Disc Brake Mounting Systems & Compatibility
IS, Post Mount, Flat Mount. How to identify them and the logic of adapters.
Disc brakes have taken over, but mounting them is not universal. We have moved from the crude tabs of IS standard to the integrated flush look of Flat Mount.
1. The Three Standards
Flat Mount (The Modern Road/Gravel Standard)
Designed by Shimano to be aerodynamic and compact. The caliper sits flush against the frame/fork.
- Rear: Bolts come through the frame chainstay from the bottom into the caliper.
- Front: Uses a mandatory thin adapter plate always. Flip the plate to switch between 140mm and 160mm rotors.
Post Mount (The Mountain Bike Standard)
Two threaded posts sticking out of the frame/fork perpendicular to the wheel axle. 74mm spacing center-to-center.
Most common on MTB and older gravel bikes. Calipers bolt directly onto these posts.
IS (International Standard - Legacy)
Unthreaded tabs sticking off the side. You cannot bolt a caliper directly to IS tabs. You MUST use an adapter (IS-to-Post or IS-to-Flat).
2. Adapter Logic & Rotor Sizes
Adapters push the caliper "out" (away from the axle) to accommodate larger rotors. You generally cannot go smaller than the native mount size (e.g., if a fork is native 160mm, you cannot fit a 140mm rotor).
Common Adaptation Rule
+20mm Adapter: Most common adapter. Moves a native 140mm mount to 160mm, or a 160mm mount to 180mm.
Can I put Flat Mount calipers on a Post Mount frame?
No. There is no room for the caliper body. (Exception: Some rare expensive niche adapters exists, but generally no).
Can I put Post Mount calipers on a Flat Mount frame?
Yes, adapters exist, but it looks bulky.