Is your shifting smooth? Are you wanting to shift and nothing is happening? What if your derailleur is shifting twice instead of once. What about that weird clicking sound? These are all signs of a derailleur that needs adjusting.
Derailleur adjustments can be intimidating for someone who has never done it before. I'd like to clear up an fear you have with this repair process by giving you a quick rundown, making things a little easier to understand. Watch this short video and read the instructions below in order to familiarize yourself with the parts you will be adjusting. This will be directed toward Shimano drivetrains, but most of the knowledge carries over to other manufacturers.
Before starting anything. Make sure to check if your derailleur hanger is not bent. The hanger is what attaches your rear derailleur to the frame. It is removable and replaceable, but if it is damaged it needs to be replaced.
Step 1: Make sure the rear derailleur is shifted all the way down into the smallest cog.
Step 2: Turn your barrel adjuster all the way tight so that you have room to adjust it later.
Step 3: Adjust the high limit by rotating the screw on your derailleur marked “H” so that it lines up with that smallest cog.
Step 4: Tighten the tension in your cable by unscrewing the cable anchor, pulling the cable tight and screwing the anchor back on tight.
Step 5: Shift your rear derailleur into the 4th or 5th cog.
Step 6: To adjust the index, turn your barrel adjuster so that the derailleur pulley lines up underneath the correct cog. With a Shimano rear derailleur, you want to line up the pulley slightly inboard of the cog. Do some practice shifts to make sure it is adjusted properly.
Step 7: Shift your rear derailleur into the largest cog.
Step 8: Adjust the low limit by rotating the screw on your derailleur marked “L” so that it lines up with that largest cog.
Step 9: Adjust the B-tension screw so that the pulley on the rear derailleur is as close to the largest cog on the cassette as possible without dragging.