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Brake vibration

After 6 track days and 11,000 miles with no issues, I replaced the front brake pads (4 mm left at time of replacement). The front rotors had a slight lip, so I had them turned. The installer assured me the rotors were still at or above minimum spec after turning. The rotors had displayed no issues with vibration at any time. New stock pads installed, I bedded in the rotors/pads per the instructions in the Boss 302 supplement provided with the car, just as I had with the original equipment when the car was new.

After Sunday's track day (4 sessions at Road Atlanta totaling 1 hour, 45 minutes) I now have a slight vibration in the brakes. By the fourth session I could feel the vibration under hard braking, and two days later it is lessened but still there. This is the longest track day I have driven, and I was harder on the brakes than at previous track days. My questions are as follows:
-Is it acceptable to turn the stock Boss rotors, or are they one set of pads and out?
-What is the original and minimum spec for front rotor thickness?
-What impact would this vibration have on braking effectiveness at future events?
-How quickly will the vibration worsen once it has begun?
-Short of turning the rotors again or replacing them, is there any way to reduce or eliminate
the vibration?
I am trying to determine if the rotors were improperly turned, or if I just overcooked them.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
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Connecticut
1) Turning today's rotors can typically end up with a warped rotor, especially if the shop doing it turns them too fast - which a lot of them do.

2) Do you have any pad buildup on the rotors? If so that can cause the front end to feel like the rotors are slightly warped.

-Pete
 
I don't see any obvious pad buildup on the rotors, and they are very smooth to the touch. As regards the speed of turning the rotors relating to warping, would that be immediately evident after resurfacing or would it manifest itself over time and use? I drove the car for about 1000 miles on the new pads/resurfaced rotors, including the bedding in process, with no vibration. The vibration only began in the second half of the track day sessions.

I had some issues with the brake job in other areas, so I might suspect poor workmanship on the resurfacing. On the other hand, I was pretty aggressive this weekend, so I don't want to blame the installer for something that happened because of my use and recognizing that resurfaced rotors are thinner than new ones and may be prone to warping.

Having just put new pads on and bedded them in, I would like to keep using the existing rotors if possible. When I replace both next time, I expect I will need rotors and pads that exceed the factory specification if I'm going to continue to track the car (and I WILL continue to track the car).
 

pufferfish

Supporting Vendor
1,094
66
Maryland
there is a phenominon that happens where the rotor will glaze in an uneven manner on the rotor faces. the result is a pulsating pedal due to varying coefficients of friction between the pad and rotor. this is often misdiagnosed as a warped rotor. the only fix is to turn or replace the rotor, so i guess the outcome is exactly the same as if it were a warped rotor. not many places will turn rotors these days and few modern rotors have the material to turn them more than once (sometimes none at all). its best to just replace them.
 
I have swapped out different pads on the same rotors, swapped pads at the track to new non-bedded pads and a few other things that are not the best way to go. I have also had (slotted) rotors cut, there are no issues cutting the rotors unless you cut then so deep it is under the recommended width. Never had a problem with any of those things.

I did have a serious vibration twice and figured out what was wrong. I was using a compound (two different brands) that were not up the heat the brakes were generating. The first time it went away when I changed the brakes, I had a different compound ready to go so I did not think too much of it. The second time this happened I was running PFC97's because I picked them up cheap and thought they would work. The vibration was bad and I assumed it was a warped rotor so I cut them. Even after cutting the vibration was still there, swapped out to different rotors and it was still there. I continued looking at other areas like tire or suspension, nothing helped.

Then I figured it had to be the pads, switched to a different compound that had a better heat range, the problem went away. I even used the same rotors that were on the car with the vibration. To answer your question about will it get worse, depends if this is the same problem, mine stayed the same. However the pads started cracking and pieces were coming loose from the backing. If a chunk ever flew completely loose I think it would have gotten worse very fast.

You also asked about what impact this vibration have on braking effectiveness. I would say a lot, I had to back off and change my braking zone for extra space. This was because I was not comfortable with the vibration but also the car would wander around in a hard brake zone.

You said you are using the stock pads and it did not happen the first time but maybe you are driving harder as you get use to the car. I do not think the stock pads are up to the task on track when driven hard.
 

PeteInCT

#LS-378 - So many Porsche's, so little time....
Moderator
2,848
14
Connecticut
1) Glazing is not always very obvious. Look for slightly different colored areas on the rotor. If you did glaze them there is no need to cut them again (and you probably cannot anyway), the glazing should wear off from track use unless your pads are too soft (not very aggressive) for the track or you are not running them in their recommended temp range (cold or hot).

2) If the vibrations started after 1000 miles it probably wasn't from the turning of the rotors. Sounds like you may have just warped them. FYI, some pads generate more heat than others for the same amount of stopping power. This may have contributed to your issue.

Also, are you running stock rotors ? OEM's rotors are fine. If not, then be careful what iron you choose to install on the Boss.


86305aee-252d-4124-928e-5f37a6bdb910_zpsd189a619.jpg
 
Take this for what its worth - I have a 2003 Mitsu Gallant as my DD. In Jersey, your always breaking HARD because of some dumb &%$# either not paying attention or giving you a break test...just because. Vibration usually means warped rotors from over heating - have changed mine out on the Mitsu over 5 times since 2003. Just an FYI
Best of luck
Rob
 

Sesshomurai

LP Heaven said:
Take this for what its worth - I have a 2003 Mitsu Gallant as my DD. In Jersey, your always breaking HARD because of some dumb &%$# either not paying attention or giving you a break test...just because. Vibration usually means warped rotors from over heating - have changed mine out on the Mitsu over 5 times since 2003. Just an FYI
Best of luck
Rob

Yes. It's either that or a front-end alignment issue such as unequal toe in both tires whenever this kind of thing happens.
 

Sesshomurai

PeteInCT said:
Toe adjustments would not show up only under braking.

Depending how off the toe angles are it will magnify under braking. But a bad toe will definitely vibrate the steering wheel while driving too.
 

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