The most challenging part of designing a diesel race bike was building a compact transmission that could simply bolt to the side of the engine block. Because of the size of the KZ400's frame, I could not use a separate gearbox or a CVT transmission. Either one of them would have required cutting and lengthening the frame. To minimize size and length, I decided to build an upgraded copy of a 2-speed minibike transmission. These transmissions were common on Rupp minibikes in the 1960's but had fallen out of popularity when CVT's became widespread. As a result, they are somewhat hard to come by now, and wouldn't really be capable of taking the beating I had planned.
Most of the parts are easy to come by. All that is required is two centrifugal clutches with the correct rpm engagement settings, a countershaft, and a sprocket with a built-in sprag clutch. One clutch and sprocket are mounted on the crankshaft, and a second clutch and the sprag sprocket are mounted on the countershaft. When the crankshaft (first gear) clutch engages at low speed, it drive the sprag clutch sprocket, which locks and drives the wheel. At the same time, the sprocket mounted on the crankshaft is spinning at a different gear ratio and driving the outer shell of the countershaft (second gear) clutch. When the countershaft reaches the correct rpm, the second-gear clutch engages. This outpaces the input from first gear, so the sprag clutch disengages and freewheels for the remainder of the speed range.
The Mk1 sprocket used an off-the-shelf sprag bearing to shift gears. This quickly failed due to the shock loads involved in the gearchange, which greatly exceeded the rated load.
 |
| Lasted barely 3 miles |
 |
| The early design with the failed sprocket |
Because I could not locate a suitable mass-produced option, I decided to design and manufacture my own clutch. The Mk2 sprocket used the same material as before, but was instead mounted on a normal bearing. A steel collar was mounted to the shaft next to it, inside the sprocket's flange. This collar contained a ratcheting pawl which would engage in a series of machined notches in the interior of the sprocket, and disengage in the opposite direction.
 |
| Machining the notches |
 |
| The finished product |
 |
| Broken ratchet |
Mk2 failed in a fairly spectacular fashion, with chunks of steel flying past my thigh and my wheel locking up at 40 mph! The sintered steel sprockets that I used, while cheap and easy to machine, proved to be extremely brittle and the flange shattered while shifting gear.
 |
| Boom |
Mk3 was virtually identical to the previous model, but used a carbon steel sprocket with a reinforcing ring press fit around the edge for strength. This version proved to be somewhat reliable, but still required frequent rebuilding due to the extreme wear experienced by the ratchet pawl.
 |
| Reinforced |
With Mk4, I did away with the ratchet design and substituted a ramping ball bearing system. The mounted collar has four ramps which contain hardened individual bearing balls, and flange of the sprocket has a series of small notches. When spun in the freewheel direction, the balls simply stay in the deep end of the ramp. When spun the opposite way, they wedge between the ramp and the notches in the sprocket.
This design improved the mechanical reliability by a factor of 40. The previous version could only last about 20 miles between rebuild, whereas Mk4 didn't fail until it had been in service for over 850 miles.
 |
| Final version |
 |
| Strengthened and improved |
When the sprocket finally wore out, I decided to simply build a strengthened copy of the same design. It is virtually identical, but the flange on the sprocket has been thickened to prevent the mushrooming seen on the worn out part above.
 |
| Final Design |
No comments:
Post a Comment