Don't start messing with the derailleur yet! Make sure your shifting trouble isn't due to something else, such as a warped, dirty, or stretched chain, gummed-up pulleys, crud in the cable housing, trapped cable, or a bent derailleur hanger. So your first job is to rule out any other cause of bad shifting before you start messing with your shifter and derailleur.

From the back of the bike, sight along the chain so the top half of the chain blocks your view of the bottom half. Look down to the derailleur pulleys. There should be a straight line through the chain down to the bottom pulley. If the derailleur hanger is bent, straighten or replace it.

As the crank is turned, watch the chain go around the cogs. Look for a bent or immobile link. Check the chain for excess wear. Replace the chain if it's worn or bent.

Clean the chain and the rear derailleur pulleys. Pull the chain away from each pulley, and turn the pulley to make sure it can move freely. A pulley that offers resistance to turning must be replaced. See the section on pulleys.

Now check the cable to be sure it slides freely. Mud and grit within the cable housing, or on a cable-guide, can keep the derailleur from moving a "full gear" during downshifts. If the cable doesn't slide freely in the housing, you may be able to clean or lube it to restore smooth shifting.

Note!  The most common cause of "missed shifts" when clicking to a smaller cog is a cable that's hanging up. Take off the cable and clean the cable housings. (See our cable-cleaning section.) Replace any sections of cable-housing that give any resistance whatsoever.

If you have a grip shift, take it apart and clean it (a cotton swab and mild soapy solution works). Grip shifts tend to get gritty inside. See our section on cable replacement for full instructions.

Start with the chain on the small cog (highest gear on the rear-shifter lever). Check the cable. If it's loose, take up the slack until the cable has no "play."

Set the high-gear limit screw. In your highest gear on the shifter, and with the chain on the smallest cog, position the derailleur so the upper pulley exactly matches the teeth of the small cog. Run the chain a bit with the front derailleur in the large ring, adjusting the limit screw until the pulley and the cog seem to match perfectly.

Once you're sure the derailleur matches the cog, move it just a whisker (1/8 to 1/4 turn) towards the other cogs. (You may need to readjust a bit if you get chain-skip.)

Now adjust the low-gear limit screw. Shift into the lowest gear (largest cog). If it won't go, loosen the limit screw. With the chain on the largest cog (and the front derailleur in the small chainring), adjust the derailleur so the upper pulley exactly matches the teeth of the large cog.

Once it's perfect, turn the limit screw so it moves the derailleur a tiny amount towards the other cogs (1/8 to 1/4 turn usually works).

Now shift to the highest gear (smallest cog). Push the shift lever to downshift one gear (go from the smallest cog to the next-smallest). If it doesn't shift, tighten the cable with the barrel adjuster, 1/2 turn counter-clockwise. Backshift and try again. Continue tightening until it shifts. If it overshifts, going from the smallest cog to the third-smallest, loosen the cable by turning 1/2 turn clockwise. Backshift and repeat until it shifts exactly from the small to the next-smallest cog.

Now go to the second-smallest ring. Tighten or loosen the cable slightly, until the outer side plates of the chain are just clearing the third-smallest cog. Shift up and down, fine-tuning until you're satisfied.

Shift to the largest cog. Look at where the upper pulley sits in relation to the cog. If the pulley touches the cog, you may not be able to backpedal freely. Using the tensioning screw (derailleur angle adjustment screw), move the pulley until it's as close as possible to the cog without touching it. Try backpedaling. If the chain rubs, the pulley is too close.