The amount of friction between
,two surfaces is expressed as a coefficient of friction (COF)
. The coefficient of friction is determined by dividing
the force required to pull an object across a surface
by the weight of the object (Figure 46-5). For
example, if it requires 100 pounds (445 Newtons) of pull
to slide a 100-pound (445 N) block of rubber across a
concrete floor, the coefficient of friction is 100 + 100 or
1. To pull a 100-pound (445 N) block of ice across the
same floor might require only 2 pounds (9 N) of pull.
The coefficient of friction then would be only 0.02. As it
applies to automotive brakes, the COF expresses the frictional
relationship between pads and rotors or shoes and
drums and is carefully engineered to ensure optimum
performance. Therefore, when replacing pads or shoes, it
is important to use replacement parts with similar COE
If, for example, the COF is too high, the brakes are too
sticky to stop the car smoothly. Premature wheel lockup
or grabbing would result. If the coefficient is too low, the
friction material tends to slide over the machined surface