actually i wonder what are the differences for foldies.... why some are faster then others
the geometry? frame stiffness? gearing?
personally, tire pressure makes quite a lot of difference....
I haven't read up enough to explain.
But basically.. geometry determines the riding position and stability. Aero vs non-aero positions, wheelbase distance determines the steering/balancing and hence the stability.
Frame stiffness determines the power transferred to the pedals. And also the degree of comfort.
Gearing wise.. not much of an issue since most cyclists are already using standard shimano/sram/campy gearings. Just stick to a groupset the cyclist is comfortable with.
Folding bikes are more difficult to understand due to their geometry limit (folding clearance) and the hinges/pivots-- which are the weakest link to the entire folding bicycle.
But generally, if the folding bicycle doesn't have a mid-frame hinge (e.g. Tyrell/BF), it is more stable with lesser vibrations at higher speed (quite noticeable when >30 kmh). That's the reason why i wanna change from tern to BF/Tyrell. The compromise is the larger folding footprint.
Other factors come into play too. Tyrell has a very aggressive riding position (may not be comfortable for ppl with neck/back problems) with very sleek and lightweight frame. The total weight is <9kg. I have seen Tyrell FX/FSX zooming past me with ease even when I was cheonging at 32kmh along coastal roads. It is generally at least 2 plus kg lighter than normal folding bikes so there will be a big advantage.
But of course, the engine does matter. Whether one can sustain high speed for long distance will be the key. If my 16" brompton can sustain 28-30 kmh along the whole of changi coastal, i am sure most other bikes can as well. The amount of effort to maintain 30 kmh on my 16" brompton is way higher than maintaining 30kmh on my 22" tern.