Ok, I did some reading, and this is what I'm going to add to the tutorial unless I am told that I'm not understanding something correctly.
There are several parameters to look at when choosing an RC motor for your e-bike application. The first is, inrunner or outrunner? With inrunner motors the shaft rotates. This is good for driving chains or belts. With outrunner motors the outside shell rotates. This is good for direct friction to tire drive applications.
KV - RC Motors are AC brushless motors and are rated by their KV. KV is the relationship between voltage and motor speed (rpms per volt. Multiply the motor KV times the battery voltage to find the maximum motor RPM. For instance a motor with a KV of 100 would run at 5000 RPMs at 50V.
E-bike RC motors run at lower than 10,000 RPM with no load. Higher motor speeds than that cause the speed reduction gearing to becomes impractical. This means with a 33V battery pack you will want a KV of 250 or lower, or with a 44V battery pack you will want a KV of 225 or lower. The Astroflight 8120-5T is a 225KV motor. 5T refers to five of turns per coil in the motor. In general, fewer turns equals higher RPM.
Next you will need to determine how much power you need. RC motors are rated in watts. It takes around 2000 watts to get up hills at a decent speed and to go over 20 MPH into a stiff headwind, so that should be the minimum to consider. Anything over 5000 watts is a monster motor and will require a very powerful battery and stronger than standard bicycle drivetrain components.
Calculator V.09 http://www.recumbents.com/wisil/e-bent/ ... ulator.asp
How do I calculate what the maximum loaded RPM is given the motor KV * volts?