Disc Brake Adaptors Unraveled

Analog’s guide to choosing the right Disc Brake Adapter for your bike.  

What is a Disc Brake Adapter? Simply put, it’s a chunk of metal and two bolts that allow you to mount different sized disc rotors on your bike frame. Basically, it’s a spacer. Bigger spacers = bigger rotors. Not all disc brakes need adaptors: If a bike frame is designed for 160mm rotors and post mount brakes, and you stick to that size of rotor, no adaptor needed. That’s pretty much only the case with Post Mount.

Disc brake adapters are a black hole of weird standards. Let’s stick to the 3 most popular ones so we don’t go crazy and reach for the Mezcal. I.S., aka International Standard is for better or worse, the current brake mounting standard for bicycle frames. It’s featured on the most bikes, and when most folks think: disc brake adapter, some sort of I.S. adaptor is what they think about. Confusingly, most disc brakes themselves are post mount. You need an adaptor to get the two to play together. I.S. brakes more or less don’t exist. Just Post mount and Flat mount.

There are two other disc brake / frame interfaces that are gaining popularity. Post mount and Flat mount. Post mount adapters are making a comeback for carbon mountain bikes. Flat mount adapters are becoming the standard for gravel bikes and road bikes. All adapters are designed to let you customize your brake rotor size. Gravel and road bikes tend to use 160mm front and 140mm rear rotors.  Mountain bikes tend to use 180mm front, 160mm rear or for the big fast bikes, 200mm rotors.  

Bigger rotors dissipate heat faster, so they work better under high loads. They also generate more leverage, so they have more stopping power. This additional power is more noticeable on the front brake than the rear. Upgrading from a 160mm rear rotor to a 180mm rear rotor will not have the same effect as upgrading to a 180mm rotor up front. They have the same theoretical impact, but in practice, the bigger rotor up front is a more noticeable change.
I.S. adapters have two advantages over the other adapters. They don’t require you to thread the brake right into a frame or fork, and you can unbolt the adapter and take the brake off without messing with its adjustment. This is only really handy for traveling, but I suppose it’s worth noting. The downside to I.S. adapters is that they are inelegant, and the bolts that mount the adapter install from the side. This means that all of the stress generated by braking force is transferred to two M6 Bolts, which are threaded. Threaded anything is basically a stress riser waiting to happen. In practical purposes, these bolts don’t snap. But why design a system with a built in stress riser.

If you have a bike with I.S. mounts, don’t sweat. Nothing is going to happen, probably. You’ll be fine. You will need one of these adapters to mount normal disc brakes, which by default are set up for ‘Post Mount’ style mounting. The only alternative to post mount is Flat Mount, which is a new, smaller ‘n lighter mounting system.

Ok so if your bike has a mount that looks like the pictures above/to the left, then you need a I.S. to post mount adapter. Here’s the code for figuring out what size adapter you need:


160mm front rotor: +0 adapter

180mm front rotor: +20 adapter  

200mm front rotor: +40 adapter

140mm rear rotor: +0 adapter

160mm rear rotor: +20 adapter

180mm rear rotor: +40 adapter

Click for Paul Component Adapters, or generic SRAM/Avid Adapters.

Don’t just go putting bigger rotors on your bike willy nilly. Frames are designed around a certain max size of rotor. Exceeding that will potentially knacker your frame. Check with the manufacturers spec before you go all 200mm rotor on your road bike.

We sell Paul’s fancy andodized I.S. adapters and some basic SRAM ones.

Post mount adapters were a big deal on mountain bikes back in the day, and recently they’ve come back into vogue, like Mom Jeans. I like post mount, because if a bike is designed right, you can run any post mount disc brake without any adapters. You just need adapters if your frame or fork designer was lazy. Adapters are best avoided if you don’t need them. They add a place for bolts to get loose, additional weight, and additional places for failure. If your frame is designed around 160mm rotors, and you want bigger rotors AND your bike can take bigger rotors, get a 20mm post mount adapter.  If it’s designed around 180 rotors, just direct mount the brake. No adapter needed. If it was designed around a 180mm rotor and you want to run 200mm rotors, get the same 20mm post mount adapter.