Build A Faster Car - helping drivers engineer

Brakes seem like simple enough devices: they stop the car. Whether this happens with traditional brake rotors and pads, a hybrid's battery-charging generator, or the drag from an air brake, the underlying physics are the same: kinetic energy is being sapped from the car and converted into an alternate type of energy. With a hybrid, that energy gets stored in a battery for later use, but with traditional brake pads, that energy gets converted to heat. The study of brakes is really the study of transfer of energy.

Disc brakes are- quite simply- a devices that convert kinetic energy to heat. The brake pad determines how quickly that happens, and the brake rotor dissipates the heat generated. Since the two are in contact with one another, the brake rotor also acts as a heat sink for the brake pad itself. More brake pressure or a higher-friction brake pad will increase the speed at which kinetic energy is converted to heat, stopping the car faster.

Thus brakes can be looked at as heat dispersing devices. After all, braking capacity is directly related to thermal capacity. How hot are the brake pads rated to? A street brake pad might work to 800 degrees F, but a race pad could work at twice that. It is the brake rotor's job to dissipate heat quickly enough so that the brake pad does not exceed its rated temperature range.

This sets up a few challenges. First of all, the amount of energy needed to stop a car goes up as a square of vehicle speed. Mathematically written:

energy = 1/2 * mass * velocity ^ 2

Thus, stopping from 85mph instead of 60mph- a 40% increase in speed- takes twice as much energy to once again stop. Going from 85mph to 120mph is another doubling of energy. Thus, the brakes must dissipate four times as much energy at 120mph than at 60mph, and this will translate into- at minimum- four times the braking distance.

So, how are brake rotors designed to cope with this? One is by simply absorbing the heat. The heavier the rotor, the more capacity it has to store heat. Raising the rotor's thermal capacity will increase the top speed of the vehicle with which the brakes can still stop the vehicle. However, the heat needs a place to go, or repeated braking will overheat the rotor and brake pads. Thus, venting a rotor allows it to shed heat faster. This means that the rotor will cool down more between applications of the brake.

Whether a street or race car, a brake rotor should be heavy enough to absorb all the heat top-speed-to-zero stop without any hint of fade from overheating. High temperature brake pads might be required to make that happen. Then, the cooling system of the rotor and brake ducting must be sufficient to make sure that rotor temperatures remain stable over time rather than increasing with each lap.

Some race cars use heavy but poorly ventilated brakes for qualifying laps. Why? Because the brakes don't need to be cool at the end of that lap- they just need to last one lap. For multiple laps, cooling capacity becomes very important indeed. Devices such as brake ducting help tremendously here by pushing ambient air through the vents of a brake rotor, and indeed it is rare to see a race car without them.