I was having a chat the other day with one of the guys on my train that has a Birdy Folder. I was moaning about climbing the hill out the station and saying I might fit a smaller chainring (which I've done), and he said can't you fit a front derailleur and two chainrings? I did a quick search to see if I could find any details and came a cross a post by @rualexander where it mentions doing it, but doesn't say how. If you're reading this, do you have any more info? Or anyone else know if this has been done successfully? What I've found on my first day of my new chainring (44T) is that it's much better for climbing, but the bike now tops out at about 20mph. I'm not sure how much of a problem this will prove to be yet. Although it was frustrating as I was chasing down some guys on road bikes, and when we hit a downhill section, I had to give up as I couldn't spin any quicker.