Patent Publication Number: US-8109084-B2

Title: Exhaust device for motorcycle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2007-095486 filed on Mar. 30, 2007 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust device for a motorcycle. More specifically, to an exhaust throttle valve arrangement structure advantageous to disturbance due to scattered stones or the like. 
     2. Description of Background Art 
     An exhaust throttle valve is known that is provided in the exhaust system of an engine to adjust an amount of exhaust flow according to operating conditions, thereby intending to reduce exhaust noise and to provide high power. 
     Examples of the provision of such an exhaust throttle valve include one in which the exhaust throttle valve is provided at a collecting portion disposed forward of an engine is disclosed in Japanese Utility Model Publication No. Hei 4-20983. In addition, another example of an exhaust throttle valve is provided at a collective exhaust pipe located near the front end of a muffler See, for example, Japanese Patent Laid-open No. Hei 4-292534. 
     Since the exhaust throttle valve is provided in the exhaust system, it is sometimes disposed close to a front wheel or a rear wheel or on the vehicle body lower portion close to the ground surface. If the exhaust throttle valve is located at any one of such positions, it tends to undergo a disturbance due to the scattered of stones or the like. 
     For Japanese Utility Model Publication No. Hei 4-20983, a rotational arm or drive wire of the exhaust throttle valve is located on the lateral surface of a collective portion forward of the lower portion of an engine, that is, located rearward of and close to the front wheel. Thus, stones or the like scattered by the front wheel tends to cause a disturbance. 
     For Japanese Patent Laid-open No. Hei 4-292534, the drive portion or the like of the exhaust throttle valve is exposed to the vicinity of the front end of the muffler located laterally of the rear wheel. Thus, it is desirable to provide a structure that protects it from being disturbed due to stones scattered by the front wheel or the rear wheel. 
     In addition, it is desired that the structure for protecting such a component from disturbances due to scattered stones or the like be realized as simply as possible. Accordingly, an object of the invention is to meet such a desire. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     To solve the above problem, according to an embodiment of the present invention an exhaust device for a motorcycle is provided that includes a plurality of exhaust pipes connected to associated exhaust ports of a multicylinder engine. A collecting portion is provided for collecting the exhaust pipes at a position close to the lower portion of the engine. A rear exhaust pipe is connected to the downstream side of the collecting portion with an exhaust chamber connected to the downstream side of the rear exhaust pipe. An exhaust outlet portion is provided on the downstream side of the exhaust chamber with a muffler connected to the downstream side of the exhaust outlet portion. An exhaust throttle valve is disposed inside a passage of the rear exhaust pipe to change an amount of exhaust flow. 
     According to an embodiment of the present invention, a lateral width of the collecting portion is greater than a lateral width of the rear exhaust pipe including the exhaust throttle valve, as viewed from above. 
     According to an embodiment of the present invention, the lateral width of the exhaust chamber is greater than that of the rear exhaust pipe including the exhaust throttle valve, as viewed from above. 
     According to an embodiment of the present invention, the muffler is formed such that a vehicle-widthwise outside thereof is slanted to externally open toward the rear. 
     According to an embodiment of the present invention, a back-and-forth length of a portion, of the exhaust throttle valve, disposed outside of the rear exhaust pipe is approximately equal to a back-and-forth length of the rear exhaust pipe. 
     According to an embodiment of the present invention, the exhaust throttle valve is disposed to recede from an external lateral portion of the collecting portion toward the inside of a vehicle body. 
     According to an embodiment of the present invention, a side stand during storage or an attachment portion overlaps the outside of the exhaust throttle valve as viewed from the side. 
     According to an embodiment of the present invention, the exhaust throttle valve is attached to the rear exhaust pipe at a position between the collecting portion disposed near the lower portion of the engine and the exhaust chamber disposed downstream of the collecting portion and connected thereto via the rear exhaust pipe. Therefore, the exhaust throttle valve is surrounded by the collecting portion and the exhaust chamber so that it can be protected from disturbance due to stones or the like scattered by the front wheel or the like. 
     According to an embodiment of the present invention, the lateral width of the collecting portion disposed forward of the exhaust throttle valve is made greater than the lateral width of the rear exhaust pipe including the exhaust throttle valve, whereby the external side of the collecting portion is made to protrude externally from the rear exhaust pipe. Therefore, the collecting portion can protect the exhaust throttle valve against disturbance due to scattered stones or the like. In particular, the exhaust throttle valve can be protected from disturbance due to stones scattered from the front of the vehicle during operation of the vehicle. 
     According to an embodiment of the present invention, the lateral width of the exhaust chamber disposed rearward of the exhaust throttle valve is made greater than that of the rear exhaust pipe including the exhaust throttle valve. Therefore, the exhaust throttle valve can be prevented from being hit by disturbance due to scattered stones bounced from the rear portion of the vehicle. 
     According to an embodiment of the present invention, the muffler is formed such that the vehicle-widthwise outside thereof is slanted to externally open toward the rear. If a disturbance due to stones scattered from the front of the vehicle hits the muffler, they can bounce obliquely externally and rearwardly from the externally opening slant surface. Thus, the exhaust throttle valve can be prevented from being hit by disturbances due to stones scattered from the front of the vehicle and bounced from the muffler. 
     According to an embodiment of the present invention, the collecting portion and exhaust chamber which are components larger than the exhaust throttle valve are disposed forward of and rearward of, respectively, the exhaust throttle valve with respect to the vehicle. In addition, the back-and-forth length of the rear exhaust pipe which corresponds to the interval between the collecting portion and the exhaust chamber is made approximately equal to the back-and-forth length of the exhaust throttle valve. Therefore, the back-and-forth interval of the exhaust throttle valve can be reduced by the collecting portion and by the exhaust chamber. This can prevent the exhaust throttle valve from being hit by disturbances due to scattered stones or the like. 
     According to an embodiment of the present invention, the exhaust throttle valve is disposed to recede from the external lateral portion of the collecting portion toward the inside of the vehicle body. Therefore, the exhaust throttle valve can easily be protected from disturbances due to scattered stones or the like. 
     According to an embodiment of the present invention, the side stand during storage or the attachment portion is made to overlap the outside of the exhaust throttle valve. Therefore, the side stand during storage or the attachment portion covers the exhaust throttle valve from the externally lateral side thereof to protect it from disturbances due to scattered stones or the like. 
     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 lateral view of a motorcycle according to an embodiment; 
         FIG. 2  is a plan view mainly illustrating an exhaust system; 
         FIG. 3  illustrates an engine from rear; 
         FIG. 4  is a lateral view of the exhaust system; 
         FIG. 5  is a plan view of the exhaust system; 
         FIG. 6  is an enlarged lateral view illustrating a portion mainly including a muffler; 
         FIG. 7  is an enlarged plan view of  FIG. 6 ; and 
         FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment will hereinafter be described with reference to the drawings.  FIG. 1  is a lateral view of a motorcycle according to the embodiment. The motorcycle is such that a V-type water-cooled 4-cylinder engine  3  is disposed between a front wheel  1  and a rear wheel  2  and supported by a body frame  4 . 
     A hanger  5  is attached to the intermediate portion of the body frame  4  so as to support the front portion of the engine  3 . A head pipe  6  is attached to the front end of the body frame  4  and a pivot plate  7  is supported by the rear portion of the body frame  4 . A fuel tank  8  is supported on the intermediate portion of the body frame  4  and an air intake box  9  is received inside the front portion of the fuel tank  8 . 
     The air intake box  9  sucks air from the front and supplies the air to front cylinders  10  and rear cylinders  11  located below the air intake box  9 . The air intake box  9  is disposed above the valley portion between the front cylinders  10  and the rear cylinders  11 . 
     Front side exhaust pipes  12  extend forward from the respective front cylinders  10 . Rear side exhaust pipes  13  extend rearwardly from the respective rear cylinders  11 . The front side exhaust pipes  12  extend downwardly along the front surface of a crankcase  14  of the engine  3  and connect, from the front, with a catalyst chamber  15  located below the crankcase  14 , via a collecting pipe. Also the rear side exhaust pipes  13  connect, from the front, with the catalyst chamber  15  via another collecting pipe as described later. 
     The catalyst chamber  15  connects with an exhaust chamber  16  and an expansion chamber disposed rearward thereof. The exhaust chamber  16  is disposed in a space defined between the lower portion of the crankcase  14  and the rear wheel  2  and connects with a muffler  17  extending on the right side of the rear wheel  2  with respect to the vehicle body. The exhaust chamber  16  is supported by the lower end of the pivot plate  7  at two, front and rear, attachment portions  18 ,  19 . 
     The muffler  17  is disposed to cross a rear swing arm  20  as viewed from the side. The rear swing arm  20  has a front end portion which is supported by the lower portion of the muffler  17  with the pivot shaft  21  so as to be swingable up and down and is suspended by a rear shock absorber  22  provided between the upper end of the pivot plate  7  and the rear swing arm  20 . 
     A suspension link  23  is provided between the lower end of the pivot plate  7  and the intermediate portion of the attachment portion  18 . The exhaust chamber  16  is disposed below the suspension link  23  so as to avoid the suspension link  23 . The front end of the muffler  17  is located at a position approximately equal to that of the suspension link  23  higher, by one step, than the exhaust chamber  16 . 
     The rear wheel  2  is of a shaft drive type and is driven by the engine  3 . A seat  24  is supported on a seat rail  25  so as to be located above the rear swing arm  20  and to the rear of the fuel tank  8 . The seat rail  25  is supported by the pivot plate  7  at its front end. 
     A radiator  26  is disposed forward of the front cylinders  10  and to the rear of the front wheel  1  and has an upper portion suspended and supported by the front portion, of the body frame  4 , close to the head pipe  6 . The radiator  26  is supported with the head bent forward in which the upper portion is slanted forward. A gap is defined between the radiator  26 , and the front cylinders  10  and the crankcase  14 . The front side exhaust pipes  12  are vertically passed through the gap. 
     The lower end of the radiator  26  is supported by the crankcase  14  via a stay  27  extending forward from the front lower portion of the crankcase  14 . A feed-water hose  28  extends to the rear from a lateral tank of the radiator  26  and connects with a feed-water pump  29  provided at the lateral portion of the crankcase  14 . 
     Water is fed from the feed-water pump  29  via a joint hose  30  to a water jacket feed-water portion provided in the valley between the front cylinders  10  and the rear cylinders  11 . The water is then fed from the water jacket feed-water portion to the respective water jackets of the front cylinders  10  and the rear cylinders  11 . The water circulates the water jackets to cool the cylinders for heat exchange and the water thus heated is fed to a return hose  32  via a thermostat  31  located above the feed-water portion in the valley between the front cylinders  10  and the rear cylinders  11 . 
     The return hose  32  extends downward to the lateral surface of the crankcase  14 , then extending forward, and connects with the lateral tank of the radiator  26 . In this way, the heated water is returned by the return hose  32  to the radiator  26 . 
       FIG. 2  is a plan view mainly illustrating an exhaust system. The catalyst chamber  15  is disposed to be offset to the left with respect to the body center C and the exhaust chamber  16  is located approximately on the body center C. A laterally-facing outlet pipe  40  is provided at a right-hand rear end portion of the exhaust chamber  16  so as to project to the right outwardly and joined to the left-lateral surface of the front end of the muffler  17 . The muffler  17  is disposed to extend in the back and forth direction and slanted so that its rear side opens outwardly toward the right side of the vehicle body. 
     The two front side exhaust pipes  12  are provided in total for the respective left and right cylinders. The front side exhaust pipes  12  extend downward and toward the right side of the vehicle body, bending at their lower portions and crossing the vehicle-widthwise direction from the right side of the vehicle body to the left side, collectively join to the collecting pipe  41  and connect with the catalyst chamber  15 . Also the two rear side exhaust pipes  13  are provided in total for the respective left and right cylinders, extending downward, and collectively join to the single rear collecting pipe  42 . The rear collecting pipe  42  extends downward of and to the right of the crankcase  14  not to interfere therewith, extends from the rear to the front, turning leftward of the vehicle body at a position close to the lower portions of the front side exhaust pipes  12 , and connects, from the front, with the catalyst chamber  15  inward of the front collecting pipe  41 . The crankcase  14  partially overlaps the catalyst chamber  15  and rear collecting pipe  42  as viewed from above. 
     A pillion step holder  33  is provided right and left of the vehicle body. The right pillion step holder  33  supports the muffler  17  at its lower end portion. A pillion step  34  is provided together with a step holder  35  supported by the seat rail  25 . A step  36 , a brake pedal  37 , a change pedal  38  and a side stand  39  are provided. These components are supported by the lower portion of the pivot plate  7 . 
       FIG. 3  illustrates the engine  3  as viewed from the rear. An oil pan  43  is attached to the bottom portion of the crankcase  14 . The oil pan  43  is formed to have a curved surface such that its bottom portion  44  is lower toward the vehicle-widthwise center. The central portion of the bottom portion  44  protrudes lengthwise downwardly to form a reservoir chamber  45 . A portion, of the bottom portion  44 , excluding the reservoir chamber  45  functions as an introduction portion for guiding oil into the reservoir chamber  45  and does not function so much to store oil thereon, so that it can be thinned. Thus, the oil pan  43  is formed thin as a whole and in an almost-T shape as viewed from the rear (or as viewed from the front). 
     The reservoir chamber  45  is formed like a recessed groove that is narrow, relatively deep and elongated in the back and forth direction (see  FIG. 5 ). An arrangement space for the rear collecting pipe  42  and the catalyst chamber  15  is sufficiently widely and is ensured to be right and left of the reservoir chamber  45 . The upper portion of the reservoir chamber  45  merges with the bottom portion  44  protruding right and left so that oil can efficiently be collected into the reservoir chamber  45 . A strainer  46  is received in the oil pan  43  and its suction port is disposed close to the bottom portion of reservoir chamber  45 . An oil pump not shown in the crankcase  14  sucks upwardly the oil collected in the deep reservoir chamber  45  and feeds it to the journal portions of the cylinders and other portions needing lubrication. Since the suction port of the strainer  46  is disposed near the bottom portion of the narrow, relatively deep reservoir chamber  45 , a necessary amount of oil can be ensured around the suction port so that the oil can stably be sucked even when the vehicle body is tilted. The rear collecting pipe  42  and the catalyst chamber  15  are disposed on the right and left, respectively, of the reservoir chamber  45  so as to be close thereto lengthwise in the back and forth direction. 
     The reservoir chamber  45  is provided at a position slightly offset from the body center C to the right so that a relatively large space is formed below the bottom portion  44  on the left side of the reservoir chamber  45 . The catalyst chamber  15  is disposed in the space. A front collecting pipe  41  and a rear collecting pipe  42  are joined to the front surface of the catalyst chamber  15  and a rear exhaust pipe  47  is joined to the rear surface of the catalyst chamber  15 . The catalyst chamber  15  is a relatively-flat cylindrical body formed in a horizontally-long circle in cross-section and internally receives a known exhaust purification catalyst with a honeycomb structure or the like. 
     A guard plate  48  is disposed on the under side of the catalyst chamber  15 . The guard plate  48  includes left and right bent lateral portions  48   a ,  48   b , which extend to surround the catalyst chamber  15  from the left and from the right. The right lateral portion  48   b  is formed like a curved surface along the catalyst chamber  15  to enter into between the catalyst chamber  15  and the reservoir chamber  15 . The guard plate  48  is disposed to slant along the left bank line  48   c.    
     A space slightly smaller than the left-hand space is provided also on the right side of the reservoir chamber  45  and below the bottom portion  44 . The single rear collecting pipe  42  is passed through this space in the back and forth direction. Also a guide plate  49  is provided to the right and obliquely downward of the rear collecting pipe  42 . The guide plate  49  is also disposed to slant along a right-hand bank line  49   a . The left and right bank lines  48   c ,  49   a  are lines each indicating the bank angle of the vehicle body. 
     The exhaust system is hereinafter described in further detail.  FIG. 4  is a lateral view of the exhaust system. Cylinder heads  50  of the front cylinders  10  are provided to the right and to the left. The two front side exhaust pipes  12  connected to the respective exhaust ports of the cylinder heads  50  extend downwardly in front of the crankcase  14 , bending at their lower ends, and connect with the approximately Y-shaped front pipe  41 . The rear side of the front collecting pipe  41  is formed as a single one, which connects with the catalyst chamber  15 . An O 2  sensor  51  is provided right before the joint portion with the catalyst chamber  15  so as to project upwardly from the upper surface thereof. 
     In addition, another O 2  sensor  51  is provided, so as to project upwardly, at a position close to the joint portion between the catalyst chamber  15  and the rear collecting pipe  42  which cannot be seen in the figure because of being superposed by the front collecting pipe  41 . The O 2  sensor  51  measures oxygen concentration in the exhaust gas before purification. 
     Cylinder heads  52  of the rear cylinders  11  are provided right and left. The two rear side exhaust pipes  13  connected to the respective exhaust ports of the cylinder heads  52  extend obliquely downwardly and rearwardly, bending at a position above the exhaust chamber  16 , extending approximately vertically downwardly, and are connect with branch portions  42   a ,  42   b  of the rear pipe  42  which is also approximately Y-shaped. 
     A drive portion of an exhaust throttle valve  53  is provided, so as to be exposed, at the lateral surface of the rear exhaust pipe  47  which connects the rear portion of the catalyst chamber  15  with the front portion of the exhaust chamber  16 . The exhaust throttle valve  53  is designed to control an amount of exhaust gas by changing the passage-sectional area of the rear exhaust pipe  47  according to the traveling conditions. 
     The respective bottom surfaces of the catalyst chamber  15  and the exhaust chamber  16  are located on respective horizontal lines H having approximately the same height to ensure a sufficient minimum ground clearance. However, the vertical width (thickness) of the exhaust chamber  16  is greater than that of the catalyst chamber  15 . The exhaust chamber  16  ensures the necessary vertical width by extending the upper surface upward. This is enabled by disposing the exhaust chamber  16  in the space put between the lower portion of the engine  3  and the rear wheel from front and from rear and by using the space formed below the rear swing arm  20  ( FIG. 1 ) located above the exhaust chamber  16 . 
     The exhaust chamber  16  is formed with a raised portion  54  at the intermediate portion of the upper surface thereof and recessed portions  55 ,  56  are respectively provided forward of and rearward of the raised portion  54 . The recessed portion  55  is adapted to avoid the lower end portion of the pivot plate  7  and similarly the recessed portion  56  is adapted to avoid the suspension link  23 . 
     The attachment portion  18  is provided at the end of a stay  18   a  and projects upwardly from the front end of the exhaust chamber  16 . Similarly, the attachment portion  19  is provided at the end of a stay  19   a  and projects upwardly from the raised portion  54  of the exhaust chamber  16 . Thus, the exhaust chamber  16  is rubber-mounted to the pivot plate  7 . The outlet pipe  40  extends rearwardly from the rear end of the exhaust chamber  16  while being reduced in diameter. To form the outlet pipe  40 , a recessed portion  57  is formed by reducing the diameter of the rear end portion of the exhaust chamber  16 . 
     The muffler  17  includes a front portion  60  formed in an approximate triangle and a main body portion  61 , which are lined and welded integrally with each other. The outlet pipe  40  is joined to the lateral surface of the front portion  60  close to an acute front end portion  60   a  thereof. An upper surface  60   b  of the front portion  60  is formed as a taper surface which extends forwardly and obliquely downwardly. The front end portion  60   a  of the front portion  60  partially overlaps the rear end portion of the exhaust chamber  16  at a portion of the outlet pipe  40 . 
     The main body portion  61  is formed like a tube having an approximately uniform diameter. A rear end portion of the main body portion  61  is closed by an end cap  62  whose upper portion projects rearward from the other portion. A stay  58  is attached to the pillion step holder  33  ( FIG. 1 ). 
       FIG. 5  is a plan view of the exhaust system. The front side exhaust pipes  12  have respective lower portions which bend vehicle-widthwise at a position forward of the rear collecting pipe  42  and overlap one on another. The rear side exhaust pipes  13  overlap one on another along the right-hand surface of the exhaust chamber  16 , extending rearwardly to the vicinity of the outlet pipe  40 , then bending downwardly, and are connect with the branch portions  42   a ,  42   b  of the rear collecting pipe  42 . 
     The outlet pipe  40  projects to the right from the internal surface of the rear end portion of the exhaust chamber  16  and is connected to a front pipe  63  with a band  64 . The front pipe  63  projects to the left from the internal surface of the front end portion of the front portion  60 . In addition, the front pipe  63  is an inlet pipe of the muffler  17 . 
     The joint portion between the outlet pipe  40  and the front pipe  63  is located slightly rearwardly from the front end portion  60   a  of the front portion  60  of the muffler  17  to form a space there. A band  65  used to secure a joint portion between the rear side exhaust pipe  13  and the rear collecting pipe  42  partially faces this space. An external surface  60   c  of the front portion  60  of the muffler  17  is formed as a taper surface which slants so that its front side faces the central side of the vehicle body. 
     An external surface  15   d  of a rear wall  15   c  of the catalyst chamber  15  is formed as a taper surface whose rear side enters inside the vehicle body and is reduced in diameter to have approximately the same diameter as that of the rear exhaust pipe  47 . A recessed space surrounded by the rear wall  15   c  and the front wall  16   a  of the exhaust chamber  16  is formed externally of the rear exhaust pipe  47 . The exhaust throttle valve  53  is received in this recessed space to be protected from scattered stones or the like. 
     A description is next made of the arrangement of the exhaust throttle valve  53 .  FIG. 6  is an enlarged lateral view of the exhaust throttle valve  53  and  FIG. 7  is an enlarged plan view of  FIG. 6 . Referring to  FIG. 6 , the exhaust throttle valve  53  is attached to the external lateral surface of the rear exhaust pipe  47  so as to be put between the catalyst chamber  15  and the exhaust chamber  16  from the front and from the back. A pulley  70  is provided in the exhaust throttle valve  53  and is turnably driven by cables  71  to turn a throttle valve  72  provided inside the rear exhaust pipe  47 , thereby adjusting an amount of exhaust gas. The cables  71  extend upward from the upper portion of the exhaust throttle valve  53  and are turned by an actuator not shown on the basis of the rotation number of the engine. 
     The throttle valve  72  is provided integrally with the pulley  70  for rotation and the passage-sectional area of the rear exhaust pipe  47  is variably controlled to adjust an amount of exhaust flow. The throttling by the throttle valve  72  is controlled by a controller not shown on the basis of the rotation number of the engine. During low-speed rotation, the throttle valve  72  is closed to reduce exhaust noise. During high-speed rotation, the throttle valve  72  is opened to enable high power. However, known control methods, types, structures and the like can be applicable. 
     A drive portion of the exhaust throttle valve  53  is a portion thereof exposed to the external lateral surface of the rear exhaust pipe  47 . A back-and-forth length L 1  of this drive portion is approximately equal to a back-and-forth length L 2  of the rear exhaust pipe  47 . In addition, the back-and-forth length L 2  of the rear exhaust pipe  47  includes the respective lengths of a connection flange  15   e  provided on a rear wall  15   c  of the catalyst chamber  15  and a connection flange  16   e  provided on a front wall  16   a  of the exhaust chamber  16 . In this way, the catalyst chamber  15  and exhaust chamber  16  which are large-size components can be compactly disposed with respect to the exhaust throttle valve  53  from front and from back. 
     Referring to  FIG. 7 , the exhaust throttle valve  53  is provided with a case  73  which projects externally to the side of the rear exhaust pipe  47  and is covered, from the external side, with a lid  74  secured thereto with bolts  75 . A rotary shaft  76  is turned integrally with the pulley  70  and passes through the rear exhaust pipe  47 , projects from the right lateral surface thereof and is secured thereto with an attachment  77 . In this way, the exhaust throttle valve  53  is integrally attached to the rear exhaust pipe  47 . The throttle valve  72  is integrally attached to the rotary shaft  76  in the rear exhaust pipe  47 . The throttle valve  72  may be of a known type such as a butterfly valve or the like. 
     If a pipe diameter of the rear exhaust pipe  47  and the attachment  77  is D and a vehicle-widthwise width including the exhaust throttle valve is W, each of the respective vehicle-widthwise widths W 1  and W 2  of the catalyst chamber  15  and exhaust chamber  16  is greater than the pipe diameter of the rear exhaust pipe  47 . 
     A drive portion which is a portion of the exhaust throttle valve  53  and includes the case  73  and the lid  74  is exposed to the outside of the rear exhaust pipe  47  and externally protrudes by a dimension d. The external surface  15   b  of the catalyst chamber  15  protrudes externally laterally from the exposure portion of the external throttle valve  53  by D 1  and the external surface  16   b  of the exhaust chamber  16  recedes from the exposure portion of the external throttle valve  53  by d 2 . However, d 2  is very small. 
     The rear wall  15   c  of the catalyst chamber  15  is reduced in diameter toward the inside of the vehicle body to form a slant surface and in particular the external surface  15   d  is largely slanted. Also the front wall  16   a  of the catalyst chamber  15  projects externally laterally and is nearly equally to the case  73 . Thus, the external surface  15   d , the rear exhaust pipe  47  and the front wall  16   a  define a recessed space  78  receding into the inward of the vehicle body. The exposure portion such as the case  73  and the like is received inside the recessed space  78 . 
     In this way, the exposure portion such as the case  73  and the like is guarded by the external surface  15   d  from the front and by the front wall  16   a  from the rear so as to be protected against disturbance due to scattered stones or the like. 
     In addition, as indicated with imaginary lines in  FIG. 7 , also the exposure portion can be protected against disturbance due to scattered stones or the like by using the side stand  39 . In  FIG. 7 , a vehicle body attachment portion of the side stand  39  is disposed externally of the exhaust throttle valve  53  by way of example. A boss portion  39   a , which is the vehicle body attachment portion provided at one end of the side stand, is turnably attached to a support plate  39   c  with a bolt  39   b . While being stored, the side stand  39  is turned to be approximately horizontal so that its rear end side is moved to the rear. The support plate  39   c  is attached to the lower portion of the pivot plate  7  ( FIG. 1 ). In this way, the exhaust throttle valve  53  can be protected against disturbance due to scattered stones by the vehicle body attachment portion of the side stand  39 . 
     The vehicle body attachment portion of the side stand  39  may be removed forward and the side stand may be approximately parallel to the rear exhaust pipe  47  during storage so as to cover the exhaust throttle valve  53  from the outside. In this case, the side stand  39  stored during operation can effectively be used to protect the exhaust throttle valve  53  against disturbance due to scattered stones or the like. 
       FIG. 8  is a cross-sectional view of the exhaust chamber  16  and muffler  17 , taken along line  8 - 8  of  FIG. 4 . Referring to  FIG. 8 , the exhaust chamber  16  and the front portion  60  of the muffler  17  are arranged at respective different levels so that the front portion  60  is located higher than the exhaust chamber  16 . In addition, the front portion  60  is formed in an appropriate inverted triangle in cross-section. The right lateral surface  60   c  is formed as a slant surface so that its lower portion faces the inside of the vehicle body, thereby providing a bank angle θ. Symbol H denotes a horizontal line coincident with the bottom surface of the exhaust chamber  16 . 
     In this way, since the right lateral surface  60   c  is formed as the downward slant surface, scattered stones  79  or the like are bounced externally downward from the right lateral surface  60   c  so that they can be prevented from moving toward the exhaust chamber  16  and further toward the exhaust throttle valve  53 . 
     Similarly, as shown in  FIG. 5 , the right lateral surface  60   c  of the exhaust chamber  16  is slanted such that the rear side thereof externally opens also as viewed from above. Thus, stones  79  or the like scattered from the front are bounced from the right lateral surface  60   c  toward the oblique outside and toward the rear, also thereby being prevented from moving toward the exhaust throttle valve  53 . 
     As shown in  FIGS. 6 and 7 , the exhaust throttle valve  53  is attached to the rear exhaust pipe  47  at a position between the catalyst chamber  15  which is the connecting portion disposed near the lower portion of the engine and the exhaust chamber  16  disposed downstream of the catalyst chamber  15  so as to connect therewith through the rear exhaust pipe  47 . Thus, the exhaust throttle valve  53  is surrounded by the catalyst chamber  15  and the exhaust chamber  16  to thereby protect the exhaust throttle valve  53  from disturbance due to stones or the like scattered by the front wheel  1  or the like. 
     In this case, the lateral width W 1  of the catalyst chamber  15  disposed forward of the exhaust throttle valve  53  is made greater than the lateral width W of the rear exhaust pipe including the exhaust throttle valve  53 . Thus, the catalyst chamber  15  can prevent the exhaust throttle valve  53  from disturbance due to scattered stones. In particular, the exhaust throttle valve  53  can be protected from disturbance due to stones or the like scattered from the front of the vehicle during operation of the vehicle. In addition, since the left lateral surface of the catalyst chamber  15  is externally protruded from the rear exhaust pipe  47  and from the exhaust throttle valve  53 , the exhaust throttle valve  53  can further effectively be protected from disturbance due to stones or the like scattered from the front wheel side. 
     Similarly, the lateral width W 2  of the exhaust chamber  16  disposed to the rear of the exhaust throttle valve  53  is made greater than the lateral width W of the rear exhaust pipe  47  including the exhaust throttle valve  53 . Therefore, the exhaust throttle valve  53  can effectively be prevented from disturbance due to scattered stones bounced from the rear portion of the vehicle. 
     The right lateral surface  60   c  which is an external lateral surface of the muffler  17  slants to externally open toward the rear. Therefore, if the disturbance due to stones scattered from the front of the vehicle hits the right lateral surface  60   c  of the muffler  17 , it can be bounced obliquely externally and to the rear by the right lateral surface  60   c  which is the externally opening slant surface. This can prevent the exhaust throttle valve from being hit by the disturbance due to stones scattered from the front of the vehicle and bounced from the muffler. 
     Further, the catalyst chamber  15  and exhaust chamber  16  which are components larger than the exhaust throttle valve  53  are disposed forward of and rearward of, respectively, the exhaust throttle valve  53  so as to put it therebetween. The back-and-forth length L 2  of the rear exhaust pipe  47  which is an interval between the catalyst chamber  15  and the exhaust chamber  16  is made approximately equal to the back-and-forth length L 1  of the exhaust throttle valve  53 . Therefore, the back-and-forth interval of the exhaust throttle valve  53  can be reduced by the catalyst chamber  15  and by the exhaust chamber  16 . This can prevent the exhaust throttle valve  53  from being hit by disturbance due to scattered stones or the like. 
     Since the exhaust throttle valve  53  is disposed to recede inside of the vehicle body from the external lateral portion of the catalyst  15 , it can further easily be protected from the disturbance due to scattered stones or the like. Further, if the side stand  39  during storage or the boss  39   a  and attachment plate  39   c  which are attachment portions of the side stand are designed to overlap the outside of the exhaust throttle valve  53 , the side stand  39  during storage or the attachment portions  39   a ,  39   c  cover the exhaust throttle valve  53  from laterally outside to protect it from disturbance due to scattered stones or the like. 
     In addition, the present invention is not limited to the embodiments described above and can be modified or applied in various ways within the principles of the invention. For example, as the collecting portion disposed below the engine, the catalyst chamber may be replaced by the exhaust chamber which is a mere expansion chamber. 
     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.