Severe Brake Shudder from speed

[fielding67]

I have had instances of brake shudder on my 2012 3.0 Tdi Quattro A7. Replaced front discs and pads sept 2022. Shudder came back a few months later. Stripped all brakes F & R two weeks ago. Pistons fine. Slide pins lubricated.
No change.
Replaced rear discs and pads last weekend. Still no change. Spent £80 on Sealey run out test gauge. All four discs are under the 0.08mm tolerance for run out.
No vibration through steering. But from higher speeds the shudder can be quite severe through the car.
I’m happy that I’ve ruled out brakes, so car running safe.
But don’t know where to look next. Car recently passed MOT. But I haven’t had the time to take a detailed look underneath to see what else could be the problem. Any sensible ideas welcome.

 

[Mickyh]

Did you ever get to the bottom of this mate?

 

[fielding67]

Did you ever get to the bottom of this mate?

I forgot to post an update! I have resolved the problem. So many blogs and chat forums immediately suggest disc run out. However I started looking a little deeper and found that the rear control arm subframe bush was virtually split through. So under braking the shudder was the control arm moving around on the mount. Further investigation. Upper control arm bushes, front and rear arms, perished. So I bought a full set of replacement arms and linkages for the R & L front suspension. Transformed the car. Now much tighter handling - shock and surprise! .
Schoolboy error though. I’m now getting tell tale signs that the strut top mount bearings need doing. I should have done them whilst I had the rest of the linkages off the car.
The frustration for me is that the car had not long been MOT’d. the splits in the bushes didn’t look recent.
But.. all sorted now

 

[Mickyh]

I forgot to post an update! I have resolved the problem. So many blogs and chat forums immediately suggest disc run out. However I started looking a little deeper and found that the rear control arm subframe bush was virtually split through. So under braking the shudder was the control arm moving around on the mount. Further investigation. Upper control arm bushes, front and rear arms, perished. So I bought a full set of replacement arms and linkages for the R & L front suspension. Transformed the car. Now much tighter handling - shock and surprise! .
Schoolboy error though. I’m now getting tell tale signs that the strut top mount bearings need doing. I should have done them whilst I had the rest of the linkages off the car.
The frustration for me is that the car had not long been MOT’d. the splits in the bushes didn’t look recent.
But.. all sorted now

Thanks for coming back. Mine has been the same for years.
I got the front bottom arms replaced 6 months ago and the discs skimed.
I'm guessing they cocked the skim up as they were grooved.
I'm on my fourth set of discs and all seems well 1000 miles in
Time will tell as its usually ao 2k that it comes back.
I'll certainly look into the rears ones now.
Thank you.

 

[fielding67]

Thanks for coming back. Mine has been the same for years.
I got the front bottom arms replaced 6 months ago and the discs skimed.
I'm guessing they cocked the skim up as they were grooved.
I'm on my fourth set of discs and all seems well 1000 miles in
Time will tell as its usually ao 2k that it comes back.
I'll certainly look into the rears ones now.
Thank you.

Oh I should also have said… whilst I had the entire front stripped I swapped out both hubs and bearings, track control arms and anti roll bar bushes. Only the struts were left as existing.
Because of the stripped threads on the steering knuckle due to seized steel bolts (top links and blot joint pinch bolt), I also replaced the entire steering knuckle too.
Given the age and mileage the rear will be next when I get some time.

 

[Mickyh]

Oh I should also have said… whilst I had the entire front stripped I swapped out both hubs and bearings, track control arms and anti roll bar bushes. Only the struts were left as existing.
Because of the stripped threads on the steering knuckle due to seized steel bolts (top links and blot joint pinch bolt), I also replaced the entire steering knuckle too.
Given the age and mileage the rear will be next when I get some time.

Crikey, so you've got a new car now then!
Non of the 'experts' I took mine to, could find a problem, and all blamed warped discs!
I think they had worn badly, but because of vibration I guess.

 

[fielding67]

Crikey, so you've got a new car now then!
Non of the 'experts' I took mine to, could find a problem, and all blamed warped discs!
I think they had worn badly, but because of vibration I guess.

It certainly feels younger now! Everything I looked at on-line all talk about ‘warped disks’. But essentially this is highly unlikely for a disc to warp. But you can get eneven run out. Which I believe is something about where pad material gets burnt onto the discs. I think you’d have to be caning it and then with red hot discs, come to a standstill for long enough for pad material to get burnt onto the disc faces. I wasn’t;t getting the vibration through the steering wheel or the brake pedal either. The entire car would shake. The Sealey ‘run-out gauge’ I bought was well worth it as it was cheaper than replacing the discs again (and it wouldn’t solved the problem. But I’m not sure it‘s something that the usual tyre fitting places bother with. The guage would also highlight worn bearings or poorly seated discs on the hub carrier. Always worth a good wire brushing to ensure the surfaces are clean. Hope you get sorted. Photos attached - The worst of the split bushes & The reassembled new suspension.

 

[Mickyh]

Well 2k in and the judder is back! I'm looking at a 'C' class AMG 300d 2 door,19 plate now.
I've only lost 6k on my A7 in 4½ years so can't grumble.
Absolutely fed up with what is a nice car that judders.
I'm not replacing 4 corners of suspension as a gamble that might not cure it.
If only you had done your work in stages! You might have pinpointed the problem, but obviously that's your business lol.

 

[fielding67]

Well 2k in and the judder is back! I'm looking at a 'C' class AMG 300d 2 door,19 plate now.
I've only lost 6k on my A7 in 4½ years so can't grumble.
Absolutely fed up with what is a nice car that judders.
I'm not replacing 4 corners of suspension as a gamble that might not cure it.
If only you had done your work in stages! You might have pinpointed the problem, but obviously that's your business lol.

Shame it had to go. But I have been in a similar place with S-Tronic gearbox issues in the past. But that’s a whole other saga.

I’ve sort of summarised a few months work in one post. But it was drawn out process. I’m too tight fisted to just throw money at a problem!
I’d already swapped all discs and pads. Checked discs run out.
But the car was an MOT failure waiting to happen with all the worn rubber and at 150k miles bearings would have been end of life.
It was more efficient and easier to swap out all the components as they’re aluminium and a lot of the threaded fixing points had been compromised through electrolytic corrosion from the steel bolts.
Good luck with the Merc.