Vehicle understructure

Abstract

The objective is to provide a vehicle understructure that can improve the vehicle's resistance to collision loads in both side and frontal collisions, especially in vehicles equipped with batteries. [Solution] The vehicle's understructure comprises a plurality of cross members 4 that extend in the vehicle width direction between a pair of side sills 1 and are spaced apart from each other in the vehicle's longitudinal direction, and a battery 5 disposed below the plurality of cross members 4. The battery 5 has at least one intermediate frame 13 that extends in the longitudinal direction within the area occupied by the battery housing 11 in a plan view. At least one of the plurality of cross members 4 is connected to the intermediate frame 13.

Description

Technical Field

[0001] The present invention relates to a lower structure of a vehicle in which a battery is mounted at the lower part of the vehicle.

Background Art

[0002] In vehicles such as electric vehicles (EVs), a structure in which a battery is disposed below a floor panel is widely known. The battery usually has a structure in which a plurality of battery modules are housed inside a battery housing, and constitutes one unit as a so-called battery pack. Each battery module is composed of a plurality of battery cells.

[0003] In order to ensure the strength of the entire battery pack, the battery pack described in Patent Document 1 is provided with a plurality of cross members extending in the vehicle width direction inside a battery housing in which a plurality of battery modules are housed. Each cross member is bolted to a metal cover frame that is a lid member of the battery pack covering above the plurality of cross members.

[0004] Such a battery pack is usually fixed to side sills that constitute both ends in the vehicle width direction of the vehicle body at both ends in the vehicle width direction of the battery back, but the central portion in the vehicle width direction is not connected to the vehicle body and has low strength. Therefore, in the above structure, the strength of the battery pack is ensured by a plurality of cross members inside the battery pack and a metal cover frame connected thereto.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, electric vehicles equipped with the aforementioned battery packs have a problem in that they have low resistance to collision loads in both frontal collisions and side collisions.

[0007] In other words, in electric vehicles equipped with battery packs, the floor tunnel, a semi-cylindrical member extending in the longitudinal direction of the vehicle through the center of the floor, is usually unnecessary from the perspective of securing interior space and maximizing battery capacity. As a result, the length of the cross members of the vehicle body extending in the width direction is increased. Moreover, the cross members inside the battery pack and the cross members of the vehicle body are arranged independently without being connected to each other. Therefore, the load during a side collision of the vehicle is mainly borne by the cross members of the vehicle body, which have increased in length as described above. This makes the cross members more susceptible to bending deformation and buckling, resulting in a problem of reduced vehicle body strength against collision loads during a side collision.

[0008] Furthermore, since both the cross member inside the battery pack and the cross member of the vehicle body extend in the width direction of the vehicle, the load-bearing capacity in the event of a frontal collision is also reduced.

[0009] The present invention has been made in view of the above circumstances, and aims to provide a vehicle understructure that can improve the vehicle's resistance to collision load in both side collisions and front collisions in vehicles equipped with batteries. [Means for solving the problem]

[0010] To solve the aforementioned problems, the vehicle substructure of the present invention is a vehicle substructure comprising: a pair of side sills extending in the longitudinal direction of the vehicle at both ends in the vehicle width direction of the vehicle; a plurality of cross members extending in the vehicle width direction between the pair of side sills and spaced apart from each other in the longitudinal direction of the vehicle; a battery cell; and a battery housing that houses the battery cell, wherein the battery is disposed below the plurality of cross members, and the battery has at least one intermediate frame extending in the longitudinal direction within the area occupied by the battery housing in a plan view, and at least one of the plurality of cross members is connected to the intermediate frame.

[0011] In this configuration, at least one of the multiple cross members extending in the vehicle width direction is connected to the intermediate frame of the battery extending in the longitudinal direction. This ensures that the entire connected cross member and intermediate frame can reliably withstand the collision load applied to the cross member during a side collision and the collision load applied to the intermediate frame during a front collision. As a result, the load-bearing capacity of a vehicle equipped with a battery pack can be improved in both side collisions and front collisions.

[0012] In the vehicle's understructure described above, it is preferable that the intermediate frame is provided within the battery housing.

[0013] With this configuration, since the intermediate frame is provided inside the battery housing, the load-bearing capacity of the battery housing can be improved during frontal and side collisions of the vehicle. Furthermore, both the improved load-bearing capacity of the battery housing and the intermediate frame inside the housing can reliably protect the battery cells inside the battery housing.

[0014] In the lower structure of the above-mentioned vehicle, it is preferable that the cross member and the intermediate frame are connected by fastening bolts.

[0015] With this configuration, the cross member and the intermediate frame can be easily and securely fastened together with bolts.

[0016] In the vehicle's understructure described above, it is preferable that the cross member is connected to the intermediate frame at least in the central part in the vehicle width direction.

[0017] In this configuration, the cross member is connected to the intermediate frame at the center in the vehicle width direction where deformation is greatest during a vehicle side impact. This ensures that deformation during a vehicle side impact is reliably suppressed, thereby improving the load-bearing capacity during a vehicle side impact.

[0018] In the vehicle's understructure described above, it is preferable that each of the multiple cross members is connected to each of the multiple intermediate frames that extend in the longitudinal direction.

[0019] In this configuration, each of the multiple cross members is fastened to each of the multiple intermediate frames extending in the front-rear direction, so that the connected multiple cross members and multiple intermediate frames form a structure with a rectangular closed space (a so-called grid structure), thereby improving the load-bearing capacity during frontal and side collisions.

[0020] In the vehicle substructure described above, the battery housing has a lid, and the cross member and the intermediate frame are fastened together by bolts that pass through the cross member and the lid, and it is preferable that the through holes in the lid through which the bolts pass are provided with watertight portions that prevent the flow of liquid in the through holes.

[0021] In this configuration, the intermediate frame is provided inside the battery housing, and the bolts fastening the cross member and the intermediate frame pass through the lid of the battery housing. However, the through-holes in the lid are provided with watertight seals to prevent liquid flow, thus preventing water or other liquids from entering the battery housing. As a result, the battery cells inside the battery housing can be reliably protected while being isolated from water and other liquids.

[0022] In the lower structure of the vehicle described above, it is preferable that the water stop portion has an annular water stop lid provided at the periphery of the through hole of the lid portion, and a cylindrical inner collar sandwiched between the water stop lid and the intermediate frame inside the lid portion.

[0023] In the water stop portion having the above-described configuration, due to the fastening force of the bolt, the annular water stop lid provided at the periphery of the through hole is firmly sandwiched between the cross member and the inner collar from the inside and outside of the lid portion. Thereby, it becomes possible to improve the water stoppage property in the through hole of the lid portion.

[0024] Further, since the cylindrical outer collar and the inner collar isolate the bolt from the outside of these collars, it is possible to reliably prevent liquid such as water from entering the inside of the battery housing along the circumferential surface of the bolt.

[0025] In the lower structure of the vehicle described above, it is preferable that a female screw portion is formed on the inner circumferential surface of the inner collar, and the bolt is fitted to the female screw portion of the inner collar.

[0026] According to such a configuration, since the bolt is fitted to the female screw portion of the inner collar inside the lid portion of the battery housing, it becomes possible to close the gap between the bolt and the inner collar, and the water stoppage property in the gap is improved.

[0027] In the lower structure of the vehicle described above, it is preferable that the water stop portion further has a cylindrical outer collar sandwiched between the water stop lid and the cross member outside the lid portion.

[0028] In the water stop portion having the above-described configuration, due to the fastening force of the bolt, the annular water stop lid provided at the periphery of the through hole is further firmly sandwiched between the cylindrical outer collar and the inner collar provided inside and outside the lid portion. Thereby, it becomes possible to further improve the water stoppage property in the through hole of the lid portion.

Advantages of the Invention

[0029] As described above, the vehicle understructure of the present invention makes it possible to improve the vehicle's resistance to collision loads in both side collisions and front collisions in vehicles equipped with batteries. [Brief explanation of the drawing]

[0030] [Figure 1] This is a plan view showing the overall configuration of the understructure of a vehicle according to an embodiment of the present invention. [Figure 2] This is a cross-sectional view taken along line II-II in Figure 1. [Figure 3] Figure 2 is a cross-sectional diagram illustrating the structure in which the cross member and the intermediate frame inside the battery housing are fastened together by bolts. [Figure 4] This is an explanatory diagram schematically showing the arrangement of the bolts in Figure 1. [Figure 5] This is a schematic diagram illustrating another example of bolt arrangement. [Figure 6] This is a schematic diagram illustrating yet another example of bolt placement. [Figure 7] Figure 2 is a cross-sectional diagram illustrating another example of a structure in which the cross member and the intermediate frame inside the battery housing are fastened together by bolts. [Figure 8] This is a cross-sectional diagram illustrating yet another example of a structure in which the cross member and the intermediate frame inside the battery housing are fastened together by bolts as shown in Figure 2. [Modes for carrying out the invention]

[0031] The lower structure of a vehicle according to an embodiment of the present invention will be described in detail below with reference to the drawings.

[0032] As shown in Figures 1 and 2, the vehicle body 1 to which the vehicle substructure of this embodiment is applied comprises a pair of side sills 2 extending in the longitudinal direction X of the vehicle at both ends of the vehicle width direction Y, a plurality of (four in Figure 1) cross members 4 extending in the vehicle width direction Y and spaced apart from each other in the longitudinal direction X between the pair of side sills 2, a floor panel 3 extending below the plurality of cross members 4 into the area between the pair of side sills 2 and constituting the bottom of the vehicle body 1, and a battery 5 disposed below the plurality of cross members 4 and the floor panel 3.

[0033] The battery 5 comprises a plurality of battery modules 15, a battery housing 11 that houses the plurality of battery modules 15, and at least one (three in this embodiment) intermediate frame 13, forming a single unit (battery pack). Each battery module 15 is composed of a plurality of battery cells 12. In this embodiment, the battery modules 15 are arranged in a matrix below the floor panel 3, with four modules in the vehicle width direction Y and multiple modules in the front-rear direction X.

[0034] The battery housing 11 has a rectangular frame body as shown in Figures 1 and 4, and a lid portion 16 and a bottom plate portion 17 that close the opening in the vertical direction Z as shown in Figure 2.

[0035] As shown in Figures 1-2 and 4, the three intermediate frames 13 extend in the front-rear direction X within the area occupied by the battery housing 11 in a plan view, and are spaced apart from each other in the vehicle width direction Y. Specifically, in this embodiment, as shown in Figure 2, the intermediate frames 13 are provided inside 11a of the battery housing 11. More specifically, the intermediate frames 13 are located below the lid portion 16 and on the upper surface of the bottom plate portion 17.

[0036] Each intermediate frame 13 is positioned between the battery modules 15 at the vehicle width direction Y shown in Figure 2. The ends of the intermediate frame 13 in the longitudinal direction X are connected to the ends of the battery housing 11 (specifically the main body consisting of a frame) in the longitudinal direction X (see Figure 4).

[0037] As shown in Figures 1 and 4, at least one of the multiple (four) cross members 4, in this embodiment, the two inner cross members 4 in the front-rear direction X of the four cross members 4, is connected to the two intermediate frames 13 at both ends of the three intermediate frames 13 in the vehicle width direction Y, specifically fastened with bolts 14 as fastening members. That is, each of the two cross members 4 and each of the two intermediate frames 13 are fastened with bolts 14 to form a rectangular closed space, i.e., a grid structure. Note that bolts 14 are a broad concept of fastening members and also include parts called screws.

[0038] As shown in Figures 2-3, the bolt 14 penetrates the cross member 4, the floor panel 3, and the cover portion 16 to reach the interior 11a of the battery housing 11, and is fastened to the cross member 4 via the inner collar 21, which will be described later.

[0039] In addition, the intermediate frame 13 located in the middle of the vehicle width direction Y as shown in Figures 1 and 2 is fastened to the cover portion 16 by bolts or the like, but it does not have to be connected to the cover portion 16.

[0040] The connection between the cross member 4 and the intermediate frame 13 may be made not only by fastening with bolts 14 as described above, but also by other means, such as fastening with blind rivets.

[0041] In this embodiment, as shown in Figure 3, the through-hole 16a through which the bolt 14 passes in the lid portion 16 is provided with a water-stopping portion 20 that prevents the flow of liquid through the through-hole 16a.

[0042] Specifically, the water-stopping section 20 includes a ring-shaped water-stopping cover 23 provided on the periphery of the through-hole 16a of the cover section 16, a cylindrical outer collar 22 sandwiched between the water-stopping cover 23 and the cross member 4 on the outside of the cover section 16, and a cylindrical inner collar 21 sandwiched between the water-stopping cover 23 and the intermediate frame 13 on the inside of the cover section 16.

[0043] The water-stopping cover 23 has a cylindrical projection 23a and a disc-shaped flange portion 23b. The projection 23a protrudes downward from the lower surface of the flange portion 23b and fits tightly onto the inner periphery of the through hole 16a of the cover portion 16. The flange portion 23b is sandwiched between the cover portion 16 and the floor panel 3.

[0044] The cylindrical outer collar 22 is a cylindrical body smaller than the outer circumference of the head 14a of the bolt 14, and is pressed downward by the bolt 14 via the cross member 4 and the collar plate 24.

[0045] The inner collar 21 is a bottomed cylindrical member. The inner circumferential surface of the inner collar 21 has a female threaded portion 21a into which the male threaded portion 14b of the bolt 14 is screwed in and fitted. When the bolt 14 is fastened (fitted) into the female threaded portion 21a of the inner collar 21, the gap between the bolt 14 and the inner collar 21 is closed.

[0046] Furthermore, the inner collar 21 has a male threaded portion 21b at its lower end that is screwed into and fitted into a threaded hole 13a formed on the upper surface of the intermediate frame 13. By fastening (fitting) the male threaded portion 21b of the inner collar 21 into the threaded hole 13a of the intermediate frame 13, the bolt 14 is fastened to the intermediate frame 13 via the inner collar 21. Alternatively, the bolt 14 may be fastened directly to the intermediate frame 13 without going through the inner collar 21.

[0047] Furthermore, the upper end surface of the inner collar 21 is in close contact with the lower surface of the lid 16. In addition, a circular recess 23c is formed on the upper end surface of the inner collar 21, which fits with the cylindrical protrusion 23a of the water-stopping lid 23. The protrusion 23a of the water-stopping lid 23 fits into the recess 23c of the inner collar 21. These structures improve the water-stopping effect of the inner collar 21.

[0048] (Features of this embodiment) (1) As shown in Figures 1 to 4, the understructure of the vehicle in this embodiment comprises a plurality of cross members 4 that extend in the vehicle width direction Y between a pair of side sills 2 and are spaced apart from each other in the longitudinal direction X, and a battery 5 disposed below the plurality of cross members 4. The battery 5 has at least one (three in the above example) intermediate frame 13 that extends in the longitudinal direction X within the area occupied by the battery housing 11 in a plan view. At least one (two in the above example) of the plurality of cross members 4 is connected to the intermediate frame 13.

[0049] In this configuration, at least one of the multiple cross members 4 extending in the vehicle width direction Y is connected to the intermediate frame 13 of the battery extending in the longitudinal direction X. As a result, the entire connected cross member 4 and intermediate frame 13 can reliably withstand the collision load LY (see Figure 1) applied to the cross member 4 during a side collision and the collision load LX (see Figure 1) applied to the intermediate frame 13 during a front collision. Consequently, the load-bearing capacity of a vehicle equipped with a battery can be improved in both side collisions and front collisions.

[0050] (2) In the vehicle's understructure of this embodiment, the intermediate frame 13 is provided inside the battery housing 11a (i.e., below the lid 16). Therefore, the load-bearing capacity of the battery housing 11 can be improved during frontal and side collisions. Furthermore, the improved load-bearing capacity of the battery housing 11 and the intermediate frame 13 inside the housing ensure reliable protection of the battery cells 12 inside the battery housing 11.

[0051] The intermediate frame 13 may be located outside the battery housing 11, that is, above the lid 16, and fixed to the battery housing 11 and / or the lid 16.

[0052] (3) In the vehicle's understructure of this embodiment, the cross member 4 and the intermediate frame 13 are connected by fastening bolts 14. Therefore, the cross member 4 and the intermediate frame 13 can be easily and securely fastened together by bolts 14.

[0053] (4) In the vehicle substructure of this embodiment, as shown in Figure 3, the battery housing 11 has a lid portion 16. The cross member 4 and the intermediate frame 13 are fastened together by bolts 14 that pass through the cross member 4 and the lid portion 16. A watertight portion 20 is provided in the through hole 16a through which the bolt 14 passes, to prevent the flow of liquid in the through hole 16a.

[0054] In this configuration, the intermediate frame 13 is provided inside the battery housing 11, and the bolts 14 that fasten the cross member 4 and the intermediate frame 13 pass through the lid 16 of the battery housing 11. However, a watertight portion 20 is provided in the through-hole 16a of the lid 16 to prevent the flow of liquid, so it is possible to prevent water or other liquids from entering the inside of the battery housing 11. As a result, the battery cells 12 inside the battery housing 11 can be reliably protected while being isolated from water and other liquids.

[0055] (5) In the vehicle substructure of this embodiment, as shown in Figure 3, the water-sealing section 20 includes a ring-shaped water-sealing cover 23 provided on the periphery of the through-hole 16a of the cover section 16, a cylindrical outer collar 22 sandwiched between the water-sealing cover 23 and the cross member 4 on the outside of the cover section 16, and a cylindrical inner collar 21 sandwiched between the water-sealing cover 23 and the intermediate frame 13 on the inside of the cover section 16.

[0056] In the watertight section 20 configured as described above, the ring-shaped watertight cover 23 provided on the periphery of the through hole 16a is firmly sandwiched between the cylindrical outer collar 22 and inner collar 21 provided on the inside and outside of the cover section 16 by the fastening force of the bolt 14. This makes it possible to improve the watertightness of the through hole 16a of the cover section 16.

[0057] Furthermore, the cylindrical outer collar 22 and inner collar 21 isolate the bolt 14 from the outside of these collars 22 and 21, thus reliably preventing liquids such as water from entering the inside 11a of the battery housing 11 along the circumferential surface of the bolt 14.

[0058] The water-stopping section 20 is not limited to the above configuration, as long as it prevents the flow of liquid through the through-hole 16a of the lid 16. For example, the water-stopping section 20 may consist only of a water-stopping lid 23.

[0059] (6) In the vehicle substructure of this embodiment, a female threaded portion 21a is formed on the inner circumferential surface of the inner collar 21. The bolt 14 is fitted into the female threaded portion 21a of the inner collar 21.

[0060] In the above configuration, the bolt 14 is fastened to the female threaded portion 21a of the inner collar 21 on the inside of the lid portion 16 of the battery housing 11. This makes it possible to seal the gap between the bolt 14 and the inner collar 21, improving the watertightness in that gap.

[0061] (7) In the vehicle substructure of this embodiment, as shown in Figures 1 and 4, each of the multiple (2) cross members 4 is connected to each of the multiple (2) intermediate frames 13 that extend in the longitudinal direction X.

[0062] In this way, each of the multiple cross members 4 is fastened to each of the multiple intermediate frames 13 that extend in the longitudinal direction X, so that the connected multiple cross members 4 and multiple intermediate frames 13 form a structure with a rectangular closed space (a so-called grid structure), thereby improving the load-bearing capacity during frontal and side collisions of the vehicle.

[0063] (modified version) (A) In the examples shown in Figures 1 and 4, each of the two cross members 4 has a structure with four fastening points, each connected to one of the two intermediate frames 13. However, as a modification of the present invention, the number of fastening points may be increased, as shown in Figure 5. That is, each of the four cross members 4 may have a structure with twelve fastening points, each connected to one of the three intermediate frames 13. In this case, the load-bearing capacity during frontal and side collisions will be further improved. The number of fastening points should be set appropriately considering the arrangement of the components of the vehicle body 1, etc.

[0064] (B) Furthermore, in the vehicle substructure of the present invention, it is sufficient that at least one of the multiple cross members 4 is connected to the intermediate frame 13, and it is not limited to a structure in which each of the two cross members 4 is connected to each of the two intermediate frames 13, as shown in Figures 1 to 4 above.

[0065] As a modified example of the present invention, for example, as shown in Figure 6, the cross member 4 may be connected to the intermediate frame 13 at least in the central part in the vehicle width direction Y.

[0066] In this modified configuration, the cross member 4 is connected to the intermediate frame 13 at the center of the vehicle width direction Y, where the deformation is greatest during a vehicle side impact. This makes it possible to reliably suppress deformation during a vehicle side impact, and thus improves the load-bearing capacity during a vehicle side impact.

[0067] (C) Furthermore, as a modified example of the present invention, as shown in Figures 7-8, the water-stopping section 20 may be configured to have a water-stopping cover 23 and an inner collar 21, omitting the outer collar 22 shown in Figure 3.

[0068] The modified version shown in Figure 7 is applied, for example, to the cross member 4 located at the foremost X1 of the four cross members 4 in Figure 1, where the central portion 4a in the front-rear direction X of the hat-shaped cross member 4 is fastened to the intermediate frame 13 by bolts 14 (more specifically, fastened by bolts 14 via an inner collar 21).

[0069] Furthermore, the modified form shown in Figure 8 is applied, for example, to the rearmost cross member 4 among the four cross members 4 in Figure 1, where the flange portions 4b on both sides of the hat-shaped cross member 4 in the front-rear direction X are fastened to the intermediate frame 13 by bolts 14 (more specifically, fastened by bolts 14 via the inner collar 21).

[0070] In these modified configurations shown in Figures 7 and 8, the fastening force of the bolts 14 firmly clamps the ring-shaped watertight cover 23, which is provided on the periphery of the through-hole 16a, between the cross member 4 and the inner collar 21 from both the inside and outside of the cover portion 16. This makes it possible to improve the watertightness of the through-hole 16a of the cover portion 16.

[0071] However, if the configuration further includes an outer collar 22 as shown in Figure 3, the outer collar 22 can locally press the area around the through hole 16a in the watertight cover 23 from the outside of the cover portion 16. This is preferable because it further strengthens the watertight cover 23 between the cylindrical outer collar 22 and inner collar 21 provided on the inside and outside of the cover portion 16, thereby further improving the watertightness at the through hole 16a of the cover portion 16. [Explanation of Symbols]

[0072] 1. Vehicle body 2 Side sills 3 Floor Panels 4 Cross Members 5 batteries 11 Battery housing 12 battery cells 13 Intermediate Frames 14 volts 15 Battery Modules 16 Lid 16a Through hole 17 Bottom 20 Water-stopping section 21 Inner Color 21a Female thread section 22 Outer color 23 Water stop cover

Claims

1. The understructure of the vehicle, A pair of side sills extending in the longitudinal direction of the vehicle at both ends in the width direction of the vehicle, A plurality of cross members are arranged between the pair of side sills, extending in the vehicle width direction and spaced apart from each other in the vehicle longitudinal direction, A battery comprising a battery cell and a battery housing that houses the battery cell, and disposed below the plurality of cross members, Equipped with, The battery has at least one intermediate frame that extends in the front-to-back direction within the area occupied by the battery housing in a plan view, At least one of the plurality of cross members is connected to the intermediate frame. A vehicle understructure characterized by the following:

2. In the vehicle substructure according to claim 1, The intermediate frame is provided inside the battery housing. A vehicle understructure characterized by the following:

3. In the vehicle substructure according to claim 1 or 2, The cross member and the intermediate frame are connected by bolt fastening. A vehicle understructure characterized by the following:

4. In the vehicle substructure according to claim 1 or 2, The cross member is connected to the intermediate frame at least in the central part in the vehicle width direction. A vehicle understructure characterized by the following:

5. In the vehicle substructure according to claim 1 or 2, Each of the multiple cross members is connected to each of the multiple intermediate frames that extend in the front-to-back direction. A vehicle understructure characterized by the following:

6. In the vehicle substructure described in claim 2, The aforementioned battery housing has a lid, With the bolt passing through the cross member and the cover, the cross member and the intermediate frame are fastened together by the bolt. The through-hole in the lid through which the bolt passes is provided with a watertight portion that prevents the flow of liquid through the through-hole. A vehicle understructure characterized by the following:

7. In the vehicle substructure described in claim 6, The water-stopping section comprises a ring-shaped water-stopping cover provided on the periphery of the through hole in the lid, and a cylindrical inner collar sandwiched between the water-stopping cover and the intermediate frame inside the lid. A vehicle understructure characterized by the following:

8. In the vehicle substructure according to claim 7, The inner circumferential surface of the inner collar has a female thread formed thereon. The bolt is fitted into the female thread portion of the inner collar. A vehicle understructure characterized by the following:

9. In the vehicle substructure according to claim 7, The water-stopping portion further includes a cylindrical outer collar sandwiched between the water-stopping cover and the cross member on the outside of the cover portion. A vehicle understructure characterized by the following:

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