Jun 2010
by larry

Tension, Integrity, and the Design of Lightweight Vehicles

The ElectraFlyer ultralight. Note the guy wires. Its electric motor weighs twice as much as my Stokemonkey motor (26 vs. 13 pounds) but it has 18 times the horsepower. The battery weighs 6.5 times my 36v 10ah Prismatic LiFePo4 (78 pounds vs. 12 pounds) but produces about 10 times the power. An Xtracycle with this monster motor and battery would add 104 pounds to the bike but would increase the range to 250 miles. Add that wing, throw in a propeller and my Xtracycle could fly!

Thomas Morse Scout showing guy wires. Incidentally this plane was manufactured in Ithaca the small town where I live.

I’ve been trying to wrap my head around guy wires. No that doesn’t mean wires made by guys. It means a tensioned cable used to brace parts of a structure or vehicle. For example have you ever noticed the cables forming Xs between an old biplanes’ wings? Those cables prevented the two wings from moving relative to each other. They also made it possible for a biplane to be lightweight. As biplanes’ motors became more powerful weight has not been so much of  a factor and you don’t see as many or any cables on a modern biplane. However you do see cables on ultralight aircraft as shown. More and more you see cables used in architecture. I argue that we need to use more cables—and parts under tension in general—in the design of our cars.

Guy wires to stabilize my solar canopy.

I recently needed to add guy wires to my Bike Wagon canopy design because when I put my solar panels on top of the canopy, the added weight made the canopy wobbly. A few cable Xs solved the problem. It got me thinking about how the design of ultralight airplanes and bicycles and lightweight vehicles in general requires using tensegrity. Say what? “Tensegrity” is a word that American architect and inventor Buckminster Fuller made up by combining the word “tension” with the word “integrity” to describe a new building technique. The basic idea is that you can make way cool lightweight structures by combining something with structural integrity (such as a hunting bow) with something that is under tension (such as the bow string).Tensegrity structures can have the unnerving property that the beams of the structure don’t actually touch each other (see the Tensegritoy below). Our bodies themselves use tensegrity: our bones provide the structural integrity and our ligaments and muscles provide the tension.

A wonderful way to get hands-on experience with tensegrity is to order a Tensegritoy kit.

The bicycle wheel is another good example of tensegrity. Back in the day wheels were constructed using wooden spokes. The spokes needed to be thick and heavy. The bicycle wheel, in contrast, uses tensegrity. The rim provides the structural integrity and the spokes provide the tension. This design creates a very strong lightweight wheel. A car wheel (excepting the occasional MG) does not use tensegrity; it is solid metal. A car’s large powerful internal combustion engine overwhelms any need for weight savings. Incidentally there are commercially successful bike frames that use tensegrity.

The Millenium Dome in London is constructed out of steel towers and tensioned fabric.

Tensegrity is a revelation in architecture. The first large buildings relied on gravity and the compression strength of stone to hold themselves up. Then with the invention of the steel I-beam, buildings could use the structural integrity of the I-beams to hold themselves up. More and more architects are experimenting with tension components such as steel cables and fabric. Some of these structures are what the layman would call a tent. However, we’re talking tents that are very large permanent structures such as the 1,200 ft. wide (365 m) Millennium Dome. Two related ideas are tensile structures (elements carrying only tension and no compression or bending) and tensairity structures (pneumatic structures that use inflated airbeams and attached stiffeners or cables).

Buckminster Fuller's Dymaxion Car. It was about 6 feet tall, seated the driver and 10 passengers, weighed less than 1000 lbs., went 120 miles/hr on a 90 horsepower engine, and got between 30-50 miles to the gallon of gas!

I believe road vehicles need to undergo a similar transition—from compression to tension components in their construction—so that they can become light enough to be powered by humans and by sunlight. Contemporary cars have structural integrity mainly by virtue of the strength of steel. I envision lightweight slow small vehicles that use aluminum poles, cables, and fabric to give them shape. Is this possible? It can’t hurt to dream big. In 1930 Buckminster Fuller’s own car invention, the Dymaxion Car, weighed less than 1000 pounds and could carry 10 passengers.

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