I'm not been able to resolve this issue and am at a bit of a dead end. I have a Phaeton W12. When I brake there is a slight shudder at most speeds unless I brake fairly hard, except when traveling at 120 km/h and 85km/h, at these speeds the shudder is severe uless I brake hard, but this could be that the car slows out of these 'bad speeds' quickly. The shudder is unnerving and severe enough to expect that it will cause damage. I did have the disks/rotors skimmed on a lathe, I thought this had fixed the problem, I can't be sure it didn't. All seemded fine initially, but I don't remember the speeds I was doing when I tested the skimmed disks for the first time.
To add to the fun, I brought with me when I moved countries and VW won't work on it before its registered locally and I need the brakes to be working to do so. Chicken and egg sort of problem. Parst are also not freely available here.
There is a rubber suspension bush in front which I noticed needs to be replaced, which I will order today, but it's likely to take a few weeks to get here unless it's used on an Audi or another car sold here, but it's the opposite front wheel which appears to have uneven wear on the disk.
if you've already had the discs skimmed so that they're true but the shake is still there and the fact that you've found a suspension bush needing replacement i would sugggest that others may also need replacing.
It sounds like it could be a control arm bush that needs replacing from the symptoms you describe. When the control arm bush has worn it causes shaking while braking but brake harder and it goes away, this is due to the wheel moving back and forth. There is a youtube clip that shows this but not really got the time to hunt it out right now......you can find it under BMW
I assume you're in the US so have the two-piston floating calipers that most Phaetons have. Although I haven't experienced this with my Phaeton, my W8 had problems with a brake wobble under heavy braking and has the ATE 2FN calipers which were the predecessors to the Phaeton's calipers.
I was worried that it was the discs that had warped (since they cost £200 each from a VW dealer ) but when I stripped them down it appeared that the brake pad connected to the pistons on one of the calipers had very uneven wear. Installing a new set of pads completely solved the problem although I suspect I might need to get the calipers refurbished to sort out the uneven wear...
I had the same problem: this is what I had done:
new tires, swapped winter tyres, no result
skimmed the discs, short time result
replaced rotors and brakepads, short time result
replaced the wheelbearings and hubs, and new rotors again, shudder almost gone, only noticable on very flat roads and only on some speeds
replaced all the control arms and that solved the whole problem.
the VW mechanic said the control arm bushes where fine, but on my advice he replaced them and he was surpriced it solved the problem, I think the heavy weight of the car asks a lot of the bushes and needs to be perfect.
Thanks again to all. I do have the floating discs, but unfortunately I'm in South Africa and have just replaced the pads (at £300 or about $450 form the UK!) Jorg I think what you say fits well. The new disks did seemed to be fine initially, but after a bit of a drive earlier the problem is back. Stefan also suggested the control arms. This would be a lot easier if I was in the US or UK.
My dealer changed already 3 times the front disks as they seems to be curved again after about 4/5000 km. I am very carefull with braking and avoid apply braking for traffic lights, however the problem still remains. Today i went back again, and now the rear disk seems to be curved, and not the frontdisks. After replacements unfortunately no improvements. Ofcourse, the dealer offers warranty and special arrangements however it seems the dealer is lost on ideas now too. So, basically now no worn disks, but still shaking brakes behaviour. I mentioned the various resolutions here in the forum, so dealer is willing to investigate.... Keep you updated.
Ps, I get the advise from Willem not to replace the curved disks, but to 'fix' the existing and settled disks.
FYI, discs don't bend. I discussed the matter with someone who was servicing racing cars, and none of the pulsating brakes cases he came across came from bent discs. Bent discs are an urban legend.
However, what often happens is that the nature of the surface is modified, for instance by keeping the pads clamped to the discs while the discs are hot. This transfers some carbon to the discs and turns them into much harder steel, meaning that they will not react in a homogeneous way any more. I the place where the pads transferred their carbon is harder than elsewhere, this means that this spot will not wear out as fast as on the rest of the disc. Then, frictional resistance will also be different, resulting in pulsating brakes.
FYI, discs don't bend. I discussed the matter with someone who was servicing racing cars, and none of the pulsating brakes cases he came across came from bent discs. Bent discs are an urban legend....
I can imagine that carbon, migrating locally into the steel disc, can cause local hardening of the steel. However, racing cars may not have the same type of steel alloy as our Phaetons have for various reasons. Fact is that VW describes a specific diagnostic method to detect bent discs. It is done with a depth gauge, which is secured in such a way that the sensing tip of the gauge contacts the disc. By rotating the disc, the gauge should indicate a fixed distance +/- some tolerance. Wouter told me that the discs were actually measured this way and that it was confirmed that the disc was bent.
Wouter, it must be very frustrating to see that replacing them doesn't fix the problem at all. Do you remember how much variation (micrometer or millimeter) was observed and what the VW specified maximum variation is?
Well, the guy I talked to was quite positive on the fact that discs didn't bend.
Now, if hardening occurs somewhere on the disc, this may result in the spot not wearing out as fast as somewhere else. This could show up on the micrometer as the discs should be thicker there, but there would not be any bending involved in what is observed...
Hence the importance of running the pads in correctly. Wouter: did you change the brake pads at the same time as the discs and did you run them in ? (the way to do this is quite precise and depends on the type of pads that are used).
I don't have the results ( i do not remember the variation) available right now, but during next time visit to the dealer ill make a copy of the results. For sure last time the frontdisks where within specicifications, and the rear disks where out of limitations. Indeed the measurements where performed using a micrometer.
I suppose it depends on the definition of warped. A disc that was perfectly straight around its axis of rotation but had an undulating surface wouldn't be warped but would produce the symptoms that people associate with "warped discs".
I suggest take a good look at the control arms, I had the same problem/discussion with the dealership, but after I had the control arms replaced the car is perfect, even when the dealership said the bushes are not worn.
I think, there is always a bit vibration while driving/braking because of the roadsurface, this vibration is controlled by the controlarms and bushes, when the rubber mounts are not tight enough, it will vibrate. you can feel in the whole car.
we even thought about the known vibration in the gearbox, but it is simply the bushes in the controlarms. the meganics are Phaeton trained but they don't know how the car exacly should feel, and don't have the experience with this weight of the car. there are just not enough Phaetons to repair to get the experience.
and we all have a phaeton that has done a few miles/km so it would be normal to replace the bushes, to have a tight running car, which you won't feel in other cars because of the weight and the perfect ride. Maybe we should contact Bentley mechanics, because they repair more bentleys with the same platform as the phaeton?
Hi Jorg, i certainly will memorize your statements as i red them with interest. You might be absolutely right here. Amazing however why this problem led to bend disks so fast, bcs they where bend all the time. Not all, but one by one. Well, April 10th we know (hopefully) more as the dealer continues their investigations. Funny however how they react on proven solutions such as mentioned here on the forum... It would save them a lot of time/money (and me).
I don't have any decent feedback, more of a question if anyone can help as the saga continues.
I've been working on the car myself. I have some experience from when I was younger, but I'm rusty at best. I've removed the wheel and loosened one end of the control arm bush, but the rubbers look perfectly fine. They actually look as if they were recently replaced which seems possible, something to that effect was done when the car was last serviced in London. I'm not sure I have the 'control arm' everyone was referring to. I've loosened a strut which I think is the control arm, longer than 1 foot, about 400mm. It's situated to the rear of the suspension quite low more or less horizontal, with the wheel side slightly more forward in relation to the other end attached to the chassis. It has a huge bush on the chassis end hinging on a bolt with the wheel end attached by what is effectively a ball joint. Is this the control arm?
There is another rubber bush which appears to be shot. Its attached to what I would describe as the torsion bar. Its short, about 150mm (6") shaped as a 'U' lying on it's side. It's seems so unlikely to be the culprit, but I'm going to replace them.
It has a huge bush on the chassis end hinging on a bolt with the wheel end attached by what is effectively a ball joint. Is this the control arm?
Terminology on the suspension components can be confusing, but it appears you are looking at the right components. I believe there are 4 "arms" suspending each front wheel - 2 upper and 2 lower.
Just a thought, but there is some discussion on whether to replace the whole control arm or just the bushings. One of the advantages of total replacement of the arm is that when you replace the entire arm, you also replace the ball joint as well as the bushing at the other end. Can you check the ball joint and make sure it is not loose or sloppy. I believe a bad ball joint could also cause your shimmy - although a rally bad one, I assume would clunk.
I had a feel, there doesn't seem to be any play. I haven't managed to remove the ball joint side, but there is an entire long weekend to play with it.
I still have my doubts that there is any wear on the control arm. Do you think it is likely to be the other rubber bush. I will replace them but the turn around time is weeks as the parts need to be imported. These bushed only deal the vertical movement/restraint.
I am assuming that your vehicle has a good number of miles on it.... 80,000 or more...
If so, I would definitely look at the control arms. The symptoms you presented are consistent with control arm component wear (bush or ball joint wear). I would suspect at a minimum that the lower rear control arms for both front wheels have bad bushings and require replacement. The drop links are another component that usually will see early signs of wear on this vehicle.
It is very important to have someone experienced diagnosing and repairing suspension and control components examine your vehicle. Examination of the bushings will require the vehicle on a rack and close visual inspection. Often, using a pry-bar between the bush and adjacent rigid structure is necessary to observe component fatigue.
The dealer performed last week a factory advisory bulletin due to brakes shudder. Latest measurements and after ar replacements resulted in;
LF 0,02 LR 0,02 RL 0,02 and RR 0,02 while the limits are 0,35. So all disks are now within their limits however sitll shudder.
The behavior of the brakes shudder is much better after last check and factory bulletin check, however still not gone. I advised the dealer again to take a look at this forum, and to have a serious look at the contol arms.... I just can advise them as we talk about guarantee issues.
Any news or progress on this shudder problem?. My car has for the 4th time this problem (servere shudder at 80-110km) and again the dealer take responsibility. Now they suggested to renew all wheel bearings, i personally doubt whether it solve the problem, but controlarms they dont want to get there (yet)... :-(