From digest.v5.n3 Fri Jul 19 09:27:58 1996
From: Pete Read <read_at_engr05.comsys.rockwell.com>
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
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).
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.
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