OK John... what are you going to accomplish with your converted mountain bike; ride the roads or go off-roading? If you want just a little pedal assistance on a flat surface road than a 250w and a 24v 10ah battery will do that. You can move along at just above normal peddling speeds with a little assist from the motor. If you have some big hills (or carry a lot of weight) then step up the motors wattage from 250 to 500 or greater depending on the hills (and weight). Do you want to go faster than peddling (hold open the throttle and ride without peddling) then step up the battery voltage (24 to 36 or 48volts); and if you want to travel a long distance then step up the battery amp hours (10ah to 20 or 30ah).
My daughter drives a hub direct drive 250w 24v pedal assist around in the yard. It is not very fast, just above pedal speed when using only throttle. It slows down a little on slight hill climbs (slows a lot on steep hills). My (110lb) 15 yr old drives an 800w 36v on the street. It moves him much faster than peddling speed and takes on all hills with great ease.
A big difference is your use... are you really going to peddle and use the motor a little (pedal assist)? do you want to sit back and let the motor carry you? How fast? How Far?? Or some combination in between. Are you going to commute, how far, how fast, what speed, what load, what terrain? Do you want to off-road.. tear up a trail, make some jumps, etc...
I suggest that you go to Youtube and watch videos of ebikes in action. Pay attention to the motor type & size, battery type & size, terrain, riders weight, bike size and the resulting effects on the bike ride.
Last edited by Pedidelic; 02-18-12 at 08:24 PM.