ONE OF THE GREAT THINGS about modern derailleur drivetrains is that they're easily fine-tuned should the need arise. How do you know? Usually, the symptom that tips you off that adjustment is needed is hesitation during shifts. You click the shifter but the chain doesn't quite engage the next gear the way it used to. The most likely cause for this is a shift cable that has stretched slightly, which happens to all cables. When the cable stretches, it does not move the derailleur far enough when you click the shift lever. Here's how to adjust the derailleur so it shifts perfectly again:
The cool thing is, derailleur designers provide a simple way for you to dial in shifting. You don't even need tools (although, it's easiest to make and check adjustments when the bicycle is supported in a repair stand). Note also that we're assuming that your derailleur is not damaged or bent. If you suspect that it is, it needs more than this simple adjustment and you should bring your bike in to us for servicing (read the sections that follow for more information).
To adjust the derailleur, look at the point where the cable enters the rear derailleur in the photo. See that black round knob-like piece where the arrow is pointing? That's a barrel adjuster, which is used to tune the derailleur adjustment.
Standing behind the bike, the barrel adjuster is turned either counter-clockwise or clockwise in half-turn increments until the shifting hesitation is cured. Which way do you turn it? It depends on what type of hesitation you're experiencing. The most common problem is slow shifting into easier gears (toward the spokes) due to the cable stretching. But, it's possible that you're experiencing the opposite.
This rule will help you remember which way to turn it: If the derailleur is hesitating when shifting toward the spokes (the more common problem), turn the barrel toward the spokes (counter-clockwise); and if it hesitates shifting away from the spokes, turn the adjuster away (clockwise) from the spokes. (Always turn it only a half turn, shift multiple times to check the adjustment, and repeat as needed to cure all hesitation.)
Another adjustment needed is an "attitude" adjustment. It's important to always remember that the rear derailleur is fragile and must be protected. This is worth emphasizing because there are many times that the derailleur is at risk, such as during flat-tire repair (always lay the bike down gently on its left side so the derailleur doesn't touch the ground), while shipping a bike (shift onto the largest cog and pad the derailleur) and even parking your bike (make sure it can't topple). All it takes is the bike falling over for the rear derailleur to get hit and bent. Usually, we can fix the damage with special alignment tools. But, you can avoid the downtime by thinking of your derailleur as a delicate object and watching out for it.
If you do manage to crash or drop your bike and bend the derailleur, you might not notice. It's important to notice however, because once the derailleur is bent, bad things can happen such as shifting into the spokes, which may ruin the derailleur and might seriously damage the rear wheel and frame. Signs of having a bent derailleur include sudden hesitation shifting into harder gears and a clicking sound when you're on your largest cog (shift out of this gear immediately if you hear this sound because the derailleur is hitting the spokes and may get pulled into the wheel at any moment). Bring your bike in immediately for us to check it if you notice these problems.
When many people decide to adjust their rear derailleur, they mistakenly try to do it by turning the small screws on the back of the derailleur. These screws are related to derailleur adjustment, however, once they're set, which we do when we assemble the bicycle, they do not change adjustment. So, it's almost never necessary to turn them. In fact, if you do turn them, it usually worsens your shifting. So, leave these screws alone.