A few people on the Human Powered Vehicles (HPV) mailing list were lamenting the lack of a good folding recumbent. Good meaning (to me) something that is comfortable to ride, and folds really small, and is affordable. So those were my design goals.
Recumbent folders are tough, not just because of the long frames, but because of the long chain lines. Chains don't take kindly to being folded out of their plane (I'm not sure how the Brompton conversion kit folds). So for folders the choices are chain removal, or to have the 'bent fold in the same plane as the chain.
I decided to see (roughly) how small a recumbent could be folded in the vertical plane. The main problem is that the tires hit each other as they fold under. Ideally you want them to overlap. So I chose to offset the hinges a few degrees from perpendicular, allowing the wheels to slide across each other as they fold under. This will offset the chain out-of-plane only slightly, well within a chain's tolerance.
You end up with a size (with seat removed) of 30x22x16 inches. Technically, this is too large for most airlines (62 inches sum of length, width, and height). But I own luggage larger than this, and the airlines don't even blink at it. It's certainly trunk-sized. It's also small enough to be convenient on commuter rail, and even subways.
It might be possible to have different sized bikes just by producing different center sections (which after all is just a single tube). But, only one center section size would give you optimum fold. Shorter would give too much overlap, longer wouldn't give enough. Also note that for shorter sections, the overlapping wheels would collide, unless you increased the hinge offset angles. This increase would mean more wasted space (between the overlapping wheels) in medium and larger frame sizes.
Since there's already a hinge in back, a rear suspension seems like it should be easier. In my design though, the hinge may be too far forward for a good suspension (I really don't know). Also, with the hinge being non-perpendicular to the frame, this may make the suspension act "weird".
Now that I've seen this design, I'd be interested in moving the rear hinge back. You'd have a longer fold. But it could be made narrower. The hinge offset angles could be smaller. It would be better positioned for a rear suspension. You could put the seat on the center section, and have a wider range of seat adjustment, and not have to worry about multiple frame sizes.
I've also thought a bit about a SWB folder. Imagine one with a rear suspension. The rear would fold under, overlapping the front (which doesn't move). The offset hinge angle could probably be very small if they only partially overlap. The boom could fold up, and back on top of the (folded down) seat. But there are already SWB folders that get really small. They are expensive though. And folders make sense for commuting and vacation tours, where IMHO most people probably would prefer medium or long wheelbase anyway.
I don't intend on building this design (lack of: time, money, tools, workspace), but I invite others to try. If you want to build it commercially, build the first 25 free. Past that, send me one free production sample for non-exclusive rights to manufacture up to 1000 units. Or contact me and we can negotiate.
Sat-R-Day folding recumbent from Bike Friday.
Recumbent Conversion Kit for the Brompton Folder.
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