Patent Publication Number: US-2006017294-A1

Title: Bumper beam attachment structure for vehicle

Description:
FIELD OF THE INVENTION  
      The present invention relates to a bumper beam attachment structure including a bumper beam adapted to be attached to the body of a vehicle via extension members for absorbing an impact force applied to the bumper beam.  
     BACKGROUND OF THE INVENTION  
      Bumper beam attachment structures including a bumper beam attached to right and left side frames of the vehicle body via shock-absorbing extension members are known. One example of such known bumper beam attachment structures is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2001-58550.  
      The disclosed bumper beam attachment structure, as shown here in  FIG. 16 , includes a bumper beam  315  attached via a shock-absorbing extension member  314  to each of the right and left rear side frames (left one  311  being shown) of a vehicle body. For attachment, the rear side frame has a reinforcement panel  312  connected by welding to a rear end thereof, and a retaining member  313  connected by welding to the reinforcement panel  312 . The extension member  314  is connected to the retaining member  313  by a screw fastener (not designated) so that the bumper beam  315  is attached to the left rear side frame  311  of the vehicle body. The extension member  314  is formed from a sheet metal and has a generally box-shaped configuration with one side  316  open and attached by welding to the bumper beam  315 .  
      With the bumper beam attachment structure  310  thus arranged, when the bumper beam  315  is subjected to an impact force applied from the back of the vehicle body, opposite sidewalls  317  of the box-shaped extension member  314  undergo buckling to thereby absorb the impact force before the impact force is transmitted to the rear side frame  311 . In this instance, however, since the impact force acts in a direction substantially parallel to respective general planes of the sidewalls  317 , the sidewalls  317  show a great resistance against yielding with the result that a desired shock-absorbing effect cannot be attained. Furthermore, the buckling of the sidewalls  317  occurs suddenly but not gradually, making it difficult to achieve a smooth sock-absorbing operation.  
      Some sophisticated bumper beams have an additional sock-absorbing member attached to a front side thereof for the purpose of absorbing a relatively small impact force by the bumper beam itself when the vehicle body encounters a minor collision, for example. One example of such bumper beams is disclosed in Japanese Patent Laid-open Publication (JP-A) No. 2004-155313.  
      The disclosed bumper beam, as shown here in  FIG. 17 , includes a bumper beam body  321 , a sock-absorbing member  322  attached to a front side of the bumper beam body  321 , and a bumper face  323  covering the front side of the bumper beam body  321  including the sock-absorbing member  322 . The sock-absorbing member  322  is so constructed as to yield or deform when subjected to a relatively small impact force. To this end, the sock-absorbing member  322  has a hollow structure blow-molded from a synthetic resin material such as polypropylene or polyethylene. The hollow sock-absorbing member  322  includes a base portion  324  attached to the bumper beam body  321 , a dome-like front portion  325  projecting forward from the base portion  324 , and two reinforcement rib  326 ,  326  extending from the base portion  324  to the backside of the front portion  325 .  
      With this arrangement, when the bumper beam  300  is subjected to a relatively small impact force at a light or minor collision with an obstacle  329 , the reinforcement ribs  326  of the sock-absorbing member  322  serve to hinder smooth yielding or deformation of the dome-like front portion  325 . Depending on a direction of collision relative to the obstacle  322 , the sock-absorbing member may be subjected to a rotational moment or torque tending to twist the bumper beam body  321  about a joint between itself and the vehicle body. This may result in a deformation or damage on the bumper beam body  321  and an extension member disposed at the joint between the bumper beam body  321  and the vehicle body.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a vehicle bumper beam attachment structure including an extension member which is capable of collapsing smoothly to ensure smooth and efficient shock-absorbing operation when a bumper beam is subjected to an impact force.  
      Another object of the invention is to provide a vehicle bumper beam attachment structure including a shock-absorbing member attached to the front face of a bumper beam and deformable to absorb a relatively small impact force without involving deformation or damage on the bumper beam or the extension member when the vehicle encounters a light or minor collision with an obstacle.  
      According to the invention, there is provided a bumper beam attachment structure for a vehicle, comprising: a bumper beam extending in a widthwise direction of the vehicle at a front end or a rear end of the vehicle; and an extension member connected to the bumper beam at a position laterally offset from a longitudinal centerline of a body of the vehicle and adapted to be connected to the vehicle body to attach the bumper beam to the vehicle body. The extension member is formed from an aluminum extruded hollow profile and includes: a rear wall for being in abutment with the vehicle body and having a first end and a second end opposite the first end, the first end being located closer to the longitudinal axis of the vehicle body than the second end does; an inner sidewall extending convexly from the first end of the rear wall toward the bumper beam; a central sidewall extending from a central portion of the rear wall in substantially the same direction as the inner sidewall; a first front wall connecting distal ends of the inner sidewall and the central sidewall and being in contact with the bumper beam; a branched sidewall branched off from the central sidewall and extending convexly toward the bumper beam; an outer sidewall extending from the second end of the rear wall toward the bumper beam; and a second front wall connecting distal ends of the branched sidewall and the outer sidewall and being in contact with the bumper beam.  
      With this arrangement, when the bumper beam is subjected to an impact force, there is created a rotational moment or torque tending to deform or collapse the extension member at about a junction between the rear wall and the inner sidewall. In this instance, because the inner sidewall and the branched sidewall have curvilinear configurations extending convexly toward the bumper beam, these sidewalls can readily cause buckling or bending in a lateral outward direction away from the longitudinal centerline of the vehicle body. By way of this buckling, the extension member can absorb the impact force to thereby block transmission of the impact force to the vehicle body. Furthermore, because the branched sidewall is branched off from an intermediate portion of the central sidewall, the force acting on the branched sidewall is partly born or retained by the central sidewall. This is effective to protect the vehicle body from damage.  
      Preferably, the rear wall has an integral end extension projecting outward from the inner sidewall in a lateral inward direction toward the longitudinal centerline of the vehicle body and is adapted for abutment with the vehicle body. By virtue of the end extension projecting from the inner sidewall in a lateral inward direction, the rear wall can retain the force applied via the inner sidewall so that the vehicle body is protected from damage or deformation.  
      The rear wall and the first and second front walls may have a first thickness, and the inner sidewall, the central sidewall and the branched sidewall have a second thickness, the second thickness being smaller than the first thickness. The sidewalls of a smaller thickness than the rear and front walls can promote yielding or buckling deformation with improved smoothness. Preferably, the second thickness is about two-third of the first thickness.  
      The bumper beam is preferably formed from an aluminum extruded hollow profile. An extruded hollow profile of aluminum alloy or magnesium alloy can be used in place of the aluminum extruded hollow profile.  
      The bumper beam attachment structure may further comprise a shock-absorbing member of elongated hollow structure disposed on a front face of the bumper beam with its longitudinal axis extending in the widthwise direction of vehicle for absorbing a relative small impact force applied to the bumper beam. The shock-absorbing member has a flat rear wall attached to the front face of the bumper beam, an arch-shaped front wall extending between upper and lower edge of the rear wall, and two ribs curved arcuately and extending convexly in a branched fashion from a vertical central portion of the rear wall to an inner surface of the arch-shaped front wall.  
      With this arrangement, when the shock-absorbing member is subjected to a relatively small impact force, the convexly curved ribs arranged to extend in a branched fashion can readily undergo uniform deformation or yielding and do not cause a difference in the amount of yielding between the two ribs. As the uniform yielding of the convexly curved ribs further continues, the shock-absorbing member becomes collapsed to the extent that a central portion of the front wall lies substantially flat in a vertical plane. During that time, the impact force is substantially absorbed through deformation or collapsing of the central portion of the front wall and the curved ribs of the shock-absorbing member. Since the deformed central portion of the front wall is substantially perpendicular to the direction of the impact force, there is no risk to generate a rotational moment or torque tending to tilt the bumper beam upward relative to the vehicle body. Thus, the vehicle body is kept free from deformation or damage.  
      Preferably, the bumper beam has a hollow structure and includes a reinforcement rib disposed in an internal space of the hollow bumper beam and extending between a front wall and a rear wall of the bumper beam along the length of the bumper beam. The reinforcement rib and the central portion of the rear wall of the shock-absorbing member lie in the same horizontal plane. The reinforcement rib acts to support a base portion of the branched ribs, allowing the ribs to bend or deform smoothly toward the bumper beam when subject to the impact force.  
      In one preferred form of the invention, the shock-absorbing member has a generally D-shaped cross section. Alternatively, the shock-absorbing member may have a semi-circular cross section.  
      The rear wall, the arch-shaped front wall and the ribs of the shock-absorbing member may have the same thickness. As an alternative, the rear wall and the arch-shaped front wall of the shock-absorbing member may have a first thickness, and the ribs have a second thickness, the second thickness being smaller than the first thickness. The second thickness is preferably about two-third of the first thickness. Use of the thinner ribs provides improved smoothness of the shock-absorbing deformation or yielding of the ribs, which enables transmission of the impact force to the bumper beam without causing local stress concentration on the vehicle body.  
      The shock-absorbing member may be formed from an aluminum extruded hollow profile or molded of a synthetic resin. Furthermore, the rear wall of the shock-absorbing member may be welded or adhesive-bonded to the end face of the bumper beam. Yet, the ribs of the shock-absorbing member may be united together at one end so as to form a straight stem projecting perpendicularly from the vertical central portion of the rear wall, in which instance, a corner formed between the rear wall and the straight stem on each side of the straight stem is preferably rounded. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A preferred structural embodiment of the present invention will be described in detail herein below, by way of example only, with the reference to the accompanying drawings, in which:  
       FIG. 1  is a perspective view of a motor vehicle incorporating a bumper beam attachment structure according to the present invention;  
       FIG. 2  is an exploded perspective view of the bumper beam attachment structure forming a part of a rear bumper assembly of the motor vehicle;  
       FIG. 3  is a top plan view of a left extension member of the bumper beam attachment structure;  
       FIG. 4  is a view similar to  FIG. 3 , but showing a right extension member of the bumper attachment structure;  
       FIG. 5  is a fragmental perspective view of a bumper beam with an impact absorption member mounted on a front face thereof,  
       FIG. 6  is a plan view of the bumper beam attachment structure;  
       FIG. 7  is a cross-sectional view taken along line VII-VII of  FIG. 6 ;  
       FIG. 8  is a cross-sectional view taken along line VIII-VIII of  FIG. 6 ;  
       FIGS. 9A  to  9 C are schematic plan views illustrative of the operation of the bumper beam attachment structure;  
       FIGS. 10A and 10B  are cross-sectional views illustrative of a problem that may occur due to the structure of a comparative shock-absorbing member when an impact force is applied to a bumper beam attachment structure including the comparative shock-absorbing member;  
       FIGS. 11A and 11B  are cross-sectional views similar to  FIGS. 10A and 10B , respectively, but showing operation of the sock-absorbing member according to the invention;  
       FIGS. 12A  to  12 C are schematic plan views illustrative of a problem that may occur due to the structure of a comparative extension member when an impact force is applied to a bumper beam attachment structure including the comparative shock-absorbing member;  
       FIGS. 13A  to  13 C are plan views similar to  FIGS. 12A  to  12 C, respectively, but showing operation of the extension member according to the invention;  
       FIG. 14  is an end view of a modified shock-absorbing member according to the invention;  
       FIG. 15  is an end view of another modified shock-absorbing member according to the invention;  
       FIG. 16  is an end view of still another modified shock-absorbing member according to the invention;  
       FIG. 17  is a cross-sectional view showing a conventional bumper beam attachment structure; and  
       FIG. 18  is a cross-sectional view showing the structure of a conventional shock-absorbing member attached to a front face of the bumper beam. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring now to the drawings and  FIG. 1  in particular, there is shown in perspective a motor vehicle  10  incorporating a bumper beam attachment structure  30  according to an embodiment of the present invention. The bumper beam attachment structure  30  generally comprises a bumper beam  40  formed from an aluminum extruded hollow profile and extending in a widthwise direction of the vehicle  10  at a rear end of the vehicle  10 , right and left extension members  33 R and  33 L connected to the bumper beam  40  and adapted to be connected to a body of the vehicle  10  for absorbing an impact force applied to the bumper beam  40  before the impact force is transmitted to the vehicle body, and a sock-absorbing member  41  disposed on a front face of the bumper beam  40  for absorbing a relative small impact force through deformation or collapsing of the sock-absorbing member  41 . The bumper beam attachment structure  30  is covered by a rear bumper face  36 . Obviously, the bumper beam attachment structure  30  may be employed in a bumper assembly disposed at the front end of the vehicle  10 . The extruded hollow profile is also called “extruded hollow shape” or “extruded hollow section”.  
       FIG. 2  is an exploded view showing the relationship between structural components of the bumper beam attachment structure  30  and the vehicle body. As shown in this figure, the bumper beam  40  is attached via the right and left extension members  33 R,  33 L to respective rear end walls  32 ,  32  of right and left rear side frames  31 ,  31  by means of two pairs of bolts  39 . The rear side frames  31  form a part of the vehicle body. For attachment to the bumper beam  40 , the rear side frames  31  each have a pair of nuts  37  welded to an inner surface of a respective one of the rear end walls  32 , and a pair of through-holes  38  formed in each end wall  32  in axial alignment with threaded holes of the nuts  37  for the passage therethrough of threaded shanks  39   a  of the bolts  39 .  
      The bumper beam  40  formed from an aluminum extruded hollow profile has a straight central portion  42  and opposite end portions  43 ,  43  bent at an angle to the central portion  42  in a forward direction of the vehicle body. The bumper beam  40  has a front face  46  on which the shock-absorbing member  41  for absorbing a relatively small impact force is disposed at the central portion  42  of the bumper beam  40 . The opposite end portions  43 ,  43  are used for mounting of the extension members  33 R,  33 L. For this purpose, the end portions  43  each have two holes  45 ,  45  extending from a front face to a rear face of the respective end portion  43  for the passage therethrough of the bolts  39 ,  39  when the bumper beam  40  is to be attached via the corresponding extension member  33 R,  33 L to the rear side frame  31 . The bumper beam  40  has a horizontal reinforcement rib  65  disposed centrally in an internal space of the bumper beam  40  and extending along the length of the bumper beam  40 . The reinforcement rib  65  is formed integrally with respective vertical central portions of a front wall  40   a  and a rear wall  40   b  of the bumper beam  40 . The bumper face  36  is molded from a synthetic resin such as polypropylene and has a profile substantially conforming to the profile of the bumper beam  40 .  
      The extension members  33 R,  33 L are disposed in symmetric relation to one another with respect to a longitudinal centerline L of the vehicle body. The extension members  33 R,  33 L are formed from an aluminum extruded hollow profile and connected by welding to a rear face of the bumper beam  40  at opposite end portions  43 ,  43  of the bumper beam  40  such that an axis of extrusion of each extension member  33 R,  33 L is disposed vertically. Each of the right and left extension members  33 R,  33 L are connected to the bumper beam  40  at a position laterally offset from the longitudinal centerline L of the vehicle body. The right and left extension members  33 R,  33 L are identical in structure so that only the left extension member  33 L will be described in greater detail with reference to  FIG. 3 .  
      As shown in  FIG. 3 , the left extension member  33 L has a flat rear wall  51  for being in abutment with the end wall  32  ( FIG. 2 ) of the left rear side frame  31 , an inner side wall  52  extending convexly from an inner end of the rear wall  51  toward the bumper beam  40 , a central side wall  54  extending from a central portion of the rear wall  51  in substantially the same direction as the inner sidewall  52  toward the bumper beam  40 , a first front wall  53  connecting distal ends of the inner and central sidewalls  52  and  54  and being in contact with the rear face of the end portion  43  of the bumper beam  40 , a branched sidewall  55  branched off from an intermediate portion of the central sidewall  54  and extending convexly toward the bumper beam  40 , an outer sidewall  57  extending from an outer end of the rear wall  51  toward the bumper beam  40 , and a second front wall  56  connecting distal ends of the branched sidewall  55  and the outer sidewall  57  and being in contact with the bumper beam  40 . The inner sidewall  52  is longer than the outer sidewall  57 .  
      The inner end of the rear wall  51  is located closer to the longitudinal centerline L ( FIG. 2 ) than the outer end of the rear wall  51  does. The rear wall  51  has an integral end extension  59  projecting outward from the inner sidewall  52  in a direction toward the longitudinal centerline L of the vehicle body. The end extension  59  is brought into abutment with the rear end wall  32  ( FIG. 2 ) of the left rear side frame  31  when the bumper beam  40  is attached to the left rear side frame  31  via the left extension member  33 L. The convexly curved inner sidewall  52  and the central sidewall  54  are inclined toward the longitudinal centerline L of the vehicle body, while the convexly curved branched sidewall  55  and the outer sidewall  57  are inclined in a direction laterally outward away from the longitudinal center line L of the vehicle body. The first and second front walls  53  and  56  are aligned with each other in a widthwise direction of the vehicle body, and the front walls  53 ,  56  extend at an angle to the rear wall  51 .  
      The left extension member  33 L has two pairs of aligned holes  61 ,  63  and  62 ,  64  for attachment to the left rear side frame  31  by the bolts  39 . The first pair of holes includes a hole  61  extending through the central sidewall  54  and a hole  63  extending through the rear wall  51 . The hole  61  has a larger diameter than the hole  63  and allows passage therethrough of one bolt  39 , while the hole  63  allows only passage therethrough of the threaded shank  39   a  of the same bolt  39  but blocks passage of the bolt  39  as a whole. Similarly, the second pair of holes includes a hole  62  extending through the second front wall  56  and a hole  64  extending through the rear wall  51 . The hole  62  has a larger diameter than the hole  64  and allows passage therethrough of another bolt  39 , while the hole  64  allows only passage therethrough of the threaded shank  39   a  of the bolt  39  and blocks the passage of the bolt  39  as a whole. The first and second front walls  53 ,  56  are welded to the rear face of the end portions  43  of the bumper beam  40 . The rear wall  51  has a thickness t 1 , and the sidewalls  52 ,  54  and  55  excepting the outer sidewall  57  have a thickness t 2 , which is smaller than the thickness t 1  of the rear wall  51 . This will ensure smooth deformation or collapsing of the sidewalls  52 ,  54 ,  56  when the bumper beam  40  is subjected to an impact force. The thickness t 2  is preferably about two-third of the thickness t 1 .  
       FIG. 4  shows in plan view the right extension member  33 R. As previously described, the right extension member  33 R is disposed in symmetric relation to the left extension member  33 L with respect to the longitudinal centerline L ( FIG. 2 ) of the vehicle body. Furthermore, the right extension member  33 R is identical in structure to the left extension member  33 L just described above with reference to  FIG. 3 . In view of the structural sameness, these parts, which are identical to those shown in  FIG. 3  are designated by the same reference characters, and no further description of the right extension member  33 R is needed.  
      As shown in  FIG. 5 , the shock-absorbing member  41  is formed from an aluminum extruded hollow profile and attached by welding to the front face  46  of the bumper beam  40  at the central portion  42  of the bumper beam  40  such that an axis of extrusion of the shock-absorbing member  41  and an axis of extrusion of the bumper beam  40  extend in the widthwise direction of the vehicle body. The hock-absorbing member  41  is profiled to have a substantially D-shaped cross section of uniform thickness and includes a flat rear wall  72  being in abutment with the front face of the bumper beam  40 , an arch-shaped front wall  73  extending between upper and lower edges of the rear wall  72 , and two ribs  75  and  76  curved arcuately and extending convexly in a branched fashion from a central portion of the rear wall  72  to an inner surface of the arch-shaped front wall  73 .  
       FIG. 6  is a plan view showing the bumper beam attachment structure  30  in an assembled condition with the bolts  39  ( FIG. 3 ) omitted for clarity. As shown in this figure, the bumper beam  40  is attached to the rear end of the right and left rear side frames  31 ,  31  via the right and left extension members  33 R,  33 L located on opposite end portions  43 ,  43  of the bumper beam  40  for absorbing an impact force applied to the bumper beam  40  from behind the vehicle body. The shock-absorbing member  41  is attached to the front face  46  of the bumper beam  40  at a central portion of the bumper beam  40  for absorbing a relatively small impact force applied from behind the vehicle body.  
       FIG. 7  is a cross-section taken along line VII-VII of  FIG. 6 . As shown in this figure, the shock-absorbing member  41  formed from an aluminum extruded hollow profile is attached by welding to the front face  46  of the bumper beam  40  for absorbing a relatively small impact force applied thereto. The convexly curved ribs  75 ,  76  extend in a branched fashion from the vertical central portion of the rear wall  72  to the inner surface of the arch-shaped front wall  73 . The vertical central portion of the rear wall  72  from which the ribs  75 ,  76  extend lies in the same horizontal plane as the reinforcement rib  65  of the bumper beam  40 .  
       FIG. 8  is a cross-section taken along line VIII-VIII of  FIG. 6 . As shown in this figure, the left extension member  33 L, which is connected by welding to the rear face of the bumper beam  40 , is connected to the end wall  32  of the left rear side frame  31  by means of two bolts  39  tightly threaded with the mating nuts  38 . The holes  45  formed in the bumper beam  40  have a diameter large enough to allow passage therethrough of the bolts  39 . Similarly, the holes  61 ,  62  formed respectively in the central sidewall  54  ( FIG. 3 ) and the second front wall  56  ( FIG. 3 ) of the extension member  33 L have a diameter large enough to allow passage therethrough of the bolts  39 . The two holes (only one  64  being shown in  FIG. 8 ) have a diameter larger than a diameter of the threaded shanks  39   a  of the bolts  39  but smaller than a diameter of enlarged heads of the bolts  39 . With the holes  45 ,  61 - 64  thus dimensioned, the bolts  39  can be inserted through the bumper beam  40  into the internal space of the extension member  33 L and then placed into threaded engagement with the nuts  38 . The bumper beam  40  can thus be attached to the rear side frame  31  from the rear side of the vehicle body.  
      Operation of the bumper beam attachment structure  30  will be described below with reference to  FIGS. 9A  to  9 C.  FIG. 9A  shows a condition in which an obstacle  79  is disposed behind the bumper beam  40  of the bumper beam attachment structure  30 . The obstacle  79  may be stationary such as a parked car or a building structure, or in motion such as a succeeding car. In the illustrated embodiment, the obstacle  79  is a succeeding car approaching the own car from behind.  
      When the succeeding car (obstacle)  79  collides with the rear end of the own car, as shown in  FIG. 9B , the shock-absorbing member  41  starts yielding or deforming to thereby absorb an impact force applied thereto. In this instance, if the impact force is relatively small, as experienced at a light or minor collision, the impact force is completely absorbed through deformation or collapsing of the hollow shock-absorbing member  41 . Thus, the bumper beam  40  and the extension members  33 R,  33 L remain unchanged or free from deformation. Since the damage at the minor collision is limited to the shock-absorbing member  41  and the bumper face  36 , the bumper beam attachment structure  30  can be repaired merely by replacing the damaged shock-absorbing member  41  and the bumper face  36  with new ones. Such repair requires only a limited cost and does not incur an undue increase in the maintenance cost.  
      In the case where the rear-end collision between the own car and the succeeding car  79  generates a greater impact force than in the case of minor collision shown in  FIG. 9B , shock-absorbing deformation or collapsing of the shock-absorbing member  41  is followed by deformation or yielding of the extension members  33 R,  33 L, as shown in  FIG. 9C . By virtue of the deformation of the extension members  33 R,  33 L, the impact force is substantially absorbed before it is transmitted to the rear side frames  31 . It is therefore possible to keep the rear side frames  31 ,  31  (i.e., the vehicle body) free from deformation.  
       FIGS. 10A and 10B  are cross-sectional views illustrative of a problem that may occur due to the structure of a shock-absorbing member  241  when a bumper beam attachment structure including the sock-absorbing member  241  is subjected to an impact force. The shock-absorbing member  241  is prepared for comparative purposes and hence will be hereinafter referred to as a “comparative shock-absorbing member”. Like the shock-absorbing member  41  of the invention, the comparative shock-absorbing member  241  is formed from an aluminum extruded hollow profile and has a substantially D-shaped cross section. Differing from the inventive shock-absorbing member  41 , the comparative shock-absorbing member  241  has only one straight rib  245  disposed horizontally and extending centrally across an internal space of the hollow shock-absorbing member  241 . The rib  245  lies in the same plane as the horizontal rib  65  of the bumper beam  40 .  
      When an obstacle such as a succeeding car  79  comes into light or minor rear-end collision with the own car, the comparative shock-absorbing member  241  is subjected to a relatively small impact force, such as indicated by a profiled arrow shown in  FIG. 10B . In this instance, because the straight rib  245  of the comparative shock-absorbing member  241  is in alignment with the horizontal reinforcement rib  65  of the bumper beam  65 , and because the impact force acts in a direction parallel to a common plane of the reinforcement rib  65  and the straight rib  245 , the reinforcement rib  65  acts to enhance the rigidity of the rib  245 . The thus reinforced rib  245  is now made highly rigid against yielding, so that under the effect of the impact force, the bumper beam  40  as a whole is driven to tilt upward about a joint portion between the bumper beam  40  and the rear side frames  31 , as shown in  FIG. 10B . With this upward tilting of the bumper beam  40 , a local stress concentration occurs at an upper edge of the end wall of each rear side frame  31 , which will eventually cause deformation or damaging of the rear side frame  31  the impact force is relatively small.  
      In the case of the inventive shock-absorbing member  41  shown in  FIG. 11A , the convexly curved ribs  75 ,  76  are not aligned with the reinforcement rib  65  of the bumper beam  40  but extend in a branched fashion to form an angle between each rib  75 ,  76  and a direction of the impact force. With this arrangement, when an obstacle such as a succeeding car  79  comes into light or minor rear-end collision with the own car, the shock-absorbing member  41  is subjected to a relatively small impact force as indicated by a profiled arrow shown in  FIG. 11B . In this instance, since the convexly curved ribs  75 ,  76  are arranged to extend in a branched fashion, they can readily undergo uniform deformation or yielding and do not cause a difference in the amount of yielding between the two ribs  75 ,  76 . As the uniform yielding of the convexly curved ribs  75 ,  76  further continues, the shock-absorbing member  41  becomes collapsed to the extent that a central portion of the front wall  73  lies substantially flat in a vertical plane, as shown in  FIG. 11B . During that time, the impact force is substantially absorbed through deformation or collapsing of the central portion of the front wall  73  and the curved ribs  75 ,  76  of the shock-absorbing member  41 . Since the deformed central portion of the front wall  73  is substantially perpendicular to the direction of the impact force, there is no risk to generate a rotational moment or torque tending to tilt the bumper beam  40  upward relative to the rear side frame  31 . Thus, the rear side frame  31  is kept free from deformation or damage.  
       FIGS. 12A  to  12 C are schematic plan views illustrative of a problem that may occur due to the structure of an extension member  233  when an impact force (indicated by profiled arrows) is applied to a bumper beam attachment structure  230  including the comparative shock-absorbing member  233 . The extension member  233  is prepared for comparative purposes and hence will be hereinafter referred to as a “comparative extension member”. Like the extension members  33 R,  33 L of the invention shown in FIGS.  2  to  4 , the comparative extension member  233  is formed from an aluminum extruded hollow profile and includes a rear wall  251  adapted to be connected to the rear end of a rear side frame  31 . Differing from the inventive extension members  33 R,  33 L, the comparative extension member  233  includes an inner sidewall  252  extending rectilinearly from an inner end of the rear wall  251  toward the bumper beam  40  and inclined toward a longitudinal centerline (not shown but identical to the one L shown in  FIG. 2 ) of the vehicle body, a central sidewall  255  extending perpendicularly from a central portion of the rear wall  251  toward the bumper beam  40 , an outer sidewall  257  extending rectilinearly from an outer end of the rear wall  251  and inclined in a direction laterally outward away from the longitudinal centerline of the vehicle body, and a front wall  253  interconnecting distal ends of the inner, central and outer sidewalls  252 ,  255 ,  257  and connected by welding to a rear face of the bumper beam  40 .  
      With this arrangement, when the bumper beam  40  is subjected to an impact force as indicated by profiled arrows shown in  FIG. 12A , there is created a rotational force or torque (indicated by the arrow) tending to deform or collapse the comparative extension member  233  at about a junction  258  between the rear wall  251  and the inner sidewall  252 . In this instance, because the inner sidewall  252  of rectilinear configuration shows a relatively high resistance to deformation or yielding, it occurs that the inner sidewall  252  tilts down in a lateral inward direction (left-hand direction in  FIGS. 12A and 12B ) while the original shape of a corner at the junction  258  remains unchanged, as shown in  FIG. 12C . Due to the tilting of the inner sidewall  252 , the rear side frame  31  may be damaged by the corner at the junction  258  between the inner sidewall  252  and the rear wall  251 . Furthermore, the central sidewall  255  extending perpendicular to an end face of the rear side frame  31  is hardly to become deformed under the effect of the impact force. As a result, the impact force is transmitted via the central sidewall  255  to the rear side frame  31 , causing local deformation or damage on the rear side frame  31 . Additionally, the rear wall  251  of the shock-absorbing member  233  is deformed to assume such a configuration that a portion extending between the inner sidewall  252  and the central sidewall  255  and a portion extending between the central sidewall  255  and the outer sidewall  257  are bent outward toward the bumper beam  40 . It appears clear that the comparative extension member  233  cannot achieve a desired shock-absorbing operation without involving damage on the rear side frame  31 .  
      In the case of the inventive extension member  33 L, as shown in  FIGS. 13A and 13B , the inner sidewall  52  has a curvilinear configuration extending convexly from an inner end of the rear wall  51 , the central sidewall  54  extends from a central portion of the rear wall  51  in substantially the same direction as the inner sidewall  52  toward the bumper beam  40 , and distal ends of the inner sidewall  52  and the central sidewall  54  are connected together by the first front wall  53 . Furthermore, the branched sidewall  55  branched off from an intermediate portion of the central sidewall  54  has a curvilinear configuration extending convexly toward the bumper beam  40 , the outer sidewall  57  extends from an outer end of the rear wall  51  toward the bumper beam  40 , and distal ends of the branched sidewall  55  and the outer sidewall  57  are connected together by the second front wall  56 . The rear wall  51  has an end extension  59  projecting outward from the inner sidewall  52  in a lateral inward direction toward the longitudinal centerline (not shown but identical to the one L shown in  FIG. 2 ) of the vehicle body. The end extension  59  of the rear wall  51  is also in abutment with the end face of the rear side frame  31 .  
      With this arrangement, when the bumper beam  40  is subjected to an impact force as indicated by profiled arrows shown in  FIG. 13A , there is created a rotational force or torque (indicated by the arrow) tending to deform or collapse the inventive extension member  33 L at about a junction  58  between the rear wall  51  and the inner sidewall  52 . In this instance, because the inner sidewall  52  and the branched sidewall  55  have curvilinear configurations extending convexly toward the bumper beam  40 , the sidewalls  52 ,  55  can readily cause buckling or bending in a lateral outward direction, as indicated by the arrows shown in  FIG. 13C . By way of this buckling, the extension member  33 L can absorb the impact force to thereby block transmission of the impact force to the rear side frame  31 . Furthermore, owing to the end extension  59  projecting from the inner sidewall  52  in a lateral inward direction, the rear wall  51  can retain the force applied via the inner sidewall  52 . By thus providing the end extension  59 , it is possible to protect the rear side frame  31  from damage or deformation. Additionally, because the branched sidewall  55  is branched off from an intermediate portion of the central sidewall  54 , the force acting on the branched sidewall  55  is partly retained by the central sidewall  54 . This is also effective to avoid damage which would otherwise occur at the rear side frame  31 .  
      FIGS.  14  to  16  show modified forms of the shock-absorbing member according to the invention. The first modified shock-absorbing member  81  shown in  FIG. 14  differs from the shock-absorbing member  41  of  FIG. 7  only in that a rear wall  82  and an arch-shaped front wall  83  have a thickness t 3  which is different from a thickness t 4  of two convexly curved ribs  85 ,  86 . The thickness t 4  of the ribs  85 ,  86  is smaller than the thickness t 3  of the rear and front walls  82 ,  83  and preferably about two-third of the thickness t 3 . By thus setting the thickness t 4  of the ribs  85 ,  86  relative to the thickness t 3  of the rear and front walls  82 ,  83 , it is possible to promote the bending of the ribs  85 ,  86  more smoothly than the shock-absorbing member  41  of uniform thickness. This will insure transmission of the impact force from a perpendicular direction to the bumper beam with improved reliability.  
      The second modified shock-absorbing member  91  shown in  FIG. 15  differs from the shock-absorbing member  41  of  FIG. 7  in that the cross-sectional shape is semi-circular rather than D-shaped. Furthermore, a rear wall  92  and an arch-shaped front wall  93  are joined together so that opposite end edges of the rear wall  92  project outward from base portions of the front wall  93  and form longitudinal flanges  99 ,  99 . Two ribs  95  and  96  extending in branched fashion from a central portion of the rear wall  92  to an inner surface of the arch-shaped front wall  93  are united together at one end so as to form a straight stem  97  projecting perpendicularly from the central portion of the rear wall  92 . Corners formed between the straight stem  97  and the rear wall  72  and located at opposite sides of the straight stem  97  are rounded. The modified shock-absorbing member  91  having a semi-circular cross section is more susceptible to deformation than the shock-absorbing member  41  of D-shaped cross section. Furthermore, the rounded corners  98  at a junction between the stem  97  and the rear wall  92  act to minimize the effect of an impact force that may be transmitted via the stem  97  to the bumper beam  40  when the branched ribs  95 ,  96  undergo deformation. This will prevent a local deformation from occurring at the bumper beam  40 . By providing the longitudinal flanges  99 ,  99 , it is possible to improve welding distortion involved in a weld connection between the rear wall  92  and the bumper beam  40 .  
      The third modified shock-absorbing member  101  shown in  FIG. 16  differs from the shock-absorbing member  41  of  FIG. 7  only in that the shock-absorbing member  101  is molded from a synthetic resin such as polypropylene or polyethylene into a hollow structure including a flat rear wall  12 , an arch-shaped front wall  103  extending between opposite end edges of the rear wall  102 , and two ribs  105 ,  106  curved arcuately and extending convexly in a branched fashion from a central portion of the rear wall  102  to an inner surface of the arch-shaped front wall  103 . The resin-molded shock-absorbing member  101  is attached by bonding to the bumper beam  40  with an adhesive layer  109  disposed between the rear wall  102  of the shock-absorbing member  101  and a front face  46  of the bumper beam  40 . The adhesive layer  109  may comprise a two-sided adhesive tape. The shock-absorbing member  101  attached by bonding to the bumper beam  40  can be replaced easily as compared to the shock-absorbing members  41 ,  81 ,  91  attached by welding to the bumper beam  40 . This will improve the maintenance efficiency and cost.  
      Although in the illustrated embodiment, the extension members  33 R,  33 L is formed from an aluminum extruded hollow profile, an extruded hollow profile of aluminum alloy or magnesium alloy can be used to form the extension members  33 R,  33 L. Furthermore, the aluminum extruded hollow profile used for forming the bumper beam  40  may be replaced with an extruded hollow profile of aluminum alloy or magnesium alloy. Alternatively, the aluminum extruded hollow profile used for forming the shock-absorbing members  41 ,  81 ,  91  can be replaced with an extruded hollow profile of aluminum alloy or magnesium alloy.  
      Obviously, various minor changes and modifications are possible in the light of the above teaching. It is to be understood that within the scope of the appended claims the present invention may be practiced otherwise than as specifically described.