Ask a Mechanic: Installing Shimano’s D-Fly Unit on Di2 Systems Ask a Mechanic: Installing Shimano’s D-Fly Unit on Di2 Systems Greg O'Keeffe2016-03-24T11:39:52-08:00 Donald wants to know, “What does Shimano's D-Fly unit do and how is it Ask a Mechanic: Installing Shimano’s D-Fly Unit on Di2 SystemsGreg O'Keeffe2016-03-24T11:39:52-08:00