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. There are several possible causes, the most likely being a shift cable that has stretched slightly, which happens to all cables.
Simple Adjustment The cool thing is, derailleur designers provide a simple way for you to dial in shifting so it works perfectly again. You don't even need tools. To make the adjustment, look at the point where the cable enters the rear derailleur. See that round knurled piece? 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. But, you might also be experiencing the opposite.
This rule will help you remember which way to turn it: If the derailleur is hesitating when shifting towardthe spokes, turn the barrel towardthe 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, check the adjustment, and repeat as needed to cure the hesitation.)
Protect That Derailleur Although it's not really maintenance, another thing to remember about derailleur-equipped bikes is 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, while shipping a bike and even parking your bike. 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.
Accidents Happen One of the most common derailleur accidents, especially for off-road riders, is falling over or dropping the bike and bending the derailleur. When this happens, 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 top 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.