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I have a bike frame that can be run as a geared or single speed bike. It has horizontal rear dropouts to adjust wheel position to tension the chain when run as a single speed. It also has slots for the disc brake caliper to slide forward and backward. Most other frames accomplish this with a combined brake/axle mount that slides in the frame, ensuring the brake always engages the rotor at the right point (e.g. IRD's sliding dropouts). However on my frame I have to set these two positions independently. How can I ensure and test that the pad and rotor are contacting evenly? I thought about putting something like permanent marker on the rotor, giving it a spin, braking, and seeing if it evenly wipes off.

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    If you have and old set of pads, you can clean the rotor after testing. Not sure what I would use it on good pads.
    – mattnz
    Aug 29, 2023 at 3:44
  • Is this something you're going to change frequently? Or is it a frame that can be used either geared or single speed but isn't really intended to be changed a lot ?
    – Criggie
    Aug 29, 2023 at 4:43
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    @Criggie worth noting that in single speed mode you have to occasionally pull the wheel back to maintain tension in a wearing chain.
    – Paul H
    Aug 29, 2023 at 5:22
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    I designed a set of dropouts for a bamboo bike a friend was building, and we faced the same problem. I was very wary indeed of adding slots given the forces involved. At the time I thought that using brakes with bigger pads (Deore MTB rather than 105 road, to use Shimano examples I have at home) would be enough, but I'm no longer sure after considering the effect of wearing a lip into the pads.
    – Chris H
    Aug 29, 2023 at 14:48
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    @Criggie That’s my understanding. I read this question as, “I’ve put my wheel into the sliding dropout (for whatever reason, maybe after repairing a puncture), how do I ensure my my brake caliper’s fore-aft position matches the wheel?”
    – Paul H
    Aug 29, 2023 at 15:10

1 Answer 1

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I wouldn't worry too much about it. Disc brakes and pads have enough surface area to tolerate some amount of misalignment, as long as you don't need all the brake-torque you can get (which you won't for a rear brake, unless perhaps if you're using it for trials-style locked-rear hopping).

Just make sure it looks roughly right. You can glimpse where the pads hit the rotor through the gaps in the caliper.
And of course if on the next pad replacement you should notice uneven wear, it's a good idea to investigate and re-adjust.

Definitely don't put anything like permanent marker on the rotor, that would likely cause more problems than a misaligned caliper.

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    Permanent marker should come off with the same solvents (alcohols) used to clean rotors. The quantity should be small enough not to contaminate pads but it would be worth keeping an old set to test as mattnz suggests in the comments.
    – Chris H
    Aug 29, 2023 at 14:33
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    ... Also you'd only need a radial line or two of marker, rather than complete coverage
    – Chris H
    Aug 29, 2023 at 14:49
  • Good idea @ChrisH I'll give that a shot.
    – tir38
    Sep 2, 2023 at 2:22

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