993 - Steering Wheel shake

[lolbert]

Hi,

I'm getting steering wheel shake when braking at about 60mph and above. No issues with wheel balance without braking so I presume this is brake-related. Is this likely to be a discs/pads issue or something more fundamental/expensive?

thanks

[POORCARDEALER]

Probably warped front discs if its nothing to do with tracking/balance of the wheels.....budget for discs and pads.

[Orangecurry]

There is no such thing as warped discs..... but that doesn't mean they don't need replacing. Almost any garage will offer to replace your discs and pads, whether they need it or not.

Quite often the reason for brake shake is caused by not bedding-in the pads/discs properly when either/both is replaced, and a wodge of pad material is transferred unevenly to the disc.

Anyway - is there a lip on the disc edge? Is there corrosion on the inner face? Do you know how old/miles the discs have done (from service records)?

If you do get them replaced, look up how to bed them in properly.

[MrCooke]

Don't believe the hype :

http://www.mossmotors.com/SiteGraphics/Pages/brake...

or do, depending whether you're a brake manufacturer or not.

[jackal]

There is no such thing as warped discs. It could be uneven material transfer onto the discs in which case you will want to rub them down first of all with some emery paper. If that doesnt work then some new pads and rebed them in propely or possibly skim the discs. That said, it could well be worn front lower wishbone bushes. Do you feel the juddering though the brakes pedal as well ? if you dont then it wont b brake related but more likely wishbone bushes or worn ball jpins on the front inner tie rods.

[With these feet]

Friend of mine brought his one to me with brake judder - we checked the disc and there was a small amount of run-out so we changed the discs. Foolishly we thought that was it until we tested it and it hadn't cured the problem.
We were changing the wheel bearing as a precaution when we spotted the cause. The hub itself was distorted causing the disc to run out when all was tightened up. Got hold of another hub and hey-presto. No judder.

Before replacing anything check simple things like wheel balance, irregular tyre wear, buckled wheel etc etc.

[thegoose]

I've heard all this before and given it due consideration but when you clean all residue from your discs and you can see them sticking against the pads in a certain place as they rotate (i.e the disc has a high spot/low spot) and the only thing that gets rid of this is new discs, it tends to point to the only possible diagnosis - the discs have been distorted out of shape i.e. warped.

This has happened on my race cars, but mainly the 964 (front discs would last 1 meeting with Pagid Blacks, half a season or more on Yellows) - the SC would get bad vibration at the end of a race but with a good clean up of the discs (compressor and flap-wheel) would be OK next time out, but also on a road going 993 (rears) and an Audi RS4 (fronts & rears).

Anyone who's done a track day will recall that part of the briefing is "cool your brakes down / don't put the handbrake on / don't leave your footbrake on" - one of the reasons being uneven cooling can result in an out of shape disc.

[Orangecurry]

thegoose said:

Anyone who's done a track day will recall that part of the briefing is "cool your brakes down / don't put the handbrake on / don't leave your footbrake on" - one of the reasons being uneven cooling can result in an out of shape disc.

No.

What happens is that if the pad is in contact with a stationary disc, a large amount of pad material is welded on at that spot.

A high spot of pad material. Not a warped disc.

[jackal]

The area in which the pad material is then has its temperature properties all messed up and then the metal underneath undergoes some sort of chemical change and hardens i think. If left too long then sanding wont remove it and you have to skim or swap the discs completely. Use your handbrake at the traffic lights and stay off the footbake ! Anyone who's owned an Evo VI will already be well versed in this. As OC says, the iscs arent warped but the surfaces are corrupted.

[With these feet]

Orangecurry said:

No.

What happens is that if the pad is in contact with a stationary disc, a large amount of pad material is welded on at that spot.

A high spot of pad material. Not a warped disc.

Are you saying that there is no such thing as a warped disc?

Care to explain why in my many years as a tech in a main dealer we would frequently bolt on a special lathe in place of the caliper to true the disc to stop both brake judder and vibration? If the disc is not out of true then you will not get kickback and vibration under braking. Mere contamination - fluid or friction material - causes reduced efficiency. Ever noticed how many fwd cars fail MOT's because of poor efficiency of rear brakes? They are not used much, the surfaces corrode and before you know it they are two discs of corrosion - yet no noticeable judder.

[jackal]

With these feet said:

Are you saying that there is no such thing as a warped disc?

Care to explain why in my many years as a tech in a main dealer we would frequently bolt on a special lathe in place of the caliper to true the disc to stop both brake judder and vibration? If the disc is not out of true then you will not get kickback and vibration under braking. Mere contamination - fluid or friction material - causes reduced efficiency. Ever noticed how many fwd cars fail MOT's because of poor efficiency of rear brakes? They are not used much, the surfaces corrode and before you know it they are two discs of corrosion - yet no noticeable judder.

You were levelling off the uneven pad transfer.

But the disc wasnt physically warped or bent.

[BertBert]

With these feet said:

Are you saying that there is no such thing as a warped disc?

Jackal has quoted the well versed argument about what happens to disks. In effect it doesn't matter all that much whether the disk surface has changed in one place, or the disk has physically warped (like we see records warped). The cure is skim or replace.

For the OP My experience of this happening is that it hasn't made my brakes judder at high speed, it's only been felt at low speed braking. So you may want to look for other causes as described.

Bert

[With these feet]

jackal said:

With these feet said:

Are you saying that there is no such thing as a warped disc?

Care to explain why in my many years as a tech in a main dealer we would frequently bolt on a special lathe in place of the caliper to true the disc to stop both brake judder and vibration? If the disc is not out of true then you will not get kickback and vibration under braking. Mere contamination - fluid or friction material - causes reduced efficiency. Ever noticed how many fwd cars fail MOT's because of poor efficiency of rear brakes? They are not used much, the surfaces corrode and before you know it they are two discs of corrosion - yet no noticeable judder.

You were levelling off the uneven pad transfer.

But the disc wasnt physically warped or bent.

Care to elaborate on exactly what "Disc run-out" is then?

[jackal]

BertBert said:

Jackal has quoted the well versed argument about what happens to disks. In effect it doesn't matter all that much whether the disk surface has changed in one place, or the disk has physically warped (like we see records warped). The cure is skim or replace.

For the OP My experience of this happening is that it hasn't made my brakes judder at high speed, it's only been felt at low speed braking. So you may want to look for other causes as described.

Bert

I agree that up to a point it just an issue of nomenclature. That said, i feel that the phrase 'warped discs' is a loaded and misleading one and has probably been used millions of times by conmen against innocent victims to force a complete change of brakes when a simple re-bedding or sanding down would have sufficed. Skim or replace are the next options but again, not always needed in my experience.

[Orangecurry]

With these feet said:

Care to elaborate on exactly what "Disc run-out" is then?

In what situation? In the examples above it's usually caused by a mechanic fitting new discs and not cleaning the hub/disc contact area properly, or not torquing the wheel bolts in the 'correct' order on top of a new disc.

',,,it took three sets of discs before we solved the problem coming back....' or perhaps the dirt was finally removed

[jackal]

With these feet said:

Care to elaborate on exactly what "Disc run-out" is then?

From what i recall, when you have uneven transfer the metal below can react and turn harder so then you have some areas that are harder and wear more slowly and i think their temperature properties are different too as the metal actually changes composition somehow. If you google it there i some ort of fancy name for the chemical change whose name escapes me. So as you use the brakes more, you then have quite significant thickness variations with these hard areas that have a different wear rate. That then is run out. The discs aren't physically warped though. 'Contaminated' would be a better word.

Of course you can also get run out from a compromised install with dirty or distorted mating surfaces, even with a brand new discs, but that's another topic I guess.

[With these feet]

jackal said:

With these feet said:

Care to elaborate on exactly what "Disc run-out" is then?

From what i recall, when you have uneven transfer the metal below can react and turn harder so then you have some areas that are harder and wear more slowly and i think their temperature properties are different too as the metal actually changes composition somehow. If you google it there i some ort of fancy name for the chemical change whose name escapes me. So as you use the brakes more, you then have quite significant thickness variations with these hard areas that have a different wear rate. That then is run out. The discs aren't physically warped though. 'Contaminated' would be a better word.

Of course you can also get run out from a compromised install with dirty or distorted mating surfaces, even with a brand new discs, but that's another topic I guess.

This though would take place over quite a distance, I have seen several cases where continuous heavy braking and heat build up has damaged discs and caused run-out leading to pad knock-off, a soft pedal and vibrations/judder.
You cant dismiss every claim of warped discs as a build up of material and poor repair is just that - a poor repair!

[thegoose]

jackal said:

From what i recall, when you have uneven transfer the metal below can react and turn harder so then you have some areas that are harder and wear more slowly and i think their temperature properties are different too as the metal actually changes composition somehow. If you google it there i some ort of fancy name for the chemical change whose name escapes me. So as you use the brakes more, you then have quite significant thickness variations with these hard areas that have a different wear rate...

That would sounds like a reasonable explanation for what most people would call warped discs, so maybe how that process is named is what's causing some disagreement.


Howwwever....it's not the same as I have seen on my own brakes, whereby there is a tight spot, seen around part of the circumfrerence of the disc as it's rotated around. It is not a spot where the disc is thicker, it's noticable that when measured against the caliper it is closer to one side AND further away from the other side, whilst still being the same thickness as the rest of the disc.

If you ever buckled a bike wheel and noticed it dragging on the brake pads as it was spun round, that's exactly what I'm trying to explain. If that's happened to a brake disc I'm not sure else you'd call it other than warped? Buckled?

[POORCARDEALER]

thegoose said:

jackal said:

If that's happened to a brake disc I'm not sure else you'd call it other than warped? Buckled?

No the term is warped everywhere but on the PH porsche forum

[thegoose]

POORCARDEALER said:

thegoose said:

If that's happened to a brake disc I'm not sure else you'd call it other than warped? Buckled?

No the term is warped everywhere but on the PH porsche forum