[200q20v] Caliper piston extension distances

Bernie Benz b.m.benz at prodigy.net
Thu Nov 8 18:24:22 EST 2001

Of course, Micheal, 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.

> From: "Michael \(Prospeed Motorsports\)" <michael at prospeedmktggrp.com>
> Organization: Prospeed Motorsports
> Reply-To: <michael at prospeedmktggrp.com>
> Date: Thu, 8 Nov 2001 11:02:30 -0600
> To: <fjamoroso at webtv.net>, "'Greg Johnson'" <gregsj2 at home.com>
> Cc: "'QList'" <quattro at audifans.com>, "'200 List'" <200q20v at audifans.com>, "'S
> Cars'" <s-car-list at yahoogroups.com>
> Subject: [200q20v] Caliper piston extension distances
> I did some measuring this morning... Here are the measurements and
> demonstration of the potential caliper/rotor issues.
> Caliper (Porsche S4, 993, and 993tt (Big Red)) designed for 32 mm rotor
> Rotor (Relevant Audi factory rotor)    30 mm
> thick
> Gap in caliper for rotor with new pads installed  35 mm
> Available Dust seal/piston travel to unseat dust seal 15 mm
> Immediate piston extension with new pads
> Based up 30 mm rotor      3.5 mm
> Remaining dust seal extension
> with "gap slack" taken up (Pad resting against rotor) 11.5 mm
> New pad thickness (PF "Z" pad)
> including backing plate     15.5 mm
> Backing plate thickness     5.2 mm
> Maximum (new) pad material    10.3 mm
> Using the 2 mm wear limit of a Porsche factory rotor applied to the Audi
> rotor...If one assumes the rotor is at the wear limit and the pad
> (friction material) has been 75% used, for the piston to have the pad
> make contact with the face of the rotor, the piston must extend 6.85 mm,
> leaving a  4.275 mm (less than 1/6 inch) margin before the dust seal
> will (potential) disengage from the caliper or rip.
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.
> At this piston extension there also remains a margin of approximately 9
> mm (about 3/8 inch) before the inner pressure seal seal will be
> compromised.
> If this same formula was applied to the 996tt caliper with a 30 mm
> rotor, the margin of would be only .275 mm (slightly more than 1/128
> inch) In this same scenario, a margin of approximately 5 mm (about 3/16
> inch) remains before the inner pressure seal seal is compromised.
> While these margins appear to allow what some would feel are reasonable
> and acceptable safety margins, What the number don't tell is the piston
> instability when extended this far.
What instability?  I just don't believe it.

> The piston/bore/inner seal
> relationship is such that a hyper extended piston does not want to
> retract easily and can, during extension, have its orientation relative
> to the bore canted slightly making retraction difficult if not
> impossible without re-alignment.
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.
> In addition, all of this is based up a rotor worn down 2 mm. If you were
> to wear the rotor down an additional 2 mm, the probability of at least a
> dust boot failure would increase substantially and the margin relative
> to the piston would decease in a similar fashion.
> On the other hand, if you replaced the rotor (in the 32/30 scenario, not
> the 34/30 scenario) after 1 mm of wear and the pads after 25% wear, I
> feel you maintain reasonable and acceptable performance and safety
> margins.
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 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.

Don't speculate on nonexistant problems, and "Don't fix it if it ain't


> 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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