CaseyH

I started to feel a vibration in the front end when braking at speeds of 50+. This started at the same time I changed the tie rods and did a little tracking. At the end of my laps my brakes were so hot I could smell them. I am going to get an alignment regardless but could the misalignment be the cause of the vibration or is more likely a warped rotor(s)?

AudiNutA8A6

I'm guessing warped rotor, I also just replaced the tie rods and need the alignment.

PaulW

When I drove home from the track, the brakes were definitely vibrating! They got so hot....they are going to cool slightly different....but after using them a few days, it's gone...no vibrations anymore.

pw

ryoung

may have left pad deposits on the rotors ... been there, done that. If you look at my rotor between the 6:00 and 8:00 spokes, the darker spots are left-overs from pads that didn't hold up on track. It took a few weeks for the deposits to wear away, but until they were gone, the brake shudder was nasty.

<img src="http://pictureposter.audiworld.com/17157/acid2.jpg"><ul><li><a href="http://www.stoptech.com/whitepapers/warped_rotors_myth.htm">Stoptech whitepaper</a></li></ul>

ryoung

You may have reached the point where you need a better pad. Those are Pagid RS4-4 Orange spots on my rotor, the same as you use? On track I'm now using the Pagid RS14 Black, a ceramic compound suitable for very high temps. Unfortunately, Pagid doesn't make an RS14 pad for our rear calipers. In the rear I use the Ferodo DS3000, which also has a 0.6 cf, to maintain brake bias.

Pagid Compounds
<img src="http://pictureposter.audiworld.com/17157/pagidcompounds.jpg">

ryoung

You mentioned in an earlier post that at Gingerman your tires got up to 56 psi, and that the air was almost hot enough to burn you as you let some out of the tire.

The thought occurred to me that it's common to have liquid water in the tire, actually, maybe it's the norm. The water can come from any air compressor you use to inflate the tires, or from mounting the tire on the wheel where the tech uses soapy water as a bead lube. You can guess what happens when your tires get hot ... the water turns to steam, and tire pressures go thru the roof. The tread of my left front tire hit 240 oF at Mid-Ohio last year.

So ... you may have come across another speedbump that comes from additional on-track seat time and pushing your A8 closer to the limit. There's really only one fix -- inflate your tires with nitrogen rather than air, which is a common practice in racing. Also, when you have your new tires mounted, you might check that they're using a water-free bead lube.

PaulW

It was funny, on Sunday afternoon at 5 P.M. on the way home at the gas station....I was next to the really, really fast S6....I was laughing how fast he was....and I showed him how perfectly worn my tires were.

I mean they have NO sidewall wear...and the flat portion of them have worn soooo evenly, they are down to almost nothing!

He was saying that his were rolling over, commented that mine had no roller (never noticed it before...now I know what to look for). I asked what kind of pressure he was running, he said 34 front and 33 rear cold. I don't think he checked them after that.

I started mine on Friday at 44 front and 42 rear and let the air out after run sessions to achieve 50 PSI hot front (maybe I would come back with 52) and 48 PSI hot rear (have about 50 when I got in).

The day on Saturday progressively warmed up and I was pushing the car harder and on the last session, about 1.4 times the normal amount of laps (maybe 16 versus 11)....it was 56 and 55 in the front and about 52 in the rears.

How should I adjust front rear pressures and should I try something like 45 hot?

BTW, another UR S6 had his front tire roll over and expose the belt.....the retread (it wasn't a retread) pulled away from the sidewall where it was attached. Didn't even think about it, but he was probably 10 PSI low at least.

I was the only one I saw check tire pressures, and it was the first thing I did when I got in.

pw

Jkay

Randy was kind enough to link the StopTech white paper on brakes; go ahead and read it.

When your rotors overheat, the pad material burns onto them. This pad material causes a raised bump on the surface of the rotor. It creates a shudder. If you are unlucky, the heat created by the extra friction of the pad material "bump" transmografies your rotor into "cementite", an overheated form of cast iron.

This creates a permanent hard spot in your rotor, which "cutting" can never repair, and a permanent "shudder spot".

PaulW

I guess it's good to keep those relatively hard material track pads on after a track event for a few days to clean those rotors! And my rotors look very clean right now.

Interesting....never put 2 and 2 together.....

Dan McBoost

It was cracked though