Description of the Problem: When the rider clicks the shifter to activate the trim function of the front derailleur, the derailleur remains in the same place.
Possible Sources Of The Problem
1. Improperly Adjusted Limits Screws
Even the cheapest derailleurs have two adjustment screws limiting the derailleur’s movement in both directions.
The “Low” or “L” screw limits the derailleur’s travel towards the bike. If the L-screw isn’t properly adjusted, the derailleur will either fail to shift from the large to the smaller chainring or throw the chain onto the bottom bracket.
The H-screw, on the other hand, limits how much the derailleur moves away from the bike. If the H-screw doesn’t have an adequate setting, the chain could fail to reach the large chainring or fall off the chainring.
The image above showcases an old Simplex front derailleur. The derailleur’s mechanism is simple and clearly illustrates the function of the limit screws.
If the L-screw is untightened, it will increase the cage’s inward trajectory.
If the H-screw is untightened, the derailleur’s outward amplitude will be amplified.
To understand how the limit screws affect the trim function of a derailleur, one first has to become familiar with the essence of the trim function.
The “trim” is a micro-shift that moves the derailleur ever so slightly so that the chain doesn’t rub against the derailleur’s cage.
The trim function cannot “override” an improper setting of a limit screw. Or in other words, if the limit screw is restricting the derailleur’s movement too much, the rider won’t be able to effectively trim the derailleur.
Therefore, it may be necessary to back off a limit screw slightly so that the derailleur has the needed “freedom” for effective trimming.
2. Improper Cable Tension
The limit screws may be perfectly set, but if the tension of the shifting cable isn’t adequate, then the derailleur will still fail to reach its full amplitude.
To avoid this issue, it will be necessary to re-tension the shift cable and then finetune the cable tension with help of the barrel adjuster.
The barrel adjuster is a knob found on the shifter or the cable housing (inline version) that allows the user to make slim adjustments to the cable tension.
3. Insufficient Clearance
If the height of the front derailleur isn’t adequate, the cage may rub against the largest chainring even if the trim function is operating properly.
Ideally, the front derailleur’s cage should be 1-3mm higher than the teeth of the large chainring. Also, the cage should be parallel to the chainring.
4. Contaminated Cable and Housing
If the shift cable and its housing are severely contaminated and/or worn, the cable will have a hard time moving. In that situation, a replacement of the cable and the housing is in order.
5. Damaged Cable
A partially torn shift cable cannot maintain as much tension as a healthy unit and will create notable shifting issues (e.g., the derailleur not moving to the largest or smallest chainring)
Usually, the damage occurs in the exposed section found the closest to the derailleur.
If this is the state of the cable, the only solution is to replace it.
6. Broken Shifter
If the shifter is contaminated or broken, its trimming mechanism will malfunction. To rule out this issue, it will be necessary to clean the shifter or replace it. Luckily, this is rarely the source of the problem since most quality shifters are very resilient.
7. Improperly Routed Cable
If the shift cable hasn’t been routed properly through the derailleur, the leverage that the shifter has will change. As a result, the derailleur will not move as much as planned during each click.
The trim function will be affected too and may fail to push the derailleur as much as needed to eliminate rubbing.
Setting Up The Trim Function Correctly (Simple Guide)
Step 1: Make sure that the derailleur’s cage is 1-3mm above the largest chainring and parallel to it.
Step 2: Shift to the smallest chainring and the largest rear cog.
Step 3: Turn the “L” screw clockwise or anti-clockwise (it depends on the initial position) until there is about 0.5mm clearance between the inner side of the derailleur’s cage and the chain.
Step 4: Click the small lever a couple of times to definitively move it to the lowest position. Untighten the anchor bolt, pull the slack out of the cable, and re-tighten the bolt.
Step 5: Shift to the largest chainring.
Step 6: Activate the trim function by clicking the small lever.
Step 7: Use the barrel adjuster to finetune the derailleur’s cage position. Ideally, there will be 0.5mm clearance between the inner side of the cage and the chain.
Step 8: Shift to the top position via the larger shifter lever.
Step 9: Shift to the smallest cog.
Step 10: Use the “H” screw to create 0.5mm clearance between the outer side of the derailleur cage and the chain.
Note: The guideline above is generic. Additional finetuning will be needed because every setup is different.
The Benefits Of The Trim Function
- Extra Gear Combinations
Without an option to trim the front derailleur, some chainring and rear cog combinations will be impractical due to significant chain rub. (Chain rubbing creates annoying noise and wears down the derailleur’s cage and the chain.)
The Downsides Of Front Derailleur Trimming
- Extra Complications
The extra clicking needed to activate the trim function could be too much for someone who is new to cycling and not proficient at shifting yet.
- Cross Chaining
Trimming encourages the use of extreme gears resulting in cross-chaining. Cross-chaining is criticized as inefficient and hard on the chain.
Interesting Fact. In the old days, many road bikes kept relying on a friction shifter for the front derailleur even after the invention of index shifters.
Unlike indexed models, friction shifters move as much as the rider wants them to. It’s up to the user to determine the position of the shifter.
This makes friction shifters harder to use when shifting up and down the cassette or freewheel.
However, front fiction shifters shine precisely because the user can very easily trim the front derailleur. If the chain is rubbing, the rider can end the noise by moving the friction shifter in the needed direction.
For example, if the rider is in the big chainring and there’s rubbing between the chain and the inner side of the FD’s cage, the needed clearance can be created by moving the shifter ever so slightly in the downshift direction. This motion will move the FD closer to the smaller chainring and eliminate rubbing.
To take advantage of this effect, some popular riders including Lance Armstrong used a downtube shifter at the front coupled with a brake-shifter in some races. (Another incentive to rely on this method was to save weight.)