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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It seems to me you could use more or less any crank; the critical thing will be to get the right length of the BB spindle to ensure the best chain line. By "any crank" I mean pretty much that; a cheap option with lots of choices in the US, for example, would be a "Bulletproof" bmx crank, available in a wide variety of crank arm lengths; and you get to chose the chain ring separately, so you can have any size there. I, personally, would avoid a MTB crank because they usually have 175 mm arms, which I consider frightfully long. Of course I have no idea what's available in Tokyo; if you go to a store, be sure to ask about BMX cranks. If you shop on ebay or internet, you should be able to find pretty much anything.
Get a chain that suits your chain ring. If you get a BMX crank, with a thick BMX chainring, you may need a BMX chain. I don't think the BMX chain is stronger than others, but the thicker chainring will wear longer.
I have a Rohloff hub retrofitted to an old (1984) tandem that originally had a 6-sp freewheel and triple crank. I am still using the original crank, with the middle chain ring (44T) moved to outer position, and the original crank, shortened to fit. So I'm running a 44-15 combination. I've gone over some pretty brutal hills with that, and never needed a lower gear. I'm sure I would have used it if I'd had it, though!