A disc brake slows a vehicle by using friction between brake pads and a spinning metal rotor. When the driver presses the brake pedal, hydraulic pressure pushes the pads against the rotor, converting the car's kinetic energy into thermal energy. This heat leaves through the rotor, pads, caliper, wheel, and surrounding air.
Understanding brake wear matters because worn parts reduce stopping performance and can become unsafe.
Key Facts
- Kinetic energy before braking is KE = 1/2 mv^2, so doubling speed makes four times as much energy to turn into heat.
- Friction force can be estimated by Ff = μN, where μ is the coefficient of friction and N is the normal force pressing the pad on the rotor.
- Brake pads wear because a small amount of pad material is rubbed away each time friction acts at the pad rotor contact surface.
- Rotors wear because heat and friction can remove metal, form grooves, create uneven thickness, or cause warping from repeated heating and cooling.
- Hydraulic brake pressure follows P = F/A, so pedal force is transmitted through brake fluid to push the caliper piston outward.
- Parts need inspection when pads are near minimum thickness, rotors are below minimum thickness, braking vibrates, squeals, pulls, or stopping distance increases.
Vocabulary
- Brake pad
- A replaceable friction part that presses against the rotor to slow the wheel.
- Rotor
- A metal disc attached to the wheel hub that spins with the wheel and is squeezed by the brake pads.
- Caliper
- The brake component that holds the pads and uses a piston to clamp them onto the rotor.
- Hydraulic pressure
- Pressure carried by brake fluid that transfers the driver's pedal force to the brake caliper.
- Friction
- A force that opposes sliding motion and converts mechanical energy into heat in a braking system.
Common Mistakes to Avoid
- Assuming brake pads and rotors last the same amount of time is wrong because pads are designed to wear faster and be replaced more often than rotors.
- Ignoring squealing, grinding, or vibration is wrong because these signs can mean the pads are thin, the rotor surface is damaged, or metal parts are contacting each other.
- Thinking harder braking only increases stopping force is wrong because it also greatly increases heat, which can speed wear and cause brake fade.
- Replacing pads without checking rotor condition is wrong because grooved, warped, or too thin rotors can damage new pads and reduce braking smoothness.
Practice Questions
- 1 A 1200 kg car slows from 20 m/s to rest. How much kinetic energy must the brakes convert mostly into heat? Use KE = 1/2 mv^2.
- 2 A brake pad is 12 mm thick when new and should be replaced at 3 mm. If it wears 1.5 mm every 10,000 km, after how many kilometers from new should it be replaced?
- 3 Two drivers have identical cars. One brakes gently and early, while the other brakes hard at the last moment. Explain which car's brake pads and rotors will likely wear faster and why.