Got Chain Rub? There Are 3 Main Ways to Adjust a Mechanical Front Derailleur
Despite having to deal with far fewer gears than a rear derailleur, a front derailleur will similarly need regular care and maintenance. Over time, you may encounter issues such as slow shifting, chain rub, or even a dropped chain (more on those below).
First, a quick refresher on how your front derailleur works: A thin metal “cage” that surrounds the chain directly above the chainrings pushes and guides your bike chain on and off of your bike’s chainrings when you switch gears. When an issue arises, there are a few different ways to adjust your front derailleur. Read on to learn the basics of those methods and more ways to troubleshoot the problem.
Common Front Derailleur Problems
Chain Rub: Your chain comes into contact with the front derailleur cage while pedaling in certain gears.
QUICK FIXES
Avoid cross-chaining: If you’re experiencing chain rub, first make sure you aren’t cross-chaining—that’s when you’re pedaling in your biggest chainring and biggest cog on your cassette, or when you’re in the smallest chainring and smallest rear cog. Cross-chaining puts too much stress on your chain and gears anyway, and should be avoided if possible. Some bikes will experience chain rub while cross-chaining regardless of how well-tuned the front derailleur is.
Trimming: You may be able to “trim” and move the derailleur cage out of the way ever so slightly by pressing on the shifter again—though this isn’t a feature on all bikes.
Slow Shifting: You can shift to different chainrings, but the shifting is slow and not as responsive as it should be.
Dropped Chain: Your chain comes off the chainring when you shift.
How to Adjust Your Front Derailleur
There are three main ways to adjust a mechanical front derailleur: the physical positioning of the front derailleur, adjusting the set screws, and setting the indexing through cable tension.
Below, we cover a basic rundown of how to do each. Some adjustments require more technical proficiency than others. Remember, when in doubt, the best thing to do is take your bike to your local bike shop.
Check Alignment
First, check the positioning of the front derailleur cage relative to the largest chainring. The cage should line up parallel with the chainrings and should sit about two millimeters above the teeth of the largest chainring. If that’s not the case, loosen the bolt where the front derailleur attaches to the frame—it could be a clamp-on or bolt-on style—and adjust as needed.
It’s also important to check if the cage is bent, since that could cause the chain to rub as well. If that’s the case, you will need a new one.
Check the Limit Screws
Similar to a rear derailleur, a front derailleur has two limit screws that are adjustable, which control how far left and right the front derailleur cage is able to move relative to the chainrings. If the cage is too restricted in its movement, it may not shift the chain; likewise, if the cage is allowed to move too far in either direction, the chain could come off.
The two limit screws are located side-by-side on top of the main body of the front derailleur and should be clearly visible. They are often labeled with an “L” for low and an “H” for high—but not always.
L Limit Screw
The L (low) limit screw restricts how far inward the derailleur cage can come towards the bike. To adjust, shift the chain onto the smallest chainring and largest rear cog. If the shifter cable is tight, slacken it with a few clockwise turns or by loosening the pinch bolt.
The cage should be positioned very close (1mm to 2mm) to the chain on the inner (bike) side, without causing it to rub. If it’s rubbing, loosen the L screw in very small increments (quarter turns or less) by turning it counterclockwise until the chain no longer rubs while pedaling. If the gap is too big, tighten the L screw by turning it clockwise in small increments.
Test out shifting and check for chain rub in all gears. Then, test how well the front derailleur downshifts from the large or middle chainring back onto the small chainring. If it’s slow to downshift, loosen the L limit screw with a counterclockwise, quarter turn, and repeat the downshift test. Loosen the screw another quarter turn or two if necessary.
H Limit Screw
The H (high) limit screw restricts how far outward the derailleur cage can move away from your bike. To prep for adjustment, shift the chain onto the largest chainring and smallest rear cog, and increase shifter cable tension by pulling on it above where it connects to the front derailleur (to make sure the cage is in its outermost position).
The outer edge of the cage should sit very close to the outside of the chain without rubbing. If there is chain rub, back out the H screw counterclockwise in small increments until there is enough of a gap—remember to pull on the shifter cable wire for accuracy. If the gap appears too large, tighten the H screw clockwise in small increments.
Once done, test for chain rub in all gears. Then, test how well the derailleur upshifts onto the largest chainring, and adjust the H screw as needed.
Check the Indexing
Once the limit screws have been adjusted, you should then check the indexing if that is a feature on your front derailleur—look for a barrel adjuster on the shifter cable. Cables stretch over time, and sometimes the solution is as simple as increasing the cable tension.
If the chain is rubbing the outer edge of the derailleur cage, try increasing the cable tension using the barrel adjuster by turning it counterclockwise one quarter to one half turn at a time—increasing the tension will pull the cage out and farther away from the bike.
Alternatively, if the chain is rubbing the inner edge of the cage when in the smaller or middle chainring and larger rear cogs, turn the barrel adjuster clockwise in small increments until the chain no longer rubs.
If you reach a point where the barrel adjuster has been unscrewed all the way (counterclockwise), you’ll need to adjust the cable tension down at the pinch bolt, located at the other end of the shifter cable where it connects to the front derailleur:
Shift to the smallest chainring; screw the barrel adjuster in almost all the way (clockwise) save for a few full turns.
Loosen the pinch bolt just enough to move the cable.
Manually pull the cable to tighten it, then retighten the pinch bolt to secure it.
Now you can use the barrel adjuster to fine-tune the tension further.
After fine-tuning with the barrel adjuster, shift through all of your gears to double-check for chain rub or other issues.
Other Troubleshooting
If shifting is still sluggish or unresponsive, check the shifter cable to make sure it isn’t frayed or rusted, and check cable housing for damage like cracks. Depending on your setup, and how much of your cable is internally routed or housed, you could apply lubricant; if not, or if it doesn’t help, you may need to run a new cable. Also, lubricate the front derailleur at the pivot point.
If your chain isn’t shifting smoothly and seems to be sticking to the chainring (you may notice this while pedaling all the time, not just while shifting), check the chainring teeth for excessive wear. If the chainring teeth look pointy and don’t release the chain smoothly, you’ll need to replace that chainring.
Electronic Front Derailleur Troubleshooting
With an electronic system, shifting is managed by firmware, so it shouldn’t go out of alignment once it’s set up—though it may be adjustable through an app or other software tool (and it may need firmware updates). If you’re having trouble with your electronic front derailleur, first check to make sure the battery has enough charge. Make sure any cables are fully connected as well.
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