Dirty bike chains
A dirty bike chain may not seem like much of a problem, aside from being the cause of greasy chain tattoos on your calf. But a dirty chain is going to cause trouble down the road. As chains pick up dirt and grit, the parts they come into contact with wear out at a faster rate than they would if they were in contact with a clean chain. To keep your chain clean, regular maintenance is essential. Before and after each ride, lube the chain lightly with a chain-specific lube. Then run the chain through a clean, lightly moistened rag by holding the rag against the chain, lifting the rear wheel and pedaling the chain through several revolutions of the pedals. Once or twice a year, remove the chain and soak it in a degreasing solution to clean it thoroughly.
A stretched chain may look perfectly fine, but it can cause problems like skipping between gears and slippage when you apply firm power to the pedals. To find out whether your chain has stretched, place a ruler against your chain, with the pin of a chain link directly on an inch mark on the ruler. Count 12 links over. If the pin of that chain link lines up with an inch mark on the ruler, your chain is fine. If the pin is more than 1/8 inches out of line with the inch mark, the chain should be replaced.
A broken bike chain isn't necessarily worn out. On occasion, the pins that hold the chain together will break while the chain is essentially still good to use. Measure the chain with the method described in Section 2. If the length is OK, extract the broken pin with a bicycle chain tool and replace the bad link with a new one.