If your bike hesitates in shifting up and down between the gears in the back (the cassette), you can make a simple adjustment by playing with the cable tension. This fix works for the majority of rear shifting problems. However, if you have a damaged derailleur (the part that moves the chain between different cogs), a bent derailleur hanger (the “fish hook”-shaped part that connects the derailleur to the rest of the bike), or are unable to do a simple fix, you’ll need to take your bike in for service.
To adjust the derailleur, find the round, knob-like piece where the cable enters the derailleur. This is the barrel adjuster, which is used to adjust the rear derailleur. Standing behind the bike, turn the barrel adjuster in half-turn increments until the problem is solved. Test the shifting between each adjustment.
The direction you’ll need to turn depends on the type of shifting problem you’re having. Most often, the bike stalls when shifting into easier gears due to the cable stretching.
To know which way to turn the barrel adjuster, remember this rule: if the shifting hesitates when moving into an easier gear (when the derailleur moves toward the spokes), tighten the cable by turning the barrel adjuster towards the spokes (counter-clockwise). If the shifting hesitates while moving into a harder gear (away from the spokes), give the cable more slack by turning the barrel adjuster away from the spokes (clockwise). You’ll know you’ve hit the sweet spot when you can shift up and down easily, with one gear shift per click.