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First off, I don't know how often this happens. I would assume, however, that it occurs, mainly, under extreme conditions and with cheap cross-drilled rotors. Many high-performance cars are sold with cross-drilled rotors right out of the showroom. Porsche, Ferrari, Lamborghini, even some Mercedes, BMWs, and Audis, among probably many others, either offer them standard or as part of an upgraded brake package. So, if it were that big of a problem, I'd assume that they wouldn't offer them so freely.
The cracking is caused by the stress on the rotors during breaking and the extreme heat/cold cycles that the rotors experience during VERY hard use. The cracks generally develop right around the holes because the holes become a weak spot in the rotor. Chamfering the holes helps prevent this cracking, but, the edge of the hole itself is where a lot of the stress from expansion and contraction concentrates during heating and cooling.
The reason why the rotors are drilled to begin with is to allow for the release of gases that build up between the face of the rotor and the pad itself. As the pad heats up, the surface of the pad gives off various gases. This is kind of similar to, say, the gases that are given off by something when it burns. These gases can build up between the pad and the rotor and can reduce the friction between them, thus, decreasing the effectiveness of the brakes. This is called fade. A way to vent these gases is to drill holes in the rotor so that the gas can be vented from between the pad and the face of the rotor and channeled to the vented center of the rotor itself. Cross-drilling can also aid in the cooling of the pads and rotor surface.
Slotting the rotors is another way of doing pretty much the same thing. Slotting, however, also shaves a tiny bit of material off the face of the pad. This can have benefits. Glazing of the surface of the pad can occur when temperatures are too great for the pad to tolerate. It's almost like the surface of the pad melts and re-hardens, thus, taking on a smooth, hard texture that doesn't create friction well. Slotted rotors actually 'sweep' the face of the pad and sort of clean it so that there's always a fresh, smooth friction surface. Of course, though, slotted rotors tend to wear out the pads a bit more quickly.
Dimpling is very similar to cross-drilling. The difference is that dimples are created instead of holes that are drilled all of the way through the rotor. This prevents cracking because the structure of the rotor isn't compromised by drilling a hole through it. Gas builds up in the dimples and then is released as soon as the dimple emerges from under the pad. So, they're strong and fade-free, and don't tend to crack like cross-drilled rotors.
Again, though, I've only heard of cracking occurring under the most extreme braking (i.e. driving the piss out of your car on a track) and even then, low quality rotors were used.
Something to think about, too, is that cross-drilled rotors tend to warp more easily and are a little noisier.
posted by 216.84.3...
, Sun, 7 Aug 2005 08:26:58
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