An easy way to check front end flex is to push the bike against a solid wall, weight the bike, and push forward on the bars while looking down the stem towards the brake. I can clearly flex the Brompton stem enough to block my view of the brake when I push forward. On my cheap Dahon Dove I can see it flexes much less because the stem is tapered and actually clamps the outside of the steerer tube. I can't detect any front end flex on my Hooligan which runs a 1.5" steerer; it's built like a tank.
It's probably not the Brompton's stem flexing much but the loose quill clamping interface that magnifies the top of the stem's movement. A quill stem only presses against a small portion of the steerer's inside (smaller diameter) surface whereas threadless stems clamp the entire outer (larger diameter) surface of the steerer. Bottom line it's a poor design that the rest of the bike industry left behind, but Brompton is too cheap to update.
Regarding the Birdy issues above:
3) not much frame clearance. Clearance for what?
4) not much tyre clearance if fit wide tyres. My Gen3 GT comes with 18x1.9" tires which is OK for me. Wider tires force a wider fold so it has to stop somewhere.
5) alloy frame get worn away if chain line incorrect Yes, biggest chainring I can fit is a 58t (still too low a gear). Any bigger and the chain will start eating the swingarm when on the 32t cassette. Hopefully whenever Gen4 is done they realize it needs gearing to match a regular 700c road bike and redo the entire rear swingarm.
8) stupid chain catcher can get bent and jam in rear cassette, breaking derailler, Gen3 moved this to the bottom bracket.
Last edited by ttakata73; 06-22-18 at 11:27 AM.