Class 2 tetrahedron

Tensegrity structures still wait for more popularity. While my fellow market traders got used to them, and spend some time playing with them, it's still rather one in hundred passers-by that identifies my work as tensegrity.

I shied away from investigating tensegrity theory since I started building sculptures, but I renewed my research lately with some surprising findings. I realised that I reinvented the wheel - the Unholy Grail uses the structure of Bob Burkhard's Wheel 2. When I browsed Bob's great site again I stumbled upon a Class 2 tensegrity tetrahedron.

I still have some models using eyebolts available for recycling, so I considered rebuilding a tensegrity structure with joints. I threaded the eyebolts of two struts together, which creates a multi-directional joint. A class 1 tetrahedral tensegrity needs 6 struts, and a bit of imagination to detect the tetrahedral shape. The hinged class 2 tetrahedron only requires 4 struts, and seven tendons.

I had no idea about the precise tendon length, nor how the joints would affect the build process. I started with elastic tendons for the edges, and a fixed tendon between the joints. The nightmare began. I hoped that the elastic cord would allow me to 'stretch' the model into a stable position, but the jointed struts kept turning and unhinging some outer tendons. The mobility of my improvised joints backfired, and after some variations of central tendon length, outer tendon length and order of attaching outer tendons I gave up.

Unlike many nicely rendered tensegrity structures one can find on the web, Bob Burkhard showed two photos of actual models showing this class 2 tensegrity. Knowing for sure that this idea can be build, using quite familiar connection types, I reflected on my difficulties during the failed attempts and devised a new strategy.

I used nylon cords with little bowline knots at either end - this should limit slippage of the outer tendons, and give equal length. Even with fewer components than most class 1 models, this build remained challenging. At first, I used a metal hook as connection between joints, with little luck. Then I limited the mobility of the joints by tying elastic cord around it several times, replacing the hook as central tendon.

After several attempts all outer tendons got connected, and shaped a tetrahedron. I didn't let go of the struts, the model didn't feel self-sustaining yet. The elastic cord made it easy to shorten the central tendon, and gave the model stability. It still collapsed, and sometimes outer tendons became loose when I played with it. It still takes me patience to pop it back into a stable 3d state after a collapse. I closed the eyebolts, so that the tendons stay in place.

The final result stunned me. The outer tendons clearly delineate a tetrahedron, and two pairs of joined struts, held together by a short central tendon, connect the corners to its central area. The joined struts give the model optically more substance, and the behaviour provided pure fun. The model balances on a triangular face, so one strut always points up. If you push back this strut, the tendon connecting it to its joint member slacks off. Once you release it, it springs forward, easily with enough momentum to tilt the model over.



If I use joints again, I make sure I use a hinge joint for the tetrahedron. Two joined eyebolts offer too much freedom of movement, which might contribute to a collapse as well. The structure feels different from most class 1 tensegrities I build, and show a surprising dynamic movement under little external stress.