Disc Mounts

Why does the adapter matter?

Disc brake adapters are small, cheap parts that solve a specific compatibility problem — matching your brake caliper to your frame or fork's mounting standard, and positioning the caliper correctly for your rotor size. Getting the wrong adapter means your brakes won't align properly, so understanding the standards matters.

IS or post mount?

There are two disc brake mounting standards on mountain bikes. Post Mount (PM) is the current standard — two threaded holes on a flat face, with caliper bolts running parallel to the rotor. International Standard (IS) is the older system — two threaded holes on a perpendicular face, with caliper bolts running at 90° to the rotor. Most modern forks and frames use post mount, but IS mounts still appear on older bikes and some rear triangles.

Which adapter do I need for a bigger rotor?

IS to PM adapters convert an IS mount to accept a PM caliper — you'll need one if your fork has IS tabs and your caliper is PM, as most modern calipers are. PM rotor size adapters adjust the caliper position for different rotor diameters: a +20mm adapter moves the caliper to fit a 180mm rotor on a 160mm post mount, or a 200mm rotor on a 180mm mount. Front and rear adapters are different, because fork and frame mounts are designed for different base rotor sizes.

How do I make sure it fits?

You need to know three things: your mount type (IS or PM), the base rotor size the mount is designed for, and the rotor size you want to run. Shimano, SRAM, Hope and Magura all make their own adapters and they're interchangeable between brands, so buy on price and condition alongside your brake set or fresh pads. Always use the correct adapter rather than shimming or spacing a caliper — improper alignment causes uneven pad wear, rotor rub and reduced braking performance.