Exhaust pipe structure

To provide an exhaust pipe structure in which the length of the exhaust pipe can be elongated while the total dimension is restricted. Provided is an exhaust pipe structure which includes an upstream-side exhaust pipe connected to exhaust ports of an engine. An exhaust chamber is provided which has an exhaust-gas inlet and an exhaust-gas outlet, and in which the upstream-side exhaust pipe is connected to the exhaust-gas inlet. In addition, the exhaust pipe structure includes a downstream-side exhaust pipe which is connected to the exhaust-gas outlet and which extends in such a direction as to move away from the exhaust ports. In the exhaust pipe structure, the exhaust-gas inlet is formed at a position farther away from the exhaust ports than the exhaust-gas outlet is positioned.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2006-151206 filed on May 31, 2006 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 pipe structure for vehicles such as motorcycles.

2. Description of Background Art

An exhaust pipe structure is known wherein an exhaust chamber is provided somewhere along the exhaust pipe extending in the front-to-rear direction of vehicle, with an exhaust-gas inlet formed at the front side of the exhaust chamber and an exhaust-gas outlet formed at the rear side. See, for example, Japanese Utility Model Application Laid-Open No. Sho 60-18839.

In the above-mentioned exhaust pipe structure, for the purpose of making a change in engine characteristics (particularly, engine power) depends linearly on the throttle opening angle. Possible ways of achieving this object include elongating the exhaust pipe, and increasing the volume of the exhaust chamber. These measures, however, accompany an increase in the length of the exhaust pipe as a whole in the front-to-rear direction, and an enlargement of the dimension of the exhaust chamber, so that it is sometimes difficult to secure a space for placing the elongated exhaust pipe or the enlarged exhaust chamber.

SUMMARY AND OBJECTS OF THE INVENTION

An embodiment of the present invention provides an exhaust pipe structure in which the exhaust pipe can be elongated while the increase in the overall dimension is restricted.

For the purpose of solving the above-mentioned problems, a first embodiment of the present invention provides an exhaust pipe structure that includes an engine25supported by a body frame5. The exhaust pipe structure also includes an upstream-side exhaust pipe36connected to exhaust ports27band28b. In addition, the exhaust pipe structure includes an exhaust chamber35that has an exhaust-gas outlet52and an exhaust-gas inlet51to which the upstream-side exhaust pipe is connected. Moreover, the exhaust pipe structure includes a downstream-side exhaust pipe37that is connected to the exhaust-gas outlet and extends in such a direction to move away from the exhaust port. In the exhaust pipe structure with the above configuration, the exhaust-gas inlet is formed in a position farther away from the exhaust port relative to the location of the exhaust-gas outlet.

A second embodiment of the present invention provides an exhaust pipe structure wherein the body frame includes a head pipe6and a right and left pair of down frames9extending from the head pipe obliquely downwardly and rearwardly. In addition, lower frames13are provided with each extending rearwardly from the bottom end of each down frame. Moreover, lower end frames14are provided with each extending obliquely upwardly and rearwardly from the rear end of each lower frame. Furthermore, the exhaust chamber is placed at the rear of the lower end frame when viewed from a side of the vehicle.

A third embodiment of the present invention provides an exhaust pipe structure wherein the exhaust port and the exhaust-gas inlet are placed in positions at the inner side in the lateral direction of the body frame. In addition, the middle section of the upstream-side exhaust pipe is placed in a position at the outer side in the lateral direction of the body frame.

A fourth embodiment of the present invention provides an exhaust pipe structure wherein the upstream-side exhaust pipe includes an inward diagonal section56in which the pipe goes inward of the vehicle body as the pipe goes down the stream of the exhaust gas. Meanwhile, the downstream-side exhaust pipe includes an outward diagonal section57in which the pipe goes outward of the vehicle body as the pipe goes down the stream of the exhaust gas. The inward diagonal section and the outward diagonal section cross each other, when viewed from the bottom of the vehicle body, in a position below the rider's seat, for example, a front seat41a, is supported by the body frame.

According to an embodiment of the present invention, the adjustment range for the engine characteristics can be made wider in comparison to the case in which the exhaust-gas inlet is placed closer to the exhaust port than the exhaust-gas is placed. This is made possible because both of the upstream-side exhaust pipe and the downstream-side exhaust pipe can be elongated while the length of the exhaust pipe structure as a whole in the front-to-rear direction is restricted.

According to an embodiment of the present invention, an adequate volume of the exhaust chamber can be secured because the space behind the lower end frame that extends obliquely upwardly and rearwardly can be used efficiently as a space for placement of the exhaust chamber.

According to an embodiment of the present invention, the adjustment range for engine characteristics can be made wider by elongating the upstream-side exhaust pipe, because the upstream-side exhaust pipe can be curved as being convex outwards of the vehicle body.

In an embodiment of the present invention, the inward diagonal section and the outward diagonal section project outwardly of the vehicle body in only a restricted amount in the crossing part of these sections when viewed from the bottom of the vehicle body. As a result, the rider can plant his/her foot more easily on the ground because the space on which the passenger can put his/her foot becomes wider below the rider's seat.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinbelow, descriptions will be given as to an embodiment of the present invention with reference to the drawings. In the descriptions below, directions with respect to front, rear, right, left and the like are the same as those in the vehicle unless otherwise indicated. In the drawings, an arrow FR points to the front of the vehicle, an arrow LH points to the left-handed direction of the vehicle, and an arrow UP points to the upward direction of the vehicle.

As shown inFIG. 1, a front wheel2of a saddle-ride type motorcycle1is rotatably supported by a right and left pair of bottom end portions of a front fork3. The upper portion of each front fork3is pivotally and steerably supported by a head pipe6at the front-end portion of a body frame5, with a steering stem4. A handlebar7for steering the front wheel is attached to the upper portion of the steering stem4. A right and left pair of main frames8extend obliquely downwardly and rearwardly from the upper portion of the head pipe6. A right and left pair of gusset pipes11, which are substantially horizontal when viewed from a side, are provided to bridge the lower portion of the head pipe6and respective middle portions of the right and the left main frames8. From portions of the respective right and the left gusset pipes near the lower portion of the head pipe6, from the front-end portions of the respective right and the left gusset pipes11, a right and left pair of down frames9extends obliquely downwardly and rearwardly, with a steeper angle than the main frames8.

The rear end portions of the respective right and the left main frames8bend obliquely downwardly and rearwardly. The bottom end portions of the right and the left main frames8are connected respectively to the upper end portions of a right and left pair of pivot frames12, which are located in the center portion of the vehicle body in the front-to-rear direction. The lower portions of the right and the left down frames9bend rearwardly in an underside of the front portion of the vehicle body, and then extend rearwardly substantially in a horizontal direction to form a right and left pair of lower frames13. The rear portions of the right and the left lower frames13bend obliquely upwardly and rearwardly in an underside of the center portion in the front-to-rear direction of the vehicle body. Then, the rear portions of the right and the left lower frames13extend upwardly and rearwardly toward the bottom end portions of the corresponding right and the left pivot frames12to form a right and left pair of lower end frames14. The upper end portions of the right and the left lower end frames14are connected to the bottom end portions of the right and the left pivot frames12. As a result, a double-cradle type main frame portion15of the body frame5is formed.

At the rear of the main frame portion15, a seat frame portion16is formed to support the rider's seat and the like. The seat frame portion16includes a right and left pair of seat rails17, which extends rearwardly, respectively, from the rear portions of the right and the left main frames8. Also included is a right and left pair of seat stays18, which extend upwardly and rearwardly, respectively, from the rear portions of the right and the left pivot frames12. Each of the right and the left seat rails17is formed in a shape of a crank with the rear portion bending upwardly when viewed from a side. The right and the left seat stays18support respectively the right and the left seat rails17, with the rear end portion of each seat stay18being connected to the rear end portion of a corresponding one of the seat rails17. It should be noted that the rear portions of the right and the left seat stays18are formed asymmetrically to each other.

A swing arm20is provided to support a rear wheel19, while the front-end portion of the swing arm20is pivotally supported, as being capable of swinging up and down, by the right and the left pivot frames12. An arm portion20a, offsetted to the left, extends rearwardly and slightly downwardly, from the front-end portion (base-end portion) of the swing arm20. At the rear end portion (tip-end portion) of the arm portion20a, which is a cantilever-type swing arm, the rear wheel19is rotatably supported. The arm portion20ais made to be a hollow member, and an unillustrated drive shaft is inserted into the inside of the arm portion20a. The rear end portion of the arm portion20ais configured to be a final gear case21, where the rear end portion of the drive shaft and the rear wheel19are engaged with each other to allow the transmission of the drive power. A rear shock absorber23, which slants rearwardly and downwardly, is placed between a middle portion of the arm portion20ain the front-to-rear direction of the vehicle and a cushion support portion22on the left side at the rear portion of the body frame5.

An engine25is mounted inside the main frame portion15of the body frame5, with the crankshaft arranged along the vehicle-width (lateral) direction. The engine25is a two-cylinder engine of the narrow-angled V-type. A forward-leaning cylinder27and a rearward-leaning cylinder28are installed upright over a crankcase26. A transmission case29is continuingly formed at the rear portion of the crankcase26. At the output portion located on the left side of the transmission case29, the front-end portion of the drive shaft and the engine25are engaged with each other as being capable of transmitting the drive power.

Intake ports27aand28aare respectively formed in a rear portion of the forward-leaning cylinder27and in a front portion of the rearward-leaning cylinder28. An unillustrated throttle body is connected to each of the intake ports27aand28a. On the other hand, exhaust ports27band28bare respectively formed in a front portion of the forward-leaning cylinder27and in a rear portion of the rearward-leaning cylinder28. Base-end portions of a first and a second exhaust pipes31and32are respectively connected to the exhaust ports27band28b. The forward-leaning- and the rearward-leaning cylinders27and28of the engine25, as well as the intake- and the exhaust ports27a,28a,27band28bof these cylinders are positioned inside the body frame5in the lateral direction.

The exhaust pipes31and32, as winding appropriately, are brought below the crankcase to the right side thereof. The exhaust pipes31and32are joined together in a joint portion33to form a joint exhaust pipe34. The joint exhaust pipe34is connected to an exhaust chamber35, which is placed below the arm portion20a, and between the lower end frames14and the rear wheel19when viewed from a side. Hereafter, the part that the first and the second exhaust pipes31and32, the joint portion33, the joint exhaust pipe34constitute is referred to as an upstream-side exhaust pipe36.

A downstream-side exhaust pipe37leads out of the exhaust chamber, and is connected to a muffler38placed at the right side of the rear wheel19. The exhaust chamber35is equipped with a catalyst54dfor exhaust-gas purification, and serves as a silencer. Use of two silencers, that is, the exhaust chamber35and the muffler38, improves the sound deadening performance, while the adjustment range of the tone of the exhaust noise can be widened.

The seat frame portion16supports, in the front part thereof, the front seat41afor the rider, and supports, in the rear part thereof, a pillion seat41bfor the pillion passenger. The seat height of the front seat41ais relatively low, and the step portion between the front seat41aand the pillion seat41bis a backrest41c. A step board42for the rider is provided on each side of the vehicle body in the front part across the crankcase26and the down frame9when viewed from a side. A pillion step holder42ais attached to the middle portion, in the front-to-rear direction, of each of the right and left seat stays18to support a step for the pillion passenger. An upper-stage fuel tank43ais placed in front of the front seat41a, while a lower-stage fuel tank43bis placed under the front seat41a.

A body cover44, made of resin, appropriately covers the vehicle body of the motorcycle1. The body cover44is mainly composed of a front cowl45, a tank cover46, a front side cover47, an under cover48, and a rear side cover49. The front cowl45covers the area extending from the portion in front of the head pipe6to both sides of the head pipe6. The tank cover46is formed continuingly from the rear of the front cowl45, and covers the upper-stage fuel tank43aand the like. The front side cover47extends downwardly from the two sides of the lower portion of the front cowl45, and covers the down frames9and the like. The under cover48covers the portion that includes the two sides of the lower portion of the engine25. The rear side cover49covers the two sides of the rear portion of the seat frame portion16.

As illustrated inFIGS. 2,4and5, the first exhaust pipe31bends and extends downwardly at a position immediately in front of the exhaust port27b, and then bends and extends obliquely downwardly to the right-hand side passing between the right and the left down frames9. The first exhaust pipe31, then, curves to the rear as running round the right down frame9at the lower portion from its front to its right-hand side. After that, the first exhaust pipe31extends rearwardly as being substantially parallel to the front-to-rear direction of the vehicle at a position obliquely upwardly to the right-hand side from the right lower frame13. Hereafter, in the first exhaust pipe31, the section that curves as running round the right down frame9is named a first curve section31a.

On the other hand, the second exhaust pipe32curves to the right at a position immediately at the rear of the exhaust port28b. Then, the second exhaust pipe32extends forward passing by the right side of the two cylinders27and28. Moreover, the second exhaust pipe32, at a position located at the right side of the front-end portion of the crankcase26, curves downwardly and rearwardly so as not to project forward beyond the down frame9. After that, the second exhaust pipe32extends rearwardly as being substantially parallel to the first exhaust pipe31at a position obliquely upward to the right. Hereafter, in the second exhaust pipe32, the section that curves at the right of the crankcase26is named a second curve section32a.

The head end portions of the exhaust pipes31and32join together at the right of the lower portion of the crankcase26. To this end, for example, the head end portion of the first exhaust pipe31bend to be connected to the head end portion of the second exhaust pipe32. The head end portions of the two exhaust pipes31and32form the joint portion33, and, from the single rear end portion of the joint portion33, the joint exhaust pipe34extends slightly downwardly and the rearwardly and slightly diagonally inwardly in the lateral direction of the vehicle body.

To be more specific, the joint exhaust pipe34extends rearwardly from the rear end portion of the joint portion33as being in parallel to the front-to-rear direction. Then, the joint exhaust pipe34bends slightly downwardly to the left, inward in the lateral direction of the vehicle body, at a position immediately at the rear of the joint portion33, and extends linearly. The joint exhaust pipe34then passes by immediately right side of the right lower end frame14, with the middle section of the joint exhaust pipe34in the front-to-rear direction crossing the right lower end frame14when viewed from a side. After that, the joint exhaust pipe34bends further inwardly in the lateral direction of the vehicle body at a point at the right of the exhaust chamber35to be obliquely connected to the right end portion of the exhaust chamber35. Hereafter, in the joint exhaust pipe34, the section bending at a position immediately at the rear of the joint portion33is named a front-side bending portion34a, while the section bending at a position at the right of the exhaust chamber35is named a rear-side bending portion34b. As described above, the joint exhaust pipe34bends at a plurality of positions (specifically, at two positions), and is connected obliquely to the exhaust chamber35. As a result, the bending of the joint exhaust pipe34is made moderate to lessen the resistance to the exhaust-gas flow.

As illustrated inFIG. 3, the exhaust chamber35extends as being substantially parallel to the lateral direction with the cross-sectional shape being substantially constant. A cylindrical body35a, a left lid member35band a right lid member35c, each of which closes each one of the two sides of the cylindrical body35aextending along the lateral direction, constitute the exhaust chamber35. Hereafter, the axial line along the direction in which the exhaust chamber35extends is expressed as C1. The exhaust chamber35has a cross-sectional shape (also the side-view shape) of a substantial isosceles triangle that is placed upside down. Each of the upper face, the front lower face, and the rear lower face, constitutes each side of the triangle, and each face is a gently-curved outward-convex surface. Each of the parts corresponding to the apexes of the triangle is chamfered in a circular-arc shape which is convex outwardly.

The exhaust chamber is formed in a flattened shape with its vertical width being made smaller than the horizontal width. The exhaust chamber35slightly slants downwardly to the rear, so that the upper face of the exhaust chamber35also slightly slants downwardly to the rear. The front portion of the swing arm20, which also slightly slants downwardly to the rear, is positioned over the exhaust chamber35. Since the upper face of the exhaust chamber35slants as being substantially parallel to the swing arm20, the space needed for the up-and-down movements of the swing arm20can be easily secured.

In the exhaust chamber35, the apex at the bottom in the cross-sectional shape, which apex forms the lower end portion of the exhaust chamber35, is chamfered in a circular-arc shape that is gentler than the shapes of other apexes. The exhaust chamber35faces the ground between the front and the rear wheel of the vehicle body. By chamfering the lower end portion of the exhaust chamber35, the ground clearance corresponding to this section can be easily secured. It should be noted that a metal plate35dis fixed by fixation means such as rivets to an area around the bottom end portion of the exhaust chamber35.

A supporting stay35eis provided in the front-end portion of the exhaust chamber35as sticking forward out at the middle in the lateral direction. With this supporting stay35e, the front portion of the exhaust chamber35is supported by a lower cross member14athat bridges the right and the left lower end frames14in the body frame5.

A water-cooling type oil cooler26ahas a cylindrical-shape and extends along the front-to-rear direction and is provided to the lower rear end portion of the crankcase26(transmission case29). To the rear end of the oil cooler26a, a cartridge-type oil filter26bwith a cylindrical shape that is substantially coaxial with the oil cooler26ais detachably attached. The oil filter26bis positioned in front of the exhaust chamber35, but a space is secured in between to use when the oil filter26bis attached or detached.

The exhaust chamber35is placed as being slightly shifted to the left of the vehicle body. More specifically, the left-hand end portion of the exhaust chamber35is positioned substantially in line with the left lower frame13in the lateral direction, while the right-hand end portion of the exhaust chamber35is located at an inner position in the lateral direction of the vehicle body than the right lower frame13.

An exhaust-gas inlet51and an exhaust-gas outlet52are formed in the right-hand end portion (right-hand lid member35c) of the exhaust chamber35. The joint exhaust pipe34is connected to the exhaust-gas inlet51, while the downstream-side exhaust pipe37leads out from the exhaust-gas outlet52. In other words, the exhaust-gas inlet51and the exhaust-gas outlet52are located at inner positions in the lateral direction of the vehicle body than the right lower frame13.

FIGS. 2,4and5shows that the exhaust-gas inlet51is positioned substantially at the center of the exhaust chamber35in the front-to-rear direction. More specifically, a protruding portion51ais formed substantially at the center in the front-to-rear direction in the right-hand end portion. The protruding portion51aprojects from the right-hand lid member35ctoward the right in a tapered shape like a funnel. At the tip end portion of the protruding portion51a, the exhaust-gas inlet51is formed so as to have an opening directed obliquely right to the front. To the exhaust-gas inlet51, the downstream-side end portion of the joint exhaust pipe34is connected from a position located ahead of the exhaust-gas inlet51on the right obliquely with respect to the axial line of the chamber C1.

On the other hand, the exhaust-gas outlet52is positioned in an upper front portion of the exhaust chamber35in a position anterior to and superior to the exhaust-gas inlet51.

Here, the exhaust chamber35is positioned at the rear of the engine25when viewed from a side of the vehicle body. The muffler38is positioned at the rear of the exhaust chamber35with the exhaust port38aat the rear end portion thereof. The upstream-side exhaust pipe36is positioned between the engine25and the exhaust chamber35, while the downstream-side exhaust pipe37is positioned between the exhaust chamber35and the muffler38(exhaust port38a). The exhaust gas produced in the engine25passes through the upstream-side exhaust pipe36, the exhaust chamber35, and the downstream-side exhaust pipe37to reach the muffler38, and then is discharged to the air from the exhaust port38a.

The exhaust-gas outlet52is positioned in a side closer to the engine25(in the front side) of the exhaust chamber35, that is, in a side farther from the exhaust port38a. Meanwhile, the exhaust-gas inlet51is positioned in a side closer to the exhaust port38a(in the rear side) than the exhaust-gas outlet52is, that is, in a side farther from the engine25than the exhaust-gas outlet52is positioned. With this configuration, the exhaust pipes36and37are made longer in comparison to a case with a configuration wherein the exhaust-gas inlet51is positioned in the front side with the exhaust-gas outlet52being positioned in the rear side.

To the exhaust-gas outlet52, the upstream-side end portion of the downstream-side exhaust pipe37is connected from a position behind the exhaust-gas outlet52on the right obliquely with respect to the axial line of the chamber C1.

The downstream-side exhaust pipe37extends out from the exhaust-gas outlet52slightly obliquely, and then bends and extends further rearwardly at a position immediately above the joint exhaust pipe34. In other words, at a position where the downstream-side exhaust pipe37crosses the joint exhaust pipe34when viewed from the bottom of the vehicle body. Eventually, the downstream-side end portion of the downstream-side exhaust pipe37is connected to the front-end portion of the muffler38. In other words, the downstream-side exhaust pipe37, as a whole, extends from the exhaust-gas outlet52obliquely rearwardly to the right, that is, to a side away from the exhaust ports27band28bof the engine25. Thereafter, in the downstream-side exhaust pipe37, the section that bends at the position above the joint exhaust pipe34is named a downstream-side bending section37a. The downstream-side bending section37asubstantially lies over the rear-side bending portion34bof the joint exhaust pipe34when viewed from the bottom of the vehicle body.

The exhaust chamber35is divided, by a partition wall53that is substantially orthogonal to the exhaust chamber in the lateral direction, into a left and a right expansion chambers53aand53b. First, second, and third communication pipes54a,54band54care provided inside the exhaust chamber35. The first communication pipe54aextends from the exhaust-gas inlet51to the left, and penetrates the partition wall53to allow communication between the exhaust-gas inlet51and the left-hand expansion chamber53a. The second communication pipe54bpenetrates the partition wall53at a position anterior to the position where the first communication pipe54apenetrates the partition wall53. The second communication pipe54bthus allows the communication between the left and the right expansion chambers53aand53b. The third communication pipe54cextends from the exhaust-gas outlet52to the right and allows communication between the right expansion chamber53band the exhaust-gas outlet52. Each of the communication pipes54a,54b, and54care arranged substantially in parallel to the direction of the axial line of the chamber C1(lateral direction). A catalyst54dfor exhaust-gas purification is provided inside each of the first and the third communication pipes54aand54c.

It should be noted that the exhaust chamber35may be formed without the partition wall53as shown inFIGS. 6 and 7. In the exhaust chamber35shown inFIG. 6, the catalyst54dis provided as being continuous to the joint exhaust pipe34(upstream-side exhaust pipe36). The flow of the exhaust gas purified by the catalyst54dis reversed, and the exhaust gas is introduced into the downstream-side exhaust pipe37. On the other hand, in the exhaust chamber35shown inFIG. 7, the flow of the exhaust gas purified by the catalyst54dis reversed. The exhaust gas is purified by a second catalyst54dthat is placed at the entrance of the downstream-side exhaust pipe37, and then the exhaust gas is introduced into the downstream-side exhaust pipe37.

The muffler38extends in the front-to-rear direction slightly upwardly and rearwardly while the cross-sectional shapes are substantially constant. The muffler38is placed as substantially lying over the rear portion of the swing arm20when viewed from a side of the vehicle body. Hereafter, the axial line along the extending direction of the muffler38is represented by C2. Each of the cross-sectional shapes of the muffler38is a triangular shape. The sides of the triangular are composed of an upper face that extends substantially along the lateral direction, an inner side face that extends substantially along the vertical direction, and a slant face that extends from the right-hand end of the upper face to the bottom end of the inner side face. With this shape, a larger bank angle is secured on the right-hand side of the vehicle body. The middle portion in the front-to-rear direction of the muffler38is supported by a right-hand pillion step holder42a, which serves as a muffler stay.

In the upper right side of the front-end portion of the muffler38, a protruding portion55ais formed in a funnel shape that projects and is tapered forward from the front-end portion. At the tip end portion of the protruding portion55a, an exhaust-gas inlet55is formed as an opening directed obliquely toward the left to the front. To the exhaust-gas inlet55, the downstream-side end portion of the downstream-side exhaust pipe37is connected from a position ahead of the exhaust-gas inlet55on the left obliquely with respect to the axial line of the muffler C2.

As described above, the exhaust-gas inlet55of the muffler38is positioned on the right side, on the outer side in the lateral direction of the vehicle body. Accordingly, the exhaust-gas inlet55is positioned away from the exhaust-gas outlet52of the exhaust chamber35which is positioned at the left side of, at the inner side in the lateral direction of the vehicle body than, the muffler38. As a result, the downstream-side exhaust pipe37is elongated with compare to a case in which the exhaust-gas inlet55is positioned on the left side.

As described before, the joint exhaust pipe34is obliquely connected to the exhaust chamber35. Accordingly, a space where some component parts can be placed is produced at the right side of the rear portion of the exhaust chamber35. By utilizing the space, the front-end portion of the muffler38extends so as to overlap the rear portion of the exhaust chamber35when viewed from a side of the vehicle body. As a result, the volume of the muffler38as a whole is increased to improve the sound deadening performance.

In addition, the middle section of the upstream-side exhaust pipe36, the section of each of the first and the second exhaust pipes31and32, respectively further downstream from the curve portions31aand31bto the middle section in the front-to-rear direction of the joint exhaust pipe34, is formed as winding to the outer side of the vehicle body in the lateral direction than the body frame5, the right down frame9, the right lower frame13, the right lower end frame14. Since the upstream-side exhaust pipe36curves as being convex outward in the lateral direction of the vehicle body, the upstream-side exhaust pipe36can be elongated.

A section of the joint exhaust pipe34on the further downstream side than the front-side bending portion34ais an inward diagonal section56in which the pipe goes inward in the lateral direction of the vehicle body as the pipe goes down the stream (rearwardly) of the exhaust gas. On the other hand, the downstream-side exhaust pipe37in this embodiment, as a whole, is an outward diagonal section57in which the pipe goes outwardly in the lateral direction of the vehicle body as the pipe goes down the stream (rearwardly) of the exhaust gas. These diagonal sections56and57are placed as crossing each other when viewed from the bottom of the vehicle body at a position substantially below the seat position of the front seat41awhen viewed from a side of the vehicle body. The widths of parts of the respective diagonal sections56and57, which parts are located outside the vehicle body in the lateral direction, are made minimum at a crossing portion58where the diagonal sections cross each other when viewed from the bottom of the vehicle body. With this structure, a larger space on which the rider places his/her foot when the rider is seated on the front seat41aor when the rider climbs on the motorcycle, can be secured directly below the front seat41aand near the vehicle body.

Inside the joint exhaust pipe34, an oxygen-concentration sensor (O2sensor)59is provided between the rear-side bending portion34band the exhaust-gas inlet51to detect the oxygen concentration of the exhaust gas. The oxygen-concentration sensor59is placed with its detection unit facing the inside of the joint exhaust pipe34from above. The upwardly protruding portion of the oxygen-concentration sensor59from the joint exhaust pipe34is surrounded by the downstream-side exhaust pipe37from the front side and the right side of the protruding portion. The exhaust chamber35is positioned at the left side of the protruding portion, while the muffler38is positioned at the rear side of the protruding portion. With this structure, the oxygen-concentration sensor59is better protected, and becomes more easily heated to reach swiftly the activation temperature.

As has been described thus far, the exhaust pipe structure in the above-described embodiment includes the engine25supported by the body frame5and the upstream-side exhaust pipe36connected to the exhaust ports27band28bof the engine25. Also included in the exhaust pipe structure is the exhaust chamber35which has the exhaust-gas inlet51and the exhaust-gas outlet52, and in which the upstream-side exhaust pipe36is connected to the exhaust-gas inlet51. The exhaust pipe structure additionally includes the downstream-side exhaust pipe37which is connected to the exhaust-gas outlet52, and which extends in a direction to move away from the exhaust ports27band28b. The exhaust-gas inlet51is formed at a position farther away from the exhaust ports27band28bthan the exhaust-gas outlet52is positioned.

According to this configuration, the lengths of the upstream-side and the downstream-side exhaust pipes36and37can be made longer, in comparison to the case in which the exhaust-gas inlet51is placed closer to the exhaust ports27band28bthan the exhaust-gas outlet52is positioned. Meanwhile, the length of the exhaust pipe structure as a whole in the front-to-rear direction is restricted. As a result, the adjustment range for engine characteristics can be widened. For example, the kind of engine characteristics in which the change in engine power is made to depend linearly on the throttle-opening angle, can be made easily reproducible. This is particularly preferable in a V-type two-cylinder engine. This is because it is more effective in such an engine to elongate the distance between the joint portion33of the two exhaust pipes respectively from the cylinders27and28and the exhaust chamber35located at the downstream side from the joint portion33, more specifically, the length of the joint exhaust pipe34.

In addition, in the above-described exhaust pipe structure, the body frame5includes the head pipe6with the right and left pair of down frames9extending obliquely downwardly and rearwardly from the head pipe6. Each lower frame13is provided as extending rearwardly from the bottom end of each down frame9. Each lower end frame14is provided as extending obliquely upwardly and rearwardly from the rear end of each lower frame13. The exhaust chamber35is placed at the rear of the lower end frames14when viewed from a side of the vehicle. Accordingly, the space at the rear of the lower end frames14extending obliquely upwardly and rearwardly can be effectively utilized as a space where the exhaust chamber35is placed. As a result, a sufficient volume of the exhaust chamber35can be secured.

Moreover, in the above-described exhaust pipe structure, the exhaust ports27band28bas well as the exhaust-gas inlet51are placed inside the body frame5in the lateral direction, while the middle section of the upstream-side exhaust pipe36is made to be located outside of the body frame5in the lateral direction. Accordingly, the upstream-side exhaust pipe36can be made to curve as being convex outwardly of the vehicle body. As a result, the adjustment range for engine characteristics can be widened.

Furthermore, in the above-described exhaust pipe structure, the upstream-side exhaust pipe36includes the inward diagonal section56in which the pipe goes inward in the lateral direction of the vehicle body as the pipe goes down the stream of the exhaust gas. Meanwhile, the downstream-side exhaust pipe37includes the outward diagonal section57in which the pipe goes outward in the lateral direction of the vehicle body as the pipe goes down the stream of the exhaust gas. The inward diagonal section56and the outward diagonal section57cross each other when viewed from the bottom of the vehicle body at a position below the front seat41asupported by the body frame5. Accordingly, the projection, outside of the vehicle body, of the upstream-side and the downstream-side exhaust pipes36and37is made small in the crossing portion58of the inward diagonal section56and the outward diagonal section57when viewed from the bottom of the vehicle body. As a result, the rider can plant his/her foot more easily on the ground by widening the space on which the rider can place his/her foot below the front seat41a.

It should be noted that the present invention is not limited to the above-described embodiment. The present invention can be applied, for example, to a single-cylinder engine or to various types of in-line engines. In addition, the present invention can be applied to a three-wheel, or a four-wheel vehicle. Moreover, the present invention can be carried out with the exhaust chamber35being the only silencer that the vehicle has. Furthermore, the present invention can have a configuration in which a part of the downstream-side exhaust pipe37is made to be the outward diagonal section57.

The configuration in the above-described embodiment is only an example of the present invention. It is obvious that various changes may be made without departing from the scope of the invention.