Abstract:
The present invention relates to an improvement in an exhaust system structure for a motorcycle. A motorcycle has an engine disposed between front and rear wheels, an exhaust pipe extending from the engine, and a silencer connected to the exhaust pipe for discharging exhaust gases. The silencer, which is a heavy object, is of the type having a gas outlet disposed near a gas inlet. The silencer is disposed below the engine with the gas inlet facing toward the rear wheel, and the exhaust pipe extends to a position near the rear wheel and is connected to the gas inlet of the silencer. By elongating the length of the exhaust pipe and placing the silencer under the engine, the exhaust system performs sufficiently with high output engines, while at the same time lowers the center of gravity of a motorcycle body and balances the weight of the motorcycle body in its longitudinal direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2001-105719, filed on Apr. 4, 2001, the entire contents thereof are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in an exhaust system structure for a motorcycle. 
     2. Description of Background Art 
     Exhaust system structures for motorcycles are arranged such that exhaust gases emitted from an engine disposed between front and rear wheels are discharged through an exhaust pipe and a silencer into the atmosphere. One such exhaust system structure for motorcycles is known from Japanese utility model publication No. 2-10071 entitled “Motorcycle with cowling” (hereinafter referred to as “prior art  1 ”). 
     As shown in FIG. 2 of the publication of the prior art  1 , the prior art  1  relates to an exhaust system structure in which an exhaust pipe  32  extends rearward from an engine  23  disposed between a front wheel  11  and a rear wheel  14 , and a muffler  34  is connected to the rear end of the exhaust pipe  32  (the reference numerals are those cited in the publication). Therefore, the muffler  34  is disposed laterally of an upper portion of the rear wheel  14 . 
     With the prior art  1 , since the muffler  34  which is a heavy object is disposed in an upper portion of the motorcycle body, the center of gravity of the motorcycle body tends to be elevated. Because the muffler  34  which is a heavy object is disposed in a rear portion of the motorcycle body, the weight of the motorcycle body needs to be balanced in its longitudinal direction in order to increase the maneuvering capability of the motorcycle. 
     To solve the above problems, an arrangement disclosed in Japanese laid-open patent publication No. 58-6912 entitled “Silencer device for motorcycle” (hereinafter referred to as “prior art  2 ”) may be employed. 
     As shown in FIG. 1 of the publication of the prior art  2 , the prior art  2  relates to an exhaust system structure in which an engine  11  is disposed between a front wheel (no reference numeral) and a rear wheel  13 , a muffler body  14  is disposed beneath the engine  11 , an exhaust pipe  12  extends downwardly from a front portion of the engine  11 , a gas inlet in the front portion of the muffler body  14  is connected to the exhaust pipe  12 , and exhaust gases are discharged from a gas outlet in the rear portion of the muffler body  14 . 
     According to the prior art  2 , since the muffler body  14  which is a heavy object is disposed in a low position near the engine  11 , the center of gravity of the motorcycle body is lowered. The layout is advantageous in balancing the weight of the motorcycle body in its longitudinal direction. 
     Motorcycles include medium and large-size motorcycles which can be driven at high speeds. For motorcycles to be able to be driven at high speeds, high-output engines may be installed thereon. For performing the capabilities of such a high-output engine, it is necessary for the exhaust pipe  12  to have at least a certain length. However, because the muffler body  14  is disposed near the engine  11  according to the prior art  2 , it is difficult to increase the length of the exhaust pipe  12  simply by elongating the exhaust pipe  12 . 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide a technique for 
     (1) maintaining the length of an exhaust pipe in order to sufficiently perform the capabilities of an engine, and 
     (2) placing a silencer which is a heavy object in a position advantageous to lower the center of gravity of a motorcycle body and balance the weight of the motorcycle body in its longitudinal direction. 
     To achieve the above object, there is provided in accordance with claim  1  an exhaust system structure in a motorcycle having an engine disposed between front and rear wheels, an exhaust pipe extending from the engine, and a silencer connected to the exhaust pipe for discharging exhaust gases, characterized in that said silencer is of the type having a gas outlet disposed near a gas inlet, the silencer is disposed below the engine with the gas inlet facing toward the rear wheel, and the exhaust pipe extends to a position near the rear wheel and is connected to the gas inlet of said silencer. 
     Since the silencer whose gas inlet faces toward the rear wheel is disposed below the engine and the exhaust pipe extends to a position near the rear wheel and is connected to the gas inlet of the silencer, the exhaust pipe extending from the engine to the gas inlet is elongated. Since a long exhaust pipe is used, the engine capability is sufficiently performed even if the engine is a high-output engine. 
     Because the silencer which is a heavy object is disposed below the engine that is disposed between the front and rear wheels, the center of gravity of the motorcycle body is lowered to reduce the inertia of rolling. Further, this layout is advantageous in keeping the weight of the motorcycle body in balance in its longitudinal direction. 
     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 right side elevational view of a motorcycle according to the present invention; 
     FIG. 2 is a perspective view of a major part of an intake/exhaust system structure of the motorcycle according to the present invention; 
     FIG. 3 is a sectional plan view of a silencer according to the present invention; 
     FIG. 4 is a front elevational view of a portion around air inlet ports of the motorcycle according to the present invention; and 
     FIG. 5 is a perspective view of a portion around the air inlet ports of the motorcycle according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the accompanying drawings. The terms “front”, “rear”, “left”, “right”, “upper”, and “lower” used in the description refer to directions as viewed from the driver of the motorcycle. The figures should be viewed in the direction in which the reference characters look in the correct directional orientation. 
     FIG. 1 is a right side elevational view of a motorcycle according to the present invention, showing the appearance of a motorcycle  10 . The motorcycle  10  has a front wheel  32  mounted on a front portion of a motorcycle frame  20  by a front fork  31 , a rear wheel  34  mounted on a rear portion of the motorcycle frame  20  by a swing arm  33 , an engine  40  mounted on a longitudinally central lower portion of the motorcycle frame  20 , and an air cleaner  53 , a fuel tank  55 , and a seat  56  which are mounted on a longitudinally central upper portion of the motorcycle frame  20 . 
     According to the present invention, the engine  40  is disposed between the front and rear wheels  32 ,  34  and a silencer  70  is disposed below the engine  40 . The silencer  70  is mounted on the motorcycle frame  20  or the engine  40 . 
     The motorcycle  10  has a cowling  100  comprising a front cowl  101  covering an upper front portion of the motorcycle body, left and right middle cowls  111  (only the right middle cowl is shown in FIG. 1) covering a front middle portion of the motorcycle body, left and right lower cowls  121  covering a lower portion of the motorcycle body and sides of the silencer  70 , and a rear cowl  131  covering a rear portion of the motorcycle body. 
     The front cowl  101 , the left and right middle cowls  111 , and the left and right lower cowls  121  are separably coupled to each other by screws. Specifically, upper front portions of the left and right middle cowls  111  (forward cowls) are separable from a lower portion of the front cowl  101 , and front end portions of the lower cowls (rearward cowls)  121  are separable from lower rear end portions of the middle cowls  111  in a position forward of the silencer  70 . 
     The front cowl  101  has a window screen  102  on its upper portion and a pair of left and right air inlet ports  103  in its front portion. 
     The middle cowls (forward cowls)  111  each have a recess  112  defined in a lower rear end thereof and extending toward the center of the motorcycle body, and the lower cowls (rearward cowls)  121  each have a front opening  122  defined in a front end thereof. The front opening  122  and the recess  112  are combined into a large air inlet port  123  for cooling the silencer  70 . 
     FIG. 1 also shows a rear fender  140  disposed below the rear cowl  131  and mounted on the swing arm  33 . According to the present invention, the rear cowl  131  has a lower edge  132  inclined substantially linearly in a rearward upward direction, and the rear fender  140  has an upper surface  141  inclined substantially linearly in a rearward upward direction parallel to the lower edge  132  of the rear cowl  131 . 
     In FIG. 1,  47  represents the crankshaft of the engine,  48  a cover member (crankshaft cover),  91  a handle,  92  a mirror,  93  a head lamp,  94  a winker,  95  a front fender,  96  a rear cushion unit, and  97  a cushion link. The silencer  70  has a rear portion extending to a position near the cushion link  97 . 
     FIG. 2 is a major perspective view of an intake/exhaust system structure of the motorcycle according to the present invention. The motorcycle  10  has an intake system  51  comprising a pair of left and right air intake ports  103  defined in the front cowl  101  shown in FIG. 1, a pair of left and right air ducts (intake ducts)  52  extending rearwardly from the intake ports  103 , and an air cleaner  53  connected to tip ends of the air ducts  52 . The air cleaner  53  is disposed between a pair of left and right main pipes  21  of the motorcycle frame  20 .  22  represents a head pipe. 
     The engine  40  comprises a four-cylinder engine having a transverse row of four exhaust ports  41  through  44  in its upper front portion and an oil pan  45  in its lower rear portion. The motorcycle  10  has an exhaust system  58  which is a device for discharging exhaust gases through an exhaust pipe  60  extending from the exhaust ports  41  through  44  of the engine  40  and the silencer  70  which is connected to the exhaust pipe  60 . 
     The silencer  70  is of the type having a gas outlet  74  in the vicinity of a gas inlet  73 . The silencer  70  is disposed below the engine  40  with the gas inlet  73  facing toward the rear wheel  34 , and the exhaust pipe  60  extends to a position near the rear wheel  34  and is connected to the gas inlet  73  of the silencer  70 . 
     The four exhaust ports are referred to as a first exhaust port  41 , a second exhaust port  42 , a third exhaust port  43 , and a fourth exhaust port  44  successively from the left (right in FIG. 2) to the right of the motorcycle body. 
     The exhaust pipe  60  comprises four pipes (a first pipe  61 , a second pipe  62 , a third pipe  63 , and a fourth pipe  64 ) and three joint pipes (a first joint pipe  65 , a second joint pipe  66 , and a third joint pipe  67 ). 
     The first pipe  61  has an end connected to the first exhaust port  41 , extends downwardly to the right, and extends rearwardly along a right side of the oil pan  45 . Similarly, the second pipe  62  has an end connected to the second exhaust port  42 , extends downwardly to the right, and extends rearwardly along the right side of the oil pan  45 . The other end of the first pipe  61  and the other end of the second pipe  62  are connected together to an end of the first joint pipe  65 . The first joint pipe  65  extends rearwardly along the right side of the oil pan  45  and then extends to the left. 
     The third pipe  63  has an end connected to the third exhaust port  43 , extends downwardly to the left, and extends rearwardly along a left side of the oil pan  45 . Similarly, the fourth pipe  64  has an end connected to the fourth exhaust port  44 , extends downwardly to the left, and extends rearwardly along the left side of the oil pan  45 . The other end of the third pipe  63  and the other end of the fourth pipe  64  are connected together to an end of the second joint pipe  66 . The second joint pipe  66  extends rearwardly along the left side of the oil pan  45 . 
     The other end of the first joint pipe  65  and the other end of the second joint pipe  66  are connected together to the third joint pipe  67 . In this manner, the four pipes  61  through  64  are connected together to the single third joint pipe  67 . The third joint pipe  67  extends rearwardly and is connected to the gas inlet  73 . 
     FIG. 3 is a sectional plan view of the silencer according to the present invention. The silencer  70  comprises a closed tubular body  71  which is slender and elongate in its longitudinal direction, the gas inlet  73  and the gas outlet  74  which are defined in a rear end  72  of the tubular body  71 , and first and second division plates  78 ,  79  dividing the interior of the tubular body  71  into three expansion chambers  75  through  77  in its longitudinal direction. 
     The first expansion chamber  75 , the second expansion chamber  76 , and the third expansion chamber  77  are disposed in the tubular body  71  successively in the order named forward (rightward in FIG. 3) from the rear end  72 . The first expansion chamber  75  communicates with the gas inlet  73 , and the third expansion chamber  77  communicates with the first expansion chamber  75  through a first communication pipe  81 . The second expansion chamber  76  communicates with the third expansion chamber  77  through a second communication pipe  82 , and the gas outlet  74  communicates with the second expansion chamber  76  through a third communication pipe  83 . A tail pipe  84  is connected to the gas outlet  74 . 
     Exhaust sounds can be attenuated by the expansion of exhaust gases as they enter the first, second, and third expansion chambers  75  through  77 . 
     Operation of the exhaust system  58  thus constructed will be described below with reference to FIG.  2 . 
     Since the silencer  70  is disposed below the engine  40  with the gas inlet  73  facing toward the rear wheel  34 , and the exhaust pipe  60  extends to the position near the rear wheel  34  and is connected to the gas inlet  73  of the silencer  70 , the exhaust pipe  60  extending from the engine  40  to the gas inlet  73  is elongated. Since the exhaust pipe  60  is long, the engine capability is sufficiently performed even if the engine  40  is a high-output engine. 
     Because the silencer  70  which is a heavy object is disposed below the engine  40  that is disposed between the front and rear wheels  32 ,  34 , the center of gravity of the motorcycle body is lowered to reduce the inertia of rolling. This layout is also advantageous for keeping the weight of the motorcycle body in balance in its longitudinal direction. 
     FIG. 4 is a front elevational view of a portion around the air inlet ports of the motorcycle according to the present invention. The left and right air inlet ports  103  are disposed in respective left and right positions outside of the front fork  31 , and the front cowl  101  has left and right recesses  104  defined outside of and adjacent to the left and right air inlet ports  103 . The air inlet ports  103  have respective screens  105  for preventing foreign matter from entering the air inlet ports  103 . 
     Because the air inlet ports  103  are disposed in the respective left and right positions outside of the front fork  31 , efforts to increase the opening area of the air inlet ports  103  do not tend to be limited by the layout of other members around the air inlet ports  103 . Therefore, the opening area of the air inlet ports  103  can be increased to supply a sufficient amount of air (ramming air) to the engine  40  (see FIG.  2 ). As a result, the capability of the engine  40  can sufficiently be performed. 
     Since the air inlet ports  103  are disposed respectively on left and right sides of the front wheel  32  which is located centrally in the transverse direction of the motorcycle body, the air inlet ports  103  do not interfere with the front wheel  32  even when the front wheel  32  is lifted the most. Therefore, there is no need to increase the height of the air inlet ports  103 , which do not impose limitations on the design of the motorcycle  10 . 
     FIG. 5 is a perspective view of a portion around the air inlet ports of the motorcycle according to the present invention. FIG. 5 shows that the recesses  104  defined in the front cowl  101  are “left dogleg-shaped” in side elevation, and air Wi which does not enter the air inlet ports  103  flows along the recesses  104 . 
     Each of the recesses  104  has a lower slanted surface  106  and an upper slanted surface  107 . The lower and upper slanted surfaces  106 ,  107  have side corners defined by relatively large curved surfaces. 
     Since the recesses  104  which are “left dogleg-shaped” in side elevation are defined in the front cowl  101  and disposed outside of and adjacent to the air inlet ports  103 , and air (ramming air) Wi which does not enter the air inlet ports  103  flows along the recesses  104 , the flow of air Wi which does not enter the air inlet ports  103  is rectified and flows rearwardly. The flow of air Wi thus rectified is advantageous in increasing the running capability of the motorcycle  10 . 
     The lower slanted surface  107  is inclined upwardly in the rearward direction from the front of the motorcycle. When the air Wi flows along the lower slanted surface  106  thus inclined, the lower slanted surface  106  serves as an air spoiler for generating a downward force to press the motorcycle  10  downwardly. 
     The air (ramming air) Wi flows upwardly in the rearward direction along the lower slanted surface  106  and passes rearwardly across the side corner. When the air Wi thus passes along the lower slanted surface  106 , it produces a downward force Fd to press the motorcycle  10  downwardly. As a result, a lifting force Fu which is generated when the motorcycle  10  is running is reduced by the downward force Fd. Accordingly, the gripping force of the tire of the front wheel  32  (see FIG. 1) is maintained to achieve a comfortable high-speed running capability. 
     In summary, the silencer of the present invention is of the type having a gas outlet disposed near a gas inlet, and is disposed below the engine with a gas inlet facing toward the rear wheel, and the exhaust pipe extends to a position near the rear wheel and is connected to the gas inlet of the silencer, and therefore the exhaust pipe extending from the engine to the gas inlet is elongated. Further, silencer which is a heavy object is disposed below the engine that is disposed between the front and rear wheels. 
     The above configuration offers the following advantages. First, since the exhaust pipe is sufficiently long, the engine capability is sufficiently performed even if the engine is a high-output engine. Second, since the silencer is disposed between the front and rear wheels, the center of gravity of the motorcycle body is lowered to reduce the inertia of rolling. Also, the layout is advantageous for maintaining the weight of the motorcycle body in balance in its longitudinal direction. 
     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.