Mike's Spacetime

So by mid June 2009 we had the design of the cube fairly well established: ripstop fabric with grommets, attached by zipties to a scaffold frame. But would it stand up in one of the storms that typically rage over the black rock desert expanse ?

A static test with weight would be a possibility but it wouldn’t take in account the rattling and typical shock wind load behavior. Only a true wind test would be required – ideally a wind tunnel – but who’s got a wind tunnel in their back yar d? Well – if you can’t make the air move around a cube, you have to move the cube through the air – right ? . So on June 14th, some post-party Sunday, hungover and fueled by Burger King and Coffee, Barry and I drove Michael H’s truck out towards the mountains,  loaded with a bunch of scaffolding, one of Eva’s proto-ripstop panels and a load of zip ties. More then one cubist and non-cubist thought this was a bad idea. Which is was. But not as bad as it was made out to

“…I am concerned about the safety of this idea. If there is a failure on the freeway, there is a
substantial chance of injury or death. There are some things that make this in my opinion
much more dangerous than its use on the playa …” E.S. 2009

Jesus Christ! We weren’t going to take it on the highway! We’re a little crazy but we’re not entirely barking mad ! Well, or maybe we are:

We found a reasonably quiet road off the main road and started assembling a scaffold cubie on top of Michael’s roof rack. It fitted perfectly. (Note, however, the assembly of spiky conducting pipes right under the high voltage over land line – clearly two science PhDs were at work here). People gave us the strangest looks as they passed in their cars. We then attached the panel and strapped the entire assembly to the lower frame of the car. What could go wrong ? With Barry driving the car and me hanging out of the front window with a camera in hand we hit the road, slowly increasing the speed. We had no idea how legal this was so we tried to get the thing over and done relatively quickly. It is quite surprising how much force even a 25 mph speed produces and how much that panel was buckling under the pressure. The grommets were being pulled through the fabric even just from the initial pressure the zipties were putting on it, but under the added tension they started tearing through the material. At about 48mph the panel ripped with a loud bang. The whole thing was extremely amusing and funny. And we took not a minute too long – after we had disassembled everything (which btw took 1/6th of the assembly time, a ratio that, interestingly, stood up for the entire structure on the playa) we drove back home, down our test route to the sight of 10 giant military trucks which had pulled up in the meantime, clearly intending to use the same stretch of road for their own practice run. I think they would have been rather surprised by a Truck with a scaffold cube strapped to the roof zipping past them at 50mph. We concluded that the fabric needed reinforcement, but not tooo much reinforcement (we *wanted* the fabric to fail at 70-80mph).

I sent the results of the test to the group:

On Mon, Jun 15, 2009 at 12:57 AM, Michael Tyka
<mike.tyka@gmail.com <mailto:mike.tyka@gmail.com>> wrote:

 a.k.a. What happens when ya stick your cubie on the roof
 and drive it down the road at 50mph  

 Expt.

 We assmebled a single cubie on the roof of a truck including a panel of
 ripstop and a reflector. We drove the whole thing down the road until it
 failed.

 Result:

 Failure occured at around 48mph. Failure mode was the grommets riping
 out of the fabric. However we accelerated reasonalby quickly and so i
 suspect we would see failures with the current designs at sustained
 30mph winds.

 Witness the carnage:

 http://www.youtube.com/watch?v=-xwV5zOZ1sI

 Conclusions:

 We *want* the fabric to fail at 50mph - this failure mode prevents other
 more tragic failure modes (such as collapse of the structre) from
 happening since once the fabric is out, the cube no longer presents much
 of a surface area. However it is questionable if the current attachment
 would even survive a sustained, gusty 30mph storm, which it should.
 We may need to add another strip of fabric into the panel design to make
 the force distribute more evenly into the fabric rather than concentrate
 aorund the grommets.

 Further insights:

 Build to disassemble ratio is 1/5 or better. I.e. it takes about 5 times
 as long to build as to disassemble. It took us about 80 minutes to build
 the cubie from scratch on the roof of MH's truck, put the panel in and
 securely guy it down. The scaffolding took maybe 1/3rd or half of the
 time at most. We took about 15 minutes to take it all down.

Prompted by the results of the above test Eva changed the design of the panels using an extra fabric layer where the grommets sit and special grommets with teeth. We tested that design using a static test one sunday afternoon in July by piling about 240 lbs worth of stuff on a panel without it ripping. That was another useful test because we had decided to replace the zipties with cord and it was unclear if shock cord or ordinary cord would be better. The static test showed how much the shock cord would stretch (up to about 8″ ! ) which would be trouble some for the black fabric coverings.

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