AUTOMOTIVE REAR UNDERBODY STRUCTURE

Abstract: A rear underbody structure comprises a pair of side sills (1) each each being of a generally closed cross-section and extending in a direction longitudinally of the automobile while spaced apart from each other in a direction widthwise of the automobile, a rear frame member (2) connected end-to-end with the associated one of the side sills so as to extend in a direction rearwardly of the automobile, a first cross member (7) connected at its opposite end to the rear frame members, respectively, so as to extend widthwise of the automobile, and a second cross member (3) connected at its opposite ends to the rear frame members, respectively, so as to extend widthwise of the automobile and positioned rearwardly of the first cross member with respect to the automobile.

Description: The present invention generally relates to an automobile body structure and, more particularly, to a rear bottom frame structure, or a rear underbody structure, for automobiles. From GB-A-890 590, a frame structure for motor vehicles is known having a pair of longitudinal side members, a rear frame member for each side member and first, second and third cross members. Additionally, a rear bottom panel comprising a forward section and rear section raised with respect to the forward section is shown, the cross members being connected in the vicinity of the step of the rear bottom panel. The third cross member is connected to the respective rear ends of the rear frame members. The Japanese Laid-open Utility Model Publication No.58-45173, published in 1983, discloses a rear underbody structure for automobiles comprising a pair of side sills each being of a generally closed cross-section and extending in a direction longitudinally of the automobile, said side sills being spaced apart from each other in a direction widthwise of the automobile, a suspension member connected at opposite ends to the respective side sills and having a generally channel-shaped cross-section opening towards the front of the automobile, a fuel tank accommodated within the hollow of the channel-sectioned suspension member, a rear frame connected at a front end with the suspension member, and a suspension arm supported for pivotal movement so as to extend from a portion adjacent the joint between each side sill and the suspension member. According to the latter prior art rear underbody structure discussed above, while the rigidity of the suspension member surrounding the fuel tank has been improved, no consideration has been paid to the physical strength and the rigidity of the rear frame relative to an automobile collision, during which the rear frame may receive considerable impacts. In addition, this prior art rear underbody structure cannot exhibit a satisfactory frame rigidity against forces transmitted through the suspension arm in a direction longitudinally, widthwise and vertically of the automobile. Accordingly, the present invention has been devised with a view to substantially eliminating the above discussed problems inherent in the prior art rear underbody structure for automobiles and has for its essential object to provide an improved rear underbody structure for automobiles which is rigid enough to substantially avoid any possible crush of a rear compartment of the automobile in the event of automobile collision. This object is solved in accordance with the present invention by an underbody structure for automobiles according to claim 1. In order to accomplish the above described object, the rear underbody structure according to a preferred embodiment of the present invention comprises a pair of side sills each being of a generally closed cross-section and extending in a direction longitudinally of the automobile, said side sills spaced apart from each other in a direction widthwise of the automobile, a rear frame member connected end-to-end with the associated one of the side sills so as to extend in a direction rearwardly of the automobile, and a first cross member connected at its opposite end to the rear frame members, respectively, so as to extend widthwise of the automobile in the same plane in which the rear frame members lie, and a second cross member connected at its opposite ends to the rear frame members, respectively, so as to extend widthwise of the automobile in said same plane, said second cross member being positioned forwardly of the first cross member with respect to the automobile. The opposite ends of the first cross member are rigidly connected to respective rear ends of the associated rear frame members so as to complete a so-called perimeter structure. Preferably, corner members may be used to connect the opposite ends of the first cross member and the respective rear ends of the associated rear frame members together. With these corner members interposed between the ends of the first cross member and the rear ends of the rear frame members, each of the corner members extends generally diagonally between the first cross member and the adjacent rear frame member. Preferably, each of the rear frame members may be in the form of a frame member having a substantially closed cross-section. Similarly, the first cross member may also be in the form of a frame having a substantially closed cross-section. Alternatively, the closed cross-sectional feature of any one of the rear frame members and the first cross members may be accomplished by employing a frame member of generally U-shaped cross-section in combination with a side plate member rigidly bolted or welded at its opposite sides to the U-sectioned frame member so as to close the opening of the U-sectioned frame member. The rear underbody structure may further comprises a suspension support member which may be fitted to the rear frame members at respective locations in the vicinity of the associated joints between the first cross member and the rear frame members. This and other objects and features of the present invention will become clear from the following description taken in conjunction with a preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of an automobile rear underbody structure embodying the present invention; Fig. 2 is is a perspective view, on an enlarged scale, of a portion of the rear underbody structure, showing the connection between one of rear frame members and a first cross member; Fig. 3 is a top plan view of that portion of the rear underbody structure shown in Fig. 2; Fig. 4 is a cross-sectional view taken along the line IV-IV in Fig. 3; Fig. 5 is a perspective view of another portion of the rear underbody structure, showing the connection between the first cross member and one of reinforcement frames; Fig. 6 is a cross-sectional view taken along the line VI-VI in Fig. 5; Fig. 7 is a top plan view of the rear underbody structure, showing the layout of a differential gear unit in relation to a rear wheel suspension system; Fig. 8 is a cross-sectional view taken along the line VIII-VIII in Fig. 7; and Fig. 9 is a fragmentary perspective view, showing the connection between a portion of the rear underbody structure and one of rear wheel suspension members. Referring first to Figs. 1 to 6, an automobile rear underbody structure shown therein comprises a pair of side sills of generally closed cross-section, generally identified by 1 and spaced apart from each other in a direction widthwise of an automobile body structure, and a pair of rear frame members also generally identified by 2 and connected rigidly with the associated side sills 1 in an end-to-end butted fashion so as to extend therefrom in a direction rearwardly of the automobile body structure. The rear frame members 2, which may be in the form of a frame having a generally closed cross-section and which are spaced apart from each other in a direction widthwise of the automobile body structure, have respective center pillars 10 and respective rear pillars 11 mounted thereon and secured at 2a and 2b so as to extend upwardly therefrom, said center and rear pillars 10 and 11 being spaced from each other in a direction longitudinally of the automobile body structure. It is to be noted that the joint 2a between each of the center pillars 10 and the associated rear frame member 2 is defined in the vicinity of the joint between the side sill 1 and the rear frame member 2 while the joint 2b between each of the rear pillars 11 and the associated rear frame member 2 is defined at a location generally intermediate of the length of the rear frame member 2. The rear frame members 2 also has respective suspension towers 12 rigidly mounted thereon, a lower end of each of said suspension towers 12 being rigidly jointed at 2c to a rear end of the associated rear frame member 2 adjacent the side sill 1. The rear frame members 2 are connected together by means of first and second cross members 3 and 7 extending transversely or widthwise of the automobile body structure. Each of said cross members 3 and 7 may be in the form of a frame having a generally closed cross-section. The second cross member 7 has its opposite ends rigidly secured to generally intermediate portions of such rear frame members 2 in the vicinity of the joints 2b while the first cross member 3 has its opposite ends rigidly connected to the respective rear ends of the rear frame members 2, thereby completing a so-called "perimeter structure". As best shown in Figs. 2 and 3 illustrating the details of connection between each of the opposite ends of the first cross member 3 and the rear end of the adjacent rear frame member 2, a corner frame 3a of generally U-shaped cross-section opening in a direction rearwardly of the automobile body structure is employed for each rear frame member 2 to render the joint between the rear end of the rear frame member 2 and a corresponding end of the first cross member 3 to represent a shape generally similar to the shape of a chamfered or beveled corner. In describing the details of the joints between the opposite ends of the first cross member 3 and the respective rear ends of the rear frame members 2, reference will be hereinafter made to only one of the corner members 3a for the sake of brevity. Referring still to Figs. 2 and 3, at least the rear end of each of the rear frame members 2 so far illustrated is of a generally U-shaped cross-section having an upright wall 2e and upper and lower walls 2f and 2g and opening in a direction laterally outwardly of the automobile body structure. Similarly, each of the first cross member 3 and the associated corner member 3a is of a generally U-shaped cross-section having an upright wall 3x or 3ax and upper and lower walls 3y or 3ay and 3z or 3az and opening in a direction rearwardly of the automobile body structure. The corner member 3a is rigidly connected at the opposite ends thereof to the first cross member 3 and the rear frame member 2, respectively, while extending generally slantwise between one of the opposite ends of the cross member 3 adjacent thereto and the rear end of the rear frame member 2 in a manner which will now be detailed. The upright wall 3ax of the corner member 3a has butt-welded at one end thereof to the upright wall 3x of the first cross member 3 and at the opposite end to the upright wall 2e of the rear frame member 2 while the opposite ends of each of the upper and lower walls 3ay and 3az of the corner member 3a are held in overlapping relation with, and rigidly connected or otherwise lap-welded to, the respective ends of the first cross member 3 and the rear frame member 2. While each of the corner members 3a are connected to the first cross member 3 and the adjacent rear frame member 2 in the manner as hereinabove described, the rear frame members 2, the corner members 3a and the first cross member 3 are all reinforced by a plurality of generally U-sectioned reinforcement frames generally identified by 5. Each of these U-sectioned reinforcement frames 5 has a cross sectional dimension somewhat undersized relative to the cross sectional dimension of each of the rear frame members 2, the corner members 3a and the first cross member 3 so that the U-sectioned reinforcement frames 5 can be accommodated within the respective hollows of the members 2, 3a and 3 thereby to render the members 2, 3a and 3 to have a generally closed cross-section as best shown in Figs. 4 and 5. With these reinforcement frames 5 accommodated within the respective hollows of the members 2, 3a and 3, the reinforcement frames 5 are spot-welded to the members 2, 3a and 3 at preselected locations. The reinforcement frames 5 may be connected together in a direction lengthwise thereof in a butted fashion. However, some of the reinforcement frames 5 which are identified by 5a and 5b in Fig. 3 in association with each corner member 3a are so bent as to extend across the joint between the corner member 3a and the adjacent rear frame member 2 and the joint between the corner member 3a and the first cross member 3, respectively, as clearly shown by the phantom lines in Fig. 3. The openings of those rear ends of the rear frames members 2 may be closed to represent a generally closed cross-section when a tire housing inner 6 is secured thereto. The rear underbody structure also comprises a pair of longitudinal reinforcement frames 8 spaced apart from each other in a direction widthwise of the automobile body structure and extending rearwardly from the first cross member 3 in generally parallel relationship. Each of these reinforcement frames 8 has one end rigidly connected, while received in the hollow of one of the reinforcement frames 5, to the first cross member 3 at a location adjacent the corresponding end of the first cross member 3, that is, adjacent the joint between the first cross member 3 and the adjacent corner member 3a, as best shown in Figs. 5 and 6. Each of the reinforcement frames 8 is of a generally U-shaped cross-section opening upwardly and having a pair of longitudinal flanges 8a for spot-welding lying in a plane generally parallel to the upper wall 3y of the first cross member 3. Referring to Figs. 1, 2, 5 and 6, a floor panel or boot panel 9 is laid over the rear underbody structure. The boot panel 9 so far shown is of two-piece construction including rear and front panel members 9a and 9b, the rear panel member 9a being mounted on, and spot-welded to, the first and second cross members 3 and 7, the rear ends of the rear frame members 2 and the longitudinal reinforcement frames 8. The front panel member 9b has a tunnel defined therein below which a propeller shaft (not shown) extends in a direction longitudinally of the automobile body structure and is mounted on frame members including front end portions of the rear frame members 2 and the side sills 1 and is placed at a level generally below the level of the rear panel member 9a. It is to be noted that the rear panel member 9a may define the bottom of a rear trunk room and is therefore formed with a circular recess, shown by 14 in Fig. 1, located between the longitudinal reinforcement frames 8 and constituting a tire casing. Alternatively, the boot panel may be of one-piece construction, in which case a step is provided in the boot panel at a location aligned with the first cross member 7. Referring to Figs. 1 and 7 to 9, positioned below the rear frame members 2 is a suspension support member 13 having four fitting corners 13a integral therewith. Two of the four fitting corners 13a which are located on one side facing the front of the automobile body structure, that is, the front fitting corners 13a, are elastically yieldably connected to the respective joints between the rear frame members 2 and the first cross member 7 in a manner which will now be described with particular reference to Fig. 8 showing only one of the front fitting corners 13a. As shown in Fig. 8, each of the front fitting corners 13a of the suspension support member 13 has an elastic mount including an inner sleeve 17, an outer sleeve 16 and a rubber bushing 14' assembled together with the rubber bushing 14' positioned around the inner sleeve 17 so as to fill up an annular space between the inner and outer sleeves 17 and 16. Each front fitting corner 13a is elastically yieldably connected to the joint between the second cross member 7 and the adjacent rear frame member 2 through a fitting bolt 15 passing through the inner sleeve 17 of the elastic mount so as to substantially isolate vibrations which would be transmitted from the suspension support member 13 to the automobile body structure. The other two of the four fitting corners 13a which are located on the opposite side facing the rear of the automobile body structure, that is, the rear fitting corners 13a, are also elastically yieldably connected to the respective joints between the rear frame members 2 and the first cross member 3 in a manner similar to that described in connection with the front fitting corners 13a. In the vicinity of each of the rear fitting corners 13a of the suspension support member 13, there is provided a suspension mechanism, generally identified by 19, for the support of a corresponding automobile rear wheel. This suspension mechanism 19 for each of the automobile rear wheels 18 includes an upper lateral link 21 having an inner end pivotally connected to the suspension support member 13 through a bracket 20 mounted thereon at a location forwardly of the adjacent rear fitting corner 13a and also having an outer end pivotally connected to an axle hub for the adjacent rear wheel 18; a trailing am 23 having a front end pivotally connected to the suspension support member 13 through a bracket 22 mounted thereon at a location rearwardly of the adjacent front fitting corner 13a and also having an outer end pivotally connected to the axle hub for the adjacent rear wheel 18; a front lateral link 24 having an inner end pivotally connected to the suspension support member 13 through a support member secured thereto at a rear portion of the suspension support member 13 and also having an outer end pivotally connected to the axle hub for the adjacent rear wheel 18; and a rear lateral link 25 having an inner end pivotally connected to a substantially intermediate portion of the suspension support member 13 and also having an outer end pivotally connected to the axle hub for the adjacent rear wheel 18. A differential gear unit identified by 26 is supported by the suspension support member 13 through a plurality of bolts and nuts 27 secured to a rearmost portion of the suspension member 13. As is well known to those skilled in the art, the differential gear unit 26 has an input shaft 28, drivingly coupled with an automotive power plant through the propeller shaft, and output stud shafts 29 extending generally perpendicular to the input shaft 28 in a direction away from each other. The output stud shafts 29 are in turn drivingly coupled via universal joints 30 with the associated rear wheels 18 through respective swimming shafts 31 so that the drive produced from the power plant can be transmitted to the rear wheels 18. The present invention having been fully described, it will be readily understood that, since the rear frame members and the cross members are connected together to provide the perimeter structure, the rigidity of the rear underbody structure can be advantageously increased enough to substantially minimize a crush of a rear end portion of the automobile body structure upon the automobile collision. Specifically, the provision of the longitudinal reinforcement frames extending rearwardly from the first cross member is advantageous in that the longitudinal reinforcement frames may undergo deformation to absorb shocks and impacts which would be imposed on the rear portion of the automobile body structure during the automobile collision, thereby to lessen the transmission of the impacts to the perimeter structure. The longitudinal reinforcement frames may also minimize the crush of the rear trunk room when impacts produced upon the automobile collision are relatively small. It is to be noted that the physical strength and the rigidity of the rear underbody structure according to the present invention can be chosen as desired by suitably selecting the wall thickness of the U-sectioned reinforcement frames accommodated within the hollows of the rear frame members and the first cross member. Also, since the suspension support member are supported with the rear fitting corners connected to the joints between the rear frame members and the first cross member, loads which would be imposed on the fitting corners of the suspension support member through the lateral links in respective directions up and down, front and rear and left and right, and a force such as a reactive force produced by a torque in the differential gear unit, can be advantageously distributed to the rear frame members 2 and the first cross member. Moreover, since the joints between the rear frame members and the first cross member are firm and rigid, the suspension support member can be supported in a stabilized fashion, thereby contributing to the increased rigidity of the rear underbody structure. Furthermore, since the front fitting corners of the suspension support member are connected to the joint between the side sills and the rear frame members, possible external force which would act on the front fitting corners of the suspension support member can be advantageously distributed to the rear frame members and the second cross member, thereby contributing to the further increased rigidity of the rear underbody structure. Although the present invention has fully been described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims.

Claims: An underbody structure for an automotive vehicle which comprises: a pair of side sills (1) each each being of a generally closed cross-section and extending in a direction longitudinally of the automotive vehicle, said side sills (1) being spaced apart from each other in a direction widthwise of the automotive vehicle; a rear frame member (2) for each side sill (1), said rear frame member (2) being connected with the associated side sill (1) so as to extend in a direction rearwardly of the automotive vehicle; and a first cross member (3) connected at its opposite ends to the rear frame members (2), respectively, so as to extend widthwise of the automotive vehicle in the same plane in which the rear frame members (2) lie; characterized in that each of the opposite ends of the first cross member (3) is connected to the rear end of the adjacent rear frame member (2) through a respective corner frame (3a); and a reinforcement frame (5) is provided in the first cross member (3) and the corner frames (3a) on respective sides of the first cross member (3). The underbody structure according to claim 1, characterized in that a second cross member (7) is provided which is connected at its opposite ends to the rear frame members (2), respectively, so as to extend widthwise of the automotive vehicle in the same plane in which the rear frame members (2) lie, the second cross member (7) being positioned forwardly of the first cross member (3) with respect to the automotive vehicle. The underbody structure as claimed in Claim 1, characterized in that the rear frame member (2) is connected with the associated side sill (1) in an end-to-end butted fashion. The underbody structure as claimed in Claim 1, characterized in that the rear frame member (2) extends to a position in the vicinity of a corresponding center pillar (10) and has a front end jointed to the center pillar (10). The underbody structure as claimed in Claim 1, characterized in that a rear end of the rear frame member (2) remote from the associated side sill (1) is connected to a suspension tower (12). The underbody structure as claimed in Claim 2, characterized in that it further comprises a boot floor member (9) having a front portion (9b) and a rear portion (9a), said front portion (9b) being positioned at a level lower than the rear portion (9a) thereby defining a step therebetween and having a tunnel defined therein for the passage of a propeller shaft thereunder, and that the second cross member (7) is connected in the vicinity of the step of the boot floor member (9) and has its opposite ends connected to respective rear ends of the rear frame members (2) remote from the associated side sills (1). The underbody structure as claimed in Claim 6, characterized in that one edge of the rear portion (9a) of the boot floor member (9) adjacent the step is connected to the first cross member (3). The underbody structure as claimed in Claim 6, characterized in that each of the rear frame members (2) and the first cross member (3) is constituted by a frame member of a generally closed cross-section. The underbody structure as claimed in Claim 1, characterized in that a portion of the first cross member (3) adjacent the corresponding corner frame (3a) is overlapped by a portion of this corresponding corner frame (3a) at the joint therebetween. The underbody structure as claimed in Claim 1, characterized in that it further comprises at least one longitudinal reinforcement frame (8) connected at one end to the first cross member (3) so as to extend rearwardly of the automobile vehicle. The underbody structure as claimed in Claim 6 and 10, characterized in that the longitudinal reinforcement frame (8) has a cross-sectional shape generally similar to the shape of an inverted figure "U" having a pair of longitudinal flanges (8a), the longitudinal reinforcement frame (8) representing a generally closed cross-section when the rear portion (9a) of the boot floor member (9) is secured to the longitudinal flanges (8a) from above. The underbody structure as claimed in Claim 1, characterized by further comprising a suspension support member (13) fitted in the vicinity of joints between the rear frame members (2) and the second cross member (7) and/or the first cross member (3). The underbody structure as claimed in Claim 12, characterized in that the suspension support member (13) is fitted in the vicinity of the joints between the rear frame members and the first (3) and second (7) cross members, and that the resepctive cross member (3, 7) is connected at its opposite ends in the vicinity of joints at which suspension towers (13) are connected.

Inventors: ASOH, SEIICHI
Applicants: MAZDA MOTOR CORPORATION
Application year: 1988
Main classification: B62D25
IPC codes: B62D21, B62D25