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BICYCLE REPAIR
Rear Derailleur Adjustment
Equipment:
Round up the following:
  • a way to suspend or hold the bike upright (a car rack works; or use your repair stand, if you have one)
  • 5-mm allen wrench
  • grease
  • diagonal cutters
  • pliers
  • small screwdriver (flat or Phillips)
  • cable end cap (to prevent fraying)
Place the bike in a repair stand. While pedaling by hand, shift the chain to the smallest freewheel cog and middle or smallest chainring (photo, right). Shift to the small cog
Housing not shown Cut off the end cap, loosen the anchor bolt, and extract the cable. If it’s kinked or rusted, replace it. Grease sections that run inside housing (photo) and reroute the cable through the housing and to the anchor bolt. Turn the derailleur adjustment barrel clockwise all the way, then unscrew it one turn. Don’t tighten the anchor bolt yet.
While pedaling with your right hand, push against the derailleur body with your left thumb (photo), causing a shift to the largest cog. Release the pressure with your thumb to shift to the smallest cog. Do this repeatedly, noting any hesitation or overshifting. The chain should move smoothly onto the smallest and largest cogs. Shift with your thumb to test the adjustment
Adjust the derailleur's range with the limit screws
If necessary, adjust the derailleur’s range of motion by turning (photo) the high- and low-gear limit screws (usually the top and bottom, respectively) to allow the derailleur to shift accurately to the largest and smallest cogs. Counterclockwise turns allow it to move farther; clockwise turns limit it. Keep shifting with your thumb and fine-tuning the screws until the chain shifts perfectly onto each cog with no hesitation or overshifting (off the top or bottom cogs).
With the chain on the smallest cog, grasp the cable with pliers and pull lightly to remove slack. While holding the cable, tighten the anchor bolt (photo). Install the cable end cap and crimp it in place with diagonal cutters. While pedaling with your left hand, shift repeatedly with your right to test adjustments. The chain should engage the largest and smallest cogs accurately. If necessary, adjust the limit screws.

If the chain won’t drop to the smallest cog despite adjusting the limit screw, you may need to loosen the cable by turning the adjustment barrel clockwise half a turn. The other possibility is that you'll need to remove slack that’s developed (new cables usually stretch a bit) by loosening the anchor bolt, and pulling on the cable with pliers to remove the slack. The adjustment is right when the cable has no noticeable slack but is not too tight, either.
 Don't pull too tightly
Use the barrel adjuster to fine-tune the derailleur For the final adjustment, while pedaling with your left hand, move the lever one click with your right. The chain should jump to the second-smallest cog and run quietly. If it hesitates, screw the adjustment barrel counterclockwise one-half turn (photo) and retry. Repeat this until it shifts immediately onto the cog. If it overshifts, screw the barrel clockwise by half turns until it doesn’t. Shift through all the gears and test ride the bike. Fine-tune again if necessary.

Note: Once the derailleur is adjusted properly, about the only adjustment necessary (assuming you don’t crash and damage the derailleur) is taking care of any cable slack that develops from stretching, which occurs over time. To remove slack and restore perfect shifting, simply turn the adjustment barrel counterclockwise in half turns.
This article is based on one I wrote for the March 1991 issue of Bicycling Magazine.
The photos are by Mel Lindstrom. I set up the shots and appear in them.
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