Description of the Problem: The front derailleur doesn’t move the chain from the large chainring to a smaller one upon command. As a result, the rider cannot use the lower gears on their bike and loses extra energy when climbing.
Possible Sources Of The Problem
1. Partially Torn Shift Cable
A few years ago, the shift cable of my hardtail MTB tore partially in the visible area the closest to the front derailleur. As a result, it became very difficult to up and downshift due to the lack of cable tension.
This problem is accompanied by a disproportionate force-to-output ratio. Or in simple terms, it will feel that you’re pressing the shifter too hard for the derailleur movement that you’re getting.
The only way to fix this problem is to replace the shift cable with a new one.
2. Improperly Adjusted Limit Screws
Derailleurs have a set of screws limiting the movement of the cage in both directions (towards the bike and away from it).
The screws are known as L-screw (low) and H-screw (high).
The image above showcases a retro front derailleur designed for road bikes.
The L-screw is on the left. By tightening it, the bolt’s body gets lower and prevents the FD’s cage from moving inward. If the screw is untightened, the cage’s inward amplitude is increased.
If the L-screw is screwed too far in, the derailleur’s cage will be prevented from fully moving the chain onto the smaller chainrings. To fix this issue, it’s necessary to back off the screw a bit.
Ideally, the limit screw will allow the cage to transfer the chain onto the smallest chainring while preventing the chain from rubbing against the spokes or falling into the bottom bracket area.
The H-screw functions similarly but limits the outward (away from the bike) trajectory of the derailleur.
3. Contaminated Derailleur Mechanism
If the bike is used off-road and in less than ideal meteorological conditions, small debris can get into the front derailleur and act as a limiter preventing the cage from covering its full trajectory. A bit of cleaning with a degreaser, disinfectant, or simply warm water should resolve the issue.
4. Contaminated Shift Cable and Housing
A contaminated cable and housing can cause shifting issues too. To rule out this issue, it’s recommended to replace both units. Cleaning them is not worth the effort unless the user doesn’t have access to new bike parts.
Note: There are two types of front derailleurs depending on the angle of cable pull – top pull and bottom pull. In the case of top pull derailleurs, the cable pulls from above the derailleur. In the second situation, the cable is pulling from below the derailleur.
Bottom-pull front derailleurs are more common on road bikes because the cable has to pass underneath the bike’s bottom bracket – an area that gets severely contaminated when riding on off-road terrain.
To eliminate rubbing against the frame, a plastic element with a channel in it is installed under the bottom bracket shell.
The image below showcases a road bike with shift cables routed along the downtube and under the bottom bracket shell.
This is one of the areas that have to be examined first because it’s prone to collecting dirt due to its proximity to the ground.
Tips On How To Diagnose A Damaged or Stuck Cable
The troubleshooting below could help you diagnose a cable that’s stuck somewhere in the system.
Shift to the largest chainring at the front. Then try to shift to a smaller chainring and examine the cable tension.
If the cable has noticeable slack, then it has enough freedom to move, and the problem is probably not the cable.
If the cable remains tensioned, the system is clogged and something is preventing the cable from moving freely.
If the cable tension is adequate, and the derailleur is still not downshifting, you can try to press the derailleur by hand to simulate normal downshifting. If the derailleur operates just fine under hand pressure, it is more than likely contaminated and has debris and other particles preventing it from moving without force.
If the cable tension remains high, the source of the problem is more than likely the cable. To truly confirm this diagnosis, you can unhook the shift cable. Once the cable is detached, the derailleur should move to its lowest position right away.
5. Improperly Routed Cable
If the cable hasn’t been routed from the shifter to the front derailleur properly, the shifter will fail to control the derailleur as expected.
Below are some of the common malpractices:
- Unnecessary loops around the frame
- Missing housing segments creating friction points between the cable and the frame
- The cable doesn’t insert into the derailleur from the right direction
Truth be told, this problem occurs only when the bike has been serviced by a non-qualified individual or a very distracted bike mechanic.
6. Broken Shifter
Index shifters have tiny mechanisms consisting of many parts. A quality shifter can survive years of exploitation. Nonetheless, time destroys everything. If a small unit in the shifter breaks, the shifter will stop pulling and releasing the shift cable as needed.
7. Shifting Under Load
Shifting while pedaling hard up a hill is known to create problems because the chain is tensioned and fights the derailleur.
Newer drivetrains are fairly forgiving and sometimes allow the rider to get away with less than ideal shifting protocols. However, older derailleurs complain.
This problem is more common when shifting at the front than the rear because the front part of the chain is loaded to a greater degree than the rear segment.
Hence why it’s recommended to slightly back off the pedals when shifting. Also, it’s a good strategy to shift down before a hill rather than when you’re in the middle of it.
8. The Derailleur’s Position Is Wrong
If the derailleur’s position on the seat tube isn’t correct, the derailleur may be unable to move the chain effectively.
The common rule is to keep the derailleur’s cage 1-3mm above the largest chainring and parallel to it. If the gap is too large, the derailleur is positioned too high for smooth downshifting.
That said, some people recommend a 1-2 gap for triple chainrings because the smallest chainring could be hard to reach otherwise.
9. Frozen Derailleur
If the bike is used in the winter, the derailleur could literally freeze in the cold.
One of the quickest fixes is to pour warm water on it. If you’re far from home but still in a civilized location, you can purchase a cup of hot water from a vending machine. If nothing else is available tea or even coffee could be an option. However, those beverages contain “contaminants” such as sugar and will make the derailleur sticky.
Note: Don’t pour boiling water on an extremely frozen bike as it may cause an unexpected crack. Instead, use moderately warm water. Of course, this scenario is a bit far-fetched, but this is the safest protocol nonetheless.