Road disc brake adaptors

How to choose a road bike disc brake adapter

The adapter connects the frame or fork mount to the caliper. On a road bike, it serves mainly one purpose: switch from a 140 mm rotor to a 160 mm rotor without changing the brake.

Three mounting standards to distinguish

• Flat Mount (FM) : the road bike standard. The caliper bolts flush to the mount; FM-to-FM adapters change the permitted rotor size.
• Post Mount (PM) : two bolts parallel to the wheel, inherited from mountain bikes. The adapters FM to PM let you fit a mountain bike caliper to a road bike frame, or vice versa.
• International Standard (IS) : two perpendicular bolts, found on older forks and frames. The adapters IS to PM remain common.

140 or 160 mm

• 140 mm is the traditional road bike rotor size: lightweight and sufficient for an average-sized rider on rolling terrain.
• 160 mm has become widespread: more stopping power, less heat build-up on long descents and longer pad life.
• Going from 140 to 160 mm requires the corresponding adapter, and checking that the frame and fork can accommodate that rotor size — the limit is specified by the frame manufacturer.
• Many bikes use 160 mm at the front and 140 mm at the rear : the two adapters are therefore different.

Front and rear are not interchangeable

The mounting geometry differs between the fork and frame. A front adapter will not fit at the rear, even for the same rotor size. Check the designation in the product title before ordering: this is the most common mistake in this category.

The alignment washers

Shimano supplies alignment washers and washers for 160 mm rotors with its adapters. They compensate for any misalignment between the caliper and rotor and are easily lost during removal. They can be ordered separately; without them, the caliper rubs.

To find out more: How to choose your disc brake adapters

See also: Road bike disc brake calipers · Road brake rotors · Road bike disc wheels