Bike Chains
The weakest component of a bike is the chain. The chain stretches with use, and as it stretches the gears are gouged out to match the stretched chain. Eventually the chain starts skipping. When you replace the chain, the gears are often too damaged to work, and the new chain skips. It's hard to tell which gears are damaged and need replacing, but you may need to replace both the cogset and crankset.
When to buy a new chain? Better sooner than later. Think of it like changing oil in your car. Cyclists training to race install new chains as often as monthly. A commuter should change chains at least several times a year. You should check for chain stretch every several hundred miles. It is a simple procedure your mechanic can handle in minutes. To check at home, remove the chain and stretch it on a flat surface. Each pivot on a new chain is one half inch apart. Using a ruler, measure from center of pivot to center of the pivot one foot away. If the measurement is approaching one sixteenth of an inch longer than a foot, replace it. If it is one eighth inch too long, your chain ring and cogset are probably ruined.
If your bike throws the chain off a front ring when it should not, it might not be a problem with your chain. It could be a chain line problem.
This chapter is divided into several sections: