Abstract:
A scooter type motorcycle includes a rear fork for supporting a rear wheel of the motorcycle. A front portion of the rear fork is attached to a crankcase of an engine in a vertically swingable manner. The engine is mounted to a body frame through elastic members. The mounting arrangement enhances the comfort of the ride of the motorcycle by reducing vibrations from being transmitted from the rear wheel to the rider.

Description:
[0001]    This nonprovisional application claims priority under 35 U.S.C. § 119( a ) on Patent Application No. 2000-272708 filed in Japan on Sep. 8, 2000, which is herein incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an engine mounting structure in a motorcycle.  
           [0004]    2. Description of the Background Art  
           [0005]    In a certain motorcycle, an engine is mounted to a body frame and a rear fork is vertically swingably attached to the engine. As this type of a motorcycle there is known, for example, “Step Device in a Sitting Type Motorcycle” (hereinafter referred to as the “background art”) disclosed in Japanese Examined Patent Publication No. Hei 1-33392.  
           [0006]    According to the background art, as shown in FIG. 1 of the aforesaid publication, an engine E is mounted to a vehicle body  1  (this and the following reference numerals and marks are quoted from the aforesaid publication). A rear fork F is vertically swingably attached to a crankcase  18  of the engine E. A rear wheel Wr is rotatably secured to a rear portion of the rear fork F.  
           [0007]    Further, in the background art, as shown in FIG. 2 of the aforesaid publication, an output shaft  19  is disposed behind a crankshaft  9  for transmitting power of the crankshaft  9  to a belt type continuously variable transmission M. A swing center of the rear fork F is aligned with the axis of the output shaft  19 .  
           [0008]    In the background art referred to above, the engine E, which is highly rigid, is fixed to the vehicle body  1  with bolts, thereby enhancing the rigidity of the vehicle body  1  to a greater extent. However, with travel of the motorcycle, the rear wheel Wr vibrates according to road surface conditions and this vibration (hereinafter “running vibration”) is transmitted directly from the rear wheel Wr to the vehicle body  1  through the rear fork F and the engine E. Therefore, there is room for improvement in damping the running vibration transmitted to the vehicle body  1  and thereby enhancing the comfortableness in riding the motorcycle.  
         SUMMARY OF THE INVENTION  
         [0009]    It is therefore an object of the present invention to provide a technique for enhancing the comfortableness in riding a motorcycle wherein a rear fork for supporting a rear wheel is swingably attached to a crankcase of an engine.  
           [0010]    According to the invention, in a motorcycle wherein a front portion of a rear fork which supports a rear wheel is vertically swingably attached to a crankcase of an engine, there is provided an engine mounting structure characterized in that the engine is mounted to a body frame through elastic members. With travel of the motorcycle, the running vibration is damped by the elastic members and is therefore unlikely to be transmitted from the engine to the body frame.  
           [0011]    According to the invention, a cylinder extends forward from the crankcase of the engine, the body frame is provided with a rising frame member which is substantially orthogonal to the cylinder in side view, and the elastic members are attached to the rising frame member in higher and lower positions respectively than the cylinder.  
           [0012]    By extending the cylinder forward from the crankcase, spaces are formed above and below the crankcase. By utilizing these upper and lower spaces, elastic members are disposed in higher and lower positions respectively than the cylinder and are attached to a rising frame which is substantially orthogonal to the cylinder in side view. It is possible to ensure a sufficient distance between the upper and lower elastic members.  
           [0013]    With running vibration, a moment acts on the case (e.g., crankcase) of the engine, centered on the upper or lower elastic member. The longer the distance between the upper and lower elastic members, the smaller the force based on the moment and acting on the upper or lower elastic member. Since the distance between the upper and lower elastic members can be set relatively large, it is possible to decrease the force acting on the upper or lower elastic member. As a result, the vibration absorbing performance of the elastic members is enhanced. The elastic members are disposed in higher and lower positions respectively than a swing center of the rear fork.  
           [0014]    The swing center of the rear fork is aligned with the axis of a crankshaft of the engine. A swing angle of a mechanism (a power transfer mechanism) for transmitting power from the crankshaft to the rear wheel coincides with a swing angle of the rear fork. Consequently, the construction of the power transfer mechanism becomes simpler.  
           [0015]    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  
       [0016]    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 limitive of the present invention, and wherein:  
         [0017]    [0017]FIG. 1 is a left side view of a motorcycle related to the present invention;  
         [0018]    [0018]FIG. 2 is left side view of an engine and a power transfer mechanism both related to the present invention;  
         [0019]    [0019]FIG. 3 is a sectional view taken on line  3 - 3  in FIG. 2;  
         [0020]    [0020]FIG. 4 is a left side view of a body frame related to the present invention;  
         [0021]    [0021]FIG. 5 is a left side view of a rear portion of the body frame, the engine, and the power transfer mechanism; and  
         [0022]    [0022]FIG. 6 is a plan view of an engine mounting portion according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    An embodiment of the present invention will be described hereafter with reference to the accompanying drawings. In the following description, “front,” “rear,” “left,” “right,” “upper,” and “lower” represent directions as seen from the rider side, and Fr, Rr, L, R, and CL stand for front side, rear side, left side, right side, and a vehicle body center (vehicle width center), respectively.  
         [0024]    As shown in FIG. 1, a scooter type motorcycle  10  includes a body frame  11 , and a front fork  13  attached to a head pipe  12  of the body frame  11 . A front wheel  14  is secured to the front fork  13 , and a handlebar  15  is connected to the front fork  13 . An engine  16  is mounted to a rear portion of the body frame  11 , and a power transfer mechanism  17  is vertically swingably attached to the engine  16 . A rear wheel  18  is secured to the power transfer mechanism  17 , and rear cushion units  19  suspend a rear end portion of the power transfer mechanism  17  from the body frame  11 . A container box  21  is attached to an upper position of the rear portion of the body frame  11 , and a double seat  22  is disposed on the container box  21  so that it can be opened and closed.  
         [0025]    In the motorcycle  10 , moreover, the body frame  11  is covered with a body cover  30 . The body cover  30  comprises a front cover  31  which covers a front portion of the head pipe  12 , an inner cover  32  which covers a rear portion of the front cover  31 , right and left floor steps  33  of a low floor type for the rider to put his or her feet thereon, right and left floor skirts  34  extending downwardly from outer edges of the floor steps  33 , a center cover  35  extending rearwardly from the inner cover  32  to cover a longitudinally central portion of the body frame  11 , and a rear cover  36  extending rearwardly from the center cover  35  to cover the rear portion of the body frame  11 .  
         [0026]    In FIG. 1, reference numeral  37  denotes a handlebar cover, numeral  38  is a front fender, numeral  39  is a rear fender, and numeral  41  is a windscreen. Reference numeral  42  is a radiator for cooling the engine, numeral  43  is a fuel tank, and numeral  44  is a passenger step. Reference numeral  45  is an air cleaner, numeral  46  is a connecting tube, and numeral  47  is an air chamber. Reference numeral  48  is an exhaust pipe for the engine, and numeral  49  is a muffler for exhaust.  
         [0027]    Referring now to FIG. 2, the engine  16  is a water-cooled, 4 cycle 2-cylinder engine in which a cylinder  54  extends forward from a crankcase  52 . The axis of the cylinder  54  is assumed to be L 1 .  
         [0028]    Referring now to FIG. 3, a case portion of the engine  16  includes a crankcase  52  comprising left and right case halves  51 L,  51 R coupled together with bolts. A cylinder block  53  is bolted to a front portion of the crankcase  52 . Two right and left cylinders  54  are disposed within the cylinder block  53 . A cylinder head  55  is bolted to a front portion of the cylinder block  53 . A pair of combustion chambers  56  are formed in the cylinder head  55 . A head cover  57  is bolted to a front portion of the cylinder head  55 . A valve operating chamber  58  is formed between the cylinder head  55  and the head cover  57 .  
         [0029]    The engine  16  includes a crankshaft  61  rotatably mounted to the crankcase  52  and extending right and left. A pair of pistons  63  are connected to the crankshaft  61  through a pair of connecting rods  62  and are adapted to reciprocate within the cylinders  54 . A valve operating mechanism  64  is accommodated within the valve operating chamber  58 .  
         [0030]    A generator cover  66  is bolted to the right side portion of the right case half  51 R to form a generator receiving chamber  67  between the right case half  51 R and the generator cover  66 , and a generator  68  is received within the generator receiving chamber  67 . The generator  68  is connected to a right end portion of the crankshaft  61 . Since the generator cover  66  is fixed to the crankcase  52 , it is assumed to be a part of the crankcase  52  in the present invention.  
         [0031]    The power transfer mechanism  17  is made up of a transfer mechanism case  72  which is vertically swingably secured at a front portion thereof to the left case half  51 L through a bearing  71  and which extends rearward. A case cover  73  closes a side opening of the transfer mechanism case  72 . A transfer mechanism receiving chamber  74  is formed between the transfer mechanism case  72  and the case cover  73 , and a transfer mechanism  75  is received within the transfer mechanism receiving chamber  74 . The transfer mechanism case  72  is formed with a boss portion  76  which projects from a longitudinally intermediate position to the transversely central side.  
         [0032]    The transfer mechanism  75  includes a combination of a belt type speed change mechanism  75 A connected to the crankshaft  61  and a gear mechanism  75 B connected to the belt type speed change mechanism  75 A. A rear wheel axle  77  is connected to an output side of the gear mechanism  75 B and is supported rotatable. Reference numeral  78  denotes a cover.  
         [0033]    A front arm  81  is vertically swingably mounted to the generator cover  66 . More specifically, a support shaft  82  is integral with a front portion of the front arm  81  and is supported by the generator cover  66  rotatably through a bearing  83 .  
         [0034]    The front arm  81  extends rearward and a boss portion  84  is projected from a rear end of the front art  81  to the transversely central side along a rear portion of the crankcase  52 . The boss portion  76  of the transfer mechanism case  72  is connected to the boss portion  84  with bolts  85 . Further, a rear arm  86  is connected to a rear portion of the front arm  81  with a bolt  87 . The rear arm  86  extends rearward and the rear axle  77  is supported rotatably by a rear end portion of the rear arm  86 .  
         [0035]    Such a combined structure of the transfer mechanism case  72  and the front and rear arms  81 ,  86  constitutes a rear fork (swing arm)  88  which is generally H-shaped in plan. Therefore, a front portion of the rear fork  88  which supports the rear wheel  18  through the rear wheel axle  77  can be vertically swingably secured to the crankcase  52 .  
         [0036]    The present invention is characterized in that a swing center P 2  of the rear fork  88 , i.e., a swing center P 2  of the transfer mechanism case  72  and the front arm  81 , is made coincident with a center P 1  of the crankshaft  61 . If the swing center P 2  of the rear fork  88  is not in alignment with (is offset from) the center P 1  of the crankshaft  61 , a swing angle of the mechanism (transfer mechanism) for the transfer of power from the crankshaft  61  to the rear wheel  18  is different from that of the rear fork  88 . Consequently, the construction of the transfer mechanism is compelled to become complicated.  
         [0037]    In contrast therewith, in the present invention, the swing center P 2  of the rear fork  88  is coincident with the center P 1  of the crankshaft P 1 . To be more specific, by connecting a driving pulley  79  in the transfer mechanism  75  directly with the crankshaft  61 , an input-side center of the transfer mechanism  75  and the swing center P 2  of the transfer mechanism case  72  are made coincident with the center P 1  of the crankshaft  61 . As a result, the swing angle of the transfer mechanism  75  becomes equal to the swing angle of the rear fork  88 . Thus, the structure of the transfer mechanism  75  (a mechanism for the transfer of power from the crankshaft  61  to the rear wheel  18 ) becomes simpler.  
         [0038]    In the foregoing background art (Japanese Examined Patent Publication No. Hei 1-33392), the power of the crankshaft is transmitted through an output shaft to a belt type continuously variable transmission and the swing center of the rear fork is made coincident with the swing center of the output shaft. In the present invention, such an output shaft is not used and therefore the structure of the transfer mechanism becomes simpler.  
         [0039]    [0039]FIG. 3 further illustrates that a pair of brackets  89  which connect lower portions of the right and left rear cushion units  19  are provided at rear ends of the transfer mechanism case  72  and the rear arm  86 .  
         [0040]    Turning back to FIG. 2, the cylinder head  55  is integrally formed with intake ports  92  contiguous to the combustion chambers  56  and having intake valves  91 , and exhaust ports  94  contiguous to the combustion chambers  56  and having exhaust valves  93 . When the engine  16  is seen sideways, the intake ports  92  are upward openings, while the exhaust ports  94  are downward openings.  
         [0041]    An inlet pipe  95  connected to each intake port  92  extends rearward of the cylinder head  55 , and a throttle valve  96  is connected to a rear end (upstream end) of the inlet pipe  95 . An upstream end of the throttle valve  96  faces rearward. The inlet pipe  95  is equipped with a fuel injector  97  for the injection of fuel toward each intake valve  91 .  
         [0042]    Such a combined structure of each intake port  92 , inlet pipe  95  and throttle valve  96  constitutes an intake passage  98 . In other words, the intake passage  98  extends rearward from an upper portion of each cylinder  54  and the intake port therein is allowed to face rearward. Further, the intake port of the intake passage  98 , i.e., the upstream end of the throttle valve  96 , is connected to the air cleaner  45  through the air chamber  47 , as shown in FIG. 1. The components identified by numeral  99  are cam shafts in the valve operating mechanism  64 .  
         [0043]    The crankcase  52  of the engine  16  is provided with upper mounting brackets  101  and lower mounting brackets  102 , and front mounting brackets  103  are provided at a lower portion of the cylinder head  55 . More specifically, the upper mounting brackets  101  are provided at a higher position than the cylinder  54 , while the lower mounting brackets  102  and the front mounting brackets  103  are positioned lower than the cylinder  54 .  
         [0044]    Referring to FIG. 4, the body frame  11  is a double cradle type frame in which a pair of main pipes  111  extend rearwardly downward from the head pipe  12 . A pair of seat rails  112  extend rearwardly upward from longitudinally intermediate positions of the main pipes  111 . A pair of down-pipes  113  extend below the main pipes  111  and rearwardly downward from the head pipe  12 . A pair of lower pipes  114  extend rearwardly from lower ends of the down-pipes  113 . Lower ends of the main pipes  111  are connected to longitudinally intermediate positions of the lower pipes  114 . Upper pipes  115  (rising frame members) extend rearwardly upward from rear ends of the lower pipes  114 . Rear ends of the upper pipes  115  are connected to longitudinally intermediate positions of the seat rails  112 .  
         [0045]    [0045]FIG. 4 shows that a pair of brackets  116  which connect upper portions of the right and left rear cushions  19  (only the left-hand one, i.e., the one located on this side in the figure, is shown) are provided at rear portions of the seat rails  112 . By connecting the rear cushion units  19  between the brackets  89  of the rear forks  88  and the brackets  116  of the seat rails  112 , it is possible to suspend a rear portion of the rear fork  88  at right and left positions thereof from the body frame  11  through the rear cushion units  19 .  
         [0046]    Referring to FIG. 5, the right and left upper pipes  115  (only the left-hand one, i.e., the one located on this side in the figure, is shown) as rising frame members of the body frame  11  are substantially orthogonal to each cylinder  54  in side view, that is, they are substantially orthogonal to the axis L 1  of the cylinder. Right and left upper brackets  121  are provided at upper ends of the right and left upper pipes  115 . A first cross member  122  is disposed bridgewise between lower ends of the upper pipes  115 , and a lower bracket  123  is provided on the first cross member  122 .  
         [0047]    A second cross member  125  is disposed bridgewise between the lower end portions of the right and left main pipes  111 , and a front bracket  126  is provided on the second cross member  125 . L 2  represents a line passing through the center P 1  of the crankshaft  61  and also through a center P 3  of the rear wheel axle.  
         [0048]    Referring now to FIG. 6, the upper brackets  121  are attached to the right and left upper pipes  115 . The first cross member  122  is mounted bridgewise between the right and left upper pipes  115 . The lower bracket  123  is attached to the first cross member  122 . A pipe-like lower pivot portion  124  is provided in the lower bracket  123 . The second cross member  125  is mounted bridgewise between the right and left main pipes  111 . The front bracket  126  is attached to the second cross member  125 , and a pipe-like front pivot portion  127  is provided in the front bracket  126 .  
         [0049]    According to a first mounting structure of the engine  16 , a pair of bushing holes  101   a  are formed in the right and left upper mounting brackets  101  provided on the crankcase  52 . Annular bushings  131  are fitted in the bushing holes  101   a  respectively, and the rubber bushings  131  and the right and left upper brackets  121  are fixed together using a single pivot shaft  132  extending in the transverse direction.  
         [0050]    According to a second mounting structure of the engine  16 , a pair of bushing holes  102   a  are formed in the right and left lower mounting brackets  102  provided on the crankcase  52 . Annular rubber bushings  133  are fitted in the bushing holes  102   a  respectively, and the rubber bushings  133  and the lower pivot portion  124  are fixed together using a single pivot shaft  134  extending in the transverse direction.  
         [0051]    According to a third mounting structure of the engine  16 , a pair of bushing holes  103   a  are formed in the right and left front mounting brackets  103  provided on the cylinder head  55 . Annular bushings  135  are fitted in the bushing holes  103   a  respectively, and the rubber bushings  135  and the front pivot portion  127  are fixed together using a single pivot shaft  136  extending in the transverse direction. It is optional whether the third engine mounting structure of the engine  16  is to be provided or not.  
         [0052]    The rubber bushings  131 ,  133  and  135  are elastic members for absorbing vibration of the engine  16 . The pivot shaft  132 ,  134  and  136  are bolt-nut combinations. In the figure, the components identified by numeral  137  are elastic members for absorbing right and left vibrations of the engine  16  and the components identified by numeral  138  are collars. The present invention is characterized in that the engine  16  is mounted through the rubber bushings  131 ,  133  and  135  (elastic members) to the body frame  11 .  
         [0053]    Turning back to FIG. 5, the present invention is characterized in that the rubber bushings  131  for the upper mounting brackets  101  are attached to the upper brackets  121  positioned higher than the cylinder  54  and are thereby attached to the upper pipes  115  in a higher position than the cylinder  54 , and that the rubber bushings  133  for the lower mounting brackets  102  are attached to the lower bracket  123  positioned lower than the cylinder  54  and are thereby attached to the upper pipes  115  in a lower position than the cylinder  54 .  
         [0054]    The present invention is further characterized in that the rubber bushings  131  for the upper mounting brackets  101  are positioned higher than the swing center P 2  of the rear fork  88  and that the rubber bushings  133  for the lower mounting brackets  102  are positioned lower than the swing center P 2  of the rear fork  88 .  
         [0055]    With travel of the motorcycle  10 , the rear wheel  18  (see FIG. 4) vibrates according to road surface conditions. Since the engine  16  is mounted to the body frame  11  through the rubber bushings (elastic members)  131 ,  133  and  135 , the running vibration transmitted from the rear wheel  18  to the engine  16  through the rear fork  88  is damped by the rubber bushings  131 ,  133  and  135  and therefore it is possible to make the vibration difficult to be transmitted to the body frame  11 . Accordingly, it is possible to enhance the comfortableness of the motorcycle  10  with the rear fork  88  attached swingably to the crankcase  52 .  
         [0056]    Further, by extending the cylinder  54  forward from the crankcase  52 , it is possible to ensure spaces above and below the crankcase  52 . By utilizing these upper and lower surplus spaces, the rubber bushings  131  and  133  are disposed in upper and lower positions respectively with respect to the cylinder  54 , and are attached to the upper pipes  115  which are approximately orthogonal to the cylinder  54  in side view. That is, the rubber bushings  131  and  133  are disposed in upper and lower positions respectively with respect to the swing center P 2 .  
         [0057]    Thus, it is easy to ensure spaces for installation of the rubber bushings  131  and  133 . In addition, the structure of the body frame  11  for installing the rubber bushings  131  and  133  are simple. Further, it is possible to ensure a sufficient distance between the upper and lower rubber bushings  131 ,  133 .  
         [0058]    Due to the running vibration, a moment acts on the crankcase  52  (cases) of the engine  16 , centered on the upper rubber bushing  131  or the lower rubber bushing  133 . With this moment, the longer the distance between the upper and lower rubber bushings  131 ,  133 , the smaller the force acting on the upper rubber bushing  131  or the lower rubber bushing  131 .  
         [0059]    In the present invention, as described above, the rubber bushings  131  and  133  are disposed in higher and lower positions respectively with respect to the cylinder  54 , that is, in higher and lower positions respectively than the swing center P 2  of the rear fork  88 . Therefore, the distance between the rubber bushings  131  and  133  can be set relatively large. As a result, the force acting on the rubber bushings  131  and  133  becomes smaller and hence the vibration absorbing performance of the rubber bushings  131  and  133  is enhanced. Thus, the running vibration transmitted to the engine  16  is further damped by the rubber bushings  131  and  133 , so that it is possible to make the running vibration difficult to be transmitted to the body frame  11  and hence possible to further enhance the comfortableness in riding the motorcycle.  
         [0060]    Although the motorcycle  10  described in the above embodiment is a scooter type motorcycle, no limitation is made thereto. The engine  16  is not limited to a 2-cylinder engine, but it may be a single- or multi-cylinder engine. The number of the rear cushions  19  is not limited to two disposed on the right and left sides, but it may be one. The front arm  81  which constitutes the rear fork  88  may be attached vertically swingably to the crankcase  52  directly without interposing the generator cover  66  therebetween.  
         [0061]    The structure of the rear fork  88  is not limited to the combined structure of the transfer mechanism case  72  and the front and rear arms  81 ,  86 . For example, it may comprise only the arms without being combined with the transfer mechanism case  72 , or the rear fork  88  may be constituted of a single member.  
         [0062]    Since the present invention is constructed as above, the following effects are obtained. Since the engine is mounted to the body frame through elastic members, the running vibration transmitted from the rear wheel to the engine through the rear fork is damped by the elastic members and hence it is possible to make it difficult to be transmitted to the body frame. Consequently, it is possible to enhance the comfortableness in riding the motorcycle in which the rear fork for supporting the rear wheel is swingably attached to the crankcase.  
         [0063]    The cylinder is extended forward from the crankcase to form spaces above and below the crankcase, and elastic members are disposed in higher and lower positions respectively than the cylinder and are attached to a rising frame which is substantially orthogonal to the cylinder in side view. Therefore, it is easy to ensure spaces for installation of the elastic members and the structure of the body frame for mounting the elastic members is simple. Further, it is possible to ensure a sufficient distance between the upper and lower elastic members.  
         [0064]    With running vibration, a moment acts on the case (crankcase, etc.) of the engine, centered at the upper or lower elastic member. The longer the distance between the upper and lower elastic members, the smaller the force based on the moment and acting on the upper or the lower elastic member. Since the upper and lower elastic members are disposed in higher and lower positions than the cylinder, the distance between the elastic members can be set relatively large. With consequent decrease of the force acting on the upper or the lower elastic member, the vibration absorbing performance of the elastic members is enhanced. Therefore, the running vibration transmitted to the engine is damped to a greater extent by the elastic members, whereby it is possible to make the vibration difficult to be transmitted to the body frame and hence possible to further enhance the comfortableness in riding the motorcycle. The elastic members are disposed in higher and lower positions respectively than the swing center of the rear fork.  
         [0065]    Since the swing center of the rear fork is aligned with the center of the crankshaft of the engine, a swing angle of the mechanism (power transfer mechanism) for the transfer of power from the crankshaft to the rear wheel can be made coincident with the swing angle of the rear fork. Therefore, it is possible to simplify the construction of the power transfer mechanism.  
         [0066]    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 to be included within the scope of the following claims.