Hi~ What is the relationship between electric bicycle power output and torque? For example, what is the difference between a bike with high torque but low power output, and one with low power output but high torque?
Welcome to the forum,
@kjd5684. This is actually a cracking question, and one that trips up a lot of people because torque and power get thrown around in marketing as if they're the same thing. They aren't.
The relationship is physics, not opinion:
Power = Torque × Rotational Speed. Specifically, Power (watts) = Torque (Nm) × Cadence (rad/s). So power is what you get when you combine how
hard the motor pushes (torque) with how
fast it's spinning. A motor can have enormous torque but if it's turning slowly, its power output is modest. Conversely, a lower-torque motor spinning quickly can produce the same wattage.
In practical eMTB terms:
high torque, lower power means the motor is strong at low speeds. Think steep, technical climbing where the cranks are grinding round slowly, the motor is still shoving you up the hill with real force. But it runs out of puff at higher speeds because it can't maintain that torque as RPM rises. The Shimano EP801 at 85Nm/600W is a good example of a motor that prioritises torque feel and low-speed grunt over raw peak power.
As
@goroncy noted, something like the Yamaha PW-X3's percentage-based assist means 400% is not maximum power in watts but 400% of rider input, so how hard
you pedal determines where in the power curve you're operating.
