Patent Publication Number: US-10787202-B2

Title: Vehicle chassis front section structure

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle chassis front section structure of an automobile or the like. 
     BACKGROUND ART 
     It is common practice that a wheel housing lower member is set to have a section having a relatively low strength in advance so as to absorb collision energy in case of a front collision of a vehicle by bringing this section to buckling. In general, a wheel housing lower member is provided with an upper curved section and a lower curved section and formed into a shape that extends in a vehicle longitudinal direction. Here, the upper curved section and the lower curved section are used collectively as a buckling section. 
     In a typical structure, each of a wheel housing lower member and a wheel housing upper member to be disposed behind the wheel housing lower member generally includes an inner panel located on the inside in a vehicle widthwise direction and an outer panel located on the outside in the vehicle widthwise direction, and an upper flange section of each outer panel and a lower flange section of the corresponding outer panel are flange-coupled to each other. In this structure, the wheel housing lower member and the wheel housing upper member are disposed in a limited space below an engine hood. For this reason, each upper flange section faces a problem of a constraint on its shape so as not to interfere with other peripheral components. 
     Patent Literature 1 to 3 describes examples of related art to a flange structure around the wheel housing lower member. Patent Literature 1 describes a technique in which a reinforcement member for suppressing inward falls is provided on the inside in the vehicle widthwise direction of a front side upper extension frame, where a flange extending downward is provided to a front upper side frame while a flange extending upward is provided to the front side upper extension frame. Patent Literature 2 describes a technique for increasing and decreasing the number of bent sections in a cross-sectional shape either by displacing a position of a flange in a widthwise direction of the frame depending on the position in the longitudinal direction of the frame or by changing the cross-sectional shape depending on the position in the longitudinal direction of the frame, which also describes the flange that extends upward. Patent Literature 3 describes a technique for providing a reinforcement panel in a region surrounded by a strut tower, a hood ridge panel, and a front side vertical wall section of a cowl top panel, in which a horizontal flange is formed in front of the hood ridge panel and a flange that extends upward is formed behind the hood ridge panel. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Publication No. Hei 4-63776 
     Patent Literature 2: Japanese Patent Application Publication No. Hei 6-135355 
     Patent Literature 3: Japanese Patent Application Publication No. Hei 7-25352 
     SUMMARY OF INVENTION 
     Technical Problem 
     A front peripheral part of the wheel housing lower member is located at a position where clearances with peripheral components such as a headlight can be easily secured. Accordingly, a shape of the flange at each flange section is relatively less constrained. 
     An object of the present invention is to provide a vehicle chassis front section structure which enables stable buckling deformation of a lateral-section frame in case of a vehicle collision by optimizing shapes of flange sections thereof. 
     Means to Solve the Problem 
     To solve the problems, a vehicle chassis front section structure includes: a lateral-section frame disposed on outside in a vehicle widthwise direction of a front side frame, the lateral-section frame including a wheel housing upper member with a rear end connected to a front pillar, and a wheel housing lower member extending forward and downward from the wheel housing upper member via an upper curved section and a lower curved section of the wheel housing lower member with a front end of the wheel housing lower member located substantially at the same position in terms of a vertical direction and a longitudinal direction as a front end of the front side frame, wherein each of the wheel housing upper member and the wheel housing lower member includes an outer panel and an inner panel to be joined to each other by using upper flange sections and lower flange sections provided to the panels, respectively, such that the wheel housing upper and lower members define a continuous hollow cross-section, and each upper flange section at a front part of the wheel housing lower member is formed from an upward flange that extends upward. A stiffener that extends forward and rearward across the upper curved section is attached to interior of the wheel housing lower member, and a fragile section is formed on an inside ridge of the upper curved section and configured to induce deformation of the upper curved section in case of input of a collision load. A fragile section is formed on an inside ridge of the lower curved section of the wheel housing lower member and configured to induce deformation of the lower curved section in case of input of a collision load. 
     In accordance with the aforementioned vehicle chassis front section structure the front part of the wheel housing lower member is located at such positions that can easily secure clearances with surrounding components including a headlight and the like. Accordingly, the upper flange sections in this zone can be formed into the upward flanges that extend upward. In this way, it is possible to ensure a large section modulus and to increase the amount of impact energy absorption for the bend of the wheel housing lower member caused by the front collision. 
     In accordance with the aforementioned vehicle chassis front section structure, the wheel housing lower member can be stably bent, at the upper curved section with the fragile section being a bent point while securing the rigidity of the upper-curved section with the stiffener. 
     In accordance with the aforementioned vehicle chassis front section structure, the wheel housing lower member can be stably bent at the lower curved section with the fragile section being a bent point. 
     In the aforementioned vehicle chassis front section structure, a portion at rear of the upper curved section of the wheel housing lower member is located lateral to a damper base, and each upper flange section of the portion at the rear of the upper curved section of the wheel housing lower member is formed from a lateral flange that extends horizontally. 
     In the aforementioned vehicle chassis front section structure an engine hood is disposed above the upper bent section of the wheel housing lower member. Accordingly, it is possible to secure a clearance between a lower surface of the engine hood and the upper flange section by forming the upper flange sections into the lateral flanges. As a consequence, it is possible to secure a concave margin when the engine food causes a downward concave deformation due to external force from an obstacle or the like. An excessive deformation of the upper horizontal straight section in case of the front, collision can be suppressed by joining the lateral flanges to the upper surface of the damper base. 
     In the aforementioned vehicle chassis front section structure, each upper flange section of the wheel housing upper member is formed from an upward flange that extends upward, and the upward flanges of the upper flange sections of the wheel housing upper member are offset outward in the vehicle width direction from the upward flange of the upper flange sections at the front part of the wheel housing lower member, and is bent such that a front part of the upward flanges of the upper flange sections of the wheel housing upper member is displaced inward in the vehicle widthwise direction relative to a rear part of the upper flanges of the upper flange sections of the wheel housing upper member. 
     In the aforementioned vehicle chassis front section structure, the upper flange sections are formed into the upward flanges. Thus, the wheel housing upper member is stably bent in the vehicle widthwise direction behind the damper base without being blocked to exert the impact energy absorption function. 
     In accordance with the aforementioned vehicle chassis front section structure, it is possible to stably bend a front part of the wheel house upper member toward a vehicle inner, side with the curved section being a bent point in response to an input of front collision load. 
     In the aforementioned vehicle chassis front section structure, each lower flange section of each of the wheel housing upper member and the wheel housing lower member is formed from a lateral flange that extends horizontally. 
     In accordance with the aforementioned vehicle chassis front section structure, even when the orientation on the upper flange section side varies in terms of the longitudinal direction, the outer panel can be easily lapped over and attached to the inner panel from the outside in the vehicle widthwise direction, and a hollow cross-sectional portion can be defined continuously inside the lateral-section frame. 
     In the aforementioned vehicle chassis front section structure, a front end of the lateral-section frame and the front end of the front side frame are connected to each other by using a gusset, and a front part of the lateral-section frame is bent substantially into a Z-shape while using the upper curved section and the lower curved section as bending points and a front part of the front side frame is bent inward in the vehicle widthwise direction substantially into a V-shape in response to a front collision load. 
     In accordance with the aforementioned vehicle chassis front section structure, it is possible to create a deformation behavior of the front part of the lateral-section frame that is bent into the Z-shape in the vertical direction and a deformation behavior of the front part of the front side frame that is bent inward in the vehicle widthwise direction into the V-shape without causing interference with each other. Thus, the amount of impact energy absorption can be secured. 
     Advantage of the Present Invention 
     In accordance with the present invention, it is possible to stably bend a side frame in case of a vehicle collision by optimizing a shape of the flange section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a vehicle chassis front section structure according to an embodiment. 
         FIG. 2  is a side view of the vehicle chassis front section structure according to the embodiment. 
         FIG. 3  is a bottom view of the vehicle chassis front, section structure according to the embodiment. 
         FIG. 4  is a cross-sectional view taken along the A-A line in  FIG. 1 . 
         FIG. 5  is a perspective view of the vehicle chassis front section structure in a state of detaching an outer panel of a lateral-section frame. 
         FIG. 6  is a cross-sectional view taken along the B-B line in  FIG. 5 . 
         FIG. 7  is a cross-sectional perspective view of a wheel housing upper member. 
         FIG. 8  is a plan view of the vehicle chassis front section structure according to the embodiment. 
         FIG. 9  is a cross-sectional view taken along the C-C line in  FIG. 1 . 
         FIGS. 10A to 10C  illustrate operational side views of a dent and an elongated hole. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A description will be given of a mode of applying the present invention to a frame structure of a vehicle chassis front section of an automobile. Referring to  FIGS. 1 to 5 , lateral-section frames  1  and front side frames  31  are disposed on two sides in a vehicle widthwise direction of a vehicle to form right and left pairs. As shown in  FIG. 3 , each lateral-section frame  1  is disposed on outside in the vehicle widthwise direction of the corresponding front side frame  31 .  FIGS. 1, 2, and 5  illustrate the lateral-section frame  1  and the front side frame  31  located on the right side of the vehicle. While the following description will discuss the lateral-section frame  1  and the front side frame  31  disposed on the right side of the vehicle, those on the left side of the vehicle have the same structures except that they are formed into symmetrical shapes to the foregoing. 
     (Lateral-Section Frame  1 ) 
     The lateral-section frame  1  includes: a wheel housing upper member  2  with its rear end being connected to a front pillar  41 ; and a wheel housing lower member  3  with its front end located at substantially at the same position as a front end of the front side frame  31  in a vertical direction and a vehicle longitudinal direction by extending forward and downward from the wheel housing upper member  2  via an upper curved section  4  and a lower curved section  5 . The wheel housing upper member  2  extends in the vehicle longitudinal direction above a wheel housing. A front end of the wheel housing upper member  2  and a rear end of the wheel housing lower member  3  overlap each other and are joined together by spot welding or the like. 
     (Wheel Housing Lower Member (Curved Frame)  3 ) 
     The wheel housing lower member  3  is formed into a shape provided with: an upper horizontal straight section  3 A horizontally extending forward by a certain amount from the front end of the wheel housing upper member  2 ; an inclined straight section  3 B extending straight forward and downward from a front end of the upper horizontal straight section  3 A via the upper curved section  4 ; and a lower horizontal straight section  3 C horizontally extending forward by a certain amount from a lower end of the inclined straight section  3 B via the lower curved section  5 . A front end of the lower horizontal straight section  3 C is located substantially at the same position as the front end of the front side frame  31  in the vertical direction and the longitudinal direction. A damper base (a damper housing)  42  to cover a not-illustrated damper from above is disposed on inside in a vehicle widthwise direction of the upper horizontal straight section  3 A. 
     The wheel housing lower member  3  is formed from an outer panel (a first panel)  6  and an inner panel (a second panel)  7 . As shown in  FIG. 4 , the outer panel  6  is a member taking on a substantially L-shaped cross-section, which is formed from: an upper surface section (a first surface)  6 A extending almost, horizontally in the vehicle widthwise direction; an outside surface section (a second surface)  6 B extending vertically downward from an outside end in the vehicle widthwise direction of the upper surface section  6 A; an upper flange section  6 C formed from an upward flange extending vertically upward from an inside end in the vehicle widthwise direction of the upper surface section  6 A; and a lower flange section  6 D formed from a lateral flange extending horizontally outward in the vehicle widthwise direction from a lower end of the outside surface section  6 B. The inner panel  7  is also a member taking on a substantially L-shaped cross-section, which is formed from: a lower surface section (a third surface)  7 A extending almost, horizontally in the vehicle widthwise direction; an inside surface section (a fourth surface)  7 B extending almost vertically upward from an inside end in the vehicle widthwise direction of the lower surface section  7 A; an upper flange section  70  formed from an upward flange extending vertically upward from an upper end of the inside surface section  7 B; and a lower flange section  70  formed from a lateral flange extending horizontally outward in the vehicle widthwise direction from an outside end in the vehicle widthwise direction of the lower surface section  7   a . Nonetheless, portions of the upper flange sections  6 C and  7 C at the rear of the upper curved section  4 , that is, at the upper horizontal straight section  3 A are formed from lateral flanges  6 C′ and  7 C′, which extend horizontally inward in the vehicle widthwise direction and are joined to an upper surface of the damper base  42  as shown in  FIG. 9 . 
     As described above, by joining the upper flange sections  6 C and  7 C together and the lower flange sections  6 D and  7 D together by spot welding or the like, respectively, the upper surface section  6 A and the lower surface section  7 A are vertically opposed to each other while the outside surface section  6 B and the inside surface section  7 B are laterally opposed to each other, thereby forming a rectangular closed cross-sectional portion  8  being a hollow cross-sectional portion inside the wheel housing lower member  3 .  FIG. 4  is a diagram viewing a cross-section of the lower curved section  5  in which the rectangular closed cross-sectional portion  8  takes on a vertically long shape. On the other hand, the rectangular closed cross-sectional portion  8  slightly reduces its vertical length at the inclined straight section  3 B, the upper curved section  4 , and the upper horizontal straight section  3 A. As described above, the wheel housing lower member  3  is formed from the outer panel  6  having the substantially L-shaped cross-section that includes the first surface and the second surface collectively forming an inside curved surface and from the inner panel  7  having the substantially L-shaped cross-section that includes the third surface and the fourth surface collectively forming an outside curved surface. Moreover, the wheel housing lower member  3  adopts the structure to overlap and join the flanges at the two ends of one of the panels to the corresponding ends on the two sides of the other panel, respectively. Thus, it is easy to form the panels and productivity of the panels is improved. 
     At the lower curved section  5 , the upper surface section  6 A forms an arc-shaped curved surface which is recessed upward while the lower surface section  7 A is formed to project downward. Schematically speaking, the center of curvature of the lower curved section  5  is located on the upper side of the upper surface section  6 A. Accordingly, the curved surface of the upper surface section  6 A is deemed to have an angle of bend (a range of bend) smaller than that of the curved surface of the lower curved section  6 B. In the present invention, the curved surface of the upper surface section  6 A having the smaller angle of bend will be referred to as an inside curved surface  9  while the curved surface of the lower surface section  7 A will be referred to as an outside curved surface  10 . In the meantime, a ridge (a corner-line) formed by the upper surface section  6 A and the outside surface section  6 B will be referred to as an inside ridge  11  while a ridge formed by the lower surface section  7 A and the inside surface section  7 B will be referred to as an outside ridge  12 . 
     In contrast, at the upper curved section  4 , the upper surface section  6 A is formed to project upward while the lower surface section  7 A is formed as an arc-shaped curved surface which is recessed downward. Accordingly, the lower surface section  7 A forms an inside curved surface while the upper surface section  6 A forms an outside curved surface. In the upper curved section  4 , a ridge formed by the lower surface section  7 A and the inside surface section  7 B will be referred to as an inside ridge  12 A while a ridge formed by the upper surface section  6 A and the outside surface section  6 B will be referred to as an outside ridge  11 A. 
     A dent  13  to induce the buckling deformation of the lower curved section  5  in case of input of a front collision load is formed on the inside ridge  11  of the lower curved section  5  in such a way as to be recessed toward the inside of the rectangular closed cross-sectional portion  8 . The dent  13  is formed over the upper surface section  6 A and the outside surface section  6 B as a dent almost in an almost oval shape which is elongated substantially in a direction of extension of the wheel housing lower member  3 . A bulge  14  is formed at a portion of the outside surface section  6 B below the dent  13  in such a way as to bulge outward in the vehicle widthwise direction as shown in  FIG. 4 . The dent  13  is formed, for example, by pressing the inside ridge  11  downward from the upper surface section  6 A. As a consequence, the portion of the outside surface section  6 B below the dent  13  is pushed outward in the vehicle widthwise direction to form the bulge  14 . 
     An elongated hole  15  is formed in a penetrating manner along the dent  13  and below the bulge  14  of the outside surface section  6 B. A bottom surface of the dent  13  takes on the arc shape that is recessed upward when viewed in the vehicle widthwise direction. Accordingly, the elongated hole  15  also takes on an arc shape that is recessed upward as with the bulge  14 . The elongated hole  15  has a function to facilitate a downward deformation of the dent  13 . 
     As shown in  FIG. 5 , a through-hole  16  is formed in the outside ridge  12  of the inner panel  7  at the lower curved section  5 . Here, as apparent from  FIGS. 1 and 2 , the upper surface section  6 A of the inside curved surface  9  is formed into a gentle arc shape from the lower horizontal straight section  3 C to the inclined straight section  3 B. Meanwhile, the lower surface section  7 A ( FIG. 4 ) of the lower curved surface  10  is formed in such a way as to intersect a range from the lower horizontal straight section  3 C to the inclined straight section  3 B while substantially retaining a flat shape. As shown in  FIG. 5 , the through-hole  16  is formed at a corner section surrounded by three surfaces of the lower surface section  7 A of the lower horizontal straight section  3 C and the lower surface section  7 A as well as the inside surface section  7 B of the inclined straight section  3 B, and from the lower surface section  7 A and the inside surface section  7 B of the lower horizontal straight section  3 C across the outside ridge  12 . The through-hole  16  formed on the outside ridge  12  side induces the buckling deformation of the lower curved section  5  in cooperation with the dent  13  formed on the inside ridge  11  side. 
     At the lower curved section  5 , substantially semicircular notches  17  are formed on the upper flange section  6 C and the lower flange section  6 D of the outer panel  6  and on the upper flange section  7 C of the inner panel  7 , respectively. The respective notches  17  on the upper flange section  6 C and the upper flange section  7 C are located at such positions overlapping each other when viewed in the vehicle widthwise direction. These notches  17  also induce the buckling deformation of the lower curved section  5 . 
     (Stiffener (Reinforcement Member)  18 ) 
     As shown in  FIG. 5 , a stiffener  18  serving as a reinforcement member, which extends forward and rearward (in the direction of extension of the wheel housing lower member  3 ) across the upper curved section  4 , is attached to the interior of the wheel housing lower member  3  by spot welding or the like. The stiffener  18  is attached around the inside ridge  12 A of the inner panel  7 , that is, attached to the lower surface section  7 A and the inside surface section  7 B across the inside ridge  12 A. Cutouts and the like are formed on the periphery of the stiffener  18  such that the stiffener  18  does not interfere with various ribs, holes, and the like provided to the inner panel  7 . Nonetheless, the stiffener  18  takes on a shape of a substantially L-shaped cross-section which is bent almost at a right angle and extends substantially from the lower surface section  7 A and the inside surface section  7 B. To be more precise, the stiffener  18  includes: a front rigid section  18 A and a rear rigid section  18 B which are attached to the lower surface section  7 A and the inside surface section  75  across the inside ridge  12 A; and a connecting section  18 C which is attached to at least the Inside surface section  7 B and connects the front rigid section  18 A and the rear rigid section  18 B to each other. 
     A recess  19  to induce buckling deformation of the upper curved section  4  in case of the input of the front collision load is formed on the inside ridge  12 A of the upper curved section  4 . As with the dent  13  on the lower curved section  5 , the recess  19  is a dent formed in such a way as to be recessed toward the inside of the rectangular closed cross-sectional portion  8 . The stiffener  18  is provided with a cutout  18 D to avoid interference with the recess  19  which will project into the rectangular closed cross-sectional portion  8 . The cutout  18 D is formed substantially into a rectangular shape from an edge section of the stiffener  18  attached to the lower surface section  7 A side to the inside surface section  7 B in such a way as to bypass the recess  19 . In other words, the stiffener  18  is disposed around the recess  19  such that the cutout  18 D defines a clearance hole for the recess  19 . The cutout  18 A also has a function as a fragile section that induces the buckling deformation of the upper curved section  4 . 
     Here, a hole to avoid contact with the recess  19  may be formed in the stiffener  18  in place of the cutout  18 D. In this case, the connecting section  18 C will be attached to both the inside surface section  7 B and the lower surface section  7 A. Alternatively, the stiffener  18  may be formed into a member having a substantially U-shaped cross-section to be attached to the lower surface section  7 A, the inside surface section  7 B, and the outside surface section  6 B. 
     (Bulkhead (Dividing Wall Member)  25 ) 
     A bulkhead  25  for reinforcement to divide the rectangular closed cross-sectional portion  8  in the wheel housing lower member  3  in the direction of extension of the wheel housing lower member  3  is attached to the front rigid section  18 A of the stiffener  13 . In  FIGS. 5 and 6 , the bulkhead  25  includes: a bulkhead plate section  25 A shaped in accordance with the rectangular closed cross-sectional portion  8 ; a lower flange section  25 B attached to the front rigid section  18 A on the lower surface section  7 A side by spot welding or the like; a lateral flange section  25 C attached to the front rigid section  18 A on the inside surface section  7 B side by spot welding or the like; and an upper flange section  25 D fitted to the upper surface section  6 A of the outer panel  6 . 
     (Fender Support Bracket  26 ) 
     A fender support bracket  26  to support a not-illustrated front fender of the vehicle chassis is disposed on the upper surface section  6 A of the outer panel  6  above the bulkhead  25 . The fender support bracket  26  includes: a horizontal surface section  26 A which is substantially horizontal; a vertical surface section  26 B extending vertically downward from a front end of the horizontal, surface section  26 A; an upper flange section  260  formed at a rear end of the horizontal surface section  26 A; and a lower flange section  26 D formed at a lower end of the vertical surface section  26 B. The upper flange section  26 C and the lower flange section  26 D are joined to the upper surface section  6 A by spot welding or the like. The upper flange section  25 D of the bulkhead  25  is located substantially below the lower flange section  26 D. 
     (Wheel Housing Upper Member  2 ) 
     The wheel housing upper member  2  is located substantially behind the damper base  42 . Referring to  FIGS. 1, 5, and 7 , the wheel housing upper member  2  includes: an outer panel  20  located on the outside in the vehicle widthwise direction; and a first inner panel  21 , a second inner panel  22 , a third inner panel  23 , and a fourth inner panel  24  which are located on the inside in the vehicle width direction. As shown in  FIG. 7 , the outer panel  20  is formed from an upper surface section  20 A extending horizontally in the vehicle longitudinal direction, and an outside surface section  20 B extending vertically downward from an outside end in the vehicle widthwise direction of the upper surface section  20 A. The upper surface section  20 A is provided with an upper flange section  20 C formed from an upward flange extending vertically upward from an inside end in the vehicle widthwise direction thereof, and the outside surface section  20 B is provided with a lower flange section  20 D formed from a lateral flange extending horizontally outward in the vehicle widthwise direction from a lower end thereof. 
     The first inner panel  21  extends horizontally in the vehicle longitudinal direction at the same position in height as the upper surface section  20 A of the outer panel  20 . An outside end in the vehicle widthwise direction of the first inner panel  21  is provided with an upper flange section  21 A formed from an upward flange extending upward and being joined to the upper flange section  20 C of the outer panel  20 , while an inside end in the vehicle widthwise direction thereof is provided with an inner flange section  21 B formed from a lateral flange being placed on and joined to an upper surface section  22 A of the second inner panel  22  and the damper base  42  ( FIG. 5 ). The second inner panel  22  includes the upper surface section  22 A that extends substantially horizontally and an inside surface section  22 B that extends downward from an outside end in the vehicle widthwise direction of the upper surface section  22 A, thus taking on a substantially L-shaped cross-section. A lower end of the inside surface section  22 B is provided with a flange section  22 C that extends downward. 
     The third inner panel  23  includes an arc-shaped lower surface section  23 A that is displaced downward to the rear of the vehicle and an inside surface section  23 B that extends upward from an inside end in the vehicle widthwise direction of the lower surface section  23 A, thus taking on a substantially L-shaped cross-section. A rear end of the inside surface section  23 B is joined to a lateral extension section  43  ( FIG. 5 ) of a dash lower panel. An outside end in the vehicle widthwise direction of the lower surface section  23 A is provided with a lower flange section  23 D formed from a lateral flange that is joined to the lower flange section  20 D of the outer panel  20 , while an upper end of the inside surface section  23 B is provided with an upward flange that is joined to the flange section  22 C of the second inner panel  22 . The inside surface section  23 B is provided with a stepped section  230  which is stepped in the vehicle widthwise direction. The stepped section  230  constitutes a bend starting point of the buckling deformation in case of the input of the front collision load. The fourth inner panel  24  extends horizontally. A front end and an inside end in the vehicle widthwise direction of the fourth inner panel  24  are provided with a front flange section  24 A and an inside flange section  24 B which are joined to the lower surface section  23 A and the inside surface section  23 B of the third inner panel  23  and the lateral extension section  43  of the dash lower panel, respectively. Meanwhile, an outside end in the vehicle width direction of the fourth inner panel  24  is provided with a lower flange section  24 C formed from a lateral flange that is joined to the lower flange section  20 D of the outer panel  20 . 
     As described above, of the wheel housing upper member  2 , an upper surface is formed from the upper surface section  20 A of the outer panel  20  and from the first inner panel  21 , an outside surface is formed from the outside surface section  20 B of the outer panel  20 , an inside surface is formed from the inside surface section  22 B of the second inner panel  22  and from the inside surface section  23 B of the third inner panel  23 , and a lower surface is formed from the lower surface section  23 A of the third inner panel  23  and from the fourth inner panel  24 , whereby a rectangular closed cross-sectional portion is defined inside. A difference in strength between the outside and the inside in the vehicle widthwise direction in a cross-sectional frame of the wheel housing upper member  2  can be easily provided by forming the outer panel  20  from a high-strength steel plate while forming the respective inner panels  21  to  24  from steel plates with a lower strength than the outer panel  20 . Accordingly, it is possible to bring the wheel housing upper member  2  into the buckling deformation that originates from the stepped section  23 C being the fragile section located on the inside in the vehicle widthwise direction in such a way as to be bent stably inward in the vehicle widthwise direction. Thus, collision energy can be absorbed efficiently. 
     As shown in  FIG. 8 , the upward flanges of the upper flange sections  20 C and  21 A of the wheel housing upper member  2  are offset outward in the vehicle width direction by a distance L or more from the upward flanges of the upper flange sections  6 C and  7 C at a front part of the wheel housing lower member  3 . Moreover, the upward flanges of the upper flange sections  20 C and  21 A are bent such that a front part  45  thereof is displaced inward in the vehicle widthwise direction relative to a rear part  46  thereof. Specifically, the upward flanges of the upper flange sections  20 C and  21 A have a bent part  44 . Of the upward flanges of the upper flange sections  20 C and  21 A, the rear part  46  bordered by the bent part  44  extends in a longitudinal direction while the front part  45  extends in such way as to be displaced inward in the vehicle width direction to the front. The bent part  44  is formed substantially at the same position as the stepped section  23 C in the longitudinal direction. 
     (Front Side Frame  31 ) 
     In  FIGS. 1 to 3 and 5 , the front side frame  31  extends in the vehicle longitudinal direction and includes a front side inner panel  32  and a front side outer panel  33 . In  FIG. 5 , the front side inner panel  32  includes a horizontal upper surface section  32 A, an inside surface section  32 B that extends vertically downward from an inside end in the vehicle widthwise direction of the upper surface section  32 A, and a lower surface section  32 C that extends outward in the vehicle widthwise direction from a lower end of the inside surface section  32 B, thus taking on a substantially U-shaped cross-section of which an outside in the vehicle widthwise direction is open. The upper surface section  32 A is provided with an upper flange section  32 D formed from an upward flange that extends upward, while the lower surface section  32 C is provided with a lower flange section  32 E formed from a downward flange that extends downward. The front side outer panel  33  includes an outside surface section  33 A that extends vertically, a lower surface section  33 B that extends inward in the vehicle widthwise direction from a lower end of the outside surface section  33 A, and a lower flange section  33 C formed from a downward flange that extends downward from the lower surface section  33 B. Accordingly, the upper flange section  32 D and an upper end of the front side outer panel  33  are flange-coupled to each other while the lower flange section  32 E and the lower flange section  33 C are flange-coupled to each other, whereby a rectangular closed cross-sectional portion is defined inside the front side frame  31 . 
     As shown in  FIG. 3 , the front side frame  31  is provided with reduced-width sections  34  which induce the buckling deformation of the front side frame  31  in such a way as to be bent inward in the vehicle widthwise direction in case of the input of the front collision load. Each reduced-width section  34  is formed such that the inside surface section  32 B of the front side inner panel  32  extends substantially straight along the vehicle longitudinal direction while the front side outer panel  33  is formed to be recessed inward in the vehicle widthwise direction into an arc shape in plan view. Thus, the reduced-width section  34  is formed so as to locally reduce a width dimension in the vehicle widthwise direction of the front side frame  31 . 
     A front end of the wheel housing lower member  3  and a front end of the front side frame  31  are connected to each other by using a gusset  35 . The wheel housing lower member  3 , the front side frame  31 , and the gusset  35  are joined to one another by spot welding or by using bolts and the like. An explanation of a specific shape of the gusset  35  would digress from the scope of the present invention and will therefore be omitted. 
     (Operations) 
     When the front collision load toward the rear is inputted to the front end of the wheel housing lower member  3  of the lateral-section frame  1 , the dent  13  at the lower curved section  5  is displaced downward as seen in a sequence  FIG. 10A → FIG. 10B → FIG. 10C . Thus, a stress is concentrated on the inside ridge  11 , whereby the wheel housing lower member  3  is bent and causes the buckling deformation such that the inclined straight section  3 B stands up forward. Since the elongated hole  15  is formed along the dent  13  in the outside surface section  6 B below the dent  13 , the dent  13  is easily displaced in such a way as to penetrate into a void space in the elongated hole  15 . Thus, the lower curved section  5  of the wheel housing lower member  3  is reliably bent at a predetermined value of the collision load and an amount of impact energy absorption is increased. 
     Meanwhile, since the bulge  14  is provided between the dent  13  and the elongated hole  15 , the bulge  14  is easily deflected so as to facilitate the displacement of the dent  13  toward the elongated hole  15 . Since the through-hole  16  is formed in the outside ridge  12  of the lower curved section  5 , the outside curved surface  10  side of the lower curved section  5  is bent easily. Moreover, the formation of the notches  17  on the upper flange section  6 C, the lower flange section  6 D, and the upper flange section  7 C also facilitates the bending of the lower curved section  5 . 
     Moreover, in case of a narrow offset collision (which is also called a small overlap collision or a fine lap collision), a rotational moment generated by the bend of the lower curved section  5  acts on the upper curved section  4 , whereby the wheel housing lower member  3  surely causes a so-called Z-shaped bend in which the inclined straight section  3 B is deformed upright by using the lower curved section  5  and the upper curved section  4  as bending points. 
     Next, operations of the respective flange sections will be described. The inclined straight section  3 B and the lower horizontal straight section  3 C of the wheel housing lower member  3  are located at such positions that can easily secure clearances with surrounding components including a headlight and the like. Accordingly, the upper flange sections  7 C and  7 C in this zone can be formed into the upward flanges that extend upward. In this way, it is possible to ensure a large section modulus and to increase the amount of impact energy absorption for the Z-shaped bend of the wheel housing lower member  3  caused by the narrow offset collision. 
     On the other hand, a not-illustrated engine hood is disposed above the upper horizontal straight section  3 A of the wheel housing lower member  3 . Accordingly, it is possible to secure a clearance between a lower surface of the engine hood and the upper flange section  6 C by forming the upper flange sections  6 C and  7 C into the lateral flanges  6 C′ and  7 C′. As a consequence, it is possible to secure a concave margin when the engine food causes a downward concave deformation due to external force from an obstacle or the like. An excessive deformation of the upper horizontal straight section  3 A in case of the front collision can be suppressed by joining the lateral flanges  6 C′ and  7 C′ to the upper surface of the damper base  42 . 
     In the wheel housing upper member  2 , the upper flange sections  20 C and  21 A are formed into the upward flanges. As a consequence, the wheel housing upper member  2  is stably bent in the vehicle widthwise direction behind the damper base  42  without being blocked to exert the impact energy absorption function. In particular, the upward flanges of the upper flange sections  20 C and  21 A of the wheel housing upper member  2  are offset outward in the vehicle width direction by the distance L or more from the upward flanges of the upper flange sections  6 C and  7 C of the wheel housing lower member  3 , and are bent at the bent part  44  such that the front part  45  is located inside in the vehicle widthwise direction with respect to the rear part  46 . In this way, when a front impact load F is received as shown in  FIG. 3 , it is possible to bend the front part  45  stably inward (in a direction of an arrow Q) while using the bent part  44  and the stepped section  23 C ( FIG. 5 ) as bending points. 
     Meanwhile, the lower flange sections  20 D,  23 D and  24 C of the wheel housing upper member  2  and the lower flange sections  6 D and  7 D of the wheel housing lower member  3  are formed from the lateral flanges over the entire length. In this way, even when the orientation on the upper flange section side varies in terms of the longitudinal direction (when both the upper flange and the lateral flange are provided as in the embodiment), the outer panel can be easily lapped over and attached to the inner panel from the outside in the vehicle widthwise direction, and a hollow cross-sectional portion can be defined continuously inside the lateral-section frame  1 . 
     Next, operations of the stiffener  18  will be explained. In the upper curved section  4 , the connecting section  13 C of the stiffener  18  is attached to the inside surface section  7 B in such a way as not to interfere with the inside ridge  12 A. Accordingly, the connecting section  18 C is easily deformed by receiving a shear load. Thus, a front side lower member  3  can be stably bent at the location where the connecting section  18 C is disposed while securing the rigidity of the front side lower member  3  with the stiffener  18 . For this reason, it is not necessary to extend the front rigid section  18 A and the rear rigid section  13 B forward and rearward merely for the purpose of creating a difference in strength distribution from that at each bending point. Thus, the stiffener  18  can be reduced in size and weight. 
     In the meantime, the recess  19  is formed on the inside ridge  12 A and the stiffener  18  is provided with the cutout  18 D in such a way as to surround the recess  19 . In other words, the stiffener  18  is disposed around the recess  19 . Accordingly, the recess  19  and the cutout  18 D collectively constitute the fragile section, whereby the upper curved section  4  is bent more stably without causing concentration of the stress on a point in front of or in the rear of the upper curved section  4 . 
     The difference in strength from the cross-sectional portion of the wheel housing lower member  3  where the connecting section  18 C is located is enhanced by disposing the bulkhead  25  at the front rigid section  18 A of the stiffener  18 . Thus, it is possible to bend the front side lower member  3  more stably at the location where the connecting section  18 C is disposed. When the fender support bracket  26  is located at the upper surface section  6 A of the wheel housing lower member  3  above the bulkhead  25 , it is possible to increase attachment rigidity of the fender support bracket  26  to the wheel housing lower member  3 . 
     Next, operations of the front side frame  31  will be explained. In case of the front collision, the front part of the front side frame  31  is bent inward in the vehicle widthwise direction (in a direction of an arrow P in  FIG. 3 ) into a V-shape while using each reduced-width section  34  as the bending point. As a consequence, it is possible to create a deformation behavior of the front part of the lateral-section frame  1  that is bent into the Z-shape in the vertical direction and a deformation behavior of the front part of the front side frame  31  that is bent, inward in the vehicle widthwise direction into the V-shape without causing interference with each other. Thus, the amount of impact energy absorption can be secured. 
     The preferred embodiment of the present invention has been described above. At the lower curved section  5  in the embodiment, the dent  13  is formed on the inside ridge  11  that is formed by the upper surface (the upper surface section  6 A) and the outside surface (the outside surface section  6 B) while the elongated hole  15  is formed in the outside surface. Instead, depending on changes in the flange sections of the outer panel  6  and the inner panel  7  and the like, the dent  13  may be formed on the inside ridge that is formed by the upper surface and the inside surface while the elongated hole  15  may be formed in the inside surface. Alternatively, the dents  13  may be formed on both of the inside ridges while the elongated holes  15  may be formed in the outside surface and the inside surface, respectively. 
     REFERENCE SIGNS LIST 
     
         
           1  lateral-section frame 
           2  wheel housing upper member 
           3  wheel housing lower member 
           4  upper curved section 
           5  lower curved section 
           6  outer panel 
           6 C upper flange section (upward flange) 
           6 C′ upper flange section (lateral flange) 
           6 D lower flange section (lateral flange) 
           7  inner panel 
           7 C upper flange section (upward flange) 
           7 C′ upper flange section (lateral flange) 
           7 D lower flange section (lateral flange) 
           8  rectangular closed cross-sectional portion 
           9  inside curved surface 
           10  outside curved surface 
           11  inside ridge (of lower curved section) 
           11 A outside ridge (of upper curved section) 
           12  outside ridge (of lower curved section) 
           12 A inside ridge (of upper curved section) 
           13  dent 
           15  elongated hole 
           16  through-hole 
           17  notch 
           18  stiffener (reinforcement member) 
           19  recess 
           20  outer panel 
           20 C upper flange section (upward flange) 
           20 D lower flange section (lateral flange) 
           21  first inner panel 
           21 A upper flange (upward flange) 
           22  second inner panel 
           23  third inner panel 
           23 D lower flange (upward flange) 
           24  fourth inner panel 
           24 C lower flange section (lateral flange) 
           31  front side frame 
           35  gusset