interesting thread. I have an older mariner (looks like a boardwalk frame, cromo) that had a worn derailleur. I ordered a sa 3 speed from Utah trikes, the wheel was laced, shifter, cable and a tire all for $ 108.00 shipped. every thing lined up perfectly except the cable was to short and I bought a new chain. I did not want a coaster brake as I am just more at ease with the old center pulls. this was about the best thing I have done to this bike except ride it. the gearing works very well, I like it much better than the 6 speed as it has been trouble free for over 50 miles. I have other bikes but this has became my go to for about all city riding. I would do this in a minute for a single speed and for the life of me do not know why dahon does not offer this for a standard boardwalk.