The Shimano FD-6870 [paid link] front derailleur is part of the Ultegra Di2 electronic shifting system, offering precise, fast, and consistent front shifting. However, installation errors are one of the most common reasons for poor performance. Even the smallest mistake can lead to chain rub, mis-shifts, or complete failure of the electronic trim system. This guide covers the critical installation mistakes to avoid when setting up the Shimano FD-6870 [paid link] front derailleur and explains how to correct them for reliable shifting.
Incorrect Mounting Height
One of the most common installation mistakes is mounting the derailleur at the wrong height. The Shimano FD-6870 [paid link] must sit close to the largest chainring without interfering. If mounted too high, the shifting becomes sluggish, and the motor may struggle to move the chain. If mounted too low, the cage will strike the chainring teeth and prevent proper movement.
The correct position is 1–3 mm above the tallest tooth of the large chainring. This spacing ensures smooth engagement and allows the derailleur’s auto-trim function to work correctly. During installation, always measure this gap carefully before tightening the clamp bolt.
Improper Alignment with the Chainrings
The front derailleur cage must be parallel to the chainrings. A common mistake is tightening the derailleur in a slightly angled position. Even a few degrees of misalignment will cause the chain to rub or derail during shifts.
When mounting the FD-6870, align the outer plate of the cage with the large chainring’s edge. Shimano recommends checking the alignment both at the top and bottom of the cage to confirm it is perfectly straight. Electronic derailleurs cannot compensate for angular misalignment, so precision at this stage is critical.
Incorrect Cable Routing and Tensioning
Although the FD-6870 is electronic, the support bolt (often called the “stabilizer screw”) must be set correctly to prevent flex under load. Many mechanics overlook this step, leading to weak shifting under heavy pedaling. The support bolt should rest firmly against the frame’s designated plate or reinforcement point.
Failure to properly set this contact point will cause the derailleur to move during shifts, leading to noise, hesitation, and poor accuracy. Always adjust the support bolt after setting derailleur height and alignment.
Skipping the Initial Positioning Setup
The Shimano FD-6870 requires an electronic setup sequence during installation. Skipping this step is one of the most damaging mistakes. Without proper setup, the derailleur won’t know its limit positions and will either overshift or undershift.
Using the Shimano Di2 adjustment mode, the derailleur must be positioned against the low and high limit stops electronically. This ensures the motorized cage moves within the correct range. Riders who neglect this step often experience chain drops or excessive rubbing.
Overlooking Limit Screw Adjustments
Some assume that electronic derailleurs automatically set their own limits. While trimming is automatic, the physical limit screws still need to be adjusted manually. Failing to do so can result in overshifting off the chainring or binding against the frame.
The low limit screw prevents the chain from falling inside toward the bottom bracket, while the high limit screw stops it from overshooting the outer chainring. Precise adjustment is required because the derailleur motor will drive against these stops during operation.
Not Checking Chainline Compatibility
The FD-6870 front derailleur is designed for Shimano’s 11-speed road chainline. Installing it on frames with non-standard chainlines or wide bottom bracket systems can create alignment issues. Mechanics sometimes attempt to compensate with extreme adjustments, but this only introduces further shifting problems.
Before installation, always confirm that the crankset, bottom bracket, and frame are compatible with Shimano’s recommended chainline. Ignoring this step leads to chronic rubbing and premature wear.
Failure to Update Firmware
Electronic derailleurs rely on firmware to function correctly. Installing the FD-6870 without updating the Di2 system may result in limited trim range or communication errors. Riders often report poor performance when using outdated firmware.
During installation, connect the bike to Shimano’s E-Tube software and update all components in the system. This ensures compatibility across derailleurs, shifters, and junctions. Skipping this process can create persistent shifting problems that appear unrelated to installation.
Over-Tightening the Clamp Bolt
Over-tightening the derailleur clamp bolt is another common mistake that can damage both the derailleur and the frame. The FD-6870 requires a specific torque setting to secure it without distorting the cage or crushing the frame tube.
Always use a calibrated torque wrench [paid link] during installation. Shimano specifies the correct torque range in its service manual, and exceeding it risks permanent damage.
Using an Incorrect Chain Length
An improperly sized chain can exaggerate derailleur setup issues. If the chain is too short, the derailleur will struggle to move across the range under load. If too long, it will sag and cause inconsistent shifting.
The correct chain length must be determined with the large chainring and largest cassette cog engaged, leaving just enough slack for proper tension. Skipping this step undermines even a perfectly installed derailleur.
Conclusion
The Shimano FD-6870 front derailleur delivers flawless electronic shifting when installed correctly. However, small installation mistakes can quickly ruin performance. Errors such as incorrect height, poor alignment, skipped setup sequences, and neglecting firmware updates are among the most common. By following precise installation steps and avoiding these mistakes, riders can achieve smooth, reliable, and long-lasting shifting performance from their FD-6870 front derailleur.