Derailer Adjustment
A variety of things may cause the derailleur to skip. Take the following measures in the order they are listed. Test the derailleur before taking the next step, which may subsequently become unnecessary.
- Replace the chain. The problem may be a stretched chain, which would cause skipping as the chain link spacing would no longer match that of the chain ring or cog spacing.
- Increase the spring tension in the derailleur. You may need to ask for help to do this. There should be a screw on the derailleur designed for this purpose.
- Replace the derailleur.
- Replace the chain ring or cogs. This should only be necessary if the chain rings or cogs are "u'ed out." Being u'ed out means that they are worn in a way that makes shifting uneasy. This usually looks like more like the silhouette of a Nike swoosh (in the sense of the Goddess of victory, not the shoe company, of course) than an actual "u." The wear will be toward the pull of the chain.
If you cannot go into your highest or lowest cog or chain ring
This is most likely a result of an improperly adjusted limit screw. Determine the appropriate scew either by its label (H or L for High or Low), a visual check to see which screw is touching the lever arm, or by watching the derailleur as you play with the screws to see which one moves it (be sure to readjust the opposite screw when you're finished with this approach). Once you have determined the appropriate screw, loosen it until it allows the chain to move onto the highest or lowest cog or chain ring. Run the bike through a couple of rounds of shifting to ensure that the chain will not fall off the cog or cassette now that the limit screw is loosened.