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Hydraulic road discs

munkstermunkster Posts: 819
edited April 2019 in Workshop
Having put a different wheelset on (with different and perhaps thicker discs) the pads now rub on the disc.

Now, it’s been about 10 years since I used to tinker with and successfully bleed my MTB (Shimano) disc brakes but I’m now rusty and can’t think how to approach this to stop the rubbing.

Is taking the pads out and pushing the pistons back in with tyre levers the way to go or will the pads reset to the same place? I don’t think it’s necessarily a case of needing to recentre the calipers (I hope anyway, that used to be irritating IIRC).

Also, what’s the current “kit” required to bleed these things? They’re Ultegra (R785 is that right?) calipers. I seem to recall I just used a small wrench, some tubing and a bag.

Posts

  • Pushing the pistons back in should get things working again.
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    2020 Voodoo Marasa
    2017 Cube Attain GTC Pro Disc 2016
    2016 Voodoo Wazoo
  • whyamiherewhyamihere Posts: 7,404
    I would guess it does need recentred - There's not really enough space in there to adapt to even quite a small difference in rotor position. Look down through the caliper with a piece of white paper/card held below - Is the disc touching one pad or the other, or noticeably off centre? If so, you need to adjust the position.
  • veronese68veronese68 Posts: 23,432 Lives Here
    pushing the pads back may help, worth a go. The discs shouldn't be thicker, I'm sure there is a standard thickness. If the brakes are hydraulic you shouldn't need to re-centre the calipers for a small difference, one piston will move in a bit pushing it's opposite number out by the same amount. this would be more of an issue with new pads as tolerances will be tighter. For a larger difference you would have to re-centre them, can't say without seeing it. Sometimes just riding a couple of miles sorts it out. How bad is the rub? Is it stopping the wheel turning freely or is it just a bit of noise?
  • joey54321joey54321 Posts: 1,297
    My guess is rotor alignment. Loosen the caliper bolts so that it can move freely on the fork/frame. Pull the brake hard so the caliper is aligned with the disc. While holding the brake (can use a zip tie or something to hold the brake lever down) retighten the bolts.

    I'd do this first before rebleeding the system, etc...
  • rafletcherrafletcher Posts: 1,235
    Read this thread....

    viewtopic.php?f=40004&t=13104912
  • timothywtimothyw Posts: 2,482
    joey54321 wrote:
    My guess is rotor alignment. Loosen the caliper bolts so that it can move freely on the fork/frame. Pull the brake hard so the caliper is aligned with the disc. While holding the brake (can use a zip tie or something to hold the brake lever down) retighten the bolts.

    I'd do this first before rebleeding the system, etc...
    Bingo.
  • munkstermunkster Posts: 819
    OK pushed pistons back, not much difference, then re-centred front caliper by the usual method: worked a treat.

    The rear however, the caliper is fixed directly to the frame at the front point, but at the back is fixed to a black mount which is then fixed to the frame, so I've got three bolts. I ended up loosening all three bolts and trying the "pull lever/tighten bolts" method and it still rubbed, albeit much less and only in one spot. So, I resorted to doing it by ear, which I remembered used to work for me before, and lo, it is now not rubbing.

    Now, obviously this is good and all, however I am now wondering if that little black mount needs a particular alignment (ie. parallel) to the caliper and have I just fudged it? I am guessing and hoping it doesn't matter as long as the rotor doesn't foul the mount and the caliper is aligned OK? Anyone know? Just checking...
  • whyamiherewhyamihere Posts: 7,404
    munkster wrote:
    Now, obviously this is good and all, however I am now wondering if that little black mount needs a particular alignment (ie. parallel) to the caliper and have I just fudged it? I am guessing and hoping it doesn't matter as long as the rotor doesn't foul the mount and the caliper is aligned OK? Anyone know? Just checking...
    Pictures would help. It's unlikely to matter as long as everything's tight and the caliper is aligned to the rotor though.
  • munkstermunkster Posts: 819
    Pictures you say? Oh, go on then ;-)

    Three bolts, numbered from front to rear. Mount is between 2 and 3.

    2.jpg

    1.jpg
  • whyamiherewhyamihere Posts: 7,404
    What an odd set up. I'll stick with my answer that as long as everything's tight and nothing's rubbing it doesn't matter.
  • munkstermunkster Posts: 819
    Well, as I say, I used to have hydros on my MTBs but haven't done that for years, so just assumed this was a normal set up for road discs, and this is hardly an obscure bike (Synapse)...

    I think you're right though: as long as it's not rubbing, surely doesn't matter. Does irk me that I couldn't centre it by just pulling lever and tightening bolts, mind. If I knew what the right way was, it would avoid the very hit and miss method of doing it by eye/ear. Still, works now!
  • veronese68veronese68 Posts: 23,432 Lives Here
    Agree with WhyamIhere, does seem an odd design. But as long as the clipper is lined up and it doesn’t rub all is well.
    Must confess I find lining up by eye/ear more reliable.
  • munkstermunkster Posts: 819
    Must confess I find lining up by eye/ear more reliable.

    I guess when I say "hit and miss" I find that you can get it to not rub fine when holding the caliper just in the sweet spot, then at some point during the bolt nipping up process you somehow displace the caliper and push it just slightly out of whack and the sweet spot is gone. I was more lucky today, all good!
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