Steering system with variable-ratio kinematic mechanism

Abstract

A mechanical type steering system with progressive variable steering ratio.

Description

[0001] DESCRIPTION

[0002] Title

[0003] Steering system with variable-ratio kinematic mechanism

[0004] Technical field

[0005]

[0001] The present invention belongs to the technical field of steering systems for vehicles equipped with at least one steerable wheel and more specifically to so-called variable-ratio steering systems, in which the ratio between the rotation imparted to the control device and the rotation actually obtained at the at least one steerable wheel of the vehicle, is not constant but variable.

[0006] Present status of the art

[0007]

[0002] In this patent text, the term "vehicles" is meant to refer to all those means of transport comprising dedicated control devices, such as steering wheels or handlebars, which act on one or more directional moving members in order to make it possible to change the direction of the vehicle itself.

[0008]

[0003] It should be pointed out that in this patent text, the term "steering device" refers to any device, whether a steering wheel or handlebar or other type of steering device, that makes it possible to control the directional wheels of a vehicle in order to achieve a change in the direction of travel.

[0009]

[0004] Hereinafter reference will be made in particular to land vehicles equipped with at least one pair of wheels on the same axis and including at least one steerable wheel operated by means of a control or steering device, such as a steering wheel or a handlebar. It is understood that the same inventive concept is also applicable to other types of vehicles, with three, four or more wheels, whether such vehicles are road or non-road vehicles, or other types of vehicles, including those vehicles in which the connection between the handlebar and the steerable wheel is achieved by means of special transmission means.

[0010]

[0005] Rotational actuation of the control device by the driver, or by a servomechanism in the case of self-driving systems, transmits a corresponding rotation to the steerable wheels, thereby allowing the vehicle to be steered in the desired direction. It should be noted that in the case of vehicles with front steerable wheels, the steerable wheels rotate in the same direction as the control device, while in the case of rear steerable wheels, a mechanism for reversing the direction of rotation is required. The conversion ratio of the transmission of rotational motion between the control device and the steerable wheels, commonly known as the "steering ratio," represents the relationship between the degrees of steering imparted to the control device and the resulting degrees of rotation at the wheels. The steering ratio isis defined as the ratio between the rotation i50Sof the steerable element and the rotation of of the steering device: is= i50S / 19S.

[0006] In general, a high steering ratio implies a greater responsiveness of the steering system, i.e., a multiplication of the value of the angle of rotation of the wheels with respect to the angle of rotation of the steering device, such a configuration being particularly advantageous when large rotations of the control device are required, for example, to perform parking maneuvers. Conversely, a low steering ratio implies a less responsive steering system, i.e., a given angle imposed on the control device will correspond to less rotation of the steerable wheels, a desirable situation when driving along a straight stretch of road at high speed.

[0011]

[0007] Constant-ratio steering systems have a number of disadvantages: first, it is not possible to adjust the vehicle's response to different possible driving situations (e.g., straight-line driving, parking maneuvers, maneuvering on narrow roads at low speed, etc.), and second, under certain driving conditions, a constant steering ratio does not allow the driver to correctly perceive the magnitude of the force coming from the steering device and promptly notice any dangerous situations such as in the case of incipient lateral rollover.

[0012]

[0008] For this reason, steering systems have been developed in which the steering ratio is not fixed but variable; in other words, in such systems the steering ratio is not always constant but varies according to the angle of rotation imposed on the steering device with respect to its neutral position, i.e., the position corresponding to a straight travel of the vehicle. In particular, in variable-ratio steering systems it is possible to define a specific steering ratio for different angular positions of the steering device: in the case of a vehicle driven by a steering wheel, it is therefore possible to assign a specific steering ratio to different angles of rotation of the steering wheel. In this way, it is possible to make it possible for small angular movements of the steering wheel with respect to its neutral position to result in limited deviations of the vehicle's trajectory, which is particularly useful when proceeding in a straight direction at high speed; on the other hand, when maneuvering at low speed, the amplitude of rotation that must be imposed on the steering wheel in order to effect steering is advantageously very small compared with conventional constant-ratio steering systems.

[0013]

[0009] A variable steering ratio thus makes it possible to improve vehicle drivability and consequently driving safety: in a variable-ratio steering system, the ratio of the transmission of rotational motion from the steering device to the wheels can be designed to achieve the most appropriate ratio under different conditions of use, for example, as mentioned earlier, by increasing the responsiveness in parking maneuvers, especially at low speeds, or by decreasing the responsiveness of the steering in case of small corrections of the steering angle when driving straight- on.

[0014]

[0010] The optimal values of the steering ratio can be determined analytically by taking into account some significant parameters such as the "Yaw Rate Gain," which is the variation of the yaw angle of the vehicle with respect to the angle imposed on the steering wheel where "yaw" refers to the rotational motion of the vehicle itself around the vertical axis passing through its center of gravity. Yaw Rate Gain measures the ratio of yaw rate (i.e., the angular velocity of rotation about the vertical axis of the vehicle) to the angle formed by the steerable wheels with the velocity vector; normally this value tends to decrease as lateral acceleration increases, and this reduces the vehicle's maneuverability in changes of direction, causing undesirable understeer behavior; the skilled person in the field is familiar with the optimum range within which said value is to be contained.

[0015] [Oil] It should be pointed out that in variable-ratio steering systems, the quantity that is changed is not the steering torque, i.e., the torque required to steer the vehicle wheel, but the kinematic ratio between the movement of the vehicle control device and the corresponding movement of the steerable wheel or wheels. The steering torque is modified, as opposed to the case of constant kinematic ratio, as a result of this change in the steering ratio.

[0016]

[0012] In the field of motor vehicles and, more generally, of those vehicles equipped with at least two steerable wheels arranged on the same axle, two main types of variable-ratio steering systems are currently known: systems employing mechanical devices of the so-called "rack-and-pinion" type and electrically driven systems referred to as "Steer-by-wire." The latter in turn can be further subdivided into systems with cascade actuation on rack-and-pinion mechanism and systems with direct actuation on the steerable wheel.

[0017]

[0013] In general, in "rack-and-pinion" mechanisms, the rotary motion applied to the control device is converted into rectilinear motion and transferred to the steering tie rods acting on the wheels, in this case the pinion receives the rotary motion from the control device and transmits an axial load on a linear rack which through lateral tie rods obtains the steering of the wheels. In its simplest realizable form, the pinion has teeth that engage with the teeth of the rack, often the pinion is of the helical type. In variable steering ratio rack-and-pinion systems, the teeth of the rack are not all equidistant from each other but are spaced differently to provide more or less responsive steering depending on the phase of rotation of the steering wheel or handlebar. A system of the aforementioned type is described in international patent application WO2020145848A1.

[0018]

[0014] Unfortunately, variable-pitch rack-and-pinion steering mechanisms, although relatively simple in construction, are too bulky and heavy to be installed on small vehicles or light or low-power vehicles such as pedal-assisted vehicles. In general, a rack-and-pinion kinematic mechanism constitutes a mechanical constraint that makes it impossible to achieve a steering system with differentiated steering ratios as desired, particularly because the pitch of the rack teeth must always remain such that precise coupling with the corresponding pinion teeth is ensured, otherwise the proper engagement of the pinion is jeopardized.

[0019]

[0015] On the other hand, electrically-operated Steer-By-Wire steering systems, such as the one described in Chinese patent application CN109726516A, although characterized by a high degree of flexibility and variability in potentially achievable steering ratios, have very high production costs and are affected by functional safety problems that make their use on road vehicles extremely complex, if not impossible, also in light of the possible failures or malfunctions to which the many electronic components that make up such systems are potentially subject. At present, all systems of the steer- by-wire type are subject to stringent regulations that require redundancy of their electronic and mechanical components, resulting in increased production costs and system complexity. The technical standard ISO 26262 defines the current international standard for functional safety applicable to road vehicles.

[0020]

[0016] Japanese patent JP4809393B2 describes a steering system for a vehicle equipped with two non-steerable front wheels, operated by pedals, and a steerable rear wheel. Such a steering system comprises a first gear called the " drive gear ' which is integrally connected to the handlebars of the vehicle and is suitable for transmitting the mechanical moment, applied to the axis of the handlebars, to a second intermediate gear called the ' transmission gear1to the upper face of which a first sprocket is integrally coupled, the latter engaging with a chain which transmits the rotary motion to a second sprocket integrally connected to the steering axis of the rear wheel. This is a very complex system in terms of structure, composed of a large number of transmission elements, which results in an increase in weight, with obvious disadvantages especially for lighter vehicles, as well as high production costs, in addition, both the first and second gear, having to engage with each other, can only have a circular conformation and this makes it impossible to achieve progressive steering with differentiated steering ratios at certain angular positions of the vehicle's handlebars.

[0021]

[0017] U.S. patent application US6125963A relates to a steering system for a vehicle with two steerable rear wheels. This system is specifically directed to achieve kinematic steering by realizing the so-called Ackermann kinematic mechanism, in which the center of instantaneous rotation of the steerable rear wheels is located on the extension of the non-steerable front axle of the vehicle. Thus, this system is not directed to achieve progressive steering, that is, progressively variable steering ratio steering. Moreover, US6125963A also shows excessive complexity, with evident disadvantages both under the functional point of view and from the point of view of production costs, needing no less than three specially conformed rotating elements, not circular in shape, specifically a first element connected to the steering wheel and two other elements each connected to the axle of a respective steerable wheel, said elements being coupled together by means of a motion transmission organ. Such rotating elements are all specially conformed so that, as the steering wheel is rotated, the motion transmission organ engages a specific portion of each of the rotating elements connected to the axles of the wheels.

[0022]

[0018] In Chinese utility model CN2601203Y, a kinematic-type precision steering system employing rotating elements with non-circular geometry, in which the steering ratio is not variable but fixed, is described.

[0023]

[0019] Therefore, there is a need for a variable ratio steering system of a mechanical type in which the kinematic ratio between the movement of the control device and the steerable wheels of the vehicle is not constant but varies according to the angle of rotation of the control device itself, said steering system not requiring complex electronic or hydraulic control systems, and being installable transversely on any type of vehicle, ensuring greater control of the lateral dynamics and steering of the vehicle compared to currently known systems.

[0024] Objects and summary of the invention

[0025]

[0020] The objective of the present invention is to provide a progressive variable-ratio steering system, of the mechanical type, particularly suitable for application to land, road and non-road vehicles.

[0026]

[0021] It should be pointed out that the present steering system can be implemented both in vehicles with a driver on board, thus a person physically interacting with the control device, and in vehicles equipped with autonomous driving systems. In the present patent text, the term "steering device" is therefore to be understood as referring to a device that allows the steerable wheels of a vehicle to be controlled in order to achieve a change in direction, regardless of whether it is operated directly by a person or not.

[0027]

[0022] Another objective of the present invention is to provide a steering system of the aforementioned type that can be applied to any type of two-wheeled or multi-wheeled vehicle, regardless of the size and total weight or maximum power output reached by the vehicle, and regardless of the mutual arrangement of the control device (steering device) and the controlled element (wheel(s)). In particular, the present invention was developed for special light three-wheeled vehicles with only one steerable wheel located at the rear and with the control device arranged at the front.

[0028]

[0023] Not the least objective of the present invention is to provide a variable-ratio steering system that is extremely versatile and highly customizable, allowing different steering ratios to be obtained at given angular positions of the control device, depending on the chosen conformation of the elements that make up the kinematic mechanism for transmitting rotary motion from the control device to each of the steerable wheels of the vehicle.

[0029]

[0024] Finally, one objective of the present invention is to provide a variable-ratio steering system that is simple and inexpensive to implement and does not require high maintenance and operating costs.

[0030]

[0025] These and other objectives that will become clear to the skilled person in the field from reading this patent text are achieved by means of a steering system for vehicles equipped with at least one steerable wheel, in which the kinematic mechanism for transmitting rotational motion from the steering device of the vehicle to the steering axle of each steerable wheel comprises a flexible element for mechanical transmission, in this case either a belt or a chain, cooperating with at least two rotating elements, either pulleys or toothed gears, respectively.

[0031]

[0026] Advantageously, the present steering system makes it possible to achieve progressive steering, i.e., a steering system with a steering ratio that changes progressively as a function of the angle of rotation of the control device, regardless of the number of wheels of the vehicle and regardless of whether the steering axle of the vehicle is the front or the rear one.

[0032]

[0027] In this patent text, the adjective "flat" is used to refer to an element presenting two dimensions prevailing over its own thickness and delimited by two opposing surfaces, or faces, that are substantially flat. The expression "steering axle" is used here to refer to the axis around which a steerable wheel rotates.

[0033]

[0028] The first of said rotating elements is called the rotating driving element being integrally connected to the distal end of a shaft to the proximal end of which the vehicle control device is integrally connected, while the second and any further rotating elements, called the rotating driven elements, are respectively integrally connected to a corresponding steering axle of each steerable wheel of the vehicle. The rotating driving element rotates as a result of the rotation imposed on the steering device; due to the flexible element for transmission, the rotational motion is transferred from the rotating driving element to the at least one rotating driven element.

[0034]

[0029] Very advantageously, at least one of said rotating elements has a special geometrical conformation with a variable radius, that is, such that its outer perimeter does not describe a circumference. In other words, at least one of said rotating elements does not have the substantially discoidal conformation typical of traditional pulleys or ring gears, but is shaped conveniently, e.g., it may be ellipsoidal or elliptical-cardioid or other more or less articulated shape, for the purpose of obtaining differentiated steering ratios at certain angular positions of the vehicle's steering device.

[0035]

[0030] For example, a rotating element can be designed so that its conformation allows for a steering ratio of 1:5 for an angle of 20° imposed on the steering device, and a ratio of 1:10 (i.e., that for every degree of rotation of the steering device there will be a rotation of 0.1° at the controlled element) for other angles of rotation of the steering device.

[0036]

[0031] The specific shape of each rotating element of the system, which is necessary to achieve the desired steering ratio at given angular intervals of the rotation of the steering device, is determined by numerical modeling using special calculation software. There are no limitations to the possible shapes of the rotating elements since, by means of appropriate tensioners, adequate tension can be maintained at all times on the "hanging" branches of the transmission elements, i.e., those between the rotating elements.

[0037]

[0032] The special conformation of at least one of said rotating elements makes it possible to obtain a progressively variable steering ratio system composed exclusively of mechanical parts, devoid of electronic control devices that, in addition to being very expensive, also require proper electronic calibration and special backup systems in case of malfunction. Such rotating elements can be made by exploiting low-cost sheet metal cutting processes or by 3D printing for shaped profiles.

[0038]

[0033] Finally, the use of flexible transmission elements ensures a very high degree of flexibility in terms of the positioning of the different components of the kinematic mechanism of the vehicle, thus overcoming the inherent limitations of a pinion-rack mechanism.

[0039] Brief description of the drawings

[0040]

[0034] Fig. 1 shows a perspective view of a special vehicle equipped with two non-steerable front wheels and one steerable rear wheel, to which the progressive variable-ratio steering system that is the subject of the present invention is applied, in an embodiment comprising a rotating driving element (1) of circular shape and a rotating driven element (2) of special shape.

[0041]

[0035] Fig. 2 shows a top view of the vehicle equipped with the variable steering ratio system shown in Fig. 1.

[0042]

[0036] Fig.3 shows a side view of the vehicle equipped with the variable steering ratio system represented in Fig. 1.

[0043]

[0037] Fig. 4 shows a detail view of just the rotating driven element (2) in an elliptical-cardioid shape.

[0038] Fig. 5 shows several possible alternative conformations of said rotating elements (1, 2), specifically, from left to right, a symmetrical conformation capable of providing progressive steering, an asymmetrical conformation capable of providing progressive and kinematic steering, an asymmetrical conformation capable of providing kinematic steering only, and a symmetrical conformation capable of providing progressivity.

[0044]

[0039] Fig. 6 shows a perspective view of a special vehicle equipped with two non-steerable front wheels and two steerable rear wheels, to which is applied the progressive variable-ratio steering system which is the subject of the present invention, in an embodiment comprising a rotating driving element (1) of circular shape and a rotating driven element (2) of special shape, one for each of said steerable wheels; also shown are means for tensioning (9) said flexible transmission element (3) useful both for maintaining an adequate tension of the flexible transmission element and for the purpose of directing said flexible element so as not to disturb the driver or any passengers.

[0045]

[0040] Fig. 7 shows a special vehicle equipped with two non-steerable front wheels and two steerable rear wheels, to which the progressive variable-ratio steering system is applied with a rotating driving element (1) of circular shape and a rotating driven element (2) of special shape, the latter being coupled to a kinematic joint suitable for obtaining kinematic steering exclusively.

[0046]

[0041] Fig. 8 shows a possible alternative embodiment of the vehicle shown in Fig. 7 in which the rotating driving element (1) is of special shape and the rotating driven element (2) is of circular shape.

[0042] Fig. 9 shows a special vehicle equipped with two steerable front wheels and two also steerable rear wheels, to which is applied a progressive variable-ratio steering system comprising two rotating driving elements (11, 12) of special shape, coaxial with each other and integrally connected to the shaft (4) of the steering device (7). The first (11) of said rotating driving elements is coupled via a corresponding first flexible element of mechanical transmission (31) to the rotating driven elements (2) of the front wheels, while the second (12) of said rotating driving elements is coupled via a second flexible element of mechanical transmission (32), in a cross configuration, to a first intermediate rotating element (13) of circular shape; the latter is coaxially connected in an integral manner to a second intermediate rotating element (14) of circular shape, and the latter is in turn coupled by means of a third flexible element of mechanical transmission (33) to the rotating driven elements (2) of the rear steerable wheels. Also visible are tensioning means (9) of said flexible transmission elements.

[0047]

[0043] Fig. 10 shows a special vehicle equipped with two non-steerable front wheels and two steerable rear wheels, to which is applied a progressive variable-ratio steering system comprising a rotating driving element (1) of circular shape coupled via a first flexible element of mechanical transmission (31) to a first intermediate rotating element (13) of special shape; the latter is coaxially connected in an integral manner to a second intermediate rotating element (14) of circular shape that is coupled via a second flexible element of mechanical transmission (32) with the rotating driven elements (2) of the rear wheels.

[0048]

[0044] Fig. 11 shows a special vehicle with two non-steerable front wheels and two steerable rear wheels, to which is applied a progressive variable-ratio steering system comprising two intermediate rotating elements (13,14), substantially circular in shape, coaxial and integrally connected to each other, the first (13) of said intermediate rotating elements receives rotary motion from the rotating driving element (1), by means of a first flexible transmission element (31), while the second (14) of said intermediate rotating elements transmits said rotary motion to the rotating driven elements (2) of the rear wheels, by means of a second flexible transmission element (32). The rotating driving element (1) and the rotating driven elements (2) are of special shape.

[0049]

[0045] Fig. 12 shows a special vehicle with two steerable front wheels and two non-steerable rear wheels, to which is applied a progressive variable-ratio steering system comprising a rotating driving element (1) of special shape and two rotating driven elements (2) also of special shape.

[0050]

[0046] Fig. 13 shows a possible alternative embodiment of the vehicle shown in Fig. 1 in which the progressive variable ratio steering system comprises a rotating driving element (1) of special shape and a rotating driven element (2) also of special shape.

[0051]

[0047] Fig. 14 shows a further possible alternative embodiment of the vehicle shown in Fig. 1 in which the progressive variable-ratio steering system includes a rotating driving element (1) of special shape and a rotating driven element (2) of circular shape

[0052] Detailed description of an embodiment of the invention

[0053]

[0048] The following detailed description refers to an illustrative, non-binding embodiment of a variable-ratio steering system applied to a three-wheel vehicle respectively equipped with two nonsteerable front wheels and one steerable rear wheel. It is, however, obvious how the same inventive concept underlying the steering system that is the subject of this present patent application can be applied to vehicles equipped with a total number of wheels less than or more than three, regardless of which steering axle the vehicle has and the actual number of steerable wheels.

[0054]

[0049] In the embodiment of the variable-ratio steering system herein described, the kinematic mechanism for transmitting rotational motion from the steering device (7) of the vehicle to the steering axle (8) of the steerable wheel (5) includes: a rotating driving element (1) integrally connected to the distal end of a shaft (4) to the proximal end of which is integrally connected said steering device (7), the center of rotation of said rotating driving element (1) lying on the axis of the distal end of said shaft (4); a rotating driven element (2) integrally connected to said steering axle (8) of said steerable wheel (5), the center of rotation of said rotating driven element (2) lying on said steering axle (8); a flexible element for mechanical transmission (3) of rotary motion from said rotating driving element (1) to said rotating driven element (2), said flexible transmission element (3) being conformed as a ring embracing a respective perimeter portion of each of said rotating elements (1, 2); wherein said rotating elements (1, 2) are substantially flat objects defined by two opposing flat surfaces. At least one of said rotating elements has a non-discoidal shape, that is, such that its perimeter edge is a closed curved line defined by a locus of points whose distance from the center of rotation of the rotating element is not constant, but varies progressively.

[0055]

[0050] In the embodiment described here and shown in the attached figures, the rotating driving element (1) basically has a circular shape, i.e., its perimeter edge defines a circumference, while the rotating driven element (2) has a special, noncircular shape, in this case it has an elliptical-cardioid shape. The non-circular shape of the rotating driven element (2) makes it possible to obtain a differentiated steering ratio depending on the angle of rotation of the steering device (7) with respect to its neutral position. Of course, other solutions are also possible in which, for example, the rotating driving element has a noncircular shape and instead the driven element is circular, or even embodiments in which both elements, driving and driven, are not circular.

[0056]

[0051] It should be noted that in some embodiments of the invention, both the rotating driven element (2) and the rotating driving element (1) may have a special non-circular shape, while in other embodiments the rotating driven element (2) may have a circular shape and the rotating driving element (1) may have a special shape.

[0057]

[0052] Advantageously, in correspondence with the outer perimeter edge of said rotating driving element (1) and said rotating driven element (2), suitable means of coupling with said flexible element for transmission (3) are defined such that rotational motion, through said flexible transmission element (3), can be transmitted from the rotating driving element (1) to the rotating driven element (2).

[0058]

[0053] In a possible embodiment, said flexible transmission element (3) is a drive chain consisting of a plurality of interconnected links, while both the rotating driving element (1) and the rotating driven element (2) are ring gears having teeth protruding perimeterwise and arranged orthogonally to the axis of rotation of the rotating element, suitable for fitting into the links of said drive chain.

[0059]

[0054] According to a possible alternative embodiment, shown in the attached figures, said flexible transmission element (3) is a transmission belt, and both the rotating driving element (1) and the rotating driven element (2) are pulleys. Such a belt may have a rectangular, circular, trapezoidal, hexagonal or other cross-sectional shape suitable for functional cooperation with said pulleys. In a particularly effective embodiment, said belt includes on its intrados a plurality of teeth or protrusions suitable to fit into corresponding grooves defined on the outer perimeter surface of each of said pulleys. Depending on the various possible embodiments falling within the same inventive concept, said teeth and corresponding grooves may be transverse or longitudinal to the direction of transmission of motion.

[0060]

[0055] In a particularly complete embodiment, the present steering system includes means for tensioning (9) said flexible transmission element (3) designed to keep the latter always in the correct position and subjected to adequate tension, whatever the form of said rotating elements.

[0061]

[0056] The shaft (4) of the steering device (7) may alternatively consist of a single tubular element or may advantageously include two half-shafts (41, 42) arranged according to axes incident to each other and mutually coupled by means of a joint (6) capable of transmitting rotational motion, as in the embodiment represented in the attached figures; in the latter case, the rotating driving element (1) is integrally connected to the distal half-shaft (42), and the latter is coupled to the proximal halfshaft (41) , to which the steering device (7) is connected, by means of a universal joint (6).

[0062]

[0057] According to an alternative embodiment of the invention, the shaft (4) of the steering device (7) consists of a single elongated element having high torsional stiffness and, at least in correspondence with a portion of it, low flexural stiffness. In a practical implementation example, said shaft (4) can be made of metal braid that is torsionally non-deformable and at the same time can be subject to flexural deformation; further solutions are also possible in which only the distal portion or only the proximal portion of the shaft (4) is made of a material with low flexural stiffness and high torsional stiffness.

[0063]

[0058] In the attached figures, the rotating driven element (2) is integrally connected to the stem of a fork supporting the steerable wheel (5) of the vehicle, said stem being rotationally connected to the vehicle frame, e.g., by bearings. In the embodiment shown in the figures, the fork comprises two arms supporting the hub of the steerable wheel (5), alternative solutions are of course possible, for example comprising a single-arm fork.

[0064]

[0059] The rotating driving element (1) and / or the rotating driven element (2) are previously suitably shaped according to the desired steering ratios at the different angular positions of the steering device (7). The appropriate shape for achieving the desired result can be determined by means of modeling software: once the input values, corresponding to the desired steering ratios at given steering angle ranges, have been entered, the software returns a simplified representation of the shape of the rotating element or elements required to achieve these steering ratios. In this way, it is possible to customize the steering ratio for different steering angles in order to optimize the dynamic behavior of the vehicle.

[0065]

[0060] The following table shows an example of variable steering ratios (denoted by the letters ivari, iVar2, i ars) achievable with the progressive variable-ratio steering system of the present invention, compared with a hypothetical example case of a system with a constant fixed steering ratio (ic): the steering ratios are defined here as the transmission ratio with respect to the steering rotation angle, denoted by 6sw, relative to the neutral position (0°).

[0066]

[0061] The following graph shows the relationship between the wheel angle, denoted by 6RW, and the angle of the steering device (6sw) for the constant-ratio configuration and the three variable-ratio configurations shown in the table above, in the case of a three-wheeled vehicle with two nonsteerable front wheels and the steerable rear wheel, such as the one described above.

[0067]

[0062] The same inventive concept described here with reference to the illustrative case of a vehicle with two non-steerable front wheels and a single steerable rear wheel is, of course, applicable to other types of vehicles, whether on-road or off-road, equipped with one or more steerable wheels.

[0068]

[0063] An alternative embodiment of the steering system, applied to a vehicle with four wheels, two of them steerable, includes a rotating driven element for each steerable wheel, in other words, a respective rotating driven element is integrally connected to the steering axle of each steerable wheel; the rotating driving element, integrally connected with the steering device, is connected by means of a single flexible transmission element, such as a belt or chain, to both rotating driven elements i.e., a same flexible transmission element embraces a respective perimeter portion of said rotating driving element and both said rotating driven elements.

[0069]

[0064] In vehicles with a rear-steering axle, i.e., with one or more steerable wheels located at the rear, the flexible element of transmission (3) may be coupled to the driving and driven rotating elements by forming a crossed configuration, in other words, by arranging the driving branch and the driven branch of the flexible element of transmission to cross at a point between the rotating driving element and the rotating driven elements. In this way, the direction of rotation of each rotating driven element is opposite to the direction of rotation of the rotating driver element, so that the steering, that is, the change of direction of the vehicle, can take place in the direction that is concordant with the rotation applied to the steering device. Such a configuration is shown in Fig. 9.

[0070]

[0065] Also included in the same inventive concept is a further possible alternative configuration, also suitable for a four-wheeled vehicle with two steerable wheels, comprising two rotating driving elements, both attached coaxially to the steering shaft, each of said driving elements being connected by a respective flexible transmission element to a corresponding rotating driven element of a respective steerable wheel. Also, solutions are possible such as the one shown in Fig. 9 comprising two or more rotating driving elements attached to the shaft of the steering device, each of said driving elements being connected through a respective flexible element of transmission to one or more rotating driven elements, in this way obtaining a vehicle with two or more steering axles, each of said rotating driven elements having a specific conformation such as to obtain different steering ratios for each of said axles.

Claims

CLAIMS1. A variable-ratio steering system for vehicles with two or more wheels having at least one steerable wheel, in which the kinematic mechanism for transmitting rotary motion from the control device (7) of the vehicle to the steering axle (8) of said at least one steerable wheel (5) includes: a rotating driving element (1), integrally connected to the distal end of a shaft (4), to the proximal end of which said control device (7) is integrally connected; the center of rotation of said rotating driving element (1) lying on the axis of said distal end of said shaft (4); a respective rotating driven element (2) for each steerable wheel, integral with the steering axle (8) of the latter; the center of rotation of said rotating driven element (2) lying on said steering axle (8) of said at least one steerable wheel (5); a flexible element for mechanical transmission (3) of the rotary motion from said rotating driving element (1) to said at least one driven rotating element (2), said flexible transmission element (3) being conformed as a ring embracing a respective perimeter portion of each of said rotating elements; wherein both said rotating driving element (1) and said rotating driven element (2) of said at least one steerable wheel (5) are substantially flat objects, defined by two opposing flat surfaces, characterized in that the perimeter edge of at least one of said rotating elements (1, 2) is a closed curved line, defined by a locus of points whose distance from the center of rotation of said rotating element varies progressively.

2. A steering system according to the preceding claim characterized in that said perimeter edge is composed of two specular half-edges.

3. A steering system according to one of the preceding claims characterized in that said rotating elements (1, 2) have, on their outer lateral edge, coupling means with said flexible transmission element (3), said coupling means being such as to provide transmission of rotational motion, through said flexible transmission element (3), from said rotating driving element (1) to said at least one rotating driven element (2).

4. A steering system according to one of the preceding claims characterized in that said flexible transmission element (3) is a belt and in that said rotating driving element (1) and said at least one rotating driven element (2) are pulleys.

5. A steering system according to the preceding claim characterized in that said rotating driving element (1) and said at least one rotating driven element (2) perimetrically have a plurality of teeth suitable for engaging in corresponding grooves defined at the intrados of said flexible transmission element (3).

6. A steering system according to one of claims from 1 to 3 characterized in that said flexible transmission element (3) is a chain comprising a plurality of interconnected links, and in that saidrotating driving element (1) and said at least one rotating driven element (2) are toothed ring gears whose teeth are suitable for engaging in corresponding links of said flexible transmission element (3).

7. A steering system according to one of the preceding claims characterized in that it comprises means for tensioning (9) said flexible transmission element (3).

8. A steering system according to one of the preceding claims characterized in that said shaft (4) comprises a proximal half-shaft (41), to which said control device (7) is integrally connected, and a distal half-shaft (42), to which said rotating driving element (1) is integrally connected, said halfshafts being mutually coupled by means of a joint (6) that transmits rotational motion.

9. A steering system according to claim 8 characterized in that said half-shafts (41, 42) are arranged according to incident axes.

10. A steering system according to one of claims from 1 to 7 characterized in that at least a portion of said shaft (4) has high torsional stiffness and low flexural stiffness.

11. A steering system according to one of the preceding claims characterized in that it comprises two rotating driven elements (2) each integral with the steering axle of a respective steerable wheel (5) and a flexible transmission element (3) embracing a respective perimeter portion of said rotating driving element (1) and of both said rotating driven elements (2).

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