Diagnosing brake disc/hub not spinning 'true'/wobbly...

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I understand, but you’ll have to take my word for it that they aren’t scored with deep grooves, if you rub your finger over them they’re smooth. I’ve attached a picture of them before the car sat for a while, they look fine to me. Not brand new, sure, but perfectly acceptable. The car was just MOT’d with no advisories. I also just had the brakes bled with no comment on the discs.

It’s normal that brakes get surface rust on the faces when sitting for a long time - I know you guys know this. The surface just needs a clean and a drive.

The fact that they were running out of true thing is more of an issue, i’ll have to see how it affects things. It’s not a big deal to replace them if there’s a problem.
 

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They don’t look awful of the outside but they’re quite scored on the inner faces though.

I think it would be good practice to replace considering the issues you’ve had and with them having been run out of true.
 
Sure, it would be ideal, but I’ve spent so much on the car recently and don’t want to needlessly spend another £180-200 if I don’t need to.

Photos below show the inside all dirty and still looks absolutely fine to me. They are not deeply grooved, the surface is smooth to the touch, and I haven’t even cleaned them yet. The pads also look fine and have even wear.

I’ll put it/them back on and if there is an issue, I’ll replace them then.

The issue I was having was actually not related to braking performance and quite minor (in terms of driving feeling), so feel that all the cleaning of the surfaces (as well as new slider pins and fresh grease everywhere) can only improve things. I need to see if it sits and spins true with the cleaned surfaces - it may not for all I know.

Cheers
 

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Finally found time to get the disc off. Lots of mini ‘mountains’ of rust everywhere. You can see it especially around the edge of the hub. Don’t think anybody who’s changed the discs has ever bothered cleaning anything. Spent some time with wire brushes (chipping away with a screwdriver on the big bits on the rear inside of the disc) and attached pics of before/after. I’ll probably be giving it another once over tomorrow and reinstalling everything, both surfaces can probably be 5% better.

The hub itself seemed to spin true, although not the easiest to tell. The disc itself seems to lie flat on a flat surface.

I’m hoping to clean and reuse the disc and pads (wear superficially looks even on pads), grease everything, along with new slide pins and boots. Bit annoying that I at least have to do the other side too, just for the sake of completeness. Should be quicker next time.

Fingers crossed….
Re the first photo in the above post. I'd place a 'straight edge' across the threaded holes to check that over zealous torqueing of the wheel bolts hasn't puckered their edges - pulling them proud of the surface (which would adversely affect disc mounting integrity/accuracy).
If so, a drill bit larger than the hole diameters will remove any lip.

I've seen discs much worse than those rescued. They'll last the life of the pads. Buff off any corrosion at the lips and refit is my advice.
 
Thanks - I'm pretty sure that this raised section of rust and gunk was the problem (attached) but I'll give everything another brush and clean and refit and see if it spins true, I think/hope it will.

I just had a shower though and can't bring myself to get covered in rust and break dust again! Maybe tomorrow...
 

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I remember you posted this the other day:


I suspect that the calipers may have been sticking for a while which could explain the condition of these discs.

Not saying they’re below minimum thickness, but the scoring of the surface on the inner side of the discs isn’t how they should be IMHO.

Out of interest, have you checked what the run out is like now you have cleaned the mating surfaces of the hubs up? :)
 
A Short history lesson- in the good old days brake disc material was hard and the brake pads were soft- [ unfortunately they contained asbestos.] Thus brake wear =d pad wear. Now brake pads are hard [asbestos free] and the discs are "relatively " softer material =s disc wear/distortion.
30,000 miles is often quoted as expected disc life before disc wear/distortion becomes unacceptable. Running the car with partially seized floating calipers will have caused disc overheating and warping - hopefully only of the disc but possibly also of the hub flange if things have been really running hot. Renew the discs sourced from a reputable manufacturer [ATE used to be oem for MB.] And get the calipers rebuilt by a company who specialises in this sort of work----brakes are components you don't want to take any chances with!
 
So - all this discussion is moot as I couldn't wait until tomorrow - I just tried to test fit the disc back onto the hub and it still has unacceptable runoff. I recorded a video on a tripod so that you can see the true amount of runoff (below) and it's better than before but still not acceptable.

Happy to go buy brembo discs and pads (£190... £220 for MB OEM but so much dust) but before I do that, how can I test if the problem is with the hub or the disc?

When I spin the hub it looks like there's no runoff but the diameter of the hub is so small you probably wouldn't notice even if it were off.

I suppose I could try to test fit the disc from the other side to the problem side, as it has to come off anyway if I'm going to replace the discs and pads, but any other ideas? I don't want to spend another £200 only to have a dodgy hub. There is no knocking or movement of the hub if you try to shake the wheel or disc, it 'feels' good and smooth.

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You could measure the trueness of the discs off the car, but regardless of whether or not the issues with run out are due to the discs I would still replace them due to their condition.

Have you got a DTI? Run out should be a fraction of a millimetre, if you can see it by eye it’s pretty bad :)
 
You can get a cheap dial test indicator for about £20 on Amazon, can’t comment on how good they are but would be a good start if you haven’t got one to hand :thumb:
 
So - all this discussion is moot as I couldn't wait until tomorrow - I just tried to test fit the disc back onto the hub and it still has unacceptable runoff. I recorded a video on a tripod so that you can see the true amount of runoff (below) and it's better than before but still not acceptable.



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Bolt the disc to the hub with all five bolts (using spacers) and repeat.
 
Bolt the disc to the hub with all five bolts (using spacers) and repeat.
Will do - what do you mean by spacers, sorry? Actual wheel spacers? I don't have any I'm afraid.

I actually thought the bolts would go in further than they did, but no
 
Just seen the video now you’ve posted it up. That runout is ridiculous, but either it’s the hub or the disc isn’t sitting flush. You can see the runout easily on the disc centre so if you removed the disc you should be able to see the hub if it’s that bad :thumb:
 
Will do - what do you mean by spacers, sorry?
You need the disc clamped to the hub. The wheel bolts are too long to do that without fouling the strut if wound in sufficiently to clamp with out the wheel being mounted.
Either, use shorter bolts (awkward) or spacers on the existing ones. Improvise - sockets, bits of metal tube, etc.
 
Just seen the video now you’ve posted it up. That runout is ridiculous, but either it’s the hub or the disc isn’t sitting flush. You can see the runout easily on the disc centre so if you removed the disc you should be able to see the hub if it’s that bad :thumb:
I'll record a video showing the same with just the hub spinning, although it doesn't spin as freely of course
 
Could you bolt the wheel to the hub without the brake disc, you'll definitely see any run off then?
 
Another way to check discs vs hub.

Bolt the discs to the hub, spin and mark the high and low point with a marker.

Carefully unbolt the disc from the hub without rotating the hub, rotate the disc a hole or two and rebolt. Check for runout.

If the high spot persists where the marker is, it's probably disc. If the high spot has 'moved' it's hub or mating surfaces.
 

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