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Rear brake discs

Discussion in 'Suspension, Steering & Brakes' started by ian-metz, Jul 24, 2010.

  1. ian-metz

    ian-metz Guest

    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.
     
  2. Naraic

    Naraic Moderator

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    Your Mercedes:
    2005 CL500.
    Have they got a distinct ridge around the outer edge?
     
  3. OP
    ian-metz

    ian-metz Guest

    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!
     
  4. Naraic

    Naraic Moderator

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    And is it otherwise smooth?
     
  5. parthiban

    parthiban Active Senior Members

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

    ian-metz Guest

    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!)
     
  7. Naraic

    Naraic Moderator

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    Your Mercedes:
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    Depends what the thickness of a new one is. Sound ok to me.
     
  8. OP
    ian-metz

    ian-metz Guest

    That's kind of what I hoped somebody might know... but thanks for the speculation and guesswork
     
  9. kth286

    kth286 Senior Member

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    Your Mercedes:
    E320 Coupe 95
    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.
     
  10. moosehead

    moosehead Senior Member

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    Your Mercedes:
    C230/2005/2.5L Avantgarde SE Sport Pack Estate, SLK230/1999 and Volvo XC60/2017 AWD
    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.
     
  11. Kompressor

    Kompressor Senior Member

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

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  12. moosehead

    moosehead Senior Member

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    Your Mercedes:
    C230/2005/2.5L Avantgarde SE Sport Pack Estate, SLK230/1999 and Volvo XC60/2017 AWD
    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.
     
  13. JonnyCJ

    JonnyCJ Senior Member

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    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 ?
     
  14. kth286

    kth286 Senior Member

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    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.
     
  15. moosehead

    moosehead Senior Member

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    Your Mercedes:
    C230/2005/2.5L Avantgarde SE Sport Pack Estate, SLK230/1999 and Volvo XC60/2017 AWD
    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
     

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