Brake Worksheet

[Richard Hudgins]

Hi Guys,

blykins said,

“Heat energy is generated from pad to rotor contact, and is disipated through air to pad relationship....”

This correct in part, however very little heat is dissipated directly from the pad to the air. The exposed surface area of the pad to the air stream is minimal at best. Rather the pad is an ablative material and most of the heat generated in pad is removed from the system by ablation.

(This is why pads are made certain sizes. A small pad will input just as much heat to the system as a large one, but it will ablate much quicker. This is the reason for 6 and 8 piston calipers. You increase the pad area to allow for this ablation and you have support this pad. Therefore, more pistons are needed to apply even force to the area. You do not increase clamping force with more pistons.)

Heat generated by the pad is typically transferred to the rotor. (Hopefully this transfer is at a rate greater than thermal conductivity rate of the pad material, Otherwise, one starts heating the pistons and therefore the fluid. )

Also, don’t forget coefficient of friction of the pad. Remember that this number is dependant on the temperature of the pad.

One must now start delving into integration.

If you really want to get confused, try running the numbers on carbon brake units. They are pure ablative systems. Less than 5% of the heat generated is stored in the system. As opposed to 65>80% in an iron/ablative system.


Brake thermodynamics are really quite complex and are really beyond my math capabilities. One of best friends and long time associates is my brake guru. He is physicist and holds many patents on brakes and brake materials. And the subject still is a bit of a black art to him when it comes heat transfer and optimal design of caliper and rotor systems.