I have a 2014 2-speed Brompton. This is completely enough for me during everyday circumstances but I would like to have the option of a bail-out gear for hilly regions (I travel a lot). I don't expect to use it often so "hand-shifting" would be fine. All instruction I have seen relates to the old pre-2013 crank.I happen to have one of those cranks at home and a 86 BCD chainring which would allow doing the conversion the old way (but would require new bottom bracket) but I am not sure this is the best way now. Would it offer any advantage over just getting a normal compact road crank? Getting a normal road compact (for tapered square) seems like a very easy solution but I haven't seen it done so I guess there is a reason for that. I am not using the Brompton pedals so the feature about stopping pedals from "overfolding" is not interesting for me.
the new spider crank is machined to only have 1 chainring on the inside face . so you would still be wise to buy a new double crankset ,
PDX Brompton Dealer's blog had stuff about him touring the OR/CA coast, using a 110 BCD double to get a second range for his 6speed M6L
a smaller one.
I use a 2 speed crankset . internally geared.
others have done that , with your 2 speed hub , perhaps as they did , a schlumpf speed drive , 1.6x overdrive .
made in Germany, now, rather than Switzerland, where the developer lives
(CHF exchange rate was getting to be too hard to be affordable in the rest of the world markets.)
so a 34t chainring acts like a 54 when the kick to shift button in the crank center is pushed ..
the swaged arm to chainring fitting of the previous manufacturing runs , as I have read ,
when using that 2nd 28t chainring on the 86 BCD
and stomping up mountains , the arm and outer chainring can be separated and the teeth between them strip.
Myself I'm using another Schlumpf crank type (of the 3) a 2.5 reduction gear with my 3 speed M3L the mountain drive.
the 3 hub gears are used twice, in the low range again . so a 50t acts like a 20t in low gear range. all 6 gears are useful .. whnthe road is steeper ..
the EZ shift plate covers the whole crankarm making the gear change simple .. I too am using a non folding pedal on both sides.
for double chainring shifting , I had bought an adapter mount,
it clamps around the frame tube and allows a road braze on FD to be installed , with it's own thumb shifter,
it sits here in a box , so I would consider selling it .