[urq] bomb test variation with S4 brakes?

Martin Pajak martin at quattro.ca
Mon Aug 7 22:45:09 EDT 2006

Show of hands to all who have done G60 upgrade (or 4 piston callipers), did
not change MC and still have great brakes (i.e. very little pedal travel)??

I’ll raise mine for three!
Good night.

Martin Pajak


1982 Audi Ur-quattro (50,000 mi) new project
1983 Audi Ur-quattro (SOLD) with G60s and stock MC
1983 Audi Ur-quattro (85,000 mi) Concourse d'Elegance
1985 Audi Ur-quattro (214,000 km) with SQ calipers and stock MC
1987 Audi 4000s quattro (212,000 km) with G60s and stock MC

-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf
Of Buchholz, Steven
Sent: Monday, August 07, 2006 10:18 PM
To: urq at audifans.com
Subject: Re: [urq] bomb test variation with S4 brakes?

... no argument from me ... in fact as I see it you are supporting my
original statement ... that it is overly simplistic to assume that you can
bolt up a new set of brakes and expect to see no difference in pedal travel.

The effect of the area of the different pistons is a fact ... paper or no
... to move a piston a particular distance you need to supply more fluid for
a larger piston diameter.  (Unless you make a similar compensation at the
M/C) more fluid needed at the caliper means that the master has to move
farther.  OK, maybe the distance will be different depending upon the
calipers ... but you could argue that even changing the pad material could
well change the amount of distance the caliper will need to move ... so even
a pad change might lead to a situation where you have a larger master
cylinder piston travel.  Bringing everything back to the original question
... you might well see a difference in the number of pedal applications to
deplete the bomb by using a different type of brake pad ... even with all
factory calipers and M/C ...

I think the message I was originally trying to get across was not to worry
too much about the observation that it seems to take less pedal applications
to deplete the bomb.  Use dynamic functionality to guide you ...

Steve B
San José, CA (USA)
> I beg to differ with you.
> Increase in piston number is theory on paper.  In real life this is
> a bit different.  There are many things to consider including how good the
> callipers are (how far they retract after brake application) and how good
> the sliders themselves are.
> Previous car was '83 with G 60s up front and the travel was minimal.
> The difference would be something your foot would not even register, not
> mention reduce # of pumps from 30 to 10!
> BTW S4/V8 rear callipers use the same piston as Ur-q and in fact use the
> same pads as well!!
> Oh and my '85 with SQ callipers (4 piston fixed) has the shortest pedal
> travel of them all.. probably because of lack of slider.

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