[DeTomaso] Wilwood Brakes WTF????
Ken Green
kenn_green at yahoo.com
Sun Apr 14 18:02:40 EDT 2013
I don't know if this has anything to do with your wear, but just about all the high end calipers are differential bore, at least partly to even out wear. The 996 turbo front calipers have 36 and 44 mm dia pistons (the trailing pistons have 50% greater area. If the leading edge of the pad wore more, this might explain it, but I have no real experience. One of the serious racers would know a lot more.
from the Internet:
* Differential bore calipers -- As the surface of the rotor heats up, the clamping force of the pistons has to be increased to avoid brake fade. If the caliper has multiple pistons (or multiple pairs of pistons), the brake rotor surface is initially heated by the pistons pushing against the brake pad at the leading edge of the caliper, making the rotor surface hotter when it rotates back to the pistons closer to the trailing edge of the caliper. Therefore it helps if the pistons closer to the rear edge of the caliper are larger. Differential-bore calipers use smaller pistons up front, larger pistons toward the back.
From: "MikeLDrew at aol.com" <MikeLDrew at aol.com>
To: detomaso at POCA.com
Sent: Sunday, April 14, 2013 2:07 PM
Subject: [DeTomaso] Wilwood Brakes WTF????
Hi guys,
So, you may recall my description of me beating the snot out of my Pantera
at Laguna Seca a couple of weeks ago. I had a super, super time!
Partway through the second session, I started to get some juddering in the
steering, from the brakes. I presumed that the cheapo rotors had warped
slightly, but I was able to drive through it, and then unwarped when the car
cooled off. I noted with some surprise that the front pads had worn down
50% in just two 30-minute sessions while the rears looked perfect. The wear
was perfectly even.
I then had an uneventful three-hour drive home. During one of my stops, I
took the time to feel the wheels for any excess heat that might be caused
by brake issues, and didn't feel anything untoward.
The next time I drove the car, I noted that the brakes were definitely
sticking (or at least, SOMETHING was sticking). I could drive down a gentle
slope, put it in neutral and the car would gently come to a stop. Obviously
not right.
Today I jacked up the car and pulled the wheels, and was shocked at what I
found:
http://www.poca.com/index.php/gallery/?g2_itemId=39244
http://www.poca.com/index.php/gallery/?g2_itemId=39260
Both brakes were clearly stuck on, with the pads badly tapered and worn
within microns of bare metal on the bottom sides.
Since I have 2 psi residual pressure valves in the system, I thought that
perhaps the front one had failed and stuck on. If that was true, then it
would act almost as if somebody was stepping on the pedal. To test that
theory, I cracked a bleeder on each caliper, expecting brake fluid to jet out.
Nothing.
So the problem is more mechanical in nature, rather than hydraulic. I bit
the bullet, pulled the pads and then removed the passenger side caliper.
The pistons were very, very difficult to move within the caliper; I managed
to compress one almost all the way before wising up to the fact that it
would be almost impossible to remove if it was flush. The other three came out
with some work, that first one came out after a LOT of work.
The rubber seals inside the calipers look and feel just fine--having said
that, I don't exactly know how they are supposed to feel, but they are soft
and pliable. The pistons were covered with brake dust, but a few minutes
spent with brake cleaner and a rag, and some emory cloth, and everything
cleaned up perfectly.
http://www.poca.com/index.php/gallery/?g2_itemId=39251
So, the question is, what caused this? Could excessive heat have caused
the pistons to expand and swell and jam in the caliper? Not what I would
expect from 'racing brakes', and the pedal never went even slightly soft, so
it's not as if the fluid was boiling.
Does anybody have any other theories as to the cause?
And more to the point, what should I do now? I suppose since they are 20
years old, and apart already, I should probably buy new seals (or new seals
and pistons?) and swap them out before reassembling everything. But this
still begs the question--why did this happen????
Looking forward to words of wisdom (along with inspired wild-ass guesses!)
Mike
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