Title of Invention

EXHAUST SYSTEM FOR MOTORCYCLE

Abstract

To provide an exhaust pipe structure enabling an increase in the rising margin of a rear wheel, in a motorcycle equipped with left and right mufflers. [Solving Means] An exhaust pipe 50 extended from an engine is extended to the left or right side (in the figure, the upper side is the right side, and the lower side is the left side) of a cushion unit 33 disposed in the vicinity of the center of a vehicle body on the rear side of the engine, and is branched in the vicinity of the cushion unit 33 into a first exhaust branch pipe 51 and a second exhaust branch pipe 52, the first exhaust branch pipe 51 is connected to a first muffler 31 disposed on the same side as the exhaust pipe 50, and the second exhaust branch pipe 52 is connected to a second muffler 32 after being passed crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body. [Effect] The second exhaust branch pipe extended crosswise in the vehicle body width direction is passed between the cushion unit and a rear wheel present on the rear side thereof. The second exhaust branch pipe does not hamper the rising of the rear wheel. As a result, it is possible to sufficiently secure a rising margin for the rear wheel. [Selected Drawing] Fig. 3

Full Text

[Name of Document] Specification [Title of the Invention] Exhaust System for Motorcycle [Technical Field] [0001] The present invention relates to an improvement in an exhaust system for a motorcycle. [Background Art] [0002] In connection with motorcycles, various layout examples as to the muffler and the exhaust pipe have been proposed (refer, for example, to Patent Document 1). [Patent Document 1] Japanese Patent Laid-open No. 2003-74337 (Figs. 3 and 4) [0003] Fig. 3 of Patent Document 1 shows an exhaust pipe 18. The exhaust pipe 18 is composed of an exhaust pipe front portion 61, a right side connecting pipe 62 extended rearwards from the exhaust pipe front portion 61 and connected to a muffler 23 disposed on the right side, and a coupling pipe 63 for connecting the right side connecting pipe 62 with a muffler 22 disposed on the left side (refer to paragraph [0015] of Patent Document 1). The coupling pipe 63 is extended crosswise directly above the rear wheel indicated by imaginary lines. [0004] Fig. 4 of Patent Document 1 is a side view of an essential part, in which the coupling pipe 63 passes through a position for securing a clearance C from the bottom of a sunken portion 43c of a tandem seat 43. Therefore, the rising limit position for the rear wheel 21 is limited by an imaginary line 82. [0005] Meanwhile, in motorcycles such as off-road vehicles and trial vehicles, large contraction/expansion amounts of suspension are required. Therefore, the exhaust pipe structure shown in Patent Document 1 cannot be adopted for these types of motorcycles, since the rising margin for the rear wheel in the exhaust pipe structure is too small. [0006] However, in connection with motorcycles equipped with left and right mufflers, there is a keen demand for an exhaust pipe structure which enables an increase in the rising margin for the rear wheel. [Disclosure of the Invention] [Problem to Be Solved by the Invention] [0007] It is an object of the present invention to provide an exhaust pipe structure which enables an increase in the rising margin for the rear wheel, in a motorcycle equipped with left and right mufflers. [Means for Solving the Problem] [0008] The invention according to Claim 1 resides in an exhaust system for a motorcycle comprising a first muffler and a second muffler disposed respectively on the upper left and right sides of a rear wheel, and an exhaust pipe extended from an engine and connected to the mufflers, characterized in that: the exhaust pipe is extended from the engine to the left or right side of a cushion unit disposed in the vicinity of the center of a vehicle body on the rear side of the engine, and is branched in the vicinity of the cushion unit into a first exhaust branch pipe and a second exhaust branch pipe; and the first exhaust branch pipe is connected to the first muffler disposed on the same side, and the second exhaust branch pipe is connected to the second muffler after being passed between the cushion unit and the rear wheel present on the rear side thereof to extend crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body. [0009] The invention according to Claim 2 resides in an exhaust system for a motorcycle comprising a first muffler and a second muffler disposed respectively on the upper left and right sides of a rear wheel, and an exhaust pipe extended from an engine and connected to the mufflers, characterized in that: the exhaust pipe is branched into a first exhaust branch pipe and a second exhaust branch pipe after being extended in the front-rear direction on one side of a vehicle body; the first exhaust branch pipe is connected to the first muffler disposed on the same side, whereas the second exhaust branch pipe is connected to the second muffler after being extended crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body; and the second exhaust branch pipe is extended in the direction along the exhaust gas flow in the exhaust pipe not yet branched, and the first exhaust branch pipe is extended in a direction of intersecting the exhaust gas flow in the exhaust pipe not yet branched. [0010] The invention according to Claim 3 is characterized in that a branching portion for branching into the first exhaust branch pipe and the second exhaust branch pipe is comprised of an integrally formed first pipe, and a second pipe having a tip end connected to a wall of the first pipe, the second pipe is on the side of the first exhaust branch pipe, and the first pipe is on the side of the second exhaust branch pipe. [0011] The invention according to Claim 4 is characterized in that the first exhaust branch pipe is smaller in diameter than the second exhaust branch pipe. [0012] The invention according to Claim 5 is characterized in that an air cleaner case is disposed between the cushion unit and the rear wheel and on the upper side relative to the second exhaust branch pipe, and a cleaner element contained in the air cleaner case is so disposed that the longitudinal direction thereof extends in the vehicle width direction. [0013] The invention according to Claim 6 is characterized in that an exhaust valve is interposed in the branching portion, the exhaust valve being so configured that, when the quantity of the exhaust gas flowing through the exhaust pipe is small, a valve body advances into the exhaust pipe to throttle the flow passages of the first exhaust branch pipe and the second exhaust branch pipe, and, when the quantity of the exhaust gas is large, the valve body recedes to cancel the throttling. [Effects of the Invention] [0014] In the invention according to Claim 1, the exhaust pipe is extended from the engine to the left or right side of the cushion unit disposed in the vicinity of the center of the vehicle body on the rear side of the engine, and is branched in the vicinity of the cushion unit into the first exhaust branch pipe and the second exhaust branch pipe, the first exhaust branch pipe is connected to the first muffler disposed on the same side, and the second exhaust branch pipe is passed between the cushion unit and the rear wheel disposed on the rear side thereof, is extended crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body, and is then connected to the second muffler. [0015] Namely, the second exhaust branch pipe extended crosswise in the vehicle body width direction is passed between the cushion unit and the rear wheel disposed on the rear side thereof, so that the second exhaust branch pipe does not hamper the rising of the rear wheel. As a result, it is possible to sufficiently secure a rising margin for the rear wheel. Claim 1 can provide an exhaust system which enables an increase in the rising margin for the rear wheel, in a motorcycle equipped with left and right mufflers. [0016] In the invention according to Claim 2, the exhaust pipe is branched into the first exhaust branch pipe and the second exhaust branch pipe after being extended in the front-rear direction on one side of the vehicle body, the first exhaust branch pipe is connected to the first muffler disposed on the same side, whereas the second exhaust branch pipe is connected to the second muffler after being extended crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body, the second exhaust branch pipe is extended in the direction along the exhaust gas flow in the exhaust pipe not yet branched, and the first exhaust branch pipe is extended in a direction of intersecting the exhaust gas flow in the exhaust pipe not yet branched. [0017] Since the second exhaust branch pipe is extended crosswise in the vehicle body width direction, the second exhaust branch pipe is liable to be longer than the first exhaust branch pipe, and a difference in exhaust gas flow is liable to be generated between the first exhaust branch pipe and the second exhaust branch pipe. It is desired that the pipe resistance be the same in both the branch pipes. In view of this, the longer second exhaust branch pipe is extended in the direction along the exhaust gas flow in the exhaust pipe not yet branched, thereby making it easy for the exhaust gas to flow therethrough, whereas the shorter first exhaust branch pipe is extended in a direction of intersecting the exhaust gas flow in the exhaust pipe not yet branched, thereby making it difficult for the exhaust gas to flow therethrough. As a result, it is possible to equalize the exhaust gas flows in the first exhaust branch pipe and in the second exhaust branch pipe. [0018] In the invention according to Claim 3, the branching portion for branching into the first exhaust branch pipe and the second exhaust branch pipe is composed of the integrally formed first pipe, and the second pipe having the tip end connected to a wall of the first pipe. The second exhaust branch pipe extended in the direction along the exhaust gas flow in the exhaust pipe not yet branched is connected to an integrally formed first pipe outlet, whereas the first exhaust branch pipe extended in a direction of intersecting the exhaust gas flow in the exhaust pipe not yet branched is connected to a second pipe outlet. The branching portion can be configured easily, and can be smoothly connected to the first exhaust branch pipe and the second exhaust branch pipe. [0019] In the invention according to Claim 4, the first exhaust branch pipe is smaller in diameter than the second exhaust branch pipe. Specifically, the shorter first exhaust branch pipe is made to be smaller in diameter, thereby making it difficult for the exhaust gas to flow therethrough, whereas the longer second exhaust branch pipe is made to be larger in diameter, thereby making it easy for the exhaust gas to flow therethrough, and, as a result, it is more easily made possible to equalize the exhaust gas flows in both the exhaust branch pipes. [0020] In the invention according to Claim 5, the air cleaner case is disposed between the cushion unit and the rear wheel and on the upper side relative to the second exhaust branch pipe, and the cleaner element contained in the air cleaner case is so disposed that the longitudinal direction thereof extends in the vehicle width direction. In the conventional motorcycles, in many cases, the cleaner element has been disposed in a vertically elongate mode in which the longitudinal direction of the cleaner element is substantially along the vertical axis. On the contrary, according to Claim 5, the cleaner element is so disposed that its longitudinal direction extends in the vehicle width direction, and the cleaner case is disposed in a horizontally elongate mode. As a result, a space can be secured between the cleaner case and the swing arm, and the second exhaust branch pipe can successfully be passed through the space thus secured, [0021] In the invention according to Claim 6, the exhaust valve is interposed in the branching portion. At the time of low output when the engine speed is low, the back pressure in the exhaust pipe is appropriately raised by the throttling action of the valve body, making it possible to enhance the engine performance. At the time of high output when the engine speed is high, the cross-sectional area of the exhaust pipe is maximized, whereby the back pressure is appropriately lowered, making it possible to enhance the engine performance. [0022] In addition, notwithstanding the two mufflers consisting of the first muffler and the second muffler are provided and the first exhaust branch pipe and the second exhaust branch pipe are coupled respectively to these mufflers, the arrangement of the exhaust valve in the branching portion makes it possible to regulate the exhaust gas pressures collectively by the single exhaust valve, to minimize the number of the valve(s), and to contrive a reduction in the number of component parts. [Best Mode for Carrying Out the Invention] [0023] A best mode for carrying out the present invention will be described below, based on the accompanying drawings. Incidentally, the drawings are to be looked at according to the posture of symbols. Fig. 1 is a side view of a motorcycle according to the present invention, in which the motorcycle 10 called off-road motorcycle has a structure in which a front fork 13 is steerably mounted to a head pipe 12 provided at a front portion of a main frame 11, a steering handle 15 is mounted to the upper end of the front fork 13, a front wheel 16 is rotatably mounted to the lower ends of the front fork 13, a single-cylinder engine 17 is suspended from the main frame 11, the engine 17 is surrounded by a down tube 18 extended downwards from a front portion of the main frame 11, the rear end of the down tube 18 is connected to a rear portion of the main frame 11, a swing arm 22 is vertically swingably mounted to a rear portion of the main frame 11 through a pivot 21, a rear wheel 23 is rotatably mounted to the rear end of the swing arm 22, a chain 25 for transmitting the power of the engine 17 is wound around a sprocket 24 belonging to the rear wheel 23, a fuel tank 26 is mounted to a front portion of the main frame 11, seat rails 27 are extended from rear upper portions of the main frame 11, the seat rails 27 are reinforced by slant sub-stays 28, a seat 29 is mounted on the seat rails 27, a first muffler 31 and a second muffler 32 (one of them is located in the depth of the drawing) are disposed along the sub-stays 28, and a cushion unit 33 for controlling the swinging motions of the swing arm 22 is disposed on the front side (the right side in the figure) of the mufflers 31, 32. Symbol 50 denotes an exhaust pipe extended from the engine 17, and the details thereof will be described later. [0024] Symbol 34 denotes a front brake caliper, 35 denotes a front fender, 3 6 denotes a cowl, 37 denotes step bars on which to mount the driver's feet, 3 8 denotes a brake pedal, 39, 39 denote protectors annexed to the mufflers 31, 32, 41 denotes side covers, and 42 denotes a rear fender. [0025] Fig. 2 is a plan view of an essential part of the motorcycle according to the present invention, and shows that, for connecting an upper portion of the cushion unit 33, a cross member 43 is bridgingly disposed between front portions of left and right seat rails 27L, 27R (L is a suffix indicating the left as viewed from the driver, R is a suffix indicating the right as viewed from the driver, which are affixed as required, here and hereinafter), a cross member 44 functioning also as a reinforcement member is bridgingly disposed between intermediate portions of the seat rails 27L, 27R, and brackets 45L, 45R are erected from rear portions of the seat rails 2 7L, 27R for the purpose of bearing a rear portion of the seat. The rear fender 42 can be seen between the brackets 45L and 45R and on the lower side (in the depth of the drawing), and the first muffler 31 and the second muffler 32 are disposed on the left and right sides of the rear fender 42. [0026] The first muffler 31 and the second muffler 32 are detachably attached to the rear portions of the seat rails 27L, 27R by bolts 46L, 46R. In the figure, symbol 4 8 denotes an exhaust pipe support portion, 4 7 denotes an air cleaner case disposed between the cushion unit 3 3 and the rear wheel, 4 9 denotes an air duct portion extending from the air cleaner case 47 to the engine, and a flexible connecting tube can be connected to the air duct portion 49. [0027] Fig. 3 is a bottom view (the rear wheel is omitted) of Fig. 2, and shows a structure characterized in that an exhaust pipe 50 extended from the engine (symbol 17 in Fig. 1) is extended to the left or right side (in the figure, the upper side is the right side, and the lower side is the left side) of the cushion unit 33 disposed in the vicinity of the center of the vehicle body on the rear side of the engine, and is branched in the vicinity of the cushion unit 33 into a first exhaust branch pipe 51 and a second exhaust branch pipe 52, the first exhaust branch pipe 51 is connected to the first muffler 31 disposed on the same side as the first exhaust branch pipe 51, whereas the second exhaust branch pipe 52 is connected to the second muffler 32 after being passed between the cushion unit 3 3 and the rear wheel (symbol 23 in Fig. 1) present on the rear side thereof to extend crosswise from one of the left and right sides of the vehicle body to the other of the left and right sides of the vehicle body. Incidentally, when it is necessary to section the exhaust pipe 50, a front portion thereof will be referred to as an exhaust pipe front portion 50a. In addition, symbol 42a denotes a rear fender front portion (see Fig. 5). [0028] Preferably, the second exhaust branch pipe 52 is extended in the direction along the exhaust gas flow in the exhaust pipe 50 not yet branched (the exhaust pipe front portion 50a) . On the other hand, the first exhaust branch pipe 51 is extended in a direction of intersecting the exhaust gas flow in the exhaust pipe 50 not yet branched (the exhaust pipe front portion 50a). [0029] Since the second exhaust branch pipe 52 is extended crosswise in the vehicle body width direction, it tends to be longer than the first exhaust branch pipe 51, and a difference in exhaust gas flow is liable to be generated between the first exhaust branch pipe 51 and the second exhaust branch pipe 52. It is desired that the pipe resistance be the same in both the exhaust branch pipes 51, 52. In view of this, the longer second exhaust branch pipe 52 is extended in the direction along the exhaust gas flow in the exhaust pipe 5 0 not yet branched, thereby making it easy for the exhaust gas to flow therethrough, whereas the shorter first exhaust branch pipe 51 is extended in a direction of intersecting the exhaust gas flow in the exhaust pipe 5 0 not yet branched, thereby making it difficult for the exhaust gas to flow therethrough. As a result, it is possible to equalize the exhaust gas flows in the first exhaust branch pipe 51 and in the second exhaust branch pipe 52. [0030] Furthermore, the condition of Dl (accurately, the inside diameter) of the second exhaust branch pipe 52. Namely, the first exhaust branch pipe 51 is made to be smaller in diameter than the second exhaust branch pipe 52. With the shorter first exhaust branch pipe 51 made to be smaller in diameter, thereby making it difficult for the exhaust gas to flow therethrough, and with the longer second exhaust branch pipe 52 made to be larger in diameter, thereby making it easy for the exhaust gas to flow therethrough, whereby it is more easily possible to equalize the exhaust gas flows in both the exhaust branch pipes 51, 52. [0031] Meanwhile, if the exhaust pipe 50 including the first exhaust branch pipe 51 and the second exhaust branch pipe 52 is short, the support rigidity thereof can be secured by supporting its front end with the exhaust pipe of the engine and supporting its rear end with the mufflers 31, 32. In the present invention, however, the second exhaust branch pipe 52 extends to cross the vehicle body in the left-right direction, so that it is long and heavy as a whole, so that it is necessary to increase the number of support points for the second exhaust branch pipe 52. [0032] Since the first exhaust branch pipe 51 is short, is light, and is located on the same side (with reference to the vehicle body center line 53) as the exhaust pipe 50, it does not have such an influence as a twisting force on the exhaust pipe 50. On the other hand, since the second exhaust branch pipe 52 is long, is heavy, and is located on the different side (with respect to the vehicle body center line 53) relative to the exhaust pipe 50, it has such an influence as a twisting force on the exhaust pipe 5 0. [0033] Since the second muffler 32 is one of the support points for the second exhaust branch pipe 52, the arrangement of the exhaust pipe support portion 4 8 on the different side relative to the second muffler 32 is effective for coping with the twisting. For coping with the property of the second exhaust branch pipe 52 of being long and heavy, it is effective to support the second exhaust branch pipe 52 at two points (at both ends) consisting of the exhaust pipe support portion 4 8 and the support point (the bolt 4 6L) for the second muffler 32. From the foregoing, the provision of the exhaust pipe support portion 48 at a position which is on a slant line 54 passing through the support point (the bolt 46L) for the second muffler 32 and intersecting the vehicle body center line 53 and which is in the vicinity of the branching portion 55 makes it possible to minimize the number of additional support point(s) and to obtain a maximum effect. [0034] Fig. 4 is an enlarged sectional view of the branching portion of the exhaust pipe in the present invention. The branching portion 55 for branching into the first exhaust branch pipe 51 and the second exhaust branch pipe 52 is preferably composed of a first pipe 56 integrally formed in a curved pipe shape, and a second pipe 58 having a tip end coupled to a large diameter side wall 57 of the first pipe 56. As a result, the second exhaust branch pipe 52 extended in the direction (arrow A) along the exhaust gas flow in the exhaust pipe 50 not yet branched (the exhaust pipe front portion 50a) can be connected to an outlet of the integrally formed first pipe 56, and the first exhaust branch pipe 51 extended in a direction (arrow B) of intersecting the exhaust gas flow in the exhaust pipe 50 not yet branched (the exhaust pipe front portion 50a) can be connected to an outlet of the second pipe 58. The branching portion 55 can be configured easily, and the first exhaust branch pipe 51 and the second exhaust branch pipe 52 can be coupled smoothly. [0035] Now, the air cleaner will be described below. Fig. 5 is a view (the rear wheel, the rear fender and the like are omitted) along arrow 5 of Fig. 1, and shows a structure characterized in that the air cleaner case 47 is disposed on the viewer's side of the drawing of the cushion unit 33 (between the cushion unit 33 and the rear wheel) and on the upper side relative to the second exhaust branch pipe 52, and a cleaner element 59 contained in the air cleaner case 47 is so disposed that its longitudinal direction (arrow C) extends in the vehicle width direction. Symbol 42a is a rear fender front portion, which is extended downwards from the air cleaner case 47. [0036] In the conventional motorcycles, in many cases, the cleaner element has been disposed in a vertically elongate mode in which its longitudinal direction is substantially along the vertical direction. On the contrary, in the present invention, the cleaner element 59 is so disposed that its longitudinal direction (arrow C) extends in the vehicle width direction, and the air cleaner case 47 is disposed in a horizontally elongate mode. As a result, a space can be secured between the air cleaner case 47 and the swing arm, and the second exhaust branch pipe 52 can be easily passed through the space thus secured. [0037] Figs. 6(a) and 6(b) show an example of disposing an exhaust valve in the exhaust pipe according to the present invention. The exhaust valve 60 is composed of a valve body 61 having a tip sharpened in a bow-like shape, a pocket portion 62 formed by bulging the branching portion 55 for the purpose of movably containing the valve body 61 therein, a valve rod 63 extended from the valve body 61 in the form of penetrating through the pocket portion 62, and a spring 64 for pushing the valve body 61 into the inside of the exhaust pipe 50. [0038] In Fig. 6(a), when the engine speed is low, the quantity Ql of the exhaust gas generated in the engine is small. When the quantity Ql of the exhaust gas is small, the force for pushing the valve body 61 against the spring 64 is small, so that the valve body 61 advances largedly into the inside of the exhaust pipe 50, whereby the first exhaust branch pipe 51 and the second exhaust branch pipe 52 are throttled, and the quantities ql and q2 of the exhaust gas flowing through the exhaust branch pipes are reduced. When viewed from the engine side, at the time of low output when the engine speed is low, the back pressure in the exhaust pipe 50 is appropriately raised by the throttling action of the valve body 61, whereby the engine performance can be enhanced. [0039] In Fig. 6 (b), when the engine speed is high, the quantity Q2 of the exhaust gas generated in the engine is large. When the quantity Q2 of the exhaust gas is large, the force for pushing the valve body 61 against the spring 64 is large, so that the valve body 61 recedes into the pocket portion 62, whereby the quantities q3 and q4 of the exhaust gas flowing through the first exhaust branch pipe 51 and the second exhaust branch pipe 52 are enlarged. When viewed from the engine side, at the time of high output when the engine speed is high, the cross-sectional area of the exhaust pipe 50 is maximized, whereby the back pressure is appropriately lowered, and the engine performance can be enhanced. [0040] In short, with the exhaust valve provided as a throttle valve, and with the exhaust passage is throttled at the time of low engine speed, a characteristic suited to the low engine speed time can be obtained. Therefore, when the exhaust valve is opened, a characteristic suited to the high engine speed time can be obtained, i.e., the exhaust branch pipes can be set short, so that the mufflers can be disposed close to the center of gravity which is near the vehicle body front side. [0041] Now, the present invention will be evaluated below from other points of view. In motorcycles such as off-road motorcycles in which the handling of the vehicle body is enhanced by reductions in size and weight, it has been the common sense that the motorcycle is equipped with a single-cylinder engine, a single exhaust pipe, and a single muffler. Since it is difficult to dispose the single muffler on the vehicle body center axis, the muffler is disposed on the right or left side of the rear wheel. The muffler is a heavy body which cannot be ignored in a small-type vehicle, and, therefore, the muffler exerts an unbalanced load on the vehicle body. Conventionally, the unbalanced load has been coped with by the skill of the driver. [0042] In this point, according to the present invention, the motorcycle such as an off-road motorcycle has a structure in which the single-cylinder engine is equipped with the left and right mufflers. The left and right mufflers can be disposed symmetrically with respect to the vehicle body center axis, so that the left-right balance of the vehicle body is enhanced. Since no unbalanced load is exerted on the vehicle body, it is unnecessary for the driver to cope with such an unbalance by his skill, so that it is easy for the driver to drive the motorcycle as he wants. [0043] In addition, making the exhaust pipe longer at the low engine speed time enhances the engine performance. It is known that the engine performance can be enhanced by making the exhaust pipe shorter at the high engine speed time. In Fig. 3, at the low engine speed time, the pressure of the exhaust gas is lowered, so that a larger quantity of the exhaust gas flows through the second exhaust branch pipe 52 in which it is easier for the exhaust gas to flow, and the sufficiently long second exhaust branch pipe 52 functions to pick up the engine performance. At the high engine speed time, the back pressure of the exhaust gas is raised, the exhaust gas flows also through the first exhaust branch pipe 51 in which it is difficult for the exhaust gas to flow, and, together with the function of the short first exhaust branch pipe 51, the exhaust gas is discharged smoothly, so that the engine performance can be enhanced. Therefore, the functions of the first and second exhaust branch pipes 51, 52 different in length make it possible to display the engine performance over the wide range from low engine speed to high engine speed. [0044] In addition, in the motorcycle such as an off-road motorcycle, the single-cylinder engine is equipped with the two mufflers, which promises the following effects. In Fig. 3, the capacity of one muffler is divided into the two mufflers 31, 32, so that the individual ones of the mufflers 31, 32 are small in size. The reduction in size ensures that the mufflers 31, 32 which are heavy bodies can be laid out close to the center of gravity (in the vicinity of the engine) of the vehicle body. [0045] Further, the combination of the short first exhaust branch pipe 51 with the second exhaust branch pipe of which the length in the longitudinal direction of the vehicle body is reduced by bending into an S shape ensures that the mufflers 31, 32 as heavy bodies can be laid out close to the center of gravity (in the vicinity of the engine) of the vehicle body. [0046] Second moment of inertia (GD2) is an inertia index when a heavy body with a weight G is located at a distance D from the center of gravity, and, as the index is smaller, the mobile body can be rotated or turned sharply. According to the present invention, the mufflers 31, 32 as heavy bodies can be disposed close to the center of gravity of the vehicle body, so that the second moments of inertia of the mufflers 31, 32 can be set smaller, and the motion performance of the motorcycle can be much enhanced. [0047] Incidentally, while the right muffler has been described as the first muffler, the first muffler may be the left muffler. Namely, since the first muffler is on one of the left and right sides and the second muffler is on the other of the left and right sides, the terms first and second have been used. This applies also to the exhaust branch pipes. [0048] In addition, while the present invention is suitable for application to an off-road motorcycle, the invention is applicable also to other motorcycles, particularly motorcycles (inclusive of scooter type vehicles) equipped with a single-cylinder engine. [Industrial Applicability] [0049] The present invention is suitably applicable to off-road motorcycles equipped with a single-cylinder engine. [Brief Description of the Drawings] [0050] [Fig. 1] Fig. 1 is a side view of a motorcycle according to the present invention. [Fig. 2] Fig. 2 is a plan view of an essential part of the motorcycle according to the present invention. [Fig. 3] Fig. 3 is a bottom view (the rear wheel is omitted) of Fig. 2. [Fig. 4] Fig. 4 is an enlarged sectional view of a branching portion of an exhaust pipe according to the present invention. [Fig. 5] Fig. 5 is a view (the rear wheel, the rear fender and the like are omitted) along arrow 5 of Fig. 1. [Figs. 6(a) and 6(b)] Figs. 6(a) and 6(b) show an example of disposing an exhaust valve in the exhaust pipe according to the present invention. [Explanation of Symbols] [0051] 10 : motorcycle; 17 : engine; 23 : rear wheel; 31: first muffler; 32: second muffler; 33: cushion unit; 47: air cleaner case; 50: exhaust pipe; 50a: exhaust pipe front portion; 51: first exhaust branch pipe; 52: second exhaust branch pipe; 53: vehicle body center line; 55: branching portion; 56: first pipe; 57: wall of first pipe; 58: second pipe; 59: cleaner element; 60: exhaust valve; 61: valve body. ® [Name of Document] Claims [Claim 1] An exhaust system for a motorcycle comprising a first muffler and a second muffler disposed respectively on the upper left and right sides of a rear wheel, and an exhaust pipe extended from an engine and connected to said mufflers, wherein: said exhaust pipe is extended from said engine to the left or right side of a cushion unit disposed in the vicinity of the center of a vehicle body on the rear side of said engine, and is branched in the vicinity of said cushion unit into a first exhaust branch pipe and a second exhaust branch pipe; and said first exhaust branch pipe is connected to said first muffler disposed on the same side, and said second exhaust branch pipe is connected to said second muffler after being passed between said cushion unit and said rear wheel present on the rear side thereof to extend crosswise from one of the left and right sides of said vehicle body to the other of the left and right sides of said vehicle body. [Claim 2] An exhaust system for a motorcycle comprising a first muffler and a second muffler disposed respectively on the upper left and right sides of a rear wheel, and an exhaust pipe extended from an engine and connected to said mufflers, wherein; said exhaust pipe is branched into a first exhaust branch pipe and a second exhaust branch pipe after being extended in the front-rear direction on one side of a vehicle body; said first exhaust branch pipe is connected to said first muffler disposed on the same side, whereas said second exhaust branch pipe is connected to said second muffler after being extended crosswise from one of the left and right sides of said vehicle body to the other of the left and right sides of said vehicle body; and said second exhaust branch pipe is extended in the direction along the exhaust gas flow in said exhaust pipe not yet branched, and said first exhaust branch pipe is extended in a direction of intersecting said exhaust gas flow in said exhaust pipe not yet branched. [Claim 3] An exhaust system for a motorcycle as set forth in claim 2, wherein a branching portion for branching into said first exhaust branch pipe and said second exhaust branch pipe is comprised of an integrally formed first pipe, and a second pipe having a tip end connected to a wall of said first pipe, said second pipe is on the side of said first exhaust branch pipe, and said first pipe is on the side of said second exhaust branch pipe. [Claim 4] An exhaust system for a motorcycle as set forth in claim 1, 2 or 3, wherein said first exhaust branch pipe is smaller in diameter than said second exhaust branch pipe. [Claim 5] An exhaust system for a motorcycle as set forth in claim 1, wherein an air cleaner case is disposed between said cushion unit and said rear wheel and on the upper side relative to said second exhaust branch pipe, and a cleaner element contained in said air cleaner case is so disposed that the longitudinal direction thereof extends in the vehicle width direction. [Claim 6] An exhaust system for a motorcycle as set forth in claim 1, 2, 3 or 4, wherein an exhaust valve is interposed in said branching portion, said exhaust valve being so configured that, when the quantity of the exhaust gas flowing through said exhaust pipe is small, a valve body advances into said exhaust pipe to throttle the flow passages of said first exhaust branch pipe and said second exhaust branch pipe, and, when said quantity of said exhaust gas is large, said valve body recedes to cancel said throttling. dated this 20 day of April 2005

Documents:

452-CHE-2005 EXAMINATION REPORT REPLY RECEIVED 11-12-2012.pdf

452-CHE-2005 POWER OF ATTORNEY 11-12-2012.pdf

452-CHE-2005 AMENDED PAGES OF SPECIFICATION 11-12-2012.pdf

452-CHE-2005 AMENDED CLAIMS 11-12-2012.pdf

452-CHE-2005 CORRESPONDENCE OTHERS 16-05-2012.pdf

452-CHE-2005 FORM-3 11-12-2012.pdf

452-CHE-2005 OTHER PATENT DOCUMENT 11-12-2012.pdf

452-che-2005-abs images.jpg

452-che-2005-abstract.pdf

452-che-2005-claims.pdf

452-che-2005-correspondnece-others.pdf

452-che-2005-description(complete).pdf

452-che-2005-drawings.pdf

452-che-2005-form 1.pdf

452-che-2005-form 3.pdf

452-che-2005-form 5.pdf

452-che-2005-priority document.pdf