Disc brake pads consist of friction material bonded onto a steel backing plate. The backing plate has lugs that locate the pad in the correct position in relation to the disc.
Calipers are usually designed so that the condition of the pads can be checked easily once the wheel has been removed, and to allow the pads to be replaced with a minimum of disassembly.
Some pads have a groove cut into the friction surface. The depth of this groove is set so that when it can no longer be seen, the pad should be replaced.
Some pads have a wire in the friction material at the minimum wear thickness. When the pad wears to this minimum thickness, the wire touches the disc as the brakes are applied. A warning light then tells the driver the disc pads are due for replacement.
The composition of the friction material affects brake operation. Materials which provide good braking with low pedal pressures tend to lose efficiency when they get hot. This means the stopping distance will be increased. Materials which maintain a stable friction co-efficient over a wide temperature range, generally require higher pedal pressures to provide efficient braking.
Disc rotors with holes or slots in them dissipate their heat faster, and also help to remove water from the surface of the pad in wet driving conditions. They also help to prevent the surface of the pad from becoming hard and glassy smooth from the friction and heat of use. However, this scraping action reduces the overall life of the brake pad, so these types of discs are generally only used in high performance or racing cars.