One of the important criteria when evaluating a brake pad is the 'coefficient of friction'. The coefficient of friction is the ratio of the force of friction between two surfaces and the force pressing them together. The smaller the coefficient of friction, the smaller the force that is required for the two surfaces to slip. The higher the coefficient of friction, the stronger the force that is necessary for the two surfaces to slip. The coefficient of friction is 1 when 100 kg of force (parallel to the ground) is required to move an object that weighs 100kg. If the same object can be moved with 50 kg of force. The coefficient of friction is 0.5. When calculating the coefficient of friction for the braking of a car, the braking torque that occurs when braking, and the fluid pressure that was required to apply the brake pad is used.
In general, OEM brake pads have a coefficient of friction of 0.3～0.4, and performance brake pads have a coefficient of friction of 0.4～0.5. The higher the coefficient of friction, the less fluid pressure (lighter push of the brake pedal) is required to create a high braking force. If the coefficient of friction is too high, there is too much friction and makes it very difficult to brake. The most important factor for the coefficient of friction is for the brake pads to reach its maximum friction level immediately after the brake pedal is stepped on. If the initial braking power is bad, it is commonly reffered to as brake pads that do not work well (Having bad initial bite). The second important factor is the stability of the coefficient of friction at various temperatures. It is common for the coefficient of friction to be lower at low and extremely high temperatures. This is a problem because there will not be enough braking power for street-use if the temperature is too low. Another problem is the decrease of the coefficient of friction at extremely high temperatures. Users who drive on race circuits need stable braking power even in those conditions. A quality racing brake pad will have stable performance from start to finish. The third important factor is the stability of the coefficient of friction at different vehicle speeds. Brake pads will be dangerous if a stable coefficient of friction was achieved during braking at 60km/h but unstable at 180km/h, selling the product would not be possible. At DIXCEL, we are constantly doing research and development to come up with a brake compound that has stable yet high coefficient of friction. To increase the coefficient of friction and stabilize the brake compound, materials like bronze, steel fibre, fibreglass, Kevlar, ceramic, titanium, carbon, etc are very popular. It is the ongoing challenge at every brake manufacturer to make the mixture of the materials to make the best brake pad.