Power Transmission Device of Motorcycles

1. Field of the Invention

The present invention relates to a power transmission device of a motorcycle, for transmitting the power of an engine mounted on the chassis frame to the rear wheel.

2. Description of the Prior Art

Generally, the power transmission device for motorcycle is adapted to transmit the power of the engine to a rear wheel through a clutch mechanism, a transmission and then through a chain or a shaft drive mechanism.

Motorcycles, however, are often required to cruise, depending on the uses and applications, at a speed which is not covered by the range of the aforesaid transmission.

Under these circumstances, the present invention aims at satisfying the above-stated requirement of the prior art.

It is an object of the invention to provide a power transmission device for motorcycles in which a sub-transmission is interposed between the aforementioned transmission and the chain or the shaft drive mechanism to provide a wider range of speed change by combining the range covered by the ordinary transmission and the range covered by the sub-transmission, thereby to fulfill the above-mentioned aim, while avoiding the deterioration of the running performance and stability of the motorcycle due to the addition of the sub-transmission by arranging the sub-transmission in a good balance of weight with other parts of the transmission system.

It is another object of the invention to provide a transmission device having a sub-transmission gear train which is designed an constructed to permit and easy replacement of the sub-transmission gear train, so that the reduction ratio of the transmission device as a whole can easily be changed without necessitating the demounting and disassembling of the transmission device which is inevitably required in the conventional transmission device.

It is still another object of the invention to provide a power transmission device having a reduced number of parts and which is easy to assemble.

FIG. 1 is a schematic plan view of a power transmission system of a motorcycle constructed in accordance with an embodiment of the invention;

FIG. 2 is a sectional view of an essential part of an embodiment of the invention, taken along line II--II in FIG. 4;

FIG. 3 is a side elevational view of the embodiment shown in FIG. 2;

FIG. 4 is a sectional view taken along line IV--IV in FIG. 2;

FIG. 5 is an exploded view of a cam groove of a shift drum cam;

FIG. 6 is a sectional view taken along the line VI--VI in FIG. 3; and

FIG. 7 is a cross-sectional plan view of a second embodiment of the invention.

A first embodiment of the invention will be described hereinunder with reference to FIGS. 1 through 6. An engine E of a motorcycle has a crank case 1 and a main transmission case 2 formed integrally with each other. A crank shaft 3 is rotatably mounted in the crank case 1. A main transmission Tm is accommodated in the main transmission case 2. The main transmission Tm includes a primary shaft 4, main shaft 5 and a counter shaft 6, which are mounted in parallel relation in the main transmission case 2. The primary shaft 4 is drivingly connected to the crank shaft 3 through a chain mechanism 7 arranged therebetween. The main shaft 5 is drivingly connected to the primary shaft 4 through a gear mechanism 8 and a clutch mechanism 9. Finally, the counter shaft 6 is drivingly connected to the main shaft 5 through a gear train 10. The power of the engine E is transmitted to the counter shaft 6, through the crank shaft 3, chain mechanism 7, primary shaft 5 and the gear train 10. The reduction ratio between the crank shaft 3 and the counter shaft 6 is changed in an ordinary manner by the changing operation of the aforesaid main transmission Tm. The output end 6' of the counter shaft 6 projects out of the main transmission case 2.

The power transmitting arrangement between the crank shaft 3 and the counter shaft 6 is not described in detail because it is well known per se.

As will be clearly understood from FIG. 2, a substantially L-shaped sub-transmission case 11 is detachably attached by means of bolts 13 to the rear side of the main transmission case 2 through a support member 12.

The sub-transmission case 11 is compose of a case body 14 and a cover 15 attached to an opened portion of the case body 14 by means of bolts 16. The case body 14 has a front half part 17 extending along one side wall of the main transmission case 2 in the front and rear directions of the chassis frame and a rear half part 18 extending in the right and left directions along the rear wall of the main transmission case 2. These half parts are formed integrally with each other. The aforementioned cover 15 is fitted to an opening formed in the front half part 17 so as to cooperate with the latter to form a gear accommodating space 20. The output end 6' of the counter shaft 6 projects into the space 20 and is rotatably carried by the cover 15.

A sub-transmission Ts is mounted in the sub-transmission case 11. The construction of this sub-transmission Ts will be explained hereafter with reference to FIG. 2. First and a second drive gears 21, 22 are rotatably mounted on the output end 6' of the counter shaft 6. A shifter boss 23 is splined to the counter shaft 6 between gears 21,22 so as to rotate as a unit with the counter shaft 6. A first hole 24 and a second hole 25 are formed in disc portions of the first and second drive gears 21,22 at the same radius. A shifter 26 is splined to the outer periphery of the shifter boss 23 and is axially slidable with respect thereto. First and second projections 27,28 formed on both sides of the shifter 26 are adapted to fit in the first and second holes 24,25, selectively.

The arrangement is such that, as the shifter 26 is moved to the left in FIG. 2, the first projection 27 is inserted into the first hole 24, whereas the rightward movement of the shifter 26 brings the second projection 28 into engagement with the second hole 25. A shift fork 29, which is rotatably carried by a shaft 30, is secured at one end to the shifter 26. The shift fork 29 has a shift pin 31 projected from the other end thereof. The shift pin 31 is received in a cam groove 34 formed in the outer peripheral surface of a shift drum 33 which is splined to a spindle 32 rotatably supported by the sub-transmission case 11.

As will be seen from FIGS. 3 and 6, an arm 36 having an engaging pin 35 extending from one end thereof is splined and secured by a stopper pin 37 to the end of the spindle 32 extending outward from the cover 15. The engaging pin 35 of the arm 36 is engaged in an elongated longitudinal slot 39 formed at an intermediate portion of the shift lever 38 by making use of one of the attaching bolts 16. Thanks to this arrangement, it becomes possible to reduce the rotation angle of the arm 36 for a given rotation angle of the shift lever 38 as compared with the case where the shift lever 38 is directly attached to the spindle 32, so that the operability is improved considerably.

As shown in FIG. 6, a tubular member 40 is formed integrally with the shift lever 38 at the base end thereof. The tubular member 40 is fitted onto a screw shaft 41 extending unitarily from one of the attaching bolts 16 and is fastened to the screw shaft 41 by means of a nut 42.

The cam groove 34 formed in the periphery of the shift drum 32 is inclined at its central portion 34a as shown in FIG. 5, and is provided at its both ends with engaging portions 34b,34c extending in the circumferential direction therefrom. One of the portions 34c is open laterally by a notch groove 34d. Therefore, as the shift drum 33 is rotated by operation of the shift lever 38, the shift fork 29 is moved in the axial direction along the cam groove 34 so that the shifter 26 is moved to the left or right to fit the first or second projection 27,28 into the first or second hole 24,25.

A drum stopper 44 rotatably carrying a roller 43 at its end is pivotally secured at 45 to the cover 15. The drum stopper 44 is biased by a spring 46 to rotate in the clockwise direction around a pivot point 45, as viewed in FIG. 4, so that the roller 43 is pressed against the outer periphery of the shift drum 33. Splined to the spindle 32 is an annular member 47 which is provided on its outer periphery with a pair of grooves 47a, 47b. When the roller 43 is in engagement with the groove 47a or 47b, the shift drum 33 is held at a predetermined rotational position. In this state, the engaging pin 35 of the shift fork 29 is held in one of engaging portions 34b or 34c formed at opposite ends of the cam groove 34.

The rear half part 18 of the sub-transmission case 11 extends laterally of the chassis frame and transversely of the portion just ahead of the rear wheel W of the motorcycle. A transmission shaft 48 extending substantially parallel with the counter shaft 6 is mounted rotatably in the rear half part 18 of the sub-transmission case 11. First and second driven gears 49,50 formed as a unit with each other are splined to one end of the transmission shaft 48 so as to rotate as a unit with the latter but is axially movably along the same.

The first driven gear 49 and the second driven gear 50 are always in mesh with the first drive gear 21 and the second drive gear 22, respectively. The opening formed in the front half part 17 of the sub-transmission case 11 opens at the lateral side of the first and second drive gears 21,22 so that the first and second drive gears 21,22 and the first and second driven gears 49,50 can be withdrawn from the counter shaft 6 and the transmission shaft 48, respectively, so that these gears are replaceable for any desired reduction ratio.

A driving bevel gear 51 is formed unitarily at the other end of the transmission shaft 48. A short driven shaft 52 orthogonal to the transmission shaft 48 is rotatably carried by the other end (right end as viewed in FIG. 2) of the sub-transmission case 11, and carries a driven bevel gear 53 formed unitarily therewith and meshing with the aforementioned driving bevel gear 51.

As will be seen from FIG. 1, a shaft drive transmission mechanism 54 is connected to the transmission shaft 48 at the side of the main transmission case 2 opposite the sub-transmission Ts. More specifically, a drive shaft 55 extending in the longitudinal direction of the chassis frame is coupled at its front end through a joint to the driven shaft 52.

The rear end of the drive shaft 55 is connected to the rear wheel W through a pair of final bevel gears. 56,57. The final bevel gears 56,57 and the drive shaft 55 are accommodated in a transmission case 58. The rear wheel W is located substantially at the mid point between the gear train of the sub-transmission Ts and the shaft drive transmission mechanism 54 in the lateral direction, so that the center of gravity of the gear train and the shaft drive transmission mechanism is located substantially on the rear wheel W.

In operation of the power transmission device of the described first embodiment, the counter shaft 6 is driven through the main transmission Tm by the crank shaft 3 of the engine E. As the shift lever 38 is operated to move the shifter 26 to the left in FIG. 2, the first projection 27 comes to be received by the first hole 24 so that the first drive gear 21 is directly coupled to the counter shaft 6, and the rotation of the counter shaft 6 is transmitted to the transmission shaft 48 through the shifter boss 23, shifter 26, first drive gear 21 and the first driven gear 49, so that the rotation speed of the counter shaft 6 is secondarily reduced before it is transmitted to the transmission shaft 48. The rotation of the transmission shaft 48 is finally transmitted to the rear wheel W through the bevel gears 51,53, driven shaft 52, drive shaft 55 and the final bevel gears 56,57.

As the shifter 26 is moved to the right as viewed in FIG. 2, the second projection 28 fits in the second hole 25 so that the second drive gear 22 is coupled to the counter shaft 6 to transmit the rotation of the latter to the transmission shaft 48 through the shifter boss 23, shifter 26, second drive gear 22 and the second driven gear 50. In this state, the rotation speed of the counter shaft is transmitted to the transmission shaft 48 at a reduction ratio which is different from that mentioned before. The reduced speed is then transmitted through the shaft drive transmission mechanism 54 to the rear wheel W to drive the latter.

For obtaining different speed reduction ratios from those explained above, the cover 15 is detached from the case body 14 and the pair of gear trains of the sub-transmission are withdrawn from the counter shaft 6 and the transmission shaft 48 for replacement with another pair of gear trains having different reduction ratios. Then, the cover 15 is attached again to the case body 14.

Referring now to FIG. 7 showing a second embodiment of the invention, a first drive gear 60 on the counter shaft 6 serves also as a shifter. Namely, the first drive gear 60 is splined to the counter shaft 6 for axial sliding motion along the shaft 6, and a second drive gear 61 is loosely mounted on the counter shaft 6 for relative rotation. A hole 62 formed in the disc portion of the second drive gear 61 is adapted to selectively receive a projection 63 formed on the disc portion of the first drive gear 60. Also, the first drive gear 60 is provided with an annular groove 64 adapted to receive the shift fork 29. When the first drive gear 60 has been shifted to the left position as illustrated in FIG. 7, this gear meshes with the first driven gear 49 on the transmission shaft 48, so that the rotation of the counter shaft 6 is transmitted to the transmission shaft 48 through the first drive gear 60 and the first driven gear 49. To the contrary, as the first drive gear 60 takes the right position in FIG. 7, the second drive gear 61 is coupled to the counter shaft 6 through the first drive gear 60, so that the rotation of the counter shaft 6 is transmitted to the transmission shaft 48 through the first drive gear 60, second drive gear 61 and the second driven gear 50.

Other portions of this second embodiment than specifically mentioned above are materially identical to those of the first embodiment and designated by the same references.

Although in the described embodiments the power is transmitted from the sub-transmission Ts to the rear wheel W through shaft drive transmission mechanism 54, it is possible, needless to say, to substitute a chain drive transmission mechanism for the shaft drive transmission mechanism.

As will be understood from the foregoing description, the transmission device of the invention provides a wide range of speed change, by the combination of the speed changing range of the main transmission and that presented by the sub-transmission which is interposed between the main transmission connected to the crank shaft of the engine and the transmission mechanism connected to the rear wheel.

At the same time, since the sub-transmission and the transmission mechanism connected to the rear wheel are arranged at opposite sides of the rear wheel, the weights of these units are balanced at a point substantially on the rear wheel so that the balance of weight of the motorcycle is maintained to ensure a high stability of the same during running.

In addition, the reduction ratio of the sub-transmission can easily be changed by replacing the gears as a unit simply by detaching the cover, so that the overall reduction ratio can be widely and quite easily changed as compared with the conventional transmission device having only the main transmission thereby to enhance the utility of the motorcycle.

Since the first and second drive gears of the sub-transmission have mutually engageable portions and a shifter for shifting the second drive gear is formed unitarily with the latter to eliminate the necessity of a separate shifter, the number of parts of the sub-transmission is reduced to simplify the construction and to reduce the production cost.