I've been riding an old Curve SL for the last couple of weeks and I've been surprised by the way it rides compared to my Vitesse. When I first got my Vitesse I was not at all happy with the harshness of the ride, it seemed as though the smallest of road imperfections were going straight to my hands/spine so I set about trying to make it more comfortable. I settled on a 2.15 BA on the front wheel and a 1.85 Maxxis tyre on the rear, along with a thudbuster ST with soften-than-recommended elastomer. I found this combination to work pretty well. But I still found that the short wheelbase and small wheels led to a seesaw-type whipping effect when traversing certain sorts of road imperfections which eventually led me to adopt a more upright, 'comfort' riding position so that the Thudbuster can do a lot of work on the rear and my weight is off the unsuspended front wheel. All in all that provides a pretty decent ride.
I expected that all these issues would be intensified with an even smaller bike; shorter wheelbase and smaller wheels. And the rear wheel is closer to the cranks, i.e. more 'under the body', meaning that up and down movements of the rear wheel will be felt more directly. Yet somehow the 16 x 2 Big Apples tyres on my Curve SL seem to work better than the 20 x 2. I don't know what it is. . . maybe it's something to do with the ratio between tyre width and wheel size? They seem to have more 'bounce'.
I do find that vibrations from the front wheel translate to more front-to-back handlebar movement than with the vitesse and there's clearly more handlepost flex, but besides that the ride is pretty much as smooth. On top of that the slightly more reclined angle of the seat tube results in the Thudbuster being more active and it really soaks up ripples effectively. I've commented before that the Thudbuster is designed for a less vertical seat tube angle than is typical on most Dahons and riding the Curve SL appears to confirm this.
My conclusion is that if there were something like a BA tyre in a 16 x 2.35 or 2.5 size the Curve SL would ride more smoothly than a Vitesse with 20 x 2 tyres and of course have the more compact fold. I wish I could get my hands on a tyre that size.