Senior Member
Join Date: Aug 2010
Location: Madison, WI
Posts: 11,237
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
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Since your bottom bracket is not centered on your seat tube axis, a derailleur might not work if you tried to install a clamp that wraps around your seat tube. That is the reason that I did not put a double or triple on my Airnimal Joey. (Now, Joeys have an way to add that, but my frame is older.)
You might want to contact the manufacturer of the bike frame.
In my case I put a Sram Dual Drive rear hub on the bike to give me the wider gearing range I wanted. Those are now discontinued, but Sturmy Archer makes a similar hub.
It looks like there is a bolt on the back of your seat tube, if you are lucky there may be a way to mount a derailleur on that.