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Adjusting A Rear Derailleur

person using a tool to adjust the rear derailleur on a red mountain bike

How To Adjust a Rear Derailleur

The rear derailleur is responsible for shifting between your sprockets, the rear set of gears on your bicycle. The derailleur consists of a guide or cage for your chain that can be shifted from side to side by means of a cable that follows the frame of your bike and attaches to your shift levers. The spring-loaded arm of the rear derailleurmaintains tension on the chain as it shifts between different gears.

On new bikes, these cables are fresh and may stretch a small amount over the first several months of riding your bike. This can cause your derailleur to become slightly loose and rub against your chain, which puts wear and tear on both parts and decreases the efficiency of your pedaling.

Fortunately, adjusting your rear derailleur is easy and only takes a basic set of tools and a few minutes. You may have to adjust the derailleur every few months if you ride a good deal, or every few years if you are a more casual rider, so it pays to learn this simple technique.

It is important to note that crossing the chain -- riding in the easiest chainring and hardest sprocket, or vice versa -- is generally a bad idea. This compromises the strength of your chain and can cause rubbing against the derailleur. If your chain is rubbing and you ride in these gears, its your shifting habits that needs to be adjusted, not the derailleur!

Also: if your rear derailleur is bent, you will need to bend it back into place or repair it. This procedure is not covered in this tutorial, so consult an expert if this is your situation. To prevent bent rear derailleurs in the first place, avoid dropping or resting your bike on the gear-side.

Tools You'll need for the Job

5mm Allen Wrench (hex wrench)

Small Philips-head screwdriver

Ruler or measuring tape (optional)

Time Required: 15 minutes

Step 1: Shift your rear derailleur to the easiest gear on the back cassette (this is the largest gear, or sprocket, in the rear set). This puts the derailleur in a relaxed position which makes for easier adjustments.

Note: some bikes, especially mountain bikes, have "high normal" derailleurs, which are relaxed when the chain is on the hardest rear sprocket (the smallest rear gear). Adjusting this type of derailleur is almost exactly the same as this guide -- except you will start at the opposite end of the cassette, with the chain on the smallest sprocket. You will also adjust the "L" screw before the "H" screw.

Step 2: At this point, let's make sure the gap between the derailleur's top pulley (also known as the "guide" pulley) and the teeth of the rear sprockets is sufficient. If there is not enough space, the derailleur will rub and make a loud noise when pedaling in this gear. If this space indeed needs to be adjusted, look for a screw marked with the letter "B" adjacent to it. Turn this screw clockwise to increase the spacing until there is 3-6 millimeters of space between the guide pulley and the sprocket; if there is too much space, turn the "B" screw counter-clockwise until the spacing is accurate.

Step 3: Find the cable exiting the rear of the derailleur mechanism. This is the shifter cable. Where the cable enters the derailleur, there is a ribbed "barrel adjuster." This can be turned counterclockwise to increase the tension on your shifting cable, or clockwise to decrease tension. Turn the barrel adjuster clockwise now until the cable is relaxed, with no tension.

Step 4: Now we need to adjust the derailleur's outer limit using the "H" screw. Shift into the most difficult rear gear (the smallest sprocket). Stand behind your bicycle and crouch down to look at the derailleur and sprockets from the rear. The guide pulley should be vertically aligned with the smallest sprocket. If the guide pulley is too far towards the frame of the bike, shifting won't be smooth. In this case, loosen the "H" screw by turning it counterclockwise. If the guide pulley is too far away from the frame of the bike, the chain may fall off entirely. Correct this by tightening the "H" screw (turn clockwise) until the teeth of the pulley line up with the teeth of the smallest sprocket.

Step 5: Now we have to re-tighten the barrel adjuster. Turn it counterclockwise to increase the tension, then try shifting a few gears. Repeat this process and fine tune the tension on the shifting cable until the chain moves smoothly.

Step 6: Now, shift to the largest sprocket. As in step 5, assess the alignment of the guide pulley and the sprocket. If the guide pulley is too far towards the frame of the bike, the chain could fall off and even destroy the spokes of your wheel. To remedy this, tighten the "L" screw by turning it clockwise. If the guide pulley is too far away from the frame of the bike, it may be difficult to shift into this gear. Correct this by loosening the "L" screw (turning counterclockwise) until the teeth of the pulley line up with the teeth of the sprocket.

Step 7: Almost done! Now is the time to fine-tune everything. Shift throughout the range of your gears. There may be problems shifting down, to easier gearing on the larger sprockets, or up, to harder gearing on the smaller sprockets. Now, use the barrel adjuster screw to adjust the tension on the shifting cable to fix this. Turn the screw clockwise in small increments to remedy problems shifting into the smaller sprockets. If you are having trouble shifting in the other direction, into the larger sprockets, turn the screw counterclockwise.

Step 8: If you have problems, review the instructions here or consult a professional bike mechanic. Enjoy your new smooth-shifting bike!