Originally Posted by Excerpt taken from the February 2001 issue of Grassroots Motorsports magazine:
(Full copy of text can be seen here: http://www.mr2sc.com/)

Crossdrilling your rotors might look neat, but what is it really doing for you? Well,unless your car is using brake pads from the ‘40s and ‘50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses somewhere to go. It was an effective solution, but today's friction materials do not exhibit the same gassing out phenomenon as the early pads. For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads - sort of like a cheesegrater rubbing against them at every stop. Want more evidence? Look at NASCAR or Fl. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it. The one glaring exception here is in the rare situation where the rotors are so oversized that they need to be drilled like Swiss cheese. (Look at any performance motorcycle or lighter formula car, for an example.) While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember that nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brakepad life, at the expense of higher weight. It’s all about tradeoffs. Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?) If you need to worry about brake pad degassing, then go with slotted rotors. Slotting is a much more superior way of preventing decreased brake performace due to brake pad degassing.

Originally Posted by RXSevenSymphonies

http://www.fc3spro.com/TECH/MODS/BRAKES/rotors.htm

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