From OCAU Wiki
- Cross drilling brake discs is bad.
- The drilling helps in out-gassing. When the pads and discs heat up, the pads start to vapourise, and basically turn to gas. With a standard disc, when you get gas between the pad and the disc, a split second at the start of each brake application is wasted pushing out the gas between the pad and disc. Slots or cross drilling allow this time to be minimised by giving the gas room to escape.
- Remember that there are two functions of a brake disc:
- To absorb heat from the braking cycle (the kinetic energy of the moving vehicle is converted to thermal energy in the disc)
- To dissipate that heat back to the atmosphere
- Cross drilling and slotting do nothing to assist in the absorbtion of kinetic energy as heat, but assist greatly in the dissipation of the heat (along with ventilation of the discs).
- The only way to increase the absorbtion of heat is to increase the swept area of the pad, either by having larger pads, or higher diameter rotors.
- Larger pads have their own problems in that a single piston cannot provide even force across the whole face of the pad, that's why there are 2, 4, 6, 8 and now 12 piston calipers making an appearance, to spread the force more evenly over the face of the pad.
- The reason cross drilling is preferred to slotting is basically aesthetic. It looks cooler and there is a perception of higher performance.
- The total heat capacity of a disc (the amount of heat, i.e. the number of hard stops it can perform before overheating) is determined by the material it is constructed from(usually cast iron), and the sheer volume of that material. Aluminium rotors were tried out on the Lotus Elise to minimise weight, but it was found that the discs had to be physically huge to achieve the same heat capacity as a cast iron disc.
- On a purely road car, cross drilling is generally fine if it is done properly and doesn't go too near the edge of the disc. Remember that drilling a bunch of holes removes quite a lot of pad material, reducing braking capacity.
- Each of the holes assists in dissipating heat back to the atmosphere, however, the edges of a cross drilled hole cool down faster than the rest of the disc and the expansion caused by different temperatures can result in a disc cracking at the edge of the hole when it is heat cycled.
- Slotting removes far less material, and there is also far less temperature difference between the edges of a slot and the rest of a disc. That is why most discs on race cars are slotted but not cross drilled.
- Braking performance is dependant mostly upon the ability of the disc to transform kinetic energy into thermal energy. This is assisted by having a greater swept area.
- The total amount of thermal energy the disc can absorb is dependant solely upon the disc material and volume.
- The dissipation of heat into the atmosphere is by far less important to the braking cycle, it is solely cooling the disc back down between applications to maximise the above braking capacity (2)
- Remember that cross drilling assists slightly in the time taken to apply the brake pedal, but decreases the ability of the disc to transform kinetic energy to thermal energy (1) by reducing the surface area of contact between the disc and pad. It also reduces the total heat capacity of the disc (2) by removing disc material. It assists slightly in the dissipation of heat back to the atmosphere (3), but brings stress and thermal cracking problems with it.
- For a race car, the slight advantages of cross drilling in steps (1) and (3) is far outweighed by the disadvantages in step (1) and (2).
- Remember that cross drilling and slotting both also assiste in cleaning the pad face to maintain a healthy pad to disc contact surface.
- Anyway, that's my essay for the day! Hope everyone can follow it :D
Note:Bear in mind that this doesn't apply to carbon / ceramic braking systems, and also is less of a problem with discs that have the holes cast into them rather than drilled (Porsche, Ferrari etc).