Much like brake pads, brake discs wear down over time and need to be replaced. The discs, which are also often called brake rotors, are what the brake pads clamp onto to stop the wheels from spinning. When the driver presses the brake pedal, the pads are signalled to close down on the rotor, which spins with the wheel. The friction generated from this action creates heat and will wear down the brake pads and rotors over time.
Brake rotors are made of metal, so they will wear out at a slower rate than brake pads. The recommended schedule for inspecting the rotors can be found in a vehicle's owner's manual, but if you're already checking the brake pads, you may as well take an extra few minutes and make sure the brake discs are in good health. The rotors can wear down over time, but they may also warp or develop deep scratches that can affect their performance.
Wear is the most common issue that can arise with brake discs, and if it is not addressed, it can lead to warping and discoloration. When they are too thin, the brake system will overheat quickly. This can cause the brake fluid to boil and the ability to brake may be lost or limited. When brake rotors warp, wobbling or pulsating vibrations can be felt in the pedal.
Any imperfection, or runout, that alters the surface of the disc can create a gap between it and the brake pads on either side. The uneven surface of the rotor may push the brake pads away from their resting positions. Drivers may notice that they have to press the brake pedal further when this occurs because the pedal must first push the brake pads to their original positions. If the brakes are applied again before the pads are pushed out beyond their open position, the pedal should feel normal.
The rotor must have a smooth, even surface for the brake pads to effectively slow them down, but sometimes the discs can become scratched. When the brakes are cool, run a finger along the brake disc surface. If you can feel grooves or other imperfections, the rotor may need to be replaced. It can be refinished if it is thick enough, but a rotor that is too thin will likely need to be replaced.
Another thing drivers can check is the thickness of the brake discs. Measure different spots around the circumference of the rotor. The measurements should all be pretty similar, and any disparities can point to warped discs.
Drivers who want to enhance the braking performance of their vehicles have a few options - the two main types of performance rotors are drilled and slotted. Drilled brake rotors have holes that allow heat, gas and liquid to dissipate easily. This can increase the brake system's longevity, and make it safer during rainy or wet weather. Wet brakes can be slippery and make it harder to stop, but drilled rotors can keep the discs dry. However, the holes also make the rotors structurally weaker, which may even lead to cracks.
Slotted brake discs go against the grain of traditional rotors. They are made with slots already carved into the surface, which serve the same function as the holes in drilled brake discs. This type of rotor is typically reserved for high-performance racetrack driving because it is more durable than a drilled brake disc.