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+--- Thread: PA safety inspection...failed!! (/showthread.php?tid=2092)
- stentech1 - 02-03-2010
The cracking that is present on the rotors is from expansion and contraction when subjected to the extreme thermal stresses created by the energy conversion process we call braking. The outer surface is expanding and contracting at such extreme rates rated compared to the body or core of the rotor that these cracks develop. Often these cracks are on the surface but only extend into the sub service a few thousandths of an inch. This happens to fly wheels as well with extreme clutch slippage. I have machined the rotors on my 996 tt that looked very bad with thermal stress cracks. After they were cut they looked almost like new. It is very difficult to evaluate the depth of these cracks with out resurfacing them. But any time a rotor is machined you are decreasing its thermal mass. Because there is less material thickness for heat to transfer to the rotor may warp easier (likely not a problem with a gt3 rotor). I did not need to take much off the rotors. Just make sure to be above the required thickness spec. Still have them on the car and they are fine. Lots of shops are just unaware of whats taking place when a rotor has these cracks and wrongfully assume they fail. It is a judgement call and many just don't have enough experience to do it properly so they often error on the side of caution which is not always a bad thing. If you need a place that will inspect it call me.
SM
- Darren - 02-03-2010
Also, the drilled rotors crack because heat accumulates on corners. So that corner between the hole and the surface of the rotor is where the heat builds up and then they crack. The holes fill up with brake dust and if you're patient enough to keep them cleaned out the rotors seem to last longer. I don't have time for that though....which is why I'd love to get rid of the holes. Unfortunately I think the holes have the best feel and they are also probably the least weight.
- Gene - 03-09-2010
An additional problem with the holes, whether they are drilled or cast (though cast is better) is that they act as stress risers in the material. As for weight I don't think the holes make a significant difference.
Weight is always a trade off on rotors... you want to reduce unsprung and rotating mass, but more mass in the rotor means it can absorb more energy before getting too hot.
- Mark Fox - 03-11-2010
Ken.............I think Sears Auto is having a sale. Seriously What a PITA. Paul Weston is a good contact source for brake/rotor. See his add in Pano. Or Joel Reiser "Reisertechnik" in central NY also in Pano I believe. Good luck.