JSD

I originally posted this on the S4 forum, but didn't get many responses. I was hoping it might get more attention in this forum. Thanks in advance for any light you can shed on the problem. :-)
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I posted last night about my problems yesterday with my stock S4 brakes after a track event. <a href="https://forums.audiworld.com/s4/msgs/556988.phtml">Here's the original post.</a> The problem started late in the day yesterday and persisted on my drive home.

Well, today I was back out at the track. I swapped out both the front and rear pads and had the rear rotors turned (thanks to Peter and Tyler for all your help!). And the problem is STILL THERE. ARGH!!!

Every time I get on the brakes the whole car vibrates as if ABS is kicking in or when you go over a rumble strip in the road. It happens regardless of how hard I'm on the brakes, but when I'm on them REALLY hard, I noticed that it feels like it gets worse when I let up a little.

There is no vibration in the steering wheel, like you would expect if the front rotors are warped.

I guess I'll take it in to the dealer this week and have them look at it, but I was hoping someone here might be able to shed some light on the situation.

Only other thing I can think of is that I just had coilovers installed 2 weeks ago. Don't know if that could somehow be part of the problem?

Rich L.

Do you get any vibration clunking over bumps or under acceleration? You might want to lift the car and check all the shock/spring attachment points. But if that's it, bumps would set it off too. If it's just brakes, maybe a caliper is loose or a mounting point broken. But I'd think you'd notice this changing all the pads.

Good luck,
Rich

xr4tic

Could be bad control arms. I wonder if a sticking caliper would cause the same thing?

zain

Front rotors hosed (since you've already replaced pads and turned the rears). Most likely thing, I think.

Caliper attachment loose/broken (hard to see how you wouldn't notice this chaning pads tho')

Some sort of brake fluid thing. Makes little sense, but perhaps if you boiled it and have bubbles in the ABS circuit somehow. Might be worth a bleed and see, but then you probably did this with the pad swap.

ABS actually IS kicking in, because a sensor has failed. Should get an ABS warning light though.

I'm betting front rotors hosed.

Pretty strange.

-Z

Billy with Car Nutz

More than likely the Front rotors are warped. You be running a high temp brake fluid also. We had a CLK 430 with a Brembo kit that had a similar problem. The Front rotors were destroyed.
Good Luck!

GregA

http://www.bira.org/neus4.html

GregA
BIRA.ORG<ul><li><a href="http://www.bira.org/neus4.html">http://www.bira.org/neus4.html</a</li></ul>

JSD

I'm hoping the front rotors are warped because thats easy to fix. Its just weird that I'm not getting steering wheel vibration under braking.

It went into the dealer today. My service manager thinks its the rotors as well and is going to replace them. Hopefully that will fix the problem. I'll post a follow-up later this week.

JWG

Question: does the vibration rhythmically slow down as the velocity of the car slows? Meaning, do you have a connection between the vibration's sound/feel and the rotational velocity of the wheel/rotor?

My vibration under braking slows as the car slows. Strangely, I feel very little vibration during twisty canyon, highway, or street driving. I most strongly feel the vibration during high speed braking (60+ on the freeway) and/or when I must slow down while fighting the vehicle's momentum at high speed, e.g., decelerating on a downward-slope where gravity pushes you down the hill.

I also lack steering wheel shaking, and the car runs beautiful under slight and low speed braking. I just put in new EBC Greenstuff kevlar pads and Powerdisc rotors several thousand miles ago; they bedded-in well and performed like Porsche brakes for thousands of miles with little pedal pressure required to stop normally and with a lot of brake pedal left in reserve if you needed it. It felt awesome until this vibration reared its ugly head.

Replacing my tires reduced the vibration but failed to eliminate it completely.

As your problem sounds similar, please let me know how you make out with a rotor replacement. I have another set of front rotors in my garage, but do not want to replace mine when no one sees anything wrong with them. If the rotors do not solve the problem, I may have to look at control arms or bushings, but it seems logical that I would feel those problems at all times, not just during braking.

JSD

The vibration is proportional to the rotational velocity of the wheels/rotors. Interestingly, this makes the vibration feel worse as you slow down because the frequency gets lower.

One Q for you - are you on stock suspension? I just had coilovers installed and I'm wondering if something could have loosened up in the suspension?

Fifth Screw

Here's the story. Track event with stock rotors and pads on my 99.5 1.8t Avant in July 2000 - Ok.

November 2000 I put on a set of EBC pads on all four corners. A few thousand miles after that I start noticing a slight vibration.

Over time the vibration gets worse, I go to the dealer and they replace the front rotors. Ok, so problem is gone, right? Yes, for about two months, then the vibration comes back. This time the dealer says it's the rear rotors but they won't do them under warranty because of my EBC pads. How's that for consistent policy? First they will, then they won't.

So, I went and replaced ALL rotors, OEM in the rear and Zimmerman x-drilled up front. I also changed my pads to Mintex pads. All vibration is gone except for my wheels neededing to be balanced. ;-)

I can't be sure, but I think the EBC pads may have contributed to the warping problem. The problem you are describing really does sound like warped rotors.