Originally Posted by ruchai
The reasons Brompton do not use deraileur is because of the small wheels. There are limit in the size of the big gear before the deraileur hit the road surface. Distance between bb and real hub in the Brompton is too short for proper deraileur operation.
I'd just thought i'd show this link, of a derailleur equipped Brompton:
http://www.ikd21.co.jp/bro/sp/
These are made by a English chap called Steven Parry.
They are made by cutting the hubs down to size, rather than spreading the rear triangle.
And look at the size of that front big ring!