Join Date: Nov 2006
Location: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
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Aside from maintanance issues, covered above, and inherent fragility of a derailleur, and the convenience of shifting a hub gear, there's the issue of gearing:
A derailleur can be made to work pretty well with 20" wheels, but on smaller wheels it becomes problematic. I customized my Super-Cheap Kent Folder to run with a 52 Tooth cog in front, and the bike was still woefully undergeared. The six gears were marked Slow, Slower, Even Slower, Even Slower than that, Awesomely Slow, and Barely Moving At All. More expensive cassette drives have smaller rear cogs, which offsets that problem but introduces others. You can have a large chain ring custom made; this will probably solve all the problems quite nicely, but not cheaply.
The Sturmey-Archer 8-speed hub is ideally suited to a folder with small wheels; a Shimano 8-speed hub also works pretty well, you just have to have a smaller rear cog and/or bigger chain wheel.