There's a lot of information we could cover here but unfortunately I don't have the super technical knowledge about this to get too in depth, nor do I think you would want to read a bunch of wordy jargon to begin with. So what will we talk about?
This has a lot to do with what's called folding bike frame geometry which simply means the relative position of the wheels, seat, and handlebars. Another primary factor which affects the ride quality of a folding bike is the material of the frame. Finally, the number, location, and quality of hinges greatly affects ride quality (a point which is of course unique to folding bikes).
First lets tackle folding bike frame geometry. Folding bikes with small wheels would be very awkward were it not for the fact that their designers very intentionally placed their handlebars, wheels, and seat in pretty much the same relative positions you'd find on a regular bicycle. There is slight variation between different folding bike frames as to the exact location of each of these critical points but on the whole they mimic a neutrally placed hybrid bicycle. In order to compensate for the quicker handling of small wheel folding bikes most designers give their folding bikes a longer wheel base. Without getting too technical, lets look at what these different folding bike frames look like.
Each folding bike frame has its advantages and disadvantages. Take the Xootr Swift for example. The Xootr Swift has no hinge in its main frame. This affords the folding bike a rigid, inflexible, and very fast ride quality. On the other hand, the Xootr Swift doesn't fold nearly as small as other folding bikes with a hinge in the main frame. Because the Swift folding bike was designed to be a sturdy, unbreakable folding bike its creators traded off compactness for stiffness. This gives the Swift a rigid, straight look.
Sometimes this kind of trade off doesn't have to be so harsh, as you can see with the Birdy folding bike. The Birdy, a performance folding bike, was also designed to be stiff and responsive and as such lacks any hinge in the main frame. However, the Birdy's designers were still able to make a compact folding bike by having both wheels swing under the frame, greatly reducing its size. With the a folding bike like the Birdy, where each wheel pivots under, you can see a bike who's frame is lower to the ground, with more pivots and moving parts. It looks much different from the Swift.
Dahon folding bikes are world famous fold-in-half bicycles. These folding bikes have one hinge in the main frame, one hinge to fold the handles over, and telescopic handlebars. Although they come in a great variety of frame shapes, the prevailing (and as far as I can tell) best design is the singularly arched frame of the MuP8. This arched, oval cross-sectioned frame is very strong and supports the rider with a stiff and relatively inflexible frame. Dahons have the advantage of being relatively inexpensive and good value folding bikes.
Their frames are made of aluminum of varying degrees of quality (dependent entirely on price) with the low end being adequate and the high end being extraordinary. Bikes like the Dahon Vector may be the fastest folding bikes on earth, but they also come at their price. This is where folding bike frame materials come into play but more on that later.
Brompton folding bikes have the most advanced frame design, yet also the simplest folding mechanism. With one hinge in the main frame, one hinge to lower the handles, and one hinge to swing the rear wheel under Bromptons maintain their stiff ride by having very tightly locking hinges and very high standards of workmanship. Their lightweight steel frames are brazed together by hand in London. This is a stamp of quality its tough for other folding bikes to beat. Because Brompton folding bikes have such tight tolerances and high-quality materials they are able to maintain their place as the worlds most compact, fastest folding, and arguably best riding folding bikes with ease. Of course, a quality folding bike frame like that comes at a price.
Dahon Midtown Mini folding bikes are excellent compromises between full size folding bikes and compact folding bikes. Their dual arch frame looks more like a standard bicycle, and with lock jaw hinges, makes for a very stiff and responsive ride. This folding bike is a bit more spread out and rigid than its smaller cousins but because it has two main tubes and stronger hinges it is more suited to hard use and long distance riding.
What makes a good hinge? Primarily it shouldn't come loose when riding, but all these folding bike makers accomplish that basic function. The other points of importance include: how fast can you release it when folding the bike? How strong is the hinge? How much flex and movement does it allow? Do you need any tools (or more than one hand) to operate it? How durable is it?
All these factors go into making a good hinge. Dahon hinges are excellent. Very fast quick release mechanisms and good solid connections are a hallmark of Dahon's hinges. Lower quality models will suffer from loose hinges as they age, but this is inevitable. Brompton hinges use a screwed locking mechanism where a clasp secures two plates together. This design is very strong and resists wear and tear but is not quite as fast as the Dahon hinge mechanism. And the strongest hinge of all? Well that would probably be the lock-jaw hinge found on full-sized Dahon folding bikes and Midtown Mini folding bikes like the Dahon Smooth Hound. With this hinge interlocking teeth are brought together in a very tight connection by using a hex wrench to twist the hinge bolt about half a turn. This is the slowest design of all but very strong and stiff.
In the end there is so much to folding bike frame design that you could write a book on the subject (as I'm sure someone already has). I hope you've gotten the idea that with so much variation there are always compromises made. Only by carefully weighing the pros and cons of each design against your needs can you determine the right folding bike for you. Of course, it helps to test ride them and talk to an expert. Come by our shop to try out all these folding bike designs.