Vibration under heavy braking.

james944 · May 13, 2014

Hi everyone,

I have an issue with quite violent vibration occurring during braking from speeds over 50mph.

I've had a look at the brakes and found that the rear disc/rotor has uneven/pitted wear. The vent holes even seem to have blocked up. Could that be brake dust or rust, or both.

I would post a picture on here but it doesn't like it. Here is a link.

https://www.flickr.com/photos/124643747@N03/13993210179/

I have no issue with vibration at any other time. Just during braking from high speed.

Thanks

James

PSH · May 13, 2014

Hi James

I don't like the look of that brake disc at all...that could be the problem but I would do a general check on the front to be sure... check wheel bearings, anti-roll bar, wishbone play etc . Also check if any bolts for the brakes and suspension are loose or sheared, I'd replace those disc's as a matter of urgency..

Pete

edh · May 13, 2014

Vibration through the wheel or just the body? I'd be looking for something loose in the suspension. How about braking on uneven or rough surfaces?

Frenchy · May 13, 2014

Those rears are completely shot, get rid i can only echo what has already been said, it may be a lazy brake piston which happened to a friends car, pistons had some small build up on them causing it not to operate correctly.

supersport · May 13, 2014

What is all the white stuff. Almost looks like it's been under the sea for a couple of years.

james944 · May 13, 2014

Thanks for your feedback guys.

I've just ordered some new discs and pads. Those old discs are probably not doing what they are supposed to looking like that anyway.

The vibration is coming through the body, hard to tell from exactly what end of the car though.

No bumpy break peddle or wobbly steering wheel during breaking. Also I do not get pulling to either side.

New front bearings this week, slightly improved steering wheel wobble at 60-70mph but hasn't removed the braking vibration.

I will have to check all possibilities like the suspension, bushes, etc.

Paul190H · May 14, 2014

Looking at the colour of that disc I'd say they were overheating? I had the same problem with my S2, had to strip down the calliper and clean the piston and housing up.

james944 · May 14, 2014

Could have warped too then, I guess?

The colour is a little misleading, its not actually white. I think the flash or dodgy iPhone may have something to do with that. This link may show a better picture.

https://www.flickr.com/photos/124643747@N03/13994133857/

bmnelsc · May 14, 2014

It is interesting that the outer part of the disc shows lots of corrosion while the part nearer the axle is quite shiny. Looks like the pads are not contacting the outer part of the disc or at least, with less force than nearer in. Might be worth checking your calipers over closely to ensure the pistons are moving freely and squarely in the bores.

I've read a number of posts here about "plate lift" but I'm not sure of the symptoms. Does plate lift interfere with free movement of the pads?

Monkeythree · May 14, 2014

The discs and pads are knackered and need to be changed anyway so you will soon learn if they were the cause of the vibration or not. If the pads are impossible to remove from the calipers then you know you've got plate lift and this will need rectifying before installing new pads. If you have patience and the right tools it is a DIY job or you can do as i did and drop them in to a specialist to do the job for you. Looking at the state of those discs, i expect plate lift to be a contributing factor in the uneven wear.

zcacogp · May 14, 2014

Chaps,

According to James's signature, the car he has is a lux - so it's the sliding-frame calipers, not the four-pot Brembo's, so there won't be any plate lift. However much still applies, and my suggestion would be;

- Remove the calipers and clean them up carefully. There was a good thread on here recently from someone who did this with the sliding-frame calipers (although I can't remember who. He painted them silver and put some logos on them as well, and they looked very good indeed once he was done.) They need to be clean, the pistons need to move smoothly and freely and the sliding bits need to be clean and well-lubricated too.
- Fit new pads and disks, ensuring that the disks fit snugly against the hubs behind them
- Re-fit calipers and wheels, and ensure that the wheel nuts are done up to the same torque, as specified in the manual (130Nm). Use a torque wrench, and buy one if you don't have one.

If that lot doesn't solve the problem then try borrowing a dial test indicator and seeing how much run-out there is on the disks. If you are local to London you are welcome to come and use mine and I'll happily lend a hand.

Worn bushes and wheel bearings can cause vibration under braking but, in my experience, it's far more commonly caused by something amiss with the braking system. New pads and disks and doing as suggested above will hopefully cure it.

Oli.

ChasR · May 14, 2014

My 205 did a similar thing to the discs (I.e They corroded quite a bit in certain places). The cause to us seemed to down to a couple of things:

- A lack of use (the car was left outside my friend's house for well over a year).
- Sticking sliding pins on the calipers (the copper grease the PO had applied had congealed and had caused the sliders to stick (indeed, when we bought the car it pulled hard to one side upon braking).

However, judging by the state of those discs, if the front are similar that would also probably cause severe juddering. I would be temped to take up Oli's offer of checking your discs for the amount of runout.

supersport · May 14, 2014

That second picture makes a lot more sense.

When I did the brakes last year on my Lux, the frames just didn't move. When I finally got them apart on pad was on the metal and the other side was barely used. A good bit of judicious banging and lubrication got them moving freely again and then I put all back together and they have been working properly again.

That rust just shows how little brake pad has been touching the disk.

colin944 · May 14, 2014

Needs Discs and pads and your top slide is seized. That's why the pad is only contacting the inner side of the disc. Remove the calipers and clean paint and grease everything then fit new discs and pads. And remember to wear your seat belt when you go out for a test drive cause if you hit the brakes like you are used to you'll go through the windscreen.

james944 · May 14, 2014

I've taken a photo of the calipers today so you can better tell if plate lift may be causing the strange wear or not.

https://www.flickr.com/photos/124643747@N03/14162500476/

Also after removing the wheel for a better look at the caliper I noticed the corrosion on the brake disc is mainly on one side. Would this suggest the disc is warped, causing the vibration under braking? The way the disc is positioned over the studs doesn't allow for the second screw that holds it to the hub to be inserted and is missing, it's the same on both sides, problem?

https://www.flickr.com/photos/124643747@N03/13999015600/in/photostream/

What is run-out?

Thanks Oli, I'm in Sussex. I'll replace the brake pads and discs this weekend and carefully clean up the calipers.

When I had the rear jacked up today I checked for play in the wheel. There is a very slight amount, would it be the bearings? Are they similar to the front wheel bearings or do you require a press? The front discs are pretty good to be honest. Even wear inside and out.

Thanks for your feedback guys.

James.

colin944 · May 14, 2014

I've taken a photo of the calipers today so you can better tell if plate lift may be causing the strange wear or not.

Your calipers don't have plates. Plate lift is a problem on Brembo alloy calipers as used on S2's and those turbo things.

Also after removing the wheel for a better look at the caliper I noticed the corrosion on the brake disc is mainly on one side. Would this suggest the disc is warped, causing the vibration under braking? The way the disc is positioned over the studs doesn't allow for the second screw that holds it to the hub to be inserted and is missing, it's the same on both sides, problem?

Corossion on one side is caused by a seized slide mechanism on your caliper.You caliper halves are not both squeezing together properly see video below

https://www.youtube.com/watch?v=ol-wDa6kAgQ

The wheel will hold the disc tight and flat against the hub. If the disc was sitting squint by any more than a millimeter or so the disc would not rotate through the caliper.So no problem.

Run out refers to the thickness of the disc being even across it's entire width. Ie if it is thinner at the outside edge then the pad would not hit the disc as much as it would at a thicker part of the disc,

Wheel bearing require a press to remove the bearing cup. If you are not 100% about doing rear bearings take it to some one who is. It's basically a modernized version of a VW Beetle back end and as such is not a straight forward job.

zcacogp · May 14, 2014

Quite a lot of questions there. However, as Colin said, your calipers don't have plates and hence won't have plate lift - so don't worry about it. Uneven wear between the two sides of the disk will indicate a sticking caliper, and that could well be the reason for your juddering during braking. As suggested, dismantle everything, clean it well, lubricate it, re-build it with new disks and pads and then see what the world looks like; you'll probably find your brakes have a similar effect to hitting a brick wall and you'll realise what you have been missing for the last few thousand miles.

ORIGINAL: colin944
The wheel will hold the disc tight and flat against the hub. If the disc was sitting squint by any more than a millimeter or so the disc would not rotate through the caliper.So no problem.

Yes, in theory. However it's quite possible to get disks cock-eyed on 944's by either having contamination between the disk and the hub (rust or a bit of grit - I have seen both) or by doing the wheel bolts up to differing torques, hence my strong recommendation of using a torque wrench to do them up with. The whole assembly is alloy, and it's easy to squash one side slightly more than the other and hence have disks which aren't held straight.

ORIGINAL: colin944
Run out refers to the thickness of the disc being even across it's entire width. Ie if it is thinner at the outside edge then the pad would not hit the disc as much as it would at a thicker part of the disc,

I always understood that 'run out' referred to the amount of wobble visible in a rotating disk, be that either due to the disk being mounted incorrectly (see above) or it being warped - certainly this is the meaning of it in Porsche manuals, hence the use of a Dial Test Indicator (DTI) to check for it. A disk that is thinner at the outside edge would be simply 'unevenly worn'.

Oli.

james944 · May 14, 2014

Ok, so plate lift is not possible with these calipers. That's one possibility removed.

Will the seized slide mechanism be whats causing the vibration then?

If so, new pads and discs and cleaning up the caliper should solve the problem, right?

I don't think there is run-out but can't be sure. I'm just going with because I don't get any pedal or steering wheel vibration. It's the whole car vibrating, the chassis!

I may have to look into getting a garage to press my bearings for me if replace them.

Thanks Colin

zcacogp · May 14, 2014

ORIGINAL: james944

Will the seized slide mechanism be whats causing the vibration then?

Could well be. Solve this problem and see whether there is an improvement.

ORIGINAL: james944

If so, new pads and discs and cleaning up the caliper should solve the problem, right?

Yup, which is why we are all telling you to do it! [

]

ORIGINAL: james944

I don't think there is run-out but can't be sure. I'm just going with because I don't get any pedal or steering wheel vibration. It's the whole car vibrating, the chassis!

Why do you think there is no run-out? Any reason? Have you measured it? As I said, solve the obvious problem (sticking calipers) before worrying about anything else.

ORIGINAL: james944

I may have to look into getting a garage to press my bearings for me if replace them.

That's almost certainly a separate issue, don't worry about it until you have changed the disks/pads and cleaned up the calipers.

Oli.

james944 · May 14, 2014

Nice one Oli. Certainly have some clarity on the issue.

Will be back to you once I've fitted new parts and cleaned the calipers. Any good links or advice for best results when cleaning the calipers. I had a look for the thread you mentioned Oli but I couldn't find it! Do you remember when it was roughly?

James