Full Speed Ahead KFX Di2 Seatpost27.2x350mm SB25 MTC. The FSA KFX Di2 Seatpost is a carbon seatpost designed for modern mountain bikes and Di2 electronic drivetrains. It features internal battery integration for a clean setup and a lightweight yet strong construction for trail and XC use. This post offers a balance of compliance and stiffness for comfort and performance on demanding terrain.