How to calculate gear ratio for electric drift

JayLock

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Hey everyone,

My son currently has a drift trike and we're planning on converting to electric, luckily for him my old commuter bike has a Bafang BBSHD electric bike kit on it with a 21ah battery so going to utilise that instead of selling.

One thing that's throwing me off is working out the final gear ratio/ top speed as the motor itself has an internal gear ratio of 22:1, how do I choose the correct sproket size for a high enough gear ratio? The sprokets I was planning on using was 42t chain and 14t sproket which would give a gear ratio of .33, is it fair to assume this would give me a gear ratio of around 7? Then from there how do I work out top speed with the motor spinning at 150rpm and ture height being 11"

Hope that makes sense
 
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