why can’t we use magnetic brakes instead of friction brakes in cars

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Couldn’t we just apply a magnetic force to the spinning wheel to slow it down?

In: Physics

Magnetic brakes don’t work well at low speeds. You need the metal to be experiencing a sudden change in magnetic fields for it to generate strong enough eddy currents to push back with much force, otherwise you’d need a large area or number of field changes to generate useful force. This means you’d still need your friction brakes to slow you down from most slower speeds in which case why add complex magnetic brakes? You still need rotors, calipers, and pads but now you need ferrous rotors and a magnetic field generator **and** your stopping force is still limited by the tires grip on the road

The one good application for magnetic brakes is trains. They have Eddy Current Brakes which help slow the train down. Trains have the ability to have a lot of eddy current brakes to help make up for the poor traction between the wheels and the rails, and since the rails are already ferrous nothing needed to change, it was just a straight improvement in the stopping power of trains because the magnets gave a second path for braking force to be applied rather than just through the already overloaded wheel/rail contact surface.

It’s called regenerative braking, and it’s used in electric and hybrid vehicles. The electric motor can also act as a generator, recharging the battery. This causes eddy currents that can provide a lot of braking resistance. You don’t want to rely on this to stop a car, because it can’t. The slower you go, the weaker the effect, so you still need a clamping force to bring the car to a complete stop.

The force of magnetic brakes is proportional to the speed of the vehicle. The brakes would be nearly worthless at low speed. You could make an active electromagnetic brake but it would be heavy and expensive and defeat the point of whatever you were trying to do with it.

You could…but you can’t slow down as fast as Friction brakes.

As mentioned, hybrid cars do use magentic brakes principle to Regen. But effectiveness of Regen is diminished as speed is reduced. Under 10mph, there’s basically no effect.