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Disc Brake Mounting adapters

Disc brakes use two different mounting styles: International Standard (IS) and Post Mount (PM). IS mounts use a 51mm bolt pattern (measured center-to-center) while PM uses a 74mm bolt pattern (center-to-center, also). These mounting styles are shared between forks, frames, and calipers. Most modern forks, calipers, and even some frames are made for PM. Previous to PM was IS mount, which you’ll see mostly on frames and occasionally on older forks and calipers. To get a particular disc brake caliper to work with a certain frame and fork, you’ll simply need the correct adapters.

International Standard (fig. 1a) and Post Mount (fig. 1b) fork mount tabs. International Standard (fig. 2a) and Post Mount (fig. 2b) brake adapters.

51mm International Standard

International Standard mounts (fig. 1a) were devised in effort to simplify disc brake mounting styles and were very common in the early day of disc brakes. While most manufactures hopped on board for standardizing frame mounts for rear disc brakes, IS mounts are also seen on select forks and some older-style calipers. IS mounts will mount to the side of the mount tabs with the mounting hardware going across the caliper, frame and fork. Due to the nature of the IS mount design, the distance between the caliper and disc will vary between manufacture models and rotor size as well as whether it’s for front or rear caliper. This requires more brake adapter options as front and rear adapters are not always interchangeable. Even though running the same model brakes and rotor size front and rear, you’ll find you’ll need two different adapters if using IS mounts.

74mm Post Mount

Post Mounts (fig. 1b) are perhaps the most common now-a-days and are seen on calipers, forks, and select frame manufactures have adopted the standard for rear disc brake mounts also. PM adapters mount directly into the frame or fork, requiring the frame or fork to be internally threaded to accept the mounting hardware. The nature of these adapters does not allow for side- to-side adjustment, however, by making all brakes mount in the same spot reduces the variance in distance from the caliper to the disc. This means front and rear adapters can be exactly the same — further simplifying the disc brake adapter market. In the classic “Grass is Always Greener” argument, there are some risks associated with PM: if the internal threads on the frame or fork ever get stripped, you’ll need to get it re-threaded or replace the fork/frame.

How to Choose the Correct Adapter

Choosing the correct disc brake mount is much easier today than it was 10 years ago. Although there are still plenty of options, here are 3 easy steps to find the correct brake adapter to fit your needs:

  • What mounting styles do you need?
    Remember, mounting styles are shared between forks, frames, and calipers. Most modern calipers, forks, and even some frames are made for PM. Select forks and calipers are made for IS mount, which are much less common. On our website, assume all disc brake calipers use the PM mounting style unless otherwise noted. Once you’ve determined the mounting style for your frame and fork, you’re off to Step 2!
  • What size rotors will you be using?
    It’s very common to run a larger rotor in the front and smaller in the rear. Either way, you’ll need to decide which size you will be using for both (they can be different or the same, doesn’t matter). Remember, if using the same rotor size front and rear and your frame or fork use IS mounts, you’ll still need 2 different adapters for front and rear. If your frame and fork use PM, you’ll be able to use the same adapters on frame and fork. If your fork or frame use PM and you’re planning on running 160mm rotors, you won’t even have to worry about adapters — just mount the caliper directly to the fork or frame and hit the trail!
  • What manufacture makes the brake caliper?
    Several manufactures make their adapters proprietary to their brake calipers. This means if you run Shimano disc brakes, you’ll need a Shimano disc brake adapter. An Avid disc brake adapter, although the correct mounting style and rotor size, will not work with your Shimano disc brakes. There are some third-party manufactures that make adapters that can be used across the board, such as North Shore Billet and Origin8; if using an adapter by one of these companies you’ll simply need to match up the correct mounting style and rotor size.

Still Unsure?

For further explanation, below is a short video giving you a bit of history on how the International Standard (IS) disc brake mount came about, and how you can tell the IS apart from a PM mount on your fork or frame. You'll also find a breakdown of the most common IS & PM adapters.

As always, if you have any questions about which adapter you’ll need our Customer Service team will be glad to assist you. Just give us a call, shoot us an email, or hop on our Live Chat!

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