How to know when your bicycle chain needs replacing
A bicycle chain stretches as it wears. The links don't actually get longer. The pins and rollers wear down, increasing the distance between them. Once a chain has stretched beyond a certain point, it starts wearing the cassette and chainrings unevenly, and replacing those is significantly more expensive than replacing a chain.
How to measure chain wear
The standard tool is a chain wear indicator, which costs about $15 at any bike shop. Place one end in a chain link and check whether the other end drops into the next link. Most indicators have a 0.5% and 0.75% marker. At 0.5% wear, replace the chain. At 0.75%, the cassette is probably worn too and will need to come off with the chain.
If you don't have a tool, a ruler works. Measure 12 links from pin centre to pin centre. On a new chain this should be exactly 12 inches (304.8mm). At 12 and 1/16 inches, the chain is due for replacement. At 12 and 1/8 inches, the cassette is likely worn.
How often does a chain need replacing?
It depends heavily on conditions and maintenance. A chain that's kept clean and lubricated on a dry-weather bike might last 4,000-5,000km. The same chain on a year-round Vancouver commuter, ridden through rain and road grit without regular cleaning, might be done at 2,000km.
The single most effective thing you can do to extend chain life is wipe it down after wet rides and re-lube it every few weeks. A dry wax lube works well in summer; a wet lube is better for the rainy season.
What happens if you leave it too long
A worn chain riding on a worn cassette will feel fine. The two have worn together and mesh reasonably well. The problem comes when you put a new chain on a worn cassette. The new chain's pins sit in the wrong position on the worn teeth and skips under load, especially on the smaller cogs. At that point you need a new cassette too, which adds $60-$120 to the bill depending on the groupset.
We check chain wear as part of every tune-up. If you're not sure where yours is at, bring the bike in and we'll measure it while you wait.