Choosing a motor
In order to complete my goal, I needed a small but powerful motor. During my research I read through and viewed ALOT of different postings and experiences people had building electric bikes and other vehicles. I figured I needed at minimum 750Watt motor because that was the legal limit for electric bikes to be used on road. But 750Watt is approximately 1hp so that isn’t very strong. But there are many electric products on the market with less power. I found these electric motors used in Remote Control applications such as large scale airplanes and helicopters. They were small and powerful but I had my doubts that they could push my 180lb body around even though they had impressive power numbers greater than 750W.
But then I saw this posting. This guy cludged one upto a bicycle and it pushed him down the road. So I figured it would work on the mountain board.
A great place to get these motors and some of the other components at a Huge discount off the MSRP is www.hobbypartz.com.
So inorder to make sure I got a motor which could fit a sprocket; I went shopping to make sure I could get a sprocket with the same I.D. as the motor shaft I was considering. For the sprocket I went to sdp-si.com. I found a Sprocket with hub with 10 teeth and 8mm shaft hole. This would fit the Monster Power 60 Brushless Motor I picked out.
Last think was to make sure, the speed of the motor would match up with the rest of the drive train. I didn’t want to be stuck going too slow. This is where the math comes in!
Wheel Diameter = 8″ if I use a 1:7.8 gear ratio on the sprocket setup using this 78T sprocket Top speed for a 18.5V supply would be about 22mph.
Ok, you probably want to know how to calculate that.
First I figured out how many revolutions of the wheel per mile = 5280/(pi * 8″) /12 = 2522.3 rev/mile
So with a 1:7.8 gearing the motor would need to turn 19,674 rev/mile.
Therefore with the Monster Power 60 having a 400Kv spec. Kv = rpm/volt would turn at maximum 400*18.5 = 7400 rpm = 444000 rev/hour. I planned on an 18.5volt Lipo battery.
Now if we take (rev/hour)/(rev/mile) we get mile/hour = 22.56mph max speed.
Another sanity check, the Monster Power 60 is rated at 1200W which is almost 2x the 750W spec mentioned earlier.
Max current is rated at 60A burst or 40A continuous so I also pick out the Exceeds Volcano 40A ESC brushless motor Electronic Speed Controller also from www.hobbypartz.com. This is ESC is designed to takes a servo control output from aR/C Receiver. So there will be some work on how to connect the throttle upto the ESC. But that is a tale for another day.