Given that 20in wheel folders exceed the 90*70*30 strict national rules of carriage, so rely on individual Train companies own rules/ disgression, but are fairly bulky and only a fold little smaller than 24in wheeled ones, why are they so ubiqutious? Surely once above the magic size, you might as well have the biggest wheels for a given fold size....
(yes I know some companies specify folded size using wheel size but thats a stupid unit for folded size IMHO and if 24in folders (of similar folded size to 20in dahons) were the majority presumably they would say 24in max...)
eg Tern Node D16 (24in wheels) fold size of 41 x 85 x 80 cm
Tern eclipse-tour(24in wheels) fold size of 42 x 89 x 76 cm
Tern Link C7 (20in wheels) fold size of 34 x 85 x 65 cm
Not a huge difference......