# Deceleration during braking is not constant

1. May 30, 2012

### cholo1968

1. The problem statement, all variables and given/known data
When you slam on the brakes your car decelerates but this is not constant as it starts at a higher value and then reduces. This has something to do with the coefficient of friction between the brakes and the discs, but what is it and what are the reasons for this?

2. Relevant equations
Not applicable.

3. I am assuming that the coefficient starts high due to the brakes being cold but then drops off as the brakes and discs heat up due to the kinetic energy changing to heat energy. There must be another factor that changes the coefficient of friction to change the deceleration rate but I am not sure what. Is it the materials, the road surface, the force on the brakes dropping off?

2. May 30, 2012

### LawrenceC

Ordinary street pads when subjected to heavy use where they get extremely hot emit gases which get trapped between the pad surface and the rotor reducing the coefficient of friction. Also during harsh and prolonged braking, the pad wears away and the powdery material gets between the rotor and the pad lowering the coefficient of friction. When the pad pressure is released the material is gradually removed until the next application. On the other hand racing pads don't work as well until the rotor reaches a rather high temperature. Then the coefficient of friction goes up considerably. I have a track/street car that uses racing pads at all times. For the first couple of miles it takes considerably more foot pressure on the pedal to lock up the wheels than it does after driving a few miles. When good and hot, the braking is very good with less pressure.

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