Abstract:
A twincam engine including a rocker arm configured to transmit rotation of a cam to reciprocate at least one of intake and exhaust valves, the rocker arm being pivotally attached to a cylinder head between a center axis of a valve stem of the intake valve and a center axis of a valve stem of the exhaust valve.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a twincam engine, and a motorcycle comprising the twincam engine. More particularly, the present invention relates to a construction for enabling an intake port and an exhaust port to have large curvature radiuses. 
   2. Description of the Related Art 
   For example, there is an inline four-cylinder four-cycle engine as an engine mounted in a motorcycle. Focusing attention on a valve system, there is a twincam engine (also referred to as DOHC (double overhead camshaft)) having rocker arms, as one type of the inline four-cylinder four-cycle engine. Such a twincam engine includes a camshaft configured to drive an exhaust valve and a camshaft configured to drive an intake valve, which are mounted between a cylinder head and a cylinder head cover and are disposed forward and rearward. These camshafts rotate in association with a crankshaft, and its rotation is transmitted through cams and the rocker arms to reciprocate the exhaust valve and the intake valve (see Publication of Japanese Examined Utility Model Application No. Hei. 3-15761). 
   A construction of the valve system will be described in detail. The exhaust valve and the intake valve are arranged forward and rearward for each cylinder, and their valve stems extend upward from their disc portions disposed at openings of the exhaust port and the intake port which are located on a combustion chamber side. A rocker arm on the exhaust valve side is disposed forward in the engine. A tip end portion of the rocker arm on the exhaust valve side is in contact with a valve lifter which is in contact with an upper end portion of the valve stem of the exhaust valve. The rocker arm is pivotally attached at a base end portion thereof to a cylinder head at a location forward of the valve stem by a pin. A rocker arm on the intake valve side is disposed rearward in the engine. A tip end portion of the rocker arm on the intake valve side is in contact with the valve lifter which is in contact with an upper end portion of a valve stem of the intake valve. The rocker arm is pivotally attached at a base end portion thereof to a cylinder head at a location behind the valve stem by a pin. The pin for mounting the rocker arm on the exhaust valve side extends in a rightward and leftward direction and is a common pin for the respective cylinders. The pin for mounting the rocker arm on the intake valve side is constructed in the same manner. 
   In the engine having the above constructed valve system, it is necessary to lay out the exhaust port and the intake port extending upward from the combustion chamber so as not to interfere with the pins located above the combustion chamber. For this reason, the exhaust port and the intake port must be respectively curved forward and rearward with relatively small curvature radiuses. If the curvature radius of the intake port is small, then it is difficult to improve air-intake efficiency. If the curvature radius of the exhaust port is small, then it is difficult to improve exhaust efficiency. This is undesirable to a high-output and high-speed engine. Such a circumstance occurs in twincam engines mounted in other vehicles as well as the engines mounted in the motorcycles. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an object of the present invention is to provide a twincam engine that is devised of a construction of a valve system to enable an intake port and an exhaust port to have large curvature radiuses, and a motorcycle comprising the twincam engine. 
   The present invention has been made in view of the above circumstances, and a twincam engine of the present invention comprises a rocker arm configured to transmit rotation of a cam to reciprocate at least one of intake and exhaust valves, the rocker arm being pivotally attached to a cylinder head between a center axis of a valve stem of the intake valve and a center axis of a valve stem of the exhaust valve. 
   In such a construction, since a mounting position of the rocker arm is located closer to a center of the engine, limitation on the layout of at least one of intake and exhaust ports can be reduced, and thus at least one of them can have a larger curvature radius. 
   The twincam engine may further comprise a plurality of cylinders each having the rocker arm pivotally attached to the cylinder head by each separate pin. In such a construction, the pin can be made lightweight as compared to a case where an elongate common pin extending over the cylinders is used. Since each rocker arm is pivotally attached to the cylinder head by each separate pin as described above, the pivot of each rocker arm is laid out without limitation by the layout of an ignition plug mounted for each cylinder in the cylinder head between a camshaft configured to drive the exhaust valve and a camshaft configured to drive the intake valve. 
   The twincam engine may further comprise a holder that is mounted to the cylinder head and is configured to mount the rocker arm by the pin. The holder may have a bolt hole through which the holder is fastened to the cylinder head by a bolt and a plug hole into which an ignition plug is inserted. The bolt hole and the plug hole may be disposed to interpose a mounting position of the rocker arm between them. In such a construction, a portion of the holder for mounting the rocker arm is fastened to the cylinder head by inserting an ignition plug into the plug holder and by inserting the bolt into the bolt hole. As a result, the rocker arm is stably mounted. 
   A head portion of the bolt inserted into the bolt hole may be disposed opposite to an end portion of the pin mounted on the holder. In such a construction, the head portion of the bolt can inhibit the pin from coming off in one direction. 
   The holder may be formed of case-hardened steel that has been subjected to surface hardening treatment (e.g., carburizing or nitriding). In such a construction, wear resistance of the holder can be improved. 
   The cylinder head may be provided with a convex base portion to which the holder is fastened by the bolt, the base portion extending along a direction in which the cylinders are arranged. In such a construction, the holder can be stably fastened to the cylinder head and the stiffness of the cylinder head can be improved. 
   A motorcycle of the present invention includes any one of the above described twincam engines. In such a construction, limitations on the lay out of the intake port and the exhaust port can be reduced, and thus the intake port and the exhaust port can have larger curvature radiuses. As a result, air-intake efficiency and gas-exhaust efficiency can be improved. If the curvature radius of the intake port is made larger, then an opening of the intake port on the outer side of the engine can be oriented upward as compared to the conventional intake port. For this reason, the intake port of the engine of the present invention is suitable for use with an air-intake system of so-called downdraft air-intake system in which an air-intake passage extending from an air cleaner box to the intake port is oriented substantially vertically. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a left side view of a motorcycle according to an embodiment of the present invention; 
       FIG. 2  is a left side view showing an engine of the motorcycle of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view showing a cylinder head and a cylinder head cover of the engine of  FIG. 2 , which is shown to be cut away; 
       FIG. 4  is a plan view showing a construction of a valve system, with the cylinder head cover removed from a cylinder of the engine of  FIG. 2 ; and 
       FIG. 5  is a perspective view showing a construction of a holder and rocker arms of the valve system of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a twincam engine according to the present invention and a motorcycle comprising the twincam engine will be described with reference to the drawings. In this embodiment, the direction “forward” means the direction in which the motorcycle is traveling, and other directions mean directions from the perspective of a rider mounting the motorcycle using forward as a reference, except for a case specifically illustrated. 
     FIG. 1  is a left side view of a motorcycle  1  according to an embodiment of the present invention. As shown in  FIG. 1 , the motorcycle  1  is of a road sport type and includes a front wheel  2  and a rear wheel  3 . The front wheel  2  is rotatably mounted to a lower end portion of a front fork  5  extending vertically. The front fork  5  is mounted on a steering shaft (not shown) by an upper bracket (not shown) attached to an upper end thereof, and an under bracket located below the upper bracket. The steering shaft is rotatably supported by a head pipe  6 . A bar-type steering handle  4  extending in a rightward and leftward direction is attached to the upper bracket. When the rider rotates the steering handle  4  clockwise or counterclockwise, the front wheel  2  is turned to a desired direction around the steering shaft. 
   A frame of the motorcycle  1  is of a twin tube type. A pair of right and left main frames  7  (only left main frame  7  is illustrated in  FIG. 1 ) extend rearward from the head pipe  6 . Pivot frames (swing arm brackets)  8  extend downward from rear regions of the main frames  7 . A swing arm  10  is pivotally mounted at a front end portion thereof to a pivot  9  attached on the pivot frame  8 . The rear wheel  3  is rotatably mounted to a rear end portion of the swing arm  10 . 
   A fuel tank  12  is disposed above the main frames  7  and behind the steering handle  4 . A straddle-type seat  13  is disposed behind the fuel tank  12 . A twincam engine (hereinafter simply referred to as an engine) E indicated by a broken line of  FIG. 1  is mounted between and under the right and left main frames  7 . The engine E is covered with a cowling  15  from the side to forward of the steering shaft (not shown). The engine E is an inline four-cylinder engine. The engine E is constructed in such a manner that a crankshaft  16  extends substantially in the rightward and leftward direction of the vehicle body. An output of the engine E is transmitted, through a chain  14 , to the rear wheel  3 , which thereby rotates. In this manner, the motorcycle  1  obtains a driving force. 
   An exhaust pipe  18  is coupled to an exhaust port  17  of the engine E to extend rearward from forward of the engine E through a region thereunder. A downstream end portion of a throttle device  20  is coupled to an air-intake port  19  of the engine E. An air cleaner box  21  is disposed between the right and left frames  7  and is coupled to an upstream end portion of the throttle device  20 . An air-intake duct  22  extends forward from the air cleaner box  21 . An upstream end of the air-intake duct  22  opens at a front portion of the cowling  15 . The engine E is configured to take in air from outside using running wind (ram pressure). An air-intake system of the engine E of this embodiment is of so-called downdraft type in which an air-intake passage extending from the air cleaner box  21  to the air-intake port  19  through the throttle device  20  is oriented substantially vertically. 
     FIG. 2  is a left side view showing the engine E of the motorcycle  1  shown in  FIG. 1 . The engine E includes an oil pan  30 , a crankcase  31 , a cylinder block  32 , a cylinder head  33 , and a cylinder head cover  34  arranged in this order from below. Each cylinder  35  is mainly composed of the cylinder block  32 , the cylinder head  33 , and the cylinder head cover  34  and is tilted forward with respect to a vertical direction of the vehicle body of the motorcycle  1 . A crankshaft  16 , a main shaft  37  and a counter shaft  38  forming a transmission  36 , gears (not shown), etc., are accommodated in the interior of the crankcase  31 . 
   The crankshaft  16  is mounted such that its center axis is oriented in the rightward and leftward direction as described above. The main shaft  37  is disposed behind the crankshaft  16  and extends substantially in parallel with the crankshaft  16 . The countershaft  38  is disposed behind and obliquely above the main shaft  37  and extends substantially in parallel with the main shaft  37 . A clutch (not shown) is mounted on one end of the main shaft  37 . With the clutch in an on-state, a rotational force of the crankshaft  16  is transmitted to the main shaft  37 . The speed of the rotational force of the main shaft  37  is reduced in a predetermined gear ratio and transmitted to the counter shaft  38 . The rotational force is output from the countershaft  38  to the chain  14  shown in  FIG. 1 . 
   A balancer shaft  25  is disposed behind and obliquely above the crankshaft  16  and is configured to be rotatable in association with the crankshaft  16 . A generator  26  for electric power generation is disposed behind and obliquely above the balancer shaft  25 . An extra space is formed above the crankcase  31 , below an intake port  19 , and behind the cylinder block  32 . A breather  27  is disposed in the space. 
     FIG. 3  is a cross-sectional view of the cylinder head  33  and the cylinder head cover  34  of the engine E of  FIG. 2 , which is cut away, showing a construction of a valve system and the exhaust port  17  and the intake port  19  as viewed from the left. As shown in  FIG. 3 , a combustion chamber  40  is formed at a lower region of the cylinder head  33  together with the cylinder block  32  so as to correspond to each cylinder. Two exhaust ports  17  (only a left exhaust port  17  is illustrated in  FIG. 3 ) extend from front portions of one combustion chamber  40  and two intake ports  19  (only a left intake port  19  is illustrated in  FIG. 3 ) extend from rear portions thereof. 
   To be specific, the exhaust port  17  extends substantially upward from an upstream opening  17 A that opens in the combustion chamber  40  and then is curved forward at a position to a downstream opening  17 B that opens forward and obliquely downward at a front wall portion of the cylinder head  33 . The intake port  19  extends substantially upward from a downstream opening  19 A that opens in the combustion chamber  40  and then is curved rearward at a position to an upstream opening  19 B that opens upward and slightly obliquely rearward at a rear wall portion of the cylinder head  33 . 
   Two exhaust valves  41  and two intake valves  42  are accommodated in the cylinder head  33  for each cylinder to open and close the two exhaust ports  17  and the two intake ports  19 , respectively. The engine E is of a four-valve type. A valve disc  41 A of the exhaust valve  41  is disposed near the upstream opening  17 A of the exhaust port  17 . The valve stem  41 B extends substantially upward from the valve disc  41 A. A valve disc  42 A of the intake valve  42  is disposed near the downstream opening  19 A of the intake port  19 . The valve stem  42 B extends substantially upward from the valve disc  42 A. As viewed from the side, the exhaust valve  41  and the intake valve  42  are disposed in such a manner that the valve stems  41 B and  42 B form a substantially V-shape. 
   An upper retainer  44  is in contact with an upper end portion of the valve stem  41 B of the exhaust valve  41  from above. A lower retainer  45  is loosely fitted at a position of the valve stem  41 B and supported on the cylinder head  33 . The upper retainer  44  and the lower retainer  45  are arranged vertically with a valve spring  46  interposed therebetween. The upper retainer  44  and an upper portion of the valve spring  46  are accommodated in a tubular valve lifter  47  (see  FIG. 4 ) having an opened lower end and a closed upper end. 
   In the same manner, an upper retainer  49  is in contact with an upper end portion of the valve stem  42 B of the intake valve  42  from above. A lower retainer  50  is loosely fitted at a position of the valve stem  42 B and supported on the cylinder head  33 . The upper retainer  49  and the lower retainer  50  are arranged vertically with a valve spring  51  interposed therebetween. The upper retainer  49  and an upper portion of the valve spring  51  are accommodated in a tubular valve lifter  52  (see  FIG. 4 ) having an opened lower end and a closed upper end. 
   The valve lifters  47  and  52  are fitted into cylindrical concave portions formed at an upper region of the cylinder head  33  from above. The valve lifters  47  and  52  are located to be vertically reciprocatable along respective center axes  41 C and  42 C of the valve stems  41 B and  42 B, and are biased upward by forces applied from the valve springs  46  and  51  through the upper retainers  44  and  49 , respectively. 
   A camshaft  55  configured to drive the exhaust valve  41  is disposed above the exhaust valve  41  and a camshaft  56  configured to drive the intake valve  42  is disposed above the intake valve  42 . The camshaft  55  and the camshaft  56  are disposed and supported between an upper portion of the cylinder head  33  and a lower portion of the cylinder head cover  34 . The camshaft  55  and the camshaft  56  are arranged forward and rearward such that their center axes extend in the rightward and leftward direction. The camshaft  55  and the camshaft  56  are coupled to the crankshaft  16  of the engine E of  FIG. 2  through a sprocket and a chain (or pulley and belt, gear train) which are not shown. The camshaft  55  and the camshaft  56  are configured to rotate in association with the rotation of the crankshaft  16  in a cycle twice as long as that of a cycle in which the crankshaft  16  rotates. 
   Cams  58  and  59  are attached to suitable locations of the camshaft  55  and the camshaft  56  so as to correspond to the exhaust valve  41  and the intake valve  42 , respectively. The cams  58  and  59  are rotatable integrally with the camshaft  55  and the camshaft  59 , respectively. The cams  58  and  59  apply forces through the rocker arms  61  and  62 , which are pivotally attached to a holder  60 , described later, to press the valve lifters  47  and  52 , respectively. 
     FIG. 4  is a plan view of the construction of the valve system, with the cylinder head cover  34  removed from a cylinder. As shown in  FIGS. 3 and 4 , a holder  60  is mounted for each cylinder at a substantially center position in a forward and rearward direction at an upper region of the cylinder head  33  to pivotally attach the rocker arms  61  and  62 . As shown in  FIG. 4 , the holder  60  is of a substantially rectangular shape in which front and rear regions and right and left regions are symmetrically cut away as viewed from above, like a butterfly with its wings spread, for example. The holder  60  of this embodiment is made of case-hardened steel that has been subjected to surface hardening treatment such as carburizing or nitriding to provide stiffness. 
   As shown in a perspective view of  FIG. 5 , the holder  60  has a base plate  60 A constructed such that a tubular member  63  that is vertically elongate penetrates a center region thereof. The tubular member  63  has a plug hole  64  into which an ignition plug  29  of the engine E is inserted. Mounting portions  66  are provided on right and left sides in front of the plug hole  64  on the base plate  60 A to mount the two rocker arms  61  on the exhaust valve side. Each mounting portion  66  is formed of right and left plate-shaped boss portions  67  protruding opposite to each other from the base plate  60 A. Holes  67 A are formed to penetrate in the rightward and leftward direction of the plate-shaped boss portions  67  to be coaxial with each other. 
   Mounting portions  68  are provided on right and left sides behind the plug hole  64  on the base plate  60 A to mount the two rocker arms  62  on the intake valve side. Each mounting portion  68  is formed of right and left plate-shaped boss portions  69  protruding opposite to each other from the base plate  60 A. Holes  69 A are formed to penetrate in the rightward and leftward direction of the plate-shaped boss portions  69  to be coaxial with each other. As shown in  FIG. 4 , a spacing  68 A between the right and left mounting portions  68  disposed at a rear region of the holder  60  (i.e., intake valve side) is larger than a spacing  66 A between the right and left mounting portions  66  disposed at a front region of the holder  60  (i.e., exhaust valve side) (see  FIG. 5 ). 
   A hole  70 A is formed on each rocker arm  61  on the exhaust valve side to penetrate in the rightward and leftward direction of a base end portion  70  thereof, and a lower region of a tip end portion  71  located forward relative to the base end portion  70  protrudes downward. The rocker arm  61  is pivotally attached to the holder  60  in such a manner that the base end portion  70  is placed between the right and left plate-shaped boss portions  67  of the front mounting portion  66  of the holder  60  and the pin  72  is inserted into the holes  67 A of the plate-shaped boss portions  67  and the hole  70 A of the base end portion  70  which are coaxially aligned. The rocker arm  61  on the exhaust valve side is pivotally attached to the holder  60  such that the tip end portion  71  is vertically pivotable around the hole  70 A formed on the base end portion  70 . 
   Likewise, a hole  75 A is formed on each rocker arm  62  on the intake valve side to penetrate in the rightward and leftward direction of a base end portion  75 , and a lower region of a tip end portion  76  located rearward of the base end portion  75  protrudes downward. The rocker arm  62  is pivotally attached to the holder  60  in such a manner that the base end portion  75  is placed between the right and left plate-shaped boss portions  69  of the rear mounting portion  68  of the holder  60  and the pin  77  is inserted into the holes  69 A of the plate-shaped boss portions  69  and the hole  75 A of the base end portion  75  which are coaxially aligned. The rocker arm  62  on the intake valve side is pivotally attached to the holder  60  such that the tip end portion  76  is vertically pivotable around the hole  75 A formed on the base end portion  75 . 
   Bolt fastening holes  80  are formed on outer sides in the rightward and leftward direction of the front two mounting portions  66  to vertically penetrate the base plate  60 A of the holder  60 . To be specific, the mounting portion  66  is interposed between the plug hole  64  formed at the center of the base plate  60 A and the bolt fastening hole  80 . Likewise, bolt fastening holes  81  are formed on outer sides in the rightward and leftward direction of the rear two mounting portions  68  of the holder  60  to vertically penetrate the base plate  60 A of the holder  60 . To be specific, the mounting portion  68  is interposed between the plug hole  64  and the bolt fastening hole  81 . 
   On the base plate  60 A, a rectangular cut portion  82  is formed between the front two mounting portions  66  and is located in front of the plug hole  64 , and a rectangular cut portion  83  is formed between the rear two mounting portions  68  and is located behind the plug hole  64 . Furthermore, on the base plate  60 A, a cut portion  84  is formed at each of right and left sides of the plug hole  64 , i.e., between the bolt fastening holes  80  and  81  formed forward and rearward at each of the left and right sides. The holder  70  is lightweight because the base plate  60 A has these cut portions  82  to  84 . 
   The above constructed holder  60  is mounted on base portions  91  that are formed at upper regions of the cylinder head  33  and have a convex shape as viewed from the side with the tubular member  63  forming the plug hole  64  fitted into a cylindrical concave portion  90  formed at a center region of the upper portion of the cylinder head  33 . The base portions  91  are formed forward and rearward with the concave portion  90  interposed between them, and extend in the rightward and leftward direction over the four cylinders  35  (see  FIG. 4 ) arranged in line, improving stiffness of the cylinder head  33 . The holder  60  is fastened to the cylinder head  33  in such a manner that bolts  92  are inserted into the bolt fastening bolts  80  and  81  (see  FIG. 5 ) formed at corner regions of the base plate  60 A from above and are threaded into threaded holes  93  formed on the base portions  91 . 
   As shown in  FIG. 3 , the base portions  91  are disposed between a center axis  41 C of the valve stem  41 B of the exhaust valve  41  located forward and a center axis  42 C of the valve stem  42 B of the intake valve  42  located rearward. The pins  72  and  77 , for pivotally attaching the rocker arms  61  and  62  to the holder  60  fastened to the base portion  91  by the bolts, are located between the center axes  41 C and  42 C. Because of the absence of any structural elements for pivotally attaching the rocker arms  61  and  62  outside (forward and rearward) of the center axes  41 C and  42 C, the exhaust port  17  and the intake port  19  can have larger curvature radiuses to easily extend upward from the combustion chamber  40 . Because the exhaust port  17  and the intake port  19  can have larger curvature radiuses, gas-exhaust efficiency and air-intake efficiency are improved in the engine E of this embodiment. 
   As shown in  FIGS. 3 and 4 , head portions  92 A of the bolts  92  are located in close proximity to the outer end portions of the pins  72  and  77  pivotally attaching the rocker arms  61  and  62 . In other words, as viewed from the side along the center axes of the pins  72  and  77  as shown in  FIG. 3 , the head portions  92 A of the bolts  92  and the pins  72  and  77  at least partially overlap with each other. With such a construction, the head portions  92 A of the bolts  92  inhibit the pins  72  and  77  from coming off outward. 
   As shown in  FIG. 4 , rectangular separating wall portions  95  and  96  extend for each cylinder from front and rear wall portions of the cylinder head  33  toward a center portion of the cylinder head  33 . The separating wall portions  95  and  96  serve to improve stiffness of the cylinder head  33  and are configured to conform in shape to the front and rear cut portions  82  and  83  of the base plate  60 A of the holder  60 , facilitating assembling of the holder  60  into the cylinder head  33 . Furthermore, since the separating wall portions  95  and  96  are opposite to inner end portions of the pins  72  and  77  with the holder  60  assembled into the cylinder head  33 , they serve to inhibit the pins  72  and  77  from coming off inward (toward the center in the rightward and leftward direction of each cylinder). 
   In accordance with the engine E, as described above, the exhaust port  17  and the intake port  19  are configured to have larger curvature radiuses to improve gas-exhaust efficiency and air-intake efficiency. Also, the motorcycle  1  of this embodiment employs a downdraft-type air-intake system, in which an air-intake passage extending from the air cleaner box  21  to the intake port  19  through the throttle device  20  is oriented substantially vertically. Therefore, the engine E, which allows the intake port  19  to have larger curvature radius, is especially suitable for the motorcycle  1  having such a construction. 
   The holder  60  is fastened to the cylinder head  33  by the ignition plug  29  inserted into the tubular member  63  and the bolts  92  inserted into the holes  80  and  81 , so as to interpose the mounting portions  66  and  68  for mounting the rocker arms  61  and  62  by the pins  72  and  77  between the ignition plug  29  and the bolts  92 , so that the holder  60  stably supports the pivots of the rocker arms  61  and  62  by using the bolts  92 . In addition, since each rocker arm  61  is pivotally attached to the cylinder head  33  by each separate pin  72  and each rocker arm  62  is pivotally attached to the cylinder head  33  by each separate pin  77 , the pivots can be laid out flexibly as compared to the case where adjacent right and left rocker arms  61  are pivotally attached by a common pin, for example. Thus, limitation on the lay out of the pivots of the rocker arms  61  and  62  because of the presence of the tubular member  63  forming the plug hole  64 , etc., can be reduced. 
   Whereas description has been made to explain that the rocker arm  61  on the exhaust valve side and the rocker arm  62  on the intake valve side are pivotally attached by the integral holder  60  between the center axis  41 C and the center axis  42 C located forward and rearward, they may be pivotally attached thereto by separate holders, or otherwise one of them may be pivotally attached between the center axes  41 C and  42 C. For example, in a case where only the rocker arm  62  on the intake valve side is pivotally attached to the cylinder head  33  between the center axes  41 C and  42 C, a portion on the intake valve side of the holder  60  of  FIG. 4  that is obtained by cutting the base plate  60 A along two-dotted line  98  may be used. 
   Whereas an engine E mounted in the motorcycle  1  of a road sport type has been described in this embodiment, the engine may be mounted to motorcycles of other types, or personal watercraft or four-wheeled all terrain vehicles. 
   Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the description is to be construed as illustrative only, and is provided for the purpose of teaching those killed in the art the best mode of carrying out the invention. The details of the structure and/or function maybe varied substantially without departing from the spirit of the invention and all modifications which come within the scope of the appended claims are reserved.