A worn chain quietly wears out the cassette and chainrings with it. You do not need a special tool to catch it in time: a steel ruler with inch marks is enough.
Why chains wear, and why it matters
A chain does not stretch the way a rope does. What happens is wear inside each link: the pins and the insides of the rollers grind against each other every time the chain bends around a sprocket, and grit carried in by old lube works like fine sandpaper. Each link gets a tiny bit longer, and across a hundred or so links that adds up to a chain that is measurably longer than new.
A longer chain no longer sits cleanly in the valleys between the sprocket teeth. It rides up on the teeth instead, and as it does it wears them into a hooked shape that matches the worn chain. For a while everything feels fine, because the worn chain and the worn sprockets suit each other. The trouble starts when you finally fit a new chain: it does not fit the hooked teeth, and it skips under load, usually when you stand up on a climb.
That is why timing matters. Replace a chain before it has worn the sprockets and a chain is all you buy. Leave it too long and you are buying a cassette as well, and sometimes the chainrings too.
What you need
A steel ruler with inch marks, twelve inches long or more. Chains are built on a half-inch pitch: from the centre of one pin to the centre of the next is exactly half an inch on a new chain. Twelve inches therefore covers twenty-four links, which is long enough for small amounts of wear to show. A rag and a good light help too, because pins are hard to see on a dirty chain in a dark hallway.
How to measure
Shift onto the big chainring and one of the middle sprockets at the back, so the chain runs fairly straight with a little tension along the top run. Wipe a section of the top run clean so you can see the pins.
Hold the ruler on its edge along the top run, close to the side plates. Line up the zero mark with the centre of one pin. Keep the ruler still and look along it to the twelve-inch mark. On a new chain, the centre of a pin sits exactly on that mark. On a worn chain, the nearest pin sits a little past it.
Measure in two or three places along the chain. Wear is not always even, and the reading you act on is the worst one. If you find it hard to judge the centre of a pin, measure from edge to matching edge instead: the leading edge of one pin to the leading edge of the pin twelve inches along gives the same answer.
Reading the result
| Pin at the 12 inch mark | Wear | What to do |
|---|---|---|
| Exactly on the mark | None to speak of | Keep riding and measure again next month |
| A sixteenth of an inch past | About 0.5 percent | Replace the chain soon |
| An eighth of an inch past | About 1 percent | Replace the chain, and expect the cassette to need replacing too |
A drop-in chain checker gives its reading as a percentage, which is where the numbers you hear at Fix-up night come from: swap the chain at 0.5, and by 0.75 the sprockets have usually worn along with it. Treat a checker with some care, though. Most of them rest on the rollers as well as the pins, so roller wear shows up as extra length and they tend to call a chain more worn than it is. The ruler measures from pin to pin, which is the wear that matters to your sprockets.
When the new chain skips
If you have just fitted a new chain and it skips under load on the sprockets you use most, the cassette is worn. There is a simple test before you spend money: put the old chain back on for one ride. If the skipping stops, the old chain and the old sprockets have worn together, and the cassette needs replacing along with the chain. Look at the chainrings as well; worn teeth there look like shark fins and cause the same problem at the front.
Before you blame the cassette, rule out the easy causes. Check that the new chain is the right length, that the quick link is fully seated on both sides, and that the gears are indexed so the chain sits squarely on each sprocket. A half-seated quick link, or a gear cable that needs a quarter turn, can feel exactly like worn teeth.
Fitting a new chain
Buy a chain made for the number of sprockets on your cassette: an 11 speed cassette needs an 11 speed chain. New chains come long, so lay the old one beside the new one, link for link, and shorten the new one to match. If the old chain is lost or was the wrong length, wrap the new one around the biggest chainring and the biggest sprocket without threading it through the rear derailleur, pull it snug and add two links.
Most chains close with a quick link. Fit it with the link on the top run, hold the rear brake on and push down firmly on a pedal until it snaps shut. Some quick links carry an arrow for the direction of travel; follow it. Joining pins, used on some chains instead, need a chain tool and a steady hand, and they are well worth learning at Fix-up night with someone watching.
Then shift through every gear with the back wheel off the ground. The chain should climb and drop across the cassette without hesitating, and the rear derailleur should still hold it taut on the smallest chainring and the smallest sprocket.
Single-speed, hub-geared and e-bike chains
Single-speed and hub-geared bikes run a wider chain on a straight line, so it wears slowly, but the ruler test is the same and so is the half-inch pitch. A mid-drive e-bike puts far more force through its chain than legs alone: measure it twice as often and expect to replace it sooner. E-bike chains are built for that load and earn their higher price.
Making a chain last
Clean chains last longest. After a wet or muddy ride, wipe the chain down with a rag while you turn the pedals backwards, let it dry, then put a drop of lube on each roller. Wipe off whatever is left on the outside. A chain that looks wet on the outside is a chain collecting grit.
Measure every few weeks if you ride every day, and every month or two otherwise. It takes a minute with a ruler, and it is the cheapest maintenance there is. Once a month, add it to the five checks before every ride.
Not sure what you are looking at? Bring the bike and the ruler to Fix-up night on a Thursday and we will measure it together, then show you how to split and join the chain if it needs replacing.