Originally Posted by
Jipe
I have a similar experience with my Bike Friday All-day: its also very stable and allow very high speed, downhill and on flat roads too.
From my several folding bikes, its the most stable one: the Brompton 1" is the worst, the Birdy is pretty good but I never reached as high speed as with the All-day. My Moulton Speed is as good as the All-day but its not a folding bike!
What is also excellent with the All-day is that its very agile, it allow to take fast very sharp curves.
I've done 41 mph (66 kph) on my stock Brompton (S type) and found it quite stable. That was down the Laguna Seca corkscrew during the Brompton World Championships, which was loads of fun, but have hit similar speeds on regular roads too. There are enough potential wiggly bits in any folding bike though that one may handle differently than another.