bearings. The hollow cartridge may be sized such that it fits inside a bottom-bracket shell whose minimum bore is less than 40mm. The bottom-bracket shell may be of a type that can be incorporated into the frame of a standard bicycle or tricycle. The plurality of bearings may include at least one needle bearing. The assembly may further comprise a controller configured to determine an imbalance of the transverse forces on the bottom-bracket axle based on readings taken from the one or more sensors. The second mounting structure may comprise an elastomer member for providing said compliant resistance to the rotation in yaw and roll of the cartridge, wherein a surface of the elastomer member that contacts a rigid element of the second mounting structure is disposed at an angle to the longitudinal axis of the bottom-bracket axle. One or more sensor members may be configured to connect, at a position which is axially displaced from the fulcrum, the cartridge, or a structure rigidly secured to the cartridge, to the frame of the HPV or to a structure rigidly secured to the frame of the HPV. Each sensor member may comprise a sensor wherein a change of state is induced by a change in strain in the sensor member which arises from radial displacement of the cartridge in a given direction relative to the frame of the HPV. The second mounting structure may comprise one or more beams which support the cartridge in the frame of the HPV and which provide elastic resistance to radial motion of the cartridge and where a sensor is mounted on one or more of said beams wherein a change of state is induced in the sensor by a change in strain in the beam arising from displacement of the cartridge in a given radial direction relative to the frame of the HPV. The bottom-bracket assembly may be used with a drive apparatus which provides assistance from a motor to the rider of an HPV, the motor comprising a rotor, wherein the level and direction of the torque which the controller applies to the rotor is a function of the determined imbalance of the transverse forces on the bottom-bracket axle. The features of the drive apparatus under the heading “Quick Unbolt” may be combined with any of the features described under the heading “Quick Isolate”, and / or with any of the features described under the heading “Cadence Drop”, and / or with any of the features described under the heading “Floating Cartridge”. The features of the drive apparatus under the heading “Quick Isolate” may be combined with any of the features described under the heading “Quick Unbolt”, and / or “Cadence Drop”, and / or “Floating Cartridge”. The features of the drive apparatus under the heading “Cadence Drop” may be combined with any of the features described under the heading “Quick Unbolt”, and / or “Quick Isolate”, and / or “Floating Cartridge”. The features of the bottom-bracket assembly under the heading “Floating Cartridge” may be combined with any of the features described under the heading “Quick Unbolt”, and / or “Quick Isolate”, and / or “Cadence Drop”. BRIEF DESCRIPTION OF THE FIGURES The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows an example of human powered vehicle in the form of a bicycle. Figure 2 shows a drive apparatus attached to a bicycle such as that of figure 1. Figure 3 shows a cross-section of the drive apparatus. Figures 4a to 4c show examples of internal formations of the housing of the drive apparatus and the drive-wheel. Figures 5a to 5e depict progressive steps of the drive apparatus being removed from a bicycle. Figure 6 depicts the method of attaching the drive apparatus to an HPV. Figures 7a and 7b show a cross-section of the drive apparatus depicting a tensioning system in a de-tensioned state and tensioned state. Figure 8 shows a cross-section of the motor and gearbox of the drive apparatus. Figure 9 shows a flow diagram of an example control algorithm for controlling the speed of the motor in response to data indicating whether motor drive is required. Figure 10 shows a first example bottom-bracket assembly of the present disclosure comprising a cartridge. Figure 11 shows an example sensing arrangement of the bottom-bracket assembly of figure 10. Figure 12 shows a second example bottom-bracket assembly of the present disclosure comprising a cartridge. Figure 13 shows an example sensing arrangement of the bottom-bracket assembly of figure 12. Figure 14 shows a third example bottom-bracket assembly of the present disclosure comprising a cartridge. Figure 15 shows an example sensing arrangement of the bottom-bracket assembly of figure 14. Figure 16 depicts the directions of pitch, yaw and roll with respect to a bicycle. DETAILED DESCRIPTION Figure 1 shows an example of a bicycle 100. The bicycle comprises front wheel 1, rear wheel 2 and a frame 3. The frame comprises a top tube 4, a seat tube 5, a down tube 6, chain stays 7 and seat stays 8. Other designs of frame are possible. A bottom-bracket shell 9 (e.g. see figure 5a) is located at the junction of the seat tube and the down tube. The bottom-bracket shell houses a bottom-bracket 10. Cranks 11 on either side of the frame are attached to each other by an axle 12 that passes through the bottom-bracket shell. The bottom-bracket provides a bearing by which the axle is mounted for revolute motion with respect to the frame. A pedal 13 is mounted to each crank to allow a rider to turn the cranks. On one side of the frame a chain wheel 14 is attached to the axle. A chain 15 runs in an endless loop between the chain wheel and a sprocket 16 attached to the rear wheel. This carries drive from the chain wheel to the rear wheel, enabling the rider to drive the bicycle to move. The side of the frame on which the chain is located is known as the drive-side, as will be explained in more detail later. The chain could be replaced by other means for conveying drive to the rear wheel, for example an endless belt or a drive shaft. Figure 2 shows a drive apparatus 200 for driving the bicycle of figure 1. Figure 2 shows the bicycle of figure 1 from the side opposite to that comprising the chain wheel 14. In other words, figure 2 shows the bicycle from the non-drive-side. The drive apparatus comprises a housing 20. The housing could couple to the bicycle in other ways: for example to the seat tube and / or to one or more chain stays. The housing contains a motor 22, a gearbox 23, a drive belt 24, a drive-wheel 25 and a control processor 26 (not shown). The housing 20 is provided with mounts 21 whereby it can be coupled to the frame of the bicycle. The mounts could couple to the frame at any suitable location. One convenient approach is for a mount (21a) to couple to the down tube 6 of the bicycle and / or for a mount (21b) and 21(c) above and below the drive-wheel respectively to couple to a structure secured to the non-drive-side of the bottombracket shell of the bicycle. A battery 27 (not shown) for powering the motor may be contained in the housing or mounted elsewhere on the bicycle. The control processor controls the operation of the motor in a manner that will be described in more detail below. The motor has a rotor. The rotor drives an input of the gearbox. An output of the gearbox can engage the drive belt 24 and the drive belt can engage the drive-wheel. In that way the motor can drive the drive-wheel to revolve. A first notable feature of the drive apparatus is that the drive apparatus does not comprise an axle on which the drive-wheel revolves. Instead, the drive-wheel has an opening 28 through its central portion. In the state in which the drive apparatus is shown in figures 5c, 5d and 5e (for example), the drive-wheel may be free to float, for example by translation in its major plane, relative to the housing. The drive-wheel may be able to translate relative to the housing. The drive-wheel may be captive in the housing so as to prevent it from leaving the housing despite its ability to float relative to the housing. To achieve that the housing may define an opening 29 through which the drive-wheel is accessible. The greatest diameter of that opening may be less than the outside diameter of the drive-wheel. When the drive apparatus is to be mounted to the bicycle of figure 1, the pedal and the crank on one side of the bicycle are passed through the opening 28 in the drive-wheel. The drive-wheel can then be attached to the crank. The housing is arranged so that when the drive-wheel is attached to the crank, the mounts can attach the housing to the frame of the bicycle. In that configuration the drivewheel can no longer float relative to the housing. The drive-wheel is rigid with crank axle 12 and can revolve about its axis. Since the translational position of the drive-wheel is fixed relative to the housing it can then be driven by belt 24 despite the drive-wheel having no axle extending directly between it and the housing. This provides a convenient way to allow the drive apparatus to be attachable to and removable from the bicycle, as will be described further below. A second notable feature is that the drive apparatus comprises a selectively actuable belt tensioner 30. The belt tensioner is movably mounted to the housing. It has a first configuration in which it bears on the belt 24 to tension the belt and a second configuration in which the belt 24 is de-tensioned. The belt tensioner can comprise an idler wheel 76 which can bear against a non-drive portion of the belt. The idler wheel can be moved between a first position where the belt is tensioned around the drive-wheel 25 and a wheel 75 that is driven by the motor and a second position in which the belt is de-tensioned. One way to implement this is for the belt tensioner to be mounted on an arm that pivots with respect to the housing. The arm could be actuated by a lever or other means, as will be described further below. The belt has inherent stiffness. Put another way, the belt resiliently resists being bent to change its natural curvature. As a consequence, the belt is inherently biased to adopt a greater radius than that of the rim (i.e. the outer circumference) of the drive-wheel. When the tensioner is actuated to bear on the belt, it tensions the belt around the output of the gearbox and the drive-wheel. In that configuration the output of the gearbox and the drive-wheel are constrained to revolve together with a ratio of speeds determined by their respective radii. When the tensioner is released so as to relax the belt, the belt is inherently biased to adopt a greater radius. A guide defines an open channel immediately outboard of the drive-wheel. When de-tensioned, the belt can expand into that channel, taking it free of engagement with the drive-wheel. In that configuration the drive-wheel can be revolved independently of the output of the gearbox. This is convenient because it allows rider to turn the cranks without any connection with the drive apparatus, and to choose between on the one hand assisted pedalling and on the other, when battery is absent or discharged or when rider doesn’t want assist, unassisted pedalling, with no resistance whatever from the transmission or motor. This is also convenient because, when a drive apparatus characterised by the first notable feature is being mounted to the bicycle, it permits an operator to turn the drive-wheel into register with the crank arm, so as to align fixings, mounting holes or the like on the drive-wheel and the crank arm. This makes it easier to attach the drive apparatus to the bicycle. A third notable feature is that, whereas there is a non-clutched drive path between each crank and the chain wheel, there is a one-way clutch 80 located between the motor and the input of the gearbox 23. The clutch could be located at other points, as will be described further below. The gearbox and the belt drive define a drive ratio between the motor and the crank axle. That drive ratio defines a motor engagement speed for any forward rotation speed of the cranks. When the motor (more precisely the rotor of the motor) is rotating at that engagement speed the clutch 80 will engage so that the rotor can deliver torque to the cranks. When the rotor is rotating forwards below that speed or backwards above that speed the clutch is slipping and the rotor cannot deliver torque to the cranks. The clutch could, for example, be a ratchet clutch. As will be described further below, the control processor 26 receives data from sensors indicating the speed of the cranks and the speed of the rotor. The controller implements a control algorithm to control the current applied to the windings and thereby control the torque applied to the rotor in response to that data and also in response to data indicating whether motor drive is required. That latter data may be received from a control switch operable by a rider of the HPV and / or, as will be described further below, from one or more sensors which respond to changes in rider force on the cranks and / or from the output of an algorithm implemented by the controller. When the motor is not driving the cranks, the rotor is usually rotating below the engagement speed for the current speed of the cranks. When the controller determines that motor drive is required, it increases the speed of the rotor to the engagement speed. That causes the clutch to engage, carrying drive from the rotor to the cranks. When the controller determines that motor drive is no longer required, it reduces the speed of the rotor to below the engagement speed. That causes the clutch to release and stops the motor driving the cranks. Because of the clutch, the controller is able to reduce the rotor-speed below the engagement speed in this way without the rider being subject to any unwanted slowing sensation from the pedals, even when the rate of reduction of rotor-speed exceeds an equivalent rate of reduction of cadence. A fourth notable feature is provided by a bottom-bracket assembly for an HPV which carries one or more sensors which respond to the level of transverse external forces applied to the bottom-bracket axle (e.g. arising from a rider’s pressing down on the pedals). In such a bottombracket assembly, support for the bottom-bracket bearing at the non-drive-side of the HPV is provided by an elastically compliant structure, whereby, when a radial load in a given direction is carried by the bearing and in turn by the compliant structure, there is a displacement of the bearing which depends on the load and which can be detected by a sensor. The radial load carried by the non-drive-side bearing in a given plane will be such as balances the moment about a fulcrum which arises from rider-induced external radial forces applied in the same plane to the bottom-bracket assembly. The fulcrum will tend to lie where the drive-side bearing for the bottom-bracket axle is positioned. At any crank angle, there will be a response from the one or more sensors to changes in rider-induced forces. A notable feature of the present disclosure is a variant of this arrangement, where the bearings which support the bottombracket axle are carried in a rigid cartridge assembly, and it is for this cartridge that the elastically compliant structure provides support, so that the whole cartridge including bearings and axle can “float” in the bore of the bottom-bracket shell of a bicycle. There is a number of ways to arrange support for the cartridge, and the geometry of the drive-side support for the cartridge can be chosen so that the fulcrum tends to a particular preferred position along the axle, which in turn affects the level, for a given external transverse force on the bottom-bracket assembly, of the balancing radial force provided by the elastically compliant structure on the non-drive-side. Other advantages, including mechanical simplification, are possible when such a cartridge is used to support the bearings. Each of the notable features identified above may be implemented independently of any of the others. One or more of them may be implemented together. References herein to a bicycle or tricycle should be understood to apply equally to any HPV. Some exemplary arrangements will now be described in more detail. QUICK UNBOLT The drive apparatus of the present disclosure is designed to be easily attached and detached from a bicycle, tricycle, or any other HPV. This can give the rider flexibility to decide whether to use their bicycle with the drive apparatus or without. A particularly notable feature of the drive apparatus 200, is that it attaches and detaches from the non-drive-side of the HPV. The drive apparatus is adapted, for example by the location of its fixings to the vehicle, to be attached on the non-drive-side of the vehicle. The drive apparatus is adapted, for example by the design and position of its coupling for attachment to the bicycle to convey drive torque thereto, to apply drive torque on the non-drive-side of the bicycle. An arrangement of this type can avoid the drive-side of the HPV being affected by, or needing to be modified to permit, attaching or detaching the drive apparatus. The non-drive-side of an HPV is the side that does not comprise an output means or power transmission element to convey drive to a road wheel of the vehicle, such as a chain set and chain or a rotary shaft running longitudinally with respect to the vehicle. The non-drive-side is often the left-hand side of a bicycle (when looking at the bicycle from behind), but it may not be. The HPV comprises a wheel (e.g. a chain wheel 14 or chain ring) for engaging a power transmission element. The power transmission element may be a chain 15, a belt, or a rigid rod or shaft in the case of shaft-driven bicycles. The wheel is connected to an axle (e.g. the bottom-bracket axle) which can turn in response to input from a rider (e.g. via pedals which are connected to cranks which connect to the axle and revolve with the axle). The power transmission element (e.g. the chain or belt) couples the wheel (e.g. the chain ring) to a road wheel of the HPV. A road wheel is a wheel of the HPV that contacts the road or ground on which the HPV moves. The wheel (e.g. the chain ring) is mounted around the bottom-bracket axle on one side of the HPV. The side of the HPV on which the wheel is mounted is termed the drive-side of the HPV. The non-drive-side is the opposite side of the HPV to the drive-side. Figure 2 shows a part of the HPV from the non-drive-side. The side view of the drive apparatus shown in figures 5a-5e is also viewed from the non-drive-side. In the right-hand diagrams in figures 5a-5e which show the bicycle from behind, the chain ring of the bicycle has not been shown (but it would conventionally be on the right-hand side, if present). The drive apparatus 200 shown in figure 2 comprises a housing 20, as mentioned above. The housing is configured to attach to the HPV, e.g. a bicycle. The housing may comprise a plurality of first mounting structures 21. Each of the first mounting structures couples to a corresponding mount, referred to as a second mounting structure 53, on the HPV (see figures 5a-5e). Any suitable means such as bolts and screws may be used to secure the first and second mounting structures together. For example, figure 5a shows a bolt 54 securing one of the first mounting structures 21 to a second mounting structure 53. Bolts 54 can also be seen being removed from the housing in figure 5b. The second mounting structures 53 may be attached to the frame of a standard HPV in any suitable manner. For example, they may be welded, brazed, bolted or clamped to the frame, or attached using an adhesive. The frame may be manufactured with suitable mounts that may be used as the second mounting structures. As an example, the bicycle frame shown in figure 5d comprises three second mounting structures 53; one on the down tube 6 of the bicycle (53a), and two, 53b and 53c, carried respectively in the upper and lower arms of a structure secured to the non-drive-side (e.g., the left-hand end) of the bottom-bracket shell. The second mounting structures could lie at any suitable location on the frame. A preferable location for the mounting structures will be described later. Of the first mounting structures 21, at least one may comprise an electrical connector (not shown) exposed at a surface of that mounting structure. When the drive apparatus is connected to the HPV, the surface of the at least one first mounting structure may contact (e.g., abut) a mating surface on the HPV for conveying electricity between the HPV and the drive apparatus. In other words, when the drive apparatus is attached to the HPV, the surface of that mounting structure may make electrical contact with a part of the HPV to allow electricity to pass between the drive apparatus and the HPV. This may be useful for conveying power from a battery located elsewhere on the HPV or for conveying control signals for controlling the operation of the drive apparatus. Examples of such control signals may be signal from sensors mounted to the HPV or from rider inputs such as switches. The housing 20 comprises a motor 22. The motor may be an electric motor. The motor is carried by the housing. In other words, the housing contains the motor. The housing comprises a drive-wheel 25. The drive-wheel is driven by power from the motor. For example, an output from the motor (e.g. an output from the gearbox) may be coupled to the drive-wheel via a belt 24. The belt transfers drive from the output of the motor to the drivewheel to thereby drive the drive-wheel. The drive-wheel 25 is carried in the housing. Figure 3 shows a cross-section of the drive apparatus 200, in which the drive-wheel 25 is visible. The drive-wheel 25 may be loosely contained within the housing, when the drive-wheel is not connected to the crank of a bicycle (as will be explained later). In other words, the drive-wheel may be free to float (e.g. move around) within the housing. The drive-wheel may float in a radial direction (e.g. in the direction of the dashed line R in figure 3) with respect to the housing. The drive-wheel may float in an axial direction (e.g. in the direction of the dashed line A in figure 3) with respect to the housing. Whilst the drive-wheel may be free to float in the housing, the housing comprises internal formations 31 which retain the drive-wheel so that it does not escape from the housing. For example, the walls of the housing can act as internal formations to restrict movement of the drive-wheel in the axial direction. Internal formations 31 such as those shown in figure 3 act to retain the drive-wheel in a radial direction. The internal formations also act to hold the drivewheel captive in a position in which the opening of the drive-wheel is accessible from opposing sides of the housing. The internal formations will be described in more detail below, with reference to figures 4a to 4c. The internal formations 31 may be configured to engage the periphery of the drive-wheel and / or portions of the drive-wheel inboard of its periphery. In the former case the internal formations may be configured to engage a part of the drive-wheel around the circumference of the drive-wheel. Figure 3 shows an example in which internal formations 31 are provided for engaging a cylindrical flange at the periphery of the drive-wheel so as to inhibit translation of the drive-wheel in a direction perpendicular to its axis. The internal formations could fulfil that function by engaging an inward-facing surface of the drive-wheel. The internal formations could fulfil that function by engaging an outward-facing surface of the drive-wheel. The drive-wheel may comprise an axially-extending drive-wheel ring structure 32. The drivewheel ring structure 32 extends in the direction of the axis of the drive-wheel (e.g. in the direction of the dashed line A). For example, figure 3 shows the drive-wheel 25 with an axially extending drive-wheel ring structure 32 on which the belt 24 lies. In the example shown in figure 3, the drive-wheel ring structure 32 is at the periphery of the drive-wheel. In the case where the drive-wheel ring structure is positioned inboard of the rim of the drive-wheel, the belt may lie on the outside surface of the rim of the drive-wheel. The drive-wheel ring structure 32 need not be positioned at the outside surface of the rim of the drive-wheel. For example, the drive-wheel ring structure may be radially inboard of the rim of the drive-wheel. For example, figures 4a to 4c show an example of a drive-wheel ring structure 32 that is radially inboard from the rim of the drive-wheel. The drive-wheel ring structure 32 may be discontinuous. For example, the drive-wheel ring structure 32 shown in figure 4c does not extend fully around the drive-wheel in one piece. The drive-wheel ring structure 32 shown in figure 4c comprises two pieces that form a ring structure around the drive-wheel. The drive-wheel ring structure may be continuous. In other words, the drive-wheel ring structure may extend around the drive-wheel in one piece to form a full circle. In the case where the drive-wheel ring structure is continuous, it may be called a drive-wheel ring. As mentioned above, the internal formations 31 of the housing act to retain the drive-wheel within the housing and position the drive-wheel so that the opening of the drive-wheel can be accessed from both sides of the housing. The internal formations may comprise a housing ring structure 31. The housing ring structure may be discontinuous. The housing ring structure 31 may be located radially inboard or radially outboard of the drive-wheel ring structure. For example, figure 4a shows a housing ring structure 31a located radially inboard of the drivewheel ring structure 32. The housing ring structure 31 could be located radially outboard of the drive-wheel ring structure 32. Either configuration retains the drive-wheel 25 from escaping the housing in a radial direction (e.g. in a direction parallel to the dashed line R) due to the axially-extending drive-wheel ring structure. In other words, it is sufficient to radially retain the drive-wheel with a housing ring structure that is either located radially outboard of the drivewheel ring structure, or radially inboard of the drive-wheel ring structure. As mentioned above, the side walls of the housing 20 may retain the drive-wheel from escaping the housing in an axial direction (e.g. in a direction along the dashed line A). The internal formations may comprise housing ring structures that are located both radially inboard and radially outboard of the drive-wheel ring structure 32. For example, figure 4b shows a first housing ring structure 31a located radially inboard of the drive-wheel structure and a second housing ring structure 31b located radially outboard of the drive-wheel ring structure. Thus, the internal formations may comprise a first housing ring structure and a second housing ring structure. Either the first or second housing ring structures may be discontinuous. Figure 4c shows an example of a first and second housing ring structure positioned around the drive-wheel 25. The first housing ring structure 31a is located radially inboard of the drivewheel ring structure 32. The first housing ring structure 31a may be formed by flanges 31a (e.g. ridges) that provide a surface against which the inside of the drive-wheel ring structure can rest. The second housing ring structure may be formed by pins 31b encircling the outside of the drive-wheel ring structure. The positions of the pins and the flange may be varied. Any suitable means may be used in place of pins, flanges, and ridges as the first and second housing ring structures. As shown in figure 4c, it is not necessary for the internal formations to be positioned all the way around the drive-wheel. The first and second housing ring structures shown in figure 4c are discontinuous. The drive-wheel has an opening 28 through its central portion. As mentioned above, this means that when the drive apparatus is not attached to an HPV the drive-wheel is not constrained to revolve on an axle or a shaft running through its central axis. The opening of the drive-wheel is shaped so as to permit a crank of the HPV to pass therethrough. The drivewheel may also be shaped so as to permit a pedal of the HPV to pass therethrough. For example, at least part of the opening of the drive-wheel may be at least 60mm across in a direction perpendicular to the axis of the drive-wheel. The opening 28 is accessible from opposing sides of the housing. This allows a crank of the HPV to be passed through the opening for mounting the crank to the drive-wheel, as will be explained in more detail later. The housing may comprise an opening 29 through which the opening 28 of the drive-wheel can be accessed. Each side of the housing may comprise a respective opening which together form the opening 29 of the housing. In other words, the opening 29 of the housing may be formed from openings on two opposing sides of the housing, though which the opening 28 of the drive-wheel can be accessed. The housing may surround the drive-wheel circumferentially thereof. For example, the housing shown in figure 2 surrounds the drive-wheel around its circumference. When the housing surrounds the drive-wheel circumferentially thereof, the housing comprises a hole through which the opening of the drive-wheel 28 can be accessed. In other words, the opening 29 of the housing is a hole. However, the housing need not surround the drive-wheel fully. The housing may extend only partially around the circumference of the drive-wheel. For example, a portion of the drive-wheel may be exposed. In this case, the opening 29 of the housing may not be a hole. The shape of the housing may be such that the opening 28 of the drive-wheel is accessible from opposing sides of the housing, without there being a hole through the housing. For example, one end of the housing may be shaped like a crescent moon. The attachment of the housing to the frame of an HPV, and the drive-wheel to the crank of an HPV will now be described, with reference to figures 5a to 5e. Figures 5a to 5e show progressive snap shots of the drive apparatus being removed from the HPV. The crank to which the drive-wheel attaches may be modified to comprise attachments to which the drive-wheel can attach. For example, as best seen in figure 5d or 5e, the crank may comprise attachments 50. The drive-wheel comprises a securing formation or securing formations 51 to secure the drive-wheel to the crank. For example, figures 5a to 5e show four securing formations 51 on the drive-wheel which correspond to the four attachments 50 on the crank for attaching the drive-wheel to the crank. The securing formations may be one or more through-holes in the drive-wheel. Bolts (not shown) may be used to connect the securing formations (e.g. through-holes) to the attachments on the crank. There may be any number of attachments on the crank, and any number of securing formations on the drive-wheel. In an example, the drive-wheel may comprise a lobe or a plurality of lobes extending radially inwards from the edge or rim of the drive-wheel. The rim of the drive-wheel may include the edge of the drive-wheel. Each lobe comprises at least one securing formation. For example, the drive-wheel shown in figures 5a to 5e comprises two lobes 52, best seen in figures 5d and 5e. In this example, each lobe 52 in figures 5a to 5e comprises two securing formations 51. In the figures 5a to 5d, the lobes 52 of the drive-wheel are visible. Once the securing formation on each lobe has been secured to the attachments on the crank, motion of the drive-wheel, and by extension, the lobes, will transfer to motion of the cranks, and vice versa. As mentioned above, the housing is attachable to the HPV. The HPV comprises a structural frame, as described above with respect to a bicycle. To facilitate attachment of the drive-wheel to the crank, the first and second mounting structures are disposed so that when the drive apparatus is mounted to the HPV with each of the first mounting structures engaged with a respective one of the second mounting structures, the drive-wheel can be positioned so that a bottom-bracket axis of the HPV extends through the central portion of the drive-wheel. As mentioned above, when the drive-wheel is not attached to the crank, the drive-wheel is free to float in the housing. So, when initially attaching the drive apparatus to the HPV, the drive-wheel may be at a variety of positions within the housing. The positioning of the first and second mounting structures ensures that, when each of the first mounting structures is connected to the corresponding second mounting structure, the centre of the drive-wheel is approximately aligned with the axis of the bottom-bracket of the HPV. When the drive apparatus is attached to the HPV, the motor of the drive apparatus may be located closer to a mid-plane of the frame of the HPV than the drive-wheel. The mid-plane of the frame of the HPV is the plane that bisects the frame down the middle when viewing the HPV front on. The mid-plane of the frame is the plane that divides the drive-side from the non-drive-side of the HPV. Some of the first mounting structures 21 shown in the figures, for example the first mounting structure 21 b in figure 2, are located outboard of the drive-wheel. In other words, some of the first mounting structures 21 may be located further away from the centre of the drive-wheel than the radius of the drive-wheel. At least one of the first mounting structures may be located outboard of the periphery of the drive-wheel. It may be desirable to reduce the conspicuousness of second mounting structures 53b and 53c on the HPV. At least one of the first mounting structures may be located inboard of the periphery of the drive-wheel. In other words, at least one of the first mounting structures may be located on the housing within the radius of the drive-wheel. The drive-wheel may define an access port (not shown) located such that when the centre of the drive-wheel is aligned with the bottom-bracket axis of the HPV, the said one of the first mounting structures can be accessed through the port in a direction parallel with the bottom-bracket axis. This allows the housing of the drive apparatus to be secured to the frame of the HPV. A method of attaching the drive apparatus to the HPV will now be described, with reference to figure 6. The HPV comprises a crank attached thereto. The attachment method may begin at step S601 by passing the opening 28 of the drive-wheel over the crank. This is performed whilst the crank remains attached to the HPV. Passing the opening of the drive-wheel over the crank may involve translating the drive apparatus along the crank at step S602. It may involve tilting the drive apparatus with respect to the HPV at step S603. Steps 602 and 603 may be done in any order. Once the drive-wheel has been passed over the crank, the next step is to secure the drivewheel to the crank at step S605. To make this easier, it may be preferable to secure the housing to the HPV at step 604, prior to securing the drive-wheel to the crank. However, steps S604 and S605 may be performed in any order. At step 604, the housing is secured to the frame of the HPV. For example, the first and second mounting structures described above are used to secure the housing to the frame of the HPV. It may be preferable to secure the housing to the HPV before the drive-wheel is secured to the crank because the locations of the mounting structures align the drive-wheel opening with the axis of the bottom-bracket. On the other hand, it may be preferable to secure the drive-wheel to the crank before securing the housing to the HPV, as securing the drive-wheel may help steady the housing and / or constrain its position to make it easier to secure. At step S605, the drive-wheel is secured to the crank. This may be done as described above using the securing formations 51 of the drive-wheel to secure to the attachments 50 on the crank of the HPV. It may be necessary to rotate and move the crank or the drive-wheel manually until the securing formations 51 of the drive-wheel align with the attachments 50 on the crank. As will be explained in more detail later, the belt 24 may be free of the drive-wheel when the drive apparatus is being attached to the bicycle due to the activation of a tensioner, so that the belt is in a de-tensioned state. This allows the drive-wheel to be rotated independently of the belt and the output of the motor, making it easier attach the drive-wheel to the crank. The internal formations 31 of the housing described above, which prevent the drive-wheel from escaping the housing, and the positioning of the first and second mounting structures, also facilitate attachment of the drive-wheel to the crank. When both the steps S604 and S605 have been completed the drive-wheel is no longer free to float relative the housing. As mentioned above, the opening of the drive-wheel may be large enough for a pedal to fit through. Optionally, the method may comprise passing, at step S600, the opening 28 of the drive-wheel over the pedal whilst the pedal remains attached to the crank. This means that the pedal does not need to be removed from the crank when the drive apparatus is being attached to the bicycle. Figures 5a to 5e depict the detachment process of removing the drive apparatus from the HPV. This process of detaching the drive apparatus follows the steps described above and in figure 6 but in reverse. For example, the detachment process starts by detaching the drivewheel from the crank at step S610. Then, the housing is detached from the HPV at step S611, as shown in figure 5b. For example, bolts 54 are removed from the first and second mounting structures to detach the housing from the frame of the HPV as shown in figure 5b. Steps S610 and S611 may be done in either order. The drive apparatus may now be tilted with respect to the frame of the HPV at step S612, as shown in figure 5c. The drive apparatus may then be translated along the crank at step S613, as shown in figure 5d. The drive apparatus is then passed over the crank at step S614, and optionally also the pedal at step S615, as shown in figures 5d to 5e. This removes the drive apparatus from the HPV. Thus, the present disclosure provides an assistance unit (drive apparatus) that is easily attached to and detached from a bicycle. The assistance unit of the present application does not require a lengthy process or special tools to attach or detach it to the bicycle. It further does not interfere with the chain-side of the bicycle. Thus, the assist unit of the present disclosure gives the rider the flexibility of upgrading their bike to an assisted one (for example if going for a hilly ride) or removing the assistance unit altogether (for example to make the bicycle lighter, or for training or exercise purposes). QUICK ISOLATE The arrangement illustrated in Figures 7a and 7b shows an exemplary mechanism for easily disengaging the assist function of the motor so that the user can power the human powered vehicle unassisted when desired, without feeling added resistance from the drive apparatus. As mentioned above, the drive apparatus comprises a selectively actuable belt tensioner, shown generally at 30 in Figures 7a and 7b. The belt tensioner is movably mounted to the housing 20. The tensioner is used to transition the belt between tensioned and de-tensioned states. The tensioner is operable to apply tension to the belt and to release tension from the belt. The detensioned state is shown in Figure 7a. The tensioned state is shown in Figure 7b. A reduced amount of tension may be applied to the belt in the de-tensioned state. In the de-tensioned state, the belt may be in compression. The belt 24 may wrap around parts contained within the housing, such as the drive-wheel 25 and wheels 75 and 76. The wheels 25, 75 and 76 may be, for example, pulleys. A motor is carried by the housing. The motor is configured to drive rotation of the wheel 75. The output shaft of the motor is connected to the gearbox. The gearbox comprises wheel 75 and may also include additional gears between wheel 75 and the output shaft of the motor. The motor causes wheel 75 to revolve about an axis parallel to the axis of the bottom-bracket axle. Wheel 75 is in contact with belt 24. When wheel 75 is driven to revolve by the motor, belt 24 is driven to move. The drive-wheel 25 in the housing is, when the belt is in a tensioned state, driven to revolve by the belt 24 and when the drive apparatus is attached to the HPV provides drive to the HPV. In the example shown in Figures 7a and 7b, there is no relative movement between the respective rotation axes of wheel 75 and drive-wheel 25. The belt 24 also wraps around wheel 76. Wheel 76 may not have teeth. The axis of rotation of wheel 76 is parallel to but offset from the axis of rotation of wheel 75. The respective axes of rotation of wheels 75 and 76 are parallel to but offset from the axis of rotation of drive-wheel 25. In the example shown in Figures 7a and 7b, wheel 76 is an idler pulley. The idler pulley provides tension to and guides the drive belt, as will be described in more detail below. The idler pulley may revolve in response to movement of the belt but is not itself driven by a motor. The drive belt 24 therefore couples the drive-wheel 25 to the motor when the belt is in a tensioned state. Output torque from the motor is transferred to the drive-wheel 25 by the belt 24. In the example shown in Figures 7a and 7b, the drive belt is a toothed belt. The teeth may be spaced regularly along the length of the belt. The teeth of the belt engage with corresponding teeth on the drive-wheel 25 and on wheel 75 which is driven to revolve by the output shaft of the motor. The belt may have other forms. The belt may have teeth on only one side of the belt. This may be the side of the belt that is in contact with the drive-wheel 25. The toothed side may be the inside surface of the belt. The teeth on the belt may engage with corresponding teeth on the drive-wheel 25. The belt may be a continuous belt (either formed as such or made from a single length of material joined together at its ends). The belt is flexible. As mentioned above, the belt 24 has inherent stiffness. Put another way, the belt resiliently resists being bent so as to change its natural curvature. As a consequence, the belt 24 is inherently biased to adopt a greater radius (a smaller curvature) than the drivewheel 25. When not in tension, the belt has a bias to form a curve with a radius larger than (a curvature smaller than) that of the drive-wheel 25 so as to release from engagement with the drive-wheel when de-tensioned. In its de-tensioned state, the belt tends towards a circular shape (or generally to a shape whose bend radius is greater than that of drive-wheel 25). As a result, on the release of tension from the belt, the belt tends to spring away from the drivewheel 25. When the tensioner is released and the belt adopts its de-tensioned state, as shown in Figure 7a, the belt is inherently biased to adopt a greater radius than in the tensioned state. In the de-tensioned state, the belt 24 may remain with its teeth partially meshed with wheel 75 and may have spare length to lift away from the drive-wheel 25 such that the teeth of the belt and the drive-wheel are not engaged. Where the belt is flexed over the idler-pulley 76, a beltrestrainer 76a, is fixed to the belt-tensioner-arm 73. When the belt-tensioner-arm 73 rotates to the de-tensioned position, the belt-restrainer 76a rotates with it and comes to bear against the belt 24, as shown in figure 7a. The belt-restrainer deflects the belt away from the adjacent drive-wheel 25, preventing it from following a path which would bring it into contact with the drive-wheel 25. Therefore, in the de-tensioned state, rotation of the drive-wheel 25 does not move the belt 24. The housing 20 defines an open channel immediately outboard of the drive-wheel 25. When de-tensioned, the belt can expand into that channel. As shown in Figure 7a, in the de-tensioned state, the belt 24 adopts a path defined by a guide. Two parts of the guide are shown at 78a and 78b. The belt may be biased so as to bear outwardly against the guide when tension is released from the belt (i.e. when the belt is in the de-tensioned state). When the belt is in the de-tensioned state, the belt may not be completely relaxed because it may be constrained by the guide from adopting its natural curvature. The guide is contained within the housing. The guide is configured to restrain the belt outboard of the drive-wheel 25 along a path where the belt 24 is free of the drive-wheel 25. The guide is spaced from the drive-wheel 25. A segment of the guide, such as that indicated at 78b, may be disposed at a constant radial offset outboard of the drive-wheel. Other segments of the guide may be oriented differently. In Figures 7a and 7b, the segment of the guide indicated at 78a extends with increasing radial offset outboard of the drive-wheel, as shown towards the left-hand side of the figures. The guide or a part thereof may be a separate component to the housing. The guide or a part thereof may be integrally formed with the housing. The guide may be fixed relative to the housing. The guide may extend from the housing 20. The guide may be a continuous component. In other implementations, the guide may be discontinuous. For example, the guide may comprise multiple pins or multiple segments (for example, linear or curved segments) for constraining the de-tensioned belt along its desired position within the housing. The guide may be spaced from the belt along the entire length of the guide when the belt is in the tensioned state. The belt comes into contact with the guide when the belt is in the detensioned state. The distance of the belt in its de-tensioned state from the path it follows when in its tensioned state may be limited by the guide up against which the belt comes to rest when in its de-tensioned state. The belt may conform to the shape of the guide when in its de-tensioned state. The guide can be shaped so as to provide a path for the belt that is clear of the drivewheel 25 when the belt is in the de-tensioned state. The guide may have a concave shape. In some implementations, the belt may in its de-tensioned state be restrained by the guide around its whole periphery. Generally, the belt may be restrained in a path where the belt does not remain engaged with the drive-wheel, such that rotation of the drive-wheel does not cause movement of the belt. The guide can therefore retain at least part of the belt in a position outboard of the drive-wheel. This guide may in some implementations be present not just around the zone where, when tensioned, the belt 24 engages with the drive-wheel 25, but also in other zones between drive wheel and the wheel 75. Generally, the guide retains the belt along a path such that rotation of the drive-wheel 25 does not cause movement of the belt. Therefore, when activated by the user, the tensioner 30 relaxes the tension on the belt so that the belt returns towards an expanded natural state to bear against the surface of the guide such that the belt 24 is not engaged with the drive-wheel 25, while remaining with its teeth partially meshed with the wheel 75 driven by the motor. The tensioner may comprise a manually operable member moveable from a first position in which it causes the tensioner to apply tension to the belt to a second position in which it causes the tensioner to release tension from the belt, and a securing mechanism for securing the handle in the first and second positions. The manually operable member can be operated by a user without the need for tools (i.e. it may be hand operable). One way to implement this is for the belt tensioner to be an arm that pivots with respect to the housing. The arm could be actuated by a lever or other means. In general, the belt tensioner comprises an element that is moveable with respect to the housing to vary the path of the belt. The element is moveable with respect to the housing for increasing the length of the path taken by the slack non-drive portion of the drive belt to apply tension to the drive belt. In the example shown in Figures 7a and 7b, the tensioner comprises an arm 73 that supports idler pulley 76. The idler pulley can revolve relative to the arm about a fixed axis on the arm. The arm 73, and thus the idler pulley 76, are moveable with respect to the housing 20 for increasing the length of the path taken by the slack non-drive portion of the drive belt to apply tension to the drive belt. The arm 73 (and the idler pulley 76 attached to the arm) rotate about an axis parallel to the axes of the wheels 25 and 75. Lever 72 can be used to actuate the arm 73 to move idler pulley 76. To transition the belt from its tensioned to its de-tensioned state, the lever 72 is rotated about an axis indicated at 40 in the direction of arrow 41 in Figure 7a. To transition the belt from its de-tensioned to its tensioned state, the lever 72 is rotated about axis 40 in the opposite rotational direction, as indicated by arrow 42 in Figure 7b. When the lever 72 is operated by the user to transition the belt to its de-tensioned state, the arm 73 is rotated about pivot point 77 (which may be a pin which rotatably fixes arm 73 to the housing 20), which moves the wheel 76 in the direction indicated by arrow 43 in Figure 7a. When the lever 72 is operated by the user to transition the belt to its tensioned state, the arm is rotated about pivot point 77, which moves the wheel 76 in the direction indicated by arrow 44 in Figure 7b. When the tensioner is actuated to bear on the belt, as shown in Figure 7b, the tensioner tensions the belt 24 around the drive-wheel 25. A spring 79 can be used to help impart a predictable tension to the belt. In the tensioned configuration, the output of the gearbox (comprising wheel 75 in this example) and the drive-wheel 25 are constrained to revolve together with a ratio of speeds determined by their respective radii. Moving to its tensioned state brings the belt away from the guide as the lower wheel 76 moves so as to increase the length of the path of slack run of the belt. The tensioner may comprise a securing mechanism for securing the manually operable member 72 in each of the first and second positions. In the example shown in Figure 7b, the tension of the belt imparts a force to the idler wheel 76 which is transmitted through arm 73 and through spring 79, whereby the lever 72 comes in the first position to rest against a stop (not shown) when the cam has moved over-centre. In the example shown in Figure 7a, the resilience of the belt imparts a force to the idler wheel 76 which is transmitted through arm 73 and through spring 79, whereby the lever 72 comes to the second position where the cam is being urged towards a centre-position. The securing mechanism may have other forms, such as a latch or lock for securing the lever at the desired position. In the example illustrated in Figures 7a and 7b, the position of the motor is fixed relative to the housing. Specifically, the motor housing and the axis of rotation of the output shaft of the motor are fixed relative the drive-wheel 25. In other implementations, the motor can be displaced with respect to the drive-wheel. Specifically, the motor housing and the axis of rotation of the output shaft of the motor can be displaced with respect to the drive-wheel in the plane of the belt to selectively apply tension to the belt. In such implementations, the additional wheel 76 may not be necessary and wheel 75 may be used to selectively apply tension to the belt and may be moveable with the motor. When the belt is in the de-tensioned configuration, the drive-wheel 25 can be rotated independently of the output of the motor and / or the gearbox. This is convenient because it allows rider to turn the cranks without any connection with the drive apparatus, and to choose between on the one hand assisted pedalling and on the other, when battery is absent or discharged or when rider doesn’t want assist, unassisted pedalling, with no resistance from the transmission or motor. This is also convenient because, when, for a drive apparatus which is designed to be easily attached and detached from an HPV, the drive apparatus is being mounted to the human powered vehicle, it permits an operator to rotate the drive-wheel into register with the crank arm, so as to align fixings, mounting holes or the like on the drive-wheel and the crank arm. This makes it easier to attach the drive apparatus to the human powered vehicle. The drive apparatus may be arranged to automatically turn off the motor in response to the tensioner being operated to release tension from the belt. For example, there may be a sensor that detects that the tensioner has been operated to release tension from the belt, so that the belt is in the de-tensioned state. For example, when lever 72 is moved into the position shown in Figure 7a, a sensor may detect that the handle has been moved to this position. In response to this detection, the motor may be automatically turned off. Therefore, when the assist function from the motor is not required (or is unavailable because of low or absent battery), the transmission used by the rider to propel the human powered vehicle can be isolated from the transmission used to deliver assist torque to the human powered vehicle (for example via the crank assembly of the human powered vehicle). The assist function can be easily re-engaged when it is available and / or required. CADENCE DROP Some mid-drive assisted bicycles have a clutch built into the mid-drive unit that allows the output which drives the road wheels of the bicycle (e.g., the chain wheel) to rotate faster forward than the bottom-bracket axle, so that the rider is able to slow pedalling or even stop pedalling altogether whilst the output (e.g. the chain wheel) is still driving the wheels. Such clutches are bulky and expensive. The assist unit (drive apparatus) disclosed herein avoids the need for such a mechanism, thus making it lighter and less expensive to manufacture. The drive apparatus of the present disclosure may be applied to HPVs which do not comprise a clutch, free-wheel, or other mechanism that allows the bottom-bracket axle of the HPV to rotate at a variable rate with respect to the rate of rotation of the output (e.g. the chain wheel) that drives the road wheels of the HPV. On an HPV to which the drive apparatus of the present disclosure can be attached, the cranks, bottom-bracket axle, and drive-side output (e.g. chain wheel) revolve at fixed rates with respect to one another. In other words, the bottom-bracket axle, the cranks, and the drive-side output revolve as one. That is, the bottom-bracket axle, the cranks, and the drive-side output always revolve as one - i.e. regardless of the direction in which they are revolving and regardless of the direction of any torque which may be applied to them. The cranks and the drive-side output both revolve around the same axis as the bottom-bracket axle. A pedal may be fitted to each crank. The drive apparatus comprises a motor with a stator and stator windings and a rotor, and a speed reduction mechanism which delivers propulsive output torque from the rotor to the bottom-bracket axle. The drive apparatus comprises a controller to determine the direction and level of the current delivered to the stator windings, which in turn determines the level and direction of the magnetic torque which is applied by the stator to the rotor. During periods when assist is required, it is important, because there is a non-clutched drive path between each crank, the bottom-bracket axle, and the drive-side output, and because forward torque from the motor is delivered directly to the bottom-bracket axle, that the controller is triggered to withdraw forward assist torque immediately when the rider wishes to slow or stop pedalling, as it would be unacceptable and / or dangerous for an assist motor to continue to drive the cranks forward against the rider’s wishes. Although a controller can respond to a change of state of a twistgrip or switch operable by the rider, it is preferable for the controller to be able continually to monitor forces induced by the rider when the rider is delivering propulsive effort through the cranks. When assist is being delivered, it is desirable for a controller to apply a level of forward torque to the rotor which is a function of the propulsive force being applied by the rider. Provided that the controller can monitor rider-induced forces, this is possible. When the controller can monitor rider-induced forces, the controller can withdraw delivery of assist torque to the rotor as soon as the controller discerns a pattern of rider-induced forces which ceases to correspond with the delivery of propulsive force by the rider: it is important for this that the controller is able to monitor the forces continually, regardless of crank angle. Accordingly it may be arranged that one or more sensors are fitted to detect displacement relative the frame of the HPV of a bearing in which the bottom-bracket axle revolves in the manner described herein for the fourth notable feature and under the heading Floating Cartridge. The controller may monitor the changes in state of the one or more sensors, and calculate therefrom changes in rider-induced forces. During periods when assist is not required, it is important that the rider can change the speed of the cranks without the forces arising from the inertia of the rotor, and felt by the rider through the pedals, exceeding an acceptable level. On an HPV with a conventional freewheel at the road wheel and without the drive apparatus herein described, a rider will feel little resistance when moving the pedals around either forwards at a speed too low to engage the freewheel or backwards. On an equivalent HPV fitted with the drive apparatus herein described, a rider would, in the absence of the one-way clutch disclosed herein, encounter resistance from the rotor inertia whenever changing the speed of the pedals in the same circumstances. The controller can apply to the rotor a torque whose level and direction is calculated by the controller as being most likely to minimise any unwanted forces felt by the rider at the pedals. The controller can make this calculation using observations of the speed of the rotor and its rate of change and also using observation of the one or more sensors which respond to changes in displacement of the bottom-bracket axle arising from rider-induced forces. There would, in the absence of the one-way clutch disclosed herein, be circumstances when the unwanted forces exceed an acceptable level. One such circumstance arises when a rider withdraws propulsive force in order to make a change to a higher gear. When the new gear engages, the bottom-bracket axle will, unless the rider has reduced cadence sufficiently, be subject to a sudden deceleration or cadence drop. Because the rotor of the motor has inertia, and is directly coupled to the bottom-bracket axle, a torque will be needed to impart the equivalent sudden deceleration to the rotor to avoid the inertia of the motor forcing the cranks forward. This torque will be transmitted by the drivetrain which directly couples the rotor via the bottom-bracket axle to the gear on the HPV itself and may give rise to an unwanted jolting sensation and / or cause stresses which are harmful to the transmission. A second such circumstance can arise when, say immediately following withdrawal of assist torque, a rider wishes to stop pedalling rapidly, within a brief interval, for example to avoid a clash between a pedal and an obstruction on the road. It can be dangerous for a rider if the inertia of the rotor prevents the rider from slowing the cranks as desired: if the controller reduces cadence too fast, this may imbalance the rider, and if too slowly this may result in an impact at the pedal. So the controller must apply a level of reverse torque to the rotor to reduce its speed at a rate, after adjusting for gearing, which is near enough the same as the rate at which the rider wishes to slow the cadence. It is unlikely that a controller can in all circumstances react fast enough to deliver the right level of deceleration. This control problem is referred to the “Cadence Drop” problem. The inventor has recognised that one way to resolve the cadence-drop problem is for the drive apparatus 200 to comprise a one-way clutch 80 along the drive path between the motor and the bottom-bracket assembly (e.g. the bottom-bracket axle, the pair of cranks, and the driveside output). The clutch 80 allows the rotor to deliver torque to the bottom-bracket axle when the rotor is revolving at an engagement speed. The engagement speed is a defined multiple of the speed at which the bottom-bracket axle is revolving. The engagement speed may be written as ve = nvb, where ve is the engagement speed, vb is the rotational speed of the bottom-bracket assembly and n is a predefined constant (e.g., 40). The value of “n” is determined by the drive ratio between the rotor and the bottom-bracket axle. The clutch 80 disengages when the rotor is revolving forwards slower than the engagement speed or revolving backwards faster than the engagement speed. The controller monitors the speed at which the bottom-bracket axle revolves. The controller also monitors the speed at which the rotor revolves. Figure 8 shows an example cut through of a part of the drive apparatus that shows the motor 22, a gearbox 23, a wheel 75 driven by the motor, and a drive belt 24. Figure 8 shows a clutch 80 located between the rotor and a rotor output shaft 84. Alternatively, the clutch may be located elsewhere in the gearbox 23. Alternatively, the clutch may be integrated into the wheel 75 or into the drive-wheel 25. Clutch 80 is a needle roller clutch in this example. The motor 22 comprises a rotor 81, a stator 82, and motor windings 83. The rotor 81 may rotate in one direction about the rotor output shaft 84. The rotor output shaft 84 carries a sun gear which is the input to a planetary gearbox. When the clutch 80 is engaged, forwards magnetic torque applied to the rotor 81 delivers forward torque to the rotor output shaft 84. When the rotor is driving the HPV forwards, the rotor output shaft 84 rotates forwards, the sun gear meshes with the large planet of a planet pair 85 and causes the planet pair 85 to rotate about a second shaft 86. The small planet of the planet pair 85 reacts off a ring gear 87 so that the planet wheels move around the ring gear 87 to rotate the planet carrier 88. Rotation of the ring gear 87 relative the housing is prevented by a flexible mounting 89 which is fixed to the housing 20 of the gearbox. The flexible mounting 89 allows a measure of radial float for the ring gear. Rotation of the planet carrier 88 imparts rotation to the output shaft 90, which is coupled to the wheel 75 which drives the belt 24. The belt 24 can transmit torque to a drive-wheel 25 connected to the bottom-bracket axle of the HPV. As mentioned above, there is a non-clutched drive path between the pedals and the chain wheel. There may be no other clutch in the HPV other than the one-way clutch described above. In other words, said one-way clutch may be the only clutch. Thus, it is always the case that the one-way clutch engages when the rotor is revolving at the engagement speed and disengages when the rotor is revolving at a speed less than the engagement speed. At regular intervals, the controller may recalculate the torque to apply to the rotor. At each such interval the controller may obtain a value for motor-speed and direction from commutator circuitry and sensors. Separately, a sensor 91 may be mounted on a printed circuit board 93: the printed circuit board may also carry circuitry comprising the controller. At each interval, the controller can obtain a value denoting crank-speed and direction from the sensor 91 which responds to changes in flux arising from rotation of the magnet 92 attached to the output shaft 90. Because of the mechanical connection provided by the belt 24, the speed at which the shaft 90 revolves is a fixed multiple of the speed at which the bottom-bracket axle revolves, and the controller can calculate rider cadence by dividing the speed of shaft 90 by the fixed multiple. The controller can in turn calculate the engagement speed that applies for the cadence observed during the interval, even though the rotor itself may not be revolving at the engagement speed. Two benefits provided by the one way clutch 80 will first be described. If there is no clutch 80 and the rotor 81 is coupled directly to the rotor output shaft 84, and a rider reduces forward cadence, the forward speed of the rotor is correspondingly reduced. The torque needed to decelerate the rotor may be applied mechanically by the rider through the pedals, which could give rise to unwanted forces felt by the rider through the pedals. The torque needed to decelerate the rotor may be applied magnetically by the controller. As it is desirable to minimise the rider’s contribution to the deceleration of the rotor, the controller may at each interval estimate an ideal level of reverse torque to apply to the rotor. The ideal level of reverse torque is that which would, when applied to the rotor during the following interval, eliminate the forces from rotor inertia felt by the rider. The estimate of ideal level of reverse torque may be made by referring, for example, to the most recent rate of change of cadence which the controller has observed and to recent readings obtained from the one or more sensors which respond to the changes in displacement of the bottom-bracket axle which arise from rider-induced forces. The controller may apply the estimated ideal level of reverse torque to the rotor. During periods of unhurried cadence drop, it may in this manner be possible for the controller to maintain the unwanted forces felt by the rider through the pedals at an acceptable level. However, during periods of rapid cadence drop, it may be difficult for a controller to react quickly enough to make a reliable estimate at each interval of the ideal level of reverse torque, and the level of torque applied by the controller to the rotor may depart from the actual ideal: the controller may determine and apply a reverse torque which is too high, or which is too low or which hunts between the two, and thereby give rise to unwanted forces at the pedals which are unacceptable to the rider. However, when the drive apparatus comprises a clutch 80, the controller can apply a reverse torque to the rotor which is higher than the estimated ideal level by an amount which exceeds any likely departure of the estimated ideal level from the actual ideal level. Because the clutch can disengage, the rider need not be subject to forces from rotor inertia transmitted through the pedals and the rider may feel little or no untoward sensation from the pedals as a result of the “overbraking” applied to the rotor by the controller. The clutch allows the controller always to err in such situations towards applying higher reverse torque to the rotor than the estimated ideal level, and this makes the task of the controller viable, especially during a period of rapid cadence drop. This is the first benefit. A second benefit when the drive apparatus comprises a clutch 80 is that, when the pedals are revolving forwards and assist is not required, the controller may maintain a tick-over speed for the rotor which is lower than the engagement speed. This tick-over speed could, for example, be zero, but this would mean that when assist is next called for, the rotor has to be spun up to engagement speed again. Instead, a tick-overspeed can be chosen which is lower than the engagement speed by a defined tick-over-difference such that, when the rider changes gear to a new higher gear, the rotor is unlikely, for a typical gear change, to be revolving, at the moment of engagement of the new gear, at a speed higher than the now reduced engagement speed, and thereby avoid the unwanted jolting sensation referred to above and avoid stresses which are harmful to the transmission. One example of a process that may, when the drive apparatus comprises a clutch 80, be followed in order to determine the level and direction of the torque to apply to the rotor will now be described, with reference to figure 9. The following describes the situations where the invention has particular relevance, namely when pedals are moving forwards and assist is not required (e.g. the answer to step S900 is “no”). Alternative processes or methods, not described here, may provide control of equivalent effectiveness when pedals are moving forwards and assist is not required and when the drive apparatus comprises a clutch 80. When the controller senses that motor speed is the same as the calculated engagement speed (e.g. the answer to step S901 is “yes”), as may typically be the case at the first interval immediately following withdrawal of assist torque, the controller recognises a state referred to herein as lockstep. During lockstep the rider may be pushing back on the pedals against rotorinertia with unwanted force, and the objective of the controller is to escape from lockstep swiftly. A straightforward procedure for the controller would be to apply to the rotor, during any interval when the controller recognises lockstep, the maximum available escape-lockstep torque. This would reduce the speed of the rotor fast enough for the rider not to feel rotorinertia during a rapid drop in cadence, but it would exceed the threshold where a rider would sense a jolt from the reaction torque carried from the stator into the frame of the HPV. As such a jolt would not be acceptable on the frequent occasions when a rider withdraws effort and lowers cadence in a normal unhurried manner, the controller, during any interval when the controller recognises lockstep (e.g. when the answer to step S901 is “yes”), calculates (e.g. at step S913) instead a reverse torque to apply to the rotor during the following interval which is the higher of a default escape-lockstep torque and a high escape-lockstep torque. The default escape-lockstep torque applied to the rotor may be a low multiple, say between 1 and 3, of the torque that would, in the absence of any other torque applied to the rotor, slow the rotor enough for the clutch 80 to disengage when the rider is reducing cadence at a rate which is typical, at the cadence in question and in the absence of any torque on the bottombracket axle arising from rotor-inertia, of a normal unhurried slowing of the pedals. The default escape-lockstep torque would be below the threshold where a rider would sense a jolt from the reaction torque carried from the stator into the frame of the HPV. A high escape-lockstep torque will be the lower of a maximum escape-lockstep torque and a bigger escape-lockstep torque. The maximum escape-lockstep torque will be the maximum available reverse torque to the rotor, as capped by the limit on reverse stator current which has been set for the controller. This will be invoked rarely, only in exceptional scenarios: the torque applied to the rotor is likely to exceed the threshold where a rider would sense a jolt from the reaction torque carried from the stator into the frame of the HPV. The bigger escape-lockstep torque will be calculated by the controller as a function of a number of variables. For example, one variable may be the level of reverse pedal force which the controller may calculate from the readings obtained during the previous interval from the one or more sensors which respond to changes in displacement of the bottom-bracket axle arising from rider-induced forces. In other words the more the rider is pushing back, the higher the rotor-braking. Another variable may, for example, be a factor which increases with the count of intervals which have elapsed since the controller first detected lockstep during the current slowing of the cranks. In other words, the longer the rider’s been pushing back, the more the controller ramps up rotor-braking. Yet another variable may be the difference between an actual measured speed of the motor with a calculated expected speed of the motor, the calculated expected speed of the motor being the speed that the controller would expect the motor to be revolving at if the clutch were disengaged. The torque applied to the rotor may or may not exceed the threshold where a rider would sense a jolt from the reaction torque carried from the stator into the frame of the HPV. When cranks and rotor come to be out of lockstep (e.g. when the answer to step S901 is “no”), the controller will sense that the rotor is revolving at less than engagement speed, and / or that the rider is no longer pushing back against the pedals, and the controller may withdraw escape-lockstep torque. While out of lockstep, the controller will continually monitor whether conditions are right for it to resume applying assist torque to the rotor (e.g., whether the answer to step S900 becomes “yes”). While this is not the case, the controller will monitor cadence (e.g. at step S912), and calculate the tick-over speed as being the calculated engagement speed less the tick-over-difference, and at each interval the controller will calculate the torque to apply to the rotor which is most likely to maintain a rotor speed equal to the calculated tick-over speed. When forward cadence rises while the rotor is revolving at less than engagement speed, the controller may apply forward torque to maintain the tick-over speed. When forward cadence drops while the rotor is revolving at less than engagement speed, the controller may apply reverse torque to maintain the tick-over speed. When forward cadence drops suddenly, the controller may apply insufficient reverse torque to the rotor to maintain a rotor speed equal to the now-lower tick-over speed which it has calculated, but as the tick-over-difference will be higher than any difference between actual rotor-speed and calculated tick-over speed which might arise when the controller applies a reverse torque to the rotor which is too low, rotorspeed can be maintained below engagement speed, and the rider will not be subject to unwanted forces at the pedals arising from rotor-inertia. Determining whether assistance is to be provided at step S900 may be based off input data from a rider, and / or data gathered by the drive apparatus itself. The said assistance may be assistance by the motor to propulsion of the vehicle. The controller may comprise one or more processors configured to determine whether assistance is to be provided to the propulsion of the vehicle by the motor. The state when assistance is not to be provided is when the one or more processors have determined (e.g. in accordance with a predefined algorithm and in dependence on one or more inputs indicative of the state of the vehicle) that assistance is not to be provided for the time being. For example, the HPV may comprise a switch operable by the rider. When the rider wishes to turn off assistance from the motor, they can operate the switch to an “off’ position (for example). When the controller receives a signal from the rider (e.g. via the control switch) that assistance is not desired, it will determine that the answer to step S900 is “no”. As another example, the controller may determine that assistance is to be switched off based on the force detected on the pedals. For example, the controller may be arranged to receive an indication of a force applied by the rider to the bottom-bracket axle, e.g. from a bottombracket assembly such as is described herein. The controller may determine that assistance is to be provided in response to the calculated propulsive torque applied by the rider exceeding a predefined threshold. The controller may further determine that assistance is not to be provided in response to the calculated propulsive torque applied by the rider being below a predefined threshold. As another example, the controller may determine that assistance is not to be provided based on the measured speed of the bottom-bracket assembly. If the speed of the bottom-bracket assembly is measured to be zero, it can be concluded that the rider has stopped pedalling and therefore assistance is not required. When assistance is to be provided (e.g. the answer to step S900 is “yes”), the controller checks at step S902 whether the motor is rotating at the engagement speed. If it is, then the clutch is engaged and the controller can apply a torque to the motor to provide assistance to the bottombracket assembly. If the motor is not rotating at the engagement speed, then the clutch is not yet engaged, so the controller applies torque to the motor to increase its forward speed until it reaches the engagement speed, following which assistance can be provided in accordance with steps S904 and S905. Thus, it is to be understood that even when assistance is not to be provided, the controller may remain configured to receive data from one or more sensors and use this information to decide a level of torque that the motor is to apply to the rotor. FLOATING CARTRIDGE The following describes a bottom-bracket assembly that can detect the pedalling force of a rider of a bicycle (or other HPV) by measuring radial displacement of the bottom-bracket axle relative to the bike frame which arises from elastic strain. Said radial displacement arises when an unbalanced transverse force is applied to the bottom-bracket axle. In particular, said radial displacement arises when rider force on a pedal on one side of the bike differs from the rider force on the pedal on the other side of the bike. Said radial displacement may also arise when the line of the force arising from the onward delivery of rider power (for instance, via a continuous chain or belt, or via a gear-wheel) is displaced to one particular side of the bike. In normal operation of a conventional bike, the force may be displaced to the side of the bike on which the chain or belt is disposed, referred to as the drive-side. In other words, said radial displacement may arise when there is tension in the drive chain. Said radial displacement may also arise from forces carried by a coupling (for instance, a continuous chain or belt, or a gearwheel) between the output of an assist motor and a drive-wheel on the bottom-bracket assembly. A transverse force refers to a force that acts in a direction transverse to the longitudinal axis of the bottom-bracket axle. Sensors are used to measure said radial displacement. A control processor for any drive apparatus described above can react to the values for rider forces which the control processor can derive from readings from the sensors. There is more than one arrangement for a bottom-bracket assembly comprising a cartridge which can provide measurable radial displacement in response to rider force. Three examples of such a bottom-bracket assembly will now be described, with reference to figures 10 to 15. The bottom-bracket assembly comprises a bottom-bracket axle and at least two bearings. The bottom-bracket axle is radially supported at two separate positions along its length by the bearings in which it may revolve. The assembly further comprises a hollow cartridge. The cartridge carries the bearings. The assembly further comprises a plurality of axially separated mounting structures. The mounting structures radially support the cartridge. At least one mounting structure provides axial support for the cartridge. At least one mounting structure prevents the cartridge from revolving. The mounting structures connect the cartridge to the bottom-bracket shell on the bike frame. They include a first mounting structure on or near to the drive-side of the bike and a second mounting structure which is nearer to the other side of the bike (the non-drive-side) than the first mounting structure. The first mounting structure can provide a fulcrum about which the cartridge may rotate in yaw and roll. The second mounting structure is radially compliant. An unbalanced transverse force arises when the moment in one direction of the forces induced by the rider about the fulcrum provided by the first mounting structure is not equal to the moment in the other direction of those forces, the difference being the rider induced moment. A radial force in the compliant second mounting structure which supports the cartridge towards its non-drive-side gives rise to a moment about the fulcrum provided at the first mounting structure. The moment is equal and opposite to the rider induced moment. Because of the compliance of the second mounting structure, there arises radial displacement of the cartridge in a given direction. The amount of radial displacement is a function of first the imbalance of the transverse forces in that direction and second the axial distance from the fulcrum. Because the cartridge is not revolving, the displacement of the cartridge can readily be measured. One or more sensors may respond to changes in the radial displacement of the cartridge that occurs due to said unbalanced transverse force on the bottom-bracket axle. Each sensor can be used to deliver an electrical signal indicating the level in a given direction of an unbalanced external force on the bottombracket axle. The assembly may measure this external force continuously or near continuously. A controller may monitor each electrical signal. A controller may use this information to determine the level and direction of the torque delivered by an electric motor which provides assistance to the rider of the HPV. As mentioned above, the bottom-bracket assembly described herein with reference to figures 10 to 15 comprises a cartridge, which houses the at least two bearings and the bottom-bracket axle. Providing the cartridge is sufficiently resistant to bending, two advantages can arise as a result of carrying the bearings in a cartridge. First, the position of the line of radial support provided by the first mounting structure can be axially offset from the line of radial force transmitted through the drive-side bearing. This allows an axial position of the first mounting structure, and so of the fulcrum, to be different from the axial position of the drive-side bearing. Secondly, the radial displacement of the bottom-bracket axle can be measured at any point along the length of the cartridge and away from the fulcrum. Further advantages can arise as a result of carrying the bearings in a cartridge, providing it has sufficient resilience in tension or compression. The direction of support provided to the cartridge by a mounting structure may be such as also provides, in addition to a radial component of force, an axial component of force. This axial component of force will normally be balanced by an equal and opposite axial component of force provided to the cartridge by the other mounting structure. Such axial force between the mounting structures gives rise to an equal axial force in the cartridge, which will be either tensile or compressive, depending on the direction of the axial forces at the mounting structures. Because these additional loads are carried between the mounting structures by the cartridge rather than through the bearings and the bottom-bracket axle, the bearings themselves (whose main role during normal pedalling is to provide radial support of the bottom-bracket axle) are not subjected to the additional axial loads induced at the mounting structures. As a result, if it is deliberately arranged that the mounting structures carry not only radial load, but also an axial load, there is no increase in the load on, nor shortening the life of, the bearings. A further advantage with such an arrangement is that, if the compliance at the second mounting structure is to be provided by elastomer, and if the surfaces abutting the elastomer are to be disposed at an angle to the bottom-bracket axis so as to give rise to an element of shear-strain in the elastomer when the cartridge is displaced radially, and so a relationship between force and displacement which is more linear than when the surfaces abutting the elastomer are parallel to the axis of bottom-bracket axle with the elastomer subject only to compressive load, then only a single elastomer element will suffice at the compliant mounting structure, rather than a pair of opposing elastomer elements. A further advantage is the possibility for further controlling the axial offset of the fulcrum provided by the first mounting structure, whereby support provided for the cartridge at the first mounting structure is geometrically equivalent to that which would be provided by a spherical bearing whose centre lies on the axis of the bottom-bracket axle at a selected offset from the centre-line of the bike or HPV, as exemplified for the first and third of the three examples described below. Three example bottom-bracket assemblies will now be described, with reference to figures 10 to 15. The following features are common to each of the three examples. The frame of the bike to which the bottom-bracket assembly is fitted comprises a hollow bottom-bracket shell 9 whose bore is threaded at each end. The bottom-bracket assembly as described in the following three examples comprises a hollow cartridge shell. The assembly also comprises a bottom-bracket axle 12. The bottom-bracket axle is constrained to revolve about its axis inside the bottombracket shell. The bottom-bracket axle can connect at one end to a crank 13 and at the other end to a crank 15. Each crank carries a pedal (not shown) to which the rider can apply force. When the bottom-bracket assembly is fitted to the frame of the bike and the axle is attached to the cranks, and the cranks attached to the pedals, a moment about the axis of the bottombracket axle arising from forces applied to the pedals is resisted by an equal and opposite moment in a wheel 14 arising from the force needed to propel the bike. The wheel 14 is on the side 101 of the bike which is referred to as the drive-side. The wheel 14 may be attached to and revolve with the crank 13. The other side 102 of the bike is referred to as the non-drive-side. Example 1 Figure 10 shows a first example of a bottom-bracket assembly 150 comprising a cartridgetube 120 positioned within a bottom-bracket shell 9 of a bicycle. The outside of the cartridgetube 120 is threaded at each end. This first example will now be described in the context of assembling and fitting it into a bottombracket shell of an HPV. First, the thrust-bearing components of the first example assembly will be described. In this example, the assembly comprises a pair of inner bearing-raceways 130, a pair of snap-rings 131, and a pair of outer-bearing raceways 129. The pair of inner-bearing raceways 130 is mounted on the bottom-bracket axle 12. The inner-bearing raceways are constrained from axially moving apart on the bottom-bracket axle by the pair of snap-rings 131. The cartridgetube 120 carries in its bore the pair of outer bearing-raceways 129. Bearing balls 132, also a part of the assembly, are trapped between the inner and outer raceways. The way in which the assembly provides constraint for the outer raceways from axially moving apart and axial location for the bottom-bracket axle will be explained further down. The bottom-bracket axle 12 is radially supported by a bearing 105 towards the drive-side 101 and by a second bearing 106 towards the non-drive-side 102. These bearings may either be plain or comprise rolling elements, and in this example, needle bearings are used. The cup of each bearing is pressed into the bore of the cartridge-tube 120. The assembly further comprises a thrust-bearing-spacer 128 which abuts the inner flank of the cup of each bearing. The opposite end of each spacer 128 abuts an outward-facing shoulder of an outer raceway 129 of the assembly. A first mounting structure 1103 at the drive-side connects the drive-side of the cartridge-tube to the bottom-bracket shell. In this example, the first mounting structure is made up of a driveside end-cup 103 and an elastomer membrane 122. The elastomer membrane is thin. The elastomer membrane may be any suitable thickness for achieving the technical effect described herein. In other words, the thickness of the elastomer membrane is sufficient to allow the membrane to deform in shear to accommodate the rotation in yaw or roll which arises when the cartridge is radially displaced at the second mounting structure. Such displacement may range up to 0.1mm (depending on the pedalling force). In an example, the elastomer membrane may have a thickness in the range 0.1mm to 1mm. For example, the elastomer membrane may be 0.2mm thick. The elastomer membrane has a conical shape. The elastomer member abuts a mating conical surface formed inside the drive-side end-cup 103 and also abuts a mating conical surface formed on the outside of the threaded clench-nut 124. Thus, a surface of the elastomer membrane that contacts a rigid element of the first mounting structure is disposed at an angle to the longitudinal axis of the bottom-bracket axle. The elastomer membrane 122 may be glued to the end-cup 103 and to the clench-nut 124. Preferably if using glue, during gluing, semi-circular holes formed in each of the end-cup and the clench-nut are aligned so as to provide circular recesses 126 with which a pin-spanner can engage during assembly. The clench nut 124 is secured tight to the threaded drive-side end of the cartridge-tube 120, so that an inward projecting flange 124a of the nut 124 abuts the end of the cartridge-tube. Thread-locking may be used here. The cartridge-tube 120, clench-nut 124, thrust-bearing-components 128, 129, 130, 131, 132, axle 12, needle bearings 105, 106, and first mounting structure 1103 (comprising the membrane 122 and drive-side end-cup 103) make up a sub-assembly A of the bottom-bracket assembly. A second mounting structure 1104 at the non-drive-side connects the non-drive-side of the cartridge-tube to the bottom-bracket shell. In this example, the second mounting structure comprises a compliant non-drive-side end-cup-sub-assembly 139. The non-drive-side end-cup-sub-assembly 139 comprises a conical elastomer ring 138, a non-drive-side end-cup 104, and a spreader-ring 137. The spreader-ring 137 is rigid. The elastomer ring abuts a mating conical surface formed inside the non-drive-side end-cup 104, and also abuts a mating conical surface on the inboard side of the rigid spreader-ring 137. The elastomer ring 138 may be glued to the end-cup 104 and may also be glued to the spreader-ring 137. Preferably if using glue, during gluing, a ring of circular holes 136 are provided respectively in each of the spreader-ring 137, and the elastomer ring 138 and the end-cup 104 are kept aligned, so that a pin-spanner can engage to secure the non-drive-side end-cup-sub-assembly. The assembly further comprises a spreader-box 140. The spreader-box 140 is addressed during assembly to the non-drive-side end of the bottom-bracket shell, and clamped firmly against it by screwing the end-cup-sub-assembly 139 into the threads in the non-drive-side end of the bottom-bracket shell. Sub-assembly A may be fitted as a complete item through the bottom-bracket shell 9 from the drive-side end of the bottom-bracket shell, with the threaded non-drive-side end of the cartridge-tube 120 passing through end-cup-sub-assembly 139. The drive-side end-cup 103 is screwed into the threads inside the drive-side end of the bottom-bracket shell. Initially, the sub-assembly A may need to be screwed in further than the position it will occupy after final adjustment. The non-drive-side clench-nut 134 may be screwed onto the threaded non-drive-side end of the cartridge-tube 120. The spreader-box 140 has a spreader-box-lid 141, which is positioned inside an outer flange 134b on the clench-nut 134. The clench-nut 134 is done up until its inward-projecting flange 134a abuts tight against the non-drive-side end of the cartridge-tube 120. The inward-projecting flange 124a and 134a of each clench-nut restricts outboard axial motion of each bearing 105,106 from the bore of the cartridge-tube 120. This in turn traps each thrustbearing spacer 128, in turn trapping each outer bearing raceway 129. When the bottombracket axle 12 is subjected to an external axial load from the drive-side, the load is transmitted via the drive-side snap-ring 131, the drive-side inner raceway 130, bearing balls 132, the non-drive-side outer raceway 129, the non-drive-side spacer 128 abutting the outer raceway, the cup of the non-drive-side needle bearing 106, the flange 134a of the clench-nut 134, and finally via sub-assembly A to the drive-side end of the bottom-bracket shell. An equivalent load path may apply when the bottom-bracket axle is subjected to an external axial load from the non-drive-side. The running clearance for the bearing balls 132 is determined by the lengths of the cartridge-tube 120, of the needle bearings 105 106, of the spacers 128, of the bearing raceways 129 130, and also by the spacing of the snap-rings 131. If axial play of the bottombracket axle 12 is to be minimised, shim washers (not shown) may be selectively deployed. As mentioned above, when assembling the bottom-bracket assembly, the sub-assembly A may need to be screwed in further than the position it will occupy after final adjustment. Said final adjustment will now be described. A pin-spanner may be used to engage in the ring of holes 126. Sub-assembly A may be unscrewed from the threaded drive-side end of the bottom-bracket shell 9, moving subassembly A, together with the clench-nut 134, towards the drive-side of the bottom-bracket shell. The result of this movement is that the conical inboard surface of the clench-nut 134 comes to abut a mating conical outboard surface on the spreader-ring 137. Lubricant may be present on the abutting conical surfaces of the clench-nut 134 and the spreader-ring 137, so as to reduce the frictional resistance to relative rotation as sub-assembly A is unscrewed from the drive-side end of the bottom-bracket shell. An axial compressive force in the conical elastomer ring 138 equal and opposite to that in the conical elastomer membrane 122 is induced. The unscrewing of sub-assembly A from the drive-side end of the bottom-bracket shell is stopped when the torque reaches a level such as induces sufficient axial preload of the elastomer ring 138. The cartridge-tube 120 becomes subject to tensile stress. With said preload, the second mounting structure at the non-drive-side end for the cartridge-tube 120 is now established, comprising the compliant end-cup-sub-assembly 139. The assembly may further comprise a lock-ring 143 to lock the drive-side end-cup 103 at the selected position. The outer flange 134b on the clench-nut 134 now bears against the spreader-box-lid 141 so as to trap it against the spreader-box 140 with inner and outer seals in place to prevent water ingress to the spreader-box enclosure 142. The way in which the first mounting structure at the drive-side end provides a fulcrum 123 tending towards the position shown is now explained. The compliance of the elastomer membrane 122 under tension and compression is low compared to its compliance in shear. As a result, the membrane provides high resistance to linear motion of the drive-side clenchnut 124 relative the drive-side end-cup 103, and low resistance to rotation in yaw and roll of the drive-side clench-nut 124 relative the drive-side end-cup. The reason that the fulcrum 123 tends towards the position shown is that this position is also the centre of a hemisphere, a slice of which is approximately defined by the conical ring of elastomer membrane 122 and it is when rotation is about this fulcrum that the compressive / tensile strain induced in the elastomer is at a minimum. When the line of net rider-induced external force on the bottombracket axle does not pass through this fulcrum, it gives rise to a moment about this fulcrum, which is resisted by an equal and opposite moment about this fulcrum resulting from a radial force at the second, non-drive-side mounting structure. The combination of the axial position and angle of the conical or part-spherical mating surfaces trapping the elastomer membrane 122 can be chosen so as to provide a preferred axial offset of the fulcrum 123. In other words, the axial position of the fulcrum is determined by the axial position of the elastomer membrane and the angle that the elastomer membrane makes with the longitudinal axis of the bottom-bracket axle. When the second non-drive-side mounting structure is subjected to a radial load in a given direction, radial displacement in the same direction of the cartridge-tube 120, together with the clench-nut 134 and spreader-ring 137 arises because of the compliance of the conical elastomer ring 138. The amount of this displacement is a function of the radial load. The displacement can be measured by a strain gauge or other sensor within the assembly. For example, figures 10 and 11 shows a sensing arrangement that can form part of the assembly. A sensing arrangement comprises a beam 145 inside the spreader-box enclosure 142 (see figure 10) which is supported in such a way as brings its centre to rest against the spreader-ring 137. The spreader-ring 137 induces an initial strain in the surface of the beam near its centre. When there is any motion of the spreader-ring 137 perpendicular to the beam 145, the strain in the beam at its centre changes. The sensing arrangement may comprise a strain-gauge 146 attached to the beam 145 to detect the change in strain in the beam. The strain-gauge 146 may be connected to a PCB 147, where circuitry is provided to deliver an electric signal indicating at all times the strain in the beam 145. This electric signal can be used by a controller (not shown) to determine the level of the unbalanced transverse force induced by the rider about the fulcrum in the direction perpendicular to beam 145. The sensing arrangement may further be fitted with a second beam 148, resting against a different part of periphery of the spreader-ring 137. The second beam 148 may carry a strain-gauge 149, also connected to the PCB 147, generating in an equivalent manner an electrical signal which can be used by a controller to determine rider-induced forces perpendicular to the second beam 148. When the controller has information about forces in more than one direction, it is possible for it to extract values not only for the difference in force applied to each pedal but also, if the fulcrum is displaced from the line of the drivechain, tension in the drive-chain, and also, similarly, for the force in any coupling between a drive-wheel on the bottom-bracket assembly and the output of an assist motor. Preferably, the sensing arrangement comprises a pair of strain gauges disposed at right angles to each other. The controller may be configured to determine the level of the unbalanced external force applied in any direction to the bottom-bracket assembly based on readings from the pair of strain gauges. Example 2 A second example of the bottom-bracket assembly 250 comprising a cartridge-tube 120 is shown in figures 12 and 13. The outside of the cartridge-tube 120 is threaded at each end. This second example will now be described in the context of assembling and fitting it into a bottom-bracket shell of an HPV. First, the thrust-bearing components of the second example assembly will be described. In the second example, the assembly 250 comprises a pair of inner bearing-rings 160, a snapring 161, and a pair of outboard bearing-rings 159. The pair of inner bearing-rings 160 is mounted on the bottom-bracket axle 12. Each bearing-ring is constrained in one direction from axial motion relative the bottom-bracket axle 12 by the snap-ring 161. The pair of outboard bearing-rings 159 are carried in the bore of the cartridge-tube 120. The bearing-rings 159 provide a plain-thrust-bearing surface each side at 162. The bearing surfaces may be lubricated. The way in which the assembly provides constraint for the outboard bearing-rings from axially moving apart and axial location in the cartridge-tube for the bottom-bracket axle is equivalent to that described above for the first example. The bottom-bracket axle 12 is radially supported by a needle bearing 105 towards the driveside 101 and by a second needle bearing 106 towards the non-drive-side 102. The cup of each needle bearing is pressed into the bore of the cartridge-tube 120. The outboard bearing ring 159 may abut the inboard flank of the cup of each needle bearing. A first mounting structure 2103 at the drive-side comprises an end-cup 153. The end-cup 153 is partially fluted in this example. The end-cup 153 carries an internal thread which may engage with the thread on the outside of the cartridge-tube 120, and an external thread which may engage with the drive-side threads in the bore of the bottom-bracket shell 9. The endcup 153 is secured tight to the threaded drive-side end of the cartridge-tube 120, so that the inward projecting flange 153a abuts the end of the cartridge-tube. Thread-locking may be used here. The cartridge-tube 120, thrust-bearing-components 159, 160, 161, 162, axle 12, needle bearings 105, 106 and end-cup 153 make up a sub-assembly 180. A spreader-box 170 is addressed during assembly to the non-drive-side end of the bottombracket shell. The spreader-box 170 is clamped firmly against the bottom-bracket shell by screwing a non-drive-side end-cup 154 into the threads in the non-drive-side end of the bottom-bracket shell. An elastically-compliant spreader-beam 167 is housed inside the spreader-box 170. The spreader-beam 167 has a hollow centre 167b which has a tapered bore 167e. The spreaderbeam carries threaded bosses 167a. The spreader-box 170 has a spreader-box lid 171 which is load-bearing and which has a tapered bore 171a. The spreader-box lid 171 is securely attached with screws 174, 175 to the threaded bosses 167a carried on the spreader-beam 167. The tapered bore 167e of the spreader-beam is disposed in the opposite sense to the bore 171a. The profile of the spreader-beam 167 is such as will allow radial compliance of its threaded bosses 167a relative to the hollow centre 167b when the spreader 167 is carrying a radial load. A circular elastomer seal is trapped between the lid 171 and the centre 167b of the spreader-beam. Sub-assembly 180 may be fitted as a complete item through the bottom-bracket shell 9 from the drive-side end of the bottom-bracket shell, with the threaded non-drive-side end of the cartridge-tube 120 passing through the non-drive-side end-cup 154.The drive-side end-cup 153 is screwed into the threads inside the drive-side end of the bottom-bracket shell. Initially, the sub-assembly 180 may need to be screwed in further than the position it will occupy after final adjustment. The lid 171, together with the spreader-beam 167 to which it has been attached and a seal 178 around its periphery, may be addressed during assembly to the spreader-box 170. The projecting end of the screw 174 engages with an elastomer bush 181 mounted in the spreaderbox, and thereby provides correct angular location relative to the spreader-box 170 of the spreader-box-lid and the spreader-beam attached to it. The non-drive-side end-cup 154 carries an external taper which can mate with the tapered bore 167e of the spreader-beam 167. A clench-nut 164 carries an external taper which can mate with the tapered bore 171a of the spreader-box lid. The clench-nut 164 may be screwed onto the threaded non-drive-side end of the cartridge-tube 120 and is done up until its inward-projecting flange 164a abuts tight against the non-drive-side end of the cartridge-tube 120. As mentioned above, when assembling the bottom-bracket assembly, the sub-assembly 180 may need to be screwed in further than the position it will occupy after final adjustment. Said final adjustment will now be described. A special spanner may be used to engage with the drive-side end of sub-assembly 180, and to unscrew sub-assembly 180 from the threaded drive-side end of the bottom-bracket shell 9, moving sub-assembly 180, together with the clench-nut 164, towards the drive-side of the bottom-bracket shell, so that the external taper on the clench-nut 164 comes to engage with the mating tapered bore 171a of the spreader-box lid. The lid 171, together with the spreaderbeam 167 to which it has been attached, is drawn inwards until the tapered bore 167e of the spreader-beam 167 engages against the external taper on the end-cup 154. Lubricant may be present on the abutting tapered surface of the clench-nut 164 and the tapered bore 171a of the spreader-box-lid 171, so as to reduce the frictional resistance to relative rotation as subassembly 180 is unscrewed from the drive-side end of the bottom-bracket shell. The unscrewing of sub-assembly 180 from the drive-side end of the bottom-bracket shell is stopped when the torque reaches a level such as induces sufficient axial preload for the tapered surfaces at the non-drive-side end to remain engaged when subject to radial load induced during riding. The cartridge-tube 120 becomes subject to light tensile stress. The assembly may comprise a lock-ring 143 to lock the drive-side end-cup 153 at the selected position. A second mounting structure 2104 at the non-drive-side comprises the spreader-box lid 171, the screws 174 and 175, the spreader-beam 167, and the non-drive-side end-cup 154. The second mounting structure at the non-drive-side end for the cartridge-tube 120 is now in place. When a radial load induced by a rider is transmitted from the non-drive-side end of the bottom-bracket axle through the bearing 106 and via the cartridge tube 120 and clench-nut 164 into the tapered bore 171a of the lid, the lid 171 has sufficient strength to transmit the load out to the screws 174 175. The load is carried by the spreader-beam 167 via the end-cup 154 into the bottom-bracket shell 9. The seal 178 around the periphery of the lid is radially trapped between the lid and the inner face of the periphery of the spreader-box 170. The way in which the first mounting structure at the drive-side end provides a fulcrum 157 tending towards the position shown is now explained. If, when an unbalanced transverse force is applied to the bottom-bracket axle, there were no radial support for the cartridge at its non-drive-side end, radial motion of the cartridge relative to the bottom-bracket shell 9 would arise from a combination of the compliance of the bottom-bracket shell near the drive-side end, the compliance of the end-cup 153 and the compliance in bending of the cartridge-tube 120.The exact axial offset of the virtual fulcrum about which transverse forces are resolved will depend on said compliances, but the offset is likely to lie at or near to the position shown for the fulcrum 157. When the second non-drive-side mounting structure is subjected to a radial load in a given direction, radial displacement in the same direction of the cartridge-tube 120, together with the clench-nut 164 and the spreader-box-lid 171 and the threaded bosses 167a on the spreaderbeam, arises because of the compliance of the spreader-beam profile. Such displacement may range up to 0.05mm (depending on the pedalling force). The displacement can be measured by a sensor such as a strain gauge. For example, figure 13 shows a sensing arrangement that can be part of the assembly. The sensing arrangement may comprise a strain gauge 183 attached to an arm 167c of the spreader-beam 167. The strain gauge is connected to a PCB 185, where circuitry is provided to deliver an electric signal indicating at all times the strain in the outer face of the arm 167c for analysis by a controller (not shown). In similar fashion, the sensing arrangement may further be fitted with a second strain gauge 186 attached to an arm 167d can be used to determine strain arising from radial displacement of the cartridge-tube 120 in a different direction, also for analysis by a controller. As in the first example, the sensing arrangement is preferably arranged to measure forces in different directions such that the controller can determine the level of the unbalanced external force applied in any direction to the bottom-bracket assembly based on readings from the pair of strain gauges. Example 3 A third example of the bottom-bracket assembly 350 comprising a cartridge-tube 120 is shown in figure 14 and 15. This third example will now be described in the context of assembling and fitting it into a bottom-bracket shell of an HPV. In this example, the second mounting structure 3104 comprises a non-drive-side end-cup 205, an elastomer ring 206, and a washer 207. The elastomer ring 206 has a conical shape. The washer 207 has a corresponding shape to the elastomer ring (e.g., a conical shape). The elastomer ring 206 abuts a mating conical surface formed inside the non-drive-side endcup 205, and with its other face the ring 206 abuts a mating conical surface formed on the outside of the conical washer 207. The elastomer ring 206 may be glued to the end-cup 205 and may also be glued to the conical washer 207. The assembly comprises a spreader-backing-plate 223. The spreader-backing-plate is addressed to the non-drive-side flank of the bottom-bracket shell 9. The end-cup 205, together with elastomer ring 206 and conical washer 207, is screwed in to the threaded bore at the non-drive-side end of the bottom-bracket shell and tightened so as to clamp the spreader-backing-plate 223 securely against the flank of the bottom-bracket shell. The assembly further comprises a cartridge sub-assembly 217. The cartridge sub-assembly 217 is made up of: a bottom-bracket axle 12, a drive-side cartridge shell 201, a non-drive-side cartridge shell 202, bearings 105, 106, a pair of inner bearing-raceways 211, a pair of outer bearing raceways 210, a pair of snap-rings 212, a set of bearing balls 213, an elastomer membrane 204, and a drive-side end-cup 203. The elastomer membrane 204 and drive-side end cup 203 make up the first mounting structure 3103. The elastomer membrane 204 has the same properties as the elastomer membrane in the first example. For example, the elastomer membrane has a conical shape. The conical elastomer membrane 204 abuts a mating conical surface formed inside the driveside end-cup 203 and on its other face abuts a mating conical surface formed on the outside of the drive-side-cartridge-shell 201. The elastomer membrane 204 may be glued to the endcup 203 and may also be glued to the cartridge-shell 201. The drive-side bearing 105 is fitted to the bore of the cartridge-shell 201, abutting an inward projecting flange 201a at the driveside end. The outer bearing raceway 210 is also fitted to the bore of the cartridge-shell 201, abutting a shoulder at 214. The pair of inner bearing-raceways 211 is mounted on the bottom-bracket axle 12, constrained from axially moving apart on the bottom-bracket axle by the pair of snap-rings 212. The bottom-bracket axle 12, including the inner raceways 211, is introduced to the non-drive-side-end of the cartridge-shell 201 carrying the set of bearing balls 213 resting on the bearing raceways 211. A second outer bearing raceway 210 is fitted to the bore of the cartridge-shell 201, trapping the bearing balls 213 between the four bearing raceways. The bottom-bracket axle 12 engages with the bearing 105. The non-drive-side bearing 106 is fitted to the bore of the non-drive-side-cartridge-shell 202, abutting an inward projecting flange 202a at the non-drive-side end. The non-drive-side cartridge-shell 202 together with bearing 106 is addressed to the driveside of the cartridge-shell 201, and moved axially until a shoulder 215 comes to abut one of the outer bearing raceways 210, in turn forcing the outer raceways into contact with each other. The outside of the shell 202 is a close fit in the bore of the shell 201. The bottom-bracket axle 12 engages with the bearing 106 at the non-drive-side end. The cartridge sub-assembly 217 is now complete, with axle 12 carried in bearings in the bores of the two cartridge parts which are now joined together. The sub-assembly 217 may be fitted as a complete item through the bottom-bracket shell 9 from the drive-side end of the bottom-bracket shell. The end-cup 203 which now forms part of the sub-assembly 217 may be screwed into threads at the drive-side end of the bore of the bottom-bracket shell. A cylindrical nose 202b at the non-drive-side end of the cartridge passes through the non-drive-side end-cup 205. A conical surface 202c formed on the outside of the cartridge-shell 202 engages with a mating conical surface on the inside of the conical washer 207. Lubricant may be present on the abutting conical surfaces of the cartridge-shell 202 and conical washer 207, so as to reduce the frictional resistance to relative rotation as subassembly 217 is screwed in. The end-cup is screwed in until a torque which introduces an appropriate level of compression of the elastomer ring 206 has been reached. The assembly may comprise a lock-ring 143 to lock the end-cup 203 at the selected position. The two now-joined cartridge shells are in compression. The bottom-bracket axle 12 is radially supported by, and may revolve in, the bearings 105 and 106. These bearings may either be plain or comprise rolling elements, and for this example, needle bearings are shown. The two halves of the cartridge-shell are now positively secured together between the conical elastomer ring 206 and the conical elastomer membrane 204, trapping the two outer bearing raceways 210 against one another. It is arranged that spacing of the snap-rings 212 is such as allows a running clearance for the bearing balls 213 between the four raceways. Any external axial force applied to the bottom-bracket axle is transmitted by the thrust bearing assembly so formed into one or other half of the cartridge shell, and on into the bottom-bracket shell via the adjacent end-cup. When the assembly of parts into each end is complete, with the lock-ring set, the first driveside mounting structure is provided by the connection through the elastomer-membrane 204 and through the end-cup 203 to the bottom-bracket shell. The second non-drive-side mounting structure is provided by the connection through the conical washer 207, the compliant elastomer-ring 206 and through the end-cup 205 to the bottom-bracket shell. The way in which the first mounting structure at the drive-side end provides a fulcrum 237 tending towards the position shown is equivalent to the way explained above for the first example. Effectively the fulcrum 237 tends towards the position shown as this position is also the centre of a hemisphere, a slice of which is approximately defined by the conical ring of elastomer membrane 204 and it is when rotation is about this fulcrum that the compressive / tensile strain induced in the elastomer is at a minimum. The assembly further comprises an elastomer washer 227 and a spreader disc 226. The elastomer washer 227 abuts the inboard face of the spreader disc 226. The washer 227 may be glued to the disc 226. The disc 226, together with the washer 227, may be pressed onto the nose 202b on the cartridge sub-assembly 217, until the elastomer washer 227 comes to bear against the end-cup 206. The bore of the disc 226 is a tight fit on the nose 202b on the cartridge sub-assembly. The assembly further comprises a spreader-box 224 and the spreader-backing-plate 223. The spreader-box 224 is secured with screws 225 to the spreader-backing-plate 223 trapping a peripheral seal 221. An inner seal 222 bears axially against the outboard flank of the disc 226, further helping retain the disc 226 in position. When the second non-drive-side mounting structure is subjected to a radial load in a given direction, radial displacement in the same direction of the cartridge sub-assembly 217, including spreader-ring 226 carried on its nose 202b, arises because of the compliance of the conical elastomer ring 206. The amount of this displacement is a function of the radial load. This displacement is measured by a sensor. For example, a sensing arrangement for measuring the displacement of the cartridge subassembly is shown in figure 15. The sensing arrangement comprises a beam 229 carried inside the spreader-box 224 that is supported in such a way as brings its centre to rest against the periphery of the spreader-disc 226, which induces an initial strain in the surface of the beam near its centre. When there is any motion of the spreader-disc 226 perpendicular to the beam 229, the strain in the beam at its centre changes. A strain-gauge 230 attached to the beam 229 is connected to a PCB 231. Circuitry (not shown) on the PCB 231 is provided to deliver an electric signal indicating at all times the strain in the beam 229. This electric signal can be used by a controller (not shown) to determine the level of the unbalanced transverse force induced by the rider about the fulcrum in the direction perpendicular to beam 229. The sensing arrangement may further comprise a second beam 232 which may be fitted to rest against a different part of periphery of the spreader-disc 226, carrying a strain-gauge 233 also connected to the PCB 231, generating in an equivalent manner an electrical signal which can be used by a controller to determine rider-induced forces perpendicular to the second beam 232. The assembly may further comprise a position sensing arrangement for determining the angular position of the bottom-bracket axle. The position sensing arrangement comprises a ring 234 mounted on, and constrained to revolve with, the bottom-bracket axle 12. The ring 234 carries one or more targets 235. The PCB 231 carries one or more sensors 236 which may respond to the passing of a target as the bottom-bracket axle revolves, so that the controller can determine the angular position of the bottom-bracket axle and of the cranks 13 15 attached to it. Protection for ring 234 is provided by a circular flange 224a projecting from the spreader-box 224. The three examples described here comprise combinations of particular features. Viable embodiments of the invention other than those described can be made using varying combinations of features described above. A drive apparatus as described in the examples above comprising a motor for providing torque to the bottom-bracket axle may be attached to an HPV comprising any of the embodiments of a bottom-bracket assembly described above. The controller of the drive apparatus may be configured to calculate, based on the determined transverse force on the bottom-bracket axle, an output torque for the motor to apply to the bottom-bracket axle both when assist is to be provided to the rider and also during periods when assistance from the motor is not to be provided. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description, it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. 09 03 26

Claims

1. A drive apparatus for providing assistance in the propulsion of an HPV, the HPV having: a frame; a bottom-bracket axle carried in a plurality of bearings which support the bottombracket axle in the frame of the HPV and allow it to revolve therein; a pair of diametrically opposite cranks, one at each end of the bottom-bracket axle, through which a rider can apply tangential force to deliver torque to the bottom-bracket axle and make it revolve; a mid-plane which is perpendicular to the axis of the bottom-bracket axle; a drive-side on one side of the mid-plane; a bottom-bracket output for onward delivery of propulsive drive which is attached to the bottom-bracket axle and is offset from the mid-plane of the HPV on the drive-side of the HPV; a bottom-bracket assembly comprising the bottom-bracket output, the bottombracket axle, and the pair of cranks, all of which are coupled so as to revolve as one relative to the frame of the HPV, wherein the drive apparatus comprises:a drive-wheel configured to connect to the bottom-bracket assembly so as to revolve with it, the drive-wheel being configured to deliver torque into the bottom-bracket axle through a connection which is axially positioned outboard of a bearing supporting the bottom-bracket axle and which, when connected, revolves in a plane which is offset from the mid-plane of the HPV;a motor comprising a rotor;a motor-output wheel which can, when the drive apparatus is connected to the bottom-bracket assembly, revolve about an axis parallel to and offset from the axis about which the bottom-bracket axle revolves;a drive-element which can couple the motor-output wheel to the drive-wheel so as to deliver torque from the motor to the drive-wheel;a one-way clutch in the drivetrain between the rotor and the bottom-bracket assembly, the one-way clutch being configured to engage when the rotor is revolving at an engagement speed, and to disengage when the rotor is revolving at a speed less than the engagement speed, the engagement speed being a predefined multiple of the speed at which the bottom-bracket assembly is revolving; anda controller configured to receive data from one or more sensors and to estimate from the data a force applied to the bottom-bracket axle by a rider through the cranks, wherein the controller is configured to use this information to decide a level of torque that the motor is to apply to the rotor when propulsive assistance is not to be provided.

2. A drive apparatus as in claim 1 and a bottom-bracket assembly, the bottom-bracket assembly comprising:09 03 26a bottom-bracket axle carried in a plurality of bearings for supporting the bottombracket axle in the frame of the HPV and allowing the bottom-bracket axle to revolve therein;a hollow cartridge having a bore and carrying in its bore the plurality of bearings, the cartridge being mountable to the frame of the HPV;a first mounting structure for the cartridge, the first mounting structure having an elastic resistance to rotation in yaw and roll of the cartridge relative to the frame of the HPV that is insufficient on its own to prevent said rotation when the bottom-bracket axle is subjected to transverse forces which are not in balance about the first mounting structure such that the first mounting structure provides a fulcrum for said rotation of the cartridge;an elastically compliant structure axially displaced from the first mounting structure when the cartridge is mounted to the frame of the HPV, the elastically compliant structure having a compliant resistance to the rotation in yaw and roll of the cartridge;one or more sensors configured to sense radial displacement of the cartridge arising from said rotation of the cartridge about the fulcrum;wherein the controller of the drive apparatus is configured to determine the torque to apply to the rotor in dependence on information from the one or more sensors relating to said sensed radial displacement of the cartridge.

3. A drive apparatus as in claim 2, wherein the one or more sensors lie on the same side of the mid-plane as that where the drive-wheel, the drive-element and the motor-output wheel lie.

4. A drive apparatus as in any preceding claim, wherein the controller is configured to monitor the speed at which the bottom-bracket assembly revolves and / or the speed at which the rotor revolves and, during periods when assistance is not to be provided, to decide from this information the torque to apply to the rotor.

5. A drive apparatus as in any preceding claim, wherein the bottom bracket axle is mounted to revolve in a bottom-bracket shell whose minimum bore is less than 40mm.

6. A drive apparatus as in any preceding claim, wherein torque from the rotor can be delivered through a speed-reduction mechanism to the motor-output wheel.

7. A drive apparatus as in any preceding claim, wherein the one-way clutch is positioned in the drive path from the rotor to the motor-output wheel.

8. A drive apparatus as in any preceding claim, wherein the drive-wheel, drive-element and motor-output wheel lie on a non-drive-side of the HPV which is opposite to the drive-side.09 03 269. A drive apparatus as in any preceding claim wherein, during periods when assist is not to be provided and the motor is revolving at the engagement-speed, the controller is configured to apply a reverse torque to the rotor at a level which is a function of the level of reverse force that the controller calculates is due to a reverse torque applied by the rider through the cranks, based on the information it has from the one or more sensors.

10. A drive apparatus as in any preceding claim wherein, during periods when assistance is not to be provided and the motor is revolving at the engagement-speed, the controller is configured to apply a reverse torque to the rotor at a level which is a function of the tension which the controller calculates is present in the drive-element coupling the motor-output wheel and the drive-wheel on the bottom-bracket assembly, based on the information it has from the one or more sensors.

11. A drive apparatus as in any preceding claim wherein, during periods when assistance is not to be provided and the motor is revolving at the engagement-speed, the controller is configured to apply a reverse torque to the motor at a level which is a function of the difference between an actual measured speed of the motor with a calculated expected speed of the motor, the calculated expected speed of the motor being the speed that the controller would expect the motor to be revolving at if the clutch were disengaged.

12. A drive apparatus as in any preceding claim wherein the controller is configured to calculate a theoretical engagement speed as being the predefined multiple of the speed at which the bottom-bracket assembly is sensed to be revolving and the controller is further configured to apply a level of torque to the motor which is likely, given a known moment of inertia of the rotor, to maintain a pre-determined difference between the speed of the motor and the theoretical engagement speed.

13. A drive apparatus as in any preceding claim, wherein the drive apparatus comprises a sensor for sensing the speed of the rotor and / or a sensor for sensing the speed of the bottom-bracket assembly.

14. A drive apparatus as in any preceding claim wherein, during periods when assist is to be provided, the controller applies forward torque to the motor at a level which is a function of the level of force that the controller calculates is due to rider torque applied to the cranks, based on the information it has from the one or more sensors.

15. A drive apparatus as in any preceding claim, wherein the drive-element is a drive belt having a bias to adopt, when de-tensioned, a curve with a radius larger than that of the drivewheel, and the drive apparatus further comprises:a tensioner being operable to apply tension to the belt whereby the belt can couple the motor to the drive-wheel, and being operable to release tension from the belt whereby the belt can follow a path which is free of the drive-wheel; anda guide for restraining the belt outboard of the drive-wheel along the path where the belt is free of the drive-wheel.

16. A drive apparatus as in any preceding claim, further comprising a housing attachable to the HPV, the motor and drive-wheel each being carried by the housing, wherein the drivewheel has an opening extending through its centre, the opening being accessible from opposing sides of the housing whereby a crank on the HPV can be passed through the opening for mounting to the drive-wheel.

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