Rotor Sizes and Mounting Types

A new rotor won't fit, or you're not sure what to order. Two details decide it: the rotor's diameter and how it attaches to the hub.

Rotor diameter

Diameter is the width of the rotor across its outside edge. Park Tool says there are four main sizes commonly used in the bike industry: 140, 160, 180 and 203mm.¹ It adds that a few outliers exist, so measure to be sure.¹

Where do you find it? Sometimes the size is listed on the rotor.¹ If not, you need a suitable measuring device for the outer diameter.¹

Why the size matters

Park Tool's advice is plain. In general, replace a rotor with one of the same size.¹ Change the size, and you will likely need changes to your calipers or adapter too, which turns a simple swap into a bigger job.¹ If you want a different size, check with the brake maker first.

How the rotor attaches to the hub

There are two common ways, and the rotor and hub must match. Park Tool says the rotor and hub should have the same interface.¹

Six-bolt

Six-bolt rotors are fastened with six bolts.¹ The hub has six holes, and the rotor lines up with them. Wikipedia says this IS (International Standard) mount is the industry standard.²

Center Lock

Center Lock rotors are fastened with a lockring, which can have external notches or internal splines.¹ Wikipedia says the system is patented by Shimano and uses a splined interface along with a lockring to secure the disc.² Park Tool notes that grease may contaminate the rotor surface and isn't necessary for Center Lock rotors.¹

Adapters

One direction works. Park Tool says it's possible to adapt a six-bolt rotor to a Center Lock hub, but not the other way round.¹ Wikipedia agrees that adaptors enable the use of six-bolt discs on Center Lock hubs.²

The caliper side has standards too

The caliper bolts to the frame or fork, and that has its own mounting standards. Wikipedia says there are many, though most manufacturers today use either IS or post mount (PM).² These differ by disc size and axle type. So rotor size and caliper mount are linked, and changing one can affect the other.

Fitting and torque

Bolt and lockring torque are set by the maker, so follow your manual. Park Tool warns against greasing or oiling the rotor bolts, since the oil can spread to the rotor face.¹ If you're replacing a rotor because of wear, the thickness page explains how that's judged: rotor wear.