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The term friction brake applies to anything using a braking system involving friction. Friction, the act of two items rubbing together, reduces movement and is one of the factors involved in making brakes work. Cars use some form of friction brake, combined with other methods, to help them stop when the driver presses the brake pedal. Part of the brake presses against the wheels and the friction of the two surfaces rubbing together eventually causes the wheels to slow and stop turning. A friction brake falls into one of two categories: drum or disc.
In a car with disc brakes, force is applied to the outside of a disc located in the middle of the wheel. This force slows the wheel until it comes to a stop. Pressing the brake pedal harder creates more friction and pressure against the wheel, thus stopping it faster. Today's cars feature a power braking system that uses both a hydraulic system, known as power braking, and friction to stop the car. Older cars did not have power braking and were both harder to stop and had slower response times.
When the driver presses on a car's brake pedal, it sends hydraulic pressure to a set of pistons which are pushed outward by the force. Hydraulic pressure simply refers to the use of pressurized liquid to move parts of the car. The hydraulic pressure on the pistons pushes them against a set of brake pads and the brake pads in turn push against the rotor to stop its motion. Disc brakes are common on the front wheels of automobiles and many cars use disc brakes for all four wheels.
Drum brakes are the second type of friction brake and use a slightly different approach to stop the car. Hydraulic pressure is triggered by the brake pedal and pushes on a cylinder located in the wheels. Protrusions from the cylinder are forced outward and press on a set of brake shoes. The brake shoes are pushed against a metal drum inside the wheel which slows its movement. Whereas the disc brakes applied force onto the outside of a rotor, the drum brakes apply force on the inside of a drum.
These systems combine the use of friction braking with hydraulics and can even incorporate other systems as well. The goal when designing an efficient braking system is to create one which responds quickly, does not lock up, and produces the least amount of heat when used. The heat released from the friction must be gradually released away from the car. Too much heat can cause problems and complications when the system can no longer deal with the excess temperature.
While today's cars may use both types of friction brake systems, disc brakes are considered the better performers in terms of how they handle heat. The heat is released more easily, helping to prevent overheating. Overheating can cause the brakes not to respond as promptly as they should. Newer cars use either all disc brake systems, or disc brakes on the front wheels and drum brakes on the back.