Abstract:
A vehicle subframe includes, at the bottom of a main body section, a lateral expansion section, a left vertical expansion section, a right vertical expansion section, and a concavity. The lateral expansion section joins a left front joining section and a right front joining section, and expands below the concavity. Of the left vertical expansion section, the front end is joined to the left end of the lateral expansion section, and the left vertical expansion section is inclined with a downward slope from the rear side of the main body section towards the front end. The right vertical expansion section is formed in a left-right symmetrical manner with respect to the left vertical expansion section, and the front end is joined to the lateral expansion section. The concavity is encircled by the vertical expansion sections and the lateral expansion section, and is formed in a concave shape facing upwards.

Description:
TECHNICAL FIELD 
     The present invention relates to vehicle subframes which are provided under a vehicle body and which support left and right suspensions by means of left and right suspension arms connected to left and right end portions thereof. 
     BACKGROUND ART 
     Examples of the conventionally-known vehicles include ones which include a cast (cast-molded) subframe provided on a vehicle body frame and which include suspension arms connected to left and right end portions of the subframe so that left and right suspensions are supported respectively by the left and right suspension arms. 
     A plurality of ribs is provided on the subframe so that a sufficient rigidity and strength of the subframe can be secured by the ribs (see, for example, Patent Literature 1). 
     With the subframe disclosed in Patent Literature 1, it is required to determine molded directions (orientations) of the plurality of ribs in such a manner as to secure a sufficient rigidity and strength of the subframe. 
     Further, in casting the subframe, it is required to smoothly direct molten metal into a cavity of a casting mold. As a means for directing the molten metal into the casting mold cavity, it is conceivable to conform or match the orientations of the plurality of ribs with the flowing direction of the ribs. However , it is difficult to determine the orientations of the plurality of ribs so as to secure a sufficient rigidity and strength of the subframe and match the molten metal flowing direction. For example, if the orientations of the plurality of ribs are determined so as to smooth the molten metal flows, it tends to be difficult to secure a sufficient rigidity and strength of the subframe by means of the plurality of ribs. 
     Thus, if the molten metal flows are to be smoothed by the plurality of ribs, it is necessary to increase a wall thickness of the subframe to thereby secure a sufficient rigidity and strength of the subframe, which cannot meet a demand for minimizing an increase of the weight of the subframe. 
     Prior Art Literature 
     Patent Literature 1: JP 2012-91693 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is therefore an object to provide an improved vehicle subframe which can secure a sufficient rigidity and yet minimize an increase of weight. 
     Solution to Problem 
     According to the present invention, there is provided an improved vehicle subframe integrally formed by casting using a core, which comprises: a subframe body formed in a hollow shape by use of the core and including an upper section and a lower section spaced from each other in an up-down direction of the subframe by a predetermined distance; and left and right suspension support sections provided on left and right end portions, respectively, of the subframe body, the left and right suspension support sections respectively having left and right suspension arms connected thereto for supporting left and right suspensions, the left and right suspension support sections including left and right front connection sections, respectively, provided on front portions of an outer peripheral portion (i.e., front outer peripheral portions) of the subframe body, and left and right rear connection sections provided on a rear outer peripheral portion of the subframe body, the lower section of the subframe body including: a transverse protruding section interconnecting the left and right front connection sections and protruding downwardly; longitudinal protruding sections connected at respective front end portions thereof to the transverse protruding section, each of the longitudinal protruding sections slanting downward from a rear portion of the subframe body toward the front end portion; and a recessed section surrounded by the longitudinal protruding sections and the transverse protruding section, the recessed section being formed in an upwardly concave shape. 
     Preferably, in the vehicle subframe, each of the longitudinal protruding sections is widened outwardly in a left-right direction of the subframe body from a rear end portion thereof, located adjacent to the rear portion of the subframe body, toward the front end portion of the longitudinal protruding section. 
     Advantageous Effects of Invention 
     In the present invention, the transverse protruding section is provided on the lower section of the subframe body, and the left and right front connection sections are interconnected by the transverse protruding section. The left and right front connection sections are provided on the outer peripheral portion of the subframe body. The longitudinal protruding sections are connected at their respective front end portions connected to the transverse protruding section, and each of the longitudinal protruding sections slants downward from the rear portion of the subframe body toward the front end portion. In addition, the recessed section surrounded by the longitudinal protruding sections and the transverse protruding section is formed in an upwardly concave shape. 
     Because the rear portion of the subframe body and the front connection sections are connected by the longitudinal protruding sections and the transverse protruding sections, the region of the subframe body from the rear portion to the front connection sections can be formed in a gentle shape with no irregularities. 
     Thus, in casting the subframe, it is possible to increase not only flowability with which molten metal poured through the rear portion of the subframe body flows to the front connection sections (toward the front of the vehicle), but also flowability with which the molten metal flows along the transverse protruding section to left and right side portions of the subframe. In this way, the molten metal can be smoothly directed to the front connection sections, so that a sufficient rigidity and strength of the front connection sections (and hence the subframe) can be secured. 
     Further, the region of the subframe body from the rear portion of the subframe body to the front connection sections can be formed in a gentle shape, so that a sufficient rigidity and strength of the front connection sections (and hence the subframe) can be secured. Thus, there is no need to form the subframe into a large thickness in order to secure a sufficient rigidity and strength of the subframe, with the result that an undesired increase of the weight of the subframe can be minimized 
     Further, because the suspensions are connected to the suspension support sections via the suspension arms, a relatively large load would be input to the suspension support sections. Thus, by securing a sufficient rigidity of the front connection sections (i.e., suspension support sections), the present invention can achieve an increased reliability of the subframe. 
     Furthermore, because the region of the subframe body from rear portion to the front connection sections is formed in a gentle shape so that the flowability of the molten metal can be increased, the present invention can eliminate a need for forming a plurality of ribs in conformity with a flowing direction of the molten metal. Thus, in the present invention, a plurality of ribs can be formed in such directions (orientations) as to secure a sufficient rigidity and strength of the subframe, with the result that the subframe can be reinforced with the plurality of ribs. Because the subframe is reinforced with the plurality of ribs like this, it is possible to even more appropriately secure a rigidity and strength of the subframe, and thus, the present invention can even more effectively minimize an undesired increase of the weight of the subframe due to the large thickness of the subframe. 
     In a preferred implementation of the present invention, each of the longitudinal protruding sections is widened outwardly in a left-right direction of the subframe body from a rear end portion thereof located adjacent to the rear portion of the subframe body toward the front end portion of the longitudinal protruding section. Thus, in casting of the subframe, the molten metal poured through the rear portion of the subframe body can be smoothly directed to outwardly in the left-right direction (i.e., to the left and right front connection sections), so that a sufficient rigidity and strength of the left and right front connection sections (and hence the subframe) can be secured even more appropriately. 
     Further, because each of the longitudinal protruding sections is widened outwardly in the left-right direction of the subframe body from the rear end portion toward the front end portion, the front end portion of the longitudinal protruding section can be formed in a relatively large shape such that the corresponding front connection section can be provided directly on the front end portion. With this arrangement, it is possible to eliminate a need for forming, on the front end portion of each of the longitudinal protruding sections, a particular part for providing thereon the connection section, so that the subframe can be significantly simplified in construction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating a front vehicle body structure equipped with a vehicle subframe of the present invention; 
         FIG. 2  is a bottom view of the subframe of the present invention with suspension arms provided thereon; 
         FIG. 3  is a perspective view showing the subframe of  FIG. 1 ; 
         FIG. 4  is a view taken in the direction of arrow  4  of  FIG. 3 ; 
         FIG. 5  is a sectional view taken along line  5 - 5  of  FIG. 3 ; 
         FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 3 ; 
         FIG. 7  is a rear lower perspective view of the subframe of  FIG. 3 ; 
         FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 3 ; 
         FIG. 9  is a view explanatory of an example manner in which molten metal is directed to a cavity in a casting process of the subframe of the present invention; 
         FIG. 10  is a view explanatory of an example manner in which the molten metal directed to the cavity is directed toward the front of the vehicle via upper and lower cavities; 
         FIG. 11  is a view explanatory of an example manner in which the molten metal directed via the upper and lower cavities is directed to a left front connection section; and 
         FIG. 12  is a view explanatory of an example manner in which the molten metal poured into a pouring port is directed to a left rear connection section. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, the terms “front (FR)”, “rear (Rr)”, “left (L)”, “right (R)”, etc. are used to refer to directions as viewed from a human driver of a vehicle. 
     Described hereinbelow is an embodiment of a vehicle subframe  15  of the present invention. As shown in  FIGS. 1 and 2 , a front vehicle body structure  10  includes: left and right side frames  11  and  12  disposed to extend in a front-rear direction of the vehicle (vehicle front-rear direction); the subframe  15  mounted to the undersides of the left and right subframes  11  and  12 ; left and right suspension arms  16  and  17  provided on left and right end portions of the subframe  15 ; and left and right suspensions  21  and  22  connected to the left and right suspension arms  16  and  17 . 
     The front vehicle body structure  10  further includes a steering gearbox  24  mounted on an upper portion  15   a  of the subframe  15 , and a torque rod  26  interconnecting the subframe  15  and a power plant  25 . A steering wheel  29  is mounted on a steering shaft  28  extending from the steering gearbox  24 . 
     As an example, the power plant  25  is an engine/transmission unit where an engine and a transmission are formed integrally to function as a drive source of the vehicle. The power plant  25  is disposed in a transverse orientation between the left and right side frames  11  and  12 . 
     Further, as shown in  FIGS. 3 and 4 , the subframe  15  is integrally formed of aluminum alloy to a thickness T 1  (see  FIG. 5 ) by high-pressure casting using a core  92  (see  FIG. 9 ). The subframe  15  includes: a subframe body  32  formed in a hollow shape by means of the core  92 ; a left vehicle body mounting section  34  and a left suspension support section  35  provided on a left end portion  32   a  of the subframe body  32 ; a right vehicle body mounting section  36  and a right suspension support section  37  provided on a right end portion  32   b  of the subframe body  32 ; a middle connection section  38  provided on a front middle portion of the subframe body  32 . 
     The left vehicle body mounting section  34  includes a left front mounting section  34   a  provided on a front portion  32   c  of the left end portion  32   a  of the subframe body  32 , and a left rear mounting section  34   b  provided on a rear portion  32   d  of the left end portion  32   a  of the subframe body  32 . 
     Similarly to the left vehicle body mounting section  34 , the right vehicle body mounting section  36  includes a right front mounting section  36   a  provided on a front portion  32   e  of the right end portion  32   b  of the subframe body  32 , and a right rear mounting section  36   b  provided on a rear portion  32   f  of the right end portion  32   b  of the subframe body  32 . 
     The left front mounting section  34   a  and the left rear mounting section  34   b  are fastened to the left side frame  11  ( FIG. 1 ) by means of bolts, and the right front mounting section  36   a  and the right rear mounting section  36   b  are fastened to the right side frame  12  ( FIG. 1 ) by means of bolts. In this manner, the subframe  15  is secured to the left and right side frames  11  and  12 . 
     Referring back to  FIGS. 1 and 2 , the left suspension support section  35  includes a left front connection section  81  provided on the front portion  32   c  of the left end portion  32   a  of the subframe body  32 , and a left rear connection section  82  provided on the rear portion  32   d  of the left end portion  32   a  of the subframe body  32 . 
     The left front connection section  81  is provided on the front portion  32   c  of the subframe body  32  to protrude from an outer peripheral portion  32   g  of the subframe body  32  forward or toward the front of the vehicle. The left front connection section  81  is located forward of the left front mounting section  34   a.    
     The left rear connection section  82  is provided on the rear portion  32   d  of the subframe body  32  to protrude outward in a vehicle width direction (i.e., leftward) from the outer peripheral portion  32   g  of the subframe body  32 . The left rear connection section  82  is provided more inward in the vehicle width direction than, or inward of, the left rear mounting section  34   b  (see  FIG. 4 ), so that the left rear connection section  82  is disposed between the left rear mounting section  34   b  and the rear portion  32   d.    
     A front mounting section  16   a  of the left suspension arm  16  is connected to the left front connection section  81  by means of a left front support pin  84 , and a rear mounting section  16   b  of the left suspension arm  16  is connected to the left rear connection section  82  by means of a left rear support pin. 
     The left suspension  21  is connected at its lower end portion  21   a  to the left suspension arm  16  and connected at its upper end portion  2  lb to a left damper housing  13  formed integrally with the left side frame  11 . Thus, the left suspension  21  is supported by the left suspension arm  16 , so that it is supported by the left suspension section  35  via the left suspension arm  16 . 
     Similarly to the left suspension arm  35 , the right suspension support section  37  includes a right front connection section  87  provided on the front portion  32   e  of the right end portion  32   b  of the subframe body  32 , and a right rear connection section  88  provided on the rear portion  32   f  of the right end portion  32   b  of the subframe body  32 . 
     The right front connection section  87  is provided on the rear portion  32   e  of the subframe body  32  to protrude from the outer peripheral portion  32   g  forward or toward the front of the vehicle. The right front connection section  87  is located forward of the right front mounting section  36   a.    
     The right front connection section  87  is provided on the rear portion  32   f  of the subframe body  32  to protrude outward in a vehicle width direction (i.e., rightward) from the outer peripheral portion  32   g  of the subframe body  32 . The right rear connection section  88  is provided more inward in the vehicle width direction than, or inward of, the right rear mounting section  36   b  (see also  FIG. 4 ), so that the right rear connection section  88  is disposed between the right rear mounting section  36   b  and the rear portion  32   f.    
     A front mounting section  17   a  of the right suspension arm  17  is connected to the right front connection section  87  by means of a right front support pin  91 , and a rear mounting section  17   b  of the right suspension arm  17  is connected to the right rear connection section  88  by means of a right rear support pin. 
     The right suspension arm  17  is substantially in left-right symmetry with the left suspension arm  16 . The right suspension  22  is connected at its lower end portion  22   a  to the right suspension arm  17  and connected at its upper end portion  22   b  to a right damper housing  14  formed integrally with the right side frame  12 . Thus, the right suspension  22  is supported by the right suspension arm  17 , so that it is supported by the right suspension section  37  via the right suspension arm  17 . 
     In the aforementioned manner, the left suspension  21  is connected to the left suspension support section  35  via the left suspension arm  16 , and the right suspension  22  is connected to the right suspension support section  37  via the right suspension arm  17 . Namely, a relatively large load is input to the left suspension support section  35  by way of the left suspension arm  16 , and a relatively large load is input to the right suspension support section  37  by way of the right suspension arm  17 . Therefore, it is preferable to secure a sufficient rigidity and strength of the left suspension support section  35  (more specifically, the left front connection section  81  and the left rear connection section  82 ) and the right suspension support section  37  (more specifically, the right front connection section  87  and the right rear connection section  88 ). 
     As shown in  FIG. 4 , the left front connection section  81  and the right front connection section  87  are provided to protrude forward from the front outer peripheral portion  32   g  of the subframe body  32 . 
     In forming the subframe  15  by high-pressure casting, molten aluminum alloy is poured through a molten metal pouring flow path formed in a rear portion  15   b  of the subframe  15 . Thus, the left front connection section  81  and the right front connection section  87  are located relatively remote from the molten metal pouring flow path. 
     Thus, left and right longitudinal protruding sections  66  and  67  and a transverse protruding section  65  are formed on the subframe  15  so that the molten metal can be smoothly directed to the left front connection section  81  and the right front connection section  87 . Such left and right longitudinal protruding sections  66  and  67  and transverse protruding section  65  will be described in detail later with reference to  FIGS. 4 to 7 . 
     The left rear connection section  82  and the right rear connection section  88  are provided near the rear outer peripheral portion  32   g  of the subframe body  32 . Thus, the left rear connection section  82  and the right rear connection section  88  are located relatively near the above-mentioned molten metal pouring flow path, so that the molten metal can be smoothly directed to the left rear connection section  82  and the right rear connection section  88 . 
     By the molten metal being smoothly directed to the left and right front connection sections  81  and  87  and the left and right rear connection sections  82  and  88  as above, it is possible to secure a sufficient rigidity and strength of the left and right front connection sections  81  and  87  and the left and right rear connection sections  82  and  88 ; namely, it is possible to secure a sufficient rigidity and strength of the left and right suspension support sections  35  and  37 . 
     Referring again back to  FIGS. 1 and 2 , the middle connection section  38  is provided on a middle front half region of the subframe body  32 , more particularly, on a middle portion  32   h,  in the vehicle width direction, of the subframe body  32 . 
     The torque rod  26  is connected at its proximal end portion  26   a  to the middle connection section  38  by means of a bolt  51  and a nut  52  and connected at its distal end portion  26   b  to the power plant  25  by means of a bolt  53 . Thus, the power plant  25  is supported by the torque rod  26 . 
     Further, as shown in  FIG. 5 , the subframe body  32  includes: an upper section  41  facing upward; a lower section facing downward; a front wall  43  interconnecting respective front end portions  41   a  and  42   a  of the upper and lower portions  41  and  42 ; and a rear wall  44  interconnecting respective rear end portions  41   b  and  42   b  of the upper and lower portions  41  and  42 . The subframe body  32  has a hollow portion  45  defined by the upper and lower sections  41  and  42  and the front and rear walls  43  and  44 . The hollow portion  45  is formed by the core  92  (see  FIG. 9 ) in a casting process of the subframe  15 . Further, the upper and lower sections  41  and  42  are spaced from each other by a predetermined distance in a vertical or up-down direction. 
     In a region  41   c  immediately inward, in the vehicle width direction, of the left suspension support section  35  (the left front connection section  81  and the left rear connection section  82  (see  FIG. 3 )), the upper section  41  of the subframe body  32  includes an upper rear flat portion  55 , an upper slant portion  56  and an upper front flat portion  57 . 
     The upper rear flat portion  55  extends substantially horizontally forward from a rear end portion  41   b  to a middle portion, in the front-rear direction, of the upper section  41 . The upper slant portion  56  extends forward in an upward slope from the front end  55   a  of the upper rear flat portion  55 . Further, the upper front flat portion  57  extends forward substantially horizontally from the front end  56   a  of the upward slant portion  56  to the front end portion  41   a  of the upper section  41 . Thus, the upper rear flat portion  55 , the upward slant  56  and the front upper front flat portion  57  are formed in a gentle shape with no irregularities. 
     In this manner, the upper section  41  of the subframe body  32  is formed in a gentle shape n the inward region  41   c  from the rear end portion  41   b  to the front end portion  41   a.  Because the upper section  41  of the subframe body  32  is formed in a gentle shape from the rear end portion  41   b  to the front end portion  41   a  as above, the molten metal of aluminum alloy can be directed smoothly in the high-pressure casting process of the subframe  15 . 
     As shown in  FIGS. 5 and 6 , a plurality of upper transverse ribs  61  and a plurality of upper longitudinal ribs  62  are provided on the inner surface  41   d  of the upper section  41  and projects into the hollow portion  45 . The transverse ribs  61  extend in the vehicle width direction, while the upper longitudinal ribs  62  extend in the vehicle front-rear direction. 
     Here, the upper section  41  of the subframe body  32  is formed in a gentle shape from the rear end portion  41   b  to the front end portion  41   a  so as to smoothly direct the molten metal from the rear end portion  41   b  to the front end portion  41   a.    
     Thus, the molten metal can be smoothly directed from the rear end portion  4  lb to the front end portion  41   a  of the upper section  41  even where the plurality of upper transverse ribs  61  and the plurality of upper longitudinal ribs  62  are not oriented in the same direction as (i.e., are not oriented to match) the flowing direction of the molten material. 
     Thus, molded directions (orientations) of the upper transverse ribs  61  and the upper longitudinal ribs  62  can be determined in such a manner as to secure a sufficient rigidity and strength of the upper section  41 . Thus, the upper section  41  can be reinforced appropriately by the upper transverse ribs  61  and the upper longitudinal ribs  62 . 
     Further, as shown in  FIGS. 4 and 7 , the lower section  42  of the subframe body  32  includes: a transverse protruding section  65  extending in the vehicle width direction along the front outer peripheral portion  32   g  of the subframe body  32 ; a left longitudinal protruding section  66  extending from a left end portion  65   a  of the transverse protruding section  65  along a left end portion  32   a  of the subframe body  32 ; a right longitudinal protruding section  67  extending from a right end portion  65   b  of the transverse protruding section  65  along a right end portion  32   b  of the subframe body  32 ; and a recessed section  68  surrounded by the transverse protruding section  65  and the left and right longitudinal protruding sections  66  and  67 . 
     Because the left longitudinal protruding section  66  and the right longitudinal protruding section  67  are symmetric with each other in the left-right direction, like elements of the left and right longitudinal protruding sections  66  and  67  are depicted by same reference numerals, and the following mainly describe the left longitudinal protruding section with a detailed description about the right longitudinal protruding section  67  omitted to avoid unnecessary duplication. 
     The transverse protruding section  65  extends along the front outer peripheral portion  32   g  of the subframe body  32  toward the left front connection section  81  and the right front connection section  87 . The transverse protruding section  65  includes a transverse bottom portion  71  extending along the front outer peripheral portion  32   g  of the subframe body  32 , and a transverse wall portion  72  formed along the rear end edge  71   a  of the transverse bottom portion  71 . 
     The transverse bottom portion  71  is a band-shaped flat portion that is located beneath the recessed section  68  and extends horizontally along the front outer peripheral portion  32   g  of the subframe body  32 . The front wall  43  (see also  FIG. 5 ) is formed integrally on the front end portion  42   a  of the transverse bottom portion  71  (i.e., the front end portion  42   a  of the lower section  42 ). 
     The transverse wall portion  72  extends obliquely upward and rearward from a rear middle portion  71   b  of the rear end edge  71   a  of the transverse bottom portion  71  to a front end portion  68   a  of the recessed section  68 . The rear middle portion  71   b  is located between the left and right end portions  65   a  and  65   b  of the transverse protruding section  65 . The transverse protruding section  65  protrudes downward relative to the recessed section  68  (see also  FIG. 5 ). 
     The left front connection section  81  is provided integrally on the left end portion  32   c  of the front wall  43  (i.e., front portion of the left end portion  32   a ) (see also  FIG. 5 ). The left front connection section  81  is located forward of the left end portion  65   a  of the transverse protruding section  65 . 
     Further, the right front connection section  87  is provided integrally on the right end portion  32   e  of the front wall  43  (i.e., front portion of the right end portion  32   b ). The right front connection section  87  is located forward of the right end portion  65   b  of the transverse protruding section  65 . Namely, the left front connection section  81  and the right front connection section  87  are connected to the transverse protruding section  65  via the front wall  43 . 
     The left longitudinal protruding section  66  is provided along the left end portion  32   a  of the subframe body  32  between the left end portion  65   a  of the transverse protruding section  65  and the left rear connection section  82 . The left longitudinal protruding section  66  is connected at its rear end portion  66   a  to a left rear end portion of the recessed section  68  (i.e., to the rear portion  32   d  of the subframe body  32 ) and connected at its front end portion  66   b  to the left end portion  65   a  of the transverse protruding section  65 . 
     More specifically, the left longitudinal protruding section  66  includes a longitudinal bottom portion  74  extending along the left end portion  32   a  of the subframe body  32 , and a longitudinal wall portion  75  extending along an inner end portion  74   a  of the longitudinal bottom portion  74 . 
     Further, as shown in  FIG. 5 , the longitudinal bottom portion  74  extends in a downward slope of a slope angle θ 1  (see also  FIG. 7 ) from the rear end portion  66   a  to the front end portion  66   b  of the left longitudinal protruding section  66 . 
     Further, as shown in  FIGS. 4 and 7 , the longitudinal bottom portion  74  is widened outwardly and inwardly in the vehicle width direction (i.e., in the left-right direction of the subframe body  32 ) from the rear end portion  66   a  to the front end portion  66   b.  Namely, the longitudinal bottom portion  74  is formed in a substantial fan shape with a width W 1  gradually increasing in a direction from the rear end portion  66   a  to the front end portion  66   b.  A left wall  47  (see also  FIG. 6 ) is formed integrally on an outer end portion  74   b  of the longitudinal bottom portion  74 . The longitudinal wall portion  75  slants upwardly and inwardly from the inner end portion  74   a  of the longitudinal bottom portion  74  to a left end portion  68   b  of the recessed section  68 . The longitudinal wall portion  75  is connected at its front end portion  75   a  to a left end portion of the transverse wall portion  72 . 
     Namely, as shown in  FIG. 5 , the left longitudinal protruding section  66  gradually protrudes downward from the recessed section  68  from the rear end portion  66   a  toward the front end portion  66   b.    
     With the front end portion  66   b  of the left longitudinal protruding section  66  connected to the left end portion  65   a  of the transverse protruding section  65  as above, the rear portion  32   d  of the subframe body  32  and the left front connection section  81  can be connected gently by the left longitudinal protruding section  66  and the transverse protruding section  65 . In this way, the region from the rear portion  32   d  of the subframe body  32  to the left front connection section  81  can be formed in a shape with no irregularities, which can achieve an increased flowability of the molten metal. 
     Similarly, as shown in  FIG. 4 , a region from the rear portion  32   f  of the subframe body  32  to the right front connection section  87  can be formed in a shape with no irreguralities, which can achieve an increased flowability of the molten metal. 
     Thus, when the subframe  15  shown in  FIGS. 4 and 7  is molded by high-pressure casting, the molten metal can be smoothly directed to the left and right front connection sections  81  and  87  even through the left and right front connection sections  81  and  87  are located relatively remote from the molten metal pouring path. 
     Further, the longitudinal bottom portion  74  is widened outwardly and inwardly in the vehicle width direction from the rear end portion  66   a  toward the rear end portion  66   b.  Thus, when the subframe  15  is molded by high-pressure casting, the molten metal can be smoothly directed outwardly and inwardly in the vehicle width direction. Namely, the molten metal poured through the rear portion  15   b  of the subframe  15  can be smoothly directed to the left front connection section  81 . Similarly, the molten metal poured through the rear portion  15   b  of the subframe  15  can be smoothly directed to the right front connection section  87 . 
     Because the molten metal can be smoothly directed to the left front connection section  81  and the right front connection section  87  as above, the left front connection section Bland the right front connection section  87  can have a sufficient rigidity and strength, with the result that the subframe  15  can have a sufficient rigidity and strength. Consequently, there is no need to form the subframe  15  into a large thickness T 1  (see  FIG. 5 ), with the result that an undesired increase of the weight of the subframe  15  can be minimized 
     Further, as shown in  FIGS. 6 and 8 , a plurality of lower transverse ribs  63 , a plurality of lower longitudinal ribs  64  and a plurality of lower slanting ribs  69  are provided on the inner surface  42   c  of the lower section  42  and project into the hollow portion  45 . The lower transverse ribs  63  extend in the vehicle width direction, while the lower longitudinal ribs  64  extend in the vehicle front-rear direction. Further, the lower slanting ribs  69  are provided on the subframe body  32  rearwardly of the middle connection section  38  and slants outwardly in the vehicle width direction. 
     Here, the lower section  42  is formed in a gentle shape from a rear end portion  42   b  to the front end portion  42   a  so that the molten metal can be smoothly directed from the rear end portion  42   b  to the front end portion  42   a.    
     Thus, even where the plurality of lower transverse ribs  63 , the plurality of lower longitudinal ribs  64  and the plurality of lower slanting ribs  69  are not oriented so as to match the molten metal flowing direction, the molten metal can be smoothly directed from the rear end portion  42   b  to the front end portion  42   a  of the lower section  42 . Thus, molded directions (orientations) of the lower transverse ribs  63 , lower longitudinal ribs  64  and lower slanting ribs  69  can be determined in such a manner as to secure a sufficient rigidity and strength of the lower section  42 . Consequently, the lower section  42  can be appropriately reinforced with the lower transverse and longitudinal ribs  63  and  64  and the lower slanting rib  69 . 
     With the lower section  42  appropriately reinforced with the lower transverse and longitudinal ribs  63  and  64  and the lower slanting rib  69  and with the upper section  41  reinforced with the plurality of upper transverse ribs  61  and the plurality of upper longitudinal ribs  62  as above, it is possible to even further increase the rigidity and strength of the sub frame  15 . As a result, it is possible to appropriately minimize an increase of the thickness T 1  (see  FIG. 5 ) of the subframe  15  and thus even more appropriately minimize an undesired increase of the weight of the subframe  15 . 
     Further, with the left longitudinal protruding section  66  widened outwardly and inwardly in the vehicle width direction from the rear end portion  66   a  toward the front end portion  66   b,  the front end portion  66   b  of the left longitudinal protruding section  66  can be formed into a relatively large size (see also  FIG. 7 ). Because the front end portion  66   b  of the left longitudinal protruding section  66  has a large size like this, the left front connection section  81  can be provided directly on the front end portion  66   b.  Thus, there is no need to form, on the front end portion  66   b  of the left longitudinal protruding section  66 , a particular part for providing thereon the left front connection section  81 , so that the subframe  15  can be simplified in construction. 
     As shown in  FIG. 4 , the right longitudinal protruding section  67  is in left-right symmetry with the above-described left longitudinal protruding section  66 , and it is provided along the right end portion  32   b  of the subframe body  32  between the right end portion  65   b  of the transverse protruding section  65  and the right rear connection section  88 . Namely, the transverse protruding section  65  is provided along the outer peripheral portion  32   g  of the subframe body  32 , the left longitudinal protruding section  66  is provided along the left end portion  32   a  of the subframe body  32 , and the right longitudinal protruding section  67  is provided along the right end portion  32   b  of the subframe body  32 . As viewed in bottom plan, the subframe body  32  is formed in a substantial C shape by the transverse protruding section  65  and the left and right longitudinal protruding sections  66  and  67 . 
     The recessed section  68  is surrounded by the transverse protruding section  65  and the left and right longitudinal protruding sections  66  and  67 . The recessed section  68  is formed in a substantial rectangular shape by the front end portion  68   a,  the left and right end portions  68   b  and  68   c  and the rear end portion  68   d.  The recessed section  68  protrudes upwardly relative to the transverse protruding section  65 , left longitudinal protruding section  66  (see also  FIG. 6 ) and right longitudinal protruding section  67 . Namely, the recessed section  68  protrudes toward the hollow portion  45  (see  FIG. 6 ). 
     The following describe, with reference to  FIGS. 4, 5, 7 and 9 to 12 , an example manner in which the subframe  15  is molded by high-pressure casting. 
     As shown in  FIG. 9 , the core  92  (depicted in imaginary line) is positioned with a casting mold  91  opened, and then, the casting mold  91  is clamped. By the clamping of the casting mold  91 , a cavity  93  is formed by the casting mold  91  and the core  92 . The cavity  93  is in communication with a plurality of flow paths (only one of the flow paths  101  is shown). With the cavity  91  clamped, the molten metal of aluminum alloy is directed to the plurality of flow paths. 
     The following describe the flow path  101  of the plurality of flow paths which communicates with a left end portion of the cavity  93 . The molten metal directed to the flow path  101  is then directed to the left end portion of the cavity  93  as indicated by arrow A. Then, a portion of the molten metal directed to the cavity  93  is directed to an upper cavity  94  as depicted by arrow B in  FIG. 10 . 
     Further, in the instant embodiment, as shown in  FIG. 5 , the upper section  41  of the subframe body  32  is formed in a gentle shape by the upper rear flat portion  55 , the upper slant portion  56  and the upper front flat portion  57 . Thus, as shown in  FIG. 10 , the upper cavity  94  is formed in a gentle shape, i.e. a shape with no irregularities, from a rear end portion  94   a  to a front end portion  94   b,  so that the flowability of the molten metal can be enhanced. 
     Thus, the molten metal directed to the upper cavity  94  can be smoothly directed along the upper cavity  94  as indicated by arrow C, so that the molten metal can be appropriately filled via the upper cavity  94  into a connection cavity  95  and a front wall cavity  96  as indicated by arrow D. 
     By the molten metal being appropriately filled into the connection cavity  95  and the front wall cavity  96  as above, it is possible to secure a sufficient rigidity and strength of the left front connection section  81  and the front wall  43  (particularly, left end portion of the front wall  43 ). The left end portion of the front wall  43  is formed by the front portion  32   c  of the left end portion  32   a  of the subframe body  32 . 
     The remaining portion of the molten metal directed to the cavity  93  is directed to a lower longitudinal cavity  97  as depicted by arrow E in  FIG. 10 . 
     Further, in the instant embodiment, as shown in  FIG. 5 , the lower section  42  of the subframe body  32  is formed in a gentle shape by the left longitudinal protruding section  66  and the left end portion  65   a  of the transverse protruding section  65  (see  FIG. 7 ) from the rear end portion  42   b  to the front end portion  42   a.  The left front connection section  81  is provided adjacent to and forward of the front end portion  42   a.  Namely, the lower section  42  of the subframe body  32  is formed in a gentle shape from the rear end portion  42   b  to the left front connection section  81 . 
     Thus, as shown in  FIG. 10 , the lower longitudinal cavity  97  is formed in a gentle shape, i.e. a shape with no irregularities, from a rear end portion  97   a  to a front end portion  97   b,  so that the flowability of the molten metal flowing along the lower longitudinal cavity  97  toward the front of the vehicle can be enhanced. 
     By virtue of the enhanced flowability along the lower longitudinal cavity  97 , the molten metal directed to the rear end portion  97   a  of the lower longitudinal cavity  97  can be smoothly directed along the lower longitudinal cavity  97  to the front end portion  97   b  as indicated by arrow F (see also  FIG. 11 ). 
     Further, as shown in  FIG. 11 , the front end portion  97   b  of the lower longitudinal cavity  97  is gently connected to a lower transverse cavity  98  that corresponds to the transverse protruding section  65  (see  FIG. 7 ). Thus, the molten metal directed to the front end portion  97   b  of the lower longitudinal cavity  97  can be smoothly directed along the lower transverse cavity  98  as indicated by arrow G. 
     In addition, the longitudinal bottom portion  74  of the left longitudinal protruding section  66  is widened outwardly and inwardly in the vehicle width direction from the rear end portion  66   a  to the rear end portion  66   b.  Thus, the lower longitudinal cavity  97  is widened outwardly in the vehicle width direction from the rear end portion  97   a  (see  FIG. 10 ) to the front end portion  97   b.    
     Further, as shown in  FIG. 9 , the flow path  101  communicating with a left end portion of the cavity  93  extends radially to the cavity  93 . Thus, the lower longitudinal cavity  97  has an outer wall  97   c  (see also  FIG. 11 ) formed on and along an extension of the left path  101 . Therefore, the molten metal directed to the left flow path  101  can be even more smoothly directed, via the lower longitudinal cavity  97  shown in  FIG. 11 , outwardly and inwardly in the vehicle width direction to the front end portion  97   b  as indicated by arrows F (particularly, by outward arrow F). 
     Thus, when the subframe  15  shown in  FIG. 4  is to be molded by high-pressure casting, the molten metal poured through the rear portion  15   b  of the subframe  15  can be smoothly directed to the left front connection section  81 . 
     By the molten metal being smoothly directed to the left front connection section  81  as above, it is possible to secure a sufficient rigidity and strength of the left front connection section  81 . Consequently, there is no need to form the subframe  15  into a large thickness T 1  (see  FIG. 5 ), with the result that an undesired increase of the weight of the subframe  15  can be minimized 
     The front end portion  97   b  of the lower longitudinal cavity  97  shown in  FIG. 11  is a portion corresponding to the left front connection section  81  (see  FIG. 5 ) of the subframe  15 . Thus, by the molten metal being smoothly directed to the front end portion  97   b  of the lower longitudinal cavity  97 , it is possible to secure a sufficient rigidity and strength of the left front connection section  81  and a sufficient rigidity and strength of the subframe  15 . 
     By forming the lower section  42  shown in  FIG. 5  in a gentle shape to thereby secure a sufficient rigidity and strength of the left front connection section  81  (subframe  15 ) as above, it is possible to eliminate the need for increasing the thickness T 1  of the subframe  15  with a view to securing a sufficient rigidity and strength of the subframe  15 . In this way, the thickness T 1  of the subframe  15  can be appropriately limited, and thus, an increase of the weight of the subframe  15  can be minimized. 
     Further, as shown in  FIGS. 4 and 12 , a cavity  93   a  corresponding to the left rear connection section  82  is located relatively near a left flow path  102 . Thus, the molten metal can be smoothly directed to the cavity  93   a  via the left flow path  102  as indicated by arrow H, so that a sufficient rigidity and strength of the left rear connection section  82  can be secured. 
     Thus, when the subframe  15  is molded by high-pressure casting, a sufficient rigidity and strength of the left front connection section  81  and the left rear connection section  82  can be secured. In this way, a relatively large load input via the left suspension arm  16  to the left front connection section  81  and the left rear connection section  82  can be borne appropriately by the left front connection section  81  and the left rear connection section  82 , and thus, an increased reliability of the subframe  15  can be achieved. 
     It should be appreciated that the vehicle subframe of the present invention is not limited to the above-described embodiment and may be modified as appropriate. 
     For example, whereas the embodiment of the present invention has been described above in relation to the case where the subframe  15  is formed of an aluminum alloy, the present invention is not so limited, and the subframe  15  of the present invention may be formed of any other suitable metal than aluminum alloy. 
     Further, whereas the embodiment of the present invention has been described above in relation to the case where the subframe  15  is formed of aluminum alloy by high-pressure casting, the present invention is not so limited, and the subframe  15  of the present invention may be formed by any other suitable type of casting. 
     Furthermore, whereas the embodiment of the present invention has been described above in relation to the case where the left and right longitudinal protruding sections  66  and  67  are widened outwardly and inwardly in the vehicle width direction from the rear end portion  66   a  toward the front end portion  66   b,  the present invention is not so limited. For example, the left and right longitudinal protruding sections  66  and  67  may be widened only outwardly in the vehicle width direction from the rear end portion  66   a  toward the front end portion  66   b.  Even in the case where the left and right longitudinal protruding sections  66  and  67  are widened only outwardly in the vehicle width direction, the same advantageous benefits as in the above-described embodiment can be achieved. 
     The shapes and constructions of the subframe, left and right suspension arms, left and right suspensions, body section, left and right suspension support sections, upper section, lower section, transverse protruding section, left longitudinal protruding section, right longitudinal protruding section, recessed section, left and right front connection sections, left and right rear connection sections, core, etc. are not limited to those illustratively shown and described in relation to the embodiment and may be modified as appropriate. 
     INDUSTRIAL APPLICABILITY 
     The basic principles of the present invention are well suited for application to automobiles equipped with a subframe which is provided under a vehicle body and in which left and right suspensions are supported by left and right suspension arms connected to a subframe body. 
     LIST OF REFERENCE SIGNS 
       10  . . . front vehicle body structure,  15  . . . subframe (vehicle subframe),  16 ,  17  . . . left and right suspension arms,  21 ,  22  . . . left and right suspensions,  32  . . . subframe body,  32   a  . . . left end portion of the subframe body,  32   b  . . . right end portion of the subframe body,  32   c  . . . front portion of the left end portion of the subframe body (front portion of the subframe body),  32   d  . . . rear portion of the left end portion of the subframe body (rear portion of the subframe body),  32   e  . . . front portion of the right end portion of the subframe body (front portion of the subframe body),  32   f  . . . rear portion of the right end portion of the subframe body (rear portion of the subframe body),  32   g  . . . outer peripheral portion,  35 ,  37  . . . left and right suspension support sections,  41  . . . upper section,  42  . . . lower section,  65  . . . transverse protruding section,  66  . . . left longitudinal protruding section,  66   a  . . . rear end portion of the left longitudinal protruding section,  66   b  . . . front end portion of the left longitudinal protruding section,  67  . . . right longitudinal protruding section,  68  . . . recessed section,  81  . . . left front connection,  82  . . . right front connection,  87  . . . right front connection,  88  . . . right rear connection,  92  . . . core