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I bought a cheap 28" steel fork to replace a rusted 26" suspension one on an old Trek MTB for my kid. The geometry looks good, but I can't align the the disc brake caliper properly. The pads end up outside the rotor.

I have a couple of different IS-adapters but none work. To me it seems that this 160 mm rotor can't work with the fork's IS-mount. Should I consider a bigger rotor? Is there a problem with the fork, or could the cheap zoom caliber be the problem? Am I missing something?

Pictured below is what I think would be the optimal position for the caliber. I haven't found an adapter online that would position the caliper as pictured.

optimal caliper position

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  • Are you able to provide Make/model and/or specs for the fork?
    – mattnz
    Commented Mar 28, 2023 at 3:01
  • It's an M-Wave Trekking Fork 28" bike-discount @mattnz
    – Panmanp
    Commented Mar 28, 2023 at 11:38

3 Answers 3

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It looks to me as if the fork was built to take a larger rotor. You might find the cheapest/simplest solution is to change the 160mm rotor for a larger one. You'll need to measure up to see how much larger.

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  • Good overview here. bicycles.stackexchange.com/questions/44161/…. @Nathan Knutson says that 160 is the minimum front, so I take that to mean 180 could be the minimum for that fork.
    – mattnz
    Commented Mar 28, 2023 at 3:00
  • @MaplePanda I think 160mm is the minimum size for a front rotor with an IS mount caliper. In this case, a larger rotor would mean that the caliper could be mounted on the existing adapter - so instead of moving the caliper in (towards the hub) with an adapter the OP can't find, I'm suggesting moving the rotor's braking surface out by using a larger diameter rotor.
    – Greg Shailes
    Commented Mar 28, 2023 at 9:59
  • @GregShailes Oh my goodness, I somehow misinterpreted the question as left-right misalignment. Total cock-up.
    – MaplePanda
    Commented Mar 28, 2023 at 18:34
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Getting a bigger rotor (180 mm) and using some cup and cone washers from old v-brakes did the trick.

I know it's not perfect, but it seems to work with my 100 kg so should be fine with my 35 kg kid.

enter image description here

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Front and rear is not the same. If you get the correct adapter it is likely to work.

You seem to be testing a few different IS adapters. It would to me make sense to use a front IS to PM 160mm adapter.

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    Hi, welcome to bicycles. In this case the difference is between two different forks, so the rear doesn't enter into it.
    – DavidW
    Commented Mar 27, 2023 at 10:59

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