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How to use a barrel adjuster to fix your derailleur on the fly.
Wouldn’t it be great if your bike shifted perfectly 100 percent of the time? Unless you ride a bike with a single gear, you’ve likely experienced the annoyance of a poorly shifting derailleur. Luckily there’s a good chance you can tune your rear derailleur on the fly without using tools, thanks to your bike’s barrel adjuster, which controls how much the derailleur moves between cogs when you shift. If you have a road bike with mechanical gears (not electronic), check your cable housing to see if there’s a barrel adjuster near your handlebars. If so, you can adjust your derailleur while riding. If you’re on a TT bike with bar end shifters, you’ll need to use the barrel adjuster near the derailleur.
If the shifting on your bike stutters while trying to shift into a bigger gear, back the adjuster out a click. If the problem occurs while trying to move into a smaller cog, turn the adjuster in a click. If you’re using the barrel adjuster near the derailleur (A), you want to make sure the derailleur pulley (B) is lined up directly under the cog. Shift into a gear in the middle of the cassette and eyeball your chain and derailleur. Turn the barrel adjuster in the direction you want it to move until it’s lined up.
You can make quick adjustments to your derailleur if you have electronic components as well. If you have a Shimano Di2-equipped bike, hold the adjustment button on the front wiring harness (C) until the red light comes on. Now your shifter will work like a barrel adjuster. Click the large shift button (D) if your shifting hesitates moving up the cassette or click the small shift button (E) if it misfires when moving down the cassette. One push of the shifter is about the same as a click of a barrel adjuster. Take the system out of adjustment mode by holding the button again.
If your bike continues to shift poorly after more than two or three clicks of your barrel adjuster, you may have a bigger problem, like a bent derailleur hanger or damaged derailleur. But before taking your bike into a shop to get worked on, try tuning your derailleur using the barrel adjuster to see if you can get your bike shifting smoothly.