Originally Posted by
fietsbob
Chris K and Phil W are both in the external bearing BB game,
but CK does not do square taper
http://chrisking.com/bottombracket so you need an the new type crankset too..
No personal experience... Schlumpf Mountain drive
is offered in a Brompton version, it's a quick install..
.. but inappropriate, I expect, for Flatlands like Florida .
Speed Drive crank, perhaps if you can turn monster gears ,
as the overdrive is 165%
but the gear system is not weightless.
The bottom bracket i mention is NOT external bearing, just a good old ISO square taper. (I should get myself to put pictures online of things I mod for others to use as a reference...)
When you ask for a PW bottom bracket that is what is usually implied. Phil Wood does make external ones as does Chris King. I am not sure what cranks are compatible with PW though...(I assume Shimano due to popularity). Chris King has 2 models Road and MTB. Road and MTB each come in two varieties Shimano (Hollowtech, think Ultegra, Dura Ace) and SRAM (GXP, Gossamer etc). As with all Chris King stuff you can convert a Shimano one to SRAM and vice versa. It holds true for all BB crank combos that the chainrings must clear your chainstays and give you the desirable chainline. On the Brompton the chainrings should not interfere with the rear triangle when parked. Using an external BB on a Brompton might work, however, I personally will not go down that route. Get a Phil Wood one in square taper and you are set for many, many miles and customer service seems to be top notch if you ever have problems. You are in the US so sending it back is even easier. That way you can keep your current cranks as well.
When it comes to the headset, if you have a CK spare I would say go for it=)
-Carcinogent