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Originally Posted by racer_d
One advantage to the cross drilled units is their ability to vent heat and gasses away from the surface area which has already been mentioned.
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Venting boundry layer gases (as well as water and dust) away, yes. Venting heat, no. Cooling is what the vents are for (between the discs). The drill holes and/or slots help keep the brake pads clean. Drilled rotors may also possibly reduce rotating weight.
To quote the raceshopper site I listed above: "What handles more heat- A cast-iron kettle or a pizza pan with holes in it?"
Darrick Dong; Director of Motorsports at
Performance Friction: "Anyone that tells you that drilling makes the disc run cooler is smoking crack."
Grassroots Motorsports (Feb 2001): "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 cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. 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 brake pad life, at the expense of higher weight. It’s all about tradeoffs."