Every bike has a chain, and they wear out. Road bike chains can get anywhere from 1000 to 2000 miles before needing replacing, which is once a year for many riders. What happens as a chain gets used, it stretches out and causes more wear on your cassette and chainrings. The spacing between the teeth on your gears and chain need to be very close. The rollers on a chain need to fit neatly onto the cassette. Shifting performance does not necessarily degrade as a chain wears, but the chain will literally jump off of worn gears. It will skip under pressure for sure and when it skips under light pressure, the damage has been done. You’ll need a new cassette at least at that point. People often describe this as “slipping” and can feel like the derailleur is skipping between gears. The chain is in fact pulling off of the cassette teeth. Chainrings wear much slower than cassette teeth, but need to be checked as well.
Worn out chains are also more flexible side to side and are more prone to breaking as well. Be proactive and keep a fresh chain to prevent expensive headaches later. Sometimes after changing an overly worn chain, you won’t know the condition of other components until you get on and start riding.
The question arises, how do I check my chain? There’s a tool for that. Chain checkers come in consumer grade “good or bad” and shop quality where it actually quantifies how far stretched a chain is. When a bike comes in for a tune-up, we check the chain for stretch since it is a fundamental part of the drivetrain. We’re happy to check your chain any time since it is quick and easy. Some chains will give you problems at a 0.75% stretch, and when it gets to 1.0% or more, it indicates that the rest of the drivetrain may be worn.