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I am putting a new caliper on and the had to get an adapter. Anybody know why it rubs and how to solve it? enter image description hereenter image description hereenter image description here

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  • Is the caliper itself in the right position relative to the rotor? I'd wonder if the frame's mounts aren't faced flat, or if the adapter's faces aren't quite parallel. Take it off and check with calipers and a straight edge.
    – Criggie
    May 18, 2021 at 3:06
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    I had no idea that there was room for adjustment when I loosened the caliper mounts. I guess I should have checked first. Thanks for all your gracious help.
    – Paul
    May 20, 2021 at 19:06

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I see the rotor hitting the caliper near the mounting area. The caliper has been moved all the way over on its slots, but the pictures don't make it clear why.

Without other information, the first thing to try is loosen the mounting bolts, reset the pistons, and re-adjust the caliper mounting with the rotor centered between the pistons in their fully reset state. Then actuate the brake a few times and make whatever small correction is needed to dial the centering in. That's how it works when things go right and there's no positioning issue weirdness happening.

If the caliper was moved over this far because it was the only way to get the inboard pad to clear the rotor, that will make the outboard piston live its life fully extended and the inboard one almost not extended at all. Having so much asymmetry tends to make the pads wear unevenly and can cause other issues with the piston function down the line, in addition to making the brake frustrating to adjust. Luckily, rotor shims are able to solve that problem. In an extreme case you can also just use M5 washers when you actually need that much thickness, but it's important they're all the same.

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  • Shouldn't there be two pairs of convex and concave washers with each bolt to adjust the alignment of the caliper?
    – Carel
    May 18, 2021 at 20:36
  • @Carel Those are only needed with Shimano PM to PM adaptors, or any other brand with two extra long bolts. IS to PM adaptors hold the caliper in the correct alignment and don't need them. The shimano adaptors, in the interest of only using two bolts, actually misalign the bolts compared to the mount faces, requiring the washers to correct.
    – MaplePanda
    May 19, 2021 at 3:38
  • @MaplePanda: They could be useful here though.
    – Carel
    May 19, 2021 at 17:37
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Ive had exactly the same issue with Deore callipers. You can get it to not strike by shifting the calliper a tiny amount but at the cost of worse braking due to the rotor not being centered well. It tends to drift back into the bad position anyway.

As mentioned by Criggie possibly due to a mis-faced rotor tab on the hub.

Try gently shaving 0.25 - 0.5 mm of material off the face of that post with a file. Just enough to stop the post fouling the rotor. Go slow, remember you can't add the material back.

Bike shops also have facing tools that can remove tiny amounts of metal from hubs and bottom brackets and will make all the rotor tabs perfectly square.

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I see that you've had to use an IS to Postmount adapter. Given that IS is one of the earlier standards for brake caliper mounts, it might be a matter of tolerances.

A subtle bend in the mounting tabs could put the whole adapter at a very-slight angle, pushing the caliper into the rotor.

So, try and find some way to accurately measure the tab's alignment. I'd try removing the adapter, and bolt some stiff metal strip through the IS hole, and compare that to the line of the brake rotor. It should be parallel to the rotor.

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