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I have custom MTB frame and I bought a pair of Shimano Hydraulic road shifters. They have on Shimano BR-RS505 Road Hydraulic Brake Calipers Flat Mount when instead my frame takes standard IS 51 mount (+adapter of course for it).

What kind of adapter should I use? I have noticed that this kind of calipers use a front adapter mount and it might work on the back too...

Thanks for your help!

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    As far as i know there is not an adapter at this point to use a flat mount caliper on a post mount frame they are not backwards compatible in that sense. There are adapters to use post mount brakes on a flat mount frame but not the other way around. At least not yet that i have seen. – Nate W Oct 23 '17 at 19:23
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    I edited the title because this question pertains equally to all flat mount road calipers. – Nathan Knutson Oct 23 '17 at 21:08
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    @PaoloGoatspeed you can use any Shimano post-mount calipers and IS adapter instead of BR-RS505. I suppose even older IS calipers might work OK. – Klaster_1 Oct 24 '17 at 3:02
  • Hi guys, so I am going to use the standard flat adapter they suggest to use on the front brake/fork, and then add to it a normal post mount IS adapter, then I will understand if it does the job, depending on the angle it will get. – Paolo Goatspeed Oct 24 '17 at 14:25
  • @NateWengert , right the market hasn't got much to use these brakes yet, and wat available is expensive. I standard flat mount is around 20/30 euro, can't believe it. – Paolo Goatspeed Oct 24 '17 at 14:28
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As of now there is no commercial product you can buy to make this possible. An adapter that did this would be awkward to design because the flat mount caliper, in its native spatial position relative to the rotor and stay/blade, is trying to occupy some of the same space as the IS tabs. I believe it's physically possible but it would have to resort to being bulky and positioning the caliper quite far away along the rotor from where the tabs sit, which is a very negative design trade-off. It may even raise questions about how much leverage the tabs or mounting bolts can handle.

  • I don't have any adapter top test it, but I will soon try one. I understand your point but I think that the position and caliper shape is less awkward that it looks. Definitely is bulky, that's why they moved it to the inside of the stays. – Paolo Goatspeed Oct 24 '17 at 14:31
  • Yes, to answer to your point: the 2 different types of caliper occupy and "bite" different areas and shapes of the rotor I think...but I will check – Paolo Goatspeed Jan 29 '18 at 12:19
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Just an update on this topic: I have now checked what is available on the market and realised (as we have discussed previously here) that the easiest way to achieve my goal is changing the calipers to post mount. I am not sure if I could build an adapter to use flat mount calipers on post mount frames, but this would mean putting the caliper farther from the rotor, affecting the positioning of it. There might be as well another problem, that the natural curve of the rotor would not fit on a flat mount caliper used as post mount. I hope this is useful (for somebody) and please let me know if you have any other question. Paolo

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A.S.Solutions makes a post mount to flat mount adapter for 140mm rotor.

  • assolutions.ca/shop/adapters/… – nori Nov 19 '18 at 22:31
  • This might just work, but Nathan will need a IS to PM adapter too. A PM-to-FM has been asked about by a lot of people, good thing it's available for sale now. – Klaster_1 Nov 20 '18 at 7:10
  • I checkd and actually the make cool stuff, useful. Thanks for the tip again ;) – Paolo Goatspeed Nov 20 '18 at 23:42

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