Warped brake disks?

[zcacogp]

Chaps,

Many of you will know I recently changed my front brake disks. The old ones juddered when I put the brakes on from high speed, and when I measured them they were below the recommended minimum thickness. They were worn out, and I assumed they were warped as a result.

New disks acquired, and fitted. (ATE from Bert - thanks Bert.) Allowed to wear in (with suggestions from people on here - thanks), and now work much much better than the old ones.

All good ... apart from the fact that the car still behaves like it has warped disks. Apply the brakes at 80+ mph (or lower, if applied firmly), and the car vibrates and rattles. (Everything is as smooth as you like at all other times - it's not a wheel balancing issue, for example.)

My thoughts are that either it is a rear brake disk that is causing this problem (unlikely, as I am positive the vibration is from the front), or I have managed to mis-seat the disks on the hubs *. Or I have managed to (somehow) bend or warp the hub ...

My plan is to find myself a dial test indicator, and test the run-out on the brake disks. If one of them appears to be out of spec, I'll take it off and do the same with the hub. This should tell me where the problem is (although the result won't allow me to differentiate between a warped disk and a mis-seated disk, although given the disks are new I doubt they will be warped.)

I also want to get on with this as soon as possible, as the disks will possibly wear in such a way as to correct the problem, without it being solved - something I don't want to have happen.

Has anyone else experienced this; if so what was it? Or have any suggestions as to what it could be?


Oli.

* - I'm using "hub" to refer to the piece which the disk bolts onto, and which carries the wheel bearing within in. I may be mis-using the terminology; if so, apologies.

 

[SteveSJS]

Oli
I had this on my 924S and after the second set that did exactly the same I approached my Inde (same one as Fen's BTW).
He bought the proper Porsche disks and when the same thing happened I took the car back to him and he took it to the local Porsche Main dealer 'As Porsche Disks 'don't' warp!!'
It is apparently a known problem with some 924's, 944's, 928's and Boxsters. OPC's have a special lather that is clamped to the wheel hub at the correctly tightened torque and the - now - distorted Disk is then skimmed in-situ.
Problem cured FOC!

But the down-side is that they must be Porsche supplied disks, they will also (possibly) have to be fitted by someone they recognise. They will not accommodate you otherwise.

 

[zcacogp]

Chaps,

Thanks for your input. It's interesting that this is not an unheard of problem, which is reassuring.

My suspicions lie firmly with the hubs - I suspect that one or the other of them is out of true. The fact that the symptoms of a warped disk were present with both the old and the new disks is what makes me think this. I do have a friend with a lathe which would be suitable for straightening up a mis-shapen hub, but would rather not go to this if I can avoid it. I guess the alternative is a new (or second-hand) hub ... ho hum.

I have ordered a DTI to check them with, and will keep you posted with what I find out.

Thanks again for your help.


Oli.

 

[mike220]

ORIGINAL: SteveSJS

Oli
OPC's have a special lather that is clamped to the wheel hub at the correctly tightened torque and the - now - distorted Disk is then skimmed in-situ.
Problem cured FOC!

But the down-side is that they must be Porsche supplied disks, they will also (possibly) have to be fitted by someone they recognise. They will not accommodate you otherwise.

There are people out there who have these machines other than OPC guys - Pumaspeed (or Pumabuild - I can never remember which) in Sheffield for example have them as Racing Puma's suffer from something similar, and benefit greatly from having the discs skimmed on the car so they match the hubs. Only know as I'm an ex Racing Puma owner.

Not much help to you I'm afraid Oli as it's miles away. If they have them here though, I'm sure there's an independant specialist (not necessarily Porsche) down south who can provide a similar service ?

Hope this helps,

Mike

 

[zcacogp]

Mike,

Thanks.

To be honest, I can't *quite* see the point of machining the disks ... if the disk is warped, it needs to be replaced. If the hub is skwiff, it needs to be machined or replaced.

I think I need to check those disks, then the hubs. I am expecting the hubs to be slightly skwiff (although I can't think how it happened), as I don't imagine that the disks were supplied warped, and the old disks were warped as well.

I'll keep you posted. Thanks for your input.


Oli.

 

[appletonn]

I don't understand how the hubs would suddenly go out of true?

Is there any play in the front wheel bearing or front suspension mountings? Possible that a wheel is out of true?

Interested to see what you find[]

 

[zcacogp]

Nick,

I don't understand how the hubs suddenly go out of true either, but it's the most logical explanation I can come up with at the moment. It is the means by which the wheel is attached to the car, so if the wheel took enough of a thump (not that I noticed any thump at all, and it would take quite a wallop) then I guess it could cause it.

I had presumed the old disks warped because I sat with my foot on the brakes for about 8 minutes after a long(ish) drive, and the symptoms of warping appeared the next time I drove it. Having said that, the drive was a leisurely one (home from work), and ended up in the small streets near where I live, which is always done slowly, so the brakes wouldn't have been anything more than slightly warm, if at all. And that's the old disks - the new ones should be straight.

Wheel out of true - I hadn't thought of that one. However, if this was the case then I would expect there to be funny behaviour at all times, not just under braking. And it behaves perfectly at all times other than when the brakes are applied. I guess another explanation could be local contamination on the brake disk - causing the pads to 'snatch' when the disk rotates, but I cleaned them clinically before fitting them and hence I doubt this is the problem. It would also seem unlikely given that the vibration symptoms happened with the old disks and pads as well ...

I will certainly keep you posted with what I find. DTI arrives (hopefully) tomorrow or Saturday, and that should allow me to explore what's going on in more detail.


Oli.

 

[mike220]

ORIGINAL: zcacogp

Mike,

Thanks.

To be honest, I can't *quite* see the point of machining the disks ... if the disk is warped, it needs to be replaced. If the hub is skwiff, it needs to be machined or replaced.

The point, is that with two components machined/manufactured seperately they'll never be perfect together. Usually if they're both well within tolerance the discrepancy will be so small it will be undetectable, but the way to get it all bang on is to effectively 'match' the run of the disc to the run of the hub - it'll make things as close as you can get in the real world on car (without going into serious engineering levels for components etc.)

Plus it's quite a cheap solution that can give remarkable results []

Hope it helps anyway bud, and you find out what your problem is. Checked balljoints on track control arms etc. ?

Mike

 

[zcacogp]

Mike,

Thanks. That point about manufacturing tolerance is a good one, hadn't thought of that. Do you think that the design of the disk (unusually large bells) has anything to do with it being harder to manufacture?

Ball joints on the control arms were replaced less than 10k miles ago as part of a suspension refresh. Good idea tho'. (Would these being worn out allow it to work perfectly under normal conditions and only vibrate under braking?)


Oli.

 

[mike220]

Shouldn't particularly make them any harder to manufacture to be honest. If you measure your runout on discs (may or may not be acceptable) you'll probably find you can alter it by turning the disc in relation to the hub (i.e. moving the disc round one hole at a time to shift the peak point if there is one)

Ball joint I meant were track control arms - e.g. steering rack ends.

And yes, ball joints etc. may feel fine when the front is unloaded/lightly loaded during acceleration or steady driving, but any error would feel magnified when you effectively load them to the hilt by throwing the cars weight on it's nose [] haha (not that any of us ever do that . . . . . . . . [8|]

Mike

 

[zcacogp]

Mike,

The idea of moving the disk around is a good one, but there are two small screws which are designed to hold it to the hub which mean there is no rotational symmetry - i.e. if you shift it to another position the small screws won't fit in. However, doing without the screws may not be the end of the world, and it may be a solution to the problem.

Ball joints - we are talking about the same thing. Good point about the loading, but I don't think this is the source of the problem.

Thanks again for your input. Any other ideas as to what could be causing this? All ideas welcome, as a 'brainstorming' (eugh [:'(] [] [:'(] [] ) exercise.


Oli.

 

[Fen]

This is an interesting discussion. My Jeep has the mother and father of a brake judder. Sometimes. Other times it's fine. I've done literally nothing to try to address it yet. I was hoping it was just from sitting around before I bought it and it would clear up through use, but it didn't and now it's doing a lot of sitting around again so it isn't going to fix itself.

I imagine I just need disks (you can actually feel a pulse through the pedal as if the thickness varies rather than runout), but it could be some of the other stuff I guess. I don't get why sometimes it's fine and once it got close to worrying from a steering perspective, then it can be fine again.

In terms of previous experience my S2 cab had mild judder when I bought it, but that really was lack of use and it cleared up. My BMW developed judder on fairly new Zimmermann disks. And the warranty replacement set. It was fine for a little while when they were new, it was fine with the old worn-out disks and it was also fine when I bought a Brembo pair rather than get another warranty set of Zimmermanns. That was a diesel estate that lived on the motorway and didn't get braked hard ever, though it was an auto so it was held on the brakes in traffic a lot. Point being the disks could have warped again.

 

[Black_JPN]

The disks warped on my S2. The new one's went the same way in no time at all. I had them machined in situ and then had no other probs with them. Make of that what you will - i.e. I have no idea if hubs had any bearing on that.

Fen, as for the Jeep I had a Grand Cherokee once with severe brake judder. Replaced the disks and pads, it made no difference. Seem to remember reading in the manual and on forums that some judder was to be expected and it was part of the ABS system working!?

 

[944s2]

Fen

Re judder in Jeep

Are all the tyres the same tread depth?

I remember one of the motoring mags years ago had a problem and it was down to mismatch in tyre circumference due to different depths of tread

Daft I know but true

 

[zcacogp]

Chaps,

Thanks for your input. Interesting - it sounds like this is not an isolated case.

My strong inclination is to get the hubs straight than machine the disks - machining the disks will solve the problem, but feels like a 'botch', for some reason. And wastes good disk thickness. And, if the hubs aren't true, then the problem will recurr when you put the next set of disks on.

DTI gauge arrives tomorrow (or it should). I'll see what it tells me.


Oli.

 

[paulf968]

another thought:

do you have the ally calipers that can suffer from the dreaded plate lift? I had exactly the same symptoms in my 968, tried everything including dial gauges, lots of new bits etc etc -- then realised one of the front pads was trapped a bit by the caliper, which then causes it to overheat, this then causes uneven glazing deposits on the disc surface especially when stopped, the uneven glaze then causes significant judder.

A rebuild of the front calipers was the total solution -- ie s/steel plates off and corrosion treated, new pistons and seals -- never looked back since.

paul f

 

[zcacogp]

Paul,

Now that is a good idea - thanks!

I don't recall there being any significant plate lift in there - the pads came in and out easily - but I'll double check that one hasn't become jammed (or worse.)

Thanks.


Oli.

 

[Fen]

At risk of taking Oli's thread off topic (sorry in advance), the judder the Jeep has is way more than I would expect if something is not awry - sometimes anyway. The tyres suggestion is interesting; it has 30" flotation tyres on it, which are something like 2" bigger than the originals and while they are all equally worn to the naked eye it could still be a tyre problem. I have on my shopping list some different wheels with a set of 31" M/T tyres of some type, but it needs a suspension lift to fit them so I'm still looking into the best way forward. There is a definite pulse through the pedal at very low speeds (like creeping in traffic) which suggests disks are a good start though I think - I just don't understand why it's sometimes fine, mostly relatively mild and occasionally horrendous.

 

[zcacogp]

Chaps,

OK, had a day to play with it, so I did.

Wheel off, caliper off, DTI gauge showed run-out on the outer face of the disk as being 0.38mm. (Porsche spec is <0.05mm). So, disk off and measure run out on the hub ... around 0.01 to 0.02mm. Well within spec.

Re-fit disk, measure run-out. 0.45mm! Remove disk, very carefully examine and clean the mating faces. Some grot there to be removed, but not muchat all. Re-fit disk, do up two countersunk screws as tight as possible AND put a couple of nuts on the wheel studs to pull the disk all the way home. (Wheel nuts don't work as they are closed, and shorter than the stud, and I only had two open-faced nuts of the right size.) In doing this I found that I could make the run-out vary quite a lot depending upon how tightly these nuts were done up.

So, I ended up re-fitting the wheel, doing the wheel nuts up tightly (and evenly) and test-driving it. And it seems fine. Not quite 100% smooth under braking, but I managed to push it hard from 80mph or so (wet, private road) and it was very nearly totally smooth until the ABS cut in. There seems to be a slight tremble at gentler braking from lower speeds, but I'm not sure whether this is disks or road surface; I can believe it's the latter.

So ... make of that what you will. Suffice it to say, "all's well that ends well", in the words of the Great Bard. I don't know why the disk wouldn't seat evenly on the hub, either the first time or later (with the open-faced nuts on), but it seems OK now.

And I am pleased!

(To make things better still, it passed an MOT yesterday, which is good. The hitch is that there is some rust on the bottom of the front wings that needs attending to, and the handbrake shoes are worn, and it will need some new tyres in the next 5000 miles, but none of these were a great surprise. And I managed to treat quite a lot of the rust today while it was up in the air, which is also good.)

Thanks everyone for your helpful suggestions. Thinking about the warped-hub problem; the hub design is such that it would take quite a lot of force to warp it (lots of bracing behind it, and it is cast alloy) so I guess I am surprised that this is a semi-common occurance. I'm glad mine was OK ... Oh, and DTI gauges are useful bits of kit, I have decided!

Fen, please go ahead and hijack this thread a much as you like. It's all yours, and I won't charge you a penny for it!


Oli.

 

[mike220]

Good news Oli - glad you've finally made some head way into the problem. I reckon a lot more of the issues out there could be due to incorrect fitting of discs (i.e. vleaning and prep before fitting) you think ?

Mike

PS - yep, DTI's are very useful indeed