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Comment by londons_explore | Hacker News Reader

I think hub motors like this are the future. Frees up space under the car for more battery. Removes the complexity of drive shafts and universal joints. Gives software full traction control. Reduces design time for new vehicles (just route a power and data cable to a wheel and you have a vehicle). Is the first step to all-electric braking.

The downside is more unsprung mass. But modern motors are getting lighter and lighter. And without a drive shaft, wheels could do crazy things like moving in and out to avoid potholes, or dynamically adjust camber to match the road you're on.

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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

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.

I wonder if centrifugal forces are useful for anything. For example, would there be any benefit in pressing battery cell plates closer together?

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