How to choose your MTB disc brake adapters?

The brake calliper adapter is the part that fits between the brake calliper and the bike to adapt it to the desired disc diameter. Apparently straightforward, there are nonetheless plenty of opportunities to get it wrong between the different diameters and the frame-side and calliper-side standards. Here is how to select the right one first time.

Standards

Mounting standard on the frame, fork and calliper: Postmount (PM) or International Standard (IS)
MTB disc diameter standard: 140, 160, 180, 200 or 203 mm

Types of disc brake adapters

IS or PM standard? That is the question you will constantly be asking yourself when choosing a component for your MTB disc brakes, so it is essential to master the subject! The best way to avoid making a mistake is to measure the centre-to-centre* distance between the 2 mounting points: 51 mm means IS, 74 mm means PM.

In the case of PM, the other point to check before choosing your adapter is the minimum mounting diameter on the frame and fork. The height of the mounting tabs inevitably gives a base mounting diameter; the standard is 160 mm, but some brands may offer a different base mounting diameter, which will create an offset when you choose your adapter.

For example, an adapter specified for a 180 mm fitting assumes that your frame is originally 160 mm. If your frame is 140 mm as standard (i.e. -20 mm), you will need to take a 200 mm adapter (i.e. +40 mm) to achieve a final fitting size of 180 mm. To find out this base value, either you deduce it from the originally fitted components, or you can measure the distance between the wheel axle and the PM mount.

Rotor size in mm	Dimension C	Dimension D
140 mm	47,5	74,2
160 mm	55,9	74,2
180 mm	64	74,2
203 mm	73,9	74,2

Finally, given that the diameters and mounting formats are completely standardised, it is in theory possible to mix adapter brands with brake brands. However, since the shape and size of callipers is not standard, it is preferable to stick to the same brand wherever possible to avoid the calliper bottoming out in the adapter before being correctly pressed against the mounting points of your MTB.

Which adapter should you choose?

No particular tips for choosing your adapter other than identifying exactly what you need — but bear in mind that they are different for the front and the rear.

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