Description of the Problem: When the rider presses the shifter in an attempt to shift to the largest chainring, the derailleur doesn’t move sufficiently to make the transition.
As a result, the rider has no choice but to remain in a lower gear.
The limitation hurts the rider’s top speed and creates an opportunity for spinning out when descending.
Possible Sources Of The Problem
- Improperly adjusted limit screws
Derailleurs have a set of screws limiting the cage’s inward and outward movement. The screws are labeled as “L” and “H”.
The “L” (low) screw limits the movement of the derailleur towards the frame. Tightening the screw reduces the amount of inward travel.
The “L” screw’s setting is not relevant to the discussed problem because the screw has a low influence on the derailleur’s outward trajectory.
The “H” (high) screw limits the movement of the derailleur away from the frame. If the “H” screw is too far in, the derailleur will fail to fully move the chain onto the large chainring.
If this is the problem, backing off the screw a bit will allow the cage to jump on the large chainring.
- Improper Position Of The Front Derailleur
If the derailleur isn’t installed on the right segment of the seat tube, it will have a hard time moving across the chainrings as expected.
The general guideline for front derailleur positioning comes with 2 requirements:
- The derailleur should be parallel to the chainrings.
- There should be a 1-3mm gap between the largest chainring and the derailleur’s cage.
- Insufficient Cable Tension
The shift to the big ring is accomplished via a cable pull initiated by the shifter. If the starting cable tension before the pull/shift is low, then the shifter will fail to move the derailleur cage sufficiently.
If the cable is slightly loose, the needed tension can be achieved via the barrel adjuster found on the shifter or near the downtube. The barrel adjuster is a small knob that can make fine adjustments to the cable tension.
- Turning the barrel adjuster clockwise tightens it and effectively shortens the cable housing and reduces the cable tension.
- Turning the barrel adjuster anti-clockwise elongates the cable housing and increases the cable path and consequently the cable tension.
If extra tension is needed, the only way to acquire it is to untighten the pinch bolt of the derailleur, pull the cable, and then re-tighten the bolt.
The procedure is easier when performed with a tool known as a cable stretcher. A cable stretcher creates an anchor point for the end of the cable and the housing and then pushes them apart.
Since the tool is fairly expensive and used infrequently, most people skip it.
The classic way to re-tension the cable without a tool is to press the derailleur towards the big ring with one hand, then pull the cable with the other, retighten the pinch bolt and finish by releasing the derailleur.
- Partially Torn Shift Cable
If the shift cable is partially torn, it won’t be able to maintain the needed tension for proper shifting. Usually, tears occur in the area the closest to the derailleur.
The only solution is to replace the cable.
- Improperly Routed Cable
The cable could be healthy and yet still create problems due to improper routing.
If the cable inserts into the derailleur from the wrong side, the leverage that the shifter has over the derailleur changes due to the new pulling angle.
If the new leverage is insufficient, the shifter will fail to move the derailleur cage onto the largest chainring.
It’s also recommended to examine the entire path of the shift cable and its housing. Sometimes inexperienced or distracted mechanics make unnecessary loops resulting in “rusty” movement of the cable.
- Broken Shifter
Index shifters contain many small parts that wear down over time. If a crucial element of the mechanism malfunctions, the entire shifting mechanism will fail.
If the shifter is failing to pull the cable after a shift, the system isn’t operating as needed. In the best-case scenario, the shifter will start working after cleaning it. If it doesn’t, it has to be replaced or repaired.
If the shifter is entry-level, buying a new one makes more economic sense. Most mechanics will refuse to play with it given that they will have to charge you almost as much or more for their labor than the price tag of the shifter.
Note: Friction shifters are a lot less likely to malfunction thanks to their simple mechanism. The simplicity, reliability, and the option to easily trim the front derailleur are among the reasons why some people continue to use friction shifters for the front derailleur.
- Contaminated Cable and Housing
If the cable and its housing are old, they may be too contaminated for smooth operation.
Cleaning them would help, but given the price of cables and housing, it’s recommended to replace them.
- Frozen Derailleur
If the bike is used in very cold conditions, the front derailleur can freeze and become non-operational.
The quickest fix is to pour warm water on it. Don’t use boiling water as the sudden change of temperature could crack or weaken the material (cold shrinks the material, hot expands it).
Try to move the derailleur outward (away from the bike and towards the big chainring) by hand. If the derailleur moves over its entire amplitude, and the cage gets above the large chainring, the cable is the more likely source of the malfunction.
If you can’t get the derailleur to move sufficiently even by hand, an improperly set “H” screw is the logical “suspect”.