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      11-26-2008, 07:58 AM   #24
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Drives: E92 M3
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Location: San Diego, CA USA

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Originally Posted by lucid View Post
The point is that you don't know if it is pure speculation or not. You don't know if there is a team of engineers behind this guy who are telling him to say what he is saying, or if the guy is an engineer himself, or if he simply learned certain things via lots of experience, which is possible.
What is most likely? Doesn't mean we can know for sure, but I definitely know what is most likely.

Originally Posted by lucid View Post
Anyway, can you explain to me why exactly there would not be any performance difference between a steel rotor and a carbon ceramic rotor in their ability to resist fade? Assume that the same amount of heat is being generated at the pad to disc contact surfaces due to friction in both cases.
I'm pretty sure it depends on which non composite system you are comparing to. All brake rotor/pad material combinations will exhibit a knee in their friction vs. temperature curve. Pass the knee and you will get fade. When properly matched, CC pads/rotors can operate in the 800-1000 C range (yes C) whereas alloy steel rotors and race pads will only get up to about 700-800 C (normal cast iron rotors will actually become so weak they will fail around 700 C). Unfortunately, I don't know exactly where the knee points are in the curves for each combination. I'd like to. But this also begs the question of the COMBINATION. The knee point is highly pad dependent. Is it fair to compare a system that works great for street use and driving events to one that is so racy it only works at the track?

Next on to temperature rise. For a given heat energy (a given bleed of speed) CC rotors will actually get much hotter than a comparable iron or steel rotor. CC has a much higher specific heat but the equivalent rotor is so light it more than makes up for the higher specific heat. Doing the arithmetic you find the temperature gain for any given heat energy is typically about 40% higher in the CC setup (assuming all heat energy goes into the rotor, which isn't a terrible first order approximation).

So does a CC setup have a knee point 40% higher than a traditional non composite metallic rotor set up? I'm not quite sure. I would guess that in fact it is more than 40% higher compared to an OEM quality cast iron rotor/sporty pad combo. So that means it would take more energy to reach the fade point with the CC system. But it is highly dependent on exactly which system you are comparing it to, the system above vs. say a much stronger and heat resistant alloy steel rotor and high temp track or race pad combination. These latter systems may be about on par with the CC system or maybe even exceed it.

But, as I mentioned, the wear/longevity and weight reduction issues are absolutely clear cut and significant. Many CC systems are rated for "lifetime" or about 300k km and the rotors can weigh as little as 1/3rd of an iron/steel rotor. These are the real advantages.
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