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here's where I get confused - I know electric motors provide the same amount of torque at any revs (i.e. they don't have a torque curve like a combustion engine - it's a flat line) - but still, using gears must multiply the torque? i.e. a 250 watt crank motor running at a three to one ratio will produce three times more torque than the same hub motor running directly as far as I understand or is this too simplistic?The BPM motor is much bigger and provides more torque in every gear except maybe first. The BPM will run typically with 20A to 22A compared with the BBS01s 15A, so it'll provide over 30% more power too.
Low powered crank motors can get you up steep hills, but you have to do a lot of pedalling whilst going slowly in first gear. The hub-motors don't care what gear you're in. They give their torque regardless, which makes them much more relaxing and comfortable to ride.
Obviously, low powered crank motors can climb better than low powered hub-motors, but there's 250w hub motors that can give a lot of power and torque.