I guess the first time riding a bike on your own felt pretty exhilarating, but this special event just started your bike riding career. Riding in different speeds, stopping exactly where you want to, doing turns with body weight shifts, stopping without blocking wheels, stopping with deliberately blocking back wheel, riding with hands off, etc, all those skills had to be mastered later. And many accidents happened until then, but if you get on the saddle again you could experience more dimensions of bike riding.
As children we understand learning as our 'work'. Improving our skills by relentlessly exercising them provides fun and gains valuable experience. As 'grown-ups' we easily lose this experimental pleasure, in a society where money can buy anything anytime our patience grew thin. I certainly notice a lack of patience in myself, and wanted to change this a bit.
I started building tensegrity models, feel free to read about the very first steps of this exploration in part 1 and part 2. I tested my patience by ordering stretchy cord via ebay. Last friday 100m of translucent beading cord arrived, after being delivered at a wrong address first.
Finally I could go ahead and build a tensegrity tower just like I had seen it in the magazine! Or so I thought. I cut and knotted my cords tensule by tensule, tuned them so that the upper triangle seemed level in both orientations. The tensuls felt rigid, but were still tuneable. I decided to use rubber bands for the 'security triangle' first, but somehow it just collapsed the tower.
I studied the photos from the instructions, and realised how much imagination and spatial thinking I need to transform the visual 2d information into a tactile 3d idea. I managed to tune a 4 story tower to keep it stable, sat back and I wondered why it still had only little resemblance to the tower I wanted to build. I turned the tower a bit and noticed its fragile balance. Instead of a lean pointy thorn I build the leaning tower of Brunswick.
I found the model lying next to the shelf the next morning, disassembled everything and started over. I wanted to figure out how the security triangles worked with only two tensuls, hoping not to collapse and entangle four level of a tower again. The way I attached the security triangle doubled some tension cords, yet all that's needed is a simple single tension line (like the ones that connect upper and lower triangle of a single tensul).
Depending on how much you entangled the different cords, it takes only little time to disassemble a tensegrity model. I reused some longer struts for the first level (and for a different visual effect) and retuned the model with the security tension cords on each level.
Attaching the connecting security cords took me a fair bit of fiddeling. They improve the balance of the tower, although I still hardly know it all works together. I learned to appreciate having rubber bands lying around handy, although I went back in this case to the builders line.
Another project awaited me: The stellated tetrahedron. I gave up when I realised that I needed more stretch than the nylon string offered, not wanting to break the dowels. Now I know that my basic problems lay somewhere else: My misconceptions how to construct the structure.
If you cut away the corners of a (space-filling) tetrahedron, you get four triangular planes instead, the triangular sides transform into a hexagonal plane. Using a cord triangle for the corners seemed obvious, but somehow I thought the remaining connections would run triangular as well.
Finding an easy way to lay out the 6 struts to have 4 corners where 3 of them meet in a specific direction proved a decent puzzle by itself. I used a zome tool model, connected the upper three triangles with stretch cord, secured the resulting mini-tetra with gaffer tape, carefully removed the model from its 'mold', and secured the final corner.
I repeated this step quite often in the last few days, until I found out how connect the for triangles to pop up into a magic tensegrity structure. Depending on how solid the model is fixed at this stage, it still can be folded (or constructed in the folded state and twisted into three dimensions.
After playing around with different ways of connecting four more triangles to my structure I went back to study images and the java applet. The final structure represents a stellated tetrahedron, I simply had to install tensors along its edges.
With a better idea of what I was doing the assembly got easier. I just had to imagine in which direction the triangles rotate, and connected the closest point of these triangles. The model twisted quite a bit while I attached the tension cords, but this time I felt confident that the model would 'emerge'.
Indeed, after cutting away the tape from the first corner the rods twisted away from each other, and I looked forward to see the miracle unfolding. As I can see now, I still got some connections wrong, I fixed the structure with two corners secured. However, I had done it!
After I bit of tuning I could balance the structures on one of the cut-away corners. It does not balance on all the corners, I think the cord triangles vary too much in length. As I used all 'recycled' materials, some of the cords could have gotten a lot of prestretching. More tuning, and fixing the cords parts of the triangles, will be the next step.
I really like the stellated tensegrity tetrahedron, and it could be fun to connect either tensuls or other tetras to it. I want to use colour as well, and/or different strut sizes. Once a structure obtained its shape, I can still tune a lot around, the stretchy cord provides enough flexibility to experiment a bit.
Size matters, in this case. Longer struts make the construction a bit easier, there's more room to cut away the securing tape without getting close to the tension cords. I might run out of display space, it doesn't appear too crowded yet.
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