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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
    Commented Oct 23, 2017 at 19:23
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    I edited the title because this question pertains equally to all flat mount road calipers.
    – Nathan Knutson
    Commented Oct 23, 2017 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 Нет войне
    Commented Oct 24, 2017 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
    Commented Oct 24, 2017 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
    Commented Oct 24, 2017 at 14:28

4 Answers 4

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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.

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  • 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
    Commented Oct 24, 2017 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
    Commented Jan 29, 2018 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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AS Solutions in Canada now sell an adapter for IS directly to flat mount, with caveats around what size rotors can be used and whether your frame has clearance.

From their site:

Rear IS mount (Frame): This adapter will only work with 160 mm. rotors. Front IS mount (Fork): This adapter will only work with 180 mm. rotors.

This will not be compatible with all IS mount frames and forks. The caliper and adapter extend out from the mounting surface. Clearance between the mounting holes is required.

They do provide a template so you can print out and test with frame/fork

https://www.assolutions.ca/shop/adapters/flat-mount-adapter-for-is-frame/

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    Welcome to the site. I see this is new since the other answer mentioning AS Solutions and is specific to the IS mounts. Can you edit in what the caveats are? That would be enlightening. A picture of the mount would also be an interesting inclusion.
    – Swifty
    Commented May 15, 2020 at 6:59
  • Likewise - what are the pros and cons of this solution? You can edit them into your answer. Welcome to SE - good first start btw.
    – Criggie ♦
    Commented May 15, 2020 at 7:43
  • At this point even Shimano made quite a few adapters... Price is around 20 euro. At the time when this post was made there were not a lot of options and often changing the calipers was the best one...
    – Paolo Goatspeed
    Commented Aug 28, 2020 at 7:20
  • @PaoloGoatspeed Do you have any part numbers? I've been unable to find an IS frame / flat mount caliper adapter from Shimano
    – tir38
    Commented May 26, 2023 at 2:50
  • Are you sure you want to buy an adapter? at this point the best I have done over the years has been changing the callipers to normal deore mtb ones.
    – Paolo Goatspeed
    Commented Jun 1, 2023 at 18:55
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A.S.Solutions makes a post mount to flat mount adapter for 140mm rotor.

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  • assolutions.ca/shop/adapters/…
    – nori
    Commented Nov 19, 2018 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 Нет войне
    Commented Nov 20, 2018 at 7:10
  • I checkd and actually the make cool stuff, useful. Thanks for the tip again ;)
    – Paolo Goatspeed
    Commented Nov 20, 2018 at 23:42

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