Warped brake disks?

[zcacogp]

Mike,

I guess it's possible. My confusion arises from two things:

1. When the disks were first fitted, I cleaned them all over with white spirit, to remove any grease, and made sure I did the back of the middle - i.e. the mating face. I didn't clean the other mating face (on the hub), but checked it for debris and it looked fine. I did all five wheel bolts up tight, and yet it clearly hadn't seated properly.

2. When I was experimenting yesterday, the run-out would change depending upon how tight the open-faced nuts holding the disk on were done up, despite the mating faces being (nearly) surgically clean.

So, yes, it would seem possible that there could be a number of issues out there which could be cured by a removal, clean and careful refit of the disks ... I guess it's therefore possible that some people have had their disks machined (as described on here) un-necessarily.

I do know that I don't quite understand what went wrong, and I don't quite trust it.


Oli.

 

[mike220]

This mat sound like a daft schoolboy error question, but how do you fit your wheels and what sequence do you tighten the wheelnuts in ?

Just a thought . . . . . . . . [8|]

Mike

 

[zcacogp]

Mike,

Not a daft Q at all ... fit wheels by putting the wheel on the studs, and screwing the nuts on in turn, finger-tight, in the sequence 1, 3, 5, 2, 4 (diagonally across the centre of the wheel, in turn), then tighten using a long handle wheel wrench (I don't have a torque wrench) in the same sequence.


Oli.

 

[Diver944]

Sounds like you need a torque wrench - 130Nm 95ftlbs

I would guess that if you have proved a slight mistightening can vary the runout quite dramatically, then without a torque wrench you could easily have one bolt looser than the others. I must remove/replace my wheels at least 20 times a year when I'm tracking and I've never felt any juddering.

 

[mike220]

2nd'd - you won't do yourself any harm using one (provided it's calibrated accurately)

You're fitting and tightening in the correct sequence (although I use 13425 myself), but few people can replicate torque accurately bolt after bolt (I only know as I used to be a Calibration engineer on torque eqpt. in a previous life [8D])

Mike

 

[zcacogp]

Hmmm. Another tool I have on my shopping list. Hey - Christmas is coming - do any of you chaps feel generous? Please? []

I take your point, but this isn't the first time I have taken the wheels off a car ... I'm not sure that I do it 20 times a year, but it must be between 5 and 10 ... maybe nearer 10 ... and I have never had this problem before.

And would a different torque affect the run-out? It shouldn't - the disk should be held against the hub by even a finger-tight bolt; tightening it further will hold it more firmly, but not more closely. (In the lack of a better explanation I think you have a point, but I can't logically understand it and remain skeptical!)


Oli.

 

[Diver944]

[&:]?

But earlier on you said:

ORIGINAL: zcacogp

2. When I was experimenting yesterday, the run-out would change depending upon how tight the open-faced nuts holding the disk on were done up, despite the mating faces being (nearly) surgically clean.

 

[mike220]

I know what you mean Oli, but look up how much load in Nm a 12mm nut on a thread can provide, then look at the compression qualities of aluminium - it only takes a few microns on one side of the hub, which is then multiplied by the radius of the disc, to ensure the very edge of the discs varies out of true by a (in relative terms) considerable amount. It's easy to get a disc cocked from incorrect torquing, in just the same way it's easy to get it cocked by tightening the wheel nuts in an incorrect sequence. You then take into account that the hub will heat at a slightly different rate dependant upon compression load on the face etc. etc.

Now these are all VERY minute changes, but can add up to a noticable feel. No one's running a race team here or anything, so it's not all that data driven or anything.

So before I get called an anorak (with reason haha) I'll shut up now . . . . . . . . [X(]

Mike
[]

 

[zcacogp]

Paul - exactly my confusion! You have put your finger directly on it.

Mike, I guess that is the best explanation. But still - I showed a run-out of (give or take) 0.4mm at the outer edge of the disk. Disk radius is around 130mm, wheel bolt radius is around 50mm. This gives a ratio of 0.38 .... so 0.4*0.38 = 0.15mm.

Am I really managing to compress the metal of the disk and hub by 0.15mm? (The only aluminium is the hub, and I think that is an alloy ... the disk is steel.) I strongly suspect that the wheel bolts (alloy) would have stripped before I got that much compression on the assembly.


Oli.

 

[appletonn]

ORIGINAL: zcacogp

Am I really managing to compress the metal of the disk and hub by 0.15mm? (The only aluminium is the hub, and I think that is an alloy ... the disk is steel.) I strongly suspect that the wheel bolts (alloy) would have stripped before I got that much compression on the assembly.


Oli.

Oli, I bet you can eat 3 Shredded Wheat too.....?!![]

Mike, I now know where to come for any torque input in future.....[]

I haven't a torque wrench either (been meaning to get one for ages - any suggestions?) and have been using a short wrench so as not to apply too much torque...

...don't ask me about the time I screwed the locking nuts on my old 964 by applying too much oomph, with the result that my friendly local specialist had to cut them off....[8|]

 

[mike220]

ORIGINAL: zcacogp
Am I really managing to compress the metal of the disk and hub by 0.15mm?

Oli.

In short. . . . .Yes ! [&:]

Don't forget you'll also get a slight opposite action of the disc/nut compressing in lift on the opposite side - It's only 150uM, it's very easy to do. Clamping force off that size of thread is a lot more than you'd think Oli.

Cheers,

Mike

 

[944Turbo]

I usualy leave the disc retaining screws quite loose or an impact driver is required to undo them later, They do little other than locate the disc.
Tony

 

[zcacogp]

OK chaps, Torque Wrench procured and applied to the wheel nuts.

They appeared to be pretty evenly done up - there was only noticable rotation on about 3 (out of 20) before the wrench clicked.

Yet to drive it since doing the tightening; I'll be VERY interested to see if it has made any difference to the slight (continued) brake vibration ...I'll keep you all posted.


Oli.

 

[mike220]

Slacken them all off and torque all of them up with the wrench ensuring rotation on each one - over torque is every bit as bad and problematic as undertorque Oli. No rotation isn't a sign it's at the right torque already - simply that it's AT LEAST that torque figure you require - it could be way past where you need.

Mike

 

[zcacogp]

Mike,

Thanks ... given the fact that the (wretched!) slight vibration on braking is still there, I'll do just as you say and let you know whether that makes any difference ... I sincerely hope it does; I'm getting a little tired of the palaver!


Oli.

 

[mike220]

Just read your other post Oli on the slack in the linkage - possible attributing factor ?

Really do hope you get to the bottom of it bud, there's nothing worse when something niggles at you when driving.

Mike

 

[zcacogp]

Mike,

Thanks. I don't *think* it is relevant, as it is simply slack in the linkage, whereas the brake wobble is just that - a brake wobble. I don't see how the slack in the linkage (i.e. a cm or so of free travel in the brake pedal before the mechanism starts operating) could produce a wobble once the mechanism is applied.

BUT, I could be wrong.

Still yet to try loosening the wheelnuts and torquing them all up correctly. In fact, I think I'll do that now. I'll be doing a couple of hundred miles in the car this weekend, so it'll be a good test.

Thanks again for your help. Your comment about it not being good when there is a niggle in the driving experience is very accurate - good to know that someone else experiences the same - thanks! (Worst niggle has to be a slightly unbalanced wheel. Maybe I am a wheel-balancing- freak, but I cannot stand the slightest wheel wobble at any speed. But that's a different topic. Rattles are the same .... [] !)


Oli.

 

[appletonn]

ORIGINAL: zcacogp


Thanks again for your help. Your comment about it not being good when there is a niggle in the driving experience is very accurate - good to know that someone else experiences the same - thanks! (Worst niggle has to be a slightly unbalanced wheel. Maybe I am a wheel-balancing- freak, but I cannot stand the slightest wheel wobble at any speed. But that's a different topic. Rattles are the same .... [] !)


Oli.

Hmm, separated at birth ???!! I also hate the vibration of poorly balanced wheels and chirps and creaks from parts of the car that shouldn't be either chirping or creaking, drive me batty... er!

Trouble is, we are talking about 17+ yr old cars and some of this irreversible (without a blank chequebook[8|]) although getting the mechanical bits on my car as they should be (ie no unwanted vibrations, leaks etc) has been my primary focus with the cosmetic bits coming a distant second. Never much funny driving a tired car that you are not able to use as it was intended.[8D]

 

[zcacogp]

Quick update .... took the off-side disk off yesterday, cleaned and re-seated the back of it, and re-fitted.

It's better; almost cured the problem, but not quite.

Very annoying. I can't believe that taking the disks off again and re-seating them will make any further difference, but it seems to be pretty much the only option. (It's also a prize faff and quite annoying.)

Ho hummm. On a different note, the stated torque for the caliper bolts, bolting back to the steering knuckle, is 85Nm. This seems very low - is this correct?


Oli.

 

[appletonn]

ORIGINAL: zcacogp

Quick update .... took the off-side disk off yesterday, cleaned and re-seated the back of it, and re-fitted.

It's better; almost cured the problem, but not quite.

Very annoying. I can't believe that taking the disks off again and re-seating them will make any further difference, but it seems to be pretty much the only option. (It's also a prize faff and quite annoying.)

Ho hummm. On a different note, the stated torque for the caliper bolts, bolting back to the steering knuckle, is 85Nm. This seems very low - is this correct?


Oli.

Oli, I happen to have in front of me the little Porsche Technical Specifications book for our cars due to me trying to sort out my suspension as per the Bilstein Cup thread [8|], so if i dip into the relevent section....

..I see that the torque for the caliper to steering knuckle ( M12x1.5 don't you know[]) is 85Nm or 63 ftlb in old money...

Hope that helps