I want to add a motor to my dahon folding bike and tie it in with my chain. I am trying to find a motor that is about 4" thick, 48-72 volts and 500-700 watts that weighs about 6 pounds. This is difficult. Yet, a year or so ago I saw a post on another forum from a guy that was using a motor that was designed to be used to make rc airplanes fly - was 2,000 watts and 72 volts (or so) that only weighed 3 pounds I think it was called an outrunner, similar to this: http://aircraft-world.com/shopexd.asp?id=4544.
What gives? Yes, it is a high rpm motor and expensive, but the low weight would make it worthwhile, if it will work.
Bottom line is, how do I find a small powerful motor to meet the specs of my application? What search words do I put into google?