Causes of vibration under braking

[kday]

I've had an ongoing problem with vibration under braking for months. It started more or less out of the blue in late December. It began as a moderate vibration that only occurred with a light-medium pedal effort; very light braking didn't cause it nor did heavy braking. Over time the vibration has been getting worse, to the point where it really shakes the car badly, and is occurring under more braking conditions. Now it's pretty much only heavy braking that doesn't lead to any vibration.

At first I thought this was pad deposits. I re-bedded in the pads multiple times. I took the brakes apart for inspection and went over them with a wire wheel. There was a minor pad imprint in one spot but nothing major. I can't imagine that pad deposits would persist this long.

I had the wheels off over the weekend and checked all of the control arms for play, but there was nothing obviously wrong. (But realistically I'm not sure what I'm looking for.)

At this point I am ready to start throwing parts at it, unless someone can suggest a way to test for specific problems.

Things I could replace (ordered by my guess at the probability of fixing the problem):
1) Control arms (never been done, have 100k miles on them)
2) Wheel bearings (ditto)
3) CV joints/axles (?)
4) rotors

Suggestions or comments?

[mik15]

my control arms are shot and i have no vibration on braking, bearings should also do some noise so i would guess the rotors or maybe the CV joints have to much play, have you checked them?

[kday]

How would I check the CV joints for play?

The rotors were the first place I looked, but they seem fine, and haven't cleaned up for months. This leads me to think that they are not the problem. Of course I could be wrong. I can't think of a way to rule them out without spending $700 for a new set.

[kismetcapitan]

check out Stoptech's website on bedding it. Sometimes, pad imprints are simply impossible to get rid of. They talk about the metal recrystalizing or something. Not sure if our rotors can be turned...?

[kday]

I've had pad deposits before, and they were nothing like this... not nearly as severe. Plus around the time the vibration started, I was driving the car very easily. And, perhaps significantly, had been over some fairly rough roads. That's part of what leads me to the control arms or wheel bearings.

[nene]

Do a test real quick. Turn off DSC and try again and see if you get less vibration.
When my rotors and pads (mainly the pads) where quite used and in need of replacement, I found that the car would vibrate under braking a whole lot, but noticed it less when the DSC was off.
Made no sense, but I found out by luck.
I have replaced the rotors and pads, and now with DSC on/off, it is all good.

[4everRS]

Front Lower rear control arms. Had the same issue.

[JSRS6]

1. Rebalance wheels
2. you can buy meyle HD bushings at auto zone to replace the upper and lower inner bushings. they are press in/out. you don't really need to worry about new upper and lower control arms all together unless you are getting vibration or wobbling just driving normally.
3. do your rotors look like they have little tiny cracks in them? Thats vitrification, or the crystallization of the pad material into the surface of the rotor. if so, that won't come out.

[4.2Crew]

How would I check the CV joints for play?

The rotors were the first place I looked, but they seem fine, and haven't cleaned up for months. This leads me to think that they are not the problem. Of course I could be wrong. I can't think of a way to rule them out without spending $700 for a new set.

You might be able to "mic" the rotors by mounting them in a rotor lathe... I know the rotors can't be machined, but you should be able to spec'em for warpage.

Just a thought. Good Luck!

[kday]

1. Rebalance wheels

Could they be out of balance in such a way that they vibrate terribly under braking, but not at all going 80 MPH without braking?

2. you can buy meyle HD bushings at auto zone to replace the upper and lower inner bushings. they are press in/out. you don't really need to worry about new upper and lower control arms all together unless you are getting vibration or wobbling just driving normally.

Interesting. That sounds worth a try. Is there an Audi part number just for the bushings? (I am having ETKA problems at the moment and can't check myself.)

3. do your rotors look like they have little tiny cracks in them? Thats vitrification, or the crystallization of the pad material into the surface of the rotor. if so, that won't come out.

Nope, the rotors look fine. I have had those cracks on other rotors though.

[kday]

You might be able to "mic" the rotors by mounting them in a rotor lathe... I know the rotors can't be machined, but you should be able to spec'em for warpage.

That's a good idea... actually I could check for runout using a dial indicator while they are mounted to the car.

[kday]

Front Lower rear control arms. Had the same issue.

Interesting -- could you elaborate on your symptoms?

[4everRS]

Interesting -- could you elaborate on your symptoms?

Sure.

Under moderate braking, I would get a vibration in the steering wheel and through the rest of the car. This didn't happen under normal driving/accelerating. This didn't happen under heavy braking. I thought rotors at first. Had the checked on a lathe, and very lightly turned (they say you shouldn't, but it didn't hurt mine ). Checked control arm bushings and they 'seemed' fine. After symptoms persisted, we put it on a lift again and checked wheel balance(it was good). Took a closer look at the front, rear lowered arms and noticed slight movement upon heavy pulling/pushing of the arm. Look closer at the bushing and saw some cracking.

After replacement, all vibration under braking ceased.

[4everRS]

Just think, those control arms are taking on a LOT of pressure under braking. Most of the vehicles weight is being pushed on them. Yes, the other arms are taking the pressure too, but the lower rear are taking the brunt of it. I think that's why they're the meatiest ones on the car.

[kday]

Ok, great -- I just ordered two Meyle 4B3407151D. (On Amazon of all places, free 2 day shipping!)
I will install them and see what happens.
Is there anything else that's a no-brainer to replace while I am doing those lower control arms? I've never had the newer Audi front suspension apart so I'm not familiar with it.

[kday]

Do a test real quick. Turn off DSC and try again and see if you get less vibration.

Interesting idea, but I tried it this morning, and there was no difference. I wonder what was up with yours...

[4everRS]

The rear bolt on that control arm, can be tricky. You need to slightly drop the cradle to get it out. Loosen the front cradle bolts just a little, to allow some movement. Put a jack under the cradle tight. Remover rear bolts. Carefully lower jack to allow rear control arm bolt to come out.

[UrS6]

My car does the same thing. I also notice that over certain bumps--like a light washboard condition--there is some play somewhere. I've had it to a dealer and to an independent and they say there's nothing wrong. But it sounds like a bad DRC shock. It was there with DRC and with my H&R coil overs so I know it's not that. I may just do the bushings too. It makes me crazy when it does it and the roads here are shite so it's quite often.