Refers to the ratio of the friction between the two surfaces and the vertical force acting on one surface thereof. It is related to the roughness of the surface, regardless of the size of the contact area. According to the nature of the movement, it can be divided into dynamic friction coefficient and static friction coefficient. The most important performance index of the brake pads is the coefficient of friction. National standard brake friction coefficient between 0.35-0.40. Qualified brake pads have a moderate and stable friction coefficient. If the coefficient of friction is less than 0.35, the brake will exceed the safety braking distance or even the brake failure.