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Question: I have my first shaft-driven motorcycle, a Yamaha FJR. I can certainly say that I don’t miss the chain, what with adjusting, cleaning, lubing, replacing, and cleaning filth off of the back of the bike. However, a belt drive seems to be even simpler still, less expensive, with more efficient power transfer, and less maintenance. Why haven’t more manufacturers embraced belt drive? What are the drawbacks?
Greg Seeger
Santa Rosa, CA
Answer: As you note, toothbelts have their attractions. But unlike a chain, they can rise up out of engagement and slip if given enough slack (note that cam belts on engines have fixed slack adjusters that remove all slack). The obvious solution for achieving zero slack in a motorcycle final toothbelt drive—making the front sprocket concentric with the swingarm pivot axis—interferes with using driveline tension to oppose the rear-end squat that otherwise occurs during hard acceleration. For these reasons, toothbelts tend to be used either on machines with very limited rear wheel suspension travel or with tensioner systems of some complexity. Chains continue to be generally used because they are not subject to these requirements and because they provide great strength in a compact form.