A problem I came across changing S8 discs/pads

Hi chaps.

This is just a little heads up for anyone who experiences the same problem, or is thinking of painting their brake calipers.

Basically, my brakes were fine. I noticed some of the pads were different makes, the rear ones needed changing. I bought the whole lot, front & rear.

I fitted the pads to the original discs (they were a bit scored but otherwise fine). When I travelled down the motorway, I discovered a juddering when braking, which then got worse until it was a juddering without braking. Warped discs I thought.

Next day, travelled to work, no judder. Got to a straight where the limit is 60, braked and got judder, developed into serious judder again without braking.

Set off from work that night, no judder at all, all the way home it was fine.

This went on for a couple of weeks until I eventually decided to get some new disks on the front (where the juddering is coming from). Ordered the discs, picked them up, took one of the old ones off, compared with new one, wrong size disc - back to swap and pay the extra £12 for 345mm instead of 323mm diameter .

Anyway, fitted the new discs. Went home, no judder. Set off to work the next day, juddering again. WTF I thought. Got to work, had a look at the discs, noticed that on O/S they were wearing in nice and even both faces of disc. Checked the N/S and noticed one face was evenly wearing, the other was wearing at the inner circumferance and outer circumferance only, about a half inch gap between the two wear bands.

Took that side apart, checked the flange was clean, checked the pads were pushing out ok, no problems visible. Went for a test run, no problems. Next day, went down motorway and the juddering was so heavy with no braking, I was forced to do 45mph until I could get off the motorway.

Now it's here that I remembered when I fitted the pads on this side, there are four metal guide plates that bolt to the inside of the caliper. The pad slides between these when you push it into it's recess. I couldn't get mine to fit, so I sanded a bit of the excess paint off the pad edges. Still couldn't get them to fit. Then I noticed that the bottom two of these metal guides were twisted up slightly, so that when the pads were worn, they fitted snugly, but new pads wouldn't slide in because the guide plates were raised at the back.

I sprayed loads of brake cleaner in there as I thought it must be chocked with brake dust which has wedged the plate out a little. I managed to tap the pads into position and thought nothing of it.

To shorten the tale a little (!!) - someone has painted these calipers and took the guides out before they painted them, but only the bottom ones. When the paint has dried, it has raised the mating face where the guides bolt in, twisting the back edge out and stopping the new pads being fitted properly. This caused the pads to pivot top and bottom on the misaligned guide plate on one side, causing uneven wear on the disk and hence the disk to warp slightly under heat.

I've had the caliper apart today, split the two halves, undone the splined bolts holding the guides in and chipped/wire brushed the paint away back to metal, refitted the straightened guides and put everything back together - the pads now slide with minimal friction into their recess, after a quick test run, the disc is now wearing evenly.

I just thought I'd bring this to your attention, as I know lots of people like to paint their calipers. My advice is to strip the calipers down totally, remove the guides but leave ALL mating faces completely untouched, try to mask the pistons off and the hex bolts that clamp the two halves of the caliper as clearances will be affected when the paint is dry and you try to reassemble.

Sorry it's so long - I should have taken pics so you know what I'm talking about. I'm gonna do the other side just to make sure it's ok, so I'll photograph the areas affected to make it clearer



Edit:- Hope this diagram makes it a bit easier to see what I'm getting at?

[Conan_the_Librarian]

Added to Sticky

[Architex_mA8tey]

Boy Conan you sure are the quickest Librarian in the west!!

Updated with a diagram - should make it clearer I hope

[Ska]

Woops! & I got a bit of a judder the other day
Thanks for the info DJ though I think mine are slightly different to yours; I run the 4 pad Floating caliper arrangment up front.

Ska I have these same symptoms. It used to judder a while back.. but then went away. Just recently its back, pretty well noticeable all the time at about 65-70, then will go away.. come back..first of all felt like wheel balance, but I recently had them all balanced when changing the wheels over so not that - definitely notably worse under braking, to the point where when the car is coming to a stop and I have my foot on the brake pedal, I can almost 'feel' the pads grabbing a bit.
Warped discs I am thinking.. hence the other question about which discs to get (I think I have the same setup as you so will be looking at 345mm - any rough price I should be paying)
Anyway will watch out for all this although dont' think my calipers have been painted at all. I'll make sure the surfaces are properly clean though when I get to changing the discs.

I got my discs from GSF down Greenford, I think they were about £120 the pair (345mm) - I haven't got the receipt handy. I had to pay another £12 on top of the price of the 323mm ones anyway, if that gives you a clue.

Incidentally, who was it in the A8 4.2Q "N117 ***" friday last week? Silver it was and looked nice and clean, somewhere near Southall I think it was. I was in a works truck so you wouldn't have noticed me.

Just as a sub-note, when mine were juddering, it wasn't like the pads were contaminated - you know, like a lack of friction judder. This was physical contact/no contact juddering, like the discs were warped big time, it felt a bit like the car was struggling to do the ABS thing as though I was on ice, but I know it wasn't an ABS fault as the light didn't come on and the juddering carried on after I released the pedal, then gradually faded as the discs obviously got cooler. Also, the outer wear band had a hint of blue (over heating) and the pads stunk like worn clutch.

The discs now look like they should, smooth even wear right across the landing zone. They bite really well now too. Shame the gearbox is kaput again...

[Ska]

Incidentally, who was it in the A8 4.2Q "N117 ***" friday last week? Silver it was and looked nice and clean, somewhere near Southall I think it was. I was in a works truck so you wouldn't have noticed me.

No it wasn't me. Though I make regular trips to Southall for groceries I hardly take the 8 there for fear of mindless delivery drivers in that area.
Now I'm thinking you used the Hayes by pass to get there? If you did you proly drove passed my house with the 8 parked in my drive.

Got discs (314mm actually.. ) and pads on order from local supplier. Hope that cures it. 40quid per disc ish.
I have felt warped discs before and this feels the same.. like an on off at low speed when you aren't pumping the brakes, and a real proper judder at high speed braking.
Had a check and they are the same as yours Ska, setup wise. Although think you said you had bigger discs? As Ian said in another post I only see 314mm listed on Vagcat - have you changed yours over the years?

SKA - got my new discs - but surprise surprise the pads they had, although 4 per side, were the wrong ones. I have the same as you - and they have to get them in.
2 questions - are the 'lumps' on the top of the pads (silver in your photo) part of the pad?
what is a decent price for a set of these - I am being quoted £60!