Q: What is a Front Derailleur?
A: The front derailleur is the mechanism that shifts your chain from chainring to chainring. It moves from side to side as you click your front shift lever, carrying the chain along with it.
Q: What Needs to Be Checked?
A: You should check your front derailleur to make sure it's functioning properly and that it's free of grime and dirt.
Q: How Often Should I Check It?
A: You should check your front derailleur quickly before each ride.
Q: What Is the Check Procedure?
A: To check your front derailleur, suspend your rear wheel off the ground. Rotate your pedals with one hand and shift your front gear lever through its full range of gear options with your other hand. If your front derailleur is properly tuned, your chain will shift crisply from one chainring to the next without hesitating between rings or overshooting them and jumping off the chainring set.
Common Front Derailleur Problems
Front derailleurs may cause sluggish or inaccurate shifting because (A) the derailleur body is not positioned properly, (B) the derailleur limit screws are not adjusted correctly, (C) the mechanism is dirty or (D) the cable is damaged or improperly tensioned.
Solution A: Reposition the Front Derailleur
To check the position of your front derailleur, shift it so that the derailleur arm is positioned over the outermost chainring. The derailleur "cage" (the long, curved section of the derailleur that the chain passes through) should be approximately 2mm above the teeth of the chainring (when viewed from the side). The outer plate of the cage should be lined up parallel with the chainring when viewed from above.
To reposition your front derailleur, loosen the derailleur mounting bolt that holds it onto your frame. This bolt is typically located on the opposite side of the seat tube from the derailleur body. Once you've loosened the bolt, reposition the derailleur by sliding it up/down and/or rotating it slightly from side to side. (Note: you may have to loosen the derailleur cable in order to move the derailleur body. To do so, read the section below titled "Setting the inner stop".) Be sure to retighten the mounting bolt carefully before riding. If during this procedure you notice that your derailleur cage is bent or damaged in any way, bring it into your local REI or other bike shop for service.
Solution B: Adjust the Derailleur Limit Screws
More often than not, sloppy shifting is caused not by a poorly positioned front derailleur, but by poorly adjusted limit screws. The derailleur case swings back and forth from the derailleur body (in response to your shift commands) to deliver your chain from chainring to chainring. It must move from side to side within a very specific range to perform well.
The inner and outer boundaries of your derailleur arm's side to side movement are controlled by limit screws. These small screws are typically located next to one another on the main front derailleur body, either stacked horizontally or positioned side by side. Each screw controls one extreme of the derailleur's movement. The "outer stop" screw determines the farthest distance that the derailleur will travel away from the frame. The "inner stop" screw determines how close the derailleur will travel in toward the frame.
Turning the screws clockwise will limit the outer range (or "throw") of the derailleur. The only time you would turn the screws clockwise is if your chain is jumping off either end of your chainraings or outer cassette. Turning the screws counterclockwise will allow the derailleur to have more range, or throw, on the outer limits. If the 2 limit screws are tuned correctly, a job that REI strongly recommends should be entrusted to professional bike mechanics only, your derailleur should perform without problems.
CAUTION: This basic guidance for adjusting limit screws is intended primarily for experienced cyclists with advanced mechanical skills. REI seriously discourages most cyclists most attempting to adjust their own limit screws. One REI bike mechanic puts it this way: "If a customer can't adjust their shifting via a barrel adjustment and cleaning, then they should bring their bike into a shop. If the limit screws are adjusted wrong, it can result in a crash. Throwing a chain can be very dangerous."
Bike manufacturers identify inner and outer limit screws in a variety of ways. Most label them with an "L" for low gear (which refers to the smallest, innermost chainring) or "H" for high gear (which refers to the largest, outermost chainring).
Setting the inner stop: The first step in adjusting your front derailleur is to see how far it swings in toward your frame. To do this, you must first shift the front derailleur to the innermost chainring and let the tension out of the front derailleur cable by loosening the small bolt that anchors the cable to the derailleur body. Loosening this cable will insure that the arm is free to swing as far in as it can go.
Next, shift your chain to its furthest inside position: the smallest chainring and the largest rear cog. Use your inner limit screw to position the inner wall of the front derailleur cage so that there is 2mm of clearance between it and your chain at the point where they come closest together.
Once you've set your inner stop, pull the derailleur cable taut and re-anchor it securely. Do this while your chain is still on the smallest chainring.
Setting the outer stop: Once you've set your inner stop, shift your chain to its outermost position: the largest chainring and the smallest rear cog. Use your outer limit screw to position the outside face of the front derailleur cage 2mm away from the outer surface of your chain (at the point where they come closest). Keep in mind as you make this adjustment that it's your derailleur cage that keeps your chain from overshooting the outermost chainring and falling off your bike. Since some shifting systems (particularly newer ones) do not automatically shift the derailleur out as far as it can go when they shift it to the largest chainring, pull outward on the front derailleur cable slightly as you set and test your outer limit adjustments. The added cable tension will ensure that the derailleur is really in its outermost position and that the chain cannot be thrown.
Solution C: Adjusting the Barrel
Most modern bicycles have fine-tuning devices called barrel adjusters built into their derailleur systems. These simple, round adjustment knobs, typically located along the derailleur cable path at the base of your shift lever or the derailleur body itself, allow you to fine-tune your derailleur adjustments by increasing or decreasing the tension of your shift cable very slightly.
Not all derailleur systems have barrel adjusters. But most bikes with "indexed" derailleur systems do. Barrel adjusters are typically used to tune the position of the rear derailleur more often than the front.
To fine tune your front derailleur using your barrel adjuster, start with your chain on the largest chainring and largest rear cog. Shift your chain down to the next smallest chainring and check to see how close the inside surface of the chain is to the inside wall of the derailleur cage. The two surfaces should be as close together as possible (approximately .5mm) without touching. Turn the barrel adjuster counterclockwise (a quarter turn at a time) to move the derailleur cage inward, away from the chain surface. Turn it clockwise to move the cage outward.
NOTE: If shifting difficulties persist despite the adjustments described above, you may have a more serious shifting problem. Persistent shifting problems should be addressed only by an experienced bicycle mechanic.
Solution D: Cleaning Your Front Derailleur
Many front derailleur problems are caused by dirt, grime or insufficient lubrication. Even small amounts of grit and grime can cause problems, so keep your derailleurs as clean as possible at all times. Re-lubricate them every month or so (depending upon riding conditions) to keep them functioning properly.
Clean your front derailleur by brushing all exposed parts with a stiff brush to remove grit or grime. Stubborn grit can be loosened by wiping the derailleur with a clean rag soaked in solvent. Be sure to clean the derailleur mechanism carefully, including the hard to reach areas of the main derailleur body and the derailleur arm.
When re-lubricating, focus on the moving pivots of the derailleur mechanism. Use a drip lubricant designed specifically for bikes, and be sure to shift the derailleur back and forth while applying it so you can work the lube into the tough to reach places. Wipe off excess lubricant carefully when you're done.