Patent Publication Number: US-6904990-B2

Title: Frame assembly for snowmobile

Description:
RELATED APPLICATIONS 
   This application is based upon and claims priority to Japanese Patent Application No. 2001-241610, filed on Aug. 9, 2001, which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a frame assembly for a snowmobile, and more particularly to an improved frame assembly for an engine compartment of a snowmobile. 
   2. Description of the Related Art 
   A typical snowmobile includes a frame assembly, a drive assembly including an endless drive belt that engages the terrain for propelling the snowmobile along the terrain, and a slide rail unit that supports the drive belt. The frame assembly carries an engine which powers the drive belt through a suitable transmission assembly. Typically, a pair of front skis depends from the frame assembly for steering the snowmobile. A steering handle extends to the rider&#39;s area from the frame assembly so that the rider can operate the steering handle. A steering linkage couples the skis with the steering handle. 
   The transmission assembly conveys the engine power to the drive assembly. The engine typically includes an output shaft. The transmission assembly can include a transfer shaft journaled on the frame assembly. The output shaft can drive the transfer shaft through a continuously variable transmission mechanism such as a v-belt transmission formed with a drive pulley, driven pulley and a v-belt. The drive pulley can be affixed onto the output shaft, while the driven pulley can be affixed onto the transfer shaft. The v-belt is wound around the drive and driven pulleys. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention includes the realization that certain mechanical difficulties with snowmobiles are the result of small fluctuations or irregularities in support portions of the frame of the snowmobile. For example, many conventional snowmobiles include engine compartments formed of numerous pieces of sheet and structural metal welded or riveted to a main frame assembly. This method of manufacturing the engine compartment of a snowmobile, however, yields irregularities in the dimensions of certain portions of the engine compartment. For example, certain portions of the engine compartment contain supports for components of the transmission of a snowmobile, which typically includes a continuously variable transmission. These types of transmissions are adversely effected by variations in the spacing and arrangement of the components, such as, for example, but without limitation, shaft spacing. Thus, it has been found that snowmobile engine compartments that are manufactured from a plurality of structural and/or sheet metal welded or riveted together yield dimensional variations that adversely effect the performance of the snowmobile transmission. 
   In accordance with another aspect of the present invention, a snowmobile comprises a frame assembly. A drive assembly depends from the frame assembly and is adapted to abut the terrain so as to propel the frame assembly along the terrain. An internal combustion engine is configured to power the drive assembly. The engine includes an output shaft. A transmission assembly is configured to transfer power of the engine to the drive assembly. The transmission assembly includes a transfer shaft driven by the output shaft. The frame assembly comprises a frame member disposed rearward of the engine. The frame member monolithically defining side sections, a rear section and a bottom section. The transfer shaft transversely extends between the side sections and journaled by the side sections. The frame member supports the engine in front of the transfer shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will be better understood with reference to a preferred embodiment, which is illustrated in the accompanying drawings. The illustrated embodiment is merely exemplary and is not intended to define the outer limits of the scope of the present invention. The drawings of the illustrated arrangement comprise 20 figures. 
       FIG. 1  is a side elevational view of a snowmobile having a frame assembly arranged and configured in accordance with certain features, aspects and advantages of the present invention. Certain portions are illustrated as being broken away and other internal portions are shown in hidden line. 
       FIG. 2  is a perspective view of the frame assembly and depending components 
       FIG. 3  is a partial perspective view of the frame assembly with the depending components and reinforcement wholly removed. 
       FIG. 4  is a top plan view of a front member that forms a portion of the frame assembly. 
       FIG. 5  is a side elevational view of the frame member. 
       FIG. 6  is a front elevational view of the frame member. 
       FIG. 7  is a front elevational view of a heat insulating cover disposed above the frame member. 
       FIG. 8  is a side elevational view of a portion of the frame member and the heat insulating cover. The heat insulating cover is illustrated in cross-section. One of exhaust runners extending through the heat insulating cover is shown in solid line. An engine snowmobile are shown in phantom. 
       FIG. 9  is a top plan view of a rear engine mount. Rear mount bosses of the engine and a collar are shown in phantom. 
       FIG. 10  is a top plan view of a front engine mount on the left hand side. A front mount boss is shown in phantom. 
       FIG. 11  is a partial side elevational view of the frame assembly generally corresponding to the part illustrated in FIG.  3 . Part of an engine cooling system is shown in this figure. 
       FIG. 12  is a front elevational view of the frame assembly. The part of the engine cooling system also is shown in this figure. 
       FIG. 13  is an enlarged side elevational view of a foot step formed at the frame assembly. 
       FIG. 14  is a front elevational view of a steering system of the snowmobile. Some portions are schematically illustrated. 
       FIG. 15  is a partial perspective view of the steering system of FIG.  14 . 
       FIG. 16  is a side elevational view of the frame assembly and a chain housing defined aside the frame assembly. 
       FIG. 17  is a cross-sectional view of the chain housing and a portion of the frame assembly taken along the line  17 — 17  of FIG.  16 . 
       FIG. 18  is a partial sectional and top plan view of the chain housing. The portion of the frame assembly also is shown. 
       FIG. 19  is a cross sectional view of a modification of the chain housing that is shown in FIG.  17 . 
       FIG. 20  is a partial sectional and top plan view of a v-belt transmission of the snowmobile. A portion of the frame assembly also is shown. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With particular reference to  FIGS. 1-3 ,  11  and  12 , an overall construction of a snowmobile  20  including certain features, aspects and advantages of the present invention is described below. 
   The snowmobile  20  generally comprises a frame assembly  22  ( FIG. 2 ) that carries a number of other components of the snowmobile  20 . The frame assembly  22  is described in greater detail below. A forward body cover  24  is disposed over a forward portion of the frame assembly  22 . As illustrated in  FIG. 1 , the forward body cover  24  covers, in part, an engine compartment  26  in which an engine  28  is mounted. The engine  28  preferably is an in-line four-cylinder, water-cooled, four-cycle engine and will be described in greater detail below. 
   A windshield  30  is disposed over a mid-portion of the body cover  24 . The windshield  30  provides some degree of protection for the riders from wind and other elements during operation of the snowmobile  20 . Rearward of the windshield  30 , a fuel tank  32  is mounted to the frame assembly  22  in a manner that allows the body cover  24  and the fuel tank  32  to appear to blend together for aesthetic and other reasons. 
   Rearward of the fuel tank  32 , a seat  34  is mounted to the frame assembly  22 . Rearward of the seat  34  is positioned a grab bar  36  that can be used to raise a rear portion of the snowmobile  20  for turning and maneuvering when the snowmobile  20  is not being ridden. 
   Forward of the seat  34  and the fuel tank  32  is a steering handle assembly  42 . The steering handle assembly  42  can carry appropriate controls and can be coupled to a pair of front skis  44  by a steering linkage  45  (FIGS.  14  and  15 ). The steering handle assembly  42  and the steering linkage  45  are described in greater detail below with reference to  FIGS. 14 and 15 . As the handle assembly  42  is turned, the skis  44  pivot clockwise and counterclockwise about an attachment location. As the skis  44  pivot, the direction of the snowmobile  20  can be altered. The skis  44  are mounted to the frame assembly  22  though a pair of swing units  46 . A pair of suspension assemblies  47  also is provided between the frame assembly  22  and the skis  44 . Each suspension assembly  47  preferably comprises a shock absorber unit (for example, a cylinder type) and a coil spring to absorb shocks and vibrations generated during travel over rough terrain, although any other construction can be applied. 
   The engine  28  is mounted onto the frame assembly  22  in any suitable manner. As illustrated in  FIG. 3 , a set of mount bosses  48 ,  49  can be used to secure the engine  28  to the frame assembly  22 . The illustrated engine  28  has resilient engine bosses cooperating with the mount bosses  48 ,  49 . Alternatively, the mount bosses  48 ,  49  can be resilient. By mounting the engine  28  with the resilient mounts, vibrations caused by operation of the engine  28  are reduced or eliminated before transmission through the frame assembly  22  to the rider. The engine mount arrangement is described in greater detail below with reference to  FIGS. 9 and 10 . 
   With reference to  FIG. 1 , the engine  28  in the illustrated arrangement is inclined rearwardly relative to vertical, and is mounted transversely within the engine compartment  26 . Cylinders of the engine  28  extend side-by-side across a width of the snowmobile  20 . In some arrangements, engines having differing numbers of cylinders, different cylinder configurations (e.g., V, W, opposing, etc.), different orientations (e.g., vertical) and different operating principles (e.g., two-stroke diesel, rotary, etc.) can be used. 
   The engine  28  also comprises a crankshaft (not shown) that extends transversely and an output shaft  50  that extends parallel to the crankshaft and is driven by the crankshaft. The output shaft  50  drives a transmission, which is a continuously variable transmission  52  in the illustrated arrangement. Other transmissions also can be used. 
   In the illustrated arrangement, the transmission  52  comprises a drive pulley  54 , a driven pulley  56  and a v-belt  58 . The output shaft  50  rotates the drive pulley  54 . The output shaft  50  and the drive pulley  54  can be connected together through a clutch, a centrifugal clutch, a sprag clutch, or they can be directly connected together. The drive pulley  54  powers the driven pulley  56  with the v-belt  58  in the illustrated arrangement. In some configurations, a drive chain can be used in place of the v-belt  58 . Other arrangements also can be used. The v-belt transmission  52  is described in greater detail below with reference to FIG.  20 . 
   The driven pulley  56  is connected to and rotates about a transfer shaft  60 . In the illustrated arrangement, the transfer shaft  60  carries a sprocket  377  ( FIG. 18 ) at the end opposite to the driven pulley  56 . The sprocket  377  is connected to a further sprocket  388  ( FIG. 18 ) that is carried by a drive shaft  62 . The sprockets  377 ,  388  are connected together by a chain in one arrangement and these three components are disposed within a chain housing  63  (FIGS.  16 - 18 ). The chain housing  63  and an inside construction of the chain housing  63  is described in greater detail below with reference to  FIGS. 16-18 . 
   The drive shaft  62  powers a drive unit  64 . The drive unit  64  generally comprises a plurality of drive wheels  68 . The illustrated drive unit  64  comprises four drive wheels  68 . The drive wheels  68  provide a motive force to a drive belt  70 , which is commonly used in the snowmobile industry. A lower surface of the drive belt  70  abuts the terrain. 
   The drive belt  70  is guided around a preferred path on a pair of slide rails  72 , a plurality of suspension wheels  74  and main rear suspension wheels  76 . The slide rails  72  preferably support the suspension wheels  74  and the main rear suspension wheels  76 . An idler roller  78  preferably is mounted to the frame assembly  22  and helps to define the preferred path for the drive belt  70 . As is known in the snowmobile industry, these components can be mounted to the frame assembly  22  with a rear suspension system  80 . Any suitable rear suspension system  80  can be used and certain portions of the rear suspension system  80  have been schematically illustrated in the illustrated arrangement. 
   The drive belt  70 , the slide rails  72 , the suspension wheels  74 , the main rear suspension wheels  76 , the idle roller  78  and the rear suspension system  80  generally form a drive assembly  84  in the illustrated embodiment. 
   With continued reference to  FIG. 1 , air is drawn into the engine compartment  26  through suitable air passages. In some arrangements, the air is drawn through ventilation openings  90  formed in the body cover  24 . The air drawn or forced into the engine compartment  26  circulates about the engine  28  and related drive components to help cool the engine  28  and the related drive components. 
   The air also is drawn into an air intake box  92 . The air intake box  92  is disposed forward of the engine  28  in the illustrated arrangement. The air intake box  92  can be mounted to the frame assembly  22  in a suitable manner. An inlet  93  into the air intake box  92  can extend upward into a lower surface of the air intake box  92 . 
   A set of intake runners  94  extends between the illustrated air intake box  92  and the engine  28 . Preferably, a charge former  96  is disposed along each of the intake runners  94 . Advantageously, the intake runners  94  extend directly rearward to the engine  28 . The charge formers  96  preferably correspond to each cylinder. In some arrangements, a single charge former can be used upstream of a separation point for runners extending to individual cylinders. In addition, in the illustrated arrangement, the engine  28  is carbureted. In some arrangements, the charge formers  96  can be fuel injectors that are mounted for direct injection, indirect injection, or port injection. 
   The air-fuel charge provided in this manner is combusted within the engine in a suitable manner. The combustion byproducts then are exhausted through a suitable exhaust system  100 . In the illustrated arrangement, the exhaust system  100  extends directly rearward from the engine  28 . In this manner, an exhaust runner  102  that extends rearward from the engine  28  can be tuned to the engine  28  for improved engine performance. Additionally, the length of each runner  102  can be lengthened prior to merging together with any other runners such that pulse effects on adjoining cylinders can be reduced. The exhaust runners  102  correspond to each cylinder and extend generally rearward from the engine  28 . Each exhaust runner  102  is coupled to the engine  28  through openings  103  ( FIGS. 3 ,  4  and  6 ) of the frame assembly  22  and a heat insulating cover  106  in the illustrated arrangement. The heat insulating cover  106  is described in greater detail below with reference to  FIGS. 2 ,  7  and  8 . In some arrangements, a single manifold can be used. 
   With continued reference to  FIG. 1 , the runners  102  preferably are joined to merge into manifold pipes  108 . The manifold pipes  108  extend rearwardly to a silencer box (not shown). The silencer box provides an enlarged volume into which the exhaust can flow. Exhaust energy is dissipated within the silencer box and the noise level of the exhaust can be decreased. The silencer box preferably is disposed rearward of the seat  34 . A pair of exhaust pipes (not shown) preferably extends rearward from the silencer box. In some arrangements, a single exhaust pipe can extend from the silencer box. One end of the exhaust pipes preferably defines an ultimate exhaust discharge from the snowmobile  20  such that the exhaust gases are discharged into the atmosphere. 
   With reference to  FIGS. 14 and 15 , the steering handle assembly  42  and the steering linkage  45  are described in greater detail below. 
   The illustrated steering handle assembly  42  preferably comprises a steering shaft  120 , a handle bar  122  and a pair of grips  124  (FIGS.  1  and  2 ). The steering shaft  120  extends generally vertically with a top portion thereof inclining slightly rearwardly. The frame assembly  22  supports the steering shaft  120  with support members  126 ,  127  in a manner that provides for pivotal movement of the steering shaft  120  about a steering axis. 
   The steering linkage  45  preferably comprises a steering column  128 , lever arms  130 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 , link rods  146 ,  148 , a connecting link  150  and steering rods  154 ,  156 . 
   The steering shaft  120  is coupled with the lever arm  130 . The lever arm  130  is connected to the lever arm  132  affixed to a top portion of the steering column  128  through the link rod  146 . The frame assembly  22  supports the steering column  128  with a support member  158  in a manner that provides for pivotal movement of the steering column  128  about a steering axis. The lever arm  134  is affixed to a bottom portion of the steering column  128  and is connected to the lever arm  136  through the link rod  148 . The connecting link  150  connects the lever arm  136  with the lever arm  138 . The steering rods  154 ,  156  extend transversely from both ends of the connecting link  150 . The lever arms  140 ,  142  are coupled with the respective outer ends of the steering rods  154 ,  156  on each one end and also are coupled with the skis  44  on each another end. 
   The illustrated steering handle assembly  42  and the steering linkage  45  are arranged generally symmetrical relative to a vertical center plane VCP which extends vertically and fore to aft in a center of the snowmobile  20  when the snowmobile  20  is laid on a horizontal plane. However, the steering column  128 , the lever arms  132 ,  134  and the link rods  146 ,  148  are offset from the center plane VCP toward the right side of the snowmobile  20 . 
   In the illustrated embodiment, the steering column  128  is affixed to a U-shaped tubular member  238  (FIG.  2 ), which is described in greater detail below, by the support member  158 . The steering column  128  also is journaled on a frame member  223  (FIGS.  2 - 6 ), which is described in greater detail below. 
   With reference to  FIGS. 1 and 2 , each ski  44  has a ski shaft (not shown) that extends generally upwardly from the ski  44  and through a holder  164  which is affixed to an outer end of each swing unit  46 . The ski shaft can pivot about a ski axis of the ski shaft which is generally consistent with an axis of the holder  164 . A coupling link  168  couples each steering rod  154 ,  156  with a top portion of the ski shaft. The ski  44  pivots about the ski axis relative to the holder  164  when the coupling link  168  pulls or pushes the ski shaft. 
   The lever arms  136 ,  138  are journaled on the frame assembly  22 . In the illustrated arrangement, the lever arms  136 ,  138  are journaled on a cross member (not shown) which connects members of a front frame  202  (FIGS.  2  and  3 ). The front frame  202  is described in greater detail below. 
   With reference to  FIGS. 1 ,  2 ,  14  and  15 , when the rider turns the handle bar  122  right or left, the steering shaft  120  and the steering column  128  pivot about respective own axes to move the link rod  148  as indicated by the arrow  176  (FIG.  15 ). The steering rods  154 ,  156  thus move as indicated by the arrow  178  ( FIG. 15 ) to pull or push the respective coupling links  168 . The ski shafts pivot about the ski axes to direct the skis  44  right or left. Accordingly, the snowmobile  20  turns to the direction that the rider desires. 
   With reference to  FIGS. 2 ,  3  and  11 - 13 , a general construction of the frame assembly  22  is described in greater detail below. 
   The frame assembly  22  generally comprises a body frame  200 , a front frame  202  and a rear frame  204 . The body frame  200  is disposed center of the frame assembly  22  and forms a main body of the snowmobile  20 . The front frame  202  is disposed forward of the body frame  200  and forms a front body of the snowmobile  20 . The rear frame  204  is disposed rear of the body frame  200  and forms a rear body of the snowmobile  20 . 
   With particular reference to  FIGS. 2 , and  11 - 13 , the body frame  200  preferably comprises an upper horizontal section  208 , side vertical sections  210  and lower horizontal sections  212 . The upper horizontal section  208  extends fore to aft and forms a center area of the body frame  200 . The side vertical sections  210  extend vertically downwardly from both sides of the upper horizontal section  208 . The lower horizontal sections  212 , in turn, extend horizontally outwardly from lower ends of the side vertical sections  210 . As shown in  FIG. 11 , the illustrated horizontal sections  212  slightly slant downwardly and forwardly. 
   The lower horizontal sections  212  preferably form foot step areas. As shown in  FIGS. 11-13 , at respective forward ends of the horizontal sections  212 , a pair of foot steps  214  is disposed. As shown in  FIG. 13 , a support member  216  is affixed to each horizontal section  212  by rivets  217 . The support members  216  are made of a sheet metal. Each sheet is bent to generally form a trapezoid that has a forward end higher than a rear end. The foot steps  214  are affixed to respective top surfaces of the support members  216  by rivets  218 . 
   A pair of support pipes  219  preferably supports the respective foot steps  214 . The illustrated support pipes  219  have a lower end affixed to the frame assembly  22  and an upper end affixed to the frame assembly  22  at a location higher than a portion where the lower end is affixed. Each support pipe  219  extends outwardly from the lower end along a bottom surface of each support member  216 , extends generally upwardly out of the step  214  and then extends inwardly above the step  214  to the upper end. Each support pipe  219  is coupled with the support member by one of the rivets  218 . The foot steps  214  thus can be rigidly affixed to the lower horizontal section  212  on the left hand side and then forwardly to the lower horizontal section  212  on the right hand side. 
   Heat exchange devices (not shown) preferably are coupled with the respective lower horizontal sections  212 . The heat exchange devices are part of an engine cooling system. Water delivery pipes  220  are provided to circulate cooling water within the cooling system. One of the delivery pipes  220  extends rearwardly to the rear frame  204  from the lower horizontal sections  212 . 
   Preferably, the upper horizontal section  208  is formed separately from the other part of the body frame  200 . The upper horizontal section  208  can be made of, for example, a sheet metal. Both sides of the upper horizontal section  208  can be lower than a center portion thereof to reinforce the section  208 . The upper horizontal section  208  can be formed by, for example, a press or stamping process. The side vertical sections  210  and the lower horizontal sections  212  preferably are unitarily formed with each other and is made of, for example, a sheet metal and is formed by a press or stamping, or is made of an extruded aluminum alloy material. 
   The sections  208 ,  210 ,  212  can be entirely unitarily formed with each other. Alternatively, the sections  208 ,  210 ,  212  can be completely separately formed. Additionally, the sections  208 ,  210 ,  212  can be produced by any methods other than the methods described above. For example, the sections  208 ,  210 ,  212 , unitarily formed or separately formed, can be produced by a mold casting, die-casting or lost-wax casting process. Similarly, the other components and/or members described below also can be formed with any metal or plastic material and can be formed by any conventional method regardless of being expressly described or not, except as noted herein. 
   With particular reference to  FIGS. 2 and 3 , the illustrated body frame  200  additionally comprises side panels  221 , a floor panel  222  and a frame member  223 . Each of the panels  221 ,  222  and the frame member  223  preferably are produced by the mold casting, die-casting or lost-wax casting process. 
   The side panels  221  are affixed to the respective side sections  210  by welding or by fasteners such as, for example, bolts or rivets. The side panels  221  define openings  224  ( FIG. 3 ) through which the transfer shaft  60  ( FIG. 1 ) extends. The driven pulley  56  is positioned adjacent an outer surface of the side panel  221  on the left side. In the illustrated embodiment, the lower ends of the support pipes  219  of the foot steps  214  are affixed to the side panels  221 . The floor panel  222  extends between lower ends of the side panels  221  to form a bottom portion of the engine compartment  26 . 
   The frame member  223  extends between upper portions of the side panels  221  and generally covers a front end of the upper horizontal section  208 . The frame member  223  defines the rear engine mounts  48  and the openings  103  for the exhaust runners  102  (FIG.  1 ). The frame member  223  also forms three bosses  228  onto which a heat insulating cover  106  (FIGS.  7  and  8 ), which is described below, for the exhaust system  100  is mounted. 
   Another heat exchange device  230  for the engine  28  is disposed below the frame member  223 . The cooling water of the engine  28  also circulates through the heat exchange device  230 . The heat exchange device  230  preferably is affixed to the side panels  221  and the frame member  223  and form part of the frame assembly  22  together with the side panels  221 . The illustrated heat exchange device  230  has flanges  232  ( FIG. 11 ) that are affixed to the side panels  221  and the frame member  223  by rivets or bolts. In the illustrated embodiment, the flanges  232  are affixed to the side panels  221  together with engine bosses as described below with reference to FIG.  9 . The frame member  223  and the heat insulating cover  106  are described in greater detail below with particular reference to  FIGS. 4-10 . 
   The body frame  200  thus is rigidly formed with the foregoing components and members. To further increase the rigidity of the frame assembly  22  and to be coupled with the front frame  202 , the frame assembly  22  preferably includes a frame reinforcement  234  ( FIG. 2 ) secured to the body frame  200 . The reinforcement  234  desirably is generally rectangularly shaped and advantageously is disposed generally above the engine  28 . Thus, the engine  28  is positioned within a cavity defined between the side panels  221 , above the floor panel  222 , forward of the frame member  223  and below the reinforcement  234 . 
   The illustrated reinforcement  234  preferably comprises a generally rectangular tubular member  238  that is bent into a U-shape. The tubular member  238  preferably is made of an extruded aluminum alloy material. A bight of the U-shape is disposed forwardly in the illustrated arrangement. Rear ends of the tubular member  238  are affixed to upwardly extending portions  239  of the frame member  223 , which is described in greater detail below. A bar  240  extends between the two legs of the U-shaped tubular member  238  at the end opposite the bight. The bar  240  reinforces the open end of the U-shape defined by the tubular member  238 . A second bar  242  extends upward and between rear ends of the tubular member  238  to further reinforce the reinforcement  234 . Furthermore, a support unit  244  extends upward and rearward from the bar  240  to the second bar  242 . The illustrated steering handle assembly  42  is affixed to the support unit  244  by the support members  126 ,  127  (FIGS.  14  and  15 ). 
   A holder member  248  is coupled with a front portion of the tubular member  238  by welding or by fasteners such as, for example, bolts or rivets. The holder member  248  preferably is made of sheet metal and is formed by a bending process. The front portion of the tubular member  238  is nested in a recessed portion defined by the holder member  248 . Four bolts  250 , for example, are planted at the holder member  248  with threaded portions thereof extending generally upwardly and forwardly. 
   With reference to  FIG. 2 , the rear frame  204  preferably comprises a generally flat shaped member  254  having a recessed portion. The illustrated member  254  of the rear frame  204  can be formed with a casting. Alternatively, the member  254  can be formed with a sheet metal by a press process. The rear frame  204  is coupled with the body frame  200  in a suitable manner. The exhaust silencer box (not shown) is disposed on the rear frame  204 . The grab bar  36  and the water delivery pipe  214  are fixed to the rear frame  204 . 
   With continued reference to  FIGS. 2 and 3 , the front frame  202  preferably comprises a pair of generally symmetrically formed front members  270 ,  271  which are coupled with each other. Preferably, the front members  270 ,  271  are made of aluminum alloy and are formed by, for example, a mold casting, a die-casting, or lost-wax casting process. The front members  270 ,  271  are connected with each other by fasteners  276  on front surfaces and by the cross member (not shown) on their rear surfaces. Each side portion of the front members  270 ,  271  is coupled with each side panel  218  by welding or by suitable fasteners such as, for example, bolts or rivets. 
   The front members  270 ,  271  support the swing units  46  on the respective side portions. Each swing unit  46  in the illustrated arrangement is formed with a pair of linked tubular members disposed above and below, although any other construction can be applied. Each side portion of the front members  270 ,  271  defines a recess (not shown) recessed inwardly. Each swing unit  46  is placed at the recess and is swingably journaled on the side portion. 
   A top side portion of each front member  270 ,  271  forms a mount boss. A top end of each suspension assembly  47  is journaled at the mount boss for pivotal movement. A lower end of each suspension assembly  47  is affixed to a portion of the lower linked member of each swing unit  46  where the holder  45  is attached, or is directly affixed to the holder  164 . 
   Each side portion of the front member  270 ,  271  is coupled with a forward-most end  272  ( FIG. 3 ) of each side panel  221  by welding or by suitable fasteners such as, for example, bolts or rivets. Each side portion of the frame member  270 ,  271  also defines the forward engine mount boss  49  ( FIG. 3 ) through which a bolt hole  274  is formed. The engine mount construction is described in greater detail below with reference to  FIGS. 9 and 10 . 
   With reference to  FIGS. 2 ,  4 - 6 ,  11  and  12 , the frame member  223  preferably is a single member and generally comprises a pair of side sections  280 , a rear section  282  and a bottom section  284 . As described above, the frame member  223  preferably is produced by a casting process so that the side, rear and bottom sections  280 ,  282 ,  284  are unitarily formed with each other. The frame member  223  thus provides enhanced rigidity. 
   A downward slope  288  is formed at a forward-most end of the bottom section  284 . Seven holes  290  are formed on the downward slope  288  and around the downward slope  288 . Rivets or bolts are inserted into the holes  290  to fix one of the flanges  232  of the heat exchange device  230  so that the heat exchange device  230  depends from the frame member  223 . The bosses  228  for affixing the heat insulating cover  106  to the frame member  223  also are formed in the area of the bottom section  284  adjacent to the holes  290 . 
   As shown in  FIG. 5 , each side section  280  has bolt holes  294 ,  295 . The rear section  282  defines a rearward extending portion  296  on each side. Each rearward extending portion  296  also defines rivet holes  298 . The side sections  280  and the rearward extending portions  296  are affixed to the side panels  221  of the body frame  200  so that the frame member  223  is affixed to the body frame  200 . As shown in  FIG. 4 , the side sections  282  define the rear engine mounts  48  on both sides of the downward slope  288 . The illustrated bolt holes  294 , which are located forwardly, are used to receive a rear engine mount bolt  299  ( FIG. 9 ) as described below. The bolt holes  294 ,  295  positioned on the left hand side preferably have a rectangular shape because the preferred bolts have a rectangular shaped portion that can prevent the bolts from rotating. 
   Each side section  280  also defines an opening  300  corresponding to the opening  224  of the side panel  221  of the body frame  200 . The transfer shaft  60  ( FIG. 1 ) extends through the openings  224  and is journaled on bearings  418  ( FIG. 20 ) at the side sections  280 . Each bearing is held by a bearing holder  301  ( FIG. 20 ) which is affixed to each side section  280  by bolts. Bolt holes  302  for the bolts are formed generally on front and rear sides of the opening  300 . The bolt holes  302  positioned on the left hand side also have a rectangular shape because the preferred bolts have a rectangular portion that can prevent the bolts from rotating. 
   The rear section  282  defines the openings  103  through which the exhaust runners  102  ( FIG. 1 ) extend. The rear section  282  extends upwardly and generally normal to the bottom section  284 . This is advantageous because the openings  103  can be small as possible and the stiffness of the frame member  223  enhanced. On both sides of the rear section  282 , the upwardly extending portions  239  are formed. 
   With reference to  FIG. 5 , the respective portions  239  define bolt holes  306  where bolts are inserted to fix the rear ends of the U-shaped tubular member  238 . Both sides of the rear section  282  defines recessed portions  308  under the upwardly extending portion  239 . The upper ends of the support pipe  219  for the foot steps  214  are affixed to the recessed portions  308  by bolts. 
   The upwardly extending portion  239  on the left hand side preferably defines a journal portion  319  (FIGS.  6  and  12 ). The steering column  128  is journaled at the journal portion  319  for pivotal movement. The illustrated journal portion  319  comprise a semicircular recess  319   a  and a bracket  319   b  that is affixed to the upwardly extending portion  239  by bolts. 
   With reference to  FIGS. 8 and 9 , a rear engine mount construction in this embodiment is described in greater detail below. 
   The illustrated engine  28  defines a pair of rear engine bosses  320  which are spaced apart from each other. Each engine boss  320  bears a metal collar  322  with a rubber tube  324  baked thereto. Each collar  322  defines a flange  326  extending outwardly beyond a side end of the engine boss  320 . The flange  326  of the collar  322  of the engine boss  320  on the left hand side faces an inner surface  328  of the mount boss  48  on the same side. The inner surface  328  is a reference surface of the rear engine mount construction in this embodiment. 
   The mount boss  48  on the right hand side includes internal female threads. A cylindrical male screw member  330  is screwed into the female threads. A position of the cylindrical member  330  thus is adjustable relative to the mount boss  48 . The cylindrical member  330  defines a flange  332  extending inwardly beyond a side end of the boss  48  toward the flange of the collar  322 . 
   The rear engine mount bolt  299  has a length that is longer than a distance between an outer surface of the mount boss  48  on the left hand side and an outer surface of the mount boss  48  on the right hand side. This elongate mount bolt  299  extends through the bolt holes  294  of the respective mount bosses  48  and collars  322  of the respective engine bosses  320 . An elongate collar  334  encloses a center portion of the mount bolt  299 . The bolt  299  defines a rotation preventing portion  336  that has a rectangular shape in cross-section next to the bolt head of the bolt  299 . A nut  338  is screwed down onto the outer end of the mount bolt  299  that extends beyond the side panel  221  on the right hand side to complete the rear engine mount construction. 
   In the illustrated embodiment, each flange  232  of the heat exchange device  230  is interposed between the side panel  221  and the mount boss  48  on the same side. The mount bolt  299  thus is used to fix the heat exchange device  230  to the side panels  221  together with the engine  28 . 
   In a preferred mounting process of the engine  28 , the cylindrical member  330  is screwed down into the female threads of the mount boss  48  to define a space between the reference surface  328  of the mount boss  48  on the left hand side and the flange  332  of the cylindrical member  330 . The space is longer than a distance between the flanges  326  of the engine bosses  320 . The engine  28  is placed such that the engine bosses  320  are positioned between both the mount bosses  48 . Then, a tool is used to rotate the cylindrical member  330  so that the member  330  approaches the flange  326  of the engine mount  320  on the same side. The cylindrical member  330  is rotated until the flange  332  thereof abuts the flange  326  of the engine boss  320 . The elongate collar  334  is positioned between the collars  322  of the engine mounts  320 . The mount bolt  299  is then inserted into the side panel  221 , the flange  232  of the heat exchange device  230 , the mount boss  48  and the collar  322  of the engine boss  320 , which are on the left hand side. The bolt  299  is further inserted into the elongate collar  334 , and the collar  322  of the engine boss  320 , the cylindrical member  330  within the mount boss  48 , the flange  232  of the heat exchange device  230  and the side panel  221 , which are on the right hand side. The nut  338  is finally screwed down onto the outer end portion of the mount bolt  334 . 
   The adjustable cylindrical member  330  is advantageous because the engine mounts  320  can be securely kept in position even though the distance between the flanges  322  are shorter than a distance between both the mount bosses  48 . The rectangular shaped portion  336  of the mount bolt  334  is useful because the nut  338  can be easily screwed down without necessitating any tool to prevent the bolt  299  from rotating. Also, the illustrated mount bolt  299  can act as a cross member that reinforces the frame member  223 . 
     FIG. 10  illustrates a front engine mount construction on the left hand side. A front engine mount construction on the right hand side can be similar to that on the left hand side. Both of the front engine mount constructions also are similar to the rear engine mount construction described above except that each mount bolt  342  is provided at each set of the mount boss  49  and an engine mount  344  and that each bolt  342  has no rectangular shaped portion. The same members and portions as those described in connection with the rear engine mount construction of  FIG. 9  are assigned with the same reference numerals and are not described repeatedly. Either one of the cylindrical members  330  on the right hand construction or the left hand construction can be omitted. In this alternative, the flange  326  of the collar  322  associated with the mount boss  49  that has no cylindrical member  330  can directly abut the mount boss  49 . 
   With reference to  FIGS. 7 and 8 , the heat insulating cover  106  is disposed in front of the rear section  282  of the frame member  223  and above the bottom section  284  thereof. The insulating cover  106  is generally formed as a box shape and preferably is formed with a top cover member  346  and a bottom cover member  348 . The top and bottom cover members  346 ,  348  preferably are made of an iron sheet metal. Each sheet is bent into a generally recessed shape in, for example, a press process. The top and bottom members  346 ,  348  have peripheral flanges  350  that can be affixed to each other with bolts (not shown). Thus, a cavity  352  ( FIG. 8 ) is defined between the top and bottom cover members  346 ,  348  when the members  346 ,  348  are coupled together. 
   The heat insulating cover  106  preferably is affixed to the frame member  223 . In the illustrated arrangement, the frame member  223  has bosses  356  ( FIG. 8 ) positioned between the rear engine mount bosses  48  and the opening  300  of the transfer shaft  66 . The bottom cover member  348  is affixed to the bosses  356  by rivets  358 . The top cover member  346  is affixed to a top surface of the rear section  282  of the frame member  223  by bolts  360 . 
   The rear section  282  defines the openings  103  and the exhaust runners  102  extend forwardly through the openings  103  toward the heat insulating cover  106  and enter the cavity  352  of the cover  106  to further extend within the heat insulating cover  106 . The respective forward ends of the exhaust runners  102  have flanges  364 . The flanges  364  are affixed to the engine  28  by bolts  366  with the exhaust passages of the runners  102  connected to inner exhaust passages  366  of the engine  28 . The exhaust runners  102  thus are entirely covered by the heat insulating cover  106  between the engine  28  and the frame member  106 . 
   With reference to  FIGS. 16-18 , the transfer shaft  60  extends outwardly beyond the side panel  221  of the body frame  200  through the opening  300  formed on the right hand side. The transfer shaft  60  preferably is journaled by a pair of ball bearings  370  affixed to inner and outer housing members  372 ,  374  of the chain housing  63 , respectively. The bearings  370  are retained in respective fixed positions by suitable fasteners such as, for example, snap rings  376 . A drive sprocket  377  is disposed between the bearings  370  and is affixed to the transfer shaft  60 . 
   The housing members  372 ,  374  preferably are coupled together and are further affixed to the side panel  221  and the frame member  223  by two bolts  378  ( FIGS. 16 and 17 ) in an upper area of the chain housing  63 . The housing members  372 ,  374  also are coupled together and are affixed only to the side panel  221  by bolts  380 . Other bolts, for example, a bolt  381  ( FIG. 180 ) can be used for coupling only both of the housing members  372 ,  374 . A seal member  380  preferably is interposed between the inner and outer housing members  372 ,  374 . 
   The drive shaft  62  extends parallel to the transfer shaft  60  and outwardly beyond the side panel  221  through an opening  382 . The drive shaft  62  preferably is journaled by another ball bearing  384  affixed to the inner housing member  372  of the chain housing  63 . The bearing  384  is retained in a fixed position by a suitable fastener such as, for example, snap ring  386 . A driven sprocket  388  is disposed at an outer end of the drive shaft  62  and is affixed thereto. The driven sprocket  388  preferably has an outer diameter larger than an outer diameter of the drive sprocket  377 . 
   A chain  390  is wound around the drive and driven sprockets  377 ,  388 . The transfer shaft  60  thus can rotate the drive shaft  62  through the chain  390 . The drive shaft  62  drives the drive belt  70  as described above. Any transmitter such as, for example, a belt can replace the chain. In this alternative, pulleys replace the sprockets accordingly. 
   A disk brake device  394  preferably is disposed at an outer end of the transfer shaft  60 . The brake device  394  generally comprises a brake disk  396  and a hydraulic caliper  398 . The brake disk  396  is rotatably affixed to the transfer shaft  60  so as to rotate together with the transfer shaft  60 . The caliper  398  is affixed to a bracket portion  400  of the outer housing member  374  by bolts  402 . The caliper  398  carries brake pads  403  that pinch the brake disk  396  therebetween to slow or stop the rotation of the brake disk  396 . Brake fluid can be supplied to the caliper  398  through a suitable hydraulic system. Supply of the hydraulic fluid can be controlled by the operator with a brake lever (not shown) preferably disposed at the handle bar  122 . 
   The inner housing member  372  of the chain housing  63  can be unitarily formed with the frame member  223 . In this alternative, as shown in  FIG. 19 , an inner housing portion  404  can extend outwardly from the frame member  223  through an opening or recess  406  formed at a portion of the side panel  221 . An outer housing member or cover member  408  can be affixed to the inner housing portion  404  by bolts  410  to complete the chain housing  63 . 
   The heat exchange device  230  also is illustrated in FIG.  16 . The heat exchange device  230  preferably has a plurality of radiator fins  412  as shown in this figure. 
   With reference to  FIG. 20 , the v-belt transmission  52  is illustrated. The transfer shaft  60  extends outwardly beyond the side panel  221  of the body frame  200  through the opening  300  of the frame member  223  and the opening  224  of the side panel  221  both formed on the left hand side. The transfer shaft  60  preferably is journaled by a ball bearing  418 . The illustrated bearing  418  is held by the bearing holder  301  that is affixed to the frame member  223  by bolts (not shown) that extend through the bolt holes  302  (FIG.  5 ). 
   The driven pulley  56  comprises inner and outer sheaves  420 ,  422 . The inner sheave  420  is affixed to the outer end of the transfer shaft  60 , while the outer sheave  422  is movable relative to the inner sheave  420 . A spring  424  urges the outer sheave  422  toward the inner sheave  420 . The v-belt  58  is interposed between the inner and outer sheaves  420 ,  422 . A diameter of the driven pulley  56  thus is continuously variable for the variable transmission. 
   The bearing holder  301  can be replaced by a combination of a recess formed at the frame member and a snap ring. 
   As described above, because the frame member is formed with a single piece, i.e., not combined pieces, in the illustrated embodiment, the frame member not only provides enhanced rigidity but also provides enhanced accuracy in distances between the shafts. 
   The same snowmobile is disclosed in co-pending U.S. patent application Ser. No. 10/214,915, filed Aug. 7, 2002, and U.S. patent application Ser. No. 10/213,726, filed Aug. 6, 2002, the entire contents of which are hereby expressly incorporated by reference. 
   Although the present invention has been described in terms of a certain embodiment, other embodiments apparent to those of ordinary skill in the art also are within the scope of this invention. Thus, various changes and modifications may be made without departing from the spirit and scope of the invention. The scope of the present invention is intended to be defined only by the claims that follow.