Checking Your Rear Derailleur
What is it and What does it Do?
The rear derailleur is the 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 your shift commands, moving the chain onto the teeth of the appropriate rear cog. Rear derailleurs also absorb the chain slack that comes from shifting from larger cog combinations to smaller ones.
What Needs to be Checked?
Rear derailleurs should be checked to make sure that they are in good working condition and properly adjusted.
How Often should I Check?
Rear derailleurs should be quickly checked before every ride.
Check Procedure
To check rear derailleur performance, suspend your rear wheel off the ground. Rotate your pedals with one hand while taking your rear shift lever through its full range of gearing options. A properly functioning rear derailleur will shift the chain crisply from one rear cog to the next, without hesitating between cogs or overshooting and dumping the chain. The cage or "hanger" of your rear derailleur (which angles back away from the main derailleur body) should extend to take up any slack that is present in the chain, then yield when more chain is required.
Common Rear Derailleur Problems and Solutions
- Sluggish or inaccurate shifting
Rear derailleurs typically malfunction because (A) they need to be adjusted, (B) they are damaged or (C) they are dirty. The first problem is addressed below. The other two are discussed in the cleaning section that follows.
Solution - rear derailleur adjustment
Limit screws
As with front derailleurs, begin any rear derailleur adjustment by checking your limit screws. These screws can be checked in much the same manner as the front derailleur limit screws, except that instead of checking to see how the derailleur cage lines up with the chain, you should compare how the derailleur guide pulleys (the toothed wheels in the rear derailleur hanger) line up with the inner and outer cogs.
Rear derailleur limit screws are typically located on the rear derailleur body, either stacked horizontally or side by side. 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
The first step in adjusting rear derailleur limit screws is to set the "H" or outer limit screw (notice that this is different from front derailleur procedures). The outer adjustment should be made with the rear derailleur cable loose, so that the derailleur is free to swing all the way outward to its limit. To loosen this cable, simply unscrew the binder bolt that holds it to the derailleur arm.
Once the cable anchor bolt is loose and the cable is free, shift your chain outward onto the smallest rear cog and the largest chainring. Set your outer limit screw so that the pulleys of your rear derailleur are directly in line with the outermost cog (as viewed from the back end of your bike).
After you've set the outer limit, pull the derailleur cable taut and re-anchor it. If your rear 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 some adjustment room if you need to do some fine tuning later on.
Setting the inner limit
To adjust your inner limit screw, shift your chain to the inside of the drivetrain system -- onto the smallest chainring and largest rear cog. Then use the inner limit screw to line up the derailleur pulleys with the largest rear cog, as viewed from the rear of the bike.
IMPORTANT: Rear derailleurs will not automatically travel as far inward as they can go when you shift the chain onto its innermost cogs. To make sure that the chain cannot be thrown off of your innermost cog, give your rear derailleur cable a good pull with your hand when lining the derailleur pulleys up with the innermost rear cog. If you can pull the rear pulleys past the innermost cog, tighten the inner limit screw (clockwise) further and test again. If the pulley won't travel inward far enough, loosen the limit screw counter clockwise.
Barrel adjustments
You may be able to fine-tune your rear derailleur adjustment using a simple feature called a barrel adjuster. If you have one of these simple, round adjustment knobs, it will be located at the base of the rear derailleur body or up on your shift lever.
To use the barrel adjuster to fine tune your rear derailleur, shift your chain out onto the smallest rear cog and the largest chainring. Shift your rear gear lever once, checking to see if your chain shifts crisply up to the second rear cog. If it does not, or if the shift is noisy and inefficient, turn your barrel adjuster counterclockwise one quarter turn to tighten the cable slightly and pull the derailleur slightly inward. Repeat this procedure until the shift is accurate (check after each 1/4-turn adjustment).
NOTE: An alternative approach is to adjust your chain inward until it begins to make noise (as it crowds the next cog), then backing off 1/4 turn at a time. If you use this method, check your shifting after each adjustment.
Rear Derailleur Cleaning Procedures
Like front derailleurs, rear derailleurs can be hampered by even small amounts of grit and grime. To clean your rear derailleur, brush off all exposed parts with a stiff brush, then wipe them down carefully with a clean rag to get as much grime off as possible. Stubborn grit can be loosened with a clean rag soaked in solvent.
When lubricating your rear derailleur, focus on the moving pivots of the derailleur mechanism and the small pulleys in the derailleur arm. Use a bicycle drip lubricant. Shift the derailleur back and forth while applying it to get the oil into tough to reach places. Wipe off extra lubricant so it can't attract new dirt.
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