A guy name Dr. Currie drafted the federal definition text. He was keenly aware that the NHTSA didn't want to release anything that would go faster than 20mph so Currie wrote the spec such that "motor alone" could sustain a 170lb rider on level surface at 20mph (elimintates the inefficiencies of the drive system). The actually makes great sense such that that power level (typ. from 300-350W depending on bike design) can continue to be provided past 20mph so long as the extra speed is the result of rider effort.You can tell the law was written long ago when perhaps the average weight of a US citizen was even close to 170lbs. Given that was their intent and I doubt that anyone that was involved in that language at the time was close to that amount. As you say, lot's of room for confusion.
So on a throttle-assist ebike the full 750W drive system rated power can be provided up to 20mph but at 20mph the motor power must drop down such that "motor alone" only sustains the 20mph.
I think everyone tries to make this definition seem poorly written but anyone with a technical background can see the elegance of it's wording (Currie was assuming that the NHTSA representatives and other law makers would not realizing he was providing for a faster LSEB with rider effort.
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