[200q20v] RE: Caliper piston extension distances

Michael (Prospeed Motorsports) michael at prospeedmktggrp.com
Thu Nov 8 21:21:15 EST 2001

First, I will apologize in advance for the "pissy" attitude I
demonstrate in the response below. I am one of few people in the country
that specialize in rebuilding and reconditioning Porsche/Brembo
calipers. I was merely attempting to share the straight facts with the
group. Once again, I apologize for my response in advance... But I tired
and its been a long day.

BTW... A big "THANK YOU" to the few guys (you know who you are!) that
took the time to call today and tell me personally how much they enjoyed
my knowledge and straight facts when it comes to these brake systems.

Everything I have shared with this list is based up personal hands on
experience. One of the services I offer is to rebuild Porsche/Brembo
calipers. I am known in the Porsche market for being the only stocking
source for individual Brembo parts (seals, bleeder screws, rubber caps,
etc.) I sell these parts and provide consultation to customers in the
USA and Europe. And I design Big Brake systems for various Porsche parts
vendors. Does this mean I know everything... No. But it certainly means
I spend an inordinate amount of time doing nothing but brake caliper and
rotor work.

My final comments are interlaced below. I will not address this topic
any longer except privately.

> -----Original Message-----
> From: Bernie Benz [mailto:b.m.benz at prodigy.net]
> Sent: Thursday, November 08, 2001 8:24 PM
> To: michael at prospeedmktggrp.com
> Cc: 200q20V mailing list
> Subject: Re: [200q20v] Caliper piston extension distances
> Of course, Micheal,

BTW, my name is spelled "M I C H A E L"

> you should practice what you preach, but
> IMO, your hypothesis is shear speculation on your part, not
> based on factual failure modes.  Further comment interlaced below.
I do practice what I preach. I don't do brakes with incorrectly sized

> Snip

> The dust boot cannot disengage from the caliper with the
> piston installed as the boot lip will not pass through the
> piston to caliper clearance, which is so designed to retain
> the boot.  Further, a good boot will not rip before its
> piston retaining lip will pull from its groove and slide down
> the piston.  So, no problem with the boot.
> >
Wrong Wrong Wrong... I pop these things all the time. If fact this
morning, when I took the measurements from an 928 S4 caliper, I put an
air hose to the caliper inlet, applied air pressure and the pistons and
dust seals popped out. Yep... That's right, the dust seals came off the
caliper body from hyper extension of the piston.

Try it sometime...  It works!

> snip <

> What forces do you think exist to retract the piston?  Beyond
> that of a warped rotor, the only piston return force is that
> of the pressure deformed elastormeric piston seal returning
> to its natural cross section.  Good for a few thou only.
> Further, there are no forces tending to cock the piston
> within the bore, short of a tapered friction surface on the
> pad.  And inasmuch as there is no metal to metal contact
> between piston and caliper bore, even a tapered friction
> surface will not cause piston binding, in any practical situation.

No again... There is a vacuum that also help to retract the piston...
Remember this is a closed hydraulic system. And for that matter, they
don't have to retract far to release the pad from the rotor. And there
are lateral forces acting on the piston... Remember, the wheel/rotor is
rotating and there is friction between the pad and the rotor, so if the
pad wasn't held in place the friction would move it in a lateral motion.
So, even a pad contained in a caliper has a slight bit of lateral
movement. This force can cock the piston. Granted, not by much, but the
clearance are so tight it doesn't take much to cock the piston in such a
manner that it will not move in the bore at all.

And, in the earliest S4 caliper designs, there was NOT a dust seal as we
now know it. There was a second pressure seal located at the top of the
piston bore. Porsche called it a "scrapper ring" So in this case there
were not be the deformed elastormeric piston seal you previously
referred to. And the Brembo calipers used by most, if not all of the
NASCAR teams do not have dust seals or scrapper rings. They have an
inner pressure seal and that's all. This would also blow your deformed
elastormeric piston seal theory. Granted they rebuild the calipers
following every set of laps (i.e. qualifying laps, practice laps, race
laps, etc.) I probably talk to the world's largest Brembo race
distributor (their self description) at least once every couple of
weeks. I have even recently spoken with the guy in charge of Brembo's
racing program as it relates to NASCAR. He's a very nice guy...

> >
> IMO, electrical pad wear sensors are totally redundant
> (unless you like to see red lights) excepting for the
> unlikely special case of worn out pads being used on new or
> remachined rotors.  The rotor/pad configuration is its own
> best warning system.  Any rotor wear will leave raised ridges
> either side of the wear surface, which will be the first
> surfaces to contact the backing plate with a loud warning
> that pads are ready to replace, long before the wear surface
> is damaged.  This system works, I use it.

I have no idea where this came from... And frankly don't tell me. I
don't care. I did not ever mention electrical pad sensors and in the
scenarios I described there was still enough pad material remaining that
there would not be any metal to metal (rotor edge to backing plate)
contact. For that matter, in the 996tt with 30 mm rotor scenario, if the
rotor was worn so much as to produce a ridge of several mm and the pad
was also wore down to the point the few mm of rotor ridge would contact
the backing plate, you would be losing the piston from its bore.

BTW, that is an extremely unsafe system you admit to using. Hard to
believe someone that thinks they know so much about the flaws in my
rationale would tell the group they basically don't inspect their
brakes... They wait until the pads/rotors are making noise from
metal-to-metal contact.

> I have seen Ford rotors worn to the point of the internal
> vents were showing through the wear surface (slotted rotors
> are good, yes!) without the hydraulic system being compromised.

So what??? We're not talking about the Ford caliper/rotor system. I have
no idea what the tolerances for such as system are and don't care. I
don't do Ford brakes.
> Don't speculate on nonexistant problems,

I'm not speculating. I am 100% positive that with the correct variables,
brake system failure will occur.

> and "Don't fix it if it ain't broke"
> Bernie
> > Michael G. Wachholz
> > Prospeed Motorsports
> >
> > 952.249.1169 office
> > 952.475.3267 fax
> >
> > 400 Pond Ridge Circle
> > Wayzata, MN 55391
> >
> > mailto:michael at prospeedmktggrp.com
> > www.prospeedmktggrp.com

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