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02-26-09, 02:10 PM
#6
cerewa

Join Date: Jun 2003
Location: southeast pennsylvania
Posts: 3,155

Bikes: a mountain bike with a cargo box on the back and aero bars on the front. an old well-worn dahon folding bike

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The amount of thrust (forward force) you need in order to match a typical hub motor is something in the realm of 12 pounds to 20 pounds.

If you're talking about a motor built in to a 24 inch wheel, that should translate to 12 foot-pounds to 20 foot-pounds of torque. (because the radius of the wheel is one foot).

If you're talking about a motor with a friction drive system, driving a small friction-wheel, 2 inches in diameter, divide by 12 to find the foot pounds required but multiply by 12 to find the RPMs required.
if the small friction wheel is 1 inch in diameter, you'd divide by 24 to find the foot pounds required and multiply by 24 to find the RPMs required.

RPMs needed:
26" wheel at 10mph : wheel turns 129RPM
24" wheel at 10mph : wheel turns 140RPM

if you double the miles per hour, double the RPMs.