Rear brake discs

[ian-metz]

Hi,

I have a 2004 CLK cabriolet and my dealer says the rear brake discs are 90% worn and need replacing. I'm doubtful - they feel like they're the best part of 1cm thick - at what point should they be replaced?

Thanks.

 

[Naraic]

Have they got a distinct ridge around the outer edge?

 

[ian-metz]

On the outer edge there is a ridge of approx 1mm, perhaps a tad more - but then again, every car I've ever had has had such a ridge!

 

[Naraic]

And is it otherwise smooth?

 

[parthiban]

Correct me if I'm wrong but isn't the difference between a new disc and a worn one only a matter of millimetres?

 

[ian-metz]

Some very shallow circular ridging, like any brake disc after a few miles.

My point really is, at what thickness do they need replacing?

(I've just done 3,600 trouble-free miles round Europe on "90%" worn discs. Which presumably means they must be 100% worn now!)

 

[Naraic]

Depends what the thickness of a new one is. Sound ok to me.

 

[ian-metz]

That's kind of what I hoped somebody might know... but thanks for the speculation and guesswork

 

[kth286]

all Merc discs are marked with the limit stamped on the outer edge of the disc rim, but of course it may be covered in rust.

Do some careful wire brushing and when found you will need a micrometer to measure disc.

The tech does not mean the disc is 90% worn, he means it is 90% worn to the limit figure.

Too thin and the disc will heat up more and your braking could worsen due to the extra heat boiling the fluid in the caliper, and this will travel back along ther brake pipes.

 

[moosehead]

The only definitive way is to measure the disc thickness with a micrometer and if it is below the minimum thickness they need replacing. You need to find out the minimum disk thickness (ask the dealer) and borrow / buy a micrometer. It's then a 30 second job to measure the disk. I've just fitted a new set of rear disks on my 2005 W203 at 80K miles if that helps.

 

[Kompressor]

Not sure about the rear discs but on the front the markings looks like this, maybe similar for the rear

 

[moosehead]

Rear disks are solid and I can't see any markings on my new ones (although it's very tricky to see the front ones and I have the vented / drilled disks as well).

The best way is a micrometer which you can normally do without removing any wheels.

 

[JonnyCJ]

If they're 90% towards the limit you should have a few thousand miles left yet. Let's say the limit would be 50,000 miles before they needed changing and you were at 90%,you'd have done 45,000 miles.

Make sense ?

 

[kth286]

If they're 90% towards the limit you should have a few thousand miles left yet. Let's say the limit would be 50,000 miles before they needed changing and you were at 90%,you'd have done 45,000 miles.

Make sense ?

BUT the limit is not measured in miles is it.

Some drivers may get 60,000 miles out of discs, whilst some may get only
40,000 miles out of identical discs.

Some people drive more aggressively than other people, and this will cause the discs to wear faster, thus wear out in less miles.

 

[moosehead]

As previously mentioned, I managed 80K from my rear disks but that was mostly motorway driving. If it was more around town I would imagine this could easily drop by 20-30K miles