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Re: Avoid cross drilled rotors on a street car (archive)

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Posted by Bill S on October 30, 2000 at 08:29:30:

In Reply to: Avoid cross drilled rotors on a street car posted by David Zeckhausen on October 29, 2000 at 08:36:17:

Agree with what you post, however I've run cross drilled for many 100K miles on varing autos, mostly BMWs w/o stress cracks, and have found that while for me they last better then oem rotors, I have them primarily because they stop better.
"It's the basic metallurgy that's critical"

Bill S
525iT x 2 with stillem drilled rotors

Cross drilling weakens rotors by introducing stress risers at the sharp edge where the holes are drilled. These stress points can result in cracks which can lead to catastrophic rotor failure. Champering the holes can reduce the stress points at the edges, but only on the outside surfaces. A vented rotor has four surfaces and there's no way to champher the two inner surfaces.

The only "cross drilled" rotors which don't have a tendency to crack are the stainless steel Porsche rotors. These rotors have holes cast in them during the manufacturing process and they are smaller. Thus, there is no tendency to crack.

If you STILL insist on cross drilled rotors, I suggest you pull the wheels at least monthly and inspect those rotors very closely on both sides for cracks. Replace the rotors if the cracks extend all the way to the edge or connect two holes together. Tiny surface cracks are normal, but larger ones are cause for concern.

The major factor in rotor longevity is not drilling holes or slotting or cryo tempering. It's the original manufacturing process. A non-drilled German rotor manufactured with quality materials and processes will always outlast a cheap, Chinese-made rotor that has been cross drilled, slotted, blessed by a holy medicine man, cryo-tempered, or drizzled with Slick 50. :-)

It's the basic metallurgy that's critical.

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