Edward

This is long-winded, so please feel free to ignore. I offer the following for one's perusal, courtesy of the late Carroll Smith, pro tuner extraordinaire and author of some great books.

"The term "warped brake disc" has been in common use in motor racing for decades. When a driver reports a vibration under hard braking, inexperienced crews, after checking for (and not finding) cracks often attribute the vibration to "warped discs". They then measure the disc thickness in various places, find significant variation and the diagnosis is cast in stone. When disc brakes for high performance cars arrived on the scene we began to hear of "warped brake discs" on road going cars, with the same analyses and diagnoses. Typically, the discs are resurfaced to cure the problem and, equally typically, after a relatively short time the roughness or vibration comes back. ...

In fact every case of "warped brake disc" that I have investigated, whether on a racing car or a street car, has turned out to be friction pad material transferred unevenly to the surface of the disc. In order to understand what is happening here, we will briefly investigate the nature of the stopping power of the disc brake system...."

FWIW, several years ago I took this advice on my (then new to me) 993 which was exhibiting some front wheel shimmy under light braking, but NO vibration under heavy braking, and no pedal pulsing at all. Replaced the pads (which needed replacing anyway), kept the same rotors and bed them in pretty good. Problem solved. Hmmm. I continued to track the car and the "problem" never resurfaced, even under track conditions. Eventually I started using track pads and was concerned about the dissimilar pad materials depositing unevenly on the same rotors. But after a good bedding after the swap (either from the OE Textars to the trackpads or vice versa), I never had any problems ...for years. Funny, now that my 993 is retired from the track, NOW the problem resurfaces, again under really light braking! But I ain't concerned because under med-hard braking, there's no vibration and the car stops straight and true. So there you go, whatever all this means, I hope it helps.

Edward

deltawedge

I have great respect for Caroll Smith (he reviewed and commented favorably on our Formula SAE entry in 1994) but have not experienced the conditions he described. I fully understand the principle, but in my experience (limited as it is), warped discs were, in fact, warped. Put them on a lathe and true the hubs (ie zero run-out) and the brake friction surface (ie rotors) were "warped". These were street cars driven by the average "Joe" or "Jane", so pad choices were generally not a factor. Just another opinion and the truth likely lies in between the two.

Bull

I, and several others in our PCA Region, have experienced the pad deposit effects. All were 993s driven in the upper run groups aggressively on track and running Pagid Orange pads. In my case, a switch to a different track pad cleared the problem very quickly. I experienced this problem twice as it takes me a while to learn! Many new rotors are sold to people who didn't need them. We have one guy who thought he was continually warping rotors, so finally installed Big Reds. Yep, still experienced the problem until he switched pad brands.

However, the quickest way to warp rotors is to have them "turned" by someone who takes grooved rotors that are close to minimal spec thickness, but still serviceable, and makes them into nice, shiny rotors that are now below minimal spec thickness.

TomF

All of the above is great advice. I would also check your tie-rods. I had the same symptoms initially and did everything mentioned above and then found out it was a failing tie-rod end. It is VERY hard to see. I will post a picture when I can dig one up. The inner rubber end fails and it is almost impossible to see.