What Are Brake Pads?

Brake pads are the part of the automotive brake system that provide friction, and are designed to wear out instead of more expensive parts like rotors. Located in the disc brake system between the caliper and rotor, the brake pad has to be of a composition that has a good coefficient of friction characteristic. The brake pad has to provide friction, but be hard enough to have an acceptable service life.

Coefficient of friction

Friction is defined as a resistance to relative motion. Co-efficient of friction is defined as a particular surface's resistance to relative motion when in contact with another surface. For example, a block of rubber on a sheet of glass resists motion better than the same weight of ice on the same glass surface. This means that the block of rubber has a higher coefficient of friction than the block of ice. The type of friction we use in the brake system is called kinetic friction. This type of friction changes kinetic energy from the moving vehicle into heat energy. The brake pad has to have a high enough coefficient of friction to convert the energy of the moving vehicle into heat, and be tough enough not to be destroyed under the high heat generated by the brake system.

Organic Pad Composition

The first type of brake pads used on passenger cars and trucks was of organic composition. Organic pads are made up of particles of organic material like carbon black, graphite and asbestos. These pads featured quiet operation and tended to be larger than today's semi-metallic pads. The larger size allowed the pads to operate at a lower temperature than today's pads due to brake fade that can occur at a relatively low temperature in this composition.

Semi-Metallic Pad Composition

Beginning in the early '80s, brake systems began to be downsized by the engineers in an effort to lower un-sprung weight. Un-sprung weight is the weight in the vehicle that isn't supported by the suspension. It includes some suspension parts, the wheel and tire assembly, and the brake system.

As the brake system became smaller, it also began to operate at elevated temperatures. The organic pad could not operate at these temperatures, and a suitable replacement compound had to be found. The answer was semi-metallic pads. The pad is made up of carbon black, and bronze, as well as other man-made fibers, held together in a resin. It last longer and resists fade due to its higher operating temperature.

Synthetic Pad Composition

Late model synthetic pads are made up of man made particles like fiberglass, Kevlar and ceramic. These pads feature smooth quiet operation, reduced brake fade, reduced dusting and extreme heat tolerance. The downside to these pad compounds is that they do not perform well at low temperatures, and are inefficient until heated up after the first few stops.

A common misconception is that they do not create brake dust. They do create dust, but less of it. The dust they do create is a light gray color instead of the familiar black dust from semi-metallic pads.

Summary

The brake pad is the functional part of the brake system that gives us the stopping power we need, while having a good service life and fade resistance.