[Biturbos4] Stock Brake Upgrade

AudiBiTurbo at aol.com AudiBiTurbo at aol.com
Wed Nov 24 00:47:45 EST 2004

In a message dated 11/23/2004 5:29:29 PM Eastern Standard Time,  
mbenno at yahoo.com writes:

Mark thanks for the objective feedback. I have some additional question  on 
this thermal capacity issue... 
...Thermal capacity of the system, which takes into account car  
aerodynamics, cooling, mass of the rotor, vane design, etc.
I thought the weak thermal link in the system was the OEM rotor and that  
going to just about any other aftermarket rotor dealt with common  warping.
...Rotor warping can be caused in various ways.  The most common is  getting 
an overly hot rotor and pad stopped and keeping your foot on the brake  (at a 
stop light, for example).  Of course, this same scenario can lead  to "pad 
dispersion" (the pad material imbedding into the surface of the rotor,  causing 
hard spots).  To most people, the resulting pulsation is  "warping."  In severe 
cases, you can see an outline of the pad on the  rotor surface.
I don't understand how the caliper has poor thermal capacity, does it  have 
something to do with the pad size vs. piston size?
...The calipers in the S4's case only limit the system's thermal capacity  by 
limiting the rotor size.  A larger (wider as well as diameter) rotor  can 
flow more air, have more mass, etc.  The piston size versus pad size  versus 
master cylinder size only has to do with bias and feel.
Also, would a OEM sized floating rotor (eg ECS tuning) deal with the  thermal 
issue as well. My understanding is that composite rotors have better  
venting. Plus the dual material prevents warping common in the stock  rotors.

...dual material (rotor hat typically being aluminum and the rotor  typically 
being a nodular iron, if I remember my materials course) has nothing  to do 
with venting.  It's all in design.  The reason for 2 pc. rotors  is that you 
can make the hat out of a lighter material, and move that mass into  the rotor 
face area, making the webs stronger.  In general, 2 pc. rotors  weigh less, but 
have more inertia, so it's not uncommon to see a dyno loss,  despite less 
dynamic weight for the suspension.  The 2 pc. design allows  the rotor to float 
(think "side to side") which makes the braking more even on  both the inner and 
outer surface.  Basically, this means no warping from an  uneven heating 
issue, but they could still warp if you take a hot rotor/pad and  keep the pad in 
contact as it cools (say, at a stop light).  As an  interesting aside, in 
testing, one of the reasons Stoptech flowed 20% more air  than the same sized 
Porsche GT-2 rotor is in the vane design and the face that  the hats are 
directional and optimized for airflow.  They patented  directional hats here in the USA.
Then brake bias comes into play.  But since you didn't ask about it, I  won't 
type any more.  But I will mention that weight, spring rate, and  master 
cylinder sizing all come into play when choosing the proper piston bore  size and 
ratio.  Although the caliper bodies are the same before machining,  an S4 
caliper isn't the same as a 2.8/3.0 caliper, nor are either the same as a  1.8T 
The point is that it's an entire system.  There's no such thing as a  magic 
rotor, but there are things which aren't expensive and will improve brake  
efficiency.  Slotted or 2 pc. rotors, proper pad composition for your  driving 
conditions, and maximizing airflow are all important.  Hanging some  hoses under 
the car and aiming them at the hub will help at speed.
Good luck, and I hope I've helped clear up some mystery and give you some  
ideas to ponder.
Mark Rosenkrantz
_AudiBiTurbo at aol.com_ (mailto:AudiBiTurbo at aol.com) 

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