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Are the mounting posts and position for road disc brake calipers the same as or similar enough to mountain bike disc calipers that you could put a set of road disc brakes on a mountain bike or vice-versa?

I'm aware that this generally requires road brake levers, and that you also need to make sure any drop bar levers are compatible with your derailleurs. However, my question here is narrowly about how you would convert the brakes of a disc-equipped mountain bike to something drop bar compatible.

If that can be done, what are the downsides to be aware of?

  • What kind of road disc? – ebrohman Mar 2 '17 at 3:31
  • Swapping a bike from flat to drop bars is considered too expensive, unless you already have all the parts from donor bikes. The caliper holes should be the same distance apart, and the rotors need to be the same size. Note "road" rotors seem to be thin little things and as small as 140mm. Your MTB probably starts at 160mm. – Criggie Mar 2 '17 at 5:58
  • One has to check compatibility of IS mount, post mount, and flat mount. The former seems to be preferred for MTB while the latter are road calliper mounts. – gschenk Mar 2 '17 at 9:50
  • Hydraulic or mechanical (or hydraulic/mechanicals)? – gschenk Mar 2 '17 at 9:51
  • @gschenk I'm interested in answers for both the mech and hydraulic scenario. From previous related questions, I got the impression that road lever to MTB brake compatibility and vise versa was not straightforward, e.g. different pull ratios for mech, different fluid displacements for hydro. That got me wondering, if you were wanting to convert a MTB to drop bar, rather than trying to get road levers working with your MTB brakes, would it be easier to just replace the MTB calipers with road calipers? – SSilk Mar 2 '17 at 15:33
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Are the mounting posts and position for road disc brake calipers the same as or similar enough to mountain bike disc calipers that you could put a set of road disc brakes on a mountain bike or vice-versa?

IS and post mount calipers are cross-compatible and can be seen on both MTBs and road bikes. Flat mount clipers, on the other hand, are a new thing that was designed specifically for road-type usage, with very limited presence on MTB frames (some small builders and Cannondale bikes). You can put a post mount caliper on a flat mount frame/fork using an adapter, similar to IS-to-post-mount.

However, my question here is narrowly about how you would convert the brakes of a disc-equipped mountain bike to something drop bar compatible.

Certainly possible, but details depend on what calipers MTB has. Shimano hydraulic calipers are compatible between road and MTB levers. In fact, BR-R785 was an XT. I don't see why you can't just get a set of any Shimano hydraulic road levers and put them on. No idea about SRAM, though. You should also be aware that mechanical MTB/road calipers might have different leverage ratio, like BB7 does.

If that can be done, what are the downsides to be aware of?

Except for existing parts compatibility I see no reason why anything might cause trouble. I'd be concerned about geometry though, drop bar will increase the reach significally, you might want a shorter stem.

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    "Shimano hydraulic calipers are compatible between road and MTB levers." A recent question asked exactly that. Replies indicated however that road and MTB hydraulics are not compatible: bicycles.stackexchange.com/a/45053/30402 Would you add an answer with regard to the compatibility of that XT caliper? I've mentioned it in a comment but do not know enough for a reliable answer. – gschenk Mar 2 '17 at 16:14
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    The linked table shows that all listed calipers are compatible. People have reported that Zee calipers work with RS685 and the fact that road hydro was launched with rebranded XT calipers. I'd argue these three points give more weight for what I said above. – Klaster_1 Mar 2 '17 at 17:10
  • @Klaster_1 I saw the previous question gschenk3 linked, and the comparison chart for Shimano. Are you aware of a chart like that comparing SRAM systems? – SSilk Mar 2 '17 at 19:28
  • @SSilk I am not. – Klaster_1 Mar 3 '17 at 2:32
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    It's totally possible that I took an overly conservative line in answering the linked question. In part I've yet to become convinced one can never ever get into trouble mismatching brake hoses between BH59 and BH90, although what's confusing is that chart seems to imply you can because the 9170 brakes use 90 and the others use 59. – Nathan Knutson Mar 3 '17 at 11:45
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Yes, you can do it. There are lots of adapters available if you need to convert between mounting styles, or if your rotors are a size larger than the mounts are designed for.

Obviously you need compatible (road) levers, but you've already addressed this.

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