Rotor Wear and Minimum Thickness

The lever feels a bit off, the brake rubs once per wheel turn, or you've gone through several sets of pads. The rotor deserves a look too. Metal wears, just more slowly than pads.

What a worn rotor looks and feels like

Park Tool gives a plain test. Run a pick or a paper clip across the braking surface. A rough feel, or a step at the end, points to a significant amount of thinning.¹

A bent rotor is a second problem. Park Tool says it can be heard as a rhythmic rub or noise as the wheel turns, or seen as the rotor moving sideways as it passes through the caliper.¹ A bend isn't thinning. It's a different fault. Still, both can end in a new rotor.

Measuring wear

Measure the thickness of the braking surface. Then compare it with an unused part of the rotor, such as a spider arm.¹ Park Tool says typical measurable wear of 2 to 4 tenths of a millimetre or more is evidence of a worn rotor.¹

For accurate numbers, Park Tool recommends a caliper (the measuring tool, not the brake part).²

The minimum thickness

There's no single figure for every rotor. Park Tool says some manufacturers provide wear specifications.¹ Its example is that Shimano recommends replacement at no less than 1.5mm.¹ That's one maker's figure for its own rotors, so don't apply it to another brand. Park Tool adds: "Check with the manufacturer for brand-specific information."¹

So where do you find your number? Look at the maker's manual or support page for your rotor model. The saved sources don't say where each maker prints its limit, so use the maker's own documents rather than a guess.

Near the limit? Plan a replacement, and follow your manual for the work. Park Tool adds that with a worn-out rotor, you should check the pads too.¹

Changing pad type can mean a new rotor too

Thickness isn't the only reason. SRAM links a change of pad compound to a rotor change.³ It also warns that a new compound on a used rotor can bring noise and weaker braking.³ See pad types for how SRAM describes them.

Reading the numbers

Two figures matter and they're easy to mix up. One is what a rotor measures when it's new. The other is the lowest thickness the maker allows. Park Tool's example gives both for two brands, so you can see the gap between them.¹ Wear is the drop from the first towards the second.

Only the second figure is the limit. A rotor that has lost some thickness isn't automatically finished. Compare your measurement to the limit for your own rotor, and do it with a caliper that reads in fractions of a millimetre.

Before you order a new one

A new rotor needs to match the old one's mount and size. If either differs, read sizes and mounting first. Afterwards the new parts go through a bed-in.