Regenerative Braking: Effect on Pads and Rotors

How does regenerative braking slow the car?

In a hybrid or EV, the electric motor that drives the wheels can also work as a generator. When you lift off the accelerator or press the brake pedal lightly, the motor resists the wheels' rotation. That resistance slows the car and turns some of its motion into electricity that goes back into the battery.

Most of these vehicles blend regen with the ordinary friction brakes. You press one pedal, and the car decides how much of the slowing comes from the motor and how much from the pads and rotors. Harder stops, a full battery or a cold battery can shift more of the work to the friction brakes.

Why do hybrid pads wear so slowly?

Because the motor does much of the everyday slowing, the pads are asked to do less. In ordinary driving, many stops are handled mostly by regen, so the pads touch the rotors for less time and with less force than on a conventional car.

The result is that pads on a hybrid or EV often last noticeably longer. How much longer depends on the vehicle, how it is driven, and how its regen is set up, so a technician's measurement is a better guide than any rule of thumb. Long pad life is not a reason to skip brake inspections, as the next section explains.

Why do EV rotors rust, and what does that feel like?

Brake rotors are made of iron, which rusts when it is exposed to moisture. On a conventional car, the pads wipe that thin surface rust away every time you brake. On a vehicle that brakes mostly with the motor, the pads may sweep the rotors only rarely, so rust and corrosion can build up on the rotor faces.

• A scraping or grinding sound: at low speed, light braking can make rough noises as the pads pass over a rusty surface.
• A rough or pulsing feel: uneven rust can make the pedal feel different or send a vibration through the car.
• Visible rust: a heavily rusted, pitted or ridged rotor face can be seen behind the wheel.

Light surface rust is common on any rotor that has sat for a few days. Pitting, ridges or a persistent rough feel are worth having inspected, since badly corroded rotors may need replacement.

Why do some EVs have rear drum brakes?

Some EVs use drum brakes at the rear wheels. Because regen handles much of the slowing and the rear brakes do the least work, a simpler, compact drum design can be enough there. Drums are also enclosed, which protects the friction surfaces from the weather and helps limit corrosion on parts that are used so rarely.

Drum brakes are explained in more detail in our comparison of disc and drum designs. If you drive a hybrid or EV and have noticed unusual noises, a rough pedal or visible rust, a brake inspection will show what the pads, rotors and fluid look like and what, if anything, needs work.