There are several ways to transmit engine power to the rear wheel of a motorcycle. A continuously variable transmission (CVT) is the simplest because it doesn't necessarily require frame modification. Alternatively, you can place a modified motorcycle transmission behind the engine. But I didn't have the money to spend on either approach, nor did I have the skills to safely stretch the frame to accommodate the extra parts. Therefore, since I didn't intend to make the bike street legal, I decided to set it up like an old minibike: with a single-speed centrifugal clutch.
Editor's Note: Click here for Part 1 of this multi-part series
Single-Speed Transmission Using a single-speed transmission has some serious drawbacks. The most significant is the bike's inability both to go fast AND go up hills unless there is a lot of power behind it. With access to only 6 hp, I had to choose my gearing carefully. Luckily, my workshop (a.k.a. my parents' garage) is located on Long Island, possibly the flattest area in the U.S. this side of the Great Plains. I chose a gear ratio of approximately 6.85:1, which would give me a top speed of 40 mph on flat land but a measly 68 foot-pounds of torque at the wheel.
Rear Sprocket Size
 One problem I hadn't considered when designing the bike was the sheer size of the rear sprocket. Using a 1600 Series centrifugal clutch from NORAM with 14 teeth, I had to order a 96-tooth rear sprocket. It was very nearly bigger than the rim of the wheel and almost hit the swing-arm. After mounting the engine, aligning the sprockets, and installing the chain, I fired up the engine and took it for a test ride.
http://www.youtube.com/watch?v=O1Cff0kgbls
After these initial test runs on pump diesel, I purchased a gallon of Wesson Canola Oil and, to my amazement, the engine ran perfectly! Starting the engine from cold was a problem, however, so I mixed in about 10% diesel fuel to ease the stress on my arm.
What I'd Do Differently
In the future, the most important improvement I'd make is to include some form of transmission. Even though the clutch was the simplest option, it severely limited both acceleration and hill-climbing. In addition, because there is so little starting torque, the clutch has a habit of overheating rather quickly during prolonged use.
Despite these drawbacks, this is one seriously fun piece of machinery, and I would recommend it as an enjoyable engineering project to anyone with the time and ability.
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Re: Designing a Diesel Motorcycle
