Sprockets and Chain
Check the sprockets occasionally for damage and wear (see Figure 5-11). Like wheels, sprockets should be true. If sprockets appear to wobble as they turn, they may have been bent in a fall. Even slightly bent sprockets may not catch the chain properly or may cause undue wear on the chain. While it's possible to repair a bent sprocket yourself by banging it back into shape with a hammer, you're just as likely to damage it as to fix it. Your best bet is to take your bike into the bike shop for sprocket repair or replacement.
A worn chain is the most common cause of worn sprockets. Chains that have been stretched will not fall precisely between the sprocket teeth. Over time the chain will wear down the sprocket teeth on one side, causing the sprocket teeth to deform into a wave shape. The only way to prevent excessive wear on the sprockets is to replace your chain before it becomes stretched. Once a sprocket becomes worn it must be replaced.
Use sprocket or freewheel removers to pull a sprocket off the freewheel. If the sprockets on your freewheel or crankset cannot be removed individually, you may need to replace the entire set of sprockets. To avoid replacing a complete set of sprockets, it's best to have individually removable sprockets. This way you'll only need to replace those that are more susceptible to bending (outside sprockets) and wear (small sprockets).
Figure 5-11 SPROCKET CONDITION
Whenever your chain starts to squeak or feel dry (every hundred miles or so) you will need to relubricate it. Check lubrication regularly by touching the chain lightly with your finger or a rag. If little oil or grease comes off, the chain probably could use some more lubrication. It is also necessary to relubricate the chain whenever you have cleaned or replaced the chain.
A spray lubricant will create the least mess and ensure that the correct part of the chain is lubricated. Spray the lubricant lightly and evenly onto the roller pins of the chain. Be very careful not to get any lubricant onto the spokes or rims because oil could make the brakes unworkable. Use a rag to wipe any excess lubricant off the chain and chain stay.
Chain Removal and Reinstallation
To replace the chain or to clean it completely, you will need to remove it from the bike. Single-gear children's bikes often have a master link that easily comes undone. On multigear bikes, though, it is necessary to remove one of the pins that holds the chain links together. To do this, you'll need a chain remover or rivet extractor (see Figure 5-12).
Figure 5-12 CHAIN REMOVAL
Extracting a pin from the chain may ruin the pin, so make sure you have a replacement pin to take its place. Some chains are easier to undo than others—higher quality chains with special high-tension links tend to be more susceptible to damage. Be careful not to damage the links while extracting the pin. To remove the chain, follow these steps.
Shift gears so the chain rests on the smallest chainwheel and smallest cog.
Align the chain remover so the chain runs through the tool and one of the pins is in line with the punch screw.
Screw the punch arm into the pin and push it through the chain links until the links come undone. Try not to push the pin all the way through; leave it sticking out the other side.
Retract the punch arm to release the chain links.
Slowly turn the pedals as you pull the chain free of the drivetrain.
When you reinstall the chain it's a good idea to put it back on inside out to switch the side that gets worn. To reinstall the chain, follow these steps.
Lay the unattached chain onto the smallest chainwheel and smallest cog, then weave it through the derailleur wheels.
Reconnect the end links at the bottom of the chain run. The extracted pin should be sticking out to the right, away from the bike. Hold the links in position while you set the chain tool in place around the link.
Screw the punch arm into the extracted pin to push it back into the chain. Continue twisting the arm until the pin returns to its original position.
Under stress, chain links become slightly stretched and no longer thread smoothly onto the sprockets. In extreme cases, chains start to skip sprocket teeth, causing a harsh crunch sound during heavy pedaling. Replacing the chain every few thousand miles ensures the chain remains the proper length and the sprockets don't become worn.
To test the chain for wear, try to lift the links off the chainwheel. If the chain does not lift, there is no need to replace the chain. If, however, you can lift the chain off the chainwheel even a few millimeters, it should be replaced. If the chain has worn to that point, chances are good some of the sprockets will need to be replaced as well.