Any bike with multiple chainrings uses a front derailleur to shift between them. In order to get smooth, accurate shifting, and avoid chain rub, it’s important to have your front derailleur properly adjusted. There are a few adjustments that can be made to the derailleur, and with the right tools you can do this maintenance yourself. Making these adjustments is easiest if you have access to a bike stand where you can turn the pedals, and shift freely while the bike is stationary.
The first thing you want to check, is the position of the derailleur itself. A clamp-on derailleur can be both moved up and down, and rotated on the seat tube. To check the height, shift the chain to the largest, outermost chainring. (If it won’t shift to the largest chainring, other adjustments are necessary but eyeball the height for now). The bottom of the derailleur cage should sit 1-2mm above the teeth of the large chainring.
If the height needs adjusting, shift down to the smallest chainring to release cable tension. Loosen the clamp holding the derailleur to the seat tube, which likely requires a 5mm allen key, and it should be free to slide.
You’ll also want to check if the derailleur is parallel to the chainrings. Shift back to the largest chainring, and sight it from above. Compare the outer edge of the derailleur to the chainring. If they are not parallel, adjust accordingly by again shifting to the small chainring to release cable tension, and loosening the clamp.
Front derailleurs have 2 screws known as limit screws, marked with an L and an H. The Low limit screw determines how far the derailleur can move in toward the small chainring. The H limit screw controls how far the derailleur can move outward toward the large chainring.
Low Limit Screw
When adjusted properly, the L screw allows the chain to move down to the smallest chainring, but not far enough that it could push the chain off the inside of the ring. Shift the chain to the innermost chainring and the innermost sprocket in the rear. Check the tension on the cable coming directly up from the derailleur, it should be fairly loose in this position. If it’s tight, turn the barrel adjuster on the shifter clockwise. The barrel adjuster is located where the cable housing enters the shifter (on the handlebars):
When resting on the small chainring, without tension on the cable, there should be only about 1 mm of space between the chain and the inner cage of the derailleur:
Loosening the L screw (counter-clockwise turn) will allow the derailleur to move in more, increasing this space. Tightening the L screw (clockwise turn) will push the derailleur out, decreasing the space. Turn the cranks a full rotation to be sure the chain doesn’t rub at any point (the ring may not be perfectly straight).
High Limit Screw
When properly adjusted, the H screw allows the derailleur to shift to the outer chainring, but stops it before it can push the chain off the edge. Shift the chain to the outermost ring in the front, and the outermost sprocket in the rear.
IMPORTANT: Keep pressure on the shift lever while sighting the derailleur, to ensure it is out against the H screw. In this position, there should be a 1 mm gap between the chain and the outer cage of the derailleur:
Loosening the H screw (counter-clockwise) will allow the derailleur to move out farther, and increase the gap. Tightening it (clockwise) will move the derailleur in and decrease the gap. Once set, turn the cranks to ensure it does not rub at any point in the revolution.
Cable Tension (Adjust Indexing)
The final part of the front derailleur adjustment is setting the cable tension. If you recall, it should be fairly loose in the rested, inner chainring position. When you shift to the larger rings, the derailleur is literally pulled by the cable. If the cable is too loose to start, it can’t pull far enough to get to that next ring, or will rub the chain while in that middle position.
Shift the chain to the middle ring in the front, and the innermost sprocket in the rear. This will allow us to make sure the chain won’t rub, even when your chosen gear creates a chain angle. There should be about 1 mm between the chain and the inner cage of the derailleur. Essentially, you want that inner side of the cage as close to the chain as possible without rubbing.
If there is a large gap, or the derailleur didn’t move far enough to push the chain to the middle ring, increase cable tension by turning the barrel adjuster on the shifter, counter-clockwise.
If the chain is rubbing the inside edge of the cage, turn the barrel adjuster clockwise to loosen the cable tension.
If the barrel adjuster is turned all the way in or out, and more adjustment is necessary, you can reset the barrel adjuster by loosening the cable from the pinch bolt on the derailleur itself. Shift to the smallest chain ring, loosen the pinch bolt holding the cable, turn the barrel adjuster back in or out according to the need, and retighten the pinch bolt.
The true test of course, is how well it shifts. The measurements here are suggestions for optimal shifting. For example, with your particular bike you might need more than 1 mm gap to the outer cage when setting your H screw just to get it to shift to the large chainring. That’s ok! If you’re unsure about any of your work, be sure to consult a professional bike mechanic.