Trevtassie

9 speed chain is wider than 8 speed and narrower than 10 speed. 20T is not a very big front ring, There aren't many FDs that can get down that low and still have clearance on the 40T over the full range of the rear cassette, the chain only just clears the bottom of a 9 speed shimano FD if it's designed for a 40T large chainring.. A 10 speed FD won't work without chain drag somewhere because the side plates are too close together, I tried, that at first because that's what was on there before I changed the crank.. 8 speed might work, but I didn't find one in my parts box that suited the 40T. So yeah, FDs can be 8, 9, 10 or 11 speed compatible, depending on the width between the side plates. I may be wrong about the FD I used being Deore, it could be Alivio, but I'm not going out to the shed in the dark to look. I do know it's side swing and 3 x 9 speed for a 40T max chain ring size.