The rear derailleur is the incredible mechanism that shifts your chain from cog to cog on the rear wheel. Like the front derailleur, it moves from side to side in response to shift commands from a gear lever. The rear derailleur also absorbs the chain slack that happens when shifting from large cogs to smaller ones. 

The rear derailleur should be cleaned, lubed and checked regularly to make sure it is in good working condition and properly adjusted.

To check the performance:

• Suspend your rear wheel off the ground. 
• Rotate your pedals while taking your rear shift lever through its full range of gearing options. 
• A properly functioning rear derailleur will shift the chain quickly and easily from cog to cog. 
• The cage or "hanger" of your derailleur should easily extend back and forth to take up or give any slack when needed.

Got a problem? Get the answer.

The rear derailleur typically malfunctions because:

1. It is dirty, needs lubrication or is damaged 
2. It needs to be adjusted

Like the front derailleur many rear derailleur problems are caused by simple dirt, grit or insufficient lubrication. Even small amounts of dirt and grit can cause problems, so keep it clean and re-lubricate every month or so, depending on riding conditions.

Clean the derailleur by brushing all exposed parts with a stiff brush. Stubborn dirt is best removed by wiping with a clean rag soaked in degreaser. Be sure to clean the derailleur mechanism thoroughly but carefully, including the hard to reach areas.

When re-lubing, focus on the moving pivots of the mechanism. It is best to use a spray lubricant designed specifically for bikes and, whilst lubing, shift the derailleur back and forth while spraying so you can work the lube into the tough to reach places. Wipe off excess lubricant.

When performing this vital function of Bike maintenance check that the derailleur is not bent or damaged in anyway. If it is take it to a competent cycle mechanic.

Problems caused by poor adjustment - Limit screws

Begin any rear derailleur adjustment by checking your limit screws. These screws can be checked in much the same manner as the front derailleur except that you check how the derailleur guide pulleys (the toothed wheels in the derailleur hanger) line up with the inner and outer cogs.

Derailleur limit screws are typically located on the main body. The "L" screw will control the inner stop of the rear derailleur, above the largest cog. The "H" screw will control the outer stop, above the smallest cog.

Setting the outer limit 

First adjust the "H" or outer limit screw; this is different from the front derailleur procedures. The outer adjustment should be made with the cable loose, so that the derailleur is free to swing all the way outward to its limit. To loosen the cable, simply loosen the binder bolt that holds it to the arm.

Once the bolt is loose and the cable is free, shift your chain outward onto the smallest rear cog and the largest chain ring at the front. Set your outer limit screw so that the pulleys of your rear derailleur are directly in line with the outermost cog. View this from the rear of the bike.

Once set, pull the cable taut and re-anchor it. If your derailleur has a barrel adjuster, turn the adjuster so that it's 1 or 2 full turns back from its most clockwise position before you reconnect the cable. This will give you room for any fine adjustment later.

Setting the inner limit.

Shift your chain onto the smallest chain ring and largest rear cog. Then use the inner limit screw to line up the pulleys with the largest rear cog, again as viewed from the rear of the bike.

The rear derailleur will not automatically travel as far inward as they can when you shift the chain onto its innermost cogs. To make sure therefore that the chain cannot be thrown off into your wheel, give your cable a strong pull when lining the derailleur pulleys up with the innermost rear cog. If you can pull the pulleys past the innermost cog, tighten the inner limit screw, clockwise and test again. If the pulley won't travel inward far enough, loosen the limit screw counter clockwise.

Problems caused by poor adjustment - Barrel adjustments

You may be able to fine-tune your adjustment using the barrel adjuster. If you have one of these simple, round adjustment knobs, it will be located either on the rear derailleur body or on your shift lever. 

Use the barrel adjuster to fine-tune the derailleur, shift your chain onto the smallest cog and the largest chain ring. Shift your gear lever once whilst checking to see if your chain shifts easily and quickly up to the second cog. If it doesn't, or if the shift is noisy and inefficient, turn your barrel adjuster counterclockwise one-quarter turn to tighten the cable and pull the derailleur slightly inward. Repeat this procedure until the shift is accurate.