Do I need an adapter for a post-mount caliper on an IS-mount frame?
By Crankworthy · Published · Updated
Yes. An IS (International Standard) mount has bolt holes running side-to-side through the tabs, while a post-mount caliper needs threaded posts facing outward. An IS-to-post-mount adapter bridges the two — choose one sized for your rotor diameter.
Why
IS mounts were common on older frames and forks: two unthreaded holes, bolted through from the side. Post mount is the current mountain standard, with two threaded posts the caliper bolts straight onto.
Almost every modern mountain caliper is post mount, so an IS frame needs an adapter. The adapter sets the caliper's distance from the hub, which is why it is specific to a rotor size.
Flat mount is a third standard, used on road and gravel bikes: the caliper bolts up from underneath onto a flat surface. Flat-mount and post-mount parts are not directly interchangeable; adapters exist for some combinations, so check with the frame and brake maker.
How to tell which you have
- IS
- Two smooth holes in tabs on the frame or fork, with the bolts entering from the side.
- Post mount
- Two threaded posts facing outward, with the caliper bolted onto them.
- Flat mount
- A flat pad on the chainstay or fork, with bolts coming up from underneath into the caliper. Road and gravel bikes.
IS-to-post-mount adapters
From our catalog. Match the adapter to your rotor size and to front or rear.
- Shimano SM-MA-F160P/S AdapterIS fork to post-mount caliper, 160mm front
- Shimano SM-MA-R160P/S AdapterIS frame to post-mount caliper, 160mm rear
Prices were last checked when the catalog was built and can change at any time. Retailer links are affiliate links; we may earn a commission at no extra cost to you.