I'm riding a Huffy beach cruiser that originally was a single speed. When I built the bike I built it for a future instillation of a jackshaft. I took both wheels off of a 26" mountain bike (3x7speed). I had to bend the frame slightly. I did this by laying the frame down, one foot on lowest part and pull by hand..and reversed procedure. I wish that the axle was a little longer as it just fits and I'm not provided with extra as I would like.
I also have a 3 speed hub I might use. I'm not looking for speed, just operating at a lower RPM.