Shimano FD-6870 Front Derailleur Limit Screw Adjustments: How to Get Them Just Right

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The Shimano FD-6870 [paid link] front derailleur is part of the Ultegra Di2 electronic shifting system, designed to deliver fast, precise, and consistent gear changes. While electronic derailleurs eliminate the need for traditional cable tension adjustments, proper limit screw tuning remains essential. Incorrect settings can cause chain rub, poor shifting, or even chain drops. This guide explains in detail how to set the high and low limit screws on the Shimano FD-6870 [paid link] front derailleur for optimal performance.

Understanding the FD-6870 [paid link] Front Derailleur

The Shimano FD-6870 is an electronic front derailleur with auto-trim technology, which automatically adjusts its cage angle to reduce chain rub. However, the limit screws still define the derailleur’s maximum inward and outward travel. These mechanical stops ensure that the derailleur does not push the chain off the chainrings, and they must be set accurately during installation or adjustment.

Tools Required

  • 2 mm Allen key
  • 2.5 mm Allen key
  • 5 mm Allen key
  • Torque wrench [paid link] with low-range capability

Initial Setup Before Adjusting Limit Screws

Before touching the limit screws, it is important to verify the derailleur’s installation and alignment:

  • The derailleur cage should be parallel to the chainrings.
  • The outer plate of the cage must sit 1–3 mm above the tallest teeth on the large chainring.
  • The derailleur must be securely fastened to the braze-on mount, tightened to Shimano’s recommended torque specification.

If the derailleur is not aligned correctly, limit screw adjustments will not produce reliable results.

Adjusting the Low Limit Screw

The low limit screw determines how far the derailleur can move inward toward the small chainring. An incorrect setting may result in chain rub on the inner plate or the chain dropping inside the bottom bracket area.

  1. Shift the chain to the small chainring and the largest rear sprocket.
  2. Using the 2 mm Allen key, locate the “L” screw on the derailleur body.
  3. Adjust the screw until the inner cage plate sits approximately 0.5 mm from the chain without rubbing.
  4. Verify that the derailleur prevents the chain from overshooting into the frame.

Adjusting the High Limit Screw

The high limit screw controls the derailleur’s outward travel when shifting to the large chainring. An incorrect setting can cause the chain to overshoot and drop outside the crankset.

  1. Shift the chain to the large chainring and the smallest rear sprocket.
  2. Locate the “H” screw on the derailleur body, also adjusted with a 2 mm Allen key.
  3. Turn the screw until the outer cage plate clears the chain by 0.5 mm without making contact.
  4. Ensure that the derailleur does not push the chain past the large chainring under forceful shifts.

Fine-Tuning with Di2 Micro-Adjustments

Even with well-set limit screws, small refinements may be necessary. The FD-6870 offers micro-adjustments via the Di2 system:

  • Enter adjustment mode using the junction box button.
  • Use the shift buttons to move the derailleur in tiny increments.
  • Confirm that the chain runs smoothly in all gear combinations without cage interference.

These micro-adjustments complement the mechanical limit settings, ensuring both protection and smooth operation.

Common Mistakes to Avoid

  • Setting the cage clearance too wide can cause noisy shifting and sluggish transitions between rings.
  • Over-tightening the limit screws can restrict derailleur movement, preventing clean shifts.
  • Ignoring derailleur alignment leads to false impressions of incorrect limit screw adjustment.

Careful attention to both positioning and screw settings is necessary for the FD-6870 to operate as designed.

Maintenance and Re-Checks

Once the Shimano FD-6870 front derailleur limit screws are set, they rarely need readjustment. However, regular inspections are recommended, especially if:

  • The bike has been transported or dropped.
  • The derailleur mounting bolt has loosened.
  • The chain or crankset has been replaced.

Checking limit screws during routine drivetrain maintenance helps maintain shifting precision and prevents premature component wear.

Conclusion

Correctly adjusting the limit screws on the Shimano FD-6870 front derailleur ensures accurate electronic shifting, prevents chain drops, and prolongs drivetrain life. By carefully setting both the high and low stops, aligning the derailleur, and using Di2’s micro-adjustment function, riders can achieve a flawless front shifting experience. This fine-tuning step is critical in unlocking the full performance potential of the Ultegra Di2 system.