Problem: Flat Mount Brake Calipers On IS Mount Frame

by Jeff Whitfield on | 10 Comments


TRP Spyre rear brake
TRP Spyre rear brake

I recently began rebuilding my bike, stripping it down to just the frame and building it back up piece by piece. One of the decisions I made was choosing the newest Shimano GRX groupset to rebuild with. Unfortunately, I found out the hard way that the new hydraulic brake calipers only work as flat mount only. 

It appears that you can take a post compatible caliper and fit it on a flat mount frame with an adapter. However, with flat mount calipers, if you're bike only accepts either post or IS then it would seem you're pretty much shit out of luck.

Now, that's not to say it can't be done. A.S. Solutions offers a few adapters that allow you to fit a flat mount caliper on a post or IS frame. There are limitations though. First, you have to have the clearance on your frame. Second, the IS adapter only works with 180 mm rotors in the front and 160 mm in the back. If these limitations work for you then you're good to go.

For me, I didn't have the clearance nor did I like the idea of having to change my rotors. As such, I ended up taking back the GRX levers and switched them out for some 105 levers that work with mechanical disc brakes. For the brakes, I decided to go with the TRP Spyre brakes. Sucks cause I was really looking forward to a hydraulic brake upgrade. Oh well! Can do that when I upgrade my frame.

In the end, be mindful of the type of brake mounts you have on your frame. If you have anything but flat mounts be prepared to be disappointed if hydraulic brake calipers won't work on your frame. 

Update (November 14, 2019)

One commenter made a great point. If you're working with a Shimano groupset, try looking up the groupset in Shimano's latest product compatibility documents. In my case, if I had stuck with a ST-RX810 brake lever then I could have used a BR-RS785 caliper, which is post-mount and can easily be adapted to an IS mount. Pays to do extra research when updating components. :P

Post a comment!


Hi, Had the same question on a Surly Straggler! Found the Shimano Product compatibility sheet

There Shimano states the ST RX400 is compatible with BR-RS785 which is post mount! adapter from Postmount to IS are easy!


Actually, what you want to know is if a ST-RX810 or ST-RX600 brake lever is compatible with that BR-RS785 brake caliper. Doesn't appear that the GRX components are reflected in that compatibility sheet. However, current 105 and Ultegra components are cross compatible so...technically, any caliper compatible with a 105/Ultegra hydraulic lever should work. Based on Page 25, a ST-R8025 (Ultegra lever) is indeed compatible with a BR-RS785 caliper. Wish I knew this early on. Doesn't matter though. I'm ok with what I got now. I can upgrade later. :P


sorry, posted the wrong link, to an outdated document: Ver. 2.3, Apr. 3, 2019, SHIMANO INC.

Correct is Ver. 2.6, May 31, 2019, SHIMANO INC.

This contains infos about the GRX compatibility


Awesome! I’m gonna update the post so other readers will know about this. Thx for posting! :D


Maybe u can use a Shimano MTB brake+GRX shifters with an adapter.

Will be following you blog as I am also looking to upgrade my Nicasio. A lot of good information. Thanks!

Regards from Malaysia


Doh! Why didn’t I think of that? That’s a really good idea. MTB brake calipers are likely to be overkill but, if they work, there you go. Probably won’t mess with it this time. The 105 levers are fine and the Spyre brakes work nicely. I plan on upgrading the frame sometime next year to modernize on things. Love the Nicasio frame but do need to take it to the next level.

Thanks for following my blog! Good luck with your adventures! Could you post a pic of your Nicasio? Would love to see what others are doing with it.


Hi Jeff, you are welcome. Yes, I have the same feeling as you. I felt that I should ve went for the Nicasio2 instead with the thru axle and hydro brakes.

Btw there are less powerful brakes lower down in the shimano hierarchy. Then again your trp sphyres are good enough in my humble opinion.

Mine is moving along slowly. Got a pair of 700x35c panaracer gravel kings, lighter stem, lighter seatpost and lighter saddle. Ordered a GRX RD-812 today and bought a set off used ultegra 6800 shifters. Yet to buy a cassette and crankset. (SLX 11-40t and 105 50/34). Plan to use the stock wheels (although they are not true) and stock brakes till i can afford to upgrade next year. Will stick to 700c wheels and upgrade my brakes to trp sphyres.

How do i attach a picture?


Unfortunately, there isn't a way to attach images to comments. However, you can post a link. Could use Instagram, Dropbox, or whatever. :P


Nice looking ride! The red is nice. You know, I considered getting the Gravel Kings myself but ended up getting the Pathfinder Pros instead. I'm looking at different road tires at the moment. Not sure about the GP5000's I'm running on. Thinking maybe something with more tread would be wiser.