Your chain is under constant pressure as you pedal. A new chain arrives exactly the right size to mesh with the other components of your drivetrain. Gradually, though, as time goes by and the miles rack up, the chain stretches. The gaps between each link grow and the chain inevitably elongates. Eventually, if you keep riding, the chain starts skipping over them instead of meshing with the teeth on the sprocket.
Joining the chain gang
If you are disciplined about measuring your chain carefully and regularly with a chain-measuring device, you can replace just the chain before it has a chance to wear the other components of your drivetrain. This tool will tell you when you have reached this point. If you are attentive, you’ll find it the cheapest option in the long term If you allow the chain to wear beyond this point, you will have to replace both the chain and the cassette at the same time. The old chain will have damaged the teeth on the cassette, so the new chain will be unable to mesh with it neatly. The consequence of changing the chain without changing the sprockets is that the new chain will slip over the old sprockets, and, even if you can make it catch, the old sprockets will wear the new chain into an old chain very quickly. If you allow the chain to wear so that it starts to slip over the cassette as you pedal, you will definitely have to change the cassette and probably some or all of the chainrings as well. Look at the pictures
on page 140 and compare them with your chainrings – if they are starting to look like the examples, change them at the same time as the chain.
Note that you cannot compensate for chain stretch by taking links out of the chain to make it shorter. The total length of the chain is not critical. It is the distance between each link that matters. If you take links out of a stretched chain, it is simply a shorter stretched chain.
“You cannot compensate for chain stretch by taking links out of the chain to make it shorter”
Step 1: A chain-measuring device is the quickest and easiest way of accurately measuring your chain. The best are from Park Tools and come complete with an easy-to-read dial. Buy one today – it saves you time and money.
Step 2: Alternatively, measure the length of 12 links. Twelve links of a new chain will measure exactly 12 inches. When it measures 121⁄8 inch or less, you can change the chain without changing the cassette. More than that, and you have to change the cassette as well.
Step 3: You can get an idea of stretch without using a chain-measuring device. Put the chain on the biggest ring at the front and the smallest sprocket at the back, then lean the bike lefthanded against a wall. Hold the chain at three o’clock and pull it outwards. If the bottom jockey wheel of the rear derailleur moves, it’s time for a new chain. If you can pull the chain off enough to see all or most of the tooth, you need a new cassette and probably new chainrings too.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Deep-cleaning your chain”