MAJOR POINTS MISSED BY THESE TWO ROTORS
There are several key points that they failed to mention, rotor warpage and the main cause of it. The main cause of warpage in any material, object, whatever is heating it beyond a certain point where plastic deformation occurs, now there’s really no force being put on the rotor that it can’t handle, our force is heat, so think of heat being a force and there is a curve that this material will follow on when it is heated and cooled. Think of warpage when the material is pushed past a certain point along this curve.
Also cyclic loading of a material will also cause failure, or warpage in our case. Heating up and cooling of a rotor is our cyclic loading. Again, the more rapidly you do this, the greater your chances are for warpage. So if you go drive like a crazy person and then park your car the rotors will cool faster then if you drove nicely around for a couple miles to slowly bring them down in temperature.
That brings me to my next point; When you drive your car hard, and park it, the part of the rotor which is exposed to air cools at a different rate then the part that is touching the pad material. The pad material will retain heat a lot longer then the air, so you will get a spot on the rotor where the pad sat that will be warped. A friend of mine who works in motor sports, Indycar to be exact, told me that during a practice session they would go through rotors, i.e. warp them faster then they would if they were in a race. The main reason, like I said above, is the cyclic cooling of the rotors from really hot to cold, and the “hot spot” from the brake pad.
The main point or problem with what they say about warpage has to do with something they talk about, heat dissipation. They say that they have increased heat dissipation with cross-drilled rotors. This is partially true. At lower to medium temperatures the holes in the rotor allow more surface area, and as any good mechanical engineer or thermal scientist knows, you can achieve better cooling with more surface area. The holes in the rotor sort of act like the fins in your radiator, they aid in cooling the rotor. However those holes are a double-edged sword. They actually do more damage then they are supposed to help. When you cross drill a rotor, you take material out of it. Well guess what, that material is what helps that rotor maintain a certain level of performance before it warps. The more material you have the better heat distribution you get through the object. Think of it as I said before, a curve that this rotor follows when it is heated and cooled, and there is a certain plateau or ceiling where warpage occurs. When you remove material from the rotor, you decrease this ceiling. So it actually takes less heat to warp the rotor then it did before you started drilling holes in it. So if you look at the cooling by the holes in comparison to the loss of heat dissipation or heat tolerance that the rotor can handle, you have a balance scale. What is more important? Well In my book, I would want that ceiling to be as high as it could be to ward off the effects of warpage. Who cares if you rotor runs slightly cooler for normal use, and may cool slightly faster. The main thing they were saying was that cross-drilled reduces brake fade. This is 1/10 correct, better pads reduce brake fade.
Another thing, heat flows through the rotor as it heats up. With a normal rotor this heat is evenly dispersed and expelled. With the holes in cross-drilled rotors you get an interruption in this heat flow through the rotor. You again get hot spots or points where the material is discontinuous, and with these discontinuous points you get added stresses or higher temperatures at these discontinuities. So if you would look at a thermal image of a normal rotor you would see an evenly spaced out heat flow in the rotor. With cross-drilled you will see cold and hot spots in certain areas of the rotor. Temperature is a cyclic force and over time, with these hot spots occurring around the holes you end up with cracks, where the material has failed.
One last thing, there is actually a formula to calculate how much material can safely be removed from a rotor without compromising it’s ability to dissipate the heat generated in it. So with larger rotors you can have more or the same amount of holes and it don’t matter, there is plenty of material there. But with smaller rotors there is less material so you either need fewer holes to maintain a safe amount of material to dissipate the heat or you will lower the “ceiling” at which the rotor will warp.
Ok to sum everything up. Cross-drilled rotors are good for fast heat dissipation and reduction in braking gasses, however they are prone to warpage because of their less amount of material, and lower peak temperature tolerance. They are good for racing applications where you need very fast cooling from high-speed stops, and where they don’t care about the longevity of the rotors. They are not practical if you want to get more life out of your rotor. Slotted rotors main advantage is that they help get rid of the braking gasses between the rotor and pad. They are good for mild to medium racing applications and for the performance minded street driver. The longevity will be greater then that of cross-drilled, yet may be a little less then stock. There are also high performance rotors that offer a combination of both slotting and cross-drilling.
I guess now you may be asking what gives him the authority or background to be saying all this. Well first off I have gone through the hassles of cross-drilled rotors myself, and had the down sides of cross-drilling happen to me. Secondly I have researched and found NON-BIAS articles and information on the two types of rotors, as well as consulted several automotive professionals on the topic. So I’m not just talking out of my ass on these things. Finally, like other performance-minded drivers I too want to get the most out of my car, and have looked into the different possibilities and options.