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I'm contemplating building up a new commuter bike, and I'd like internal gearing and disc brakes.

Internal gears require horizontal dropouts. Disc brake rotors however must be aligned with the caliper fairly exactly. As the chain wears and the wheel migrates backwards, the caliper could lose alignment with the rotor.

Is this an issue in practice? Does it mean that the hub must find a single spot and never move without requiring a brake adjustment?

I've got several single speed bikes, where I move the axle around in the dropouts routinely..

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There are some high-end IGH/single-speed frames where the dropout is reconceptualized: there is a part that carries the through-axle and brake, and this part mounts to a slot in the frame, so it can slide fore/aft. So the caliper’s location relative to the wheel never changes, but the chain tension can be adjusted

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  • And those with an eccentric bottom bracket
    – Noise
    Sep 5, 2023 at 13:57
  • + 1 what are some examples and or what is this called so I can search for it? Sep 5, 2023 at 16:59
  • Looks like these are generally called "sliding dropouts." A number of frame builder-supply companies make these; here's one example. Here's a frame set up this way.
    – Adam Rice
    Sep 5, 2023 at 22:46
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IGH do not require horizontal dropouts, a large part of the bikes sold with IGH are not using horizontal dropouts. Such dropouts are a very simple solution to a problem that arises when there's no derailleur: to offer chain tension adjustment.

But this problem can be solved by other means, the most common I've seen are chain tensioners or eccentric bottom brackets. In both cases, the hub remains exactly at the same position, and therefore no alignment problem between the rotor and the caliper.

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    People do often build IGH setups on frames with horizontal dropouts, and most freewheeling singlespeed bikes I see also use such frames (perhaps because it looks tidier than a chain tensioner). But an eccentric bottom bracket is a very nice solution, and they do exist for normal frames (I have one on a tandem, and that needs a specially-made frame to lock its rotation, as well as to fit its larger diameter)
    – Chris H
    Sep 5, 2023 at 12:49
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    @ChrisH thanks for the correction, text updated.
    – Renaud
    Sep 5, 2023 at 12:56
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There are, in general, four approaches that are commonly taken.

The first, mostly used with solid axle designs, is a floating rear caliper mount with horizontal dropouts. In this type of design, the rear caliper mount fits into a slot aligned directly with the rear dropout. Part of it nests around the axle so that the caliper maintains a fixed distance from the axle, ensuring correct alignment with the rotor front-to-back. There is usually a bolt and nut that go through both a hole in the chainstay and a slot in the rear caliper mount which is used to fix it in place once you have the wheel installed and the belt tensioned properly. This design makes it a bit tedious to remove the rear wheel, and it requires a frame that’s designed for it, but it’s one of the most inexpensive options from what I’ve seen.

The second is a variant of the floating mount concept commonly called slotted dropouts. These are used with QR and thru-axle designs, and essentially just let you adjust the linear distance between the rear axle and the bottom bracket, allowing for adjustment of chain/belt tension. Most of the good ones have caliper mounts for a rear disk brake caliper on the sliding part so that it will stay the correct distance from the axle as you adjust tension. Just like with a floating caliper mount, these also require a frame designed for them.

The third approach is an eccentric bottom bracket, which is mostly used with QR and thru-axle systems. Again, this is mostly just providing a way to adjust chain/belt tension, but unlike slotted dropouts or a floating caliper mount, it doesn’t change the position of the rear axle, so you can just mount your rear caliper directly on the frame. I’ve not dealt with an EBB in such a setup myself, but from what I’ve heard from friends who have, they’re also a bit of a pain to handle when removing the rear wheel, because you have to loosen the BB to remove the wheel, then adjust it correctly again when reinstalling the wheel.

The fourth option is to just use a chain tensioner. This obviously only works with a chain drive, but it’s likely the most practical option if adapting a frame designed for a RD setup to an IGH. Just like an EBB setup, the rear axle never moves, so the caliper is just mounted directly to the frame.

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One answer is to use a belt drive rather than a chain to avoid the stretch in the first place (and as a bonus you also get to skip chain lubrication)

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    You still have to get the belt tension correct, which often involve horizontal/sliding dropouts or eccentric bottom brackets
    – Paul H
    Sep 6, 2023 at 6:39
  • Belt drives are also MUCH more sensitive to tension and alignment than chain drives. So belt drives don't do anything to address the issues in this question.
    – jayhendren
    Sep 6, 2023 at 16:41

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