Abstract:
A motortricycle with oscillation mechanism including left and right suspension arms respectively swingably mounted onto a vehicle body frame, rear wheels mounted respectively onto the outer portions of the left and right suspension arms. An oscillation mechanism permitting leftward and rightward oscillation of the vehicle body frame relative to the side of the suspension arms is provided between the suspension arms and the vehicle body frame, and an engine for driving the left and right rear wheels is mounted onto the vehicle body frame. Since the engine of the motortricycle is mounted on the vehicle body frame rather than on the suspension arms, weight on the suspension arms can be largely reduced, and the riding comfort can be further enhanced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2002-197423 filed on Jul. 5, 2002 and Japanese Patent Application No. 2002-287344 filed on Sep. 30, 2002, the entire contents thereof is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a motortricycle with an oscillation mechanism, more particularly to an engine support structure and a vehicle body frame structure for supporting the engine. 
   2. Description of Background Art 
   As a motortricycle with an oscillation mechanism, those in which the engine can swing with the rear wheels have been known. For example, Japanese Patent Laid-open No. Sho 59-153674 and Japanese Patent Publication No. Hei 1-23356 have been known. The contents of Japanese Patent Laid-open No. Sho 59-153674 will be described below referring to  FIG. 18 , and the contents of Japanese Patent Publication No. Hei 1-23356 will be described below referring to  FIG. 19 . 
   A vehicle body frame for a vehicle, is known wherein a driving device support frame has a substantially box shape for seated type four-wheel buggy car. For example, Japanese Utility Model No. 2521705 has been known. The contents of Japanese Utility Model No. 2521705 will be described below referring to  FIG. 20  and  FIG. 21 . 
     FIG. 18  shows a motortricycle in which a connection shaft  203  is mounted onto a rear portion of a front frame  201  through a bracket plate  202 , a drum  204  is turnably mounted onto the connection shaft  203 , a back tube  205  is attached to the drum  204 , an engine unit  207  is mounted onto the back tube  205  through a support shaft  206 , left and right rear wheels  208  are mounted onto rear portions of the engine unit  207 , and a shock absorber  211  is disposed bridgingly between the engine unit  207  and the rear end of the back tube  205 . 
   Both the engine unit  207  and the rear wheels  208  are vertically movable relative to the back tube  205 . With this structure, there arises no problem in the case of a vehicle having a comparatively small displacement. However, in the case of high-speed running of a vehicle having an intermediate to large displacement, the weight under the spring  211  will be large. Therefore, in this situation, it is difficult for the rear wheels  208  to follow the surface of a very rough road on which the vehicle is running. 
     FIG. 19  shows a motortricycle  226  in which an oscillation joint  218  is vertically movably mounted onto a rear portion of a front vehicle body including a front wheel  215  through a pin  217 , a rear frame  222  constituting an essential part of a rear vehicle body  221  is attached to a rear portion of the oscillation joint  218 , Further, an engine  223  is mounted onto the rear frame  222 , and left and right rear wheels  225  are mounted onto an output shaft of a transmission case  224  provided integrally with the engine  223 . 
   In the motortricycle  226 , since the engine  223  is mounted onto the rear frame  222  swingably mounted onto the front vehicle body  216 , the under-spring weight will be large. Therefore, in the same manner as in the case of the motortricycle shown in  FIG. 18 , it is difficult for the rear wheels  225  to follow up to the ruggedness of the ground surface in the case of a vehicle having an intermediate to large displacement. 
   As shown in  FIG. 20 , a seat mount frame  231  L-shaped in side view is mounted onto a rear portion of a main frame  230 , an engine support frame  232  L-shaped in side view is mounted onto the rear end of the main frame  230 , the rear end of the seat mount frame  231  is attached to a front portion of the engine mount frame  232 , and an intermediate portion of the engine support frame  232  is supported by a slant frame  233  extended rearwardly upwards from the rear end of the main frame  231 . Further, a rear frame  234  roughly L-shaped in side view is attached to the rear end of the engine support frame  232 , a lower frame  235  is arranged bridgingly between the front end of the rear frame  234  and a lower portion of the slant frame  233 , and an engine  236 , a non-stage speed change type belt converter transmission  237 , and a differential gear  238  are surrounded by the main frame  230 , the seat mount frame  231 , the engine support frame  232 , the rear frame  234 , and the lower frame  235 . 
     FIG. 21  shows the condition where the lower frame  235  roughly V-shaped in plan view is arranged bridgingly between left and right L-shaped pipes  241 ,  241  constituting the engine support frame  232 , rear swing arms  242 ,  242  are swingably attached to the lower frame  235  through brackets  243 ,  243 , and rear wheels  244 ,  244  are disposed on the end portion sides of the rear swing arms  242 ,  242 . 
   In the above-mentioned prior art of  FIG. 20  and  FIG. 21 , for example, the engine support frame  232  has a structure in which the L-shaped pipes  241 ,  241  disposed on the left and right sides are connected by a plurality of cross members, so that the weight thereof will be large although the rigidity thereof will be high. If a reduction in weight can be contrived while securing the rigidity for supporting the engine  236  and other power transmission mechanisms, the inertial weight of a rear portion of the vehicle body frame can be reduced. 
   In addition, when the L-shaped pipes  241 ,  241  are disposed on the left and right sides, the vehicle width is enlarged. For example, where the above buggy car is a vehicle having such a structure as to permit large oscillations to the left and right sides, the L-shaped pipes  241 ,  241  are liable to interfere with the rear swing arms  242 ,  242  upon oscillations to the left and right sides. Therefore, the rear swing arm  242  must, for example, be curved largely so as to obviate the interference with the L-shape pipe  241 , so that the total length thereof is increased, resulting in an increase in weight. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   In view of the above, it is an object of the present invention to provide a motortricycle with oscillation mechanism enhanced in riding comfort even if an engine with a comparatively large displacement is mounted thereon, and so as to reduce weight while securing the rigidity of the vehicle body frame for supporting an engine and the like. In particular, an objective is to reduce the inertial weight of a rear portion of the vehicle body frame and, further, reduce the weight of suspension arms. 
   In order to attain the above object, according to an aspect of the present invention, an oscillation mechanism is provided for a motortricycle wherein left and right suspension arms are respectively swingably mounted onto a vehicle body frame, rear wheels are mounted respectively onto the tip ends of the left and right suspension arms, an oscillation mechanism permitting leftward and rightward oscillation of the vehicle body frame relative to the suspension arm side is provided between the suspension arm side and the vehicle body side, and an engine for driving the left and right rear wheels is mounted onto the vehicle body frame. As a result, the engine can oscillate together with the vehicle body frame. 
   In the above-mentioned motortricycle with an oscillation mechanism, the rear wheels are mounted onto the vehicle body frame through the suspension arms, and the engine is mounted onto the vehicle body frame. Therefore, when a suspension spring is attached to the suspension arm side, the engine is not present on the suspension arm side, so that the under-spring weight can be reduced greatly, and riding comfort can be further enhanced. 
   According to another aspect of the present invention, the engine is preferably supported on the vehicle body frame through a rubber mount. In the motortricycle, preferably the rubber mount makes it difficult for vibration to be transmitted from the engine to the vehicle body frame, and also restrains generation of noise. 
   According to another aspect of the present invention, the engine is preferably supported on the vehicle body frame through a plurality of links. With the links made different in length, the links differ in resonance frequency due to the difference in length, so that the vibration transmitted from the engine to the vehicle body frame can be reduced. 
   According to another aspect of the present invention, the vehicle body frame may surround the front and rear sides and the upper and lower sides of the engine. At least a rear portion of the vehicle body frame is composed of a single pipe. 
   Since the vehicle body frame surrounds the front and rear sides and the upper and lower sides of the power unit, the rigidity of the vehicle body frame can be enhanced, as compared, for example, with a structure in which a vehicle body frame is extended in the manner of cantilevers on the left and right sides on the upper side of a power unit. 
   In addition, since at least a rear portion of the vehicle body frame is composed of a single pipe, the inertial weight of the rear portion of the vehicle body frame can be reduced, whereby turning performance of the motortricycle can be enhanced. 
   Further, since the size in the vehicle width direction of the rear portion of the vehicle body frame can be reduced, even where the vehicle body frame, particularly in a motortricycle with an oscillation mechanism, is largely oscillated to the left and right sides and the suspension arms are moved vertically, sufficient clearances can be secured between the vehicle body frame and the suspension arms. Therefore, the suspension arms need not be curved largely for obviating interference with the vehicle body frame, the total length of the suspension arms can be reduced, and the weight of the suspension arms can be reduced. 
   According to the further aspect of the present invention, the single pipe of the rear portion of the vehicle body frame may be connected to a front frame through a Y-shaped branch portion on the upper side and/or the lower side of the engine. The front frame and the single pipe of the rear portion of the vehicle body frame can be firmly connected to each other. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a side view of a motortricycle with oscillation mechanism according to the first embodiment of the present invention; 
       FIG. 2  is a side view of an essential part of the motortricycle according to the first embodiment; 
       FIG. 3  is a plan view of the motortricycle according to the first embodiment; 
       FIG. 4  is a plan view of an essential part of the motortricycle according to the first embodiment; 
       FIG. 5  is a perspective view of a vehicle body frame structure of the motortricycle according to the first embodiment; 
       FIG. 6  is a back elevation of the motortricycle according to the first embodiment; 
       FIG. 7  is a perspective view of a rear portion of the vehicle body structure of the motortricycle according to the first embodiment; 
       FIGS. 8A to 8C  are illustrations showing a structure and function of the oscillation mechanism according to the first embodiment, in which  FIG. 8A  is a side view of the oscillation mechanism according to the first embodiment,  FIG. 8B  is a sectional view taken on line B—B of  FIG. 8A , and  FIG. 8C  is a function diagram of the oscillation mechanism according to the first embodiment; 
       FIG. 9  is a perspective view of the vehicle body frame structure and a structure for mounting portions of suspension arms of the motortricycle according to the first embodiment; 
       FIG. 10  is a plan view of the vehicle body frame according to the first embodiment; 
       FIG. 11  is a back elevation of a rear suspension according to the first embodiment; 
       FIG. 12  is a function diagram of the rear suspension when a rear wheel on a left side is moved upward according to the first embodiment; 
       FIG. 13  is a function diagram of the rear suspension when the rear wheels on the right and left sides are both moved upwards according to the first embodiment; 
       FIG. 14  is a function diagram of the rear suspension when the rear wheels on the right and left sides are both lowered according to the first embodiment; 
       FIG. 15  is a function diagram of the rear suspension when an upper portion of vehicle body frame is oscillated to a left side of the vehicle body frame according to the first embodiment; 
       FIG. 16  is a function diagram of the rear suspension when the rear wheel on the left side is moved upwards and the upper portion of the vehicle body frame is oscillated to the left side of the vehicle body according to the first embodiment; 
       FIG. 17  is a side view of an essential part of the motortricycle with oscillation mechanism according to the second embodiment; 
       FIG. 18  is a side view of a motortricycle with oscillation mechanism according to the background art; 
       FIG. 19  is another side view of the motortricycle with the oscillation mechanism according to the background art; 
       FIG. 20  is a side view showing a vehicle body frame of a vehicle according to the background art; and 
       FIG. 21  is a plan view of an essential part of the vehicle body frame of the vehicle according to the background art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , the motortricycle with an oscillation mechanism  10  (hereinafter referred to as “motortricycle  10 ”) includes a front fork  12  steerably mounted onto a head pipe  11  through a handle shaft not shown, a front wheel  13  mounted onto the lower ends of the front fork  12 , a handle  14  integrally attached to the front fork  12 , a vehicle body frame  16  attached to a rear portion of the head pipe  11 , a power unit  17  mounted onto a rear portion of the vehicle body frame  16 , left and right rear wheels  18  and  21  driven by the power unit  17 , a luggage box  22  mounted onto an upper portion of the vehicle body frame  17 , and a seat  23  openably and closably mounted onto an upper portion of the luggage box  22 . 
   The vehicle body frame  16  includes a down pipe  25  extended rearwardly downward from the head pipe  11 , a left-right pair of lower pipes  26  and  27  extended rearwardly from a lower portion of the down pipe  25  and then extended rearwardly upwards, a center upper frame  28  connected to rear portions of the lower pipes  26  and  27 , a center pipe  31  extended rearwards from the down pipe  25  and connected to the center upper frame  28 , and a J frame  32  J-shaped in side view connected to rear portions of the lower pipes  26  and  27  and to the rear portion side of the center upper frame  28 . 
   The center upper frame  28  is a member for supporting the luggage box  22  and for hanging the power unit  17 . 
   The J frame  32  is a member for mounting a rear suspension for suspending the rear wheels  18  and  21  and for mounting an oscillation mechanism permitting leftward and rightward oscillation of the vehicle frame  16  side relative to the rear suspension side. The rear suspension and the oscillation mechanism will be described in detail later. 
   The power unit  17  includes an engine  34  disposed on the front side of the vehicle body, and a power transmission mechanism  35  for transmitting the power of the engine  34  to the rear wheels  18  and  21 . 
   Also shown are a front fender  41  for covering the upper side of the front wheel  13 , a battery  42 , a signal light  43 , a tail lamp  44 , an air cleaner  46 , and a muffler  47 . 
     FIG. 2  shows the condition where connection pipes  52 ,  52  (the connection pipe  52  on the other side is not shown) bridgingly disposed between the J frame  32  and the center upper frame  28  so as to connect an upper portion of the J frame  32  and the rear end of the center upper frame  28  to each other, reinforcing plates  53 ,  53  are attached to the connection pipes  52 ,  52  and the center upper frame  28 , and an L pipe  54  L-shaped in side view is attached to the inner side of a rear portion of the J frame  32 . Brackets  56 ,  56  (the bracket  56  on the other side is not shown) are attached to the center upper frame  28 , a front upper portion of the power unit  17  is attached to the brackets  56 ,  56  through a relay member  57 , a support rod  58  is extended rearwardly downwards from the reinforcing plates  53 ,  53  so as thereby to support a rear portion of the power unit  17 , and a projection portion  61  is extended forwards from a front portion of the L pipe  54  so as thereby to attach a rear end portion of the power unit  17 . The relay member  57  functions as a link for connecting the brackets  56 ,  56  with the power unit  17 . The supports rod  58  functions as a link for connecting the reinforcing plates  53 ,  53  with the power unit  17 . Incidentally, symbols  32 A,  32 B, and  32 C denote a lower horizontal portion set to be substantially horizontal, a rear end slant portion with its upper end side moved to the rear side than its lower end side, and an upper slant portion with its front end side moved to the upper side than its rear end side, respectively, of the J frame  32 . 
   The relay member  57  is composed of a main body portion  57   a  oscillatably mounted onto the side of the engine  34  through a support shaft  34   a  and oscillatably mounted onto the sides of the brackets  56 ,  56  through a support shaft  56   a , and stopper rubbers  57   b ,  57   b  mounted onto the main body portion  57   a  so as to be brought into contact with the lower surface of the center upper frame  28 . 
   Rubber bushes  62 , are respectively provided between the support shaft  34   a  and the side of the engine  34 , between the support shaft  56   a  and the bracket  56 , between the support rod  58  and the reinforcing plate  53 , between the support rod  58  and a non-stage transmission  78 , and at a connection portion between a gear box  81  and the L pipe  54 . 
     FIG. 3  shows the condition where a rear portion of the J frame  32  is constituted of a single pipe, and the rear suspension  63  (details will be described later) is attached to the J frame  32 . Also shown are a brake lever  65  for the rear wheels, and a brake lever  66  for the front wheels. 
     FIG. 4  shows a structure in which suspension arms  71  and  72  are attached to left and right portions of the J frame  32 , holders (not shown) are attached respectively to the tip ends of the suspension arms  71  and  72 , the rear wheels  18  and  21  are rotatably mounted on the holders, and the rear wheels  18  and  21  are driven by drive shafts  73  and  74  extended from the power transmission mechanism  35  of the power unit  17 . 
   Symbol  76  denotes a shock absorber composed of a damper  77  and a compression coil spring (not shown) and is connected to the sides of the left and right suspension arms  71  and  72 . 
   The center upper frame  28  is a member having substantially elliptic shape, and the luggage box  22  (see  FIG. 1 ) having a bottom with substantially the same shape is mounted on an upper portion thereof. 
   The power transmission mechanism  35  of the power unit  17  includes a belt type non-stage speed change gear  78  extended rearwards from a left rear portion of the engine  34 , and a gear box  81  connected to a rear portion of the non-stage speed change gear  78 . The drive shaft  74  is connected to an output shaft on the front side of the gear box  81 , whereas the drive shaft  73  is connected to an output shaft on the rear side of the gear box  81 . 
     FIG. 5  shows the condition where a front portion of the J frame  32  is attached to rear portions of the lower pipes  26  and  27  of the vehicle body frame  16 . Incidentally, symbol  83  denotes the holders (the holder  83  on the other side is not shown). 
   As shown in  FIG. 6 , the rear end slant portion  32 B of the J frame  32  is a portion set substantially vertical in the condition where the rider is not seated on the motortricycle  10 , and rear portions of the suspension arms  71  and  72  are attached to the rear end slant portion  32 B. Incidentally, a rear swing shaft  85  is provided for swingably mounting the rear portions of the suspension arms  71  and  72  onto the rear end slant portion  32 B. 
     FIG. 7  shows the rear suspension  63  in which the suspension arms  71  and  72  are extended leftwards and rightwards from the J frame  32 , the holders  83  are attached respectively to the tip ends of the suspension arms  71  and  72 , arcuate links  88  and  89  are swingably mounted onto upper portions of the suspension arms  71  and  72  through mount brackets  86  and  87 , and bell cranks  90  and  91 , which are roughly L-shaped in side view and which functions as a connecting means, are swingably mounted onto the tip ends of the arcuate links  88  and  89 . The shock absorber  76  is bridgingly disposed between upper end portions of the bell cranks  90  and  91 , a connection member  92  in the shape of a bar is bridgingly disposed between side end portions of the bell cranks  90  and  91 , and the connection member  92  is attached to the rear end slant portion  32 B of the J frame  32  through the oscillation mechanism  93 . 
   The arcuate links  88  and  89  are members each including a side projection portion  95  at an intermediate portion thereof, and brake devices  97 , 97  for braking the swinging of the arcuate links  88  and  89  are attached to the side projection portions  95 . Incidentally, brake devices  97 ,  97  each include a brake caliper  96 , with disks  98 ,  98  clamped between the brake calipers  96 ,  96  by a hydraulic oil pressure. The disks  98 ,  98  are members attached respectively to the suspension arms  71  and  72 . A bolt  100  functions as a swing shaft for the arcuate links  88  and  89 . 
   The bell clamps  90  and  91  are each composed of two crank plates  102 ,  102 , and each include a first bolt  103 , a second bolt  104 , and a third bolt  106 . Incidentally, symbol  107  denotes a fourth bolt used as a stopper pin for restricting the stretching and shrinking of the shock absorber  76  and the swinging of the connection member  92 . Nuts  108  are screw-engaged with the first to fourth bolts  103  to  107 . 
   The oscillation mechanism  93  permits leftward and rightward oscillation of the vehicle body frame  16  relative to the suspension arms  71  and  72  at the time of cornering and the like. As the oscillation inclination increases, a reaction force is increased by an elastic element incorporated therein so as to return the vehicle body frame  16  to an original position. 
   As shown in  FIG. 8A , the oscillation mechanism  93  is the so-called “Neidhard damper” including a case  111  attached to the rear end slant portion  32 B of the J frame  32  and a rear portion of the L pipe  54 , damper rubbers  112  contained in the case  111 , a pressing member  113  pressing the damper rubbers  112  and attached to the connection member  92 , and a penetrating pin  116  penetrating through the pressing member  113  and the connection member  92  and having both end portions supported by a tip end support portion  114  provided in the L pipe  54  and the rear end slant portion  32 B. A mount portion is provided in the pressing member  113  for mounting the pressing member  113  onto the connection member  92  by a bolt, and a swing restricting portion  118  is provided integrally on the tip end support portion  114  for restricting the swinging amount of the connection member  92 . 
   As shown in  FIG. 8B , the case  111  includes a left case  121  and a right case  122  mated with each other, a damper containing chamber  123  is provided therein, the damper rubbers  112  are disposed in four comers of the damper containing chamber  123 , and the damper rubbers  112  are pressed by projected form pressing portions  124  of the pressing member  113 . 
   In  FIG. 8C , when the vehicle body frame  16  is oscillated to the left side of the vehicle body (arrow “left” in the figure indicates the left side of the vehicle body) relative to the connection member  92  connected to the suspension arm side and the L pipe  54  is inclined by an angle, θ ( theta) the case  111  of the oscillation mechanism  93  is rotated relative to the pressing member  113 , and the damper rubbers  112  contained in the case  111  are compressed while being clamped between the case  111  and the pressing member  113 , whereby a reaction force for returning the case  111  and, hence, the vehicle body frame  16  to the original position (the position in  FIG. 8A ) is generated. 
     FIG. 9  shows the condition where the J frame  32  is provided with a rear portion mount portion  127  for swingably mounting rear portions of the suspension arms  71  and  72  (see  FIG. 7 ) and a front portion mount portion  128  for swingably mounting front portions of the suspension arms  71  and  72 . 
   The rear portion mount portion  127  is composed of the rear end slant portion  32 B, and a vertical bracket  131  extended downward from the L pipe  54  to the lower horizontal portion  32 E (described later). The rear portion swing shaft  85  (see  FIG. 6 ) for supporting the rear portions of the suspension arms  71  and  72  is mounted to the rear portion slant portion  32 B and the vertical bracket  131 . 
   The front portion mount portion  128  is composed of a front riser portion  133  and a rear riser portion  134 , which are raised from the lower horizontal portion  32 E with an interval therebetween. A front portion swing shaft  136  for supporting the front portions of the suspension arms  71  and  72  is mounted to the front riser portion  133  and the rear riser portion  134 . 
   Also shown are an oil tank  138 , engine mount vibration-isolating links for mounting the engine  34  onto the vehicle body frame  16 , and a U-shaped U pipe  144  attached to rear lower portions of the lower pipes  26  and  27  for attaching the tip end of the lower horizontal portion  32 E of the J frame  32 . 
   While  FIG. 5  shows the embodiment in which the front ends of the lower horizontal portion  32 A branched in a Y shape are attached directly to the lower pipes  26  and  27 ,  FIG. 9  shows another embodiment in which the J frame  32  is composed of the lower horizontal portion  32 E branched in a Y shape, the rear end slant portion  32 B, and the upper slant portion  32 C, and the front ends of the lower horizontal portion  32 E are attached to the lower pipes  26  and  27  through the U pipe  144 . 
     FIG. 10  shows the condition where the lower horizontal portion  32 E of the J frame  32  is branched in a Y form at an intermediate portion thereof and connected to a rear portion of the U pipe  144 , and the connection pipes  52 ,  52  are extended in a Y form from the upper slant portion  32 C of the J frame  32  to the center upper frame  28 . The U pipe  144  and the center upper frame  28  function as the front frame. 
   The lower horizontal portion  32 E (and the lower horizontal portion  32 A (see FIG.  5 )), more specifically, is formed by bending a single elongate first pipe  151  at an intermediate portion thereof and connecting a second pipe  153  to a portion near the bent portion  152  of the first pipe  151 . A Y-shaped branch portion  154  branches in a Y shape by connecting the second pipe  153  to the first pipe  151 , and a Y-shaped branch portion  155  branches in a Y shape by connecting the connection pipes  52 ,  52  to the upper slant portion  32 C. 
   The first pipe  151  is a member including the rear end slant portion  32 B and the upper slant portion  32 C, and is a member obtained upon excluding the second pipe  153  from the J frame  32 . 
   Thus, by forming the lower horizontal portion  32 E in the Y shape, the connection between a lower front portion of the J frame  32  and the U pipe  144  can be made to be firm, and by disposing the connection pipes  52 ,  52  in a Y shape, the connection between a rear upper portion of the J frame  32  and a rear portion of the center upper frame  28  can be made to be firm. In addition, in  FIG. 5 , by forming the lower horizontal portion  32 A in the Y shape, the connection between a lower front portion of the J frame  32  and the lower pipes  26  and  27  an be made to be firm. 
     FIG. 11  shows the rear suspension  63  in the condition where one rider (driver) is riding on the vehicle (this condition will be referred to as “1G condition”). Incidentally, the rear slant portion  32 B and the upper slant portion  32 C of the J frame  32  shown in  FIG. 9  are omitted here. The right case  122  of the oscillation mechanism  93  shown in  FIG. 8B  is indicated by imaginary line. In this case, the L pipe  54  of the vehicle body frame  16  is in a roughly vertical condition, and the connection member  92  is in a roughly horizontal condition. 
   The connection member  92  includes fan-shaped portions  156  and  157  at both ends thereof, with arcuate slots  158  and  159  provided in the fan-shaped portions  156  and  157  respectively, and fourth bolts  107 ,  107  as stopper pins are passed through the arcuate slots  158  and  159 , whereby the inclination angles of the bell cranks  90  and  91  relative to the connection member  92  are restricted. The inclination angles of the bell cranks  90  and  91  vary according to the inclination angles of the suspension arms  71  and  72 , i.e., the vertical movement amounts of the rear wheels  18  and  21 . In other words, the arcuate slots  158  and  159  are portions for restricting the vertical movement amounts of the rear wheels  18  and  21 . 
   The functions of the rear suspension  63  described above will be described in  FIG. 12 . 
   In  FIG. 12 , for example, when the rear wheel  18  on the left side is moved upwards by a movement amount M 1  from the condition shown in  FIG. 11 , the suspension arm  71  swings upwards as indicated by arrow a with the rear swing shaft  85  and the front swing shaft  136  (see  FIG. 9 ) as a center. Associated with this, the arcuate link  88  moves upwards as indicated by arrow b to swing the bell crank  90  in the direction of arrow c with the second bolt  104  as a fulcrum, thereby pressing and shrinking the shock absorber  76  as indicated by arrow d. In this manner, transmission of the shock to the side of the vehicle body frame  16  (see  FIG. 10 ) caused by the upward movement of the left-side rear wheel  18  is moderated. 
   At this time, the suspension arm  72  on the other side is in the same condition as in  FIG. 11 , so that the connection member  92  is in the same roughly horizontal condition as in  FIG. 11 . 
   In  FIG. 13 , when the rear wheels  18  and  21  are both moved upwards by a movement amount M 2 , or the vehicle body frame  16  is lowered by the movement amount M 2  relative to the rear wheels  18  and  21 , from the condition of  FIG. 11 , the suspension arms  71  and  72  swing upwards about the rear swing shaft  85  and the front swing shaft  136  (see  FIG. 9 ) as indicated by arrows f, f. Associated with this, the arcuate links  88  and  89  move upwards as indicated by arrows g, g to swing the bell cranks  90  and  91  in the directions of arrows h, h with the second bolt  104  as a fulcrum, thereby pressing and shrinking the shock absorber  76  as indicated by arrows j, j. As a result, a shock-absorbing function of the shock absorber  76  takes place. 
   In  FIG. 14 , when the rear wheels  18  and  21  are both lowered by a movement amount M 3 , or the vehicle body frame  16  is moved upwards by the movement amount M 3  relative to the rear wheels  18  and  21 , from the condition of  FIG. 11 , the suspension arms  71  and  72  swing downwards as indicated by arrows m, m with the rear swing shaft  85  and the front swing shaft  136  (see  FIG. 9 ) as a center. Associated with this, the arcuate links  88  and  89  are lowered as indicated by arrows n, n to swing the bell cranks  90  and  91  in the directions of arrows p, p with the second bolt  104  as a fulcrum, thereby stretching the shock absorber  76  as indicated by arrows q, q. As a result, a shock-absorbing function of the shock absorber  76  takes place. 
   In  FIG. 15 , when the vehicle body frame  16 , here the L pipe  54 , is oscillated by an angle φ 1 (phi  1 ) to the left side of the vehicle body from the condition of  FIG. 11 , the connection member  92  connected to the L pipe  54  through the penetrating pin  116  undergoes a parallel movement to the left side as indicated by arrow s. Associated with this, the arcuate links  88  and  89  are inclined as indicated by arrows t, t, and the bell cranks  90  and  91  undergo a parallel movement as indicated by arrows u, u. In this case, since the distance between the third bolts  106 ,  106  of the bell cranks  90  and  91  remains unchanged, the shock absorber  76  is not stretched nor shrunk. 
   At this time, since the vehicle body frame  16  oscillates relative to the connection member  92 , the oscillation mechanism generates a reaction force for returning the vehicle body frame  16  to its original position (namely, the position in  FIG. 11 ), in the same manner as in the case shown in  FIG. 8C . 
   In  FIG. 16 , when the rear wheel  18  is moved upwards by a movement amount M 4  and the vehicle body frame  16 , here the L pipe  54 , is oscillated by an angle φ  2  (phi  2 ) to the left side of the vehicle body from the condition of  FIG. 11 , the suspension arm  71  swings upwards as indicated by arrow v with the rear swing shaft  85  and the front swing shaft  136  (see  FIG. 9 ) as a center, and the connection member  92  is moved leftwards as indicated by arrow w. Associated with this, the arcuate link  88  is moved upwards and inclined to the left side, the arcuate link  89  is inclined to the left side as indicated by arrow x, the bell crank  90  swings clockwise with the second bolt  104  as a fulcrum and is moved leftwards, and the bell crank  91  is moved leftwards, resulting in that the shock absorber  76  is pressed and shrunk, to display a shock-absorbing function. 
   As shown in  FIG. 17 , the same components as those in the embodiment shown in  FIG. 2  are denoted by the same symbols as used above, and detailed descriptions thereof are omitted. 
   The motortricycle with oscillation mechanism  170  is a vehicle in which a rear bracket  171  for reinforcing the connection portions between the connection pipes  52 ,  52  and the center upper frame  28  and for supporting a rear upper portion of the power unit  17  is attached to the connection pipes  52 ,  52  and the center upper frame  28 , and left and right front brackets  172 ,  172  for supporting a front upper portion of the power unit  17  are attached to the center upper frame  28 . 
   The front upper portion of the power unit  17  is a portion that is rubber-mounted on the front brackets  172 ,  172 . Namely, the front upper portion of the power unit  17  is connected to the front brackets  172 ,  172  through a support shaft  173  and the rubber bush  62  provided in the periphery of the support shaft  173 . 
   In addition, the rear upper portion of the power unit  17  is also a portion, which is rubber-mounted on the rear bracket  171  through the rubber bush  62 . 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.