The Braking Equation
STOP!!! Do you really know about brakes? Or do you read too many magazines?
Let's look at some common rotor "modification" and "performance" upgrades that you may have been exposed to and try to separate the marketing from the engineering. We'll start with size. Bigger isn't always better.
Brake Rotor Size
Bigger rotors will make your friends think you are cool. Bigger rotors look sexy. But bigger rotors do not stop the car. What a bigger rotor will do is lower the overall operating temperature of the brakes - which is a GREAT idea IF your temperatures are causing problems with other parts of the braking system. Take, for example, an F500 racecar - a small 800 pound single seat formula car. While the brakes are certainly much smaller than those found on a 3,000 pound GT1 Camaro, that does not necessarily mean that they need to be made larger. In fact, swapping on a GT1 brake package would probably do more harm than good - that's a lot of steel hanging on the wheel that needs to accelerate each time the "go" pedal is pushed. So, the moral of this story is “Bigger is better until your temperatures are under control.” After that point, you are doing more harm than good...unless you really like the look.
Cross-drilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads that are 50 to 60 years old, 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 and lightening feature than a performance feature. Contrary to popular belief, they don't lower temperatures. In fact, by removing weight from the rotor, the temperatures can actually increase. The holes can actually create stress risers, allowing 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 (look at any performance motorcycle or lighter formula car) that the rotors are drilled like Swiss cheese. While the issues of stress risers and brake pad wear are still present, these racing teams are replacing brake pads on a much more frequent basis - sometimes every race weekend. Drilling is used to reduce the mass of the rotor in spite of these concerns. (Remember – 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 trade-offs.)
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. This, in turn, helps 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?)
Last year a top race team bought 4 rotors. Two were bone stock, and two were subjected to a process know as Cryogenically Treating - one of the high-tech buzzwords floating around the paddock. The rotors were run back-to-back on the same track on the same car on the same day with temperatures taken to make sure that they saw the same level of heat. Following the track session, the parts were removed and we had them literally dissected by a materials lab.
The testing conducted included surface hardness, grain structure analysis, density, and surface scanning with an electron microscope. Guess what - after seeing the heat of use, the rotors looked identical in every regard. This is not to say that there is not a benefit from treating other parts that see lower temperatures and/or have different material properties. However, treating the rotors on the racecar showed no tangible benefits (note that it didn't seem to hurt anything either). Come to your own conclusions…
So, what's the secret recipe? Again, there is no absolute right or wrong answer. Like most modifications, there are those that appear to be well founded and those that “look cool.” If ultimate thermal performance is your goal, look to what the top teams are running (relatively large, slotted rotors). However, if “image” is your thing, break out the drill press – and be prepared to replace your brake pads on a regular basis.