Now, if this was a common problem, you'd think all the people with modified R20s, bromptoms, moultons and bike fridays would have come forward already.
Maybe they're not as fast as me or maybe their wheels aren't as small as mine.
Also, if your brake ability exceeds the limits of tire traction how can you tell if the performance is reduced unless it drops below the threshold of tire traction? So if you have a very high quality brake as one surely would on a bike friday or moulton and/or if you only use the best brake pads (koolstop) then perhaps you might not notice? But I've seen lots of complaints about the brake performance of Bromptons and other folders and if it is indeed related to wheel curvature then I wouldn't expect folders with larger wheels like R20s to be as affected. The braking action of V-brakes and calipers are also different in their vertical profiles, so that may be another variable obscuring user experiences.