Cross-Drilled /Slotted Rotors

The second thing you can do to improve your brake performance is often to go to a larger rotor. We all know that this gives the rotor further ability to dissipate heat away from the pads through itself and through the air (conductive and convective heat transfer). So obviously a larger pad, a larger rotor, or both result in better brake performance by avoiding brake fade.

But what about cross drilled or slotted rotors? Well the common belief in the main stream is that somehow slotted or cross-drilled rotors allow for better performance by handling heat. This is 100 percent false. The individuals involved in such fallacies mention that air through the holes or slots work to cool the rotor (convective heat transfer into the air from the rotor). The issue is that from physics we know that metal transfers heat better then air by a significant amount. As such the larger mass of the rotor becomes more important then the larger surface area of the rotor in any situation other then the optimal. Cross drilling and slotting rotors are not optimal manners of creating metal to air transfer through larger surface areas. There is not much airflow through the holes or slots. Furthermore for cross drilling the holes will fill with brake dust in effect lowering the cooling ability of the rotors vanes they pass through.

Rigidity

From the information above we can glean that the rotor begins to work as a heat sink. Now by cross drilling or slotting we are decreasing the overall amount of metal to transfer this heat to. Clearly we are decreasing performance of the rotor to dissipate heat amongst itself. Furthermore, the holes of a cross-drilled or slotted rotor decrease the area of the pad that contacts the rotor. This concentrates the heat more on certain areas of the pad, which has similar effects to that of using a smaller pad. As such the pad heats up more quickly.

We are also damaging the brakes structural rigidity. The iron in a brake rotor is made of a crystalline structure. By drilling holes in said surface we cut the end grains creating a situation that breeds cracks. Furthermore, even if we were to cut the rotors correctly to avoid cutting the end grains structural rigidity is still decreased. The temperature around the holes will be slightly less then that of the entire rotor leading to temperature stress. Moreover, the decreased mass will result in lowered rigidity.

Advantages

So what do cross drilled and slotted rotors accomplish? The main original purpose of slotted and cross-drilled rotors was to vent gases that buildup between the pads and the rotors. However, this reasoning is no longer valid. As the years have gone by pads have been designed that produce very little gas. Furthermore many pads come with groves in themselves that allow for the removal of any minor gas that is created. A slotted or drilled rotor always decreases the rotors capability to dissipate heat amongst itself. A slotted or drilled rotor will also clean off the brake pad as it passes the slots at the expense of faster pad wear. As such there are benefits for rally and dirt tracks. Furthermore, the slots or holes themselves can serve to wipe off the top layer of glaze that tends to appear on your brake pads. Some racers say this last part is beneficial while others question whether the slots will fill before the deglaze affect is ever helpful. I have yet to determine the answer to this question.

The answer of slotted and cross-drilled rotor usefulness seems to lie with whether the benefit of cleaning the pads outstrips the loss in heat dissipation. In terms of cross drilling there are so many costs that nothing is accomplished beyond perhaps giving you a certain bling look. In a motorcycle or other extremely light vehicle the decrease in rotational inertia and unsprung mass might perhaps be useful (once other more efficient avenues are exhausted). However, in a street car or race car the speeds and weight of such vehicles will make the relatively miniscule decrease be outweighed by the need for more heat dissipation. Slotted rotors, meanwhile, share the positives of cross drilling but notably are slightly less subject to the costs. They do not impede airflow through the rotors vanes, nor do they have as large an affect on structural rigidity. Therefore, the need for slotting depends on your application.