Shimano Disc Brake Adaptors – What’s the Difference?

Shimano disc brake adaptors - what's the difference? Shimano disc brake adaptors can get confusing quite quickly, and trying to makes sense of them and figure out the right one you need can be frustrating. With different standards and options it can be hard to make sense of it all. Don't worry though, because the following article will discuss the different standards as well as focus on which Shimano adaptor to choose for your brakes and rotors.

Shimano Disc Brake Adaptors - What's the Difference?

Disc Brake Mounting Standards

Before we get into the different adaptors, it is important to first understand the different mounting standards. The three main mounting standards are "I.S" (International Standard), "P.M" (Post Mount), and flat mount. Since the flat mount standard is primarily used on road bikes, we are just going to focus on the international and post mount standards as these are the two main standards used in mountain biking.

"I.S" (International Standard)

The international standard, sometimes referred to as IS tabs, are unthreaded eyelets (tabs) located on the frame or fork. They will almost always require an adaptor, which gets bolted perpendicularly to the IS tabs. The brake caliper then gets bolted to the adaptor. Due to the design and geometry of this standard, it requires different adaptors to adapt to the same caliper for the same size rotor for the front and rear. For instance, the adaptor for a 180mm rotor for the rear would differ then the adaptor you would need for a 180mm rotor for the front.

Shimano Disc Brake Adaptors - What's the Difference?
                        IS Tabs on a Fork


Shimano Disc Brake Adaptors - What's the Difference?
                        IS Tabs on a Frame

"P.M" (Post Mount)

The post mount standard consists of two threaded holes on the frame or fork, and are positioned in a way that a caliper can be bolted directly to the frame or fork without the need for an adaptor. With most mountain bike frames and forks setup for a rotor size of 160mm in the rear and 160mm in the front, the caliper would be mounted directly to the frame or fork without the use of an adaptor. However, there are many exceptions to this with an example being that some downhill forks require a minimum rotor size of 203mm.

How To Install and Adjust Disc Brakes | Bike198
                Post Mount on a Fork


                     Post Mount on a Frame 

Now that you have more knowledge of the mounting standards and the differences between IS tabs and post mount, we can look at which adapter to use when.

Frames with I.S Tabs

160mm Rotor Use: SM-MA-R160P/S 

180mm Rotor Use: SM-MA-R180P/S

203mm Rotor Use: SM-MA-R203P/S

Forks with I.S Tabs

160mm Rotor Use: SM-MA-F160P/S

Shimano Disc Brake Adaptors

180mm Rotor Use: SM-MA-F180P/S

Shimano Disc Brake Adaptors

203mm Rotor Use: SM-MA-F203P/S

Shimano Disc Brake Adaptors

Frames and Forks with 160mm Post Mount

160mm Rotor: No Adaptor Needed

180mm Rotor Use: SM-MA-F180P/P2

203mm Rotor Use: SM-MA-F203P/P

Frames and Forks with 180mm Post Mount

180mm Rotor: No Adaptor Needed

203mm Rotor Use: SM-MA-F203P/PM

The below chart also shows which adapters are needed for the various post mount sizes and rotor sizes for Shimano, SRAM, and Hope.

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