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I am struggling with a rubbing disc rotor on a new bicycle. Which bolts should I use when trying to align the caliper? The ones I marked in red, or the blue ones?

enter image description here

I tried to use the red ones so far, but could not get rid of the rubbing. The rotor looks true, but the space on each side is extremely tight.

Later I realized that the lower blue bolt is simply not accessible. The space is too tight to fit a screwdriver. However, should I encounter a similar situation in the future, where the calipers are held on a bracket with accessible screws: Should one adjust the caliper on top of the bracket? Or should one adjust the bracket on the fork?

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  • @Criggie The answer was that one of the lower bolts (the blue ones) is simply not accessible with the screwdriver. So there was no way to adjust it. I don't know why I didn't realize this until I turned the bike upside down to take a close look in good light ...
    – Foor
    Commented Aug 1, 2020 at 12:21
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    @Foor FYI Magura (the brand of your brakes) uses relatively tight clearances between the pad and rotor. At some point you just need to accept the rubbing and move on.
    – MaplePanda
    Commented Aug 1, 2020 at 17:32
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    @Foor Yup, I'd say go ahead and move your comment to a real answer. That way, folks will know that it's the answer and you can get the rep if anybody finds it useful.
    – jimchristie ♦
    Commented Aug 2, 2020 at 2:54

2 Answers 2

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Should one adjust the caliper on top of the bracket? Or should one adjust the bracket on the fork?

For Magura parts, you'll adjust the caliper. It's impossible to predict what would be the case for every brake assembly that will ever be manufactured, so your question about the future cannot be answered with certainty. But if you ever see an intermediate bracket like that, engineering concerns dictate a very high probability that you'll always want to adjust the caliper itself, rather than the mounting of the bracket it's secured to.

The part secured by the blue screws is the QM26 adapter used to allow the caliper to work with a 203 mm rotor: enter image description here
The red screws then secure the caliper to the adapter. It is those screws that you need to loosen before aligning the caliper:
enter image description here
The diagrams above are from the owner's manual. The instructions that go with the second picture read in part:

Installing brake calliper

  • …
  • Screw in retaining screws top (M6x35)(11) and bottom (M6x33)(12).
  • The brake calliper can still be moved slightly but only wobbles slightly
  • Pull and release the brake lever several times
  • Pressure point is clearly noticeable and does not change.
  • Pull and hold brake lever.
  • Tighten retaining screws alternately and in stages to a tightening torque of max. 6 N-m (53 lbf-in). [A7][A2]

Note particularly the last two points. Disc brakes in general are finicky to get adjusted right (well, even many rim brake designs are too), but it's important to make sure you are holding tension on the brake lever to secure the caliper in line with the rotor, and tightening the screws carefully so as to not disturb this alignment.

Assuming you've already done all these things, then you may have things aligned as best as you're going to. The brake pads ride very close to the rotor, and minute vibrations resonate well through the brake parts and bike frame, so the tiniest irregularities produce noticeable noise. Often once the pads have been worn a bit, things get evened out and the noise abates or disappears completely.

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Additional information:

The root cause is that you appear to have a 203mm rotor on a fork designed for a 180mm rotor. To get the extra spacing required, your bike has an adapter bolted in between the caliper and the fork.

The adapter would have been bolted securely to the fork using the original disk caliper mounts, and then the caliper would have been fitted second, obscuring one of the blue bolts.

Adaptor is marked in red here:
enter image description here

There's nothing inherently wrong with this setup, its just one way to get a larger rotor than the fork was designed for.

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    A larger rotor will have more flex and come out of true more often, creating rub. Will need trueing more often.
    – Jeff
    Commented Aug 8, 2020 at 19:08

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