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How to install flat mount road disc brake calipers on IS mount bike?

Bicycles Asked by Paolo Goatspeed on December 23, 2020

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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4 Answers

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.

Correct answer by Nathan Knutson on December 23, 2020

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

Answered by Paolo Goatspeed on December 23, 2020

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

Answered by nori on December 23, 2020

AS Solutions in Canada now sell an adapter for IS directly to flat mount, with caveats.

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

Answered by T0xic Tim on December 23, 2020

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