V8 Brake swap for UrQ - saga continues..

[Ben Swann]

Thanks for the reply - I suspected something like this.  If that is the case, then
probably the proportioning valve won't work, except to divert all braking to the front.
I still probably ought to install one.
 
There are two different master cylinders for the early UrQ.  I ordered the earlier 443
611 021 (23, 81X19/17) . Should I have ordered 447 611 021 (25,40X18/18) ?  I'm not sure
what the numbers mean exactly, but presume the bore size is larger in the later version.
Maybe there is another one I ought to get - perhaps from a different car.
 
I originally tried to fit the V8 Quattro Master, but it did not fit the booster - which
prompted me to open up the booster (idiot!) thinking the plastic retainer was a spacer
sleeve.
 
I could really use some help on this - I want to stay the course with the V8 brakes and
ultimately put the Porsche monoblocks in front.
 
Ben

  _____  

From: Mark Rosenkrantz [mailto:speedracer.mark at gmail.com] 
Sent: Sunday, November 07, 2010 9:14 PM
To: Ben Swann
Cc: urq at audifans.com; quattro at audifans.com
Subject: Re: V8 Brake swap for UrQ - saga continues..


Ben,

Without doing the calculations, I suspect you have a swept area problem. Going to
"bigger brakes" often leaves a long pedal.
When doing brake modifications, the size of the new caliper pistons have to be taken
into account when choosing a brake master cylinder. With no other changes, when
increasing piston size, often the result is a master cylinder primary piston which will
sweep right past the correct operating area and either bottom out on the secondary
piston, allow cross circuit hydraulics, etc. I'm tired... hardly the right term but I
just got home after frostbiting the J-24 all day.
Brakes are too often thought of as individual pieces and not as a system. Big mistake.
Brembo, Stoptech, etc.... they're not taking a single caliper and making different
brackets. The caliper bodies might be the same across different applications, but the
piston bores sizing is application specific for, bias, area and to minimize pad taper.
Take for example a Stoptech application for a 2000 A4 bodied car. All use the same
caliper body, but the piston size and radio is different for a 1.8T, a 2.8, and the S4
to match the spring rates (for bias) and master cylinder volume.
A couple of remedies: A proportioning valve might be enough. Properly sizing the master
cylinder. Sleeving the calipers with smaller bore pistons.
Good luck,
Mark Rosenkrantz