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Every so often the Shimano 105 7020 hydraulic disc brakes on my road bike start to make a metallic grinding noise and I can feel additional resistance when riding. The culprit has always been misaligned brake calipers on both my front and rear wheels. The fix is simple: throw the bike on the stand, loosen the caliper bolts, spin the wheel, hit the brakes, and tighten the bolts.

I'm wondering why the calipers fall out of alignment in the first place? When using the brakes while riding you're forcing them into the correct, centred position - basically the technique I described above to fix them.

I can also imagine that the calipers might move around due to other forces (bumps, etc) and hitting the brakes doesn't apply enough force to centre them. In that case it probably bends the rotor a bit. The solution in this case is to tighten the caliper bolts more but I already use full force with a screwdriver and don't want to risk doing any damage to the fork posts with a wrench.

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There could be a few possible issues here:

1: You’re not tightening the bolts enough. Take the bike to a bike shop, or even a car garage, and have them torque the bolts down properly with a torque wrench. Additionally, make sure to use threadlocker on the bolts, which is a special type of glue that prevents them from loosening due to vibrations.

2: Your bike’s caliper mounts are not faced properly, meaning that they are not flat and square. This is a more complex issue that can be resolved either by getting the bike warrantied, or having a bike shop machine the mounts down to get them flat. See Weiwen’s comment for details.

3: Your rotor isn’t true. Realigning the caliper won’t fix this. To diagnose this issue, watch the gap between the rotors and brake pads as the wheel spins. If it pulsates or the rotor briefly rubs, then the rotor is not true. Carefully use a clean adjustable wrench to bend it straight again, a little at a time.

4: Your brake piston seals are dirty, and one or both pistons is not operating as it should. This can be remedied by extending the pistons and cleaning them with alcohol.

5: As mentioned by Nathan in the comments, if your bike uses quick release hubs, your wheels may twist and reposition themselves under hard braking if your QR skewer is inadequately clamping on the fork/frame. Make sure you are properly tightening the QR skewer, that it is fully closed and not contacting part of the frame, and that you are indeed flipping it and not just using it as a wrench. If the problem persists, consider switching to an internal cam skewer.

If you have thru axles instead, make sure the axles are properly fastened. The exact procedure depends on the axle system your bike uses.

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    #2: the process is called facing the frame. It can be done to frames of any material, including carbon, and I believe manufacturers are usually fine with this (I.e. they won’t deny warranty because of it). It’s labor intensive and requires special tools, so many bike stores don’t do it. Relevant article by James Raison: lavelocita.cc/guides-page/facing-carbon-fibre-bike-frames
    – Weiwen Ng
    Commented Dec 25, 2020 at 18:54
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    +1 but it can also be axle squirm if they're QR hubs, particularly if external cam QR and extra particularly on all-carbon dropouts.
    – Nathan Knutson
    Commented Dec 25, 2020 at 20:12
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    @NathanKnutson I would assume that a bike with 105 7020 would come with thru axles, but I’ll add it to the answer!
    – MaplePanda
    Commented Dec 25, 2020 at 20:56
  • Thanks for the detailed answer! Considering I currently have no brake rub after realigning the calipers, and my QR is properly set, I think my problem is probably answered by either pt #1 or 2. I always use thread locker but will reapply and re-tighten the bolts. Pt #2 is further complicated by the fact that I'm using post-to-flat-mount adapters on my bike. It introduces a new level of things that can go wrong, although I've had this issue even before I upgraded to my current setup.
    – valsidalv
    Commented Dec 26, 2020 at 0:25
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    One thing I've seen more than once that's sadly common is this: bike with all-carbon dropouts comes with internal cam QRs, they're ridden in undertightened state or are just bad and don't clamp tight enough ever, dropout material erodes under brake/drive forces, and then getting the wheels to stay put with any skewer becomes problematic. You can look for evidence of this by holding a bare hub/wheel without a QR up in the dropouts.
    – Nathan Knutson
    Commented Dec 26, 2020 at 4:05
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One more solution which I applied with success

On my front wheel the disc rotor wasn't sitting true to the wheel. Having exhausted other options, I believe this to be a manufacturing fault, an issue with the wheel hub rather that the disco brake rotor. Reluctant to bend a top end Shimano rotor (new from box) I resolved this by putting a thin metal shim between the rotor and the hub, on the inside of the rotor. The shim I used was the metal thin washer than comes with the rotor lock nut. Cut out about 1/3 of the circular shim/washer and placed it on the opposite side of the part of the rotor that was catching. Thus lifting one side of the rotor very slightly and making the rotor true. Cut the lugs off the inside circle of the shim. You can cut the shim metal using strong scissors.

Realigned the caliper be loosening from mount, applying brake and re tightening the caliper to the fork mount.

Result is totally perfect.

I must have spent three hours fiddling before arriving at this solution.

rotor

rotor shim

rotor shim

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That happened to be on my road bike rear brake.

I tried aligning the caliper but it didn't help.

After further investigation, the cause seems to be a "lazy piston". One of the pistons is not moving fully, only a little whereas the other piston on the other side is moving fully. When I have time, I have to do this fix procedure:

  1. Remove the wheel
  2. Remove the brake pads
  3. Push the non-lazy piston fully in
  4. Pump the brake while continuing to push the non-lazy piston fully in, don't pump too many times or else you will find all your brake fluids have leaked as the piston becomes fully detached from the brake
  5. Clean the lazy piston with a cotton swab that has brake fluid (in my case mineral fluid) on it -- you only need to clean the surface that moves on the seal, not the surface that touches the brake pads
  6. Push both pistons fully in
  7. Ensure the pistons move by equal amount by pumping the brake, if not then you need to continue exercising the lazy piston more (exercise means you fully push both piston in, pump the brake while pushing the non-lazy piston in, and repeating that multiple times)
  8. Reattach brake pads
  9. Reattach the wheel
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