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An exhaust pipe connected to an exhaust port of an engine and an expansion chamber is disposed in the vicinity of the engine. A main tube is provided that extends rearwardly from a head pipe disposed at the front side of a main body. An engine having a cylinder extending upwardly from a crankcase is

An exhaust pipe connected to an exhaust port of an engine and an expansion chamber is disposed in the vicinity of the engine. A main tube is provided that extends rearwardly from a head pipe disposed at the front side of a main body. An engine having a cylinder extending upwardly from a crankcase is disposed under the main tube with an exhaust port open at the front side of the cylinder. A first exhaust pipe is provided that is connected to the exhaust port. A down tube is disposed on the center line in the left-right direction of the main body. The exhaust port disposed behind the down tube is deviatingly open at one side from the center line. The first exhaust pipe extends forward from one side of the down tube, and to the other side of the down tube, subsequently turns, and extends again to one side.

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1. An exhaust structure of a motorcycle, in which a main tube is provided for extending rearwardly from a head pipe disposed at a front side of a main body, an engine is disposed under the main tube, the engine includes a cylinder extending upwardly from a crankcase, a down tube is disposed on a cen

1. An exhaust structure of a motorcycle, in which a main tube is provided for extending rearwardly from a head pipe disposed at a front side of a main body, an engine is disposed under the main tube, the engine includes a cylinder extending upwardly from a crankcase, a down tube is disposed on a center line in a left-right direction of the main body, a lower frame extending downwardly from the down tube and having left and right side lower frames which are parallel to each other and which extend rearwardly under the engine, the exhaust structure comprising:an exhaust port open at a front side of the cylinder, the exhaust port being disposed behind the down tube and being deviatingly open at a right side from the center line in the left-right direction of the main body,a first exhaust pipe connected to the exhaust port and extending downwardly and forwardly at an angle to the right side of the main body;a first expansion chamber directly connected to a downstream end of the first exhaust pipe;a second expansion pipe directly connected to a downstream end of the first exhaust chamber via a joint, the joint being at a position directly above a clutch storage portion protruding to the right side in the crankcase and near a lower portion of a throttle body, anda muffler having a second expansion chamber directly connected to a downstream end of the second exhaust pipe,wherein the first exhaust pipe includes at least first, second, third, and fourth bending portions,the first exhaust pipe connected to the exhaust port bends and extends down toward a left side of the main body at the first bending portion in front of the down tube, andthen the first exhaust pipe bends along a curve of the lower frame at the second bending portion at a front side of the lower frame at the left side of the main body, andthen the first exhaust pipe extends rearwardly and thereafter extends to the front side of the main body while being bent at the third bending portion to an inside of the main body, andthen the first exhaust pipe bends to an upper one side of the main body at the fourth bending portion under the first bending portion, and is connected to the first expansion chamber,wherein the third bending portion has a U-shape, is arranged on the center line in the left-right direction of the main body, and opens toward the front of the motorcycle, and portions of the first exhaust pipe extending forwardly from left and right sides of the U-shaped third bending portion are arranged between and are substantially parallel to portions of the left and right side lower frames extending rearwardly under the engine, andwhen the first expansion chamber is viewed in side view, an upper-most portion of the first expansion chamber can be seen to be disposed entirely below where a head cover is attached to a cylinder head of the engine,wherein the first expansion chamber has an inner sidewall facing the cylinder head and the exhaust port, and at least a portion of the inner side wall facing the cylinder head is substantially flat. 2. The exhaust structure of a motorcycle according to claim 1, wherein the inner sidewall of the first expansion chamber is inclined to be widened outside toward a front side, a portion of the first exhaust pipe which extends forward from the exhaust port is widened outward accordingly as it extends toward the front side along the inner sidewall, andwherein the first exhaust pipe is connected to the first expansion chamber on the right side of the down tube at a position that is lower than either the exhaust port or the third bending portion, and when the engine is viewed in a right side view, the first expansion chamber is seen to be positioned entirely below the head cover and to be mounted at an angle sloping upwardly and rearwardly from front to back thereof. 3. The exhaust structure of a motorcycle according to claim 1, wherein the second exhaust pipe extends rearwardly and substantially horizontally from the joint with the first expansion chamber, through an inner side of a pivot plate connected to a rear end of the main tube, and then continues rearwardly and substantially horizontally and is connected to the muffler which is disposed behind the pivot plate. 4. The exhaust structure of a motorcycle according to claim 1, wherein the exhaust port is open to a front side of a cylinder head behind the down tube and faces down at an angle at a right side of the main body. 5. The exhaust structure of a motorcycle according to claim 4, wherein the exhaust port is positioned higher relative to a joint of the down tube and a lower frame in the height direction of the main body and positioned in the vicinity of a middle portion in an up-down direction of the down tube. 6. The exhaust structure of a motorcycle according to claim 1, wherein the first exhaust pipe includes a first pipe, a second pipe and a third pipe, the second pipe is larger in diameter relative to the first pipe, and the third pipe is larger in diameter relative to the second pipe. 7. The exhaust structure of a motorcycle according to claim 6, wherein the diameter of the first expansion chamber is larger than the third pipe wherein exhaust gas flowing into the first expansion chamber expands and a pressure drop occurs. 8. The exhaust structure of a motorcycle according to claim 1, wherein as a diameter of the first exhaust pipe increases in stages from an upper stream to a lower stream of the exhaust structure, resistance to exhaust gas is reduced. 9. An exhaust structure adapted to be used with a motorcycle, in which a main tube is provided for extending rearwardly from a head pipe disposed at a front side of a main body, an engine is disposed under the main tube, the engine includes a cylinder extending upwardly from a crankcase, a down tube is disposed on a center line in a left-right direction of the main body, a lower frame extending downwardly from the down tube and having left and right side lower frames which are parallel to each other and which extend rearwardly under the engine, the exhaust structure comprising: an exhaust port open at a front side of the cylinder, the exhaust port being disposed behind the down tube and being deviatingly open at the right side from the center line in the left-right direction of the main body,a first exhaust pipe connected to the exhaust port and extending downwardly and forwardly at an angle to the right side of the main body;a first expansion chamber directly connected to a downstream end of the first exhaust pipe;a second expansion pipe directly connected to a downstream end of the first exhaust chamber via a joint, the joint being at a position directly above a clutch storage portion protruding to the right side in the crankcase and near a lower portion of a throttle body, anda muffler having a second expansion chamber directly connected to a downstream end of the second exhaust pipe,wherein the first exhaust pipe includes at least first, second, third, fourth, and fifth bending portions,wherein the first exhaust pipe adapted to be connected to the exhaust port extends down forward of the main body, bends toward a left side of the main body at the first bending portion in front of the down tube, then extends downward toward the left side of the main body substantially horizontally across the center line in a left-right direction of the main body,the first exhaust pipe bends upwardly at an angle to the left side of the main body at the second bending portion in front of the left side lower frame at the left side of the main body, then turns in a U-shape at the third bending portion and then extends toward the right side of the main body,the first exhaust pipe bends toward the right side of the main body at the fourth bending portion in front of the lower frame at the left side of the main body and then extends substantially horizontally under the second bending portion, thereafter the first exhaust pipe bends to the right side of the main body at the fifth bending portion, and is connected to a first expansion chamber disposed at a side of the cylinder at the right side,wherein the third bending portion has a U-shape, is arranged on the left side of the main body, and opens in a forward direction that slopes downwardly and at an angle toward the right side relative to a length of the motorcycle, and portions of the first exhaust pipe extending forwardly from left and right sides of the U-shaped third bending portion are substantially parallel to each other,wherein the second exhaust pipe extends rearwardly and substantially horizontally from the joint with the first expansion chamber, through an inner side of a pivot plate connected to a rear end of the main tube, and then continues rearwardly and substantially horizontally and is connected to a second expansion chamber of a muffler which is disposed behind the pivot plate. 10. The exhaust structure of a motorcycle according to claim 9, wherein an inner sidewall of the first expansion chamber is inclined to be widened outside toward a front side, a portion of the first exhaust pipe which extends forward from the exhaust port is widened outward accordingly as it extends toward the front side along the inner sidewall, wherein the first exhaust pipe is connected to the first expansion chamber on the right side of the down tube at a position that is lower than either the exhaust port or the third bending portion, and when the engine is viewed in a right side view, the first expansion chamber is seen to be positioned below a head cover and to be mounted at an angle sloping upwardly and rearwardly from front to back thereof. 11. The exhaust structure of a motorcycle according to claim 9, wherein the second exhaust pipe is connected to a rear end of the first expansion chamber at a position directly above a clutch storage portion protruding to the right side in the crankcase, wherein when the engine is viewed in a front view,a portion of the first exhaust pipe extending between the exhaust port and the third bending portion can be seen to extend from the right side of the down tube, across a front of the down tube, and to the left side of the down tube, anda portion of the first exhaust pipe extending from the third bending portion to the first expansion chamber can be seen to extend from the left side of the down tube, across a front of the left and right side lower frames connected to a lower end of the down tube, and to the right side of the down tube, wherein the first exhaust pipe is arranged entirely above portions of the lower frames extending under the engine. 12. The exhaust structure of a motorcycle according to claim 9, wherein the exhaust port is positioned higher relative to a joint of the down tube and the lower frame in the height direction of the main body and positioned in the vicinity of a middle portion in an up-down direction of the down tube. 13. The exhaust structure of a motorcycle according to claim 9, wherein the first exhaust pipe includes a first pipe, a second pipe and a third pipe, the second pipe is larger in diameter relative to the first pipe, and the third pipe is larger in diameter relative to the second pipe. 14. The exhaust structure of a motorcycle according to claim 13, wherein the diameter of the first expansion chamber is larger than the third pipe, and wherein exhaust gas flowing into the first expansion chamber expands and a pressure drop occurs. 15. The exhaust structure of a motorcycle according to claim 9, wherein as a diameter of the first exhaust pipe increases in stages from an upper stream to a lower stream of the exhaust structure, resistance to exhaust gas is reduced.