50x11 on 20"/406x1.75" is plenty high for me, 85 gear inches, I can pedal down mild descents, and steeper down I can coast.

Yes, FD mount is tricky. I just detailed on a different thread perhaps a week ago. Let me try to find that.... ah, here it is:

If you go with a folder and want to put on a 2X crankset, here are the challenges, not huge, just be aware:
- You'll need a front derailleur adapter, UNLESS your frame already has a welded ("braze-on") FD bracket/hanger for a road style FD, if so, the following *does not apply*, everything is a LOT easier! If no welded bracket, you need an FD adapter, because no one makes a clamp-style FD in that large a seat tube diameter. If Dahon steel frame or clone, probably 40mm seat tube diameter. If Dahon or Tern aluminum frame or clone, probably 41mm seat tube diameter (because both use a 33.9mm seatpost, and the aluminum needs a touch thicker wall seat tube for strength). My 40mm adapter worked well, sort of: The fore-aft position of the derailleur was perfect, the adapter mounted the FD generally forward, and had sliding adjustment for that, HOWEVER, the large seat tube diameter, plus thickness of the FD adapter, combined with an overly large top inner lip on the FD cage, prevented the FD from going in far enough for the 2X inner chainring (and totally out of the question for a 3X crank). Spacing the crank out 4mm caused chain drops, the chainline needs to be perfect (see below). The solution was to grind off that top inner lip on the FD, only in the area of interference, which was small and well forward on the inner cage, FD worked perfect. 41mm adapters for aluminum frame were different in mounting the derailleur much further aft, I believe these were designed for the different frame configuration of the Dahon Mu which had rear monostays; If your frame is a traditional rear triangle like mine, that config may not be optimum. I'll try to post pics on this.
- Check chainline for the cassette, i.e., dimension from the centerline of the rear dropouts/hub Outer Locknut Distance (OLD) to the centerline of the cassette. 2X crankset must match cassette chainline. My 7-speed cassette on 130mm dropouts is 43.5mm I think, because the 2X crank I bought that specifies 43.5mm chainline works perfect; As mentioned above, spacing the crank out even a little caused chain drops, but once set up as designed, worked perfect.
- My FD adapter fit Microshift 3X 9-speed FD fine (I originally bought to fit 3X, but liked because of long cage, so chain never drags on small/small combo). However, spring on that is ridiculously strong, could not shift with gripshift. Changed to Microshift trigger shifter, broke after 1 year, replaced with Shimano trigger, durable, but still need to push with right thumb straight for upshifts, cannot sweep with left thumb, force too high. Shimano front derailleur with lighter spring, linkage sticks forward enough that it won't mount straight onto 40mm adapter (first pic below). (Microshift is cantilever linkage, Shimano linkage is double-shear, more durable, but larger linkage.) This issue still unresolved.

Those above are the big challenges, but not insurmountable. While upgrading the crank to 2X, I also tried the "2-piece hollow-spindle" crankset with "ISO External" bearings (most common external standard), i.e., a clone of Shimano Hollowtech 2; LOVE IT, LOVE IT, LOVE IT! Main reason: As bearings wear so slight slack in bearings, I can simply loosen left crank arm, retorque spindle cap to restore bearing preload, tighten left arm, takes 5 minutes. Maintaining preload is critical for improved bearing life; With taper spindle cartridges, once begins to loosen, unable to retighten, so shorter life. Crank is also lighter, and can be removed at home or in the field without a "crank tool". But you will need to buy a special wrench for the external bearings; I bought a 4-way wrench so can fit that many different bearing styles. If you have a bike shop do the conversion, they will have the correct wrench. I also was able to find a "road" crankset so has a "low Q-factor", the pedals closer together laterally than a "mountain" crank. And, in 5x110mm spider which I prefer over 4 arm spiders, however 110mm means smallest chainring is 34T, no smaller; If you need smaller, the inner chainring must be on a smaller BCD.

Everything else is easy, mounting the left shifter, running the cable to the front derailleur, all same as any other bike. Oh and my front derailleur is typical "bottom pull", so I run the cable under the bottom bracket shell and then up to the FD adapter.

Any questions, feel free to ask.

I'll look now for pics of those FD adapters. Got'em:

Steel frame 40mm adapter; Note forward mount and fore-aft slider:


Aluminum frame 41mm adapter; Note more rearward mount and fore-aft slider:


Newer style adapter; Note forward mount and fore-aft adjustment. Vertical mount is held in place with one bolt, I've heard this is not as strong/durable as above designs, however there *might* (don't know) be less interference with the FD cage: