A two-speed derailleur set-up might win me points for style, but it won't help get me up to Whitestone Pond - not with gearing that'll help me crack the 40mph barrier (I've wound the current Brompton up to 37mph with the assistance of gravity, so 40mph is the next step).
I'll be using a chain tensioner, the standard Brompton one. As mdcc_tester has correctly identified, the effective chainstay length on the Brompton changes when you fold the rear wheel under, so some sort of tensioner is necessary.
As for the chainrings/cranks, the plan at the moment is to dig out a suitably-sized sheet of titanium from my stores, give it to a nice man with a water cutter, and have a titanium chain guard made up and fit that in the outer ring position. I can't see that the ramps and pins will have any effect, but if the lowered teeth are a problem (and at present I suspect they won't be) then a combination of another sheet of titanium, said nice man, and my CAD package (which has a function for plotting chainring teeth) will result in a new titanium chainring with full-height teeth. All other issues should be capable of being overcome with time, and the assistance of my milling machine and lathe...
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