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One may want to ride an electric bike with the electronics turned off. There are many reasons to do this, i.e. flat battery, fitness (who would have thought)

When riding an ebike conventionally, is there a resistive friction caused by the DC motor?

If so, do e-bikes have a way to 'disengage' the motor from the wheel? Potentially, a clutch

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Different electric assist bikes will present different circumstances. For instance, a standard hub motor (front or rear wheel) will have permanent magnets which react with the iron in the stator. You can feel what is called cogging when rotating the wheel by hand, without power. This is energy expended by hand, but also applies when pedaling without applying throttle. It's not zero, but it's also not a substantial amount.

Geared hub motors will have an internal freewheel, which prevents this cogging. One can rotate the wheel (in the forward direction) and not feel interaction with the magnets. This particular design has less energy loss to consider when pedaling without applying throttle.

Mid-drive motors can be considered as geared hub motors, as they also have freewheels that permit pedaling without motor rotation and the description above applies there as well.

My now-sold velomobile used a Stokemonkey electric assist which required me to pedal whenever power was applied by battery/throttle, but would not spin when I pedaled without application of throttle. There is a freewheel on the motor which had minimum resistance, but not zero.

stokemonkey drive system with freewheel

Typically, there will be very little additional friction or load to consider, although the non-geared hub motor design is going to be the least efficient when operating without battery power applied.

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Friction? Always, bearings do that plus air resistance.

Then there may be a magnetic drag - depends on the motor.

And as for a clutch, consider the free-wheel as used on most bicycles.

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