The short answer
- A chain doesn't stretch, it wears — and the pitch grows. That growth is what you measure, and it's the only honest way to know.
- Kilometres tell you nothing. In one South African thread riders reported the same job at 500 km and at over 10,000 km. Measure it instead.
- Lay a ruler along 12 links. New, that's 304.8 mm. Past 306.4 mm the chain is due. Past 308 mm you're probably buying a cassette too.
- There isn't one percentage. The general rule is 0.5% on an 11-to-13-speed chain; SRAM publish 0.8% for their own 12-speed. Both are right, for different chains.
- Catch it in time and one cassette outlives several chains. Leave it and the chain takes the cassette, then the chainrings, with it.
- Not sure? Bring it in. We still don't sell a chain-wear gauge, and measuring it on the stand takes a minute.
The question usually arrives late. Someone comes in because the gears have started skipping under load, and by then the chain isn't the problem any more — the cassette is. A chain is a wear part. It's supposed to be replaced. The whole trick is replacing it before it takes the expensive parts with it.
So: what wear actually is, how to measure it on your kitchen table with a ruler, what the numbers mean, and what it costs if you leave it. I dodged the percentage question in the piece on chain lube because it needed more room than a paragraph. This is the room.
"Chain stretch" is the wrong name for it
Nothing on your chain is being pulled longer. BikeRadar put the mechanism plainly: "Chain wear is commonly referred to as 'chain stretch', because the chain's pitch grows in length as it wears… the growth comes from the bushings wearing with the chain pins." The inner diameter of the bushings opens up, the pins groove out, and every link ends up fractionally longer than it left the factory.
Fractionally is the word. A new chain has a half-inch pitch — 12.7 mm from pin to pin — and the point at which it's finished is roughly one per cent longer than that. One per cent of 12.7 mm is a tenth of a millimetre per link. You cannot see it. You can only measure it, which is why people ride on a dead chain for months without noticing anything except that shifting has gone a bit vague.
The quoted advice on measuring below is all from that same guide. Where a component maker publishes a figure for their own chains, I've used theirs instead and said so.
Why a tenth of a millimetre costs you a cassette
Here's the part worth understanding, because it explains why the timing matters more than the part. A new chain sits deep in the valley between two teeth. As the pitch grows, it stops fitting. BikeRadar again: "As the pitch increases, the chain rolls higher up on the tooth and causes rapidly increased cog wear as the point of contact is reduced."
So a worn chain isn't just a worn chain. It's a tool that is slowly re-cutting the teeth on your cassette to match its own wrong spacing. Do that for long enough and the two only work together — which is why a brand-new chain on a worn cassette can skip, and why the bill arrives all at once.
SRAM say the same thing in their own maintenance notes, and they're the ones selling the parts: "Replacing the chain before it reaches 0.80% of wear and routine cleaning is the best way to extend cassette and chainring life."
The upside of getting it right is real. Replace on time and "it's possible to run multiple chains on your bike without having to replace your cassette or chainrings". Two or three chains through one cassette, instead of both at once, every time.
What that's worth depends entirely on what you're running, so here are two real numbers off our own shelf rather than a made-up saving. A 12-speed Shimano chain is R999 and a 12-speed Shimano cassette is R2,100. Catch the chain at the right moment and you spend the first number; miss it and you spend both. That arithmetic doesn't hold everywhere — our cheapest 8-speed cassette is R450, which is less than some of our chains — but on a modern 12-speed drivetrain it holds comfortably.
How to measure it with a ruler, in millimetres
Every guide on this subject gives you the measurement in inches and sixteenths, which is no use to anyone who owns a South African ruler. So here it is in millimetres, converted from BikeRadar's figures and nothing else.
The method itself: "a new chain should measure exactly 12 inches across 12 links, from middle of pin to middle of pin". Twelve inches is 304.8 mm. The two figures that matter after that are 12 1/16in and 12 1/8in, which come out at 306.4 mm and 308 mm.
Three things that ruin the reading
Measuring across the quick link. Skip it. Quick links "wear slower than conventional links", so a section of chain containing one will read healthier than the chain actually is.
Measuring once. Chains don't wear evenly. Take the reading in "at least three different parts of the chain" and believe the worst one.
Measuring a filthy chain. Grit packed into the rollers fills the gaps you're trying to measure, so a dirty chain flatters itself. Clean it first — and while you're there, the dirt is also the cause: it "can act as a grinding paste in the drivetrain and accelerate the rate of wear".
If you own a chain checker, use it — it's quicker and it gives you a straight go or no-go. We don't stock one, so I'm not going to send you shopping for a tool when a ruler answers the same question. One warning if you do buy one: not every checker fits a SRAM Flattop chain, because "the chains use oversized rollers".
Four minutes from the Global Cycling Network on making the measurement, if you'd rather watch it than read it.
The percentage depends on whose chain it is
This is where most of the confusion on the internet comes from. People quote a number without saying which chain it belongs to, and the numbers genuinely differ.
The general workshop rule, by speed: "A 6 to 10-speed chain should be replaced when it measures 0.75 per cent on a chain checker. If it is stretched to 1 per cent, the cassette should be replaced with it. 11 to 13-speed chains should be replaced at 0.5 per cent on a chain checker." Tighter tolerance, narrower chain, less room for error — that's why the modern number is the strict one.
Then there's what the manufacturer says about their own product. SRAM publish "Replace the chain when it reaches 0.8% on an approved chain checker" for their 12-speed Eagle mountain chains, and the same figure for their 12-speed road and gravel chains: "Flattop chains last longer than 10/11 speed road chains and should be replaced when they measure .8% on an approved chain checker."
So a 12-speed chain is either due at 0.5% or at 0.8%, depending on whose it is. That isn't a contradiction, and it isn't something to average out. BikeRadar flag it themselves — "the manufacturer's recommended wear limit can differ from the above".
My advice, and it's the boring one: if the maker of your chain publishes a figure, that's your figure. If they don't, or you don't know what you're running, use 0.5% and you will never be wrong in the direction that costs money. Replacing a chain slightly early costs you a chain. Replacing it late costs you the drivetrain.
Forget the kilometres
Everyone wants an interval, and there isn't one. "Riding distance is not an accurate indicator of chain wear" — and the spread in practice is wider than most people expect.
There's a thread on Bike Hub, the South African cycling forum, where riders compare what they actually got out of a drivetrain. One rider: "i've killed SLX chains in 500km." Another rider, on an entry-level 12-speed Deore setup, reported over 10,000 km. The verdict in between is the useful bit:
Conditions matter. Type of soil/dust, lube, cleaning, vertical climbing meters, weight, type of bike, type of riding, etc etc etc.
That's a twentyfold range on the same question, from riders in this country on the kind of bikes we sell. It's why I won't give you a kilometre figure across a counter, and why a measurement settles in a minute an argument that a number never will. If you ride the sandy stuff behind Hout Bay through summer and don't wash the bike, you're at the fast end of that range. If you're on tar in the dry and you look after it, you're at the slow end.
What if it's already past the line?
Depends how far past. Slightly over, and you may get away with the chain alone: "fit a new chain and if you experience any skipping on the cassette or chainrings, you know the old chain has worn the cassette and chainring and these now need to be replaced". The new chain is the test. It either runs quietly or it jumps under power on the cogs you use most.
Well past, and you can see it without measuring anything — "the teeth of the cogs will begin to look hooked", leaning over like a wave about to break instead of sitting symmetrical. At that point the chain on its own isn't an option and you're replacing the cassette, and often the chainrings, with it.
And don't be tempted to leave the new chain on the old cassette to save money now. It goes the other way: "If you continue to run a new chain on an old cassette or chainring, the chain will wear prematurely". You buy the same cassette in the end and a spare chain on top of it.
If you're fitting it yourself
Two things, both of which come up every week. The first is that a cassette needs a lockring tool and a chain whip — you cannot undo one with what's in a multi-tool, and improvising usually ends with a scarred cassette body.
The second is the quick link, and it's the one people get wrong after doing everything else right. SRAM are unambiguous about theirs: "The PowerLock is designed for single use only… Install a new PowerLock each time a chain is installed." A new PowerLock is R45. A chain that comes apart under load on Suikerbossie is not.
Common questions
How do I know if my bike chain needs replacing?
Measure it. Lay a ruler along twelve links, pin centre to pin centre: a new chain reads 304.8 mm, and past 306.4 mm it's due. Do it in three places on the chain, skip any section with a quick link in it, and clean the chain first — a dirty one reads better than it is. Without a ruler there are two symptoms worth taking seriously: the chain slipping when you stand on the pedals, and chain suck, where it clings to the small chainring and gets dragged upwards. Either one means it's already late.
How often should you change a bike chain?
Whenever the measurement says so, which for most people is more often than they'd guess. There's no calendar answer, because the rate depends on where you ride and how clean you keep it. The sensible habit is to check it every few weeks — it takes less time than pumping your tyres — and to check it after any wet or gritty spell, which in Cape Town means most of the winter.
How many kilometers can a bike chain last?
Anywhere from a few hundred to more than ten thousand, which is exactly why nobody sensible answers this with a number. On one Bike Hub thread South African riders reported killing chains in 500 km and getting over 10,000 km from an entry-level 12-speed setup. Soil, dust, rain, how much climbing you do, your weight and how often you clean the thing all move it. Distance is a prompt to check, not an answer.
Is it worth replacing a bike chain?
Yes, and it's the cheapest maintenance decision on the bike. A chain is a consumable that protects two things that cost much more: a 12-speed chain on our shelf is R999 against R2,100 for a 12-speed cassette, and chainrings are dearer again. Replace on time and one cassette will see out several chains. Replace late and you buy all of it at once — plus you've ridden the last few hundred kilometres on vague shifting and a chain that's measurably weaker than it was.
Bring it in and we'll measure it
If you're not sure whether your chain has had it, that's a minute on the stand and we'd rather check it than have you guess. We'll measure the wear, tell you honestly whether it's the chain alone or the chain and cassette, and if it's the chain alone we'll say so — that's the cheap answer and it's usually the true one if you've come in early enough.
We're at 35 Main Road, Scott Estate, and the chains and cassettes are on the drivetrain shelf if you'd rather look first. Prices in this article are what we charge today; they move when our suppliers move them.