Instead of traditional "expanding" brakes that act in opposite directions on a round drum, disc brake systems use discs with brake pads mounted on both sides. An easily observable resemblance is to a handbrake on a bicycle. The pads compress the rim of the bicycle wheel, slowing down the movement. Automotive disc brakes use the same principle, but the braking force is not applied to the wheel, but to a separate disc.
The disk, or rotor, is a single unit installed right inside the wheel. Discs are solid and ventilated. Ventilated discs have fins around the circumference between the two braking surfaces to allow air to enter, which cools them and makes them less susceptible to heat breakdown and aging. Brake discs are slightly affected by dirt and water, since all sediment is discarded by the centrifugal action of the rotor, or is cleaned by the pads. Also, the equal pressing action of the two brake pads provides an even, confident stop, while the uneven pressure of the pads of the left and right wheels can cause unwanted traction when braking. All disc brakes are self-adjusting.
There are three main types of disc brakes. In fixed caliper design
two pistons are used, mounted on either side of the rotor (on each side of the caliper). The caliper is rigidly mounted and does not move. Such a braking system is very effective, but the size of the caliper and its mounts add to the weight and size of the car.
Sliding and floating structures are very similar. In both types, one brake shoe comes into contact with the rotor under the action of hydraulic force. The caliper, which is not rigidly fixed in a certain position, moves somewhat and ensures contact of the other pad with the rotor. There are various ways to attach floating calipers. Some pivot around the base or top, others slide along the mounting bolts. Various causes of uneven brake wear can be caused by dirt or sticking in slip or turn areas.