Brake/Steering Wheel shimmy during DE.

[autobonrun]

My first two sessions at a high speed DE event today went well except I know the brakes got hot from the smell. During hard braking on the third session, the steering wheel shook very badly. So badly, after I finished the third session, I skipped the final session due to safety concerns; one more run was not worth the risk. On the drive home with the brakes cool, no more shimmy. Following each DE session, I took several cool down laps, used the transmission to stop the car and did not apply the hand brake, so the rotors may have been warped before the DE and only showed up when the car was pushed hard.

The symptoms seem like a warped rotor, but is it possible that a warped rotor will only exhibit a shimmy in the wheel once it gets hot? Is there anything else that could cause the steering wheel to shake only when the brakes are hot?

Despite this issue, the day was a blast.

[anthony]

I'd try different pads for DE. When you over cook pads they leave deposits of the pad material on the rotors and then the grab of the pads over the rotors is uneven - giving the warped feeling.

BTW, I'm not sure you can actually warp a rotor. They break, crack, and wear unevenly top to bottom but I don't think you could actually warp one like a pretzeled bicycle wheel.

[jaydubya]

I had this problem a few years ago on my SC. During driver's ed at Phoenix they would heat up and the front end would buck like a bronco during braking. Once the brakes cooled down all was fine.

I had just installed brand new aftermarket rotors - not stock Zimmermans, the ones with the oval slots in them, can't remember their name - Power sluts? . Anyway, we checked the dimensions of the new rotors very carefully and then ruled them out as the possible cause. I subsequently replaced brake pads, front wheel bearings, added a cool brake kit... one at a time with another DE at Phoenix between each change. Problem persisted at each DE. Finally out of desperation I decided to switch back to the stock rotors. Got a brand new pair of Zimmermans and installed them. All was well after that.

I can't say for sure what was going on with those other rotors other than they were possibly heating unevenly and becoming out of round when hot. Again the dimensions were fine as measured with a micrometer when they were cool.

If your rotors are new, they are suspect. If your rotors are old and thin, they are also suspect. You can try changing the pads but if they are not terribly thin I bet it has nothing to do with them.

By the way - still have those power sluts if anyone wants them

[Nine9six]

Anthony,
Yes, you can warp a rotor. Usually due to excessive heat, and or poor pad choice. I have seen my share of warped stainless rotors, but not so much with the cryogenically processed cast iron tidbits. Much better heat dissipation, with superior micro granular structure. Decent rotors should run true and parallel within .001"

[ttweed]

read this:
http://www.stoptech.com/whitepapers/warped_rotors_myth.htm

and then this:
http://www.stoptech.com/whitepapers/warpaway.htm

TT

[jaydubya]

Originally posted by ttweed
read this:
http://www.stoptech.com/whitepapers/warped_rotors_myth.htm

and then this:
http://www.stoptech.com/whitepapers/warpaway.htm

TT

Those are good descriptions of pad deposits and how to get rid of them. However, it is unclear how such deposits would result in a transient problem.

(my rotors were clean, new, and properly broken in).

[ttweed]

Originally posted by jaydubya
However, it is unclear how such deposits would result in a transient problem.

Because it only happens when the temps are high enough, perhaps, or the bedding of new rotors/pads was accomplished differently in one instance compared to another?

TT

[jaydubya]

Originally posted by ttweed
Because it only happens when the temps are high enough, perhaps?

TT

You mean the transfer of material to the rotors? The material would then presumably still be there at low temps. Yet this poster's symptoms (and my own) only showed up at track temps.

[Formerly Steve Wilkinson]

Those Carroll Smith papers are highly regarded among people who build braking systems for racecars. And no, you're never going to "warp" a heavy, high-quality Porsche rotor. It is a myth. I've had exactly the same thing happen at a couple of DEs, but the uneven deposits do eventually seem to get worn away during normal road use of the brakes. At least the steering-wheel shake that my PCA "instructor" attributed to "warped rotors" is gone.

Stephan

[jaydubya]

Originally posted by Formerly Steve Wilkinson
Those Carroll Smith papers are highly regarded among people who build braking systems for racecars. And no, you're never going to "warp" a heavy, high-quality Porsche rotor. It is a myth. I've had exactly the same thing happen at a couple of DEs, but the uneven deposits do eventually seem to get worn away during normal road use of the brakes. At least the steering-wheel shake that my PCA "instructor" attributed to "warped rotors" is gone.

Stephan

I agree that warping a rotor would be nearly impossible - warping implies plastic deformation and that would require heating the material to a temperature that caused local stresses to exceed the yield point of the steel.

However, after 3 months of this problem and multiple attempted fixes, my working theory was that the ATE power discs/slots were more affected by uneven heating than the stock zimmerman rotors, resulting in transient changes in dimensions at high temps.

[autobonrun]

Thanks for the input and excellent articles. I read both articles. I plan to take my system apart this week to inspect the rotors and pads. I'm using Ferrodo pads as I always have but must admit, I was running faster on the track than I ever have done. In fact, the instructor said my afternoon lap times were 10 seconds faster than my morning times. Interstingly this was when the vibration occurred and I had to take the last 5 or so laps at reduced speeds. As would be expected, the greatest vibration occurred at the end of the straight when braking from about 110 mph. There was vibration at the other curves but not as severe. I think it shook the entire front end. As far as brake fade, I experienced none that was discernible, but as I stated earlier, I know they got very hot from the smell.

It just happened that immediately after the track time and cool down laps, there was a slalom and threshold braking session set up in another part of the facility. When I did the 60 mph to 0 tests after cool, the car stopped straight and true with no vibration or fade and I ran this section maybe 5 or 6 times. So, I know that the vibration is purely high temperature dependent. At the moment, the only thing I can think of that may have caused this is that I'm sure I did not initially bed them in properly based on the description in the article. I made maybe three or four stops from a speed of 40 mph, not ten from 60 mph.

If I don't visually see a problem, I may just replace both the rotors and pads at the same time I bleed the system. This may be the safest if I do another high speed DE in the near future. There's no rush since the brakes work great on the street. By the way Jaydubya, have the Zimmermans always been stock rotors on Porsches; I have a 79SC? Where can I get them and any suggestions on pads for mainly street and occasional track use? Also, where can I get an instrument for measuring runout on rotors?

The good news about this event was I got a chance to really test my new sport Bilsteins, increased diameter torsion bars, and Bridgestone SO3's for the first time since installation. The car is unbelievably balanced at speed through curves and on the slalom. I'm very happy with the suspension setup; now to get the brakes right.

[ttweed]

Originally posted by jaydubya
The material would then presumably still be there at low temps.

Did you read the article? It says, among other things: "In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming... the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures."

This indicates to me that the problem can easily occur only at higher temperatures due to the adherent nature of the braking interface, and not at lower temps, thus the "transient" nature of the symptoms. This is not to say that you didn't experience something different due to the specific metalurgy of the rotors you changed out. It is certainly possible that there was some difference in the casting of the metal that affected your performance.

TT

[Todd Simpson]

EDIT-Removed a bunch of quotes, Tom beat me to it!

The pad deposition isn't permanent, it's constantly being wiped away then reformed. When everything cools there are no new deposits and it's easy to wipe off the existing ones.

[jaydubya]

A porsche mechanic told me to avoid replacing both rotors and pads at the same time. The specific phrase was "new-new is a no-no". Can't remember why but I'm sure there is a wealth of reliable info on the web

You didn't say what type of 911 that you have. Assuming that your car has the dust shields on the back of the rotors (All SCs have them) and you decide to replace your rotors, remove the dust shields. They impede airflow to the rotors and can contribute to uneven cooling. If you are serious about doing more DEs, I strongly encourage you to install a cool brake kit in conjunction with rotor replacement. Finally, bleed your entire brake system if you haven't already and replace with ATE Type 200 or Super Blue (same ***** different dye).

Make sure that you "season" your rotors before your next high speed event.

If you replace the pads there are a number of options. I have been very happy with Performance Friction (PFC) carbon metallic pads. You have a number of options for compounds (80, 83, 90, 93, etc). The folks at PFC are very knowledgeable about brakes - give them a call and they will answer any and all questions that you have.

Good luck!

Jeff

[autobonrun]

Originally posted by ttweed
Did you read the article?

Yes, but obviously not in the context that you have highlighted. If the run out of the rotors is ok, it sounds like the problem is the transient buildup. I hated studying transient response even in college.

Jeff, thanks for the additional response. I did replace the rotors and pads at the same time. Not sure why this would be a problem but it could be an issue I guess. To eliminate the transient issue, I'll try using some abrasive pads to remove the buildup then start over bedding them in correctly using some new pads. I'll contact Performance Friction as you suggest and see about additional cooling in the process. I do have the dust shields on the back as you stated.

If I have to, I'll consider changing rotors as well after I've worn the new pads in some.

79SC

[jaydubya]

Originally posted by ttweed
Did you read the article? It says, among other things: "In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming... the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly.

Hi - no I didn't read the whole article, I read the first few paragraphs in detail and then scanned the rest. I read this first part twice:

"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."

If the rotors have measurable variation in thickness, then yeah high temp transfer of pad material is certainly a possible cause.

In my case I think that it is an unlikely explanation. I ran the same pads on both sets of rotors.

[jaydubya]

Forgot to answer your question about rotors - Zimmerman are the stock rotors. You can buy them from Pelican. I strongly encourage you to stick with the stock rotors - not stock drilled, aftermarket drilled or aftermarket slotted. Lots of info on this forum about pros and cons of drilling/slotting and opinions both ways. You know what they say about opinions... including mine!

To measure the rotors you need a micrometer:

http://www.pelicanparts.com/cgi-bin/ksearch/pel_search.cgi?command=DWsearch&description=micrometer

Measure three locations radially and 4-8 circumferential locations. There is a factory spec for allowable runout but can't seem to find it right now.

[jakermc]

I had the exact same symptoms after installing shaved, unused Toyo RA-1s. I mentioned it to another racer friend and he mentioned that he will often have the same problem after a new tire install and the problem goes away after the tires get scrubbed a bit. Sure enough, after a couple of sessions all was good.

Did you have new rubber put on just before the problem?

[autobonrun]

Originally posted by jakermc
I had the exact same symptoms after installing shaved, unused Toyo RA-1s. I mentioned it to another racer friend and he mentioned that he will often have the same problem after a new tire install and the problem goes away after the tires get scrubbed a bit. Sure enough, after a couple of sessions all was good.

Did you have new rubber put on just before the problem?

Nope, my tires are nearly a year old and broken in well. I did notice some faint lines on the front rotors and they are slightly different in color now from the rears. The rears are shiny with no lines discernible. I'm sure that is from the heating. Although you can't feel the lines in the rotors, you can see them. I plan to take a good photo as soon as I get one of the wheels removed. Both front rotors have the same faint pattern of circumferential lines with some discoloration noticed.