Disc Brake Mount Adaptors Explained

by Spectrum Bike Parts

Posted on April 20, 2024, 8:22 p.m.

With more than 100 types of disc brake mount adaptors on the market it can be intimidating to figure out which one is the right one for your bike. We'll try to simplify things here.

What are disc brake adapters used for?

A common use of disc brake adaptors is to adjust disc brake locations to fit different brake rotor sizes. For example, if a rider had a bike with a 140mm diameter disc brake rotor and decided to swap that out for a 160mm rotor, the contact area (where the brake pads squeeze the rotor) would be further out. In many cases, the disc brake pads / calipers will still work as long as they're given a boost.

The adaptor sits between the bike frame (or fork) and the brake caliper. It's like a booster seat for brake calipers.

Which disc brake adapter do I need?

This is the tricky part.

Rotor Diameter

The net change in diameter between rotors is an obvious factor. For example, if you're swapping 160mm for 180mm you'll need a 20mm adaptor. But, it's not quite that simple.

For flat-mount systems, you need to know the diameter of the new rotor to determine the right adaptor.

For post-mount systems, you need to know the diameter of your current rotor and the diameter of the new one to determine the boost you'll need. If your post-mount system already has an adaptor in place (example: you're running 160mm rotors and plan to upgrade to 180mm rotors but realize that you already have a 20mm adaptor installed) you'll want to pick up a 40mm adaptor to replace the 20mm.

Mount Type - Bike

You also need to figure out what type of mount your brake calipers use. The three most common options are IS (International Standard), Post mount, and Flat mount. Post mount is common on mountain bikes and Flat mount on road and gravel bikes.

Post mount systems are recognizable by the two posts on the frame or fork that the brake caliper bolts into. You'll often see these on mountain bikes.

Flat mount systems are much more flush with the frame. They're common on carbon bikes and gravel bikes.

IS (International Standard) mounts are most common on lower end bikes and steel bikes.

Adaptors can be used to allow a bike to use brake calipers with different mounting systems than what the bike originally had. For example, someone might choose to switch from IS to Post Mount brakes.

Mount Type - Caliper

The second mount type to consider is the design of the brake caliper (the same specs as above, but now we're talking about the caliper that mounts on the bike). If it's a post-mount caliper you'll want an adapter that support that. If you're swapping brakes from flat mount to post mount, there are adaptors for that, but putting flat-mount calipers on a post-mount bike is not done.

Keep both mount types in mind when filtering for appropriate adaptors.

Mount Location

Some adaptors are designed specifically for the rear (frame) or fork (front) so keep that in mind for your project.

Advice

Identify what you have today, and what your new rotors or calipers need, before digging in. If you're not sure what's on your bike, snap a bike and send it to a bike geek friend. They'll be able to tell you right away.

Also, the rotors list their size on them, and you can look up your brake caliper here to learn about that product's specs.

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