Power transmission apparatus

A power transmission apparatus includes a shift drum, a clutch, a clutch lifter, and a transmission body. The shift drum makes a dowel be extracted/inserted between a shifter and a shift gear. The clutch has first friction plates and second friction plates. The first friction plates rotate around a main shaft by receiving power from a crankshaft. The second friction plates are disposed alternately with the first friction plates and are relatively non-rotatably supported by the main shaft. The clutch lifter is displaced between a connection position at which power is transmitted by the clutch and a disconnection position at which the transmission of the power is disconnected. The transmission body transmits a driving force to the shift drum while the clutch lifter moves from the connection position to the disconnection position, in accordance with rotation of a shift spindle that rotates in accordance with a driving force.

BACKGROUND

1. Technical Field

The present invention relates to a power transmission apparatus including a main shaft, a counter shaft, a shift drum, a clutch, a clutch lifter, and a shift spindle. The main shaft rotates on a first rotation axis. The counter shaft rotates on a second rotation axis parallel to the first rotation axis. The shift drum rotates to drive a shifter of at least one of the main shaft or the counter shaft and makes a dowel be extracted or inserted so that a shift gear will be relatively non-rotatably joined to the at least one of the main shaft or the counter shaft, thereby selectively establishing a speed stage between the main shaft and the counter shaft. The clutch has driving-side first friction plates and driven-side second friction plates. The first friction plates are supported rotatable around the main shaft and rotate by power transmitted from a crankshaft. The second friction plates are disposed alternately with the first friction plates and are relatively non-rotatably supported by the main shaft. The clutch lifter is displaced between a connection position and a disconnection position. The clutch lifter at the connection position makes the first friction plates and the second friction plates be mutually pressed so as to transmit power at the maximum. The clutch lifter at the disconnection position makes the first friction plates and the second friction plates be mutually separated so as to disconnect transmission of the power. The shift spindle rotates in accordance with a driving force supplied from an electric motor and thereby applies a driving force to the clutch lifter.

2. Description of the Background

Patent literature 1 discloses a shift spindle that rotates in accordance with a driving force supplied from a shift motor and thereby applies a driving force to a clutch lifter. The clutch lifter is displaced between a connection position and a disconnection position. The clutch lifter at the connection position makes alternately arranged driving-side first friction plates and driven-side second friction plates be mutually pressed so as to transmit power at the maximum. The clutch lifter at the disconnection position makes the first friction plates and the second friction plates be mutually separated so as to disconnect transmission of the power. The first friction plates are relatively non-rotatably supported by a clutch outer that rotates by power transmitted from a crankshaft. The second friction plates are relatively non-rotatably supported by a main shaft that is incorporated in a transmission. When the clutch lifter reaches the disconnection position in accordance with rotation of the shift spindle, a shifter arm is displaced in response to rotation of a shift drum, and a dowel is extracted between a shift gear and a shifter. As a result, gear shift operation is started.

CITATION LIST

Patent Literature

BRIEF SUMMARY

When the clutch lifter starts to move from the connection position to the disconnection position, the first friction plates and the second friction plates come to slide on one another. The power that is transmitted from the crankshaft to the main shaft becomes reduced. Thus, the power, which is output from the counter shaft and is to be transmitted to a driving wheel, is partially lost or is reduced. The period of time of losing the power is desirably shortened to the least.

The present invention has been achieved in view of these circumstances, and an object of the present invention is to provide a power transmission apparatus in which loss of power output from a counter shaft is reduced to the least degree.

A first aspect of the present invention provides a power transmission apparatus including a main shaft, a counter shaft, a shift drum, a clutch, a clutch lifter, a shift spindle, and a transmission body. The main shaft is configured to rotate on a first rotation axis. The counter shaft is configured to rotate on a second rotation axis parallel to the first rotation axis. The shift drum is configured to rotate to drive a shifter of at least one of the main shaft or the counter shaft and to make a dowel be extracted or inserted so that a shift gear is relatively non-rotatably joined to the at least one of the main shaft or the counter shaft, thereby selectively establishing a speed stage between the main shaft and the counter shaft. The clutch includes driving-side first friction plates and driven-side second friction plates. The first friction plates are rotatably supported around the main shaft and are configured to rotate by power that is transmitted from a crankshaft. The second friction plates are disposed alternately with the first friction plates and are relatively non-rotatably supported by the main shaft. The clutch lifter is configured to be displaced between a connection position and a disconnection position. The clutch lifter at the connection position is configured to make the first friction plates and the second friction plates be mutually pressed so as to transmit power at the maximum. The clutch lifter at the disconnection position is configured to make the first friction plates and the second friction plates be mutually separated so as to disconnect transmission of the power. The shift spindle is configured to rotate in accordance with a driving force supplied from an electric motor and thereby apply a driving force to the clutch lifter. The transmission body is configured to transmit a driving force to the shift drum in accordance with rotation of the shift spindle while the clutch lifter moves from the connection position to the disconnection position.

In a second aspect, in addition to the configuration of the first aspect, the shift spindle may be configured to drive the clutch lifter to the disconnection position.

In a third aspect, in addition to the configuration of the first or the second aspect, ignition may be cut in an internal combustion engine that supplies power to the crankshaft, in rotating the shift drum.

In a fourth aspect, in addition to the configuration of the first or the second aspect, an injection amount of fuel may be reduced in the internal combustion engine that supplies power to the crankshaft, in rotating the shift drum.

In a fifth aspect, in addition to the configuration of the third or the fourth aspect, the power transmission apparatus may further include a throttle valve. The throttle valve may be provided to an intake path of the internal combustion engine and be coupled to an operation member to be used for throttle operation, by a wire.

In a sixth aspect, in addition to the configuration of any one of the first to the fifth aspects, the power transmission apparatus may further include a shift arm, a master arm, and a lost motion elastic body. The shift arm may be fixed to the shift spindle so as to swing around a rotation axis of the shift spindle and may be coupled directly to a clutch lifter lever that is coupled to the clutch lifter. The master arm may be supported in a manner swingable around the rotation axis of the shift spindle and may be configured to cause rotation of the shift drum. The lost motion elastic body may have an elasticity that provides a driving force for following swing of the shift arm, to the master arm.

In a seventh aspect, in addition to the configuration of the sixth aspect, the transmission body may be configured to come into contact with the master arm by receiving an elastic force of the lost motion elastic body, when the shift spindle rotates in a first direction around the rotation axis in shifting up. In addition, the shift arm may be configured to come into contact with the master arm after moving by a predetermined play angle, when the shift spindle rotates in a second direction opposite to the first direction, around the rotation axis in shifting down.

In the first aspect, although the clutch lifter does not yet reach the disconnection position, the first friction plates and the second friction plates start to slide on each other, whereby transmission of torque between the main shaft and the counter shaft is reduced accordingly. As a result, the dowel is extracted or inserted by the action of the driving force applied to the shift drum. Thus, a gear shift operation is completed in a half-clutch state. This requires only small rotation amount of the shift spindle and only a short time for the clutch to move between the connection position and the disconnection position, whereby loss of power from the counter shaft is reduced.

In the second aspect, when the extraction or insertion of the dowel is not completed in the half-clutch state, the shift spindle drives the clutch lifter to the disconnection position. As a result, the clutch is disconnected, and the dowel is extracted or inserted. Thus, the gear shift operation of the transmission is completed.

In the third aspect, although the clutch lifter does not yet reach the disconnection position, power that is supplied from the crankshaft is reduced, and transmitted torque is decreased. This facilitates extraction and insertion of the dowel.

In the fourth aspect, although the clutch lifter does not yet reach the disconnection position, power that is supplied from the crankshaft is reduced, and transmitted torque is decreased. This facilitates extraction and insertion of the dowel.

In the fifth aspect, the configuration is inexpensive but can reduce the time of the gear shift operation.

In the sixth aspect, rotation of the shift spindle makes the shift arm swing, and the clutch lifter lever starts to drive the clutch lifter accordingly. While the master arm receives the driving force from the lost motion elastic body, rotation of the shift drum is prevented, and the master arm is restrained from swinging, unless the driving force of the lost motion elastic body exceeds the joining force of the dowel. In this situation, the shift spindle continues to rotate further, and the clutch lifter also rotates further. This results in accumulation of an elastic force in the lost motion elastic body in accordance with rotation of the shift spindle. Then, transmission of torque between the first friction plates and the second friction plates is reduced as the clutch lifter is displaced. The elastic force that is accumulated in the lost motion elastic body comes to exceed the joining force of the dowel, whereby the shift drum starts to rotate. Thus, the gear shift operation is rapidly completed after the clutch comes to the half-clutch state.

In the seventh aspect, the shift drum rotates by the elastic force accumulated in the lost motion elastic body in shifting up. Thus, the gear shift operation is rapidly completed after the clutch comes to the half-clutch state. While the torque required to extract or insert the dowel increases in accordance with back torque in shifting down, it is difficult to reduce the back torque at a desired timing without operation of a passenger. In view of this, the shift drum is configured to rotate after the clutch lifter reaches the disconnection position. This reduces the burden on the components such as the dowel and the shifter in performing the gear shift operation.

DETAILED DESCRIPTION

An embodiment of the present invention will be described with reference to the attached drawings hereinafter. An up-down direction, a front-rear direction, and a right-left direction of a vehicle body are defined based on a line of sight of a passenger riding on a small vehicle.

FIG. 1schematically shows a configuration of a saddled four-wheeled buggy11as a small vehicle of an embodiment of the present invention. The four-wheeled buggy11includes a vehicle body frame12made of steel pipes that are assembled by welding to one another. The front part of the vehicle body frame12supports right and left front wheels WF in a manner rotatable around a horizontal axis. The rear part of the vehicle body frame12supports right and left rear wheels WR in a manner rotatable around a horizontal axis. The front wheels WF are coupled to a bar handle13. The bar handle13is rotatably supported by the vehicle body frame12and turns the axle of the front wheels WE A fuel tank14for storing fuel is supported by the vehicle body frame12on a rear side of the bar handle13. A passenger seat15to be ridden by a passenger is supported by the vehicle body frame12on a rear side of the fuel tank14.

A power unit16is mounted on the vehicle body frame12between the front wheels WF and the rear wheels WR. The power unit16extends frontward to be coupled to the axle of the front wheels WF via a differential and also extends rearward to be coupled to the axle of the rear wheels WR via a differential. The front wheels WF and the rear wheels WR are driven based on power supplied from the power unit16.

As shown inFIG. 2, a left grip17is fixed at a left end of the bar handle13. In a range reachable by a finger of a hand that is gripping the left grip17, a starter switch18, a stop switch19, a shift-up switch21, and a shift-down switch22are arranged. The starter switch18activates the power unit16. The stop switch19stops the power unit16. The shift-up switch21causes shifting up in the transmission (described later) incorporated in the power unit16. The shift-down switch22causes shifting down in the transmission.

A right grip23is attached to a right end of the bar handle13in a manner rotatable around an axial center. The right grip23functions as an operation member for operating a throttle. A passenger is allowed to adjust output of the power unit16by handling the right grip23.

As shown inFIG. 3, the power unit16includes an internal combustion engine25that generates power in accordance with combustion of fuel supplied from the fuel tank14. The internal combustion engine25includes a crank case25a, a cylinder block25b, a cylinder head25c, and a head cover25d. The crank case25asupports a crankshaft26in a manner rotatable on a rotation axis Rx extending in the front-rear direction of the vehicle body. The cylinder block25bis joined to the crank case25aand defines a cylinder bore28that guides linear reciprocation of a piston27along a cylinder axis C. The cylinder head25cis joined to the cylinder block25band closes the cylinder bore28. The head cover25dis joined to the cylinder head25cand covers the cylinder head25c. The piston27is coupled to a crank of the crankshaft26by a connecting rod29. The linear reciprocation of the piston27is converted into rotation motion of the crankshaft26. A combustion chamber31is defined between the cylinder head25cand the piston27. An ignition plug32is attached to the cylinder head25cfrom a ceiling of the combustion chamber31and has a tip facing the combustion chamber31.

The cylinder head25cis formed with an intake passage33that opens to the combustion chamber31. The intake passage33is opened and closed by the action of an intake valve34. The intake valve34is axially displaceably supported by the cylinder head25c. The cylinder head25cis also formed with an exhaust passage35that opens to the combustion chamber31. The exhaust passage35is opened and closed by the action of an exhaust valve36. The exhaust valve36is axially displaceably supported by the cylinder head25c.

A throttle body37is joined to the cylinder head25cand defines a flow path37athat is communicated with the intake passage33. A butterfly throttle valve38is disposed to the throttle body37and opens and closes the flow path37a. The throttle valve38is connected to the right grip23by a wire (not shown). Handling the right grip23causes axial displacement of the wire. The throttle valve38is opened in accordance with the axial displacement of the wire.

A fuel injection valve39for injecting fuel to air that flows through the flow path37ais attached to the throttle body37. The injected fuel generates air-fuel mixture. The fuel injection valve39is supplied with the fuel from the fuel tank14. The injection amount of the fuel is set by an electronic control unit (ECU), for example.

In the internal combustion engine25, the piston27repeatedly undergoes an intake stroke, a compression stroke, a combustion stroke, and an exhaust stroke, in this order, and thereby reciprocates linearly.

In the intake stroke, the piston27moves down in accordance with rotation of the crankshaft26based on an inertial force. This increases the capacity of the combustion chamber31. The intake valve34is made to open, whereby the air-fuel mixture is introduced from the intake passage33into the combustion chamber31.

In the compression stroke, the piston27moves up while the intake valve34and the exhaust valve36are closed. This decreases the capacity of the combustion chamber31. Thus, the air-fuel mixture is compressed. In the combustion stroke, the air-fuel mixture is ignited by the ignition plug32. Combustion of the air-fuel mixture lowers the piston27. As a result, power is generated. In the exhaust stroke, the exhaust valve36opens, and the piston27moves up. This discharges exhaust gas from the combustion chamber31to the exhaust passage35.

The power unit16includes a multistage transmission42. The multistage transmission42is incorporated in the crank case25aand transmits power from the crankshaft26to an output shaft41. The multistage transmission42includes a main shaft43and a counter shaft44. The main shaft43is supported by the crank case25ain a manner rotatable on a rotation axis Mx that is parallel to the rotation axis Rx of the crankshaft26. The counter shaft44is supported by the crank case25ain a manner rotatable on a rotation axis Cx that is parallel to the rotation axis Mx of the main shaft43. Selectively establishable gear trains of multiple speed stages are arranged between the main shaft43and the counter shaft44. The gear trains include a first-speed gear train G1, a second-speed gear train G2, a third-speed gear train G3, a fourth-speed gear train G4, a fifth-speed gear train G5, and a sixth-speed gear train G6.

The first-speed gear train G1includes a driving gear45aand a driven gear45b. The driving gear45ais fitted to the main shaft43in a manner relatively rotatable on the same axis as the main shaft43. The driven gear45bis fixed to the counter shaft44in a manner relatively non-rotatable on the same axis as the counter shaft44and meshes with the driving gear45a. The driving gear45ais coupled to the main shaft43in a manner relatively non-displaceable in the axial direction of the main shaft43.

The sixth-speed gear train G6includes a driving gear46aand a driven gear46b. The driving gear46ais joined to the main shaft43in a manner relatively non-rotatable on the same axis as the main shaft43. The driven gear46bis fitted to the counter shaft44in a manner relatively rotatable on the same axis as the counter shaft44and meshes with the driving gear46a. The driving gear46ais joined to a first shifter47that is joined to the main shaft43by a spline in a manner relatively displaceable in the axial direction. The driven gear46bis coupled to the counter shaft44in a manner relatively non-displaceable in the axial direction of the counter shaft44. Upon moving in a first axial direction MR1from a neutral position, the first shifter47is coupled to the driving gear45aof the first-speed gear train G1by its dowel. This results in transmission of power from the main shaft43to the counter shaft44via the first-speed gear train G1. Thus, the first speed stage is established.

The fourth-speed gear train G4includes a driving gear48aand a driven gear48b. The driving gear48ais fitted to the main shaft43in a manner relatively rotatable on the same axis as the main shaft43. The driven gear48bis fitted to the counter shaft44in a manner relatively non-rotatable on the same axis as the counter shaft44and meshes with the driving gear48a. The driving gear48ais coupled to the main shaft43in a manner relatively non-displaceable in the axial direction of the main shaft43. The driven gear48bis joined to a second shifter49that is joined to the counter shaft44by a spline in a manner relatively displaceable in the axial direction. Upon moving in the first axial direction MR1from a neutral position, the second shifter49is coupled to the driven gear46bof the sixth-speed gear train G6by its dowel. This results in transmission of power from the main shaft43to the counter shaft44via the sixth-speed gear train G6. Thus, the sixth speed stage is established.

On the other hand, while the second shifter49stops at the neutral position, and the driving gear48aand the driven gear48bof the fourth-speed gear train G4mesh with each other, the first shifter47is coupled to the driving gear48aof the fourth-speed gear train G4by its dowel upon moving from the neutral position in a second axial direction MR2opposite to the first axial direction MR1. This results in transmission of power from the main shaft43to the counter shaft44via the fourth-speed gear train G4. Thus, the fourth speed stage is established.

The second-speed gear train G2includes a driving gear51aand a driven gear51b. The driving gear51ais coaxially fixed to the main shaft43. The driven gear51bis fitted to the counter shaft44in a manner relatively rotatable on the same axis as the counter shaft44and meshes with the driving gear51a. The driving gear51ais integrally formed with the main shaft43. The driven gear51bis coupled to the counter shaft44in a manner relatively non-displaceable in the axial direction of the counter shaft44. Upon moving in the second axial direction MR2from the neutral position, the second shifter49is coupled to the driven gear51bof the second-speed gear train G2by its dowel. This results in transmission of power from the main shaft43to the counter shaft44via the second-speed gear train G2. Thus, the second speed stage is established.

The third-speed gear train G3includes a driving gear52aand a driven gear52b. The driving gear52ais joined to the main shaft43in a manner relatively non-rotatable on the same axis as the main shaft43. The driven gear52bis fitted to the counter shaft44in a manner relatively rotatable on the same axis as the counter shaft44and meshes with the driving gear52a. The driving gear52ais supported by the main shaft43in a manner non-displaceable in the axial direction of the main shaft43. The driven gear52bis coupled to the counter shaft44in a manner relatively non-displaceable in the axial direction of the counter shaft44. Similarly, the fifth-speed gear train G5includes a driving gear53aand a driven gear53b. The driving gear53ais joined to the main shaft43in a manner relatively non-rotatable on the same axis as the main shaft43. The driven gear53bis fitted to the counter shaft44in a manner relatively rotatable on the same axis as the counter shaft44and meshes with the driving gear53a. The driving gear53ais supported by the main shaft43in a manner non-displaceable in the axial direction of the main shaft43. The driven gear53bis coupled to the counter shaft44in a manner relatively non-displaceable in the axial direction of the counter shaft44.

The counter shaft44supports a third shifter54that is joined thereto by a spline in a manner relatively displaceable in the axial direction of the counter shaft44. Upon moving in the first axial direction MR1from a neutral position, the third shifter54is coupled to the driven gear52bof the third-speed gear train G3by its dowel. This results in transmission of power from the main shaft43to the counter shaft44via the third-speed gear train G3. Thus, the third speed stage is established. On the other hand, upon moving in the second axial direction MR2from the neutral position, the third shifter54is coupled to the driven gear53bof the fifth-speed gear train G5by its dowel. This results in transmission of power from the main shaft43to the counter shaft44via the fifth-speed gear train G5. Thus, the fifth speed stage is established.

A driving gear55is fitted to the crankshaft26in a manner relatively rotatable around the rotation axis Rx of the crankshaft26. A start clutch56is arranged between the crankshaft26and the driving gear55. The start clutch56includes a clutch inner56a, a clutch outer56b, and a clutch shoe56c. The clutch inner56ais relatively non-rotatably fixed to the crankshaft26. The clutch outer56bis relatively non-rotatably coupled to the driving gear55so as to surround the clutch inner56aaround the crankshaft26. The clutch shoe56cis supported by the clutch inner56ain a manner swingable on a swing axis parallel to the rotation axis Rx of the crankshaft26. As the number of rotation of the crankshaft26increases, the clutch shoe56cswings in a direction away from the rotation axis Rx by the action of a centrifugal force acting on a flyweight and comes into frictional contact with the clutch outer56b. The driving gear55is automatically joined to the crankshaft26in accordance with increase in the number of rotation.

A driven gear57is fitted to the main shaft43in a manner relatively rotatable around the rotation axis Mx of the main shaft43. The driven gear57meshes with the driving gear55on the crankshaft26. A gear shift clutch58is arranged between the main shaft43and the driven gear57. The gear shift clutch58includes a clutch inner58aand a clutch outer58b. The clutch inner58ais relatively non-rotatably fixed to the main shaft43. The clutch outer58bis coupled to the driven gear57around the rotation axis Mx so as to surround the clutch inner58aaround the main shaft43. The clutch outer58bsupports multiple first friction plates59in a manner relatively non-rotatable around the main shaft43and axially displaceable. The first friction plates59are supported rotatably around the main shaft43and rotate by power that is transmitted from the crankshaft26. The clutch inner58asupports multiple second friction plates61in a manner relatively non-rotatable around the main shaft43and axially displaceable. The first friction plates59and the second friction plates61are alternately disposed in the axial direction of the main shaft43.

The clutch inner58aaxially displaceably supports a driving plate62. The first friction plates59and the second friction plates61are arranged in the axial direction between the driving plate62and a pressure receiving plate63of the clutch inner58a. The driving plate62is driven in a direction toward the pressure receiving plate63by the action of a coil spring64. As a result, the first friction plates59and the second friction plates61are alternately put on each other, whereby the clutch outer58bis joined to the clutch inner58a.

A clutch lifter65is coupled to the driving plate62in a manner relatively rotatable around the rotation axis Mx of the main shaft43. The clutch lifter65is displaced between a connection position and a disconnection position. The clutch lifter65at the connection position makes the first friction plates59and the second friction plates61be mutually pressed so as to transmit power at the maximum. The clutch lifter65at the disconnection position makes the first friction plates59and the second friction plates61be mutually separated so as to disconnect transmission of the power.

An output gear66ais joined to the counter shaft44in a manner relatively non-rotatable on the same axis as the counter shaft44. The output gear66ameshes with a driven gear66bthat is coaxially fixed to the output shaft41. Thus, power of the crankshaft26is transmitted from the counter shaft44to the output shaft41.

As shown inFIG. 4, the multistage transmission42includes a gear shift control system42afor sequentially switching the speed stages among the neutral stage, the first speed stage, the second speed stage, the third speed stage, the fourth speed stage, the fifth speed stage, and the sixth speed stage, in this order. The gear shift control system42aincludes a shift drum67and a guide shaft69. The shift drum67rotates on a rotation axis Dx parallel to the rotation axis Mx of the main shaft43and the rotation axis Cx of the counter shaft44. The guide shaft69guides displacement of a first shift fork68a, a second shift fork68b, and a third shift fork68cin parallel to the rotation axis Dx of the shift drum67. The shift drum67includes a first cam groove71a, a second cam groove71b, and a third cam groove71cthat are formed on an outer circumferential surface. The first cam groove71aengages with the first shift fork68aand displaces the first shift fork68aalong the guide shaft69in accordance with rotation of the shift drum67. The second cam groove71bengages with the second shift fork68band displaces the second shift fork68balong the guide shaft69in accordance with rotation of the shift drum67. The third cam groove71cengages with the third shift fork68cand displaces the third shift fork68calong the guide shaft69in accordance with rotation of the shift drum67. The first cam groove71aand the second cam groove71bhave structures similar to the structure of the third cam groove71c. For this reason, the first shift fork68aand the second shift fork68bare omitted inFIG. 4.

As shown inFIG. 3, the first shift fork68ais coupled to the first shifter47in a manner relatively rotatable around the main shaft43. Displacement of the first shift fork68ain accordance with rotation of the shift drum67causes displacement in the axial direction of the first shifter47on the main shaft43. The second shift fork68bis coupled to the second shifter49in a manner relatively rotatable around the counter shaft44. Displacement of the second shift fork68bin accordance with rotation of the shift drum67causes displacement in the axial direction of the second shifter49on the counter shaft44. The third shift fork68cis coupled to the third shifter54in a manner relatively rotatable around the counter shaft44. Displacement of the third shift fork68cin accordance with rotation of the shift drum67causes displacement in the axial direction of the third shifter54on the counter shaft44. The shift drum67rotates to drive the first shifter47of the main shaft43and to drive the second shifter49and the third shifter54of the counter shaft44. This makes the driving gear and the driven gear be relatively non-rotatably joined to each other between the main shaft43and the counter shaft44in accordance with extraction or insertion of the dowel. Thus, the shift drum67selectively establishes the speed stage between the main shaft43and the counter shaft44.

As shown inFIG. 4, the shift drum67is connected to a potentiometer72. The potentiometer72measures a rotation angle around the rotation axis Dx of the shift drum67. The speed stage (e.g. the neutral stage, the first speed stage, and the second speed stage) of the multistage transmission42is determined in accordance with the rotation angle. The determination of the speed stage can be performed by other means.

As shown inFIGS. 4 and 5, the gear shift control system42aincludes a shift spindle74. The shift spindle74rotates on a shaft center (rotation axis) Sx in accordance with a driving force that is supplied from the electric motor73and thereby applies a driving force to the clutch lifter65. The electric motor73generates a driving force around a driving shaft in accordance with supply of electric power. The driving force of the electric motor73is transmitted to the shift spindle74via a reduction gear mechanism. The shaft center Sx of the shift spindle74is arranged in parallel to the rotation axis Mx of the main shaft43and the rotation axis Dx of the shift drum67.

A shift arm75is fixed to the shift spindle74in a manner relatively non-rotatable around the shaft center Sx. A clutch lifter lever76is coupled to the shift arm75, and the clutch lifter lever76is supported so as to be swingable on an axis parallel to the shaft center Sx of the shift spindle74. To couple the clutch lifter lever76to the shift arm75, a pin roller77having an axial center parallel to the shaft center Sx is supported by the shift arm75. The pin roller77is inserted in a long hole76athat linearly extends in a centrifugal direction in the clutch lifter lever76and moves in the long hole76a. When the electric motor73drives to rotate the shift spindle74on the shaft center Sx in response to a shift-up signal, the shift arm75swings from a neutral position in a first circumferential direction DR1around the shaft center Sx of the shift spindle74. The swing of the shift arm75immediately causes the clutch lifter lever76to swing from a neutral position in a third circumferential direction DR3. When the electric motor73drives to rotate the shift spindle74on the shaft center Sx in response to a shift-down signal, the shift arm75swings from the neutral position in a second circumferential direction DR2opposite to the first circumferential direction DR1around the shaft center Sx of the shift spindle74. The swing of the shift arm75immediately causes the clutch lifter lever76to swing from the neutral position in a fourth circumferential direction DR4opposite to the third circumferential direction DR3.

A lifter driving member78is coupled to the clutch lifter lever76. The lifter driving member78is supported in a manner rotatable on the same axis as the rotation axis Mx of the main shaft43. To couple the lifter driving member78to the clutch lifter lever76, a pin roller79having an axial center parallel to the shaft center Sx is supported by the clutch lifter lever76, at a position away from a rotation axis of the clutch lifter lever76. The pin roller79is inserted in a guide path78athat linearly extends in a centrifugal direction in the lifter driving member78and moves in the guide path78a.

The lifter driving member78and a cam plate81have a ball82therebetween in the axial direction of the rotation axis Mx. The cam plate81makes the ball82move in the axial direction of the rotation axis Mx in accordance with the position in the circumferential direction around the rotation axis Mx. When the clutch lifter lever76swings in the third circumferential direction DR3from the neutral position, and the ball82is displaced around the rotation axis Mx in accordance with rotation of the lifter driving member78, the lifter driving member78drives the clutch lifter65from the connection position to the disconnection position. Similarly, when the clutch lifter lever76swings in the fourth circumferential direction DR4from the neutral position, and the ball82is displaced around the rotation axis Mx in accordance with rotation of the lifter driving member78, the lifter driving member78drives the clutch lifter65from the connection position to the disconnection position.

A torsion spring83is fitted to the shift spindle74so as to be wound around the shaft center Sx. The torsion spring83includes a linear body83athat is integrally formed at each end. The linear bodies83aextend parallel to each other in a direction away from the shaft center Sx. The torsion spring83is applied with an elastic force for making the linear bodies83acome close to each other around the shaft center Sx. The linear bodies83ahave a shaft body84therebetween. The shaft body84has a shaft center parallel to the shaft center Sx and is fixed to the crank case25a. The shift arm75is integrally formed with an engaging piece75athat is disposed between the linear bodies83a. Swing of the shift arm75makes the engaging piece75adrive one of the linear bodies83ain a direction away from the shaft body84. This accumulates an elastic force in the torsion spring83. Upon being released from a binding force, the shift arm75returns to the neutral position by the action of the torsion spring83.

A pin holding plate86, as shown inFIG. 6, is fixed to the shift drum67and supports multiple pins85having axial centers parallel to the rotation axis Dx. The pins85are arranged on the pin holding plate86, on the same circle at an even interval in the circumferential direction around the rotation axis Dx. The pin holding plate86includes recesses88that are formed at an outer edge, and the recesses88are configured to engage with a driving roller87. The driving roller87engages with the recess88along a cylindrical plane coaxial with an axis parallel to the rotation axis Dx. The driving roller87is applied with a pressing force to the outer edge of the pin holding plate86by the action of an elastic body. The recess88receives the pressing force from the driving roller87and stepwisely positions the shift drum67at a predetermined angle position around the rotation axis Dx.

An engaging plate89is put on the pin holding plate86. The engaging plate89is supported by the shift spindle74in a manner displaceable in a centrifugal direction and relatively rotatable around the shaft center Sx of the shift spindle74. The engaging plate89is formed with a first hook91aand a second hook91b. The first hook91aapplies a rotation force around the rotation axis Dx to the pin holding plate86by engaging with the pin85when the engaging plate89swings in the first circumferential direction DR1around the shaft center Sx. The second hook91bapplies a rotation force around the rotation axis Dx to the pin holding plate86by engaging with the pin85when the engaging plate89swings in the second circumferential direction DR2around the shaft center Sx.

As shown inFIG. 7A, a master arm92is relatively rotatably supported around the shaft center Sx of the shift spindle74. The master arm92is coupled to the engaging plate89in a manner relatively non-rotatable around the shaft center Sx of the shift spindle74. To couple the master arm92to the engaging plate89, a coupling shaft93having a shaft center parallel to the shaft center Sx is supported by the master arm92, at a position away from the shaft center Sx of the shift spindle74. The coupling shaft93is inserted in a long hole89athat linearly extends in a centrifugal direction in the engaging plate89. Swing of the master arm92causes the engaging plate89swing around the shaft center Sx of the shift spindle74. The rotation force of the shift spindle74is transmitted to the master arm92.

A coil spring94that exerts a tensile force in a linear direction is coupled between the master arm92and the engaging plate89. The coil spring94exerts an elastic force that pulls the engaging plate89toward the shaft center Sx of the shift spindle74.

The master arm92is integrally formed with an engaging piece92athat is disposed between the linear bodies83aof the torsion spring83. Swing of the master arm92makes the engaging piece92adrive one of the linear bodies83ain a direction away from the shaft body84. This accumulates an elastic force in the torsion spring83. Upon being released from a binding force, the master arm92returns to the neutral position by the action of the torsion spring83.

As shown inFIG. 7B, when the master arm92swings in the first circumferential direction DR1in shifting up, the pin holding plate89rotates around the rotation axis Dx by engagement of the first hook91awith the pin85a. The swing of the master arm92thus causes rotation of the shift drum67. The shift forks68a,68b, and68care then driven. The dowel is extracted or inserted between the first shifter47, the second shifter49, or the third shifter54and the shift gear.

The shift up is completed when the driving roller87goes over the top between the recesses88and is contained in the adjacent recess88, as shown inFIG. 7C. Thereafter, when the master arm92returns to the neutral position, the first hook91agoes over the pin85cby the action of a cam edge95acontinuous with the first hook91a. The engaging plate89swings to the neutral position while being separated from the shaft center Sx of the shift spindle74.

As shown inFIG. 7D, when the master arm92swings in the second circumferential direction DR2in shifting down, the pin holding plate89rotates around the rotation axis Dx by engagement of the second hook91bwith the pin85b. The swing of the master arm92thus causes rotation of the shift drum67. The shift forks68a,68b, and68care then driven. The dowel is extracted or inserted between the first shifter47, the second shifter49, or the third shifter54and the shift gear.

The shift down is completed when the driving roller87goes over the top between the recesses88and is contained in the adjacent recess88, as shown inFIG. 7E. Thereafter, when the master arm92returns to the neutral position, the second hook91bgoes over the pin85dby the action of a cam edge95bcontinuous with the second hook91b. The engaging plate89swings to the neutral position while being separated from the shaft center Sx of the shift spindle74.

As shown inFIGS. 8A to 8F, the shift spindle74supports a support arm (transmission body)96in a manner swingable around the shaft center Sx. The support arm96supports a torsion spring (lost motion elastic body)97that is wound around the shaft center Sx of the shift spindle74. The torsion spring97includes a linear body97athat is integrally formed at each end. The linear bodies97aextend parallel to each other in a direction away from the shaft center Sx. The torsion spring97is applied with an elastic force for making the linear bodies97acome close to each other around the shaft center Sx. An arm body75band an engaging piece96aare disposed between the linear bodies97a. The arm body75bis formed to the shift arm75and supports the pin roller77at an end. The engaging piece96ais integrally formed with the support arm96. The linear bodies97acouple the shift arm75and the support arm96with each other around the shaft center Sx of the shift spindle74. In the state in which the support arm96is not restrained around the shaft center Sx of the shift spindle74, the support arm96swings in accordance with swing of the shift arm75.

The support arm96is integrally formed with a driving piece96b. The driving piece96bcomes into contact with the master arm92upon receiving an elastic force of the torsion spring97when the shift arm75swings in the first circumferential direction DR1around the shaft center Sx of the shift spindle74. When the shift arm75swings in the first circumferential direction DR1around the shaft center Sx of the shift spindle74in shifting up, the torsion spring97applies a driving force for following the swing of the shift arm75, to the master arm92.

As shown inFIG. 8B, the shift arm75rotates in the first circumferential direction DR1around the shaft center Sx of the shift spindle74in shifting up. Meanwhile, the clutch lifter lever76starts to swing in the third circumferential direction DR3. The first friction plates59and the second friction plates61do not smoothly slide on each other at the time the clutch lifter65starts to move. Thus, a large torque acts on the driving gear of the main shaft43or the driven gear of the counter shaft44in the gear train for the speed stage, whereby the axial displacement of the dowel is restrained. As a result, the angle position of the shift drum67is not changed. This prevents the master arm92from swinging around the shaft center Sx of the shift spindle74.

At this time, the driving piece96bof the support arm96receives the elastic force of the torsion spring97and comes into contact with the master arm92, thereby preventing the support arm96from following the swing of the shift arm75. The arm body75bof the shift arm75drives the linear body97aof the torsion spring97in a direction away from the engaging piece96aof the support arm96. This accumulates an elastic force in the torsion spring97.

When the shift arm75further rotates in the first circumferential direction DR1around the shaft center Sx of the shift spindle74, as shown inFIG. 8C, the clutch lifter lever76further swings in the third circumferential direction DR3. During movement of the clutch lifter65from the connection position to the disconnection position, the first friction plates59and the second friction plates61start to slide on each other, whereby transmission of torque between the main shaft43and the counter shaft44is reduced accordingly. At this time, the elastic force that is accumulated in the torsion spring97makes the support arm96provide a driving force around the shaft center Sx of the shift spindle74, to the master arm92. Then, the master arm92swings, and a rotation force is thereby applied to the shift drum67. As a result, the dowel is extracted or inserted in the gear train for the speed stage. Thus, a gear shift operation is completed in a half-clutch state. This requires only small rotation amount of the shift spindle74and only a short time for the gear shift clutch58to move between the connection position and the disconnection position, whereby loss of power from the counter shaft44is reduced. In response to detection of establishment of the speed stage by the potentiometer72, the shift spindle74returns to the initial angle position.

In this state, ignition in the internal combustion engine25, which supplies power to the crankshaft26, is cut in shifting up. To cut ignition, supply of electric power to the ignition plug32is stopped. As a result, although the clutch lifter65does not yet reach the disconnection position, power that is supplied from the crankshaft26is reduced, and transmitted torque is decreased. This facilitates extraction and insertion of the dowel in the gear train for speed stage. Alternatively, instead of cutting ignition, the injection amount of fuel may be reduced in the internal combustion engine25. The cut of ignition and the reduction of the injection amount may be performed together.

When the shift arm75further rotates in the first circumferential direction DR1around the shaft center Sx of the shift spindle74, as shown inFIG. 8D, the clutch lifter lever76further swings in the third circumferential direction DR3. Then, the clutch lifter65reaches the disconnection position. This releases joining between the first friction plates59and the second friction plates61, and transmission of torque between the main shaft43and the counter shaft44is interrupted. The engaging piece75aof the shift arm75comes into contact with the engaging piece92aof the master arm92. The support arm96provides a driving force around the shaft center Sx of the shift spindle74, to the master arm92in accordance with a driving force applied to the shift spindle74. Then, the master arm92swings, and a rotation force is thereby applied to the shift drum67. As a result, the dowel is extracted or inserted in the gear train for the speed stage. Thus, when the extraction or insertion of the dowel is not completed in the half-clutch state, the shift spindle74drives the clutch lifter65to the disconnection position. This results in disconnection of the gear shift clutch58and completes extraction or insertion of the dowel. The gear shift operation of the multistage transmission42is thus completed. In response to detection of establishment of the speed stage by the potentiometer72, the shift spindle74returns to the initial angle position.

As shown inFIG. 8E, the shift arm75rotates in the second circumferential direction DR2around the shaft center Sx of the shift spindle74in shifting down. Meanwhile, the clutch lifter lever76swings in the fourth circumferential direction DR4. Then, the clutch lifter65reaches the disconnection position. This releases joining between the first friction plates59and the second friction plates61, and transmission of torque between the main shaft43and the counter shaft44is interrupted. At this time, the engaging piece75aof the shift arm75comes into contact with the master arm92after moving by a predetermined play angle. While the shift arm75rotates by the play angle, the clutch lifter65reaches the disconnection position.

As shown inFIG. 8F, upon further rotating in the second circumferential direction DR2around the shaft center Sx of the shift spindle74, the shift arm75applies a driving force around the shaft center Sx of the shift spindle74to the master arm92in accordance with the driving force applied to the shift spindle74. Then, the master arm92swings, and a rotation force is thereby applied to the shift drum67. At this time, the support arm96does not transmit power to the master arm92. As a result, the dowel is extracted or inserted in the gear train for the speed stage. The gear shift operation of the multistage transmission42is thus completed. In response to detection of establishment of the speed stage by the potentiometer72, the shift spindle74returns to the initial angle position.

In this embodiment, the throttle valve38is provided in an intake path (the flow path37aof the throttle body37) of the internal combustion engine25. The throttle valve38is coupled to the right grip23, which is used for throttle operation, by a wire. This configuration is inexpensive but can reduce the time of the gear shift operation.

The gear shift control system42aof this embodiment includes the shift arm75, the master arm92, and the torsion spring97. The shift arm75is fixed to the shift spindle74so as to swing around the shaft center Sx of the shift spindle74and is coupled directly to the clutch lifter lever76, which is coupled to the clutch lifter65. The master arm92is supported in the manner swingable around the shaft center Sx of the shift spindle74and causes rotation of the shift drum67. The torsion spring97has the elasticity that provides a driving force for following swing of the shift arm75, to the master arm92. When the shift spindle74rotates, the shift arm75swings accordingly, and the clutch lifter lever76starts to drive the clutch lifter65. While the master arm92receives the driving force from the torsion spring97, rotation of the shift drum67is prevented, and the master arm92is restrained from swinging, unless the driving force of the torsion spring97exceeds the joining force of the dowel. In this situation, the shift spindle74continues to rotate further, and the clutch lifter65also rotates further. This results in accumulation of an elastic force in the torsion spring97in accordance with rotation of the shift spindle74. Then, transmission of torque between the first friction plates59and the second friction plates61is reduced as the clutch lifter65is displaced. The elastic force that is accumulated in the torsion spring97comes to exceed the joining force of the dowel, whereby the shift drum67starts to rotate. Thus, the gear shift operation is rapidly completed after the clutch comes to the half-clutch state.

In this embodiment, the support arm96comes into contact with the master arm92by receiving the elastic force of the torsion spring97when the shift spindle74rotates in the first circumferential direction DR1around the shaft center Sx in shifting up. On the other hand, the shift arm75comes into contact with the master arm92after moving by the predetermined play angle, when the shift spindle74rotates in the second circumferential direction DR2around the shaft center Sx in shifting down. At this time, the support arm96does not come into contact with the master arm92. The shift drum67rotates by the elastic force accumulated in the torsion spring97in shifting up. The gear shift operation is rapidly completed after the clutch comes to the half-clutch state. While the torque required to extract or insert the dowel increases in accordance with back torque in shifting down, it is difficult to reduce the back torque at a desired timing without operation of a passenger. In view of this, the shift drum67is configured to rotate after the clutch lifter65reaches the disconnection position. This structure reduces the burden on the components such as the dowel and the shifter in performing the gear shift operation.

REFERENCE SIGNS LIST