Yup, I know of a lot of people using a 19T driver with a lot of success over the past 3 years. Around twisty tracks like rye, bayford and buckmore its supposedly worth 0.2seconds over a 20T, there are a number of theories as to why its quicker, for me its more to do with the reduced inertia (flywheel-effect) of the rear sprocket once the chain is wrapped around it........ apologies for being a bobble hatter, but the inertia calculation is:
Inertia = mass x radius squared
So only a tiny reduction of the rear sprocket radius will make a significant reduction to the overall inertia figure as the radius is squared (multiplied by itself), consider its the second biggest diameter on the rear axle so its not insignificant.
To back-up this theory, last year one of the big cheats in Iame-cadets was to run a hollow axle, on the basis that it was low-inertia and it spun up much quicker.
The reduced rear sprocket size also is less prone to being damaged due to kerbing......on the honda cadet with a 20T I find when using really big sprockets like 82 in the wet, it all gets far too close to the engine guard bracket, and generally a bit too tight for comfort.
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