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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 rotor-
cracking 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. 
 
 
Regards,  
 
Pete Read 
'88 M5