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From digest.v5.n3 Fri Jul 19 09:27:58 1996
From: Pete Read <>
Date: Fri, 19 Jul 1996 10:10:02 -0700
Subject: Re: Cool Carbon Pads, Rotor Cracking/Warping

I can't resist adding a little more of my own limited experience to the Cool Carbon versus Metalmaster brake pad discussion. Both brand pads have worked fine for me. Cool Carbons just cost more and perform better (wider temperature range) at the track, but have some downsides for street use. I posted my Cool Carbon versus Metalmaster impressions a while back, so I'll try to say something new.

It's tough for any of us to make perfect assessments after one or two samples. I can understand why Ken Sax didn't want to try Cool Carbon pads again after the first rotorcracking experience with his Acura NSX. He just stated his single-sample conclusion a little too strongly (which he has since admitted).

Rotor Cracking

Given enough heat cycles, in theory, all rotors will develop cracks. Cracks aren't common because the minimum wear limit is usually reached before they crack. This may not be the case for driver's school cars because track-use gives brake rotors much more severe heat cycling than street driving.

Rotor cracks are caused by stress from uneven heating of the material. When brakes are first applied, the rotor surface heats up very quickly, while the inner material remains relatively cool. The hot surface tries to expand while the cool inner material doesn't want to budge. This puts the outer surface in compression (pushed together) and the inner material in tension (pulled apart). Once the inner material heats-up, the process is reversed as the rotors cool. The inner material is at high temperature while the surface cools first, putting the surface in tension and the inner material in compression.

Cast iron works good in compression but not in tension. Eventually the material fatigues and cracks from the back and forth tension/compression cycles. Ideally, for less rotor stress, you'd warm-up and cool-down your brakes slowly (no 150 mph first stops in below-zero weather). Cracks can form even faster from stress-raisers like imperfections when the rotor is cast and drilled holes (especially if the holes aren't chamfered to reduce the stress).

Rotor Warping

I've warped rotors with both Cool Carbon and Metalmaster pads, so I'm guessing it has been my lack of judgment when I've pushed my brakes too hard. My advice is to back-off when the brakes fade and be sure to cool down your brakes before you stop. Proper cool-down is probably the most important factor for preventing rotor warping.

Brakes fade when the pads have gone beyond their maximum effective temperature range. Not only does your car not stop, the pads also wear very quickly at these elevated "fade" temperatures. Just to clarify, fade is when the brake pedal feels fine but the car won't stop. Brake fluid boiling is when the pedal goes to the floor.


Pete Read
'88 M5

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