How to check and replace your bike chain

Why chains wear out (and why it gets expensive)

A chain is 100 to 120 links rubbing against each other with every pedal stroke. Kilometre after kilometre, the pins erode and the chain gets longer. What people call "stretch" isn't really stretching: it's play that develops between each pin and each roller.

As the chain lengthens, it no longer meshes properly with the cassette and chainrings. It starts carving its own grooves into the teeth. The result: the cassette and chainrings wear fast, and a brand new chain will skip on them. Replacing a worn chain at the right time costs 30 euros. Ignoring it for a few hundred kilometres can add 150 more in cassette and chainrings.

So the real move is to measure regularly. Here's how.

How to monitor wear

One method measures actual wear, the other predicts when to check. They complement each other.

The chain checker (Park Tool CC-3.2, Shimano TL-CN42) is the reference tool. Drop it between two links: at the 0.5% mark, the chain is starting to tire; at 0.75%, time to replace; at 1%, the cassette is probably already cooked. Note: these thresholds are specific to Park Tool and Shimano models. Other checkers use different graduations, so always check the instructions.

Mileage tracking doesn't measure the chain itself: it predicts when it'll likely need checking. By default, ChainLog alerts around 3,000 km. A clean, well-maintained road chain can reach 5,000 km; a muddy gravel or MTB chain gives up much earlier.

When should you actually replace it?

Simple rule: replace the chain as soon as it passes 0.75% on the checker. Below that, it's still doing its job without damaging the rest. Above, every extra kilometre eats into the cassette.

And if you're already past? Two scenarios:

• Checker between 0.75% and 1%: replace the chain alone, ride 100 km, check that the new one doesn't skip under load. If all good, you got away with it.
• Checker past 1%, or a new chain that skips: the cassette needs replacing too. Chainrings often survive one more cycle, but not always.

In practice, on a well-tracked road bike, you'll replace the chain 2 or 3 times before replacing the cassette. On an MTB or a commuter exposed to the weather, the ratio can drop to 1:1.

How to replace your chain (without the hassle)

With basic tools, the swap takes about twenty minutes.

What you need: a new chain in the right speed count, a chain tool (~€15), a compatible quick link (or two links + pin if your chain comes with one), and a rag.

For chain length, the easiest method is to lay the new chain next to the old one on a flat surface and cut to the same number of links. From scratch, the reference is: big ring + big sprocket + 2 links, without routing through the rear derailleur.

For direction, most modern chains (Shimano 11-12sp, SRAM) have an install direction. Engraved logos must face outward. Check before installing the quick link.

The quick link goes on by hand first, then you "pop" it by pressing on the pedals (brakes held). It should click crisply and sit perfectly flush with the rest of the chain.

Small detail that matters: some quick links are single-use (SRAM PowerLock, Shimano Quick-Link 11/12sp). Throw them away after removal. Others are reusable (KMC Missing Link). Check the packaging.

The default intervals we recommend

At ChainLog, we start every bike type from the same interval by default. Not an absolute truth, just a cautious starting point you can adjust in the settings.

These numbers are deliberately conservative: we'd rather trigger a check a bit early than a bit late. The 200 to 500 km "lost" by replacing too early cost nothing; 200 to 500 km too many can cost you a cassette.

In practice, real chain life varies wildly: from 2,000 km on a muddy winter MTB to over 5,000 km on a dry, well-maintained road bike. To match the interval to your riding, just tweak it in the bike settings; the checker stays your safety net.

Two levers that can double a chain's life: a lubricant matched to conditions (dry lube for the road, wax or wet lube for rain) and regular cleaning to remove the abrasive paste that builds up between the rollers. Worth a dedicated article, probably the next one.

How to tell if your cassette needs replacing too →

Automatic tracking, so you don't have to think about it

Pulling out a chain checker every month on every bike: in real life, nobody does that. What works better: get reminded at the right time, and only then pull out the tool to confirm.

ChainLog connects to Strava (or Geovelo, or manual mode), counts kilometres, and alerts you when your chain approaches the threshold. You check with the tool, replace if needed, log it in the app, and the counter resets.

Not a perfect measurement. Just a reminder that lands at the right time.