Join Date: Jul 2010
Bikes: too many of all kinds
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Building a Kit, I would stick with something new and proven. Some motors turn out to be junk after a couple of months use. Off the top of my head, Cute (Q100), Bafang, MAC, BMC, eZee are pretty reliable geared motors.
Basically when choosing the Motor RPM, you just match it with the wheel and battery voltage. The vendor should have a kit or recommendation for you. Roughly speaking a 250watt 36v motor on a 26” wheel would be around 230rpm, while on a 20” wheel it may be closer to 280rpm. Those are very rough numbers though, whomever sells you the motor should know what works best for your wheel size.
With that said, the 235 rpm motor would work best on most 26” (or 700c) bikes, but even so that motor will not give you much help above 15mph, and no help above 18mph.
There is a strong argument for only using a front motor on a steel fork and the alloy forks are prone to breaking with a motor and breaking a front fork makes an ugly and painful crash. I have never heard of a front fork breaking with a small 250watt hub motor, but anything bigger definitely requires steel and a torque arm.