Judder under braking - fixed

[nfearn]

I have been suffering from some brake judder when braking (worse when the brakes are hot) for a couple of years or so, and I've tried the following in a series of attempts to fix it:

- new disks front and rear, plus new pads (Pagid, from Euro car parts)
- another new set of front and rear disks, plus new pads (Porsche-supplied)
- careful check of all bushes
- ditto steering rack, track-rod ends and so on

Disks were very carefully mounted with ultra-clean hub and disk bell faces, and with run-out of 0.04mm or less (allowable limit is 0.08mm) when measured with a dial test indicator.

Result - brakes were fine for a couple of thousand miles and then quickly developed the problem of vibration when used, especially when hot.

However, speaking with the owner of the garage where my day-to-day BMW is serviced, he mentioned that he's had the same problem on several customer cars. He finally noticed that all of the affected cars had had their wheels refinished. On checking the wheels, all had been painted such that the surface of the wheel's web that mates to the disk/hub assembly had a layer of paint on them. He removed this on the wheels of one car, fitted new disks of the same brand that had gone bad before and the problem went away permanently.

Well, I checked my wheels and they had paint on the mating surface. I've now removed this and the problem is diminishing with every drive (I've done about 1,200 miles since).

I cannot be absolutely certain yet that this was the cause, but it is looking very likely given that the problem is subsiding now whereas previously it just kept on getting worse. The only absolute test would be to fit new disks but given the amount of cash and time sunk into the previous sets I am hoping to avoid that if possible.

The moral of the story is obvious, and I hope this information will help someone with the same problem.

 

[JB 3.2]

Nick very interesting,

One would presume that a factory fresh wheel has no paint just a machined surface where it mounts on the hub.

So the addition of paint is uneven which means the wheel rotates off centre as its not mounted square to the hub.

What I don't understand is why this is noticeable under braking as opposed to normal driving before applying the brakes.

I think your observations make sense, and its something I had not considered when trying to solve the source of occasional vibration through the steering wheel during driving rather than under braking.

 

[nfearn]

John

I believe there are a couple of effects at work here.

i) The wheel might be marginally offset by the paint thickness variation but I don't believe that this is a significant effect. It could, though, produce something along the lines of a mild tyre imbalance effect at motorway speeds.

ii) The clamping forces acting at the five points around the brake mounting surface could pull the disk into a mildly out-of-true position which would, in time, lead to scuffing of the "high" points on the disk surface and consequent disk thickness variation. This is, I believe, the mechanism by which the painted surface causes this problem.

As I have said, this is a bit of an embryonic theory but it seems to me to have "legs" and is worth pursuing.

 

[sue11sam]

ORIGINAL: nfearn

I have been suffering from some brake judder when braking (worse when the brakes are hot) for a couple of years or so, and I've tried the following in a series of attempts to fix it:

- new disks front and rear, plus new pads (Pagid, from Euro car parts)
- another new set of front and rear disks, plus new pads (Porsche-supplied)
- careful check of all bushes
- ditto steering rack, track-rod ends and so on

Disks were very carefully mounted with ultra-clean hub and disk bell faces, and with run-out of 0.04mm or less (allowable limit is 0.08mm) when measured with a dial test indicator.

Result - brakes were fine for a couple of thousand miles and then quickly developed the problem of vibration when used, especially when hot.

However, speaking with the owner of the garage where my day-to-day BMW is serviced, he mentioned that he's had the same problem on several customer cars. He finally noticed that all of the affected cars had had their wheels refinished. On checking the wheels, all had been painted such that the surface of the wheel's web that mates to the disk/hub assembly had a layer of paint on them. He removed this on the wheels of one car, fitted new disks of the same brand that had gone bad before and the problem went away permanently.

Well, I checked my wheels and they had paint on the mating surface. I've now removed this and the problem is diminishing with every drive (I've done about 1,200 miles since).

I cannot be absolutely certain yet that this was the cause, but it is looking very likely given that the problem is subsiding now whereas previously it just kept on getting worse. The only absolute test would be to fit new disks but given the amount of cash and time sunk into the previous sets I am hoping to avoid that if possible.

The moral of the story is obvious, and I hope this information will help someone with the same problem.

Hi Nick, mating surfaces between alloy wheels and hubs should always be metal to metal,most manufacturers say you should not even introduce any substance which would allow easier removal of the wheels at a later date, as it is the clamping effect of the two machined surfaces which is your interface and takes the torque effect of the wheels in operation, all the studs/bolts are doing is supplying the pressure to maintain that joint

 

[nfearn]

That sounds about right considering the problems I have had. Thankfully they now seem to be well on the way to being resolved.

 

[sue11sam]

glad to hear it[]

 

[IndischRot]

I have had freshly refurbished wheels stick to the hub because of the paint thickness on the wheel where it mates up the hub. By this I mean the circular section, not the flat mounting surface. Careful removal of the paint on the wheel and a super thin layer of grease meant that the wheel sat properly on the hub and could later be removed. One check that can be done - you should be able to place the wheel on the hub and firmly push it in place and still be able to rotate it against the hub. If it binds or you're able to rock it around slightly there's possibly paint or corrosion build-up in the way. Obviously someone will need to be stepping on the brake pedal to prevent the front hub from turning if you're doing this at the front of the car.

 

[sue11sam]

Don't think you are supposed to put anything between wheel machined surface and hub, see my earlier post.

 

[IndischRot]

yes, but... in the real world a thinner than thin layer of grease or the like can make all the difference between a wheel that's virtually welded to the hub and one that comes off with ease. Particularly over the winter when salt is used. And if kept thin it shouldn't affect the balance or centring of the wheel.

 

[sue11sam]

Yes, you are right, the wheel will come off more easily, we are all aware of that, but why do you think the car manufacturers advise against applying anything between wheel and hub? because what you are advising is perceived as unsafe! please read my earlier post under this topic, if you do not understand what I am saying I suggest you google the issue, and become informed. I should add, as I suggested earlier, I have not checked Porsches stance on this, but I would think it in line with the laws of physics, and the suggestions of all other manufacturers who's cars I have owned.

 

[IndischRot]

Sorry but I don't agree. The clamping force exerted by the wheel's fasteners is enormous and the resulting "stiction" prevents any movement, rotational or otherwise, of the wheel against the hub. Sound engineering practice is to never put bare ali against bare steel/cast iron because when you add moisture and heat and perhaps road salt the two materials will all but weld themselves to one another. I put a thin film of copper grease on the mounting interface and on the threads of the bolts/studs and in 30 years I have never had a wheel come loose or a stud/bolt strip or break. And the wheels come off without a fight when it's time to replace the tyres or work on the brakes. In my book that sort of empirical evidence is worth far more than what a google search can dredge up.

 

[sue11sam]

Okay, you go ahead and put whatever you want on your aluminium wheels, the worst thing you can use is a different metal, ie copper , the definitive advice , which I have now sought, is not to use anything at all between your alloy wheels and hub, and indeed if it has been employed, remove it. I will say that grease on steel wheels and studs works well avoiding difficulties removing wheel nuts. Sticktion you mention is a rotational force and will be diminished by applying a lubricant, I repeat, it seems for the umpteenth time, that the wheel nuts ON ALLOY WHEELS ARE EMPLOYED TO CLAMP THE WHEEL TO THE HUB and using the sticktion you mentioned earlier to transfer the torque to the wheels and prevent shock loading shearing the studs/bolts off. If you wish to apply a smear of ALUMINIUM grease to the spigot section of the hub, combined with the perfectly cleaned metal to metal machined surfaces of the wheel/hub interface, your wheels should be easier to remove. I agree that plastering your wheels with grease will allow easier removal but the main idea is to keep them on the car! The Sound engineering practice you mention should have told you never to introduce, wherever possible , dissimilar metals into an environment where galvanic corrosion is likely to occur , for example copper grease on aluminium wheels. I would be even more concerned if you were introducing copper grease into Porsche aluminium wheel nuts, the threads of which could easily corrode, and your wheels simply fall off.

 

[IndischRot]

If there was an issue with what I was doing I would have seen signs of it by now - either a collection of wheels at the roadside, loose wheel bolts or simply fretting on the mounting surfaces.

You're quoting theory; I'm stating real world results. The chap I learned this from bought his first 356 new and still drives 911s today - none have ever reverted to Reliant Robin mode.

"In theory, theory and practice are the same. In practice, they are not."

 

[sue11sam]

No, I am not quoting theory, this stuff really happens, luckily not often. If you continue to use a lubricant on the wheel/hub interface and alloy wheel nuts, I strongly advise you not to use a copper compound for all the reasons I mentioned earlier. I say all these things, not from hearsay from an old gentleman who drove a 356(steel wheels, by the way! perfectly fine greasing those) but from 48 years I have been an HNC qualified mechanical engineer, and the first Porsche I drove was a 1956 carrera 4 cam(also with greased steel wheels)and have owned several since. So whatever you decide to do please enjoy your car, but remember the dissimilar metals issue, and the opinion, if you choose to check, of the performance car manufacturers, and all the marine engineers in the world.

 

[Simon P]

ORIGINAL: JB 3.2

Nick very interesting,

One would presume that a factory fresh wheel has no paint just a machined surface where it mounts on the hub.

So the addition of paint is uneven which means the wheel rotates off centre as its not mounted square to the hub.

What I don't understand is why this is noticeable under braking as opposed to normal driving before applying the brakes.

I think your observations make sense, and its something I had not considered when trying to solve the source of occasional vibration through the steering wheel during driving rather than under braking.

 

[Melv]

The paint melts on the mating face. I ALWAYS put some copper slip on to avoid wheel
bonding to hub. Have had cars in past where I couldn't get wheel off due to corrosion.

 

[Steve Brookes]

Here's a tip for any one that finds they have a wheel refusing to budge after loosening the nuts or bolts. Crack the nuts or bolts before you jack the car up and just roll the car backwards and forwards a few feet. That will break the sticktion. Saved me a lot of grief on my boxster that used to almost self weld its wheels on if they had been on a long time.