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What does the front derailleur do?
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The front derailleur pushes the chain from one chainring to another. It works very similarly to the rear derailleur by shifting the chain so that it falls onto a smaller chainring, or so that is pushes against a larger chainring and eventually catches the teeth. A properly adjusted front derailleur will shift between chainrings without allowing the chain to slip off in either direction.
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When do I need to adjust my front derailleur?
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If you find your chain has a tendency to leave the chainrings, either by falling inward onto the bottom bracket shell, or outward toward the drive-side pedal, it may be time to adjust the front derailleur. Hesitation while shifting between chainrings may also signal the need for this adjustment. Never adjust the front derailleur until you are sure the rear derailleur is properly adjusted.
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- A bike stand is the best way to work
- Small screwdriver, probably a Phillips head
- The appropriate allen wrench or screwdriver for the bolts holding the front derailleur to the seat tube
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Messing with the height and limit stops of the front derailleur 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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Locate the point where the derailleur cable enters the front derailleur. Where these parts meet you will find a knob that is threaded into the derailleur. This knob is known as a barrel adjuster. If there is no barrel adjuster at this junction, check the shift lever.
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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 the larger chainring. 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 a smaller chainring, 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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It is essential the height position of the front derailleur is adjusted before you move on to the limit screws.
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To determine the correct height alignment of the front derailleur, start by positioning the derailleur over the largest chainring. The gap between the largest chainring and the derailleur should be no more than 2mm.
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If the front derailleur needs to be raised or lowered, start by shifting to the smaller chainring. This will alleviate the tension in the front derailleur cable.
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Loosen the bolts that hold the front derailleur in place. You will probably notice a mark on the seat tube which you can use to gauge the amount of movement you make.
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After tightening the bolts which hold the front derailleur in place, shift back to the larger chainring and re-measure for the proper (1-2mm) height. Repeat this process if necessary.
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As with the rear derailleur, the limit screws on the front derailleur prevent it from throwing the chain too far to either extreme of the chainrings. The optimal adjustment for the limits of the front derailleur will have the chain just clearing the inside of the cage of the derailleur when the chain is on the smallest chainring, and just clearing the outside of this cage when the chain is on the largest chainring.
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- The “L” limit screw prevents the derailleur from throwing the chain inward onto the bottom bracket shell.
- The “H” limit screw prevents the front derailleur from moving outward past the largest chainring.
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While pedaling with one hand, shift the chain to the smallest chainring and the largest cog with your other hand. These will both be the options closest to the frame of the bicycle.
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Look through the front derailleur cage and find the place where the chain is closest to the inside of the cage.
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The “L” limit screw is one of a pair of small screws that you will find on the front derailleur. It is generally the screw closest to the frame of the bicycle. Adjust the “L” limit screw until the gap between the chain and the front derailleur cage is about 1mm at the closest point.
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Pedal the bike slowly with your hand while looking through the cage to make sure that the chain never rubs on the inside of the cage.
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If the chain does rub, loosen the “L” limit screw in quarter-turn increments until the optimal 1mm gap is achieved.
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Shift to the larger chainring and back again several times to make sure that the chain is not trying to go past the smaller chainring.
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To adjust the “H” limit screw, shift to the largest chainring and the smallest cog. These will both be the outermost options.
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As with the “L” limit screw, the gap between the chain and the cage should be about 1mm at the tightest point, this time on the outside of the cage.
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Repeat the process for the “L” limit screw, sighting down the cage and tightening until the 1mm gap is established.
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Pedal slowly to make sure the chain never rubs on the outside of the cage. If it does, loosen the “H” limit screw until no rubbing occurs.
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Your front derailleur should be properly adjusted! Lubricate all moving parts and go for a ride.
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Never ride with the chain on the smallest (innermost) chainring and the smallest (outermost) cog. The unnatural path created for the chain in this gearing jeopardizes the strength of the chain and is potentially dangerous.
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