[200q20v] Re: Caliper piston extension distances

QSHIPQ at aol.com QSHIPQ at aol.com
Mon Nov 19 14:51:47 EST 2001

Man you guys crack me up!  I'm watching B*ra bail on offering brake packages
based on an incorrect (rather:  Incomplete) post from MGW.  I'm now gettting
phone calls from those with 30mm rotors asking WTF?!?

First, to the crowers and crowees, if you don't know what you are doing,
getting out is a good idea.  Second, Michael, grab a pair of Porsche factory
spec Textars and walk thru your numbers again.  I got the same as you for PF,
but massively different from both Porterfield and Textar.  Please feel free
to confirm my mitutoyo, the only relevent numbers to your lengthy post appear
to be:

Porterfield AP594-R4 (Bedded, but not run)
New pad thickness including backing plate:  18mm
Backing plate thickness:  5.0mm
Maximum (newer) pad material:  13mm

Textar T428FF (new dealer 993.351.930.01)
New pad thickness including backing plate:  17.8mm
Backing plate thickness: 5.2mm
Maximum (new) pad material:  11.1mm
ADDITIONAL MEASURE:  1.50mm = anti sqeal plates

(A/S plates are *not* found on most aftermarket pads, but can be added, even
stacked.  FI, my toyota 4runner 4piston stock calipers use 2 per pad)

I think just the above makes your numbers below N/A.  However, now that
you've scared the trick and treaters with claims of "risk", how bout we just
add an additional 1.50 A/S plate to each pad?  All I read from your post is
that I wouldn't use a PF "Z" pad with Big Reds.  Seems to me that the
manufacturing differences of aftermarket pads are the problem, not what
porsche delivers from the factory, nor necessarily clamping a 30mm rotor.

Jimmy, please feel free to edit copy whatever you feel is appropriate for the
Urs4 site.  I'll charge Dave Jones for the consulting fee (Fat Tire Ale) on
his 30mm applications.

My .02 arbitraged thru the peso.

Scott Justusson
QSHIPQ Performance Tuning

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

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.

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

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. 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.

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

Michael G. Wachholz

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