Heat is speed and vice-versa, plain and simple. In fact, plainer and simpler than most things once we’re talking about brakes, because the brakes translate the kinetic energy – the speed or momentum – of a large-scale object – the car or truck – into the kinetic energy – the speed again – of very small-scale objects, the molecules of the brake parts and the air they heat. If you stop a car from a high cruise speed, you have increased the speed of the molecules of the air, the heat, by exactly the same amount of energy the car gave up to stop.