Vehicle lower structure

Abstract

A vehicle lower structure according to the present disclosure comprises a side sill (1), a center pillar (12), and a reinforcement (3). The side sill (1) extends in the front-rear direction at the lower part of the side surface of the vehicle, and is formed in a closed cross section by joining an inner member (6) and an outer member (7). The center pillar (12) is connected to an intermediate part of the side sill (1) in the front-rear direction. The reinforcement (3) is attached inside the side sill (1) along the outer side surface of the side sill (1) and extends in the front-rear direction. The reinforcement (3) has a first weak part (8) formed so as to have a lower torsional rigidity than other parts between the front-rear-direction end part of the side sill (1) and the lower end part of the center pillar (12).

Description

Vehicle undercarriage

[0001] This case concerns the vehicle undercarriage around the side sill.

[0002] A side sill is provided at the lower part of the side of a vehicle. The side sill is formed into a cylindrical shape with a closed cross section by joining an inner member and an outer member, and is provided so as to extend in the front-to-rear direction in the foot area of ​​the side door. A reinforcing member or a reinforcing structure may be provided inside the side sill. For example, a side sill having a structure in which a reinforcing panel is sandwiched between an inner panel and an outer panel with a generally U-shaped cross section is known. Also known is a side sill having a partition member (bulkhead) attached inside the closed cross section (see Patent Documents 1 and 2).

[0003] JP 2008-207650 A JP 2023-149585 A

[0004] Deformation of the side sill can be suppressed by increasing the moment of inertia. However, increasing the rigidity of the side sill can actually reduce the vehicle's occupant protection performance. For example, external forces input to the center pillar during a side collision can cause the entire side sill to rotate in the roll direction. The more rigid the side sill, the more likely it is to rotate while maintaining its cross-sectional shape. This can cause the joint between the inner and outer members of the side sill to separate, significantly changing its cross-sectional shape and increasing the amount of penetration of the center pillar into the interior of the vehicle.

[0005] One of the objects of the present invention was invented in light of the above-mentioned problems, and is to provide a vehicle underbody structure that can improve the occupant protection performance of a vehicle. However, in addition to this object, another object of the present invention is to achieve effects derived from the respective configurations shown in the "Mode for Carrying Out the Invention" described below, which are effects that cannot be obtained with conventional technologies.

[0006] The disclosed vehicle undercarriage structure can be realized as the following disclosed embodiments (application examples), which solve at least part of the above-mentioned problems. Each of the embodiments from embodiment 2 onwards is an embodiment that can be selected as an additional element, and each of the embodiments from embodiment 2 onwards is an embodiment that can be omitted. None of the embodiments from embodiment 2 onwards discloses an embodiment or configuration that is essential to the present invention.

[0007] Aspect 1. The disclosed vehicle undercarriage structure includes a side sill extending in the longitudinal direction at a lower portion of a side of a vehicle and formed into a closed cross section by joining an inner member and an outer member, a center pillar connected to a longitudinally intermediate portion of the side sill, and a reinforcement attached inside the side sill along the outer surface of the side sill and extending in the longitudinal direction. The reinforcement also has a first weakened portion formed to have lower torsional rigidity than other portions between the longitudinal end of the side sill and the lower end of the center pillar.

[0008] Aspect 2. With regard to aspects including the above-described Aspect 1, it is preferable that the first weak portion is formed to have a vertical dimension shorter than the other portions of the reinforcement. Aspect 3. With regard to aspects including the above-described Aspect 1, it is preferable that the reinforcement has a first reinforcement that extends in the front-rear direction through the lower end of the center pillar, and a second reinforcement that is joined to the front-rear direction end of the first reinforcement from the outer side in the vehicle width direction and has the first weak portion provided therein.

[0009] Aspect 4. With regard to aspects including the above-mentioned Aspect 3, it is preferable that the second reinforcement has a front cutout portion formed in a shape in which the end portion on the first reinforcement side is cut out at the joint portion with the first reinforcement. Aspect 5. With regard to aspects including the above-mentioned Aspect 3, it is preferable that the second reinforcement has a second weak portion formed to have lower horizontal bending rigidity than other portions on the end side in the front-to-rear direction than the first weak portion.

[0010] Aspect 6. With regard to aspects including the above-mentioned Aspect 5, it is preferable that the second weakened portion is formed to have a vehicle width direction dimension shorter than that of the first reinforcement. Aspect 7. With regard to aspects including the above-mentioned Aspect 3, it is preferable that the second reinforcement has an upper surface portion joined to the upper surface of the side sill, a lower surface portion joined to the lower surface of the side sill, and rear cutout portions formed in a shape in which end portions of the upper surface portion and the lower surface portion on the first reinforcement side are cut out.

[0011] Aspect 8. With regard to aspects including Aspect 1 above, it is preferable that the vehicle further includes a bulkhead attached inside the side sill near the first weak portion, the bulkhead having a wall portion whose normal direction is in the front-to-rear direction. Aspect 9. With regard to aspects including Aspect 8 above, it is preferable that the wall portion includes a front wall portion and a rear wall portion that are arranged substantially parallel to each other with a gap between them in the front and rear directions, the bulkhead has a planar connecting surface portion that connects inner end edges of the front wall portion and the rear wall portion in the vehicle width direction, and the first weak portion is arranged between the front wall portion and the rear wall portion in a side view.

[0012] Aspect 10. In relation to the aspects including Aspect 8, it is preferable that the vehicle undercarriage includes a cross member connecting the left and right side sills in the vehicle width direction. It is also preferable that the bulkhead is provided in a position overlapping with the cross member in a side view.

[0013] According to the disclosed vehicle undercarriage structure, a first weak portion with low torsional rigidity is provided in the reinforcement between the longitudinal end of the side sill and the lower end of the center pillar, thereby promoting torsional deformation at the first weak portion against external forces from the side of the vehicle. This suppresses torsional deformation of the side sill rearward of the first weak portion, thereby suppressing fracture of the welded joint between the inner member and outer member of the side sill and preventing a decrease in the cross-sectional performance of the side sill. Therefore, vehicle occupant protection performance can be improved.

[0014] 1 is a perspective view of a vehicle body to which the vehicle undercarriage structure according to the present invention is applied; FIG. 2 is a side view of a vehicle body with the interior of a side sill seen through; FIG. 3 is a perspective view showing a portion of the structure of the side sill; FIG. 4 is an exploded perspective view showing a portion of the structure of the side sill; FIG. 5 is a perspective view showing a portion of the side sill inner and side sill outer; FIG. 6 is a perspective view showing a rear end portion of a first reinforcement; FIG. 7 is a perspective view showing a front end portion of a second reinforcement; FIG. 8 is a perspective view of a bulkhead; FIG. 9 is a cross-sectional view showing the structure of a joint between a first reinforcement and a second reinforcement, where (A) is a vertical cross-sectional view of the side sill and (B) is a horizontal cross-sectional view of the side sill; FIG. 10 is a vertical cross-sectional view of the vehicle undercarriage structure for illustrating the deformation state of the side sill during a side collision of the vehicle; and FIG. 11 is a horizontal cross-sectional view of the side sill for illustrating the deformation state of the side sill during a side collision of the vehicle.

[0015] The disclosed vehicle undercarriage structure is a structure for the lower side of a vehicle, and is applied to, for example, the lower part of a monocoque body or the lower part of a body in a frame vehicle. Regarding the definitions of directions in the embodiments, the longitudinal direction (vehicle length direction) is defined based on the forward / reverse direction of the vehicle, and the left / right direction (vehicle width direction) is defined based on the longitudinal direction. The up / down direction is defined based on the state in which the vehicle is stopped on a flat road surface. Regarding the inward / outward directions of the vehicle, the direction from the outside of the vehicle toward the passenger compartment in the vehicle width direction is defined as the inward direction, and the direction from the passenger compartment toward the outside of the vehicle is defined as the outward direction. To the right of the vehicle's median plane (the vertical plane dividing the vehicle into left and right halves), the right direction is the outward direction, and the left direction is the inward direction. To the left of the vehicle's median plane, the left direction is the outward direction, and the right direction is the inward direction.

[0016] [1. Configuration] Figure 1 is a perspective view of a vehicle body 10 to which a vehicle undercarriage structure according to an embodiment is applied. Side sills 1 are provided at the lower portions of both sides of the body 10 in the vehicle width direction. The side sills 1 extend in the front-to-rear direction at the lower portions of the sides of the vehicle and serve as the sills of the side doors. A pair of side sills 1 are provided along the lower edges of the left and right side doors, and are arranged substantially horizontally to connect the front wheel housing and the rear wheel housing.

[0017] The lower ends of the front pillars 11 (A-pillar), center pillars 12 (B-pillar), and rear pillars 13 (C-pillar), which are columnar structures, are connected to the side sill 1. The upper ends of the front pillars 11, center pillars 12, and rear pillars 13 are connected to roof side rails that form the left and right sides of the ceiling surface of the vehicle compartment. The front pillars 11 are pillars located in front of the front side doors. The lower ends of the front pillars 11 are connected to the front ends of the side sills 1. The hinges of the front side doors are fixed to the front pillars 11. The lower ends of the front pillars 11 are connected to the front ends of the side sills 1.

[0018] The center pillar 12 is a pillar located between the front side door and the rear side door. The lower end of the center pillar 12 is connected to the middle part of the side sill 1 in the longitudinal direction. The striker of the front side door and the hinge of the rear side door are fixed to the center pillar 12. The lower end of the center pillar 12 is connected to the middle part of the side sill 1 in the longitudinal direction. The rear pillar 13 is a pillar located behind the rear side door. The lower end of the rear pillar 13 is connected to the rear end part of the side sill 1. The striker of the rear side door is fixed to the rear pillar 13. The lower end of the rear pillar 13 is connected to the rear end part of the side sill 1.

[0019] Additionally, a front floor panel 14, a rear floor panel 15, and a rear seat cross member 16 are provided under the body 10. The front floor panel 14 is the floor of the passenger compartment where occupants sit, and is provided, for example, as a substantially horizontal surface between the left and right side sills 1. The rear floor panel 15 is the floor of the luggage compartment provided at the rear of the passenger compartment. The rear floor panel 15 is provided at a higher position than the front floor panel 14, is substantially horizontal, and forms a floor surface that is continuous with the front floor panel 14 via a step.

[0020] The rear seat cross member 16 is a portion that connects the left and right side sills 1 in the vehicle width direction, and is provided at the step between the front floor panel 14 and the rear floor panel 15. The rear seat cross member 16 is connected to a portion of the side sill 1 that is sandwiched between the lower end of the center pillar 12 and the lower end of the rear pillar 13. In this embodiment, the base of the rear seat (back seat) is fixed to the rear seat cross member 16.

[0021] 2 is a side view of the body 10 showing a perspective view of the interior of the side sill 1. A bulkhead 2 and a reinforcement 3 are provided inside the side sill 1. The bulkhead 2 is a partition member provided to divide the space inside the side sill 1 in its extension direction. The bulkhead 2 has a wall portion whose normal direction is the front-to-rear direction. In this embodiment, two bulkheads 2 are provided inside each of the left and right side sills 1, and each bulkhead 2 has two wall portions.

[0022] The position of the front bulkhead 2 is set between the lower end of the front pillar 11 (the front end of the side sill 1) and the lower end of the center pillar 12. The position of the rear bulkhead 2 is set between the lower end of the center pillar 12 and the lower end of the rear pillar 13 (the rear end of the side sill 1). It is preferable that the position of the rear bulkhead 2 is set so as to overlap with the rear seat cross member 16 in a side view.

[0023] The reinforcement 3 is a reinforcing member attached inside the side sill 1 along the outer surface of the side sill 1 and extending in the front-rear direction. The reinforcement 3 has a length spanning almost the entire length of the side sill 1 from the front end to the rear end. In this embodiment, the reinforcement 3 is composed of two members: a first reinforcement 4 located on the front side and a second reinforcement 5 located on the rear side. The first reinforcement 4 extends in the front-rear direction through the lower end of the center pillar 12. The second reinforcement 5 is joined to the front-rear end (the rear end in this embodiment) of the first reinforcement 4 from the outside in the vehicle width direction and extends rearward. The first reinforcement 4 is provided with a first weak portion 8 (first weak portion, torsional deformation portion) and a second weak portion 9 (second weak portion, bending deformation portion), which will be described later.

[0024] Fig. 3 is a perspective view showing a portion of the left side sill 1, and Fig. 4 is an exploded perspective view showing a portion of the side sill 1 and its internal structure shown in Fig. 3. The side sill 1 is formed into a cylindrical shape with a closed cross section by joining a side sill inner member 6 (inner member) and a side sill outer member 7 (outer member). The side sill inner member 6 constitutes the vehicle inner portion of the side sill 1, and the side sill outer member 7 constitutes the vehicle outer portion.

[0025] The reinforcement 3 is disposed on the outside of the vehicle along the side sill outer 7 inside the side sill 1. Meanwhile, the bulkhead 2 is disposed on the inside of the vehicle so as to abut against the side sill inner 6 and the reinforcement 3 inside the side sill 1. A gap is provided between the bulkhead 2 and the side sill outer 7, and the reinforcement 3 is inserted into this gap. In addition, the joint between the first reinforcement 4 and the second reinforcement 5 is set near the bulkhead 2 on the rear side.

[0026] [A. Side Sill] Figure 5 is a perspective view showing a portion of the side sill inner 6 and the side sill outer 7. The side sill inner 6 has a hat-shaped cross section that protrudes toward the inside of the vehicle and extends in the fore-and-aft direction. On the other hand, the side sill outer 7 has a hat-shaped cross section that protrudes toward the outside of the vehicle and extends in the fore-and-aft direction. By butting and joining (e.g., spot welding) these flanges together, a substantially rectangular closed cross section is formed inside the side sill 1.

[0027] The side sill inner 6 has an inner wall portion 61, an inner upper surface portion 62, an inner lower surface portion 63, an inner upper edge portion 64, and an inner lower edge portion 65. The inner wall portion 61 is a planar portion that is provided substantially vertically. The inner wall portion 61 constitutes the inner wall surface of the side sill 1 having a rectangular cross section. The inner upper surface portion 62 is a planar portion that extends substantially horizontally from the upper edge of the inner wall portion 61 toward the outside of the vehicle, and the inner lower surface portion 63 is a planar portion that extends substantially horizontally from the lower edge of the inner wall portion 61 toward the outside of the vehicle.

[0028] The inner upper edge 64 is a planar portion (flange) extending substantially vertically upward from the outer edge of the inner upper surface portion 62, and the inner lower edge 65 is a planar portion (flange) extending substantially vertically downward from the outer edge of the inner lower surface portion 63. The inner upper edge 64 and the inner lower edge 65 are provided, for example, to be located on the same plane and are provided parallel to the inner wall portion 61.

[0029] The side sill outer 7 has an outer wall portion 71 (outer surface), an outer upper surface portion 72, an outer lower surface portion 73, an outer upper edge portion 74, and an outer lower edge portion 75. The outer wall portion 71 is a planar portion that is provided substantially vertically. The outer wall portion 71 constitutes the outer wall surface of the side sill 1 having a rectangular cross section (i.e., the outer surface of the side sill 1). The outer upper surface portion 72 is a planar portion that extends substantially horizontally from the upper edge of the outer wall portion 71 toward the inside of the vehicle, and the outer lower surface portion 73 is a planar portion that extends substantially horizontally from the lower edge of the outer wall portion 71 toward the inside of the vehicle.

[0030] The outer upper edge portion 74 is a planar portion (flange) extending substantially vertically upward from the inner end edge of the outer upper surface portion 72, and the outer lower edge portion 75 is a planar portion (flange) extending substantially vertically downward from the inner end edge of the outer lower surface portion 73. The outer upper edge portion 74 is joined to the inner upper edge portion 64 in surface contact, and the outer lower edge portion 75 is joined to the inner lower edge portion 65 in surface contact.

[0031] [B. Reinforcement] Fig. 6 is a perspective view showing the rear end portion of the first reinforcement 4, and Fig. 7 is a perspective view showing the front end portion of the second reinforcement 5. As shown in Fig. 6, the first reinforcement 4 has a U-shaped vertical cross section that opens toward the inside of the vehicle and is provided to extend in the front-to-rear direction. The first reinforcement 4 has a first wall portion 41, a first upper surface portion 42, and a first lower surface portion 43.

[0032] The first wall portion 41 is a planar portion that is provided substantially vertically, and is joined to the outer wall portion 71 of the side sill outer 7. A first opening 44 is drilled in the first wall portion 41 near the rear end portion of the first reinforcement 4. By providing the first opening 44 near the rear end portion of the first wall portion 41, the second moment of area and the second polar moment of area near the rear end portion of the first wall portion 41 become smaller than those of other portions (portions of the first wall portion 41 other than the rear end portion), and the torsional rigidity and bending rigidity are relatively reduced.

[0033] The first upper surface portion 42 is a planar portion extending substantially horizontally from the upper end edge of the first wall portion 41 toward the inside of the vehicle, and is joined to the outer upper surface portion 72 of the side sill outer 7. The first lower surface portion 43 is a planar portion extending substantially horizontally from the lower end edge of the first wall portion 41 toward the inside of the vehicle, and is joined to the outer lower surface portion 73 of the side sill outer 7.

[0034] As shown in FIG. 7 , the second reinforcement 5 has a front portion 51 joined to the first reinforcement 4 and a rear portion 52 joined to the side sill outer 7 on its rear side. The front portion 51 and the rear portion 52 are integrally formed. The boundary position between the front portion 51 and the rear portion 52 is set to be located rearward of the rear end of the first reinforcement 4 when the second reinforcement 5 is joined to the first reinforcement 4. The front portion 51 is provided with a joint portion 53 and a front cutout portion 54. The joint portion 53 is a planar portion that is provided approximately vertically, and is joined from the outside in the vehicle width direction while being in surface contact with the outer surface of the rear end portion of the first wall portion 41 of the first reinforcement 4.

[0035] The front cutout 54 is a portion formed in a shape where the front end portion (the end portion on the first reinforcement 4 side) of the joint 53 joined to the first reinforcement 4 is notched. In the example shown in FIG. 7 , the front cutout 54 is formed in a shape where the front lower portion of the joint 53 is notched. By providing the front cutout 54 at the joint 53, the second moment of area and the polar second moment of area at the front portion 51 are smaller than those of other portions (portions of the second reinforcement 5 other than the front portion 51), and the contact area and the number of welding spots between the first wall portion 41 of the first reinforcement 4 and the joint 53 are reduced. This reduces the torsional rigidity and bending rigidity at the front portion 51 of the second reinforcement 5 joined to the first reinforcement 4. In addition, the moment in the torsional direction of the reinforcement 3 (moment around an axis extending in the front-to-rear direction) is reduced.

[0036] The rear portion 52 has a U-shaped vertical cross section that opens toward the inside of the vehicle and extends in the front-to-rear direction. The rear portion 52 has substantially the same cross section as the first reinforcement 4. The rear portion 52 has a second wall portion 55, a second upper surface portion 56, a second lower surface portion 57, and a rear cutout portion 59. The second wall portion 55 is a portion that is provided substantially vertically and in the same plane as the joint portion 53, and is joined to the outer wall portion 71 of the side sill outer 7. A second opening 58 is drilled near the front end of the second wall portion 55. By providing the second opening 58 near the front end of the second wall portion 55, the second moment of inertia and the polar moment of inertia near the front end of the second wall portion 55 are smaller than those of other portions (portions of the second wall portion 55 other than the front end), and the torsional rigidity and bending rigidity near the front end of the rear portion 52 are reduced.

[0037] The second upper surface portion 56 is a planar portion extending substantially horizontally from the upper end edge of the second wall portion 55 toward the inside of the vehicle, and is joined to the outer upper surface portion 72 of the side sill outer 7. The second lower surface portion 57 is a planar portion extending substantially horizontally from the lower end edge of the second wall portion 55 toward the inside of the vehicle, and is joined to the outer lower surface portion 73 of the side sill outer 7. The second wall portion 55, the second upper surface portion 56, and the second lower surface portion 57 form a U-shaped vertical cross section that is slightly larger than the first reinforcement 4 and that is open toward the inside of the vehicle.

[0038] The rear cutout portion 59 is a portion formed by cutting out the front end portions of the second upper surface portion 56 and the second lower surface portion 57. In the example shown in Fig. 7, the rear cutout portion 59 is formed by cutting out the inner front ends of the second upper surface portion 56 and the second lower surface portion 57. By providing the rear cutout portion 59, the second moment of area and the polar second moment of area near the front end of the rear portion 52 become smaller than those of other portions (portions other than the vicinity of the front end of the rear portion 52), and the torsional rigidity and bending rigidity near the front end of the rear portion 52 decrease.

[0039] [C. Bulkhead] Figure 8 is a perspective view of the bulkhead 2. The bulkhead 2 is formed in a U-shape that opens toward the outside of the vehicle when viewed from above. The bulkhead 2 is provided with a front wall portion 21, a rear wall portion 22, a connecting surface portion 23, a front upper edge portion 24, a front lower edge portion 25, a rear upper edge portion 26, and a rear lower edge portion 27.

[0040] The front wall 21 and the rear wall 22 are both walls whose normal direction is the fore-and-aft direction. The front wall 21 and the rear wall 22 are arranged substantially parallel to each other and are provided, for example, to partition the interior space of the side sill 1 in the direction of extension thereof. The front wall 21 and the rear wall 22 are preferably formed to a size that covers the entire cross section of the side sill 1 so as to completely partition the interior space of the side sill 1. However, the partition by the front wall 21 and the rear wall 22 may be incomplete; for example, the front wall 21 and the rear wall 22 may be formed to be smaller than the cross-sectional shape of the side sill 1.

[0041] The front wall portion 21 is disposed in front of and spaced apart from the rear wall portion 22. The front wall portion 21 of the rear bulkhead 2 shown in Figure 1 is disposed forward of the front surface of the rear seat cross member 16. In addition, the rear wall portion 22 of the rear bulkhead 2 shown in Figure 1 is disposed in a position that overlaps with the rear seat cross member 16 in a side view (rearward of the front surface of the rear seat cross member 16 and forward of the rear surface of the rear seat cross member 16).

[0042] The front upper edge 24 is a planar portion (flange) extending substantially horizontally forward from the upper edge of the front wall 21, and the front lower edge 25 is a planar portion (flange) extending substantially horizontally forward from the lower edge of the front wall 21. Similarly, the rear upper edge 26 is a planar portion (flange) extending substantially horizontally rearward from the upper edge of the rear wall 22, and the rear lower edge 27 is a planar portion (flange) extending substantially horizontally rearward from the lower edge of the rear wall 22.

[0043] The front upper edge portion 24 and the rear upper edge portion 26 are joined in surface contact with, for example, the inner upper surface portion 62, the outer upper surface portion 72, the first upper surface portion 42, the second upper surface portion 56, etc. Furthermore, the front lower edge portion 25 and the rear lower edge portion 27 are joined in surface contact with, for example, the inner lower surface portion 63, the outer lower surface portion 73, the first lower surface portion 43, the second lower surface portion 57, etc. The connecting surface portion 23 is a planar portion that connects the inner end edges of the front wall portion 21 and the rear wall portion 22 in the vehicle width direction and is provided substantially vertically. The connecting surface portion 23 is joined in surface contact with the inner wall portion 61.

[0044] The front wall portion 21 of the bulkhead 2 is preferably arranged so as to overlap the rear end portion of the first reinforcement 4 in a side view. This makes it easier for the rear end portion of the first reinforcement 4 to be supported by the front wall portion 21 when a side collision load is input to the vehicle. Furthermore, the rear wall portion 22 of the bulkhead 2 is preferably arranged so as to overlap the rear portion 52 of the second reinforcement 5 in a side view. This makes it easier for the rear portion 52 of the second reinforcement 5 to be supported by the rear wall portion 22 when a side collision load is input to the vehicle.

[0045] [D. Weak Portion] The reinforcement 3 is provided with a first weak portion 8 and a second weak portion 9. The first weak portion 8 is a portion formed to have lower torsional rigidity than other portions (portions of the reinforcement 3 other than the first weak portion 8) between the front-rear end (the rear end in this embodiment) of the side sill 1 and the lower end of the center pillar 12. The first weak portion 8 is formed in a cross-sectional shape having lower torsional rigidity than other portions. For example, the first weak portion 8 is formed in a cross-sectional shape having a smaller polar moment of area than other portions. The first weak portion 8 can be considered to be a portion that is allowed a predetermined torsional deformation in response to a side collision load.

[0046] Furthermore, the second weak portion 9 is a portion formed on the front-rear end side (rear side in this embodiment) of the first weak portion 8, and has lower horizontal bending rigidity than other portions (portions of the reinforcement 3 other than the second weak portion 9). The second weak portion 9 is formed in a cross-sectional shape with lower horizontal bending rigidity compared to other portions. For example, the second weak portion 9 is formed in a cross-sectional shape with a smaller moment of inertia related to horizontal deformation compared to other portions. The second weak portion 9 can be considered to be a portion that is allowed a predetermined horizontal bending deformation in response to a side collision load.

[0047] The first weak portions 8 in this embodiment are formed to have a shorter vertical dimension than other portions of the reinforcement 3. In the example shown in Fig. 7 , the portion of the joint 53 of the second reinforcement 5 that is not joined to the first reinforcement 4 (the portion of the joint 53 that is close to the rear portion 52) functions as the first weak portion 8. That is, in a side view of the state in which the second reinforcement 5 is joined to the first reinforcement 4, the reinforcement 3 has a portion that is recessed below its upper edge and a portion that is recessed above its lower edge (see Fig. 9(A)). The portions that are shorter in the vertical dimension due to such recesses are the first weak portions 8.

[0048] The second weakened portions 9 in this embodiment are formed to have a shorter vehicle width direction dimension than the first reinforcement 4. In the example shown in Fig. 7 , the front end portions (portions cut out by the rear cutout portion 59) of the second upper surface portion 56 and the second lower surface portion 57 function as the second weakened portions 9. That is, in a top view or bottom view of the state in which the second reinforcement 5 is joined to the first reinforcement 4, the reinforcement 3 has a portion that is recessed outward in the vehicle width direction from its inner end edge in the vehicle width direction (see Fig. 9(B)). The portion whose vehicle width direction dimension is shortened by such a recess is the second weakened portion 9.

[0049] In this embodiment, the first weak portion 8 and the second weak portion 9 are both disposed in a portion sandwiched between the lower end of the center pillar 12 and the lower end of the rear pillar 13 (rearward of the lower end of the center pillar 12 and forward of the lower end of the rear pillar 13). Note that the first weak portion 8 and the second weak portion 9 are preferably provided so as to overlap with the bulkhead 2 in a side view.

[0050] 9(A) and 9(B) are cross-sectional views showing the structure of the joint between the first reinforcement 4 and the second reinforcement 5, with FIG. 9(A) being a vertical cross-sectional view of the side sill 1 and FIG. 9(B) being a horizontal cross-sectional view of the side sill 1. The x marks in FIGS. 9(A) and 9(B) represent the positions of the spot welding points. As shown in FIG. 9(A), the vertical dimension H of the first weakened portion 8 １ is the vertical dimension of the reinforcement 3 other than the first weak portion 8, H ０ 9B, the vehicle width direction dimension W of the second weak portion 9 is set to a value smaller than １ is the vehicle width direction dimension of the reinforcement 3 at a portion other than the second weak portion 9, W ０ is set to a value smaller than

[0051] The first weak portion 8 is disposed near the bulkhead 2, and preferably disposed between the front wall portion 21 and the rear wall portion 22 in a side view. The front wall portion 21 is located in an overlapping region between the first wall portion 41 and the joint portion 53 in a side view. The second weak portion 9 is also disposed near the bulkhead 2. The second weak portion 9 is disposed adjacent to and rearward of the first weak portion 8, and preferably disposed between the front wall portion 21 and the rear wall portion 22 in a side view.

[0052] The first weakened portion 8 acts to allow more rotational movement in the roll direction of the first reinforcement 4 than the second reinforcement 5 in response to an external force input from the side outside the vehicle toward the inside of the vehicle compartment. For example, when an external force is input to the center pillar 12 during a side collision of the vehicle, the entire side sill 1 may tend to rotate in the roll direction. At this time, the first reinforcement 4 passing through the lower end of the center pillar 12 tends to rotate in the roll direction.

[0053] On the other hand, because the first weak portions 8 are formed to have lower torsional rigidity than other portions, the amount of rotational movement of the first reinforcement 4 increases in the vicinity of the first weak portions 8. Furthermore, due to deformation of the vicinity of the first weak portions 8, the amount of rotational movement of the second reinforcement 5 located rearward of the first weak portions 8 decreases relatively. As a result, at the joint between the first wall portion 41 of the first reinforcement 4 and the joint portion 53 of the second reinforcement 5, the first wall portion 41 tries to peel off from the joint portion 53, accelerating the rotational movement of the first reinforcement 4 in the roll direction.

[0054] In the first weakened portion 8 of this embodiment, the joint portion 53 of the second reinforcement 5 is joined to the first wall portion 41 of the first reinforcement 4 from the outer side in the vehicle width direction. This makes it difficult for the second reinforcement 5 to hinder the rotational movement of the first reinforcement 4, and makes it easier to promote the rotational movement of the first reinforcement 4. Furthermore, the first opening 44 provided near the rear end portion of the first reinforcement 4 also contributes to the ease of torsional deformation of the first wall portion 41. In this embodiment, these structures make it easier to promote the rotational movement of the first reinforcement 4.

[0055] Due to the torsional deformation of the first reinforcement 4, a load acts on the front wall portion 21 of the bulkhead 2 inward in the vehicle width direction. The front wall portion 21 absorbs the impact while being compressed and deformed in the vehicle width direction. This reduces the amount of penetration of the center pillar 12 into the interior of the vehicle compared to when the front wall portion 21 is not compressed and deformed. In this way, promoting torsional deformation at the first weak portion 8 due to an external force during a side collision of the vehicle improves the occupant protection performance of the vehicle.

[0056] Furthermore, because the amount of rotational movement of the second reinforcement 5 in the roll direction is reduced, impact fracture between the side sill inner 6 and the side sill outer 7 is less likely to occur. This impact fracture will be explained using Figures 10(A) to 10(C). Figure 10(A) is a cross-sectional view taken along line A-A in Figure 9(B), and Figure 10(B) is a longitudinal cross-sectional view showing the deformation state of the side sill 1 during a side collision. Figure 10(C) is a longitudinal cross-sectional view (comparative example) showing the deformation state of the side sill 1 during a side collision, assuming a structure in which the second reinforcement 5 is not present and the first reinforcement 4 extends from the front end to the rear end of the side sill 1.

[0057] 10(A), the inner upper edge portion 64 of the side sill inner 6 is spot welded to the outer upper edge portion 74 of the side sill outer 7, and the inner lower edge portion 65 of the side sill inner 6 is spot welded to the outer lower edge portion 75 of the side sill outer 7. If the second reinforcement 5 were not present and the first reinforcement 4 were to extend from the front end to the rear end of the side sill 1, the side sill outer 7 would be pulled upward due to torsional deformation of the first reinforcement 4, as shown in FIG. 10(C), and the welded point between the inner lower edge portion 65 and the outer lower edge portion 75 would be prone to fracture.

[0058] This impact fracture is likely to occur near the rear end of the side sill 1. On the other hand, in the vehicle undercarriage structure of this embodiment, as shown in Fig. 10(B) , the amount of rotational movement of the second reinforcement 5 in the roll direction is reduced, so that the portion of the side sill outer 7 reinforced by the second reinforcement 5 (near the rear end of the side sill 1) is less likely to be torsionally deformed, and impact fracture between the inner lower edge portion 65 and the outer lower edge portion 75 is efficiently suppressed.

[0059] 11 is a horizontal cross-sectional view of the side sill 1 illustrating the deformation of the side sill 1 during a vehicle side collision. The second weakened portion 9 acts to allow the side sill 1 to bend in the vehicle width direction in response to an external force from the side outside the vehicle toward the interior of the vehicle. When the front wall portion 21 of the bulkhead 2 is compressed in the vehicle width direction due to torsional deformation of the first reinforcement 4, the entire first reinforcement 4 rotates counterclockwise in FIG. 11 .

[0060] On the other hand, since the second weak portions 9 are formed to have lower bending rigidity in the horizontal direction than the other portions, the second reinforcement 5 bends in the horizontal direction near the second weak portions 9, and the entire second reinforcement 5 rotates clockwise in Figure 11. This deformation mode promotes compressive deformation of the front wall portion 21 and the rear wall portion 22 of the bulkhead 2, further absorbing the impact. In this way, promoting bending deformation at the second weak portions 9 due to external forces during a side collision of the vehicle improves the occupant protection performance of the vehicle.

[0061] In the second weakened portion 9 of the present embodiment, the rear cutout portion 59 is formed in a shape obtained by cutting out the front end portion of the rear portion 52 of the second reinforcement 5 (the front end portions of the second upper surface portion 56 and the second lower surface portion 57). This makes it easier to promote horizontal bending deformation between the front portion 51 and the rear portion 52 of the second reinforcement 5. Note that, as shown in FIG. 11 , the rear cutout portion 59 is formed only in a range rearward of the front wall portion 21 and forward of the rear wall portion 22. This limits the position where bending deformation of the second reinforcement 5 occurs to a range rearward of the front wall portion 21 and forward of the rear wall portion 22, suppressing excessive deformation.

[0062] In addition, since the boundary position between the front portion 51 and the rear portion 52 of the second reinforcement 5 is located rearward of the rear end of the first reinforcement 4, bending deformation of the second reinforcement 5 is easily promoted. Furthermore, the second opening 58 provided near the front end of the rear portion 52 also contributes to the ease of bending deformation of the second reinforcement 5. In this embodiment, these structures make it easier to promote bending deformation of the second reinforcement 5.

[0063] [2. Effects] (1) The vehicle undercarriage structure of this embodiment includes a side sill 1, a center pillar 12, and a reinforcement 3. The side sill 1 extends in the longitudinal direction at the lower part of the side of the vehicle, and is formed into a closed cross section by joining a side sill inner 6 (inner member) and a side sill outer 7 (outer member). The center pillar 12 is connected to a middle part of the side sill 1 in the longitudinal direction. The reinforcement 3 is attached inside the side sill 1 along an outer wall portion 71 (outer surface) of the side sill 1 and extends in the longitudinal direction. In addition, the reinforcement 3 has a first weakened portion 8 formed to have lower torsional rigidity than other portions between the longitudinal end (rear end) of the side sill 1 and the lower end of the center pillar 12.

[0064] In this way, by providing the first weak portion 8, which has low torsional rigidity, in the reinforcement 3 between the longitudinal end of the side sill 1 and the lower end of the center pillar 12, it is possible to promote torsional deformation of the first weak portion 8 in response to an external force (e.g., a side impact load) applied to the center pillar 12 from the side of the vehicle. This suppresses torsional deformation of the side sill 1 rearward of the first weak portion 8, thereby suppressing fracture of the welded joint between the side sill inner panel 6 and the side sill outer panel 7 and preventing a decrease in the cross-sectional performance of the side sill 1. This therefore improves the vehicle's occupant protection performance.

[0065] (2) In the vehicle undercarriage structure of this embodiment, the first weak portions 8 are formed with a shorter vertical dimension than other portions of the reinforcement 3. This more reliably promotes torsional deformation of the first weak portions 8 in response to external forces from the side of the vehicle. Therefore, fracture of the welded joints of the side sill 1 can be effectively suppressed, and the vehicle's occupant protection performance can be improved. In addition, the configuration is simple, and the vehicle's occupant protection performance can be improved while suppressing increases in manufacturing costs.

[0066] (3) In the vehicle undercarriage structure of this embodiment, the reinforcement 3 includes a first reinforcement 4 that extends in the front-rear direction through the lower end of the center pillar 12, and a second reinforcement 5 that is joined to the front-rear end (rear end) of the first reinforcement 4 from the outside in the vehicle width direction and is provided with a first weak portion 8. By configuring the reinforcement 3 from two members in this way, it is possible to promote torsional deformation at the first weak portion 8 of the second reinforcement 5 in response to an external force from the side of the vehicle while maintaining the structure of the first reinforcement 4, and it is possible to increase the efficiency of absorbing impact energy.

[0067] In addition, since the influence of torsional deformation of the first reinforcement 4 is less likely to be transmitted to the second reinforcement 5, fracture at the welded joint between the side sill inner panel 6 and the side sill outer panel 7 can be effectively suppressed. Therefore, the vehicle's occupant protection performance can be improved. Furthermore, it becomes easy to torsionally deform only the vicinity of the first weakened portion 8 without deforming the portion of the side sill outer panel 7 reinforced by the second reinforcement 5 (near the rear end of the side sill 1). This more reliably suppresses fracture at the welded joint of the side sill 1, preventing a decrease in the cross-sectional performance of the side sill 1. Therefore, the vehicle's occupant protection performance can be further improved.

[0068] (4) In the vehicle undercarriage structure of this embodiment, the second reinforcement 5 has a front cutout 54 formed by cutting out the end of the second reinforcement 5 on the first reinforcement 4 side at the joint 53 with the first reinforcement 4. This reduces the moment in the torsional direction of the reinforcement 3 (the moment around the axis extending in the front-rear direction) and suppresses fracture of the welded joint between the side sill inner 6 and the side sill outer 7. This prevents a decrease in the cross-sectional performance of the side sill 1 and improves the vehicle's occupant protection performance.

[0069] (5) In the vehicle undercarriage structure of this embodiment, the second reinforcement 5 has second weak portions 9 formed to have lower horizontal bending rigidity than other portions at the end side (rear end side) in the fore-and-aft direction than the first weak portions 8. In this way, by providing the second weak portions 9 having lower horizontal bending rigidity, horizontal bending deformation in the second weak portions 9 can be promoted in response to an external force from the side of the vehicle. This makes it possible to efficiently absorb impact energy and further improve the occupant protection performance of the vehicle.

[0070] In the vehicle undercarriage structure of this embodiment, the second weak portions 9 are disposed adjacent to and rearward of the first weak portions 8. This allows torsional deformation in the first weak portions 8 and bending deformation in the second weak portions 9 to be simultaneously achieved. In addition, in this embodiment, the first reinforcement 4 is responsible for torsional deformation, and the second reinforcement 5 is responsible for bending deformation. This division of roles allows each deformation to occur more reliably, and impact energy can be absorbed efficiently. Therefore, the occupant protection performance of the vehicle can be further improved.

[0071] (6) In the vehicle undercarriage structure of this embodiment, the second weak portions 9 are formed to have a shorter dimension in the vehicle width direction than other portions (e.g., the first reinforcement 4). This more reliably promotes bending deformation of the second weak portions 9 in response to external forces from the side of the vehicle. This improves the vehicle's occupant protection performance. In addition, the configuration is simple, and the vehicle's occupant protection performance can be improved while suppressing increases in manufacturing costs.

[0072] (7) In the vehicle undercarriage structure of this embodiment, the second reinforcement 5 has a second upper surface portion 56 (upper surface portion) joined to the outer upper surface portion 72 (upper surface) of the side sill 1, a second lower surface portion 57 (lower surface portion) joined to the outer lower surface portion 73 (lower surface) of the side sill 1, and rear notch portions 59 formed by cutting out the ends (respective front end portions) of the second upper surface portion 56 and the second lower surface portion 57 on the first reinforcement side. This makes it possible to reduce the bending rigidity of the respective front end portions of the second upper surface portion 56 and the second lower surface portion 57, promote bending deformation of the second weak portion 9, and further improve the occupant protection performance of the vehicle.

[0073] (8) The vehicle undercarriage structure of this embodiment includes a bulkhead 2 attached to the inside of the side sill 1 near the first weakened portion 8 and having a wall portion whose normal direction is the longitudinal direction. This allows the bulkhead 2 to be compressively deformed in the vehicle width direction in response to an external force from the side of the vehicle, thereby increasing the efficiency of absorbing impact energy. This further improves the vehicle occupant protection performance.

[0074] (9) In the vehicle undercarriage structure of this embodiment, the wall portions of the bulkhead 2 include a front wall portion 21 and a rear wall portion 22 that are arranged substantially parallel to each other with a gap between them in the fore-and-aft direction, and the bulkhead 2 has a planar connecting surface portion 23 that connects the inner edges of the front wall portion 21 and the rear wall portion 22 in the vehicle width direction. That is, the bulkhead 2 is formed in a U-shape that opens toward the outside of the vehicle in a top view. Furthermore, the first weakened portion 8 is disposed between the front wall portion 21 and the rear wall portion 22 in a side view. This allows the front wall portion 21 to be more reliably compressively deformed in response to an external force from the side of the vehicle. This increases the impact energy absorption efficiency and further improves the vehicle's occupant protection performance.

[0075] In this embodiment, the rear cutout portion 59 is formed only in a range rearward of the front wall portion 21 and forward of the rear wall portion 22. This limits the area where bending deformation of the second weakened portion 9 is promoted to a range rearward of the front wall portion 21 and forward of the rear wall portion 22, thereby suppressing bending deformation in the portion rearward of the rear wall portion 22, for example. This more reliably suppresses impact point fracture near the rear end portion of the side sill 1.

[0076] (10) The vehicle undercarriage structure of this embodiment includes a rear seat cross member 16 (cross member) that connects the left and right side sills 1 in the vehicle width direction. The bulkhead 2 is also positioned so that it overlaps with the rear seat cross member 16 in a side view. This increases the rigidity of the connection between the side sill 1 and the rear seat cross member 16. Furthermore, the provision of the rear seat cross member 16 increases the floor rigidity on the passenger compartment side of the bulkhead 2, allowing the bulkhead 2 to be more reliably compressively deformed against external forces from the side of the vehicle, thereby efficiently absorbing impact energy. This further improves the vehicle's occupant protection performance.

[0077] In this embodiment, a first opening 44 is provided near the rear end of the first reinforcement 4. This reduces the torsional rigidity and bending rigidity near the rear end of the first reinforcement 4. Therefore, it is possible to further promote torsional deformation in the first weakened portion 8 in response to an external force from the side of the vehicle, increase the efficiency of absorbing impact energy, and further improve the occupant protection performance of the vehicle.

[0078] Furthermore, a second opening 58 is provided near the front end of the rear portion 52 of the second reinforcement 5 in this embodiment. This reduces the torsional rigidity and bending rigidity near the front end of the rear portion 52. Therefore, bending deformation in the second weak portion 9 can be further promoted in response to an external force from the side of the vehicle, the efficiency of absorbing impact energy can be increased, and the occupant protection performance of the vehicle can be further improved.

[0079] [3. Other] The above-described embodiments are merely illustrative and are not intended to exclude various modifications or applications of techniques not explicitly described in the present embodiments. Each configuration of the present embodiments can be modified in various ways without departing from the spirit of the present embodiments. Furthermore, each configuration of the present embodiments can be selected as needed, or can be appropriately combined with various configurations included in known techniques.

[0080] 12(A) and 12(B) are perspective views illustrating a modified vehicle undercarriage structure. In the above embodiment, the bulkhead 2 is shaped like a U (or a C) when viewed from above. However, the shape of the bulkhead 2 is not limited to this. The bulkhead 2 may have a wall whose normal direction is the longitudinal direction and be attached to the inside of the side sill 1. The bulkhead 2 shown in FIGS. 12(A) and 12(B) includes only the front wall 21, front upper edge 24, and front lower edge 25 of the above embodiment. Even with this configuration, the front wall 21 can be compressively deformed to absorb impact during a side collision, thereby achieving the same functions and effects as the above embodiment. The bulkhead 2 is optional and is not an essential element.

[0081] In the above-described embodiment, the first reinforcement 4 has a U-shaped vertical cross section that is open toward the inside of the vehicle, but the shape of the first reinforcement 4 is not limited to this. The first reinforcement 4 shown in Fig. 12(A) is formed in a flat plate shape and includes only the first wall portion 41 in the above-described embodiment. Even with this configuration, by promoting torsional deformation of the first weak portion 8, it is possible to achieve the same actions and effects as in the above-described embodiment.

[0082] In the above embodiment, the second reinforcement 5 includes a rear portion 52 having a U-shaped (C-shaped) longitudinal cross section that is open toward the inside of the vehicle, but the shape of the second reinforcement 5 is not limited to this. The second reinforcement 5 shown in Fig. 12(A) is formed in a flat plate shape and includes only the joint portion 53 and the second wall portion 55 in the above embodiment. Even with this configuration, the same actions and effects as those of the above embodiment can be achieved by promoting torsional deformation of the first weak portion 8 and bending deformation of the second weak portion 9.

[0083] In the above embodiment, the reinforcement 3 is configured from two members, the first reinforcement 4 and the second reinforcement 5, but the shape of the reinforcement 3 is not limited to this. The reinforcement 3 shown in Fig. 12(B) is configured from a single member. This reinforcement 3 includes only the first wall portion 41, first upper surface portion 42, first lower surface portion 43, joint portion 53, second wall portion 55, second upper surface portion 56, and second lower surface portion 57 in the above embodiment.

[0084] The first wall portion 41, the joint portion 53, and the second wall portion 55 are formed on the same plane. The height dimension of the joint portion 53 is set smaller than the first wall portion 41 and the second wall portion 55. This allows the joint portion 53 to function as the first weak portion 8 and the second weak portion 9. Even with this configuration, by promoting torsional deformation of the first weak portion 8 and bending deformation of the second weak portion 9, it is possible to achieve the same actions and effects as in the above-described embodiment.

[0085] In the above embodiment, a structure in which the first weak portion 8 and the second weak portion 9 are provided rearward of the center pillar 12 on the side sill 1 has been exemplified, but the first weak portion 8 and the second weak portion 9 may also be provided forward of the center pillar 12. For example, as shown in FIG. 13 , a structure in which the first reinforcement 4 and the second reinforcement 5 are joined forward of the center pillar 12 may be used. In this case, the structure of the reinforcement 3 (the first reinforcement 4 and the second reinforcement 5) may be a structure inverted in the front-rear direction from that shown in FIG. 3 (a mirror-symmetrical shape). Even with such a structure, the same actions and effects as those of the above embodiment can be achieved.

[0086] The present invention is applicable to the manufacturing industry of vehicles having side sills on the lower sides of the vehicle body, and also applicable to the manufacturing industry of vehicle bodies having side sills on the lower sides.

[0087] REFERENCE SIGNS LIST 1 Side sill 2 Bulkhead 3 Reinforcement 4 First reinforcement 5 Second reinforcement 6 Side sill inner (inner member) 7 Side sill outer (outer member) 8 First weakened portion 9 Second weakened portion 10 Body 11 Front pillar 12 Center pillar 13 Rear pillar 14 Front floor panel 15 Rear floor panel 16 Rear seat cross member (cross member) 21 Front wall portion (wall portion) 22 Rear wall portion (wall portion) 23 Connection surface portion 24 Front upper edge portion 25 Front lower edge portion 26 Rear upper edge portion 27 Rear lower edge portion 41 First wall portion 42 First upper surface portion 43 First lower surface portion 44 First opening 51 Front portion 52 Rear portion 53 Joint portion 54 Front cutout portion 55 Second wall portion 56 Second upper surface portion (upper surface portion) 57 Second lower surface portion (lower surface portion) 58 Second opening 59 Rear cutout portion 61 Inner wall portion 62 Inner upper surface portion 63 Inner lower surface portion 64 Inner upper edge portion 65 Inner lower edge portion 71 Outer wall portion (outer surface) 72 Outer upper surface portion (upper surface) 73 Outer lower surface portion (lower surface) 74 Outer upper edge portion 75 Outer lower edge portion

Claims

1. A vehicle undercarriage structure comprising: a side sill extending in the longitudinal direction at the lower part of the side of a vehicle and formed into a closed cross section by joining an inner member and an outer member; a center pillar connected to the middle part of the side sill in the longitudinal direction; and a reinforcement attached inside the side sill along the outer surface of the side sill and extending in the longitudinal direction, wherein the reinforcement has a first weak part formed to have lower torsional rigidity than other parts between the longitudinal end of the side sill and the lower end of the center pillar.

2. A vehicle undercarriage structure according to claim 1, characterized in that the first weakened portion is formed to have a shorter vertical dimension than the other portions of the reinforcement.

3. A vehicle undercarriage structure as described in claim 1, characterized in that the reinforcement comprises a first reinforcement extending in the longitudinal direction through the lower end of the center pillar, and a second reinforcement joined to the longitudinal end of the first reinforcement from the outside in the vehicle width direction, and having the first weak portion provided therein.

4. A vehicle undercarriage structure as described in claim 3, characterized in that the second reinforcement has a front cutout portion formed by cutting out the end portion on the first reinforcement side at the joint with the first reinforcement.

5. A vehicle undercarriage structure according to claim 3, characterized in that the second reinforcement has a second weak portion formed to have lower horizontal bending rigidity than other portions at the end side in the fore-and-aft direction from the first weak portion.

6. A vehicle underbody structure according to claim 5, wherein the second weakened portion is formed to have a shorter dimension in the vehicle width direction than the first reinforcement.

7. A vehicle undercarriage structure as described in claim 3, characterized in that the second reinforcement has an upper surface portion joined to the upper surface of the side sill, a lower surface portion joined to the lower surface of the side sill, and rear cutout portions formed by cutting out the ends of the upper surface portion and the lower surface portion on the side of the first reinforcement.

8. A vehicle undercarriage structure according to claim 1, characterized in that it comprises a bulkhead attached to the inside of the side sill in the vicinity of the first weakened portion and having a wall portion whose normal direction is the longitudinal direction.

9. A vehicle undercarriage structure as set forth in claim 8, characterized in that the wall portion includes a front wall portion and a rear wall portion that are arranged approximately parallel to each other with a gap between them in the fore-and-aft direction, the bulkhead has a planar connecting surface portion that connects the inner end edges of the front wall portion and the rear wall portion in the vehicle width direction, and the first weakened portion is arranged between the front wall portion and the rear wall portion in a side view.

10. A vehicle undercarriage structure as claimed in claim 8, further comprising a cross member connecting the left and right side sills in the vehicle width direction, and the bulkhead is provided in a position overlapping with the cross member in a side view.

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