Now, the other factors that you need to consider are the weight of the rider, the desired top speed, and the acceleration desired. All other things being equal, a heavier rider will take longer to get up to speed than a lighter rider. Decreasing the top speed increases the acceleration. Likewise, increasing the top speed will reduce the acceleration. Acceleration is also directly related to the bike's hill-climbing ability. The greater the acceleration the motor can provide, the greater the slope that the engine can overcome on its own. (However, this is

*not* a linear relationship; it is a trigonometric relationship, proportional to the inverse-tangent of the slope percentage)

A roller diameter of 1.25 inches will give you a speed of about 26 MPH at 7000 RPM. A 1 inch roller results in a speed of 20.8 MPH at the same engine RPM.

It's all a compromise. The faster you can go, the slower you will get up to speed, and the less slope you can take unassisted. The slower the top end, the better the take-off and the steeper the slope.

Considering all the above, let's use me as an example.

I'm a big guy (ex-defensive tackle, over 300 pounds) who wanted decent acceleration, but didn't need a top speed a lot over 20 MPH, as Arizona's laws can REALLY penalize you if you go over 20. I'm happy with a friction drive with a 1.25 inch roller.