Custom lots-of-titanium Brompton

Another idea is using a Schlumpf 2 speed crankset.

It's also another way of adding 2 speed to your fixed wheel bicycle.

this requires modding the BB though and not a particularly cost effective way of doing it (they cost a packet) - probably cheaper is buying an ebay hammerschmidt and buying the 130bdcd ring adapter, i'll be doing it if i decide to remove from the mtb