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Comment by namdnay | Hacker News Reader

Modern motors get lighter in part because they can offer lower torque with a smaller rotor but compensate with higher rpm. That’s not possible for a hub motor

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The hub motors on the car in the OP run at ~24k RPM (faster than any production electric car AFAIK) and have a multistage planetary gearbox for reduction.

Pretty much all top Formula Student cars now use hub motors. Mainly for packaging reasons. Putting the motor in the unused space in the wheel means you can make a smaller chassis and have more space for wings and underbody aero.

wow, interesting! i didn't imagine you could fit the "gearbox" in the hub too

Can you compensate smaller rotor with larger stator (magnet), so rpm could stay the same?

And why you cannot have small hub motor with high voltage (kilovolts) and very high rpm?

Torque is directly proportional to radius, all other factors being equal. There are practical limits on the strength of the magnetic force because you can only cram so many amps through a confined space, which is even more confined on a small motor.

It will still make high power at high RPMs, but you have to gear it down to convert that power into more torque at low speeds. A gearbox makes it not really a Hub Motor at all any more. Terminology aside, a gearbox also adds even more weight and complexity inside the wheel. For less weight and complexity you can use a half-shaft to the center of the vehicle and use whatever motor you want inboard.

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