Motor is from a razor scooter, the battery is temporary till I put a better basket, "throttle" consists of pressing the positive wire to the handlebars to complete the circuit, if it gets hot the duty cycle has been reached and the rider will let go. Drive sprockets are 11:80, I could make it re-gen by taping a stick to the tall sprocket but that complicates the design 
I took the rear wheel from a similar bike and the tall sprocket from the scooter and used a lathe to turned both to pressfit into the bore of the 80 tooth with spot welds to hold it together and it came within 1.5mm of square/flat/correct. A small amount of hammer time persuaded it to be even better. I notched the fork and welded in some material from another bicycle to pass the small chain through, structurally is not great but the rider is light and will outgrow this frame in 2 years.
The nice thing about the simplicity is when one wire came off the battery a pair of 1st graders were able to diagnose and repair the problem, they accidentally learned a little bit about electric motors in the process.
simple, dangerous, fun.
I took the rear wheel from a similar bike and the tall sprocket from the scooter and used a lathe to turned both to pressfit into the bore of the 80 tooth with spot welds to hold it together and it came within 1.5mm of square/flat/correct. A small amount of hammer time persuaded it to be even better. I notched the fork and welded in some material from another bicycle to pass the small chain through, structurally is not great but the rider is light and will outgrow this frame in 2 years.
The nice thing about the simplicity is when one wire came off the battery a pair of 1st graders were able to diagnose and repair the problem, they accidentally learned a little bit about electric motors in the process.
simple, dangerous, fun.
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