My Airnimal Joey with 24 inch wheels had flex that bothered me a lot during the first few miles of riding it, but I quickly got used to it and it no longer bothers me. Specifically, the seatpost and extension appears to flex fore and aft, as does the steerer tube/stem extender. I am guessing that it is just a function of long seatposts, etc. My Joey is over a decade old, not sure if they use the same parts now or not. I have never ridden a Brompton, or any folder with wheels smaller than 20 inch.