Brake disc vibration after skimming (Paul G discs) - why!?

[lockwood77]

Bought some second-hand Paul G discs the other week, fitted them last weekend and found that they had a much bigger lip than my old ones (also Paul G's), and as a result the pads were not totally meeting the face of the disc - you could see a shiny ring on the outside, a matt ring in the middle and another shiny ring on the inside of the disc, both sides.

So, I had them skimmed by a pro machine shop who do lots of brake discs and fitted them yesterday. They feel terrible, over 50mph they vibrate the pedal and the wheel badly, under 50mph you can feel the modulation in the pedal. At slow speeds you can feel it "pulsing", i.e. braking more/less/more/less as the wheel rotates.

The discs also look like they did before they were skimmed - two shiny rings and a big matt ring in the middle, same both sides. I can only assume they're warped somehow, I don't know how though as I've hardly used them!

I am using the same 1144 pads I had on before with the old discs, could that be it? There's loads of meat on them and they've worn very evenly, so I can't see how that would create a "warping" effect. Could it be anything other than warped discs?

 

[C&S Evo7]

You should ensure the hub itself is clean and rust free and the inner mating face of the bell is the same, any small debris trapped between the bell and hub will cause it to run out of true,
If they are clean then it is likely you need to change the pads or take them out and reface them , they are bedded to the disc how it was before machining so likely the face wont be square or flat with the new discs
to reface-
remove pads and inspect- look for material to be parrallel with backplate and flat.
if not, get some 40-60 grit production paper and a flat surface (workbench) and remove a mm or so of the material to get flat and straight again. this is a graft, and wear a face mask and old clothes, brake dust is a mare to get off anything.

its cheaper to try and reface them before buying new.

 

[lockwood77]

Thanks Simon. Checked inside of bell and hub face, scrubbed them down with a bit of emery.

From what you say it sounds like it could be the pads then, which is good, as I can't see how the discs would be dodgy unless the guy skimming them, 20 years running his own machine shop, cocked it up!

I have a trackday at Silverstone tomorrow when I'll be using other pads, although they too have been used with the previous discs. I might give them a go and see if I get the same result, if so, I'll have my old discs with me that should stand another trackday at least.

 

[alucardo]

Cant the pads be skimmed aswell? Then they should mate up better...

 

[tron2k3]

Were they vibrating as soon as you fitted the discs or has it started occurring after a few miles of road use?

 

[lockwood77]

Started after a few miles of road use, they were OK on my five mile test route last night. What are you thinking?

 

[tron2k3]

could be pad deposits :?:

Maybe give them a good hammering then let them fully cool to bed them in again.

A few 60-10 stops to build up the temp gradually
Then about ten 80(I mean 70) > 40 with moderate to high pressure.
Then try not to use the brakes and let them fully cool for a few hrs.

 

[EndaITR]

Could it be when you bolted the disc back onto the bell that there off by a very very tiny bit? like just enough to cause vibrations?

Have heard of it happening with discs and bells before.

 

[lockwood77]

Hmm, I had a dial test indicator on them to check run-out, it seemed within a couple of thou around each disc. Might check again to see if anything's moved.

Had a motorway journey Nottingham to Leicester today, return journey in a few hours, will see what it's like after that! Should get a few high-speed braking attempts in...