Anyone used AmD for "on hub" brake disk skim?

[Dave_M]

Evening all

I've got brake judder on front disks which have only done maybe 15k miles. My car's booked in with AmD for a decoke later this month, so I thought I'd see if their website mentioned anything about brake disk skimming as it would be convenient for me to get it done whilst they've got the car for the week.

Their website - under "technical FAQs" - has a section on brake judder to which their solution is "RMS" - which is an "on-hub" lathe to skim them.

Has anyone had this done by them? Were you happy with the results, and how much was it?

I'll call them on Wednesday to find out more.

Thanks
Dave

[adsgreen]

Hmm - 15k miles is an odd number.
If its brake pad material transfer then you would generally expect that much sooner.
Similarly with a bad disk but disks should last more than that.

What might a first check would be to get the disks measured to see how much material is left.

[grizz]

After a skim they might be too thin ....

[DaveB1970]

You can't skim a floating disc unless you disassemble it completely and then mount the rotor to a mandrel in a lathe big enough to take a 365mm rotor, some lathes can handle the diameter but don't have the geometry to handle the back face.

Be very suspicious about any Rs4 disc that has been skimmed it just doesn't work

[P_G]

I wouldn't totally subscribe to that last comment. Front discs can be skimmed if your skimmer (person) is good; just not many of those around. The only thing I haven't seen is front discs successfully skimmed on hub but IIRC there ws someon on here who had a mobile guy do theirs with no issues. If you have them taken off then it can be done.

Again if the person skimming is good yyou can skim front or rear discs theoretcially up to 6 times before being under minimum tolerance.

[Dave_M]

Oops - just checked the service records and the disks have actually done 26k miles not 15k. There isn't any noticeable wear lip on the outer edge of the disk, but I guess they'd need to be mic'd up to check how much meat is left in them. I'm generally not that hard on brakes, and most of my mileage is (sadly!) a rather mundane commute.

Dave / PG - I thought there was a mobile operator that a couple of people on this forum had used to get a front disk skim done on the hub?

Thanks
Dave

[P_G]

Oops - just checked the service records and the disks have actually done 26k miles not 15k. There isn't any noticeable wear lip on the outer edge of the disk, but I guess they'd need to be mic'd up to check how much meat is left in them. I'm generally not that hard on brakes, and most of my mileage is (sadly!) a rather mundane commute.

Dave / PG - I thought there was a mobile operator that a couple of people on this forum had used to get a front disk skim done on the hub?

Thanks
Dave

Correct, but I don't know how well the discs have performed after that skim baecause the person that used them hasn't written any feedback post event. Didcs can be skimmed, seem OK but once bedded in still have issues. Some people would say that you can never skim discs to get rid of high spots are it has metallically altered the intergrity of the discs.

[Dave_M]

OK - thanks.

I'll speak to AmD on wednesday. If it's fairly cheap for them to do a skim (and they can do it for the floating disk and there's enough meat on my disks) I've got little to lose by giving it a try. I can then provide feedback on here as to whether it works and how long before the judder returns.

Car goes in on 23rd and I collect it the following saturday.

Thanks
Dave

[Dave_M]

Related question - if I do need new front disks, do I have to put new pads on at the same time. Last service indicated they are only 25% worn (ebc yellow stuffs). On previous cars I've always put new pads on as a matter of course when replacing disks, but at £120 I'll only replace if I have to.

Thanks
Dave

[PJC]

New disks and pads for me.

[DaveB1970]

I still beg to differ.

Quite how you can machine something that is designed to move 3/10ths of a mm and expect it to be straight is beyond me.

Strip it down, do it properly, rebuild it using cleaned components and they'll be ok.

If you skim them in situ, the first time you get them hot the datum changes and you feel they're warped again

[P_G]

What I differed on was that you implied having to further take the rotor off the hat to skim with the disc was off the car which isn't the case. I've had fronts skimmed off the car with the rotor hat in place on 3 separate occassons with zero issues.

[adsgreen]

I would replace the pads too.

25k is a reasonable amount if wear.

However - if I hear 'my discs are vibrating but I am really easy in the brakes and only do commuting' one more time I think I will scream!

In short you can cause more damage to discs by being too light on then than too hard. Warping a modern floating disc through overuse is very hard- you'll trash the pads or boil the fluid (or both!) before that point... Just ask sonny (!)
However being too light which is normal for day to day driving will cause uneven pad material transfer and generate a 'wobble' that feels identical to a warped disc. The brakes on the rs4 are well over the top for a normal car do its easy to just feather them... They need to be worked with a reasonable amount of force otherwise you get high spots building up.

I also have over 25k on my discs and I'm bloody hard on them bit they are in great shape.

[DaveB1970]

What I differed on was that you implied having to further take the rotor off the hat to skim with the disc was off the car which isn't the case. I've had fronts skimmed off the car with the rotor hat in place on 3 separate occassons with zero issues.

Whether the disc is on the car or off the car the float is between the bell and rotor.

If you mount the rotor in the lathe with the bell then by design the rotor has radial and axial float, if it's not floating due to corrosion or contamination with pad material then it may not move when being machined....it isn't right however.

What then happens is the rotor expands under load displacing the corrosion and dust and you end up having to get the discs skimmed a second or evidently a third time. At a useable thickness of 1.6mm and £600 per pair every time you have a swipe at them with the lathe and take 250 microns a side it's just cost you £200 onto the machine shop floor plus machine time

Same happens on S55 discs which share the same crap design, they put the float in the bell but then don't allow you to buy bobbin kits or replacement rotors or service them properly

[ArthurPE]

I tend to agree, turning floating rotors is probably a waste

if they vibrate/shudder I would look elsewhere, since by their very design they are much less susceptable to doing so

clean them up, especially the pins, with some brake cleaner, wd40, etc, whack them around the edge and both sides of the face with a rubber mallet to knock everything loose...then wash them good with soap and water
re-bed the pads, most of the time the issue is deposits