Rusty Brake Discs On Electric Cars: The Little-Known Side Effect Of Regenerative Braking

Electric cars do most of their slowing through regenerative braking, so the friction brakes rarely work and the discs rust easily in humid rainy-season air. A few small driving habits and routine checks at service time keep the brakes ready for the moment they are truly needed.

Many electric car owners feel proud when a technician tells them the brake pads are still thick after tens of thousands of kilometers. That is one of the real advantages of regenerative braking: when the driver lifts off the accelerator or presses the brake pedal lightly, the electric motor works in reverse as a generator, slowing the car while charging the battery. The friction brakes, meaning the pads and discs, only do serious work during hard braking or as the car comes to a stop.

The catch is that brake discs are made of cast iron, which rusts easily. On a petrol car, the thin layer of rust that forms after rain or a car wash is scraped off by the pads within the first few stops. On an electric car the pads rarely touch the discs, so the rust is left to build up. In a humid country like Indonesia, especially in the rainy season when cars often drive through standing water, the process can be much faster, particularly on the rear wheels, which do a smaller share of the braking.

The symptoms tend to creep in slowly. There may be a rough grinding sound on the first few stops of the morning, the disc surface may look brown and speckled instead of shiny, or the steering wheel and pedal may vibrate slightly when braking from high speed. In worse cases, rust pits the disc surface and the pads only make contact across part of it. As a result, emergency stopping distances can be longer than the driver expects, at the very moment the friction brakes are needed most.