Originally Posted by Duragrouch

Bromptons with 2 cogs and 3 IGH, run a narrower than standard O.L.D. (outer locknut distance, the width of the hub in the rear dropouts) in back, I think(?) 110mm. However there are conversions (new rear triangle) that convert Bromptons to 135mm O.L.D. (a MUCH more common standard), to use an Alfine 11 (all gears internal), and I think they fold fine though I haven't seen one, others on here can say for sure.

Once you have a 135mm O.L.D. envelope in back, externally, everything is the same, with a wide (distance between spoke flanges) IGH hub and 2 cogs, taking up the same space as (only) 9 external cogs (on a much narrower hub). HOWEVER, with one qualifier, and that is how much the 9 speed rear derailleur sticks out beyond the dropout (when fully "in" on the large cog), versus the 2 speed derailleur on a traditional Brompton, and whether any additional width causes a wider fold, or if that is just enveloped in complimentary gaps in the front section that folds over it, given the drivetrain is on the inside of a trifold like a Brompton.

The external 9-speed Dahon Curl is designed to compete with the Brompton. That (latest) Curl also has a single sided fork for a more compact fold (versus a double blade fork, not necessarily the Brompton). The previous Curl, all IGH, had a conventional double fork. So that may tell you something.