When to replace a bike cassette, and whether to replace it together with the chain

How to tell if a cassette is worn

The Park Tool, SRAM and Shimano pages we read give neither a limit nor a distance for the cassette, so you have to look at it. These signs are practical judgement; no maker we read publishes them as a spec:

1. Fit a new chain and pedal hard in the cogs you use most. If it skips under load, the cassette is the first suspect.
2. Look at those cogs in good light. Teeth shaped like hooks or shark fins point to wear.
3. Compare them with a cog you barely use: the difference is easier to see.
4. Before you blame the cassette, check that the shifting is adjusted and the chain is clean and lubed, the same check Park Tool asks for before calling a chain worn.

TOUR describes the same failure: on heavily worn cogs, a new chain jumps over the tips of the teeth as you pedal.

A worn chain is what wears out a cassette

Park Tool puts it this way: a worn chain meshes poorly with cogs and chainrings, shifts badly and wears them out early. Shimano says the same. SRAM, for its Eagle groups, adds that replacing the chain before 0.8% and cleaning it routinely does the most to extend cassette and chainring life.

So looking after a cassette starts with measuring the chain and cleaning and lubing it. The limits depend on the source: Park Tool gives 0.5% at 11 speeds or more and 0.75% up to 10; SRAM gives 0.8% for its Eagle and AXS chains. Park Tool reminds you that a cassette costs far more than a chain, and TOUR that replacing the chain late multiplies the cost.

New chain on an old cassette, or both together

You can put a new chain on a used cassette: it is the most direct way to find out what state it is in. If it does not skip under load, that gives you no reason to replace the cassette.

Should you replace them together? The sources do not agree. SRAM explains that on AXS the chain, cassette and chainrings wear together, asks you to check the manual for the other parts, and says replacing all three can be the way to restore performance. TOUR's cost estimate assumes a new cassette after two to four chain lifetimes, depending on the design, which means several chains per cassette. That is an assumption for road bikes, made within a chain test. Our own judgement: if you measured in time, try the chain on its own first.

TOUR pointed out another difference in 2023: Shimano sells single cogs, while SRAM's Red cassette is milled from one piece, so one broken cog means a whole new cassette. Check what is available for your model.

Chainrings: when to replace them

The pages we read give no distance for chainrings either. SRAM says that on Eagle the chainring and chain are the first to show wear, and Park Tool includes chainrings among the parts a chain past its limit wears out early. What to look for, as practical judgement:

• Teeth shaped like hooks or shark fins, as on the cassette.
• A chain that clings to the chainring when you shift under load: Park Tool lists this as a possible sign of a worn chain, so measure the chain first.
• Check the rings when you replace the cassette, since SRAM says that on AXS the chain, cassette and chainrings wear together.
• TOUR's estimate assumes new chainrings after three chain lifetimes. That is an assumption in its cost sums, with no measurement behind it.

What the 12,000 km and 18,000 km in Pedalytics are

They are prompts to inspect, with low confidence, and they say nothing about service life. No maker we read publishes them. The cassette figure comes from three 4,000 km chains. The chainring figure is a prompt to look at the teeth, and it works out at four and a half chains.

A made-up case shows the weakness: someone riding through a lot of mud who hits the chain's limit at 2,000 km has used three chains by 6,000 km, so a 12,000 km prompt would come too late. TOUR assumes two to four chains per cassette, a range that includes our three; for chainrings it assumes three, and we use four and a half. These are different assumptions, and none of them measures your drivetrain.

In Pedalytics

Pedalytics is not out yet: it is listed as «Coming soon». Each ride stores a snapshot of the setup and adds wear to the parts that were on the bike, so the cassette and chainring keep adding up when you swap the chain. The app warns between 80% and 90% of the 12,000 km and 18,000 km, as a reminder to inspect. A part can start as new, used with its distance, or unknown; if it is unknown, the percentage is a minimum.