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MG MGF Technical - Wedge-shape brake pad wear

As you may have heard from the Treffen reports, I had to replace the rear pads on my F in the Nurburgring car park (thanks again for your assistance Dieter! :o))

The pattern of wear was pretty weird though. Whilst the outer pads were in perfectly good shape, still with plenty of meat on them (a useful lesson there: don't expect the inner pads to be in the same condition as the visible outer ones), the inner pads had worn down into a wedge shape - with the leading edge of the pad completely down to the metal whilst the upper, trailing edge of the pad still had >5mm of material left.

Tim noticed that the pad material itself was very friable, crumbling under finger...

All this is in the context of having, a couple of weeks before, completed about 100 track miles at Donington (and a couple of other track days earlier in the year, along with the usual diet of sprints and hillclimbs).

I know that Paul's HiSpec pads have gone in a similar fashion.

Questions are:
1. is there a problem with the rear calipers (both sides behaved in a similar fashion) - but I've yet to confirm the pattern of wear on the front pads
2. is there a problem with these particular pads (a production batch problem???)
3. is this merely a symptom of too much heat - and I need to consider upgrading the whole braking system if I wish to continue playing on Trackdays?

Anyone else come across this problem? Have contacted Techspeed, so will chat with them about it...
Rob Bell

Throw in another idea...

How does the handbrake operate? could it push out only the inner pads? could you have a binding handbrake?
Will Munns

Binding callipers - yup, been thinking about that Will - and something that I'll be investigating in due course (problem #1 above). But as you know, the sliding design of the caliper would mean that both the inner and the outer pads would be brought into contact with the discs (although it could be that the inners would bear more of the frictional load?)
Rob Bell

>But as you know
I know very little of the clipers on the F, but if they have a sliding design (did you know the Astra has one pot calipers!) then I assume that the caliper is held on with two sliding bolts, wedge shape wear would show up if one of the pins were binding, and this might be caused by one of them bending slighly, causing the caliper to only slide on one.
Will Munns

(note this is from Astra caliper knowlage, not F)
Will Munns

The Lucas-Varity design of the MGF calliper is very widely used (1 in 4 cars produced world wide was a figure I read): I suspect that the Astra calliper is practically identical - except in dimentions - to the MGF items (which are single piston items too).

No problem with the sliding bolts encountered on pad replacement Will - and the chances of this occuring on both callipers seem small.

I think that the problem was definitely worse after Donington: I was starting to get a definite pad 'clonk' when the wheels changed direction when reversing for example - probably indicating pad movement. Probably just a symptom of the "wedging" that had occured.
Rob Bell

For illustration, a piicture of Rob's Pads :
http://www.mgfcar.de/event/treffen2003/dsc02091.html
weird.
T

It might not help Rob directly, but I thought I'd share this rather interesting Link I found :

http://www.carquest.com/product/brake/B001-2995%20CQ%20padWear%20Cat.2..pdf

@Rob: The correct description for your problem (as found in the link above) seems to be called "Tapered Wear".
T


My first call to my brake suppliers resulted in a comment that track day use often wears the trailing edge more quickly as there is more heat in the pads there.

Doesn't explain either the "inside only" that Rob got, or "diagonal wedge" shape that I got on all my four front pads.

I suspect that the my answer may turn out to be "exadurated feature" and Rob's may be "binding caliper".

P.
Paul Nothard

Interesting page you found there Thierry. Oddly, none of those explanations really covers the pattern of wear on my pads...

The friability of the pad material indicates overheating. Whether this is due to the handbrake binding or to general "track induced wear" remains to be determined... :o/
Rob Bell

>Oddly, none of those explanations
>really covers the pattern of wear on my pads
Tapered wear looks like your pattern, at least what I can see from the single unit on picture (http://www.mgfcar.de/event/treffen2003/dsc02091.html), depends on the other pad, does it have the same shape (mirrored) or different ?
T

If I remember correctly it was the 'inside' pad only BUT on both rear wheels - very odd.

Ted
Ted Newman

Exactly right Ted - the outer pads did not have the wear pattern mirrored (as it shown in the example of 'taper wear' on that web page).
Rob Bell

Quite a nice explanation of "Taper wear" here: http://www.stoptech.com/whitepapers/glossary/t.htm
Rob Bell

Don't forget the outer pad is only forced against the disc after the inner pad is already in contact so you will always wear the inner pad first. So if you have a small amount of caliper stick it might be just enough to keep the pressure on the inner pad but not enough to pull the outer pad onto it?
R Baker

Could be Richard. I am popping up to Techspeed sometime next week to get the brakes thoroughly looked over - and will let you all know what is found.
Rob Bell

Rob, I could have a simple answer - but I may be wrong.

I had identical wear and it was because the inner pad was not seated correctly.

You'll know that on the rears you have to screw the piston in rather than just pressing it.

I believe that it is possible that after rotating a piston 'in' (to make space for the new pad) you can leave it in a position such that it does not seat fully flush against the back of the pad.

As I recall the piston face is not completely flat, a section of its circumference is raised and doesn't bear on the pad.

If incorrectly aligned this raised section can act as a lever applying more pressure on one side of the pad and leading to the nice pic Ted found.

Hope I'm right - worth checking out anyway.


jt
John Thomas

Unfortunately, may be difficult to know one way of the other now John - and it is a possibility I agree. But what are the chances of this occuring on both sides?
Rob Bell

Hmm, I think I maybe talking a lot of tosh again Rob.

Making use of Dieter's excelent resources I found pics of the rear calipers.

http://www.mgfcar.de/sales/dcp_3821.jpg

The end of the piston clearly has indentations to allow you to be able to rotate it but it is hard to see how it could fail to mate properly against the back of the pad.

All I know is that several years ago now I must have made some sort of error when removing and refitting the rear pads such that I somehow misfitted one and it led to wedgeshaped wear.

It does sound a pretty remote posibility that someone would have made the same mistake on your car - twice!

jt

John Thomas

I've just been looking at the Draper website, http://www.drapertools.com . Is the stock item 52334 the correct tool to wind back the pistons in the F? I assume that the rear brakes on the ZS need to be wound back in a similar fashion to the F.

Ralph
Ralph

You can do it easilly without a tool Ralph.
Just lodge the end of a decent screwdriver in one of 4 posible slots and apply tangential rotational force.
jt
John Thomas

Hmm, don't know much (euh nothing) about the F's brakes and their pads, but that shape could be caused (not sure on the F) by using the metal clip (sold with the brake pads) in the wrong way.

It would surprise me if the clip/spring (if there are in the brake config of the F) is incorrectly fitted on Rob's car.

Hope this makes some sense
Erik

Just had a look in the workshop manual... the clips aren't that big and I couldn't clearly see if both sides are different.
Erik

When I bought my TF, it went clearly to the right when braking. The dealer said that it was strange, but one rear pad, was started to worn in a very irregular way (free cost exchange of the pad and the disc was the solution, and I didn't had again), starting in one side. I never saw the pad, but i'll take a picture of the Rob's one, so he can confirm me if that was teh way it worn.

Just for knowing if it's possible that it could be badly fitted...
Spinder00

Rob i work in one of the dreaded MGR dealers, and see the wedge shape problem almost every day. it occurs on a lot of rear disc cars not only the F. I think john is right when he says it is the piston on the caliper not sitting right. if you look on a inner pad you will see two small nipple one at 12 o'clock and one at 3 o'clock, these nipples have got to sit in the cross on the piston, otherwise the pressure from the piston is only force on at the top of the pad. i hope this helps.


colin

Thanks for the feedback Colin - and it is one of the hypotheses that I'm working on (and one that JT mentions a little earlier in the thread). I didn't install these particular pads (they were put in by Tech-speed at the beginning of the season), so whilst I think that installation is not the cause, I can't be 100% sure.

Colin, you mention that if the pads are not correctly installed, all the pressure is applied at the top of the pad? This must mean that the top of the pad wears first? The pattern of 'wedging' on my car was wear long it's lower (leading) edge... Inconsistent with a fitting problem perhaps?
Rob Bell

This thread was discussed between 15/09/2003 and 26/09/2003

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