the magic formula

[bandit5050]

got a gsxf750 trike with standard cog on front with chain going to a beval box onto the propshaft of a reliant axle? is there a way of working out what wheel and tyre size to use.also what cog size on the beval box?.......think that makes sence .cheers ady

[tbone]

Need a lot more information than you have supplied but the formula is

MPH = RPM x ERWR / GR x 168

ERWR is the Effective Rear Wheel Radius, that is, the wheel radius + height of the tyre

GR is the Gear ratio and that`s the  total gear reduction. Most bikes have a secondary reduction to the input shaft as well as the primary + (in your case) the bevel box reduction + the axle reduction.

so working the other way, ERWR (that`s wheel and tyre size) = MPH x GR x 168 / RPM

I forgot to add that you really want your bevel box output 1:1 with the engine output shaft cos then you can eliminate it from the equation.