JohnF88

Hi,

Over the last couple of months I have begun to get a judder under braking through both the pedal and the wheel in my 2003 STI PPP. This has got worse recently and is worse when the brakes get warmer. From searching around it would appear that the discs are likely warped. How this has happened I'm not sure, as I pretty much never drive the car hard or brake harshly.

Is there any way I can tell if the discs are warped by taking them off and having a look at them or would I really need to get a garage to look at them? Is there anything else that could cause this that I could diagnose by simply having a look at them?

If it turns out they are warped then I guess they will need replacing. Where is the best place to get replacement standard front discs?

If I do need to replace them I would quite like to do it myself as

A) I like to give things ago and
B) I don't want to get raped by a subaru dealer for parts and labour (anyone know how much roughly they would charge to replace the discs compared to doing it yourself?)

The only issue I've got with that is I have never done anything like that before and I don't really want to break my car. Having said that I have looked at the guide for replacing discs and pads in the archive and it looks fairly straightforward. I appreciate this was written for a classic but I assume it would be fairly similar?

Also, I don't want to go through the hassle of replacing the discs only to find it hasn't fixed the problem and then have to go through the hassle of taking it to a garage to have it looked at and then having to fork out the parts and labour I was trying to avoid paying in the first place.

Thoughts?

dunx

iTrader: (3)

1. You need "kit" to measure the disc for run-out/warping, looking won't help.
2. Pictures are nice...
3. Pad deposits can cause similar problems, and can be "burnt" off with enthusiastic use
4. Are they heavily worn and thus a bit on the thin side ?
5. The bolts can often seize in the threads and will then need a helicoil insert to repair.

dunx

budd

iTrader: (5)

as Dunx says it's likely to be either warped discs or pad deposits, pad deposits are more common than people think and are often mistaken for warped discs.

In fact almost all cases of "warped brake disc" , whether on a racing car or a street car, eventually turns out to be friction pad material transferred unevenly to the surface of the disc. This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures.

Regardless of pad composition, if both disc and pad are not properly broken in, material transfer between the two materials can take place in a random fashion - resulting is uneven deposits and vibration under braking. Similarly, even if the brakes are properly broken, if, when they are very hot or following a single long stop from high speed, the brakes are kept applied after the vehicle comes to a complete stop it is possible to leave a telltale deposit behind that looks like the outline of a pad. This kind of deposit is called pad imprinting and looks like the pad was inked for printing like a stamp and then set on the disc face. It is possible to see the perfect outline of the pad on the disc.

It gets worse. Cast iron is an alloy of iron and silicon in solution interspersed with particles of carbon. At elevated temperatures, inclusions of carbides begin to form in the matrix. In the case of the brake disk, any uneven deposits - standing proud of the disc surface - become hotter than the surrounding metal. Every time that the leading edge of one of the deposits rotates into contact with the pad, the local temperature increases. When this local temperature reaches around 1200 or 1300 degrees F. the cast iron under the deposit begins to transform into cementite (an iron carbide in which three atoms of iron combine with one atom of carbon). Cementite is very hard, very abrasive and is a poor heat sink. If severe use continues the system will enter a self-defeating spiral - the amount and depth of the cementite increases with increasing temperature and so does the brake roughness

There is only one way to prevent this - follow proper break in procedures for both pad and disc and use the correct pad for your driving style and conditions. All high performance after market discs and pads should come with both installation and break in instructions. The procedures are very similar between manufacturers. With respect to the pads, the bonding resins must be burned off relatively slowly to avoid both fade and uneven deposits. The procedure is several stops of increasing severity with a brief cooling period between them. After the last stop, the system should be allowed to cool to ambient temperature. Typically, a series of ten increasingly hard stops from 60mph to 5 mph with normal acceleration in between should get the job done for a high performance street pad. During pad or disc break-in, do not come to a complete stop, so plan where and when you do this procedure with care and concern for yourself and the safety of others. If you come to a complete stop before the break-in process is completed there is the chance for non-uniform pad material transfer or pad imprinting to take place and the results will be what the whole process is trying to avoid. Game over

Fortunately the procedure is also good for the discs and will relieve any residual thermal stresses left over from the casting process (all discs should be thermally stress relieved as one of the last manufacturing processes) and will transfer the smooth layer of pad material onto the disc. If possible, new discs should be bedded with used pads of the same compound that will be used going forward. Again, heat should be put into the system gradually - increasingly hard stops with cool off time in between. Part of the idea is to avoid prolonged contact between pad and disc. With abrasive pads (which should not be used on high performance cars) the disc can be considered bedded when the friction surfaces have attained an even blue color. With the carbon metallic type pads, bedding is complete when the friction surfaces of the disc are a consistent gray or black. In any case, the discoloration of a completely broken in disc will be complete and uniform.

Depending upon the friction compound, easy use of the brakes for an extended period may lead to the removal of the transfer layer on the discs by the abrasive action of the pads. When we are going to exercise a car that has seen easy brake use for a while, a partial re-bedding process will prevent uneven pick up.

The driver can feel a 0.0004" deposit or TV on the disc. 0.001" is annoying. More than that becomes a real pain. When deposit are present, by having isolated regions that are proud of the surface and running much hotter than their neighbors, cementite inevitably forms and the local wear characteristics change which results in ever increasing TV and roughness.

Other than proper break in, as mentioned above, never leave your foot on the brake pedal after you have used the brakes hard. This is not usually a problem on public roads simply because, under normal conditions, the brakes have time to cool before you bring the car to a stop (unless, like me, you live at the bottom of a long steep hill). In any kind of racing, including autocross and "driving days" it is crucial. Regardless of friction material, clamping the pads to a hot stationary disc will result in material transfer and discernible "brake roughness". What is worse, the pad will leave the telltale imprint or outline on the disc and your sin will be visible to all and sundry.

The obvious question now is "is there a "cure" for discs with uneven friction material deposits?" The answer is a conditional yes. If the vibration has just started, the chances are that the temperature has never reached the point where cementite begins to form. In this case, simply fitting a set of good "semi-metallic" pads and using them hard (after bedding) may well remove the deposits and restore the system to normal operation but with upgraded pads. If only a small amount of material has been transferred i.e. if the vibration is just starting, vigorous scrubbing with garnet paper may remove the deposit. As many deposits are not visible, scrub the entire friction surfaces thoroughly. Do not use regular sand paper or emery cloth as the aluminum oxide abrasive material will permeate the cast iron surface and make the condition worse. Do not bead blast or sand blast the discs for the same reason.

The only fix for extensive uneven deposits involves dismounting the discs and having them Blanchard ground - not expensive, but inconvenient at best. A newly ground disc will require the same sort of bedding in process as a new disc. The trouble with this procedure is that if the grinding does not remove all of the cementite inclusions, as the disc wears the hard cementite will stand proud of the relatively soft disc and the thermal spiral starts over again. Unfortunately, the cementite is invisible to the naked eye.

Taking time to properly bed your braking system pays big dividends but, as with most sins, a repeat of the behavior that caused the trouble will bring it right back.

So whether it is warped discs or pad deposits new discs or resurfacing and pads will cure it, once you've fitted your new kit (pretty easy even for a novice but the golden rule is don't take chances with your brakes, if your not sure ask on the forum, post pics, ask a knowledgable friend etc, but make absolutely sure they are fitted correctly), the warping issue is sorted due to the new discs/grinding so now a llyou need to do is make sure you don't get any pad deposits.


A majority of this info was found at the Stoptech technical information page.



http://www.stoptech.com/