How To Index Your Gears - Adjusting Your Rear Derailleur
Why Bicycles Stack Exchange recommends it
Bicycles Stack Exchange answerers point people to it when gears skip, shift poorly or overshift, since it shows barrel adjuster and limit screw adjustment step by step.
Recommended by 6 people across 8 different months, 2015 to 2019.
What it is
A Global Cycling Network maintenance tutorial on indexing a rear derailleur. It explains how cable tension and the barrel adjuster fix skipping or unresponsive shifts, and how the limit screws stop the chain dropping off the cassette at either end.
What people said
Recommends David Richerby · 2015-12 · Bicycles Stack Exchange · on “gears overshifting issue”
If adjusting the indexing caused you to lose gears at one end, you probably need to adjust the limit screws, too. There's a GCN video [this video] on what you need to do.
Recommends c10yas · 2016-06 · Bicycles Stack Exchange · on “Changed shifters and chain isn't in same spot”
This happens everytime one replaces a component of the drivetrain since the original cable position is rarely restored. He needs to do something called indexing the gears. It is a common procedure and there are many good tutorials online explaining how to do it much better than I could. Here is one I'd trust-
Recommends John Hunt · 2017-01 · Bicycles Stack Exchange · on “MTB wont shift into 7”
There are many possible causes for this: Cable tension needs adjusting (see Virus_7's answer) Bent mech hangar / bent mech Worn cable Dirty derailleur Broken shifter See this video:
Recommends David Richerby · 2017-08 · Bicycles Stack Exchange · on “How do I tune slipping gears?”
GCN has a video on adjusting rear derailleurs [this video] . They show it on an 11-speed derailleur on a road bike but the principles are exactly the same.
Recommends Baratier ErebusDuHalm · 2017-09 · Bicycles Stack Exchange · on “Rear derailer/chain skipping/shifting issues”
I think your indexing might be off. Before trying Argentis answer, try adjusting the derailur's barrel adjuster to get the jockey wheels in line with the cogs. Watch this for reference
Recommends Carel · 2018-09 · Bicycles Stack Exchange · on “What does this barrel adjuster do on the rear derailleur (SRAM Apex 1)?”
Better than a long answer watch this video for a comprehensive explanation of the settings of the RD:
Recommends David Richerby · 2019-07 · Bicycles Stack Exchange · on “Chain getting in between sprocket and rear dropout”
Yes, you need to adjust the limit screws of the derailleur so it can't pull the chain farther over than the last cog. There are various videos on YouTube, such as GCN's:
Recommends Andy P · 2019-08 · Bicycles Stack Exchange · on “Mountain bike not shifting properly”
It sounds like after the bike has been in the shop the gears work correctly for a short time, but then the indexing goes out again. In my experience, the most likely cause of this is poor quality cables. Usually after cables are fitted they stretch and need the tension adjusting, but after this tend to be problem free. However I have had experiences on a cheaper bike where the cable would just keep stretching - in the worst example it took 3 months of being adjusted twice a week before they finally started to behave. I highly recommend learning how to index gears yourself as it's fairly easy to do and saves you a lot of time/money going to the bike shop. GCN do a whole range of bike maintena