2ndof2

Happy weekend,

Thought I'd go ahead and roll the ol' 9-3 to work today. Unfortunately, on the way home a problem recurred after I thought it had been resolved.

If this sounds familiar, I posted some questions trying to trouble-shoot a strange vibration I was getting on braking from speeds of about 50+ a while back but finally took the car in to my mechanic.

Background on this is that the rotors are well within spec as are the pads. In the process of trying to solve this the first time, my mechanic had two new front tires installed (it was time), replaced the control arm bushings and had it aligned (tie rods, PSS9s are all in great shape, wheels are straight etc). Contrary to normal protocol we had the rotors turned for good measure (or bad). In the process of pulling the rotors off the car they found one of the brake pad dampeners on the driver's side rear had somehow dislodged itself from the piston and was deformed. I did the install and we figured that maybe I had installed it improperly when I replaced the pads and rotors as a DIY a while back.

The car had no problems for many thousands of miles after my DIY job before this problem started. My mechanic described the feel of the pedal after he had completed doing all of the recent work as "like glass" on braking from 90mph. I hadn't tried that myself until today (not quite 90 but def north of 75). The shudder from the pedal and what feels like the rear of the car is just as bad as before the work was done. I have put about 1500 miles on since I got it back. Haven't yet had time to pull the wheel and take a look but I suspect that the new pad dampener has suffered the same fate as the last one (not sure - just a guess).

One last couple of points is that there are no warning lights or codes but strangely, when my mechanic hooked up his laptop with the diagnostics software when I first brought it in to try to sort this problem before all the work he did, he said the computer link to the wheel sensors would disconnect the moment the car started rolling. I assume he was testing the ABS system?

Anyway, sorry for the long message, but this problem is quite annoying and worst of all, my very experienced mechanic has no idea what the problem is. He and I decided we would each throw a shout out and see what other experienced wrenches, techs, Porsche Doctors had to say on the matter and then circle back to develoop a game plan. Any insight into what we may be missing is greatly appreciated as always!

Linnm

I had a problem a couple of years ago that was similar. After a lot of trials we determined that the rotor braking surfaces (each side) were not parallel causing irregular brake contact/pressure. My dealer technician turned the rotors (on the car) with a cutting device that he employed that ensured parallelism with the surfaces. (this was sort of an "mounted on the wheel" lathe) It worked well.

earossi

A shudder almost always is a warped rotor. When you take off the calipers be sure to look for any damaged seals that may have allowed corrosion in the caliper that would lead to a dragging puck. Look to see if the pads are wearing evenly....wheel to wheel, and from each side of the rotor on each wheel. If you have a dragging puck in one of the calipers, you will end up with more clamping force on one side of the rotor which can deform the rotor and lead to a warped condition.

NC TRACKRAT

+2 for a warped rotor. If you have a dial micrometer, you can check for run-out if you remove the wheels, use a couple of nuts on opposing studs to snug the rotor to the hub, then rotate to check the amount of run-out. If warped and still within thickness spec., you can have them trued, then Blanchard-ground. (A fine, cross-hatching of the surface).

BobbyT

Another point of view, The "Warped" Brake Disc and Other Myths of the Braking System
by Carroll Smith, here: http://www.stoptech.com/tech_info/wp...akedisk.shtml#

vincer77

I read the Carroll Smith article, but I think that the article is written from the perspective of a race car, not a street car - i.e. lower mileage wear. Every machine shop I've spoken to that turns rotors tell me that the rotors actually do warp. I have seen rotors where the metal has been removed from the pads and a lip is formed. There is not reason to think that the metal will be removed evenly like on a lathe.

Blanchard grinding the rotors may be an option to be assured that warped rotors aren't the problem.

chris walrod

Uneven pad deposition from an event such as a panic stop on cold rotors is often times the culprit for pulsing brakes or pedal judder.

Sometimes the fix is as simple as going out and rebedding pads or changing pads to something of a different compound and bedding them in. Then, install the original pads.

mhm993

Dollars to donuts it's pad deposits. I keep a box of abrasive hawk blue pads that i use to scrub rotors clean by driving for 5 miles braking a lot, and then swap the original pads back in. Works like a charm.

Read about it in that stoptech link

Steve Weiner-Rennsport Systems

A few thoughts on this subject,

While other marques do suffer from warped rotors (due to insufficent rotor mass), Porsche's do a lot better in this regard. I always check them for axial runout whenever someone complains about brake vibrations and shuddering.

In most cases, its due to uneven pad deposits and just as Marc said, a short trip with a set of used (or new) Hawk Blues fixes that problem almost immediately. Reinstall the original pads and voila',.....problem resolved.

Its impossible to Blanchard grind a one-piece rotor with an offset hat. This process is only for two-piece rotors where the disc can be removed from the hat and then magnetically attached to the table to allow the grinding process. If a rotor is truly warped, it must be either turned with the right kind of brake lathe, or discarded.

Hope this helps,

DanL993

The Carroll Smith article really got my attention because I had a lot of experience with older Acura (Legend and an early TL) brakes "warping" repeatedly--sometimes quite soon after servicing. Of course I had an indy that was more than happy to turn or replace rotors. Now Chris and Steve (two guys who definately know what they are talking about) show up and confirm it.

I'm convinced. Judgement for the plaintiff.

NC TRACKRAT

"Its impossible to Blanchard grind a one-piece rotor with an offset hat. This process is only for two-piece rotors where the disc can be removed from the hat and then magnetically attached to the table to allow the grinding process. If a rotor is truly warped, it must be either turned with the right kind of brake lathe, or discarded"

Steve is right. I stand corrected. Just a hold-over from my '95 Cobra R days.

vincer77

Very interesting method.

Thanks Steve. Haven't worked on brakes on this car yet - did not appreciate that they are 1 piece.

So the take away is that rotors sometimes warp, but pulsation is normally due to pad deposition - correct? Great thread - I learned something.

But back to the OP's problem - he has turned the rotors and he still has pulsation. How necessary are the dampeners (I assume these are the thin plates that sit between the piston and the pad). When working on cars in the "old days" they did not use these. Would a good test be to remove them to see if they cause problems? Are they used mostly to reduce squel?

jhummel68

I've used the bedding process on my 993 and several non-Porsche cars. So far this procedure has solved all the vibration problems. It doesn't cost very much to give this a try, just time and gas. Just make sure you don't come to a stop after all the acceleration / slow cycles until the brakes have cooled down to normal operating temperature.

Steve Weiner-Rennsport Systems

Thats the time to carefully measure the rotor's axial runouts on each side. If its more than .006" and the bearings are in perfect condition, its time for new rotors.

The anti-vibration dampers on these cars are the self-adhesive, spring-center discs that snap into the piston recesses. These are simply noise control devices and damned effective ones at that,...

Older cars didn't need them since the pads were much smaller and those didn't tend to vibrate as easily.

2ndof2

Very interesting commentary so far...thanks!

I pulled the rear wheels today and looked for any sign of a damper out of place with a mirror and flashlight. the pistons appear to be sitting up against the damper bases in each case which are clearly well seated against the backing plates. My mechanic said he thought it was the right rear that had an issue last time, but I could see nothing wrong on either side today.

The car has about 1500 miles on it since the rotors were turned. I'm assuming that if this symptom has come back, its something that happened fairly quickly. Is it possible that one of my pistons is binding up and causing this issue? I'm really confused as to how one of those dampers could have gotten out of place and deformed as was described by my mechanic. The rotors in the rear and the pads have probably 10,000 easy miles on them at the most, and look hardly worn to my eye.

The rotors also are factory OEM stuff from the stealership as are the textar pads. Seems unusual that with moderate use this problem has reappeared. Also seems like whatever is causing the problem wasn't fixed, only masked by resurfacing and then came back to haunt me!

Any other ideas besides trying the swap pads then back strategy?