Originally Posted by
northernlights
I'm sure some people are fine with the ergonomics of the bike, but 20 inch is pushing it already in terms of riding comfort, speed and ergonomics.
Looking at these pics you may understand the ergonomic aspect. Wheelbase, distance saddle-bar etc. are pretty normal on a Brompton while they are often not on other folders (in this case the Birdy and a Dahon Curve).
Ergonomics have absolutely nothing to do with the wheel size but with the frame geometry.
Originally Posted by
northernlights
Generally speaking, the smaller the wheel the lower your speed,
This is wrong. Smaller wheels have a slightly higher rolling resistance but they are both, lighter (important for rotating mass) and stronger, plus they offer better aerodynamics and acceleration. Try it at a traffic light of your choice.
Moulton and Brompton both overcome the rolling-resistance issue by using high-pressure-tires. Alex Moulton has done years of engineering work on the topic and - according to him - small high-pressure-wheels in combination with suspension are the best of breed. The fact that the land-speed record for upright bikes is still (and for ages) held by a Moulton bke suggests that he may have been right.
If you do not pump up your Brompton's tires to the suggested 100 PSI but run on low pressure you'll suffer from a significant loss of efficiency (and therefore speed). If you do you are just fine. The only modern Brompton tire that is designed for less pressure is the Scorcher (due to it's width it has lower rolling resistance is therefor fast AND comfy already at lower pressures).
Originally Posted by
northernlights
the rougher the ride due to less shock absorption over rocks, bumps and potholes, more twitchy handling, etc. I might be somewhat biased but I have heard plenty of such complaints about Bromptons. Generally speaking, going over a pothole or unexpected bump on any 16 inch wheeler is likely to send you flying. Some people are fine with those limitations, and that's fine. I'm sure there are those who would never ride a 20 inch bike for similar reasons.
Surely smaller wheels are more affected by potholes and riding a cobblestone road with a Brompton is no fun. But on the other hand bikes with smaller wheels are more agile, therefore avoiding potholes by navigating around them is far easier than with a bigger bike. Still, a Brompton obviously is no Mountain Bike and also you won't reach the comfort level of a 20" bike with 55mm Big Apples on it. Still from my experience most of the judgements about "bad ergonomics", "bad ride quality" etc. come from people that have never ridden a Brompton for a while - they are just prejeduces, based on emotion, not on objectives.