Vibrations with brakes

[Massive Lee]

One of the most common complaint from people who track their car is vibrations. We've heard they can come from warped discs (very unlikely), uneven deposit on the friction ring (may happen with certain types of pads that perform by first laying some deposit, and then using that layer against the pads to actually create a better friction (like carbon-to-carbon).

Another source of vibrations can be worn out suspension components. Look at ball joints and rear control arm bushings. Up to now, it is pretty common knowledge.

And what about alignment? Hmmm... Raising or lower the suspension will greatly affect toe, especially at the front. If the car has excessive toe out or toe in, when under hard braking, the wheels want to go in different directions... And that will create vibrations too, wether you have stock brakes or any BBK.

And if a car locks only one wheel, even in straight line? Hmmm.. Well, put your car on scales and measure how much each corner weights. Chances are that one of the front wheels checks much lighter and therefore doesn't apply the same pressure as the other on the ground. So, that tire can lock easily. What could be the cause of uneven front weight? Bent chassis, wrongly adjusted coil-overs.

[jlevie]

Well said!

I've seen exactly one case of warped rotors (proved by a dial indicator check), but lots of cases of uneven pad deposits. While more commonly encountered in cars that see track time, uneven pad deposits can also occur on street cars when the pads used aren't compatible with driving style/conditions. Uneven pad deposits are the result of approaching or exceeding the Maximum Operating Temperature (MOT) of the pads and then bringing the car to a stop before the pads and rotors have cooled. In that case the hot pad will leave a layer of material on the rotor (pad sized) that will cause vibration under braking.

[eLKuRLy]

You learn something new everyday.
So if you do have vibration caused by the layer of material, how do you fix it?

[jlevie]

You learn something new everyday.
So if you do have vibration caused by the layer of material, how do you fix it?

In some cases this can be fixed by re-bedding the pads. In other cases you may be able to sand off (use 120grit SC paper) the adherant layer from the rotors. But I have seen cases where only replacement of the rotors worked. Turning the rotors would work, but it isn't much of an option with BMW rotors as the total allowable wear is only 2mm in thickness and even a light cut is going to get you close to or below the wear limit.

Uneven pad deposits isn't that much of a problem on a pure track car. You almost always have some vibration from marble pickup and rotors don't last all that long anyway. I only get about 4 weekends out of front rotors before they crack or are about to crack. Fortunately, E30 rotors are cheap.

On a street car, or one that sees street and track time, a better choice of pads when on the track and making sure that the brakes are cooled off before bring the car to a stop avoids the problem. Sometimes, like when a track is red flagged, you don't have the option of cooling down the brakes. In that case you just have to live with the results. True track junkies have been known to have two sets of rotors and pads. One for the track and one for the street to avoid having to live with vibration under braking on the street after a red flag event.

If this is happening on a street only car, the remedy is to switch to a higher performance pad or change the way you drive the car. Low dust pads have a lower friction level and lower MOT. It isn't hard to exceed the MOT of those pads in spirited street driving and there may be no opportunity to cool down the pads. In that case you either have to drive the car slower or live with the dust that higher performance pads cause.

[eLKuRLy]

thanks!