| Tuning the Rear Deraileur |
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As the shifter cable stretches on a new bike, the chain may begin to skip around on the rear derailleur. After many miles of vibration, the limiting screws may allow the derailleur to move outward or inward further than desired, mismatching the chain and cog. This section teaches you how to tune a derailleur. Warning: messing with your rear derailleur is not for the mechanically inept. But knowing how to adjust your shifting can be very helpful when the chain starts skipping around in the middle of the Slickrock Trail.
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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, trapped cable, or bent derailleur hanger. 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. Sighting down the derailleur pulleys to detect a bent hanger. |
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.
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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. Cleaning buildup off the pulleys. |
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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. Checking the cables for hangups. |
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.
The grip shifting knob pulls away from the body, exposing the cable and ratcheting mechanism. |
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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." Taking up slack in the derailleur cable.
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Let 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. Setting the high-gear (small cog) limiting screw. |
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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.) Closeup of setting the limiting screw. |
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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). Setting the low-gear (largest cog) limiting screw. |
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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. Fine adjustment of tension in the derailleur cable. |
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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. Checking the position of the chain on the cogs.
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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 backpedalling. If the chain rubs,
the pulley is too close. Adjusting the derailleur angle adjustment screw. |
| Ride the bike up and down a few hills, shifting frequently. Fine-tune the cable tension or limiting screws until your bike shifts the way you like it. |  |
