Tensegrity Bicycle?

cabiria · Post by **cabiria** » Mon Feb 16, 2004 7:45 pm

Anyone seen this and know who did it? Looking for photos...

Muchos Thanks.

Wind_Borne · Post by **Wind_Borne** » Tue Feb 17, 2004 6:38 pm

I have nothing about an actual bicycle, but here some background on the concept:

Spoked bicycle wheels are an everyday example of tensegrity structures -- the strength of the wheel comes from the opposing tensions of each spoke working against the others (spokes have almost no compressive strength). B. Fuller took the spoked wheel as a point of departure for architectural structures; experimenting with buildings that relied heavily on tensile elements. Sometimes a tensegrity structure is refered to as a bicycle wheel structure. Below is a photo of a velodrome with a tensigrity roof:

http://www.ppeng.ch/aigle.htm

Bob · Post by **Bob** » Tue Feb 17, 2004 7:50 pm

Did it look like this? Try Image Gallery under bicycle, bike, etc?

http://images.burningman.com/

Wind_Borne · Post by **Wind_Borne** » Tue Feb 17, 2004 8:19 pm

Here is guy a has been thinking about Tensegrity Bicycles (from http://lists.canonical.org/pipermail/kr ... 00704.html ) :

I want a tensegrity bicycle
Kragen Sitaker [email protected]
Sat, 13 Apr 2002 03:24:36 -0400 (EDT)

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Materials get cheaper and better every day.

In my backpack, I carry a Mylar blanket which is quite large and warm
enough for one person to sleep under comfortably (if a bit noisy),
weighs perhaps an ounce, and easily fits in my pocket.

Beatrice and I are busily trying to reduce our possessions; we just
moved into a smaller apartment and don't have room for what we have.
So I was pondering; if I reduced the size of my possessions
sufficiently, I could live nomadically. If I could reduce their size
without reducing their benefits, I could live nomadically as
comfortably as I live in a house today.

So I began pondering how to build a vehicle in which to roam. I'd
like it to be a bicycle or something similar; but traditional bicycles
are relatively hard to fix when they break, often needing a welding
shop and expensive spare parts.

So I started thinking about tensegrity bicycle frames. If your
tensegrity frame broke on the road, it might still be ridable; and if
not, you could fix any part of it with a very small supply of spare
parts.

There seems to be a mailing list about human-powered vehicles ('hpv')
run by the International Human Powered Vehicle Association [6] with
some interesting posts on related topics, including a 1997 post by
Chuck Knight about building a tensegrity bicycle and considering using
PVC pipe. [7]

$1.39 IIRC gets you 250 yd. of 8-lb nylon monofilament at the hardware
store, for 0.56 cents per yard or 0.2 cents per foot. Fishing stores
offer 4000 yards of 12-lb line or 2600 yards of 20-lb line for $23,
weighing a pound; this 12-lb price is about the same price per yard I
was getting for the 8-lb line at the hardware store, and this price
remains the same per pound of fishing line. [8]

3/4" PVC pipe is 6 cents a foot; 1/2" PVC pipe is 5 cents a foot. You
can cut PVC pipe quickly with a tubing cutter from any hardware store
that costs $6.

Further info on PVC pipe:
- DR is 'dimension ratio', the ratio between average outside diameter
and minimum wall thickness; typical DRs are the SDRs, 'standard
dimension ratios', some of the more common of which are 21, 26,
32.5, 41, and 51. 35 is the most common DR for sanitary sewer PVC pipe
in the US. [0]
- modulus of elasticity for PVC is minimum 400kpsi [0] [2] [3] (compare
30Mpsi for steel [2])
- so for DR 32.5 3/4" pipe, we have .027" cross-sectional area, meaning about
10 000 pounds would be enough to compress it to 0 length, or 100 pounds to
compress or extend it by 1%
- cold PVC pipe becomes brittle and loses impact resistance [1]
- questions: what's the density? Buckle ratio? yield stress? (yield stress
for mild steel is 35 kpsi, so probably in that neighborhood)
- specific gravity is 1.4; tensile strength is 7.4kpsi; compressive strength
is 9.6kpsi; flexural strength (???) is 14.4kpsi; coefficient of linear
expansion is 2.9e-5 in/in/degf [3]
- so for DR 32.5 3/4" pipe, we have 7.4 grams per foot; a 100' mast
with four feet of PVC per foot of mast would be 3 kg (and cost $25)
- so compressive strength for this pipe is 257 pounds, or 116 kg; if
compressive strength for the whole mast was 200 kg, that would allow
a 6 km mast to remain upright.

So suppose we wanted to build a bicycle of 3/4" PVC pipe and nylon
fishing line. The bicycle might be roughly three feet by seven feet
by four feet, and might require, generously, 10x7 + 10x4 + 10x3 feet
of pipe --- or 140 feet. It would weigh about 1 kg and cost about
$8.40 worth of pipe, plus a negligible amount of money and weight for
the line.

I'm not sure how to make the joints between the pipe and the line;
metal hooks of some kind would work, I suppose.

On tensegrity:

It's hard to find information about tensegrity on the Web. Kenneth
Snelson, a sculptor, seems to be a big creator of it; Robert Connelly,
a mathematician, has done a bunch of math on tensegrities, as well as
designing tensegrity toys.

Robert W. Gray has put Synergetics on the Web [4], and Synergetics has
a large section on tensegrity [5].

[0] http://www.uni-bell.org/faq.htm
[1] http://www.madisongroup.com/Services/Fa ... lysis.html
[2] http://www.roscoemoss.com/specs.html
[3] http://www.harvel.com/PVCpipe.html
[4] http://www.rwgrayprojects.com/synergeti ... etics.html
[5] http://www.rwgrayprojects.com/synergeti ... toc07.html
[6] http://www.ihpva.org/pipermail/hpv/index.html or news:saar.lists.hpv
[7] http://groups.google.com/groups?selm=19 ... 40juno.com
[8] http://www.charkbait.com/cs/cshL.htm

--
<[email protected]> Kragen Sitaker <http://www.pobox.com/~kragen/>
This radically anti-cynical approach to life is not just a shared
disposition but also an act of conscious dissent. -- Alan Bershaw, on the
attitude of Jewel fans ("everyday angels")

Bob · Post by **Bob** » Tue Feb 17, 2004 8:42 pm

cabiria · Post by **cabiria** » Fri Feb 20, 2004 9:08 pm

Ok, thanks Wind_Borne. I've actually seen the website where the guy is talking about building a bicycle, but was hoping that someone had actually done it and written about it online.

The curiosity is about how a tensegrity structure ( a tall column) would hold up to stress, not about a bicycle in particular...

cabiria · Post by **cabiria** » Fri Feb 20, 2004 9:11 pm

Bob,

If you ain't seen a picture of it online, then it must not exist.

I can stop looking for it now...Thanks!

Wind_Borne · Post by **Wind_Borne** » Sat Feb 21, 2004 1:46 am

cabiria,

I appreciate the asthetics behind your quest. The problem is that towers are essentially resisting compression from gravity, and not tension as would a suspended foot bridge. Tension in a tower arises mostly from wind or siesmic loads. The loads can be directed downward through the tower by making it very heavy (as in the Washington Monument), or carried by cable stays (as in high radio towers).

If you're looking to construct the lightest, most easily transported tower, then look to cable-stayed truss-work spars as your model.

I can envision a completely tensile tower... Imagine a huge helium balloon pulling upwards against a set of cable spokes tied to the ground. If you want to go that way, check out the FAA regs on moored balloons: Title 14 Part 101 >>>http://ecfr.gpoaccess.gov/cgi/t/text/te ... 15&idno=14

DVD Burner · Post by **DVD Burner** » Sat Feb 21, 2004 2:03 am

Really good Bob. nekid Sub genius and all.

Beat me to it with the 3ds man.

darn.

Bob · Post by **Bob** » Sat Feb 21, 2004 11:20 pm

Guy I usually camp with in July did up a page on one of his tensegrity-related projects.
[url]http://daredevil_30.tripod.com/tensegrityshade/tensegrityshade.html[/url]
He had more, including a tensegrity tower he built c.1998 at the burn, but apparently took it down over political conniptions.

The other pages I had cached seem to be dead links. Will post more if I can track anything down.

cabiria · Post by **cabiria** » Tue Mar 02, 2004 11:04 pm

To be more specific, if we were to build a structure such as this: (Middle one)

http://www.bergen.org/AAST/Projects/Eng ... grity1.htm

The want being the beauty of a structure built of rods that do not connect, but held by rope with tensegrity.

Say, 200 hundred pounds stepped on the top of it, bending it like a young sapling tree that will bend and snap back again if you climb it and jump off of it.

Do you think that would work, repeatedly? And if so, what would be the best materials to keep repeating the motion?

Wind_Borne · Post by **Wind_Borne** » Wed Mar 03, 2004 1:57 am

I like that tower. Very ethereal.

You should build a scale model to investigate they way the tower responds to loads. I'm wondering, upon reaching a critical side load, will itl buckle like a tube would or bend like a sapling?

For an actual structure consider true aircraft cable and spruce or steel spars. You might want to go to Aircraft Spruce and Speciality for that stuff. The reason I suggest that is to ensure known material strength and consistent quality (since you may have someone up on top).

Tap some structural engineer to give you a hand with the strucural analysis and to recommend cable and spar sizes. If you don't get a response from an engineer here, then drop by your local university engineering department.

Tensegrity Bicycle?

Tensegrity Bicycle?

Please, Bob and Wind_Born: