The Shimano FD-6770 [paid link] front derailleur, part of the Ultegra Di2 electronic shifting system, is a precision component designed to deliver smooth and consistent front shifts. While the system is electronic, correct mechanical alignment and precise limit screw adjustment remain essential for optimal performance. Incorrect limit screw positioning can cause chain rub, missed shifts, or even chain drops. This guide explains how to correctly set the FD-6770 [paid link] limit screws to ensure reliable shifting.
Understanding the FD-6770 Limit Screws
The FD-6770 [paid link] front derailleur uses two mechanical limit screws, even though it operates electronically. These screws do not directly move the cage; instead, they establish the maximum range of movement for the motorized derailleur.
- Low Limit Screw (L): Controls the inward travel of the derailleur, ensuring the chain does not overshoot the small chainring and drop onto the bottom bracket.
- High Limit Screw (H): Controls the outward travel, preventing the chain from overshooting the big chainring and falling off to the outside.
Both screws must be set precisely, as the electronic system relies on these hard stops to know when the derailleur has reached its limits.
Tools Required
- 2mm Allen key
- 5mm Allen key (for derailleur mounting)
- Torque wrench [paid link] (recommended for accurate adjustment)
- Bike stand [paid link] (optional but helpful)
Preparing for Adjustment
Before adjusting the limit screws, confirm the derailleur is properly mounted. The FD-6770 should sit with the outer cage plate parallel to the chainrings. The lower edge of the cage must clear the largest chainring teeth by 1–3 mm. Correct alignment is critical, as limit screw adjustments cannot compensate for a poorly installed derailleur.
Check that the derailleur is powered on and functioning. Run through the shifts to verify the cage moves smoothly. If shifting is inconsistent, address cable routing, battery charge, or firmware issues before proceeding with screw adjustments.
Setting the Low Limit Screw
Shift the chain to the small chainring and the largest sprocket on the cassette. This position places the chain at its most extreme inward angle.
- Observe the clearance between the inner cage plate and the chain.
- Adjust the Low (L) screw until the cage sits approximately 0.5–1 mm from the chain without rubbing.
- Test the shift several times by moving between small and large chainrings to confirm smooth inward shifting without overshooting.
If the cage is set too tight, it will rub when cross-chained. If set too loose, the chain risks dropping inside the crankset.
Setting the High Limit Screw
Shift the chain to the large chainring and the smallest sprocket on the cassette. This creates the most extreme outward chain angle.
- Check the clearance between the outer cage plate and the chain.
- Adjust the High (H) screw until there is 0.5–1 mm of space without contact.
- Test shifts by moving between the small and large chainrings under load. The cage should move outward enough to allow smooth shifts but stop before the chain can overshoot.
An overly tight H screw will cause hesitation or incomplete shifts to the big chainring. Too loose, and the chain may drop off the outside.
Fine-Tuning the Adjustment
Once the limit screws are set, run through all gear combinations to confirm there is no rubbing at extreme angles. Pay particular attention to small-to-large and large-to-small cross-chain positions, as these are where clearance is most critical.
If issues persist, recheck derailleur alignment. Limit screw adjustments cannot fully correct a cage that is rotated or set too high above the chainrings.
Troubleshooting Common Problems
- Chain drops inside: Loosen the Low limit screw slightly.
- Chain drops outside: Tighten the High limit screw slightly.
- Slow shift to big chainring: The High screw may be too tight or the derailleur misaligned.
- Excessive chain rub: Recheck mounting height and cage parallelism.
Always make adjustments in small increments, typically one-quarter turn at a time, to avoid overshooting the correct position.
Why Proper Limit Screw Adjustment Matters
The FD-6770 relies on both electronic control and mechanical stops. While the motor ensures precise movement, the limit screws define the safe boundaries. Proper adjustment ensures:
- Smooth and reliable front shifting.
- Prevention of chain drops and drivetrain damage.
- Maximum lifespan of chain, chainrings, and derailleur.
By correctly setting both the Low and High screws, you establish a reliable foundation for the entire Di2 shifting system.
This concludes the detailed guide to FD-6770 front derailleur limit screw adjustments.