Jake

I have a slight wobble/vibration through the steering wheel when braking at high speed and very slight at low speeds [40mph]. The thing is I am running AP 6 pots and I doubt that the discs are warped as they havent done any track work, could it be the pads? or does the geometry need redoing???

Any help would be great as this is really annoying.

Jake.
MY00
AP 6
18" SuperLegs
Eibachs
Toyo T1s

StopTech

You may just have a bit of uneven pad deposition...

Virtually all modern brake pads are what are referred to as an Adherent type of pad. The pad is designed to transfer a layer of pad material onto the rotor. When a sufficient and EVEN layer of pad material is adhered to the rotor face, the pad material on the rotor, interacting with the similar material on the pad, creates the most efficient friction mechanism. These like materials, breaking against each other on a molecular level are what really stops the car well. To emphasize, there is supposed to be a layer of material pad material on the rotor.

The problems occur if the pads are not properly bedded-in (an even layer of pad material on the rotor) and run aggressively, OR if the pads are overheated. The pad transfer occurs most efficiently at the pads optimal operating temperature. That means a higher temperature pad needs to be hotter to properly transfer material. If you have a high performance pad and never run it hot enough to get a proper layer of material onto the rotor, it will never be properly bedded-in. Thus, even after 1000 miles of "normal" street driving, when you blast your favorite canyon and heat the brakes, you can get uneven deposits on the rotor causing a vibration.

The other common scenario is over-heating the pads even if they are properly bedded-in. In this case, the pad material starts to break down and smear onto the rotor face, again causing the uneven deposits.

The other problem that occurs is if the system is really hot and you come to a complete stop and leave your foot on the brake pedal. In this instance, we get what is called "pad imprinting" where a small layer of material breaks off the surface of the pad and literally can be seen as an imprint of the pad on the rotor face. This can occur no matter the state of bed-in.

All these scenarios leave very small, uneven layers (we call it TV, Thickness Variation) of material on the rotor. We're talking a few 10/1000's of an inch, like a TV of 0.0003". It starts out almost imperceptibly, but as the pads start to skip over the high spots, more material is deposited on those areas, ever increasing the vibration until it becomes quite noticeable, even days after the event that started it occurred.

This is our most populare technical white paper: "The Warped Brake Disc and Other Myths of the Braking System" http://www.stoptech.com/whitepapers/...otors_myth.htm

Ways to cure:

Turning the rotors will take care of it, but you will be shortening the life of the rotor and decreasing its ability to absorb and control heat, as there will be less mass in the rotor after turning. Also, turning a 2-piece rotor that uses floating attachment hardware between the rotor and hat can be tricky. We have had very good success running an aggressive track pad at lower temperatures on the street in order to scrub off the rotor surface. We have found the Hawk Blue 9012 race pad to be very effective. At lower temperatures it is very ABRASIVE, not becoming ADHERENT until it reaches it's optimal operating temperature. If it is used with a few firm stops at a time, not getting too hot (we want to remove material, not transfer more), it will often remove the source of vibration.

A WARNING: Do not leave an abrasive pad in the caliper longer than necessary to solve the problem. We have had rotors destroyed in under a week by leaving the abrasive track pads in on the street.

Hoppy

Excellent post from ST there.

Just to say that I have had this problem when using DS2500 pads, which are excellent, but I've never had it with other pads. My brakes are AP 6-pts, too.

The cure is as ST says, but I didn't need to change pads to clean the discs up. I just went through the basic bedding-in procedure again and all was well within 20 minutes.

Richard.

Jake

Thanks ST very informative , it sounds as though I am suffering from the "never run it hot enough to get a proper layer of material onto the rotor" problem as I dont use my brakes in anger very often.

Hoppy,
What was the procedure that you did in 20 mins? I fancy giving that a go tonight


Cheers
Jake.

StopTech

You asked Hoppy, so he can answer (of course)... but this may save him reinventing the wheel. We have a fairly detailed/standard bedding-in procedure documented. You can go to: http://www.stoptech.com/installation...2005-12-03.pdf

Go all the way to page 20 (is a .pdf file)

Have fun.

Hoppy

Edited to add, just read the StopTech link above. Good reading. (nb Rotor = disc and hat = bell LOL). Oh, and The Hoppy Corporation does not endorse speeding on public roads, but you knew that already

Jake - To be honest, it's not the bedding-in procedure, but similar. Sorry to mislead anyone. Bedding in requires you to get a whole bunch of cold components to work together when red hot. Basically, start slow and build up speed, braking pressure, and length of application - you know the sort of thing.

But here, it's a slightly different issue. I wouldn't be so aggressive with stock brakes but APs can take plenty of abuse

First, and probably most importantly, find yourself a (private) smooth, dry, straight and very quiet stretch of road, with a complete absence of pedestrians, cyclists, horses, hedgehogs, plod etc, and preferably where you can just turn around and do the same thing in the other direction. Check your brakes - are they in good shape, with plenty of pad material front AND back? Reomove anything loose in the car or the boot or it will get completely smashed. Do a few dummy runs with mild braking, and if the car pulls to one side or shows any signs of instability, abort the mission or you'll end up in the ditch. I'm serious. This can easily get dangerous. Only brake in a dead straight line. At your own risk, etc etc.

Okay, so your brakes have already gone through the heat-cycle bedding-in thing, so build up some speed and then hit the brakes hard - say from 80-40. Allow the brakes to cool off, then do it again, and again and again. You want high brake pressure, but low heat, so keep the applications hard but brief, then allow to cool. I think I did this eight times before the brakes were smooth again - you might need more. Then give 'em another couple of runs for luck.

If your brakes are now smooth again (if they're not, forget this bit as you'll need to find another solution) you need to coat the discs with pad material once more. For this you need high pressure and heat. So as before, but with moderate braking and holding the brakes on longer, down to walking pace, and don't let the discs cool off as much. Build up the pedal pressure on each run. You'll find the ABS will kick in immediately when you're up to max braking and, depending on tyres/road etc, that's around 1.2g. Violent.

Six runs from 80-5, pretty much back to back, should do it for sure. APs can take this treatment, with their separate alloy bells, but I wouldn't recommend it for solid discs. Finally, let the brakes cool thoroughly.

IMPORTANT - Throughout this whole procedure, do not let the car come to rest at any time if you can avoid it. If you can't, never sit with your foot on the brake (or handbrake on) and try to move again, even very slowly, as soon as possible. If you don't, you'll only make the whole problem worse as it's something like this that has caused the problem in the first place!!!

Take care.

Richard.

Jake

Richard, Thankyou will give it a try later.

Jake.

drb5

iTrader: (4)

check this out for more info
http://bbs.22b.com/cgi-bin/ultimateb...;f=10;t=000211

petrv

to StopTech & all: is it possible that the disc runout is increasing with temperature or not?

I have a Honda Prelude, not as powerfull as Subaru, but the problems with disc judder are the same (so the brakes are still a problem ).

I have some ATE blanks with DS2500 pads with about 500miles on them and (of course) I get disc judder when the brakes are hot. With cool brakes the braking is perfectly smooth.

I hope that it is the pad deposition and I will try the re-bedding procedure. If I check my runout (with cool disc), both sides say 0.05mm and I thing this is OK. What if I get the brakes hot, so the disc material "expands" so I feel the judder in my steering wheel and when the disc cool down, it "shrinks" back to it's original smooth shape? I'm just wondering if there are any tests with measuring the runout with cold/hot disc.

Thanks for your replies,
Petr Veit.

StopTech

Hey Petr,

It's possible that you are suffering from pad deposition, but if you are running a 1 piece rotor (or a fixed two piece rotor), your may be suffering from coning. Here is an excerpt from a white paper that discusses this and other topics:

All metals "grow" when heated. The diameter of cast iron brake discs can increase as much as 2mm (0.080 inch) at elevated braking temperatures. When the disc is radially restrained from growing (as in all one-piece discs) the friction plates are forced into a cone shape as temperature increases, adversely effecting both temperature and pressure distribution within the pads and the feel of the pedal. Racing and high performance street discs are mounted on separate hats or bells [two piece floating rotors], usually of Aluminum. The fastening system is designed to allow radial growth and minimal axial float resulting in a mechanically stable system....Subsequently due to the coning, the pad contacts unevenly when the brakes are applied or remains in contact with the disc in the regions mentioned and even higher temperatures and wear are the result.

For more information feel free to check out this link:
http://www.stoptech.com/whitepapers/...ons_122701.htm
You might be most interested in the sections on Tapered Pad Wear and Floating Discs.

This link also has pictures of fixed vs. floating rotors: http://www.stoptech.com/faq/data/faq18.html

Hope this helps!

petrv

Thank you very much for your info!

From your expirience, are there any differences in materials used by various manufacturers or all of them use basically the same metal, so all one-piece rotors expand the same?

StopTech

You're welcome and to answer your next question...

I would never say that all rotors are the same because metal chemistry, manufacturing process and vane design will all determine the quality of the end product. The precise metal chemistry can, for example, effect durability of the rotor and/or make the rotor more or less sensitive to pad deposition. A rotor that is cooled more rapidly during the manufacturing process will cost less to produce but will also be more brittle/ more inclined to crack, etc. Since brakes turn momentum into heat, a vane design with better airflow and cooling leaves your brake system in a better position to absorb and dissipate the heat created by the next braking event. But in terms of coning, I have not personally seen or heard that there are any significant differences between the various one piece or two-piece fixed rotors on this particular issue.

petrv

Thank you for your valuable info.

One more question about cementite. Is the cementite layer temperature dependend? Look at this "session":





Note that the sparks are made by Textar pads, it is common for them.

Starting with cold brakes - absolutelly no shudder, discs have 0.05mm runout.

Preparing the brakes for this test - my steering wheel is shaking like crazy, so I think that when red-hot, my steering wheel will fly out of my window.

Pushing brakes hard at 200kmh (130mph) - rotors are getting red and there is no, I say NO vibrations, nice smooth braking.

When the brakes cooled, again no shaking. When hot a bit, there is shaking again. . It looks like it is temperature dependent. Or maybe the disc is softer when red-hot?