Making Sense of Shimano Disc Brake adapters

Disc brake adapters are rabbit hole of different standards and options which can be downright impossible to make sense of. Since at Bikecomponents.ca we deal primarily in Shimano parts, we’ll focus on shedding some light on which Shimano adapter to choose for you Shimano brakes and the various rotor sizes.

Before we get into the details it’s useful to discuss the current disc brake mounting standards, "I.S." (international standard), "P.M." (post mount), and flat mount. Flat mount is used primarily on road bikes, with some manufacturers starting to experiment with this standard on cross country race mountain bikes, but here we’ll focus on "I.S." (international standard) and "P.M." (post mount), the two most common standards in the MTB world. 

The "I.S." (international standard), also sometimes referred to as IS tabs, are unthreaded eyelets (tabs) in the frame or fork. In almost all cases they require an adapter which is first bolted to the IS tabs. The brake caliper then bolts onto the adapter. The design and geometry of this standard also requires the use of different adapters to adapt the same calliper to the same rotor size front and rear. For example, the adapter for a 160mm rotor in the front differs from the one that would need to be used for a 160mm rotor in the rear.

IS Tabs on a frame

IS Tabs on a fork

The "P.M." (post mount) standard consists of two threaded M6x1 holes on the frame or fork, oriented such that a caliper can be bolted directly to the frame or fork without the need for an adapter. Most mountain bike frames and forks are setup for a rotor size of 160mm front and 160mm rear, where the caliper would be mounted directly to the frame or fork without the use of any adapters. That being said, all sorts of exceptions exist, with some downhill forks for example being setup for a minimum rotor size of 200mm. There are also both frames and forks setup for 140mm rotors. Post mount doesn’t suffer from the geometry issue of IS tabs, as such the same adapters are used front and rear to adapt to the same rotor size.

Post mount on a frame

Post mount on a fork

Now that we’ve covered some the differences in intricacies of IS tabs vs. post mount, let’s try to demystify which adapter to use in which case.

Forks with IS tabs:

160mm rotor use: SM-MA-F160P/S

180mm rotor use: SM-MA-F180P/S

203mm rotor use: SM-MA-F203P/S

Frames with IS tabs:

160mm rotor use: SM-MA-R160P/S

180mm rotor use: SM-MA-R180P/S

203mm rotor use: SM-MA-R203P/S

Forks & Frames with 140mm Post mount:

160mm rotor use: SM-MA-F180P/P2

Forks & Frames with 160mm Post mount:

160mm rotor use: NO ADAPTER

180mm rotor use: SM-MA-F180P/P2

203mm rotor use: SM-MA-F203P/P

Forks & Frames with 180mm Post mount:

180mm rotor use: NO ADAPTER

203mm rotor use: SM-MA-F203P/PM

Please note that you shouldn’t just put bigger rotors on your bike without first checking the frame and fork manufacturer specifications. Bigger rotors do have several advantages such as faster heat dissipation, they also generate more leverage, and therefore have more stopping power. However, frames and forks almost always specify a maximum rotor size, which has to do with the amount of forces they can safely deal with due to the increased leverage bigger rotors provide. Exceeding the maximum specified size can damage the frame and/or fork, so again, check with the manufacturers specifications before upgrading your rotors to a larger size.