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Rear Brake Disc Warp - Advice Needed

Discussion in 'Technical Help' started by ck_uk, Sep 7, 2018.

  1. Hi all

    When you place a bike on the rear stand, should there always be absolutely no contact with the brake pads at any point?

    My original rear disc developed a slight warp, evidenced via the MOT as "slight fluctuation of braking effort". The rear caliper was found to be in need of a refurb, so this was sent away for a professional rebuild to ensure smooth operation of the pistons.

    A new replacement disc was purchased, but this also seems to have slight play in it. I placed the new disc itself on a sheet of thick glass, to check how it behaved when pressure applied at various points. It did seem to rock slightly on one side. I double checked this on a glass tabletop also, to make sure the results were the same. The company this was purchased from won't accept it as a return, as it was purchased some time ago.

    With the wheel and disc in place, if I turn the chain adjuster on the caliper side, there is some improvement. However, I can't get it so that the disc doesn't touch the pads at some point during rotation.

    Has anyone else had any similar issues? Not sure how to resolve this.
     
    #1 ck_uk, Sep 7, 2018
    Last edited: Sep 7, 2018
  2. Should be no more than 0.2mm either side of centreline for runout. How much runout do you have?
     
  3. I will try and measure it..
     
  4. Hold a ruler or screwdriver resting it on fixed part of bike touching the disc. Rotate wheel so it pushes ruler away from disc. When disc starts to move away from ruler keep going to the point when it starts to return to ruler .The gap you now have is the total run out of the disc.Obviously it would be nice if it was virtually nothing but if it is within 0.4 mm it os adequate.
     
  5. Just tried to measure this, but can't get an accurate measurement. It's between 0.5mm and 1mm from what I can tell. But it's enough to prevent the wheel from spinning freely at the point of contact.

    I don't know whether that tolerance it's acceptable or not, for road use.
     
  6. Take the bike for a belt around, and carefully check the disc when you get back home ( spit on your finger before touching it !) If it is any hotter than a low luke warm then you have a problem.
     
    #6 drax dave, Sep 8, 2018
    Last edited: Sep 8, 2018
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  7. 0.5 is max 1m m is to much. If you have a lot of run out you can feel it pulsing though the lever as the disc pushes the pads back and forth. Usually as well you get a lot of lever travel as the disc run out pushes the pads back slightly so you need to push the lever down farther to take this up. It should only really heat the disc up if the pistons in caliper are seizing in caliper and not retracting back properly when lever is released. This spell check is driving me nuts!
     
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