Abstract:
Disclosed is a seat rail ( 30 ) of a motorcycle having a pair of left and right rail sections ( 45 L,  45 R) which do not have a tendency to store water and which have a sufficient rigidity. Each rail section comprises a first groove section ( 64 ) and a second groove section ( 65 ). A drain hole ( 71 ) for draining the water stored at the bottom ( 69 ) of the second groove section downward is formed in each rail.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to motorcycle seat rails, and more particularly motorcycle seat rails made by casting. 
       BACKGROUND ART 
       [0002]    Many of the conventionally-known automotive two-wheeled vehicles or motorcycles employ a structure where a seat rail extends from a vehicle body frame and a seat is supported on the seat rail. Among the conventionally-known motorcycle seat rails are ones made of pipes, plates, etc. as well as ones made by casting as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2007-261587 (“patent literature 1”). 
         [0003]    As disclosed in patent literature 1, a seat frame, which constitutes the seat rail, is made by die-casting and extends obliquely rearward and upward from a rear arm bracket that is a rear-part member of the vehicle body frame. 
         [0004]    The seat frame comprises left and right rail sections interconnected at their upper ends by a cross member. Each of the rail sections includes a large-thickness base portion, an extension portion extending from the large-thickness base portion, and a large-thickness edge portion formed at the lower end of the extension portion. 
         [0005]    The seat frame has a generally U sectional shape opening downward, and thus, it can be readily released from a mold; particularly, the seat frame has a sectional configuration suited for die-casting. As loads of a passenger and goods onboard act downwardly on the seat frame, stress would increase in the edge portion. As measures against such a stress increase, the edge portion is formed to have a large thickness as noted above in order to provide an increased sectional area so that the stress increase can be limited to below a predetermined level. Namely, it is effective to increase the thickness of the edge portion. 
         [0006]    So far, the dominating seat frames of this type have been ones made by die-casting an aluminum alloy (i.e., aluminum-alloy die-cast seat frames), because such an aluminum alloy can achieve a lighter weight than an iron-based alloy. 
         [0007]    In recent years, more and more seat frames made by die-casting a magnesium alloy (i.e., magnesium-alloy die-cast seat frames) are being brought into practical use, for purposes of further weight reduction etc. Magnesium has a high specific strength and superior workability and vibration absorbing performance as compared to aluminum. However, because a magnesium alloy is easily subject to oxidation, it took a long time to establish a technique for making a seat frame by die-casting a magnesium alloy, and thus, practical realization of a seat frame made by die-casting a magnesium alloy has been delayed until recent years. 
         [0008]    The following explain specific gravities and Young&#39;s moduli of such an aluminum-alloy die-cast seat frame and magnesium-alloy die-cast seat frame, with reference to Table 1 below. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Name of 
                 Specific 
                 Young&#39;s 
               
               
                   
                 Alloy 
                 Gravity 
                 Modulus 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aluminum-alloy 
                 ADC3 
                 2.7 
                 70 GPa 
               
               
                   
                 Die-cast 
               
               
                   
                 Magnesium-alloy 
                 AM60B 
                 1.8 
                 44 GPa 
               
               
                   
                 Die-cast 
               
               
                   
                   
               
             
          
         
       
     
         [0009]    The magnesium alloy is lighter in weight than the aluminum alloy but much smaller in Young&#39;s modulus than the aluminum alloy. Because the Young&#39;s modulus influences an amount of bending or deflection, the magnesium alloy bends more greatly than the aluminum alloy in response to a same load. To limit the amount of bending to below a predetermined value, it is necessary to increase second moment of area. For that purpose, there is a need to further increase the thickness of the above-mentioned edge portion. However, if the thickness of the edge portion is increased, the seat frame would increase in overall weight and size, so that manufacturing cost of the seat frame would increase. 
         [0010]    Further, if the edge portion is formed to project horizontally, water tends to accumulate on/in the upper surface of the horizontally projecting edge portion. Water accumulation on/in the upper surface of the horizontally projecting edge portion is undesirable in that corrosion is likely to occur in the edge portion. Such water accumulation is also undesirable from the viewpoint of operability. 
         [0011]    Therefore, there has been a demand for a motorcycle seat rail where water is difficult to accumulate and which has a sufficient rigidity. 
       PRIOR ART LITERATURE 
       [0000]    
       
         Patent Literature 1: Japanese Patent Application Laid-open Publication No. 2007-261587 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0013]    It is therefore an object to provide a motorcycle seat rail which is a magnesium-alloy die-cast seat rail, where water is difficult to accumulate, and which has a sufficient rigidity. 
       Solution to Problem 
       [0014]    According to one aspect of the present invention, there is provided a seat rail of a motorcycle which includes a pair of left and right rail sections extending rearward from a vehicle body frame for supporting thereon a seat, the seat rail being a magnesium-alloy cast seat rail, each of the left and right rail sections having a first groove section formed in an upper inner side region thereof as viewed in a vehicle width direction and opening downward, and a second groove section formed in a lower, outer side region thereof as viewed in the vehicle width direction and opening upward, the second groove section having a drainage hole formed in a bottom portion thereof for draining accumulated water in a downward direction. 
         [0015]    Preferably, each of the left and right rail sections has a lower surface, and an annular protruding portion formed integrally on the lower surface so as to surround a lower opening end of the drainage hole. 
         [0016]    Preferably, the seat rail of the invention has a threaded through-hole for allowing another member to be fastened to the seat rail by means of a bolt, and which further has an increased-thickness portion formed thereon around the threaded through-hole such that the threaded through-hole has a length equal to two times a diameter of the through-hole plus a length in a range of 2 mm to 4 mm. 
         [0017]    Preferably, the second groove section has an outer wall portion extending upward from an outer side region, in the vehicle width direction, of the bottom portion, and the outer wall portion has a boss formed integrally thereon so that the distal end of a push pin abuts against the boss at the time of mold release during manufacturing of the seat rail. 
         [0018]    Preferably, the second groove section is segmented in a plurality of segmented groove portions, and the drainage hole is provided in each of the segmented groove portions in such a manner that the drainage hole is located at the lowermost position in the segmented groove portion in a state where the seat rail is mounted in place on the vehicle body in a slanting posture. 
       Advantageous Effects of Invention 
       [0019]    According to the present invention, the first groove section opening downward is provided in an upper region of each of the rail sections, and the second groove section opening upward is provided in a lower region of each of the rail sections. It is known that a groove-shaped sectional configuration has a particularly great section modulus and second moment of area as compared to a rectangular sectional configuration. Each of the rail sections, where such groove-shaped sectional configurations are disposed in vertically-spaded-apart relation to each other, has a great bending rigidity and deflection rigidity as a whole. Namely, even when the seat rail bends due to a load applied from above, stress produced in a lower region of the rail section can be lowered below an acceptable value because the rail section has a great rigidity. In addition, through the drainage hole, water accumulating on/in the second groove section can be promptly drained from the second groove section, so that occurrence of galvanic corrosion can be suppressed. 
         [0020]    The annular protruding portion is formed integrally on the lower surface so as to surround the lower opening end of the drainage hole. When the seat rail bends due to a load applied from above, stress may concentrate in a region around the drainage hole. However, because that region is reinforced by being formed as the annular protruding portion, it is possible to sufficiently lower the stress level. 
         [0021]    The increased-thickness portion is formed around the threaded through-hole such that the threaded through-hole has a length equal to two times the diameter of the through-hole plus a length in a range of 2 mm to 4 mm. JIS B 1181 prescribes that a nut made of carbon steel should have a thickness that is about 85% of the diameter of a threaded hole. A magnesium alloy is more liable to present a more noticeable permanent strain due to heating than other metals. As measures against such a permanent strain, the seat rail of the invention has the increased-thickness portion around the threaded through-hole such that the length of the threaded through-hole is equal to two times the diameter of the through-hole plus a length in the range of 2 mm to 4 mm. Further, if a distal end portion of the bolt projects out of the threaded through-hole, galvanic corrosion may occur in the projecting portion. Thus, in the present invention, the length of the threaded through-hole is set to be equal to two times the diameter of the through-hole plus a length in the range of 2 mm to 4 mm as noted above, so that a sufficient axial force can be secured. 
         [0022]    The outer wall portion has a boss formed integrally thereon so that the distal end of a push pin abuts against the boss at the time of mold release. The provision of the boss can achieve an enhanced mold release performance. At the same time, the outer wall portion can be reduced in thickness in the other region than the boss, and thus, the second groove section can be reduced in thickness and weight as a whole. 
         [0023]    Because the second groove section is segmented in a plurality of segmented groove portions, the groove section can have an increased rigidity in the width direction of the groove section. In addition, because the drainage hole provided in each of the segmented groove portions is located at the lowermost position in the segmented groove portion in a state where the seat rail is mounted in place on the vehicle body in a slanting posture, water can be caused to drop reliably from each of the groove portions. Namely, the second groove section in the present invention is constructed to reliably drop water while securing necessary rigidity of the groove section. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a side view showing a rear section of a motorcycle provided with a seat rail of the present invention; 
           [0025]      FIG. 2  is a sectional view taken along line  2 - 2  of  FIG. 1 ; 
           [0026]      FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 1 ; 
           [0027]      FIG. 4  is a graph showing results of a corrosion test; 
           [0028]      FIG. 5  is a plan view of the rear section of the motorcycle provided with the seat rail of the present invention; 
           [0029]      FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 5 ; 
           [0030]      FIG. 7  is a sectional view taken along line  7 - 7  of  FIG. 5 ; 
           [0031]      FIG. 8  is a side view of the seat rail; 
           [0032]      FIG. 9  is a plan view of the seat rail shown in  FIG. 8 ; 
           [0033]      FIG. 10  is a sectional view taken along line  10 - 10  of  FIG. 9 ; 
           [0034]      FIG. 11  is a sectional view of a die-cast mold including core pins; 
           [0035]      FIG. 12  is a sectional view taken along line  12 - 12  of  FIG. 9 ; 
           [0036]      FIG. 13  is a sectional view of a die-cast mold including push pins; and 
           [0037]      FIG. 14  is a sectional view showing behavior of the push pins. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0038]    A description will be given about preferred embodiments of the present invention with reference to the accompanying drawings. 
         [0039]    As shown in  FIG. 1 , a seat rail  30  of the present invention, which is constructed to support thereon a driver&#39;s seat  12  and a pillion passenger&#39;s seat  13 , is fixedly mounted to a vehicle body frame  11  of a motorcycle  10  so as to extend rearward from the vehicle body frame  11 . An exhaust pipe  15  and muffler  16  are disposed beneath the seat rail  30 . Thus, the seat rail  30  tends to assume relatively high temperatures. 
         [0040]    The seat rail  30  is made by casting a magnesium alloy that, for example, consists of Al making up 5.5-6.5%, Mn making up 0.24-0.6%, Zn making up 0.22% or less, Si making up 0.1% or less, RE making up 0.2% or over, minute amounts of Cu, Ni and Fe, with the remaining percentage being Mg. 
         [0041]    Further, a pillion step holder  17  extends downward from a portion of the seat rail  30  located forward of the middle of the seat rail  30 . A heat-shield-plate rubber mount fastening portion  18  of a muffler and a reserve tank mounting portion  19  of a rear brake master cylinder extend from the bottom of the seat rail  30 . 
         [0042]    A rear fender is provided over a rear wheel  21 , and this rear fender comprises a front portion  47  ( FIG. 6 ) and a rear portion  22 . 
         [0043]    As shown in  FIG. 2 , the pillion step holder  17  of a V shape, which is a die-cast member, includes a step mounting portion  24  located in a valley portion  23  thereof, and two leg portions  25  extending upward from the valley portion  23 . 
         [0044]    The seat rail  30  has two threaded through-holes  31  and  32 , and a region around one of the threaded through-holes  31  is formed as an increased-thickness portion  33  such that the threaded through-hole  31  has a length equal to two times a diameter d of the through-hole  31  plus a length in a range of 2 mm to 4 mm. Namely, in order to have an increased total thickness, the seat rail  30  has the increased-thickness portion  33  integrally formed thereon around the through-hole  31 . The other threaded through-hole  32  per se has a length L 2  equal to two times a diameter d of the through-hole  32  plus a length in a range of 2 mm to 4 mm. Namely, because the seat rail  30  has, around the through-hole  32 , an increased thickness equal to two times the diameter d of the through-hole  32  plus a length in the range of 2 mm to 4 mm, no increased-thickness portion  33  is formed around the through-hole  32 . 
         [0045]    The pillion step holder  17  is fixed to the seat rail  30  by the leg portions  25  being abutted against the underside of the seat rail  30  via alumite-coated washers  34  and by chrome-plated bolts  35  being screwed into the threaded through-holes  31  and  32 . Because alumite has Al 2 O 3  as its main component, it has electric insulating performance. Thus, the alumite-coated washers  34  can serve to prevent galvanic corrosion between the seat rail  30  and the pillion step holder  17  that is a separate member from the seat rail  30 . 
         [0046]    JIS B 1180 prescribes that a nut made of carbon steel should have a thickness that is about 85% of a diameter of a threaded hole. Namely, if the nut and the threaded hole are each made of carbon steel, the threaded hole only need have a length that is about 0.85 times the diameter of the threaded hole. 
         [0047]    A magnesium alloy is more liable to present a more noticeable permanent strain due to heating than other metals. As measures against such a permanent strain, the seat rail  30  has the increased-thickness portion  33  around the threaded through-hole  31  in such a manner that the length L 1  of the threaded through-hole  31  is equal to two times the diameter d of the through-hole  31  plus a length in the range of 2 mm to 4 mm. 
         [0048]    If the seat frame  30  is made of the magnesium alloy and the plated bolts  35  are made of chrome-plated steel, and if water is present between the seat frame  30  and any of the plated bolts  35 , galvanic corrosion would occur, because the seat frame  30  and the plated bolts  35  are made of different materials. 
         [0049]    The inventors of the present invention conducted an experiment to find that the galvanic corrosion between the seat frame  30  and the plated bolt  35  is below 2 mm at most. 
         [0050]    As measures against such galvanic corrosion, the lengths L 1  and L 2  of the threaded through-holes  31  and  32  are each set to equal two times the diameter d of the through-hole  31  or  32  plus a length in the range of 2 mm to 4 mm. Thus, even where some galvanic corrosion has occurred, the length L 1  of the threaded through-hole  31  can secure a bearing width two times the hole diameter d. Namely, with the lengths L 1  and L 2  of the threaded through-holes  31  and  32  set appropriately, it is possible to achieve two advantageous benefits, i.e. securement of necessary screw-fastening performance and galvanic corrosion measures. Note that the thickness of the increased-thickness portion  33  only need be set such that the length of the threaded through-hole  31  is equal to or greater than two times the diameter d of the through-hole  31  (i.e., 2d+2 mm), more preferably (2d+a value in the range of from 2 mm to 4 mm). 
         [0051]    As shown in  FIG. 3 , a distal end portion of the seat rail  30  is lapped on the vehicle body frame  11  made by casting, and a plated bolt  37  is passed through the distal end portion of the seat rail  30  and vehicle body frame  11  via an alumite-coated washer  36  of a 2.5 mm thickness. Then, a distal end portion of the plated bolt  37  is screwed into a nut  38 , to thereby fix the seat rail  30  to the vehicle body frame  11 . 
         [0052]    Because the plated bolt  37  made of chrome-plated steel and the seat rail  30  made of the magnesium alloy are of different materials, unwanted galvanic corrosion could occur. Thus, the alumite-coated washer  36  is disposed between the plated bolt  37  and the seat rail  30 , to electrically insulate between the plated bolt  37  and the seat rail  30 . However, if a water drop  39  lies astride between the head of the plated bolt  37  and the seat rail  30  as indicated by imaginary line, galvanic corrosion might occur. Therefore, the outer diameter of the alumite-coated washer  36 , i.e. dimension w, becomes important. 
         [0053]    The inventors etc. conducted a CCT (combined cyclic corrosion test), focusing on the distance w between the outer periphery of the alumite-coated washer  36  and the outer periphery of the plated bolt  37 . Results of the test are shown in graphs of ( a ) and ( b ) of  FIG. 4 . 
         [0054]    In ( a ) of  FIG. 4 , the horizontal axis represents the number of cycles of the combined CCT (“number of CCT cycles”), while the vertical axis represents an amount of corrosion (i.e., corrosion loss depth). Where the distance w is 0.5 mm, the amount of corrosion exceeds 600 μm. Where the distance w is 1.5 mm, the amount of corrosion stays at 250 μm. Further, where the distance w is 2.5 mm, the amount of corrosion is not much different from that in the case where the distance w is 1.5 mm. 
         [0055]    The inventors etc. further examined correlationship between the dimension w and the thickness of the alumite-coated washer, results of which were shown in the graph of ( b ) of  FIG. 4 . 
         [0056]    In ( b ) of  FIG. 4 , the horizontal axis represents the thickness of the alumite-coated washer, while the vertical axis represents the dimension w. It could be confirmed that, where the thickness of the alumite-coated washer  36  is 1.0 mm, a small-large boundary of the corrosion amount exists at a point where the dimension w is 3.5 mm, and that, where the thickness of the alumite-coated washer  36  is 2.5 mm, the small-large boundary of the corrosion amount exists at a point where the dimension w is 1.5 mm. For example, by setting the thickness of the alumite-coated washer at 2 mm and setting the dimension w at 3 mm, it is possible to limit the corrosion of the alumite-coated washer within a small corrosion amount region, as indicated by broken line. Namely, by selecting (designing) a thickness of the alumite-coated washer  36  and a dimension w such that the amount of corrosion of the alumite-coated washer can fall within the small corrosion amount region, it is possible to suppress galvanic corrosion. 
         [0057]    As shown in  FIG. 5 , the driver&#39;s seat  12  is placed on the seat rail  30 , indicated by broken line, via four cushion rubbers  41 , and the driver&#39;s seat  12  is fastened to the seat rail  30  by means of a plurality of bolts  42 . The pillion passenger&#39;s seat  13  located behind the driver&#39;s seat  12  is surrounded by a rear cowl  43 , and the muffler  16  having left and right tail pipes  44  are accommodated inside the rear cowl  43 . 
         [0058]    As shown in  FIG. 6 , the seat rail  30  includes a pair of a left rail section  45 L and a right rail section  45 R. The exhaust pipe  15  and heat shield plate  46  are disposed between the left rail section  45 L and the right rail section  45 R and closer to the right rail section  45 R, the front portion  47  of the rear fender spans between the left rail section  45 L and the right rail section  45 R, and the driver&#39;s seat  12  is placed on the front portion  47  of the rear fender. 
         [0059]    The driver&#39;s seat  12  includes a seat bottom plate  48 , a cushion material  49  placed on the seat bottom plate  48 , and a seat skin  51  wrapping the cushion material  49 . The seat bottom plate  48  is fastened to the left and right rail sections  45 L and  45 R by means of bolts  42 , so that the driver&#39;s seat  12  is fixed to the seat rail  30 . As shown, the front portion  47  of the rear fender is kept pressed downward by the seat bottom plate  48  in such a manner that it does not float up beyond the seat rail  30 . Attachment/detachment of the bolts  42  can be facilitated by left and right end portions of the driver&#39;s seat  12  being pushed up as necessary. 
         [0060]    As shown in  FIG. 7 , the left rail section  45 L and the right rail section  45 R are interconnected via a fourth cross section  54  with the front portion  47  of the rear fender placed thereon, and a seat hook engaging member  55  is fastened to middle portions of the front portion  47  and fourth cross section  54  by means of bolts  56 . As shown in  FIG. 5 , the seat hook engaging member  55  is a member for supporting the pillion passenger seat  13 , and the seat hook engaging member  55  also functions to keep the front portion  47  and fourth cross section  54  pressed so as not to lift up beyond the fourth cross section  54 . A heat shield cover  57  is placed over the upper surface of the muffler  16 . 
         [0061]    The following describe in more detail the seat rail  30 . 
         [0062]    Referring to  FIG. 8 , the seat rail  30  is a magnesium-alloy die-cast seat rail having a generally triangular shape as viewed in side elevation. The seat rail  30  has bolt holes  57  formed in its front end portions, and bolt holes  58  formed in its rear end portions for fastening the rear portion  22  ( FIG. 1 ) of the rear fender to the rear end portions. 
         [0063]    Further, as shown in  FIG. 9 , the seat rail  30  is a generally ladder-shaped rail in where the left rail section  45 L and the right rail section  45 R are interconnected via first to fourth cross sections  61 ,  62 ,  63  and  64 . Such a seat rail  30  will be described below in greater detail. 
         [0064]    Referring to  FIG. 10 , the left rail section  45 L has: a first groove section  64  formed in its upper, inner side region as viewed in a vehicle width direction right side region in  FIG. 10  and opening downward; and a second groove section  65  formed in its lower, outer side region as viewed in the vehicle width direction (left side in  FIG. 10 ) and opening upward. Namely, the first groove section  64  includes an upper end portion  67  extending inward in the vehicle width direction from the upper end of a main, vertical wall portion  66 , and an inner wall portion  68  extending downward from the upper end portion  67 . 
         [0065]    The second groove section  65  includes a bottom portion  69  extending outward in the vehicle width direction from the lower end of the vertical wall portion  66 , and an outer wall portion  70  extending upward from the bottom portion  69 . Although not particularly described here, the right rail section  45 R has a sectional structure similar to the above-described sectional structure of the first groove section  64 . 
         [0066]    It is known that a groove-shaped sectional configuration has a particularly great section modulus and second moment of area as compared to a rectangular sectional configuration. The rail section  45 L, where such groove-shaped sectional configurations (groove sections  64  and  65 ) are disposed in vertically-spaded-apart relation to each other, has a great bending rigidity and deflection rigidity as a whole. Namely, even when the seat rail  30  ( FIG. 9 ) bends due to a load applied from above, stress produced in a lower region of the rail section  45 L can be lowered below an acceptable value because the rail section  45 L has a great rigidity. 
         [0067]    Further, the bottom portion  69  of the second groove section  65  has a drainage hole  71  formed therethrough. Through the drainage hole  71 , water accumulating on/in the second groove section  65  can be promptly drained from the second groove section  65 , so that occurrence of galvanic corrosion can be suppressed. 
         [0068]    Referring again to  FIG. 8 , the rail section  45 L has an annular protruding portion  72  formed integrally on the lower surface thereof so as to surround the lower opening end of the drainage hole  71 . When the seat rail  30  bends due to a load applied from above, stress may concentrate in a region around the drainage hole  71 . However, because that region is reinforced by being formed as the annular protruding portion  72 , it is possible to sufficiently lower the stress level. 
         [0069]    In addition, as shown in  FIG. 9 , the second groove section  65  is segmented in four groove portions by a plurality of, e.g. three, ribs  73  formed on one side thereof. By the reinforcing function of the ribs  73 , the second groove section  65  can have an increased rigidity in its width direction. The above-mentioned drainage hole  71  is provided in each of the segmented groove portions. In each of the segmented groove portions of the groove section  65 , as shown in  FIG. 8 , the drainage hole  71  is located at the lowermost position in the segmented groove portion in a state where the seat rail is mounted in place on the vehicle body in a slanting posture. In this way, water can be caused to drop reliably from each of the segmented groove portions of the groove section  65 . 
         [0070]      FIG. 11  shows a die-cast mold including core pins. The die-cast mold  74  comprises a fixed mold  75  and a movable mold  76  movable relative to the fixed mold  75 . The core pins  77 , each having a tapering shape, are provided on the fixed mold  75 , and the movable mold  76  has recessed portions  78  for receiving respective distal ends of the core pins  77 . After mold clamping, high-pressure magnesium alloy melt is poured into a cavity  79 , and the movable mold  76  is separated or detached from the fixed mold  75  after the magnesium alloy melt has coagulated. Because the core pins  77  are left on the fixed mold  75 , the movable mold  76  is detached from the fixed mold  75  with the cast, having an opening formed therein as the drainage hole  71 , attached to the movable mold  76 . 
         [0071]    As shown in  FIG. 12 , a plurality of bosses (five bosses in the instant embodiment)  81  (see also  FIG. 9 ) are formed integrally on and along the outer wall portion  70  of the second groove section  65 . The outer wall portion  70  has a relatively small thickness, and only the bosses  81  project locally toward the vertical wall portion  66 . Because the outer wall portion  70  has a relatively small thickness, the seat rail will not become heavy in weight. These bosses  81  are very helpful in separating the molds  75  and  76  from each other. 
         [0072]      FIG. 13  shows a die-cast mold including push pins. The die-cast mold  74  shown in  FIG. 13  includes first push pins  82  provided on the fixed mold  75  for being opposed to the bosses  81 , and second push pins  83  provided on the movable mold  76  for pushing the bottom portions  69 . 
         [0073]    When the movable mol  76  is to be detached from the fixed mold  75 , as shown in ( a ) of  FIG. 14 , the bosses  81  are pushed by the first push pins  82 . Then, die-cast items  84  can be smoothly detached from the fixed mold  75  together with the movable mol  76  without being left on the fixed mold  75 . 
         [0074]    Then, as shown in ( b ) of  FIG. 14 , the die-cast items  84  can be detached from the movable mol  76  by being pushed by the second push pins  83 . The thus-obtained die-cast items  84  are subjected to additional processing, such as machine work, to thereby provide the seat rail  30  shown in  FIG. 9 . 
         [0075]    Whereas it is preferable that the seat rail be made by die-casting, the seat rail may be made by any other desired type of casting, such as gravity die-casting or metal mold casting or sand mold casting. 
         [0076]    Whereas the above-described embodiment of the seat rail is designed to collectively support thereon the driver&#39;s seat and pillion passenger&#39;s seat, the seat rail of the present invention may be constructed to support thereon only the driver&#39;s seat. Namely, the number of seats to be supported on the seat rail of the present invention is not limited to a particular number. 
       INDUSTRIAL APPLICABILITY 
       [0077]    The seat rail of the present invention is preferably a magnesium-alloy cast seat rail. 
       LEGEND 
       [0000]    
       
         
           
               10  motorcycle 
               11  vehicle body frame 
               12  seat (driver&#39;s seat) 
               13  seat (pillion passenger&#39;s seat) 
               17  pillion step holder 
               30  seat rail 
               31  threaded through-hole 
               35  bolt (chrome-plated bolt) 
               64  first groove section 
               65  second groove section 
               69  bottom portion 
               70  outer wall portion 
               71  drainage hole 
               72  annular protruding portion 
               81  boss 
               82  push pin (first push pin) 
               84  die-cast item 
             d diameter of the threaded through-hole 
             L 1  length of the threaded through-hole