Some of the Bromptons are just 3 gears i.e. just the SA 3 speed hub but others have the 2 speed derailleur system which gives them a wide range of gears similar to the Nexus 7 or Nexus 8. It's how Brompton got around the much reduced reliability of the Nexus 7 and 8 for long term use. Some may not like the 6 speed system with 2 shifters but its their engineering solution to the problem. The Nexus 7 is actually a 9 speed hub with 2 gears mapped out so is actually a lot less reliable than the Nexus 8 and only has a 244% gear range compared to something like 308% for the Nexus 8. I personally wouldn't use a Nexus 7 on a bike although admit I do have a folding bike with a Nexus 7 but I don't use it very much. I think I'm right in saying the Brompton has wider range gears than the Nexus 7 (with the 2x derailleur) but not as much as the Nexus 8. I haven't bothered to google to check though. Actually changed my mind just checked. 302% for Brompton and 307% for Nexus 8. Even the standard 3 speed Brompton isn't far off the Nexus 7 as its a wide ratio 3 speed hub.It's the welds that are failing on some Terns, not the frame material. Perhaps aluminium's low melting point, might make poorly skilled factories err too much on the side of caution? It's easy to mess up an aluminium weld. I certainly hope the OP reports back about his conversion. I think the steel framed Dahon is the Boardwalk. The Brompton fold patent has expired, I'm surprised more new folders aren't using that folding mechanism design. I can't cope with 16 inch wheels, and three gears are useless everywhere but parts of Lincolnshire, also while cycling on gigantic billiard tables which stretch forever in all directions - the former I actually fell asleep while cycling on, the latter I've had nightmares about... in both cases there was far too much endlessly boring green.
I wouldn't do any jumps with Dahon's Pandemic Special - no rebar: