Description of the Problem
When the rider tries to shift onto the largest cog, the front derailleur doesn’t move sufficiently to make the transition. As a result, the cyclist cannot use the highest gear of the bicycle.
Possible Causes of The Problem
- Improperly Adjusted Limit Screws
Derailleurs have a set of screws “hard” limiting the movement of the front derailleur in both directions.
There are two limit screws – Low (L) and High (H).
The L-screw limits the movement of the derailleur inward towards the small chainring and prevents the chain from dropping in the space between the cranks and bottom bracket.
The H-screw limits the movement of the derailleur outward towards the large chainring and stops the chain from falling outside of the cranks.
If the derailleur cannot physically move far enough even when you’re bypassing the shifter and moving the cage with your hands, the source of the problem is more than likely an improperly set H-screw.
To fix this issue, unscrew the H-screw a 1/4 of a turn and check if the upward amplitude of the derailleur has increased. Proceed to unscrew the H-screw in 1/4 to 1/2 rotations until the movement of the derailleur has been increased sufficiently.
For a detailed guide on how to set the limit screws of a front derailleur, consider reading this guide.
- Insufficient Cable Tension
If you can move the derailleur sufficiently by hand (bypassing the shifter), then the most likely source of the problem is low cable tension.
The usual solution is as follows:
- Shift into the lowest chainring and tighten the barrel adjuster on the shifter all the way in or close to it.
- Untighten the pinch bolt on the derailleur holding the gear cable.
- Keep the derailleur in its lowest position. (A set of extra hands will be helpful)
- Pull the gear cable until it’s tight.
- Tighten the pinch bolt.
- Torn Gear Gable
With time gear cables wear down and experience partial tears. It’s also possible to directly damage the cable, especially if it’s located under the downtube. When that happens, the derailleur cable becomes too weak to maintain full tension.
The most likely place for a gear cable to tear is in the section closer to the derailleur. However, it may fail at other locations too.
Inspect the cable. If you see a tear, replace it.
- Contaminated and Worn Cable and Housing
If the gear cable and housing are contaminated and worn, they will make it more difficult to shift onto the larger chainring. Replacing them will render shifting smoother and easier. That said, if the derailleur is missing by a lot, and the gear cable is intact, the new cable is unlikely to fix the problem.
- Extreme Cross-chaining
Cross-chaining occurs when riding in extreme gear combinations (e.g., largest chainring + smallest rear cog). During cross-chaining, the chain is stretched to the extreme and starts fighting the front derailleur even more.
To see if this is the issue, shift closer to the outer cog and only then try to jump on the largest chainring.
- Worn Chainring Ramps
Chainrings designed for multi-speed bicycles have small ramps which guide the chain onto the next chainring during a shift. If the chainring is old and worn, there’s a chance that the ramps have flattened to the point where they are no longer effective.
Inspect the ramps of the large chainring and replace the chainring if they’re worn.
- Misrouted Gear Cable
If the gear cable is routed improperly (on the opposite side of the derailleur), the leverage that the shifter has will be reduced. As a result, it will take extra force to move the derailleur towards the large chainring.
- Worn Front Derailleur
It’s unlikely, but technically possible for the derailleur to wear down to the point where it has too much flex for full shifting.
- Bent Derailleur Cage
If the derailleur cage is bent inward, the derailleur will have a harder time moving the chain towards the large chainring. In this case, the diagnosis will be easy because the deformation of the cage will be obvious.
- Excessively Wide Bottom Bracket
If the bottom bracket in question is too wide for the frame, it may be positioning the cranks and respectively the chainrings too far away from the frame. If the bike is relatively new, this is unlikely to be the problem.
- Improperly Positioned Front Derailleur
If the front derailleur is positioned too low on the seat tube, the cage will fail to clear the large chainring. Ideally, the cage of the front derailleur will be ever so slightly taller than the chainring.
- Old and Worn Shifters
Sometimes older shifters make it harder to jump onto the large chainring. A new gear cable and cleaning of the shifter could improve the performance.
That said, sometimes the problem is the ancient mechanism of the shifter which is difficult to fully service.
It’s also possible that the shifter was hard to use even when it was completely new. The only way to circumvent this issue is to get a modern unit.
- Wrong Shifter
If the derailleur is coupled with a non-compatible shifter, the shifter’s pull may be too short to move the derailleur’s cage onto the large chainring.
Summary: What You Need To Know
While there are many possible factors that could make it difficult for the front derailleur to move onto the large chainring, the two most common culprits are an improperly set H-screw and/or a poorly tensioned gear cable.