Derailer Adjustment

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A variety of things may cause the dérailleur to skip. Take the following measures in the order they are listed. Test the dérailleur before taking the next step, which may subsequently become unnecessary.

Ensure the dérailleur and chain are clean.
Ensure the dérailleur moves freely.
Check the chain for wear. 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. Replace the chain
Ensure there isn't a tight/frozen chain link.
When the dérailleur is at the highest gear setting (smallest cog) ensure the cable is loose enough to deflect but not so loose that moving the shifter doesn’t do anything. Adjust if necessary. Sometimes "loose" isn't loose enough.
Increase the spring tension in the dérailleur. You may need to ask for help to do this. There should be a screw on the dérailleur designed for this purpose.
Check dérailleur hanger for proper alignment vertical and horizontal. A bent dérailleur hanger can change the alignment resuling in the dérailleur not moving enough for an index shift to complete. You may need to ask for help to do this.
Replace the dérailleur.
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 = High gear/smallest cog or L = low gear/biggest cog), a visual check to see which screw is touching the lever arm, or by watching the derailer 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.

If your chain falls off when you try to go into your highest or lowest cog or chain ring

The most likely cause of this problem is a limit screw that is too loose.

Determine the appropriate screw 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 derailer 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, tighten it until the derailer moves directly above the cog or chain ring on which you wish the chain to ride. Run the bike through a couple of rounds of shifting to ensure that the chain will easily slide into the appropriate cog or chain ring without falling off.