JP7764798B2 - Vehicle lower body structure - Google Patents

Description

The present invention relates to a vehicle underbody structure.

Various technologies have been developed to suppress the up and down vibration of floor panels while a vehicle is in motion.

The structure described in Patent Document 1 has a closed cross section extending in the vehicle width direction at the lower rear end of the floor tunnel in order to suppress vertical vibration of the floor panel. This closed cross section is formed by an L-shaped cross member extending in the vehicle width direction, a front floor panel located in front of the cross member, and a rear floor panel located behind the front floor panel.

Specifically, the L-shaped cross member has a vertical portion rising approximately vertically and a horizontal portion extending forward from the bottom of the vertical portion. The rear floor panel is positioned higher than the front floor panel. The rear floor panel has a step portion that hangs downward at its front end and a ridge portion that juts forward in an inverted L shape from the bottom of the step portion. The vertical portion of the cross member is joined to the step portion of the rear floor panel, and the horizontal portion of the cross member is joined to the rear end of the front floor panel and the bottom end of the ridge portion. As a result, the L-shaped cross member and the ridge portion of the inverted L-shaped rear floor panel form the above-mentioned closed cross section extending in the vehicle width direction.

This closed cross section improves the rigidity of the boundary between the front and rear floor panels, thereby suppressing vertical vibration of the floor.

JP 2015-98263 A

In the above structure, a cross member with an L-shaped cross section extending in the vehicle width direction is joined to the front and rear floor panels, respectively, to form a closed cross section extending in the vehicle width direction. However, in vehicles with a large floor area (for example, a three-row vehicle with three rows of seats arranged in the fore-and-aft direction of the vehicle), the above closed cross section structure alone is not enough to sufficiently suppress vertical floor vibrations (i.e., floor membrane vibrations), and further ingenuity is required to suppress floor vibrations.

The present invention was made in consideration of the above-mentioned circumstances, and aims to provide a vehicle underbody structure that can sufficiently suppress floor membrane vibration.

To solve the above problem, the vehicle underbody structure of the present invention comprises at least one floor panel, a cross member extending in the vehicle width direction and joined to the front-rear end of the floor panel, a pair of frame members extending in the vehicle length direction and joined to both widthwise ends of the underside of the floor panel to form a closed cross section extending in the front-rear direction in cooperation with the floor panel, and overlapping the opposite widthwise ends of the cross member from below, and a pair of reinforcing members extending in the vertical direction and connecting the cross member to the frame member in the area where the opposite widthwise ends of the cross member and the frame member overlap in a plan view, each reinforcing member having a first portion that protrudes above the lower end of the cross member and overlaps with the cross member in the front-rear direction, and a second portion located within the closed cross section below the first portion, and the first portion is joined to the cross member and the second portion is joined to the inner surface of the frame member, thereby connecting the cross member to the frame member in the vertical direction.

In other words, the lower body structure of the present invention includes a cross member joined to the longitudinal end of the floor panel, and a pair of frame members extending in the longitudinal direction of the vehicle and forming a closed cross section with the floor panel. The structure also includes a pair of vertically extending reinforcing members connecting the cross member to the frame member in the areas where both transversely oriented portions of the cross member overlap the frame member in a plan view. The reinforcing members vertically connect the cross member to the frame member by having a first portion joined to the cross member and a second portion below the first portion joined to the inner surface of the frame member within the closed cross section. As a result, when the floor panel vibrates vertically during vehicle travel, causing floor membrane resonance and resulting in a vertical bending deformation mode of the cross member's central portion in the transverse direction, the reinforcing members vertically push or pull the transverse end of the cross member, thereby suppressing vertical displacement of the cross member. This suppresses the vertical deformation mode of the cross member and floor membrane resonance. As a result, floor membrane vibration can be sufficiently suppressed.

In the above-mentioned lower body structure, it is preferable that the floor panels are disposed on both sides of the cross member in the vehicle longitudinal direction, and that the floor panels are joined to the cross member so that they are at different heights, thereby forming a step between the front and rear floor panels.

With this configuration, a step can be formed by joining floor panels to both sides of the cross member in the vehicle's fore-and-aft direction, making it possible to change the height of the floor inside the vehicle depending on the position in the vehicle's fore-and-aft direction.

In the above-mentioned vehicle underbody structure, it is preferable that the cross member has a front wall and a rear wall spaced apart from each other in the longitudinal direction of the vehicle, the frame member has a bottom wall that forms the bottom surface of the closed cross section, and the reinforcing member connects the cross member and the frame member in the vertical direction by having the first portion joined to the front wall or the rear wall of the cross member and the second portion joined to the bottom wall of the frame member.

With this configuration, the first portion of the reinforcing member is joined to the front or rear wall of the cross member, and the second portion is joined to the bottom wall of the frame member. Therefore, when the cross member enters a vertical bending deformation mode due to vertical vibration of the floor panel while the vehicle is in motion, a vertical shear load is input from the cross member to the first portion of the reinforcing member. This allows the reinforcing member to withstand a larger input load than when a bending load is input to the reinforcing member, which is advantageous in terms of the strength of the reinforcing member. Furthermore, during a vehicle side collision (i.e., when a vehicle collides with another vehicle from the side), the cross member, which extends in the vehicle width direction, enters a vertical bending deformation mode due to the input of a compressive load from the vehicle width direction. However, even in this case, the input load can be distributed from the cross member to the frame member via the reinforcing member, thereby suppressing deformation of the cross member.

In the above vehicle lower body structure, the reinforcing member preferably has a rigidity-enhancing portion that increases rigidity against vibration input in the vertical direction.

With this configuration, the rigidity-reinforcing section can increase the rigidity of the reinforcing member against vibration input in the vertical direction.

In the above vehicle underbody structure, the rigidity reinforcement portion is preferably a bead extending in the vertical direction.

With this configuration, by providing the reinforcing member with a bead as a rigidity-enhancing portion, it is possible to easily and reliably increase the rigidity of the reinforcing member against vibration input in the vertical direction.

In the above vehicle underbody structure, it is preferable that the bead extend continuously in the vertical direction to the upper end of the reinforcing member.

This configuration makes it possible to increase the rigidity of the reinforcing member against vertical vibration inputs across its entire height.

In the above-mentioned vehicle underbody structure, it is preferable that the frame member has a pair of side walls spaced apart in the vehicle width direction, and the second portion of the reinforcing member is respectively joined to the pair of side walls of the frame member.

With this configuration, the second portion of the reinforcing member is joined to each of a pair of side walls of the frame member, which reliably suppresses vertical displacement of the cross member and floor membrane resonance.

It is preferable that the above-mentioned vehicle lower body structure further includes a floor tunnel extending in the fore-and-aft direction of the vehicle at the center of the vehicle width, the floor panel being joined to each end of the floor tunnel on both sides of the vehicle width, and the cross member being joined to the rear end of the floor tunnel.

With this configuration, the floor tunnel extends in the fore-and-aft direction of the vehicle at the center of the vehicle's width, and the floor panels are joined to both ends of the floor tunnel in the vehicle width direction. Because the cross member is joined to the rear end of the floor tunnel, vertical vibrations of the floor panel are smoothly transmitted to the cross member via the floor tunnel when the vehicle is moving. By suppressing the resulting vertical displacement of the cross member using the reinforcing member, floor membrane resonance can be smoothly suppressed.

In the above vehicle underbody structure, it is preferable that the reinforcing member have a bent portion that bends in the fore-and-aft direction of the vehicle between the first portion and the second portion.

With this configuration, the bent portion of the reinforcing member bends in the fore-and-aft direction of the vehicle between the first portion joined to the cross member and the second portion joined to the frame member. This bent portion makes it possible to accommodate the tolerance in the relative position between the cross member and the frame member, and makes it easy to join the first and second portions of the reinforcing member at two points.

As described above, the vehicle lower body structure of the present invention can achieve sufficient suppression of floor membrane vibration.

1 is a rear view of a cross member joined to the rear end of a floor tunnel of a vehicle body and its surrounding area, showing the overall configuration of the lower body structure of a vehicle according to an embodiment of the present invention. 2 is an enlarged view of an overlapping portion of the cross member and the rear side frame in FIG. 1 and the surrounding area thereof; 2 is a view of the vehicle body of FIG. 1 as seen obliquely from above the front and from the outside left side of the vehicle, and is an enlarged cross-sectional explanatory view of a left-side reinforcing member connecting a cross member and a rear side frame and its surrounding area. FIG. 2 is a view of the vehicle body of FIG. 1 as seen obliquely from above rear and from the outside left side of the vehicle, and is an enlarged cross-sectional explanatory view of a left-side reinforcing member connecting a cross member and a rear side frame and its surrounding area. FIG. 5 is a cross-sectional explanatory view of a right-side reinforcing member joined to the right end of the cross member of FIG. 4 and its surrounding area, as viewed from the rear side inside the vehicle. FIG. 4 is a cross-sectional explanatory view of a right-side reinforcing member joined to the right end of the cross member of FIG. 3 and its surrounding area, as viewed from the front side inside the vehicle. FIG. 7 is a cross-sectional explanatory view of the reinforcing member and its surrounding area of FIG. 6, as viewed from the outside front side of the vehicle. FIG. 2 is a view of the right side of the vehicle body of FIG. 1 as seen obliquely from above and in front, and is an enlarged cross-sectional explanatory view of an end portion in the vehicle width direction of a cross member and its surrounding area. FIG. 9 is a cross-sectional view taken along line IX-IX in FIG. 8.

The vehicle underbody structure according to an embodiment of the present invention will be described in detail below with reference to the drawings.

As shown in Figures 1 to 4, a vehicle body 1 to which the vehicle lower body structure of an embodiment of the present invention is applied comprises a pair of left and right front floor panels 2 that form the floor portion of the passenger compartment 9 on the front side X1 of the vehicle, a rear floor panel 3 that forms the floor portion of the passenger compartment 9 on the rear side X2 of the vehicle, a semi-cylindrical floor tunnel 4 that is arranged between the pair of front floor panels 2 and extends in the fore-and-aft direction X of the vehicle at the center of the vehicle width direction Y of the vehicle body 1, a rear cross member 5 (so-called No. 3 cross member) that extends in the vehicle width direction Y and is connected to the rear end 4a of the floor tunnel 4, a pair of rear side frames 6 (frame members) that extend in the fore-and-aft direction X of the vehicle at positions spaced apart on both sides of the vehicle width direction Y of the vehicle body 1, a pair of side sills 7 that are arranged on the outer sides of the pair of rear side frames 6 in the vehicle width direction Y (outside side Y2 of the vehicle), and a pair of reinforcing members 8 that respectively connect the end portions 5a on both sides of the vehicle width direction of the rear cross member 5 to the pair of rear side frames 6. In this specification, with regard to the direction in the vehicle width direction Y, the side toward the center of the vehicle interior (i.e., the side approaching the floor tunnel 4) is referred to as the inside side Y1, and the side away from the center of the vehicle interior (i.e., the side away from the floor tunnel 4) is referred to as the outside side Y2.

Note that Figures 3 and 4 show a central cross member 10 (the so-called No. 2.5 cross member) that extends in the vehicle width direction Y on both sides of the floor tunnel 4 in the vehicle width direction Y as a cross member located further forward in the vehicle X1 than the rear cross member 5.

As shown in Figures 3-4 and 8-9, the rear cross member 5 is composed of a front member 5b with an inverted L-shaped cross section extending in the vehicle width direction Y, and a rear member 5c with an L-shaped cross section. The front member 5b and rear member 5c form a closed cross section 5d extending in the vehicle width direction Y. The front member 5b has a front wall 5b1 extending in the vertical direction Z, and the rear member 5c has a rear wall 5c1 extending in the vertical direction Z. The front wall 5b1 and rear wall 5c1 are positioned spaced apart from each other in the vehicle's fore-and-aft direction X. Both ends of the rear cross member 5 in the vehicle width direction Y are joined to the side sills 7 (specifically, the side sill inners 7a).

As shown in Figures 1-4 and 8-9, the end of the rear cross member 5 on the vehicle's front side X1 in the vehicle's fore-and-aft direction X is joined at its center to the rear end 4a of the floor tunnel 4, and is joined at both sides of the center to the ends of the pair of front floor panels 2 on the vehicle's rear side X2. Meanwhile, the end of the rear cross member 5 on the vehicle's rear side X2 in the vehicle's fore-and-aft direction X is joined to the end of the rear floor panel 3 on the vehicle's front side X1.

Note that while the joining in this structure is primarily done using spot welding, other joining methods are also acceptable. The same applies to the other joining methods described below.

The pair of front floor panels 2 are also joined to the ends of the floor tunnel 4 on both sides in the vehicle width direction Y. Therefore, vibrations of the pair of front floor panels 2 are also transmitted via the floor tunnel 4 to the center of the rear cross member 5 in the vehicle width direction Y.

In this embodiment, as shown in Figures 3 and 4, the front floor panel 2 and the rear floor panel 3 are disposed on either side of the rear cross member 5 in the vehicle's fore-and-aft direction X, and are joined to the rear cross member 5 so that the front floor panel 2 and the rear floor panel 3 are at different heights, thereby forming a step portion 12 between the front floor panel 2 and the rear floor panel 3.

As shown in Figures 1 to 9, the pair of rear side frames 6 extend in the longitudinal direction X of the vehicle and are joined to both ends in the vehicle width direction Y of the undersides of the front floor panel 2 and the rear floor panel 3, thereby forming a closed cross section 11 extending in the longitudinal direction X of the vehicle in cooperation with the front floor panel 2, the rear floor panel 3, and the side sill inners 7a described below. Furthermore, the pair of rear side frames 6 are arranged overlapping each other from below Z2 of the end portions 5a on both sides in the vehicle width direction Y of the rear cross member 5. The rear side frames 6 correspond to the frame members in this invention.

The pair of rear side frames 6 are members that extend in the vehicle longitudinal direction X and have a generally U-shaped cross section that opens upward Z1. Specifically, as shown in Figures 3 to 7 and 9, the rear side frame 6 has a bottom wall 6a that forms the bottom surface of the closed cross section 11, and a pair of side walls 6b, 6c that are spaced apart in the vehicle width direction Y (specifically, the outer wall 6b on the vehicle exterior side and the inner wall 6c on the vehicle interior side).

As shown in Figure 5, the upper end of the inner wall 6c is joined to the underside of the rear floor panel 3. Furthermore, a flange portion 6c1 (see Figure 5) is provided on the upper portion of the inner wall 6c at a portion X2 rearward of the vehicle relative to the rear cross member 5. The flange portion 6c1 is joined to the rear member 5c of the rear cross member 5.

The outer wall 6b has a smaller width (height) in the vertical direction Z than the inner wall 6c, and is joined only to the side sill inner 7a, which is the inner portion of the generally cylindrical side sill 7 in the vehicle width direction. The outer wall 6b may also be made to have the same width (height) in the vertical direction Z as the inner wall 6c, and joined not only to the side sill inner 7a but also to the underside of the rear floor panel 3.

As shown in Figures 3 to 7 and 9, each of the pair of reinforcing members 8 is formed by pressing a material such as a metal plate, such as steel. The reinforcing members 8 extend in the vertical direction Z and are configured to connect the rear cross member 5 and the rear side frame 6 in the region R where the ends 5a of the rear cross member 5 on both sides in the vehicle width direction Y overlap with the rear side frame 6 in a plan view.

Specifically, the reinforcing member 8 has a first portion 8a that protrudes upward Z1 from the lower end 5e of the rear cross member 5 and overlaps with the rear cross member 5 in the vehicle longitudinal direction X, and a second portion 8b that is located below the first portion 8a at a position Z2 within the closed cross section 11.

The first portion 8a is a flat plate-shaped portion. The second portion 8b is configured to be joinable to the inner surface of the rear side frame 6. Specifically, the second portion 8b has a flat plate-shaped main body portion 8b1 that is continuous with the lower end of the first portion 8a, a lower flange portion 8b2 that extends from the lower end of the main body portion 8b1 toward the vehicle front side X1, an inner flange portion 8b3 that extends from the inner end of the main body portion 8b1 in the vehicle width direction Y (the side end on the vehicle interior side Y1) toward the vehicle front side X1, and an outer flange portion 8b4 that extends from the outer end of the main body portion 8b1 in the vehicle width direction Y (the side end on the vehicle exterior side Y2) toward the vehicle front side X1.

In the reinforcing member 8 configured as described above, the first portion 8a is joined to the front wall 5b1 or rear wall 5c1 of the rear cross member 5 (in this embodiment, the rear wall 5c1 of the rear member 5c). Meanwhile, the lower flange portion 8b2 of the second portion 8b is joined to the bottom wall 6a of the rear side frame 6. The inner flange portion 8b3 is joined to the inner wall 6c of the rear side frame 6. Furthermore, the outer flange portion 8b4 is joined to the outer wall 6b of the rear side frame 6 and the side sill inner 7a, respectively.

As a result, the reinforcing member 8 is able to connect the rear cross member 5 and the rear side frame 6 in the vertical direction Z by having the first portion 8a joined to the rear cross member 5 and the second portion 8b joined to the inner surface of the rear side frame 6.

Furthermore, the reinforcing member 8 of this embodiment has a bead 8f extending in the vertical direction Z as a rigidity-reinforcing portion that increases rigidity against vibration input in the vertical direction Z. The bead 8f extends in the vertical direction Z continuously to the upper end of the reinforcing member 8, and specifically, extends over the entire length of the first portion 8a and the main body portion 8b1 of the second portion 8b of the reinforcing member 8 from the widthwise center of the first portion 8a and the main body portion 8b1. The bead 8f has a shape that protrudes toward the rear side X2 of the vehicle and extends in the vertical direction Z, and is molded integrally with the reinforcing member 8 when the reinforcing member 8 is press-molded.

Furthermore, in this embodiment, the reinforcing member 8 has a bent portion 8e that bends in the vehicle longitudinal direction X between the first portion 8a and the second portion 8b. Specifically, the bent portion 8e extends in the vehicle width direction Y at the boundary between the first portion 8a and the main body portion 8b1 of the second portion 8b. When the main body portion 8b1 of the second portion 8b is erected vertically, the first portion 8a is inclined toward the vehicle rear side X2, starting from the bent portion 8e. As a result, even if the rear wall 5c1 of the rear cross member 5 on the vehicle rear side X2 is slightly inclined toward the vehicle rear side X2, the reinforcing member 8 can join the first portion 8a in surface contact with the inclined rear wall 5c1 while maintaining the main body portion 8b1 of the second portion 8b in the vertical position. This bent portion 8e makes it possible to accommodate tolerances in the relative positions between the rear cross member 5 and the rear side frame 6.

(Features of this embodiment)
(1)
The lower body structure of the vehicle of this embodiment is configured to include a rear cross member 5 joined to the ends of the front floor panel 2 and the rear floor panel 3 in the fore-and-aft direction X of the vehicle, and a pair of rear side frames 6 extending in the fore-and-aft direction X of the vehicle and forming a closed cross section 11 with the floor panels 2, 3, and is further provided with a pair of reinforcing members 8 extending in the up-down direction Z and connecting the rear cross member 5 and the rear side frames 6 respectively in a region R (see Figure 2) where the ends 5a of the rear cross member 5 on both sides in the vehicle width direction Y and the rear side frames 6 overlap in a planar view.

As shown in Figures 3 to 7 and 9, the first portion 8a of the reinforcing member 8 is joined to the rear cross member 5, and the second portion 8b, which is lower than the first portion 8a, is joined to the inner surface of the rear side frame 6 within the closed cross section 11, thereby connecting the rear cross member 5 and the rear side frame 6 in the vertical direction Z.

As a result, when the floor panel vibrates in the vertical direction Z while the vehicle is in motion, causing floor membrane resonance and causing the rear cross member 5 to enter a mode in which its central portion in the vehicle width direction Y bends and deforms in the vertical direction Z, the reinforcing member 8 braces (pushes) or pulls the end of the rear cross member 5 in the vertical direction Z in the vehicle width direction Y, thereby suppressing the vertical displacement of the rear cross member 5. This makes it possible to suppress the vertical deformation mode of the rear cross member 5 and suppress floor membrane resonance. As a result, it is possible to sufficiently suppress vertical vibration of the floor, i.e., floor membrane vibration, even in vehicles with a large floor area.

(2)
In the lower body structure of the vehicle of this embodiment, as shown in Figures 3 and 4, a front floor panel 2 and a rear floor panel 3 are respectively arranged on both sides of the rear cross member 5 in the vehicle's fore-and-aft direction X, and the front floor panel 2 and the rear floor panel 3 are respectively joined to the rear cross member 5 so that they are at different heights, thereby forming a step portion 12 between the front floor panel 2 and the rear floor panel 3.

With this configuration, the front floor panel 2 and rear floor panel 3 are joined to both sides of the rear cross member 5 in the vehicle's fore-and-aft direction X, forming a step portion 12, making it possible to change the height of the floor inside the vehicle cabin depending on the position in the vehicle's fore-and-aft direction X.

(3)
In the vehicle underbody structure of this embodiment, as shown in Figure 9, the rear cross member 5 has a front wall 5b1 and a rear wall 5c1 that are spaced apart from each other in the vehicle fore-and-aft direction X. The rear side frame 6 has a bottom wall 6a that forms the bottom surface of the closed cross section 11. The reinforcing member 8 has a first portion 8a joined to the front wall 5b1 or the rear wall 5c1 (the rear wall 5c1 in this embodiment) of the rear cross member 5 and a second portion 8b joined to the bottom wall 6a of the rear side frame 6, thereby connecting the rear cross member 5 and the rear side frame 6 in the up-down direction Z.

With this configuration, the first portion 8a of the reinforcing member 8 is joined to the rear wall 5c1 of the rear cross member 5, and the second portion 8b is joined to the bottom wall 6a of the rear side frame 6. Therefore, when the rear cross member 5 enters a mode in which it bends in the vertical direction Z due to vertical vibration of the floor panel while the vehicle is in motion, a shear load in the vertical direction Z is input from the rear cross member 5 to the first portion 8a of the reinforcing member 8. This allows the reinforcing member 8 to withstand a larger input load than when a bending load is input to the reinforcing member 8, which is advantageous in terms of the strength of the reinforcing member 8.

Furthermore, during a vehicle side collision (i.e., when a vehicle collides with the side of the vehicle), the rear cross member 5, which extends in the vehicle width direction Y, enters a mode in which it bends and deforms in the vertical direction Z due to the input of a compressive load from the vehicle width direction Y. However, even in this case, the input can be distributed from the rear cross member 5 to the rear side frame 6 via the reinforcing member 8, thereby suppressing deformation of the rear cross member 5.

The first portion 8a of the reinforcing member 8 may also be joined to the front wall 5b1 of the rear cross member 5. In this case, the above-mentioned effects can also be achieved.

(4)
In the vehicle lower body structure of this embodiment, the reinforcing member 8 has the bead 8 f as a rigidity enhancing portion that increases the rigidity against vibration input in the vertical direction Z. With this configuration, the bead 8 f can increase the rigidity of the reinforcing member 8 against vibration input in the vertical direction Z.

(5)
In the vehicle lower body structure of this embodiment, the rigidity reinforcing portion is a bead 8 f extending in the vertical direction Z. According to this configuration, by providing the bead 8 f as the rigidity reinforcing portion on the reinforcing member 8, it is possible to easily and reliably increase the rigidity of the reinforcing member 8 against vibration input in the vertical direction Z.

(6)
In the vehicle lower body structure of this embodiment, the bead 8f extends continuously in the vertical direction Z up to the upper end of the reinforcing member 8. With this configuration, it is possible to increase the rigidity of the reinforcing member 8 over the entire height thereof against vibration input in the vertical direction Z.

(7)
5 to 7 , in the vehicle lower body structure of this embodiment, the rear side frame 6 has an outer wall 6b and an inner wall 6c as a pair of side walls spaced apart from each other in the vehicle width direction Y. The second portion 8b of the reinforcing member 8 (specifically, the outer flange portion 8b4 and the inner flange portion 8b3) is joined to the outer wall 6b and the inner wall 6c of the rear side frame 6, respectively.

With this configuration, the second portion 8b of the reinforcing member 8 is joined to the outer wall 6b and inner wall 6c of the rear side frame 6, respectively, which reliably suppresses vertical displacement of the rear cross member 5 and floor membrane resonance.

(8)
1 to 4, the vehicle lower body structure of this embodiment includes a floor tunnel 4 that extends in the vehicle front-rear direction X at the center of the vehicle body in the vehicle width direction Y. The front floor panel 2 is joined to each end of the floor tunnel 4 on both sides in the vehicle width direction Y. The rear cross member 5 is joined to the rear end 4a of the floor tunnel 4.

With this configuration, the floor tunnel 4 extends in the vehicle's fore-and-aft direction X at the center of the vehicle's width direction Y, and the front floor panel 2 is joined to both ends of the floor tunnel 4 in the vehicle's width direction Y. Because the rear cross member 5 is joined to the rear end 4a of the floor tunnel 4, vibrations of the front floor panel 2 in the vertical direction Z while the vehicle is moving are smoothly transmitted to the rear cross member 5 via the floor tunnel 4. By suppressing the resulting vertical displacement of the rear cross member 5 using the reinforcing member 8, floor membrane resonance can be smoothly suppressed.

(9)
In the vehicle underbody structure of this embodiment, as shown in Figures 5 to 7 and 9, the reinforcing member 8 has a bent portion 8e that bends in the vehicle fore-and-aft direction X between the first portion 8a and the second portion 8b.

With this configuration, the bent portion 8e of the reinforcing member 8 is bent in the vehicle's fore-and-aft direction X between the first portion 8a joined to the rear cross member 5 and the second portion 8b joined to the rear side frame 6. This bent portion 8e makes it possible to absorb the tolerance in the relative position between the rear cross member 5 and the rear side frame 6, and makes it easy to join the first portion 8a and the second portion 8b of the reinforcing member 8 at two locations.

(Modification)
(A)
In the above embodiment, the rear cross member 5 (so-called No. 3 cross member) joined to the rear end 4a of the floor tunnel 4 has been described as an example of a cross member in the lower body structure of the present invention, but the present invention is not limited to this. For example, in the case of a vehicle without a floor tunnel (such as a front-wheel drive vehicle (FF vehicle) or an electric vehicle (EV)), the present invention can also be applied to a cross member (such as a front cross member (so-called No. 1 cross member)) located further forward in the vehicle X1 than the rear cross member 5. In this case, it is possible to suppress vertical displacement of the cross member caused by floor membrane vibration and floor membrane resonance.

However, as in the above embodiment, a rear cross member 5 joined to the rear end 4a of the floor tunnel 4 is preferable in that it allows the front floor panel 2 and rear floor panel 3 to form a step portion 12 as described in (2) above (Features of this embodiment), and that it allows vibrations from the front floor panel 2 to be smoothly transmitted via the floor tunnel 4 as described in (8).

(B)
In the above embodiment, the reinforcement member 8 is provided with the bead 8f extending in the vertical direction Z as a rigidity reinforcing portion, but the present invention is not limited to this. The rigidity reinforcing portion may be anything that can increase the rigidity of the reinforcement member 8 against vibration input in the vertical direction Z, and as a rigidity reinforcing portion other than the bead 8f, for example, a patch made of a strip-shaped metal plate material extending in the vertical direction may be provided on the reinforcement member 8.

1 Vehicle body 2 Front floor panel 3 Rear floor panel 4 Floor tunnel 5 Rear cross member 5a End portion 5b in the vehicle width direction Front member 5b1 Front wall 5c Rear member 5c1 Rear wall 5d Closed cross section 6 Rear side frame (frame member)
6a Bottom wall 6b Outer wall 6c Inner wall 7 Side sill 8 Reinforcing member 8a First portion 8b Second portion 8e Bent portion 8f Bead (rigidity reinforcement portion)
11 Closed section

Claims (9)

at least one floor panel;
a cross member extending in a vehicle width direction and joined to an end of the floor panel in a vehicle front-rear direction;
a pair of frame members extending in the vehicle longitudinal direction and joined to both end portions of the underside of the floor panel in the vehicle width direction to form a closed cross section extending in the longitudinal direction in cooperation with the floor panel, and overlapping the end portions of the cross member on both sides in the vehicle width direction from below;
a pair of reinforcing members extending in the vertical direction and connecting the cross member and the frame member in a region where the ends of the cross member on both sides in the vehicle width direction and the frame member overlap in a plan view;
The reinforcing member has a first portion that protrudes above the lower end of the cross member and overlaps with the cross member in the fore-and-aft direction of the vehicle, and a second portion that is located inside the closed cross section at a position lower than the first portion, and the first portion is joined to the cross member and the second portion is joined to the inner surface of the frame member, thereby connecting the cross member and the frame member in the vertical direction. 2. The vehicle underbody structure according to claim 1,
The floor panels are disposed on both sides of the cross member in the vehicle longitudinal direction, and the floor panels are joined to the cross member so that they are at different heights, thereby forming a step between the front and rear floor panels.
A vehicle underbody structure characterized by: 3. The vehicle underbody structure according to claim 1,
The cross member has a front wall and a rear wall spaced apart from each other in the vehicle longitudinal direction,
the frame member has a bottom wall that forms a bottom surface of the closed cross section,
The reinforcing member connects the cross member and the frame member in the vertical direction by having the first portion joined to the front wall or the rear wall of the cross member and the second portion joined to the bottom wall of the frame member.
A vehicle underbody structure characterized by: 3. The vehicle underbody structure according to claim 1,
The reinforcing member has a rigidity enhancing portion that enhances rigidity against vibration input in the vertical direction.
A vehicle underbody structure characterized by: 5. The vehicle underbody structure according to claim 4,
The rigidity reinforcing portion is a bead extending in the vertical direction.
A vehicle underbody structure characterized by: 6. The vehicle underbody structure according to claim 5,
The bead extends continuously in the vertical direction to the upper end of the reinforcing member.
A vehicle underbody structure characterized by: 3. The vehicle underbody structure according to claim 1,
The frame member has a pair of side walls spaced apart from each other in the vehicle width direction,
the second portion of the reinforcing member is joined to each of the pair of side walls of the frame member.
A vehicle underbody structure characterized by: 3. The vehicle underbody structure according to claim 1,
The vehicle further includes a floor tunnel extending in the front-rear direction of the vehicle at the center of the vehicle width direction of the vehicle body,
The floor panel is joined to each of the ends of the floor tunnel on both sides in the vehicle width direction,
The cross member is joined to the rear end of the floor tunnel.
A vehicle underbody structure characterized by: 3. The vehicle underbody structure according to claim 1,
The reinforcing member has a bent portion that bends in the fore-and-aft direction of the vehicle between the first portion and the second portion.