Getting a Di2 front derailleur to play nice with a round seat tube is one of those small compatibility puzzles that stops a build cold—Shimano's electronic front derailleurs are all braze-on units, which means without a mounting tab on the frame, the derailleur has nowhere to go. The SM-AD91 solves that problem by wrapping the seat tube with a purpose-built clamp band that presents a Shimano braze-on interface on the outside. It's the current-generation adapter for Dura-Ace and Ultegra Di2 front derailleurs, and it's what we reach for when a customer brings in a titanium, steel, or older aluminum frame that wasn't spec'd with a braze-on tab.

The SM-AD91 comes in two versions to cover the range of seat tube diameters you'll encounter. The SM-AD91-MS handles 28.6mm and 31.8mm seat tubes—the smaller unit ships with a shim that lets a single clamp fit both diameters—while the SM-AD91-L is a dedicated 34.9mm clamp for larger-diameter seat tubes typical of oversized aluminum and some carbon frames. Check your seat tube diameter before ordering, since the two versions aren't interchangeable and the wrong size will either slip under load or refuse to close entirely.

Compatibility covers the full modern Shimano braze-on front derailleur lineup. The R9250 Dura-Ace and R8150 Ultegra 12-speed Di2 derailleurs mount directly, as do the previous-generation R9150 and R8050 11-speed Di2 units. It also works with mechanical braze-on derailleurs across the R9100, R8000, and R7000 series, and with GRX braze-on derailleurs including the RX815 and RX810. If you're building a classic steel frame around a modern Di2 group or replacing a worn adapter on an older setup, this is the part.