The Tribike is an attempt to create a “hexayurt for transport” – something minimally functional that can be made with common parts. The core idea is to use a tetrahedron as the basic form – the most minimal shape for enclosing space, and one of the strongest. Steel tube would be an obvious fabrication choice. A wheel is added at each corner.
Inside of the tetrahedron, a seat is suspended. It hangs inside of the frame, rather than being directly joined to it. For strength, the seat has multi-point attachments to the corners of the frame so that it cannot rotate in space or shift forwards or backwards. However, if the frame sustains a shock, flexibility in the steel frame and in the seat cables will cushion the impact. Clearly a seatbelt is required for riding in the tribike!
In the event of a crash, the frame will sustain the impact of an oncoming vehicle. The frame struts protect the occupants. In the event of a roll-over, people remain inside the interior volume of the tetrahedron, keeping them safe.
For the drive train, there are two basic approaches: two independent motors on the rear wheels, for tank-style steering, or a front wheel assembly which rotates and has the motor attached. There are benefits to both approaches, but for sheer simplicity, tank-style steering is to be preferred.
The entire assembly can be made to fold. If the three bottom bars of the frame are hinged, the entire vehicle can fold nearly flat, depending on the construction of the seat. If the seat is mainly made of tension elements, so that the frame stretches the seat out to make it strong enough to sit on, then the entire thing should go into a vertical stack about 3m high with a 1m square footprint.
Another approach is to have four “connectors” – the four points of the tetrahedron – containing all the welded components and mechanicals. The tubes which form the frame slot into these connectors, and are removable, so that they can be taken off for storage, packing the whole tribike down into six tubes plus three wheel assemblies, a chair, and the roof point. This approach is probably a lot easier to manufacture than a complete folding version, but would require some work when the tribike was being reassembled.
For weatherproofing, a canopy could be added over the seat, or possibly over the tribike as a whole. A floor for carrying cargo, or cargo nets, could also be added.
Right now the tribike is only an idea, but I would not be surprised if we take a shot at making some prototypes over the summer. Interestingly, Marcin Jakubowski and I both came up with more or less this exact design independently. The same basic conclusions came from the same basic knowledgebase about the tetrahedron as a fundamental structure, and the widespread availability of strong, cheap tubes.
http://hexayurt.com/tribike is the canonical URL for this project.
PS: I forgot to mention bamboo as a promising tube material. Thanks to Lucas for the reminder!