During the braking action, kinetic energy is converted into thermal energy by means of the friction created. Up to 90% of this converted energy is absorbed by the brake disc and then emitted into the ambient air. Thus, under extreme conditions, temperatures reaching as high as 700°C can prevail at the wheel brakes.
As well as being affected by physical stresses and strains, brake discs are also exposed to environmental influences, to dirt, water and salt. All these issues have to be factored into the equation when structural design is being planned by the manufacturers of brake discs.
The two-piece brake discs are internally ventilated and, therefore, because of their greater mass they possess better heat storage capacity and they also cool down more quickly through the air flow in the channels. These radial channels are located between the two friction rings. The rotation of the brake disc brings about a fan effect, which then creates a permanent air flow throughout the brake disc. An added feature is that two-piece brake discs can also have slits or grooves or they can be axially perforated. Brake debris, water and dirt gather in the slits or grooves and such material is propelled outwards by the rotary movement. The axial bores increase heat dissipation, but they are only slightly self-cleaning as brake debris can accumulate in the holes.