ScotD

When I step on the brakes hard at speeds greater than 40 mph, the car shudders.

I replaced all 4 rotors last year, along with the pads.

Any ideas? I would prefer to not have to change it all out again, but will if I have to.

skiracer

Slightly warped rotor? Are all lug bolts properly torqued?

ryoung

of thoughts/ideas,

1. ABS activates under normal braking -- If you don't see the ABS warning light, I doubt that this is the problem.

2. Warped rotor, excessive run-out -- This can be checked with a dial indicator.

3. Swapping to a dissimilar pad -- The pads are designed to leave a VERY thin deposit on the rotor. If you change to a different pad, at some point the rotor will have deposits from both. If the coefficient of friction between the new pad is different with the two different pad deposits, you'll experience vibration until the old deposit is worn away (I've had this problem). Or worse yet, the different pad deposit can result in severe galling of the rotor, which I've also seen (but thankfully not on my car). Since you used only Audi pads with the first 5 rotors, or a Brembo pad with your current set-up, this is not the problem.

4. Using a race only pad on the street -- Especially if the pad has not been properly bedded, you may have an uneven pad deposit on the rotor, which causes vibration as in #3 above.

5. Rotor distorts under pressure -- I've never seen this, but others have reported this can occur if the wheel bolts are not properly torqued in the proper order when the wheel is mounted. Half of the mechanics I've seen either don't properly torque wheels or don't know how. If I didn't personally witness a mechanic torque my wheels, I'll redo it myself when I get home.

6. Bad wheel bearing -- This can cause vibration under braking loads. Since you've had a vibration problem with 6 rotors now, perhaps this is the problem?

7. An A6 owner reported that his problem with vibration under braking was caused by worn tie rods.

8. Cracked rotor -- Cracks can grow radially and cause vibration especially when the rotor is very hot. The crack will tend to close when the rotor is cold, and the crack can occur on either side of the rotor.

This photo of one of my Alcon rotors shows what can happen. The driver's side rotor cracked from overheating at the track, and it felt like a brake shudder when braking from high speeds. Needless to say, this rotor wasn't far from a catastrophic failure, a major safety issue. Even though the crack was on the outside, it was still very difficult to see in most lighting conditions. Both rotors would need to be removed from the car, and both sides carefully examined, to determine if this is the problem.

<img src="http://pictureposter.audiworld.com/17157/alcon_cracked.jpg">

Surface cracks, like those in the photo of the rotor on my S8, are not a problem.

<img src="http://pictureposter.audiworld.com/17157/rotorcracks.jpg">

Well ... this is about all I can think of. Good luck!

ryoung

The disc is spun on it's axis, any "wavyness" (sp?) or the uneven build-up of pad material can be measured with a dial indicator that measures variations. According to this <A HREF="http://www.stoptech.com/whitepapers/warped_rotors_myth.htm">Stoptech Article</a> written by Carroll Smith, as little as 0.0004" of a variance can be felt in the brake pedal. Note that this article says warped rotors are extremely rare and that most pedal vibration is caused by the uneven build-up of pad material.