Originally Posted by
Retro Grouch
Way back when I remember a similar rig done the other way.
I vaguely remember something that was even commercially available to enable you to connect a freewheel cog to the inside of your crankset as a 4th chainring. Unfortunately, I can't remember the maker's name, what the part was called, or how well it actually worked.
That's exactly what I thought of when I saw this thread. There was an adapter to allow you to use a freewheel cog as either a granny for a triple or as a 4th chainring. That was before MTB cranks with their 20 and 22T granny rings were available and it allowed a really low low gear you couldn't get any other way.