Originally Posted by
feijai
Derailleurs are not chosen based on the size of the wheel. They are chosen based on (1) the total slack that might occur between the smallest-tooth gear combination and the largest and (2) the size of the largest rear gear.
Tell that to Dahon, Tern and virtually every other bike company out there. Their 20" folders are made with compact derailleurs designed for smaller wheels, which is called common sense.