Brake pads and area swept

[zcacogp]

Chaps,

This is a photo of the front brakes from my car.

It's not a great photo, but you can see that the disk isn't shiny all the way to the edge - there is a band around 6 or 7 mm wide all the way around the edge which is cruddy (and slightly rusty). The pads are more worn in this area - as in, they don't actually touch this part of the brake disk. They used, to, but don't now.

What causes this? I can see that grot build-up on the surface of the disk should be wiped off by the pad, but grot like this will be missed by the pad. The question is whether the pad became worn away in this area first, or whether the grot built-up which caused it to wear away.

What is the solution? (Do I need one?) New pads will surely just be worn away on the grot, and not solve the problem. Is the only way forward therefore new disks and pads? (A very expensive caper, and not one I would consider lightly!) The brakes work and feel fine - lots and lots of stopping power from any speed you care to mention. It was mentioned (verbally) at the last MOT by the tester, who pointed it out but said that the braking test performance was 'excellent'. That was about 9 months ago.


Oli.

 

[Mike_Pollock]

Its quite common on lots of vehicles, on 944s would think it could be related to plate lift.

If the discs are still within limits you can get them skimmed and with new pads this will sort the problem, unless you have plate lift.

 

[George Elliott]

(Do I need one?)

In a word, No, Oli - the Lip is caused by the fact that the disc overlaps the pad, not the pad overlapping the disc. As you said - the brake performance is fine, it just looks a bit bad. My car is the same. If you skim the disc in the normal manner, i.e. the full surface on both sides, it will cause other problems because a wheel hub was not designed to be a lathe headstock & chuck, but if you knew a handy turner, he could remove the lip and tidy up the appearance of the disc in a lathe.

Mike, its nothing to do with plate lift if you don't mind me saying.

Other than that, PCCB will not do this[]

George
944t

 

[ukmastiff]

For what its worth the discs on my v70 t5 look exactly the same as that and ut stops on a sixpence.
I'd definately live with it if i were you unless its a show car.

 

[Mike_Pollock]

I was just speculating on a possible cause.

The rough area is far larger than any lip I have seen on a brake working properly so there is something wrong, however 944s with opposed piston callipers have surplus braking capacity.

They should give an almost complete and even coverage of the swept area as the MOT man mentioned, with a mm or two of unswept area on the outside.

A single piston caliper tends to give those sorts of rough areas because of twisting and wear of the sliding components, something that shouldn't happen with opposed pistons.

Having said that if the performance is ok then there is no need to do anything about it.

 

[eddieedmo]

just been checking my S2 over and the discs on both front and back have only 1mm of non swept area , they are 12mths old/5K miles so I would say something not right and you are right to investigate. I know mine are 4 pot but still yours should have a better swept area than that.

Nick []

 

[zcacogp]

Interesting answers - thanks chaps. The pads do overlap the edge of the disks, but the pads seem to have worn away in the non-swept area, thus allowing this problem to occur. However I don't know which happened first (pad wore away allowing the unswept area, or the unswept area became rough thus wearing the pad away). Given the continued braking effect (as in, they work very well) I am disinclined to worry too much about it, so thanks for the reassurance chaps.

Nick, mine is an S2 as well, so should be as you describe!


Oli.

 

[Super_Marv]

I tend to agree with Mike, that the discs probably need skimming and new pads. That'd sort the problem.

IME it gets worse the longer you leave it, but then you could just leave it, run the discs into the ground then eventually replace both pads and discs. Not much difference really, what ever gives you peace of mind.

 

[eddieedmo]

sounds as though you may have sticking pots/pistons giving uneven pressure onto the disc.

Nick

 

[robwright]

I wouldn't worry too much about it Oli. If it passed MOT OK with good efficiency then they will be fine for normal road use. The discs don't look as though they are that far off a change anyway.

 

[Diver944]

ORIGINAL: robwright

The discs don't look as though they are that far off a change anyway.

Difficult to tell from a photo but I think Rob's right. Get a measuring calliper across the disk to measure the thickness, if its less than 26mm it needs changing. New ones are 28mm thick and you are allowed 1mm wear each side (hence 26mm), that crusty lip looks more than a mm to me.

What brand are the pads?

 

[sulzeruk]

Oli,it will probably be caused by plate lift. The pads will not be pushed squarely due to being caught in the caliper with the lift. The disks look pretty old. You will probably find the inside face much worse. Everyone checks the outside, few check the inside which gets corroded worse. I have seen disks that are shiny all over on the outside but have a 5mm polished strip on the back! Cars used regularly are always better than cars used infrequently due to the corrosion being cleaned off the disks and not getting a chance to fester.
Alasdair

 

[zcacogp]

Chaps,

More replies, thanks.

The disks and pads were changed in August 2008, at 141k. Disks are by ATE (from Bert), pads are Mintex. The car is now on about 171k, so both are 30k miles old - I'd expect a reasonable chunk more than 30k miles out of a pair of front disks, and usually reckon on getting through two sets of pads to one set of disks. (I think the picture is a little deceptive as the lip on the outside isn't that big at all. Wide, yes, but not deep.) If the disks were nearer the end of the road I'd be less worried about this current issue; it's the fact that I think there should be a lot more life in them that is making me wonder what to do.

The calipers were stripped and plate lift resolved about a year ago (thread here: http://www.porscheclubgbforum.com/tm.asp?m=626776&mpage=1&key=plate%2Clift%F2%99%81%98 ). I am fairly confident there is no plate lift on them at the moment. There certainly was plate lift, which is why I rebuilt the calipers; perhaps that was what started it, and it has yet to completely wear off.

I've looked at the back of the disks and they are much the same as the fronts - similar ring around the outside, but not significantly worse (or better).

I wonder inf Alasdair's comment is spot on tho' - "Cars used regularly are always better than cars used infrequently due to the corrosion being cleaned off the disks and not getting a chance to fester. " The car has gone from doing 15-18k miles a year to less than 7k, and that is having a (negative) effect on many aspects of it. Perhaps this is simply another bit that needs to be used more.

I think I'll take it all apart as soon as possible, and see if I can grind the rusty ring off the outside of the disks with a flappy pad grinding wheel on an electric drill, and grind the pads flat at the same time. That may be a good idea .... I'll update this thread when I have done so.

Thanks again for your suggestions chaps.


Oli.

 

[Mike_Pollock]

Using hand tools like those is a bad idea on brake discs / pads, you'll never get everything flat and square enough.
If discs are thick enough still, get them reground with new pads, nothing else makes sense other than to leave them as is and keep an eye on them. Which is what I'd do.

 

[Copperman05]

I would have thought seeing the disk and pad area is reduced braking efficiency will also be reduced somewhat, that fact that you dont notice it is probably due the them being very effective brakes in the first place. Looks like uneven pad/piston pressure on the disk to me, the 'uneven' part has then corroded and made the issue worse perhaps?


Edd

 

[eddieedmo]

If you have got 30k miles then definitely time for a change.
Most I can remember getting out of a set of front discs on any car is 20k miles.
You will be amazed by the difference they will make , we don't realise how braking efficiency changes with age as it is a slow progress.When I changed mine last year only done 10k and difference was amazing.

Nick []

 

[Mike_Pollock]

The Reno Traffics I have had for work over the 250k miles have done 18k front pads 36k front discs and rear pads and 54k rear discs, figures I think that are a fair average.

 

[zcacogp]

30k miles normal for a set of brake disks? Surely not! That sounds stupidly low ... in fact, I got over 30k miles out of the set which were on the car when I bought it, and those weren't new! And the history only shows them being changed three times in 170k miles ...

Had a good look at them today; the lip on the outside is hardly any depth at all (the picture is slightly deceptive), so I got out the angle grinder with a rust-removal thingummie to take the rust off. I then managed to nearly cut my finger off using aforementioned angle grinder which make a hell of a mess, lots of pain and Mrs zcacogp to think I should go to hospital. So I think I'll probably take the most common advice on here which seems to be to not worry about it.

Get Well Soon cards to the usual address - thanks. []


Oli.

 

[Mike_Pollock]

The van gets used hard and stops a couple of tons at a time, so 30K is ok - they are leased so the lease company would know if that wasn't right,

Get well soon Oli. [:-]

 

[zcacogp]

Thanks Big Boy.

Just taking some oral anaesthaeseisia (or however you jolly well spell it) in the form of something alcoholic. Apparently it doesn't interfere with the antibiotics and is the same colour as the stuff that came out ...

(And it's making me care a heck of a lot less about the state of my brakes!) [&:]


Oli.