Originally Posted by
ttakata73
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.
I've been saying this for years on this forum. This aspect needs a redesign, but this flex is in fact softening the ride considerably. So once frame stiffness is optimized a new rim format with wider tyres would be necessary to improve the ride quality and with that rolling resistance. It's accepted wisdom now that wider tyres roll better, and on such small rims running narrow tyres is plain silly. I'd summarize the thread dedicated to figuring out why Brompton's are slower than other folding bikes roughly in this way: the Brompton frame is providing the suspension rather than the tyres, the rider's energy is going into flexing the frame and the tyres roll poorly.