Rear sub-frame and sub-assembly

Abstract

A sub-assembly 104 for a vehicle has a pair of rear longitudinal members 106, 108, with a rear crossmember 118 connected to, and extending between, the rear longitudinal members and having a rear engagement portion 143. A rear subframe (128, Figure 3) is attached to longitudinal members 106, 108, and has a front engagement portion (140, Figure 3) with at least part disposed rearwardly of and / or lower than rear engagement portion 143. If a rear impact of the vehicle causes forward movement of the subframe, the forward engagement portion moves towards rear crossmember 118 to come into contact with rear engagement portion 143, so rear crossmember 118 resists further forward movement of at least the subframe 128. At least part of the subframe may be arranged to rotate in a rear impact, and the subframe may be arranged to bend in such an impact. A subframe and a vehicle are also provided.

Description

TECHNICAL FIELD The present disclosure relates to a sub-assembly having a rear crossmember and a rear subframe. Aspects of the invention relate to a sub-assembly, a rear subframe for use in such a sub-assembly, and a vehicle comprising such a sub-assembly. BACKGROUND It is known to provide rear longitudinal members as part of a vehicle sub-assembly such as a unibody chassis. Rear longitudinal members impart significant longitudinal strength and rigidity. One or more crossmembers may extend between the rear longitudinal members to increase strength. A rear subframe is mounted to the rear longitudinal members. Suspension and other components are mounted to the rear subframe. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a sub-assembly, a rear subframe for use in such a subassembly, and a vehicle comprising such a sub-assembly, as claimed in the appended claims. According to an aspect of the present invention, there is provided a sub-assembly for a vehicle, the subassembly comprising: a pair of rear longitudinal members; a rear crossmember connected to, and extending between, the rear longitudinal members, the rear crossmember having a rear engagement portion; and a rear subframe attached to the longitudinal members, the rear subframe comprising a front engagement portion, at least part of the front engagement portion being disposed rearwardly of and / or lower than the rear engagement portion, wherein the forward engagement portion is arranged to, in the event that a rear impact of the vehicle causes forward movement of the rear subframe relative to the rear crossmember, move towards the rear crossmember such that the forward engagement portion comes into contact with the rear engagement portion, such that the rear crossmember resists further forward movement of at least a portion of the rear subframe relative to the rear crossmember. The rear crossmember may impede or prevent further forward movement of at least a forward part of the subframe, which may help protect an area forward of the rear crossmember. That is, (ignoring the front of the vehicle that may have its own configuration for frontal collisions), in any vehicle rearend collision it is desirable to protect the integrity of the volume of the vehicle forward of the crossmember as much as possible given the dynamics of any given collision. On the other hand, it is of course well known to be desirable for at least a proportion of the kinetic energy of the collision to be absorbed by deformation of components of the vehicle rear of the crossmember. This reduces the maximum acceleration of the volume of the vehicle forward of the crossmember, which is desirable for components of the vehicle such as the battery and its associated electrical components. Thus a balance is struck in the construction of the vehicle between these two requirements in that, once the crumple zones of the vehicle have absorbed as much energy as their design and construction allows in a rearend collision, further collision energy only accelerates the passenger compartment and battery components without deformation, or at least insofar as this is reasonably possible depending on the energy and circumstances of a collision. The rear engagement portion may comprise a rearward-facing surface, which may optionally be rearwardly canted. The front engagement portion may comprise a forward-facing surface, which may optionally be rearwardly canted. The subframe may be configured to, in the event that a rear impact of the vehicle causes forward movement of the subframe relative to the crossmember, rotate the forward-facing surface such that it becomes more parallel to the rearward-facing surface as they come into contact with each other. This may increase a surface area where the surfaces come into contact with each other, which may reduce point loads and increase friction, whereby the crossmember may be accelerated rather than deformed. A lower portion of the rearward-facing surface may terminate at a first vertical position, and an upper portion of the forward-facing surface terminates at a second vertical position that is higher than the first vertical position. The subframe may be mounted to forward mounting points on the rear longitudinal members, the forward mounting points being positioned at a lower vertical position than the rearward-facing surface. This may assist in controlling a movement or pivoting direction of at least a portion of the subframe relative to the mounting points during a rear impact. At least a portion of the subframe adjacent to the forward mounting points may be configured to pivot around the forward mounting points during the rear impact, thereby to contribute to the forward movement of the forward-facing surface. This may assist in bringing the forward-facing and rearward-facing surfaces into contact. The rear longitudinal members may be configured to deform during the rear impact so as to lift a rear of the subframe, thereby to at least contribute to the pivoting of the portion of the subframe adjacent to the forward mounting points. The subframe may be configured to bend during the rear impact, thereby to at least contribute to the pivoting of the portion of the subframe adjacent to the forward mounting points. According to a further aspect of the present invention, there is provided a subframe for use with the subassembly of the preceding aspect, the subframe comprising the engagement portion. According to a further aspect of the present invention, there is provided a vehicle comprising the sub-assembly of the preceding aspect. According to a further aspect of the present invention, there is provided a sub-assembly for a vehicle, the subassembly comprising: a rear crossmember having a rear engagement portion; and a rear subframe disposed rearwardly of the rear crossmember, the rear subframe comprising a front engagement portion, at least part of the front engagement portion being disposed rearwardly of and / or lower than the rear engagement portion, wherein the forward engagement portion is arranged to, in the event that a rear impact of the vehicle causes forward movement of the rear subframe relative to the rear crossmember, move towards the rear crossmember such that the forward engagement portion comes into contact with the rear engagement portion, such that the rear crossmember resists further forward movement of at least a portion of the rear subframe relative to the rear crossmember. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of a vehicle in accordance with an embodiment of the invention; Figure 2 shows an underside view of a rear subframe, for use with a sub-assembly according to an embodiment of the invention; Figure 3 shows a rear perspective view of the rear subframe of Figure 2, for use with a sub-assembly according to an embodiment of the invention; Figure 4 is an underside view of a rear portion of the vehicle of Figure 1 showing its rear sub-assembly; Figure 5 shows a partial longitudinal section of the vehicle of Figure 1; Figure 6 shows the partial longitudinal section as of Figure 5, following a rear impact; Figure 7 is a side view showing a rear longitudinal side of the rear subframe of Figures 2 and 3; Figure 8 is a side view showing a rear longitudinal side as of Figure 7, following a rear impact; Figure 9 shows a detailed view of the longitudinal section of Figure 5; Figure 10 shows the longitudinal section of Figure 9, following a rear impact; Figure 11 is a schematic underside view of the rear subframe of Figures 2 and 3, showing its relationship to a rear crossmember of the vehicle of Figure 1; Figure 12 is a schematic underside view as of Figure 11, following a rear impact; Figures 13, 15, 17, and 19 are schematic sections of different crossmembers and forward lateral members of a subframe according to an embodiment of the invention, prior to a rear impact; Figure 14, 16,18, and 20 are schematic sections of the crossmembers and forward lateral members of respective Figures 13,15,17, and 19, following a rear impact; and Figure 21 is a section through a forward subframe mounting point and forward mount of the rear portion of the vehicle and its sub-assembly of Figure 4. DETAILED DESCRIPTION Sub-assemblies at the rear of a vehicle and rear subframes in accordance with various embodiments of the present invention are described herein with reference to accompanying Figures 2 to 20. The sub-assemblies can be incorporated within a vehicle, such as vehicle 200 as shown in Figure 1, in accordance with other embodiments of the invention. Figures 5 and 6 show a longitudinal section through a midline of the vehicle 200. Only the rear section of the vehicle 200 is shown. The vehicle 200 includes a battery pack 100 that includes a plurality of rechargeable cells (not shown) arranged in a generally flat, rectangular array disposed underneath a floor of the vehicle 200. Supporting circuitry 102 is positioned to the rear of the battery pack 100. The supporting circuitry 102 can include wiring and bus terminations, switchgear, and the like, depending upon the implementation. The battery pack 100 is configured to protect the cells and circuitry, etc., from the ingress of dust and moisture, as well as to physically protect the cells during driving and in the event of an impact. As shown in Figures 2 to 12, the vehicle 200 includes a vehicle sub-assembly in the form of a unibody chassis 104. The chassis 104 forms part of the vehicle 200, but many of the vehicle’s other components, such as suspension and drivetrain components, axles, wheels, and tyres, are omitted from the drawings for clarity. In Figure 4, the chassis 104 includes a left rear longitudinal member 106 and a right rear longitudinal member 108 (noting that Figure 4 shows the underside of the vehicle 200). The left rear longitudinal member 106 and the right rear longitudinal member 108 are indicated with hatching to show them more clearly relative to the surrounding chassis. The left rear longitudinal member 106 and the right rear longitudinal member 108 extend rearwardly between respective wheel arches 110 and 112. A body panel in the form of a subfloor panel 114 extends between inner edges of the longitudinal members 106 and 108. A rear crossmember 118 extends between inner edges of the longitudinal members 106 and 108, near a forward region of the wheel arches 110 and 112. The rear crossmember 118 increases stiffness and strength of the chassis 104. The rear crossmember 118 has a rear engagement portion 143 comprising a rearward-4 facing surface 144, indicated by hatching in Figures 11 and 12. In the embodiment of Figures 2 to 12, the rearward-facing surface 144 is substantially planar, extends along a central portion of the rear crossmember, and is rearwardly canted (as best shown in Figures 9 and 10). The wheel arches 110 and 112, subfloor panel 114, and rear crossmember 118 are stamped or otherwise formed from steel and / or aluminium alloy, although other manufacturing techniques and / or materials may be employed. In this embodiment, the longitudinal members 106 and 108 are cast from an aluminium alloy, but again, other manufacturing techniques and / or materials can be employed. The various components are integrally joined to each other and to other components to form the unibody chassis 104. The components can be joined to each other by way of, for example, welding, adhesives, connectors such as rivets, screws, or bolts, or any other suitable joining method, including any suitable combination of such joining methods. At least some of the components can be formed from a single sheet or piece of metal or other material. The longitudinal members 106 and 108 include respective rear subframe mounting points 120 and 122, and respective forward subframe mounting points 124,126. As best shown in Figure 3, a rear subframe 128 comprises generally arch-shaped side portions 130 and 132 on the respective left and right sides of the rear subframe 128. The front ends of the arch-shaped portions 130 and 132 are connected by a forward lateral member 136, and the rear ends of the arch-shaped portions 130 and 132 are connected by a rear lateral member 138. The rear subframe 128 is die cast in aluminium alloy but other material(s) and / or construction methods can be used in other implementations. In vehicle 200, driveshafts (not shown) for rear wheels 134 (see Figure 1) pass underneath the respective arch-shaped portions 130 and 132. Referring to Figures 2, 3, and 9 to 12, the forward lateral member 136 includes an engagement portion 140, comprising a forward-facing surface 142 indicated by hatching in Figures 11 and 12. In the embodiment of Figures 2 to 12, the forward-facing surface 142 is substantially planar, extends along a laterally central portion of the forward lateral member 136 (as best shown in Figures 2 and 3), and is rearwardly canted (as best shown in Figures 9 and 10). As best shown in Figure 2, the rear subframe 128 includes rear mounts 174, 176, and forward mounts 178, 180, which are positioned and configured for connection to respective subframe mounting points 120, 122, 124, 126 by way of bushings 180 (see Figure 21). For example, a bolt 182 may connect the forward mount 178 to the forward subframe mounting point 124. The bolt 182 is sleeved by a bushing 180,184, which provides some isolation between the chassis 104 and the rear subframe 128. Similar bolts and bushings are used to connect the other forward and rearward mounts to their respective subframe mounting points. As best shown in Figures 9 and 10, the forward-facing surface 142 of the sub-frame 128 is disposed rearwardly of the rearward-facing surface 144 of the chassis 104 sub-assembly. In this context, “rearwardly” means that at least a portion of the forward-facing surface 142 is disposed such that it would contact the rearward-facing surface 144 if the forward-facing surface were to move directly forwards. A lowest point, vertically, with respect to the vehicle in a normal horizontal disposition, of the forward mounting points 124,126 is at a lower vertical position than a lowest point of the rearward-facing surface 142. The lowest point of the forward mounting points 124,126 can be considered to be the lowest point of the forward mounting points 124,126 with the rear subframe 128 removed, excluding any mounting hardware and connectors (such as bolt 182 and bush 184). For example, the lowest point in the embodiment of Figures 2 to 12 is shown by dashed line 198 in Figure 21. The lowest point of the rearward facing surface 144 in the embodiment of Figures 2 to 12 is lower edge 199 in Figure 9. The angle of the rearward cant of the forward-facing surface 142 to the vertical is shown by dashed line 156 and the angle of the rearward cant of the rearward-facing surface 144 to the vertical is shown by dashed line 154 (see Figures 9 and 10). Dashed line 154 is closer to the vertical than dashed line 156. However, in other implementations, the angles that the dashed lines 154 and 156 make to the vertical are the same. In yet other implementations, the dashed lines can be at different angles to each other, with either of the dashed lines being closer to the vertical than the other. The engagement portion 140 is arranged to, in the event that a rear impact of the vehicle causes forward movement of the rear subframe 128 relative to the crossmember 118, move towards the rear crossmember 118 such that the forward-facing surface 142 comes into contact with the rearward-facing surface 144. Figures 5 and 7 show the subframe 128 and rear longitudinal members 106, 108 of the vehicle sub-assembly before a rear-end collision. Figures 6 and 8 show the subframe 128 and rear longitudinal members 106, 108 after a rear-end collision. In this example, the rear-end collision takes the form of a rear impact by a test rig 202 during a rear-end collision test. However, it will be appreciated that similar principles and behaviours will apply during other rear impacts, such as a rear vehicular impact (whether or not the vehicle 200 is stationary) and an impact caused by the vehicle 200 reversing into an object. As shown in Figures 6 and 8, during the rear impact, a rear portion 146 of the vehicle 200 is significantly deformed. For clarity, Figure 6 only shows deformation of the subframe 128, and deformation of the chassis 104 and body panels is not shown. Figure 6 also does not show the rear longitudinals, again for clarity. Part of the deformation of the rear portion 146 involves crush regions, which are designed to deform in a specific manner so as to absorb at least some of the energy of impact and reduce the peak acceleration experienced by vehicle occupants. When the impact is across most or all of the rear portion 146 of the vehicle 200, the left rear longitudinal member 106 and the right rear longitudinal member 108 will accept a considerable proportion of the impact’s energy, causing the longitudinal members 106 and 108 and surrounding chassis and bodywork to crumple. Due to the geometry and construction of the longitudinal members 106 and 108, surrounding chassis components, and bodywork in the embodiment of Figures 2 to 12, the longitudinal members 106 and 108 will tend to bend upwards at their rear end as they crumple during a rear impact. As described above, the rear mounts 174, 176 of the rear subframe 128 are attached to the longitudinal members 106 and 108 at the respective rear subframe mounting points 120 and 122. As a result, the upward movement of the longitudinal members 106 and 108 in the vicinity of subframe mounting points 120 and 122 tends to raise the rear end of the rear subframe 128. This results in rotation of the rear subframe 128 about the forward subframe mounting points 124, 126 in the direction of arrow 170 in Figure 6. This is also shown by the higher position of circled region 192 in Figure 8 as compared with Figure 7. In addition (or, in other cases, alternatively), compression of the rear subframe 128 between the forward subframe mounting points 124 and 126 and the rearward subframe mounting points 120 and 122 tends to move circled region 192 of the subframe 128 forwards relative to the forward subframe mounting points 124, 126. This tends to bend the rear subframe 128, particularly along the arch-shaped portions 130 and 132. This change in shape, as best shown in Figures 6 and 8, results in further rotation of at least the forward region of the rear subframe 128 about the forward subframe mounting points 124 and 126 in the direction of arrow 190 in Figure 6. This bending is also shown by the higher position of circled region 194 in Figure 8 as compared with Figure 7. As described above, the forward mounts 178, 180 of the rear subframe 128 are connected to the forward subframe mounting points 124, 126 by way of tough but resilient bushings, such as bushing 184 described above and shown in Figure 21. These allow for predictable compliance while the vehicle is being driven under ordinary conditions. However, the high force of a rear impact can cause the forward mounts 178,180 to move forward relative to the forward subframe mounting points 124,126 as a result of compression and deformation of the bushings, as well as bending of the bolts (such as bolt 182) that connect the forward mounts 178, 180 of the rear subframe 128 to the chassis 104. This is shown by the more forward position of circled region 196 in Figure 8 as compared with Figure 7. Figure 8 also shows the forward rotation of the forward mount 180 as a result of the rotational forces described above. Figure 6 shows the post-impact rear subframe 128’ overlaying the pre-impact rear subframe (i.e., as shown in Figure 5) 128, illustrating the change to the position and shape of the rear subframe after impact. As best shown in Figures 6, 8, and 10, the net result of the deformation of the rear longitudinals 106 and 108, the bending of the rear subframe 128, and forward movement of the rear subframe 128 as a result of bushing compression and bolt deformation, is forward and upward movement of the forward lateral member 136 of the subframe, as shown by arrow 172 in Figure 10. This brings the forward-facing surface 142 of the forward lateral member 136 into contact with the rearward-facing surface 144 of the crossmember 118. The movement and bending of the subframe 128 also causes the forward-facing surface to rotate slightly as it moves upwards and forwards. The result of this rotation is that the forward-facing surface 142 is substantially parallel to the rearward-facing surface 144 as the two surfaces come into contact. In other implementations, rotation can cause the forward-facing surface to become more parallel to the rearward-facing surface as they come into contact with each other, without actually becoming completely parallel. Figure 11 shows an underside view of the rear subframe 128 and rear crossmember 118 before a rear impact (all other components are omitted for clarity). Figure 12 shows the same view after a rear impact. It can be seen that the rear subframe 128 is compressed along the central axis of the vehicle (for clarity, deformation is shown as simple compression along the longitudinal axis rather than the more complex deformation shown in Figures 6 and 8), such that the distance between the forward lateral member 136 the rear lateral member 138 is reduced after impact. In addition, the rear mounts 174,176 have been pushed closer to the forward mounts 178, 180. The crossmember 118 is a relatively stiff and strong component, and significantly impedes further forward movement of the rear subframe 128, particularly at the point of contact. Additional energy imparted to the rear subframe 128 during the rear impact will be directed into further deformation of the rear subframe 128 and other components of the chassis and bodywork, and will further be converted into kinetic energy (in the event the rear impact is from a car or other object moving towards the vehicle 200). This may help protect the region forward of the rear crossmember. Vehicular impacts can be unpredictable. Factors such as the angle and location of the impact, the shape, size, and mass of the impacting vehicle or object, and the interaction of crumple zones and crush components can make the precise trajectory of particular components of the vehicle during an impact difficult to predict. It will therefore be appreciated that the two surfaces may not end up exactly parallel as a result of every rear impact. Nevertheless, even non-parallel contact helps transfer forces from the rear subframe 128 to the crossmember 118. Additionally, the forward subframe mounting points 124, 126 receive considerable force during a rear impact, as a result of their connection to the subframe 128. After engagement of the forward-facing surface 142 with the rearward-facing surface 144, at least some of the forward forces imparted to the subframe 128 will be directed into the crossmember 118. As a result, less of those forces are imparted to the mounting points 124, 126 through the mounts 178, 180. This may reduce the chance of shearing of connectors that connect the mounting points 124,126 to the mounts 178, 180, which in turn reduces the chance of subframe 128 moving towards the battery in an unconstrained manner. Optionally, a lower portion of the rearward-facing surface 144 terminates at a first vertical position, and an upper portion of the forward-facing surface 142 terminates at a second vertical position that is higher than the first vertical position. For example, as best shown in Figure 9, rearward-facing surface 144 terminates at a first vertical position in the form of lower edge 150 and forward-facing surface 142 terminates at a second vertical position in the form of upper edge 152. The lower edge 150 can be, for example, the lowest point of the rearward-facing 144, and the upper edge 152 can be the highest point of the forward-facing surface 142. The lower edge 150 is positioned vertically lower than the lower edge 152. The relationship between the first vertical position and the second vertical position can exist at least at a laterally central point on the rearward-facing surface 144 and the forward-facing surface 142. Alternatively, or in addition, this relationship can exist across at least a majority of the lateral extents of the rearward-facing surface 144 and the forward-facing surface 142 respectively, as shown in Figures 11 and 12. Figures 13 to 20 show embodiments with different relationships between the forward-facing surface 142 and the rearward-facing surface 144. In Figures 13 and 14, the rearward-facing surface 144 and the forward-facing surface are rearwardly canted and parallel to each other. During the rear impact, rotation of the forward portion of the rear subframe as described above causes upward and forward movement of the forward-facing surface 142. The rotation is such that the forward-facing surface 142 ends up at a closer angle to the vertical than the rearward-facing surface as they come into contact. This causes an upper edge 160 of the forward-facing surface 142 to engage first with the rearward-facing surface 144. In Figures 15 and 16, the rearward-facing surface 144 is substantially vertical, while the forward-facing surface 142 is rearwardly canted. During the rear impact, rotation of the forward portion of the rear subframe as described above causes upward and forward movement of the forward-facing surface 142. The rotation is such that the forward-facing surface 142 ends up substantially parallel to the rearward-facing surface 144 as they come into contact. The contact across the forward-facing surface 142 and the rearward-facing surface causes greater friction between them than would be the case if, for example, there was only a point contact, such as at the upper edge 160 as shown in Figure 10. In Figures 17 and 18, the rearward-facing surface 144 and the forward-facing surface 142 are forwardly canted. During the rear impact, rotation of the forward portion of the rear subframe as described above causes upward and forward movement of the forward-facing surface 142. The rotation is again such that the forward-facing surface 142 ends up substantially parallel to the rearward-facing surface 144 as they come into contact. In other implementations, the forward-facing surface is not parallel with the rearward-facing surface as they come into contact with each other. The forward-facing surface and rearward-facing surface can be parallel prior to impact, or at an any other angle that does not result in the surfaces being parallel when they come into contact. Either or both of the forward-facing and rearward-facing surfaces can be forwardly canted, rearwardly canted, or vertical. Optionally, either or both of the forward-facing and rearward-facing surfaces can include interlocking and / or friction-increasing formations that tend to reduce sliding between the surfaces after they come into contact with each other. For example, one or more ribs, spikes, bosses, or the like, can be provided on either or both of the forward-facing surface and the rearward-facing surface. Optionally, complementary formations can be provided on the surfaces. For example, spikes or ribs can be provided on one surface and complementary recesses can be provided on the other. Such complementary formations tend to engage with each other and reduce sliding between the surfaces. As one example, Figure 19 shows an embodiment in which both the forward-facing surface 142 and the rearward-facing surface 144 include lateral ribs 162. Spaces 164 between the ribs 162 on each surface are configured to receive ribs 160 from the other surface. As shown in Figure 20, the ribs 162 fit within the spaces 164 as the surfaces 142 and 144 come into contact during a rear impact. This locks the surfaces 142 and 144 together, making it much more difficult forthem to slide relative to each other once contact is made. The front engagement portion need not define a planar surface. For example, the front engagement portion can be curved in one or more dimensions, or take any other form. Similarly, the crossmember can take any suitable form. The forward-facing and rearward-facing surfaces can have generally complementary profiles, which may increase contact area and improve performance during a rear end impact. Where terms such as “rotate” and “rotation” are used, it will be understood that translation may also be involved. For example, in the illustrated embodiments, the rotation of the forward-facing surface 142 is accompanied by forward and upward translation. Depending upon the implementation, and the nature and characteristics of any particular rear impact that the sub-assembly experiences, forward movement of the forward-facing surface can include any combination of forward translation, upward translation, and rotation in any direction. The front engagement portion of the described embodiments is configured to move forwards and upwards in the event a rear impact causes forward movement of the subframe. In other embodiments, the geometry of the subframe and surrounding components is configured such that forward movement of the subframe causes forward and downward movement of the front engagement portion towards the rear engagement portion of the crossmember. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A sub-assembly for a vehicle, the sub-assembly comprising:a pair of rear longitudinal members;a rear crossmember connected to, and extending between, the rear longitudinal members, the rear crossmember having a rear engagement portion; anda rear subframe attached to the longitudinal members, the rear subframe comprising a front engagement portion, at least part of the front engagement portion being disposed rearwardly of and / or lower than the rear engagement portion,wherein the forward engagement portion is arranged to, in the event that a rear impact of the vehicle causes forward movement of the rear subframe relative to the rear crossmember, move towards the rear crossmember such that the forward engagement portion comes into contact with the rear engagement portion, such that the rear crossmember resists further forward movement of at least a portion of the rear subframe relative to the rear crossmember.

2. The sub-assembly of claim 1, wherein the rear engagement portion comprises a rearward-facing surface.

3. The sub-assembly of claim 2, wherein the rearward-facing surface is rearwardly canted.

4. The sub-assembly of any preceding claim, wherein the front engagement portion comprises a forwardfacing surface.

5. The sub-assembly of claim 4, wherein the forward-facing surface is rearwardly canted.

6. The sub-assembly of claim 4 or 5 when dependent on claim 2 or 3, wherein the rear subframe isconfigured to, in the event that a rear impact of the vehicle causes forward movement of the rear subframe relative to the rear crossmember, rotate the forward-facing surface such that it becomes more parallel to the rearward-facing surface as they come into contact with each other.

7. The sub-assembly of any one of claims 4 to 6 when dependent on claim 2 or 3, wherein a lower portion of the rearward-facing surface terminates at a first vertical position, and an upper portion of the forward-facing surface terminates at a second vertical position that is higher than the first vertical position.

8. The sub-assembly of any preceding claim, wherein the rear subframe is mounted to forward mounting points on the rear longitudinal members, a lowest point of the forward mounting points being at a lower vertical position than a lowest point of the rearward-facing surface.

9. The sub-assembly of claim 8, wherein at least a portion of the rear subframe adjacent to the forward mounting points is configured to pivot around the forward mounting points during a rear impact, thereby to contribute to the forward movement of the forward-facing surface.

10. The sub-assembly of claim 9, wherein the rear longitudinal members are configured to deform during the rear impact so as to lift a rear of the rear subframe, thereby to at least contribute to the pivoting of the portion of the rear subframe adjacent to the forward mounting points.

511. The sub-assembly of any preceding claim, wherein the rear subframe is configured to bend during the rear impact, thereby to at least contribute to forward movement and / or pivoting of the portion of the rear subframe adjacent to the forward mounting points.10 12. A rear subframe for use with the sub-assembly of any preceding claim, the rear subframe comprisingthe front engagement portion.

13. A vehicle comprising the sub-assembly of any one of claims 1 to 11 and / or the rear subframe of claim 12.

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