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Recently we were asked, what gear ratio shoud I select for my electric scooter I am building? Why do I need to gear the electric motor down?
We have a support page that will provide a lot of help.
http://www.superdroidrobots.com/shop/custom.aspx/how-to-build-a-robot/70/#motorsYou need to gear a electric motor down for the same reason you have a transmission on a car or motorcycle. To get the HP the RPM is typically brought way up, especially in electric motors. If you run it 1 to 1 it will not have enough torque to move you and if it could it would be way to fast. (it would tare the bike apart, do the calculation) So a normal transmission you start is 1st gear, which is the most reduced so you have the low end torque to start moving, then as you speed up you change gears to get more speed. With a single speed (an electric bike, golf cart, or robot) you have to find the right balance so you have enough torque to get started from zero speed, but then enough top end speed. If you gear it down too much you will have plenty of power, but never get any speed. If its geared too high, you will have trouble getting it started, especially up a hill, etc, but then you will have a fast top end. You need to find the balance where it works well for both. The big electric motors will help because they will mussel through it if you don’t have it gear just right, but if it works too hard you will be drawing a ton of current and burning up your batteries…
So say you want to get 25mph and you have a 2ft diameter tire, 25mph = 25*5280/60= 2200ft/min. For every revolution of the tire: 2ft*3.14 you go ~6.25 ft. So you want the tire to rotate at 2200/6.25=352 RPM. Let’s say your motor is 4000RPM full speed, therefore you need to gear it down by 1:6.25 So if you are using chain, you put the smallest sprocket on the motor, say 10 tooth (this will be driven by the pitch of the chain and the size of the motor shaft) therefore you will need a 62 tooth sprocket at the wheel.
To Figure out how much HP or toque you need is a lot more complicated. Trial and error and experience are the best tools for this…
Some more useful links to help you with your robot project with regards to motor sizing:
Motors Sizing and robot speed calculator
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