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What does a rear derailleur do?
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The rear derailleur is designed to move the chain up and down on the sprockets (or cogs) connected to your rear hub. Together these cogs are known as the rear cassette. By moving to a cog with fewer teeth, the derailleur gives the rider a faster (or higher) gear which is appropriate for flat areas or descents. By moving to a cog with more teeth, the derailleur offers a gear more appropriate for climbing hills (or a lower gear).
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- A high gear gets its name from the larger distance covered with less pedal strokes.
- A low gear gets less distance per pedal stroke, but it makes hill-climbing much easier.
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When do I need to adjust my rear derailleur?
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It may be time to adjust your rear derailleur if you find your bicycle jumping between gears unexpectedly or if the chain fails to move to the highest or lowest gears. However, if your derailleur is bent or damaged, it is time to visit the local bike shop.
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There are many moving parts in the rear derailleur that need to be fine-tuned to work properly. Messing with most of these parts is not a task those less mechanically inclined should attempt, but this simple adjustment often does the trick and is easily reversible.
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- A bike stand is the best way to work, but it is not strictly necessary.
- Your hands
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Locate the point where the derailleur cable enters the rear derailleur. Where these parts meet you will find a knob that is threaded into the derailleur. This knob is known as a barrel adjuster.
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- Turning the barrel adjuster clockwise decreases tension in the derailleur cable.
- Turning the adjuster counterclockwise increases tension in the cable.
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The most common adjustment that needs to be made is an increase in cable tension because cables stretch through repeated use. The need for this adjustment is signaled by hesitation when shifting to larger cogs. This adjustment can be made by turning the barrel adjuster counterclockwise. This should be done in quarter-turn increments between tests of the derailleur.
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If you find the derailleur is hesitating while shifting to cogs with fewer teeth, cable tension may need to be decreased by turning the barrel adjuster clockwise. Again, do so only in quarter-turn increments.
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Limit screws are aptly named because they limit the amount of movement the rear derailleur attempts to make beyond the highest or lowest gears. Adjusting these screws is a somewhat advanced procedure, but patience and a little trial-and-error should be enough to see the mechanically inclined through to a successful adjustment. The idea behind this procedure is to find the tightest settings for the limit screws that will allow the chain to shift to the largest and smallest cogs.
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- Limit screws only control chain movement to the extremes of the cassette (the largest and smallest sprockets). They do not effect movement between these cogs.
- Noisy shifting will not be fixed by adjusting these screws.
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- As always, a bike stand is the best way to work.
- Small screwdriver, probably a Phillips head.
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You will find the limit screws in a pair on the rear derailleur. A third screw the same size as the limit screws, known as the chain gap adjustment screw, will be found on the derailleur, but if the derailleur pulleys are not grinding against the cassette and they are not near the spokes when the bike is shifted to the lowest gear, disregard this third screw for now.
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Loosen the tension in the derailleur cable by turning the barrel adjuster clockwise as far as it will go.
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While pedaling with one hand, shift to the highest gear with your other hand. Watch the chain as it rides on the smallest cog and determine if it looks like the chain wants to jump outward onto the axle. If this is the case, tighten the top of the two screws, which is known as the “H” screw. If the chain looks like it wants to travel toward the next gear, loosen the “H” screw. Make these adjustments only in quarter-turn increments. Observe the effect of each adjustment after it is made. Watching the derailleur move should make it obvious whether or not you are tightening the correct screw in the correct direction.
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Shift to the lowest gear while pedaling with your hand and determine whether or not the chain wants to jump from the largest cog. If this is the case, tighten the lower screw, which is known as the “L” screw. If the chain looks as if it is moving toward the next gear, loosen the “L” screw. Again, quarter-turn increments are the best way to prevent over-adjustment.
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- The “H” screw keeps the chain from leaving the highest gear (smallest cog) and jumping outward onto the axle.
- The “L” screw keeps the chain from leaving the lowest gear (largest cog) and jumping inward toward the spokes.
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Pedal with your hand and attempt to shift back and forth from the highest to the lowest gear a few times. If the bike is hesitating to shift up or down, follow the instructions in “Easy Adjustment” to find a proper cable tension.
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Your rear derailleur is adjusted! Don’t forget to lubricate all screws and pivot points before your next ride.
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