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May 27, 2013 1 Comment
A bicycle wheel's single biggest accomplishment is simply being a wheel, a device that enables ultra-low friction terrestrial mobility. Unchallenged as one of humanity's greatest achievements, it brought huge change to civilization upon its appearance.
This is a larger topic than you might think. What is a wheel, precisely? Why did it appear in Sumer and not sooner or later? Why are there none in nature? Is it a non-intuitive invention and who is the inventor? Why is it so popular? Why am I nominating the bicycle wheel as the most significant and evolved form of this contrivance? What does fulfilling this role,"being a wheel," require of design?
The conversation has drawn some of the best minds of our times: evolutionary biologists Stephen Jay Gould and Richard Dawkins, anthropologist and ecologist Jared Diamond, legendary British archeologist Stuart Piggott, and countless others: philosophers, engineers, sci-fi writers, and mechanics. A fabulous discussion of rotation in living systems is hosted on Wikipedia.
Most agree that wheels represent rotation around a fixed center, something unknown in nature. Rotation of a planet implies a theoretical center but it's not part of the dynamic, just a concept. Rotation is not rolling. Many living things (and rocks) roll in obeisance to gravity, to travel, or to escape. But they do not have wheels. Recently, however, a quaint sidebar to the topic notes certain bacteria have this rotation.
Certainly for humans, the wheel was an invention that archeologists agree appeared quite suddenly in Sumeria (present-day Iraq) in 3500BC. Sumerians built their wheel a particular way, so the concept is easy to track around the globe. That style wheel in prehistoric Great Britain was undoubtedly inspired by the Sumerian invention.
Stuart Piggott's classic study, The Earliest Wheeled Transport (1983), documents the incredible speed at which this concept spread across the prehistoric planet. Perhaps the fastest idea dissemination of all time. Within a relative blink, wheels were in North Africa, Switzerland, Great Britain, Bavaria, Russia, the Far East, everywhere. Trade was prolific from early times and it seems every trader who saw wheels when visiting Sumeria understood the possibilities. Or, at least, the fun. As soon as they were home, they made their own.
The planet's ultimate nomadic and mobile life form (that's us) took a nano-second to understand the potential. In fact, the next big trend was being buried with your wheels. Burial sites with skeletons and wheeled vehicles began appearing all over and make it possible for archeologists to precisely document the spread.
A number of important, related factors must be mentioned but not discussed for brevity. Availability of traction domestic animals much increased wheeled vehicle usefulness. Oxen permitted these carts to be heavy. Later came faster, more trainable horses, which drove a quest for lighter carts and wheels. King Tut's rig shows how far development had come by 1290BC.
Egypt was a late comer to the wheel scene (except for pottery wheels) but when introduced, they went crazy. King Tut's wheels are 100X lighter and faster than Sumerian. Spoked wheels and the ultra lean machine they supported (see previous post) was a hot rod by any measure.
Two healthy horses hauling this 80lb basket at 40mph across the desert must have been a thrill. In Tut's case, perhaps his last. In 2010, DNA study suggests he suffered a fracture to his left leg a few hours before death. Given his royal schedule, hard to imagine another catastrophe besides high siding it at speed. Speed is addictive, light wheels enticing, and young men (pharaohs or not) will push the limits.
A side note, maybe you can tell from the crude image above that each spoke consists of two slats, side-by-side. When the wheel is built, these are sized and attached with a wedge jammed between them in the center. At each end, the slats are bound together by leather. The spokes are fitted as tight as possible. Once in place and wrapped securely with damp leather (that dries and tightens like steel), the wedges are popped out and the slats pulled together. Increasing their length, the spokes go into substantial compression; making a wheel much stronger than without.
The wheel entered a relative stasis for development until the miracle of single track vehicles was discovered. That's the bicycle. Single track (one wheel follows the other) = lighter weight (fewer wheels) + less strength needed (cornering by leaning makes low side force) = more fun. Again, the best engineers and thrill seekers attacked the challenge.
First breakthrough was the tensioned spoke. For that we owe huge debt to Sir George Cayley, father of flight and aerodynamics. He wanted a lighter wheel for heavier than air flight (100 years in his future) that would prove essential to the bicycle (40 years away). In 1808 he said no to clever compression spokes and proposed tensioned cord instead. There was no steel wire yet available.
In 1816, he said balloons and blimps weren't aerodynamic and created a shape that is and generates lift. Since it has been verified as matching perfectly N.A.C.A. aerfoil section 63A016-LB N-0016. Later that year he unveiled a design for a steam powered airplane 432ft long, weighing 163,000lbs. He calculated lift, drag, engine power, water recovery, fuel consumption and said it would carry 50 people 962 miles on one fuel load. Later study shows the plane would have flown. But it took 100 years for a pair of bicycle mechanics to take the risk of flight and succeed.
OK, we now have a super light engineering marvel and the cycling world explodes into the world consciousness. The first tensioned wire wheel saw production on George Starley's Ariel in 1874. The father of the English bicycle industry, Starley was also having trouble getting drive to the twin wheels of an adult tricycle in 1877. He puzzled for a time, was heard shouting "I have it!" then sat down to design the differential gear mechanism seen on virtually every motor vehicle to this day. My oh my, bicycle mechanics can be pretty darned clever.
Last big improvement to the bicycle wheel ensures its place as the most significant wheel of all time. It saw the first commercial pneumatic tires, key to today's wide use of wheels for mobility. In 1888, John Dunlop capitalized on work by Charles Goodyear (1844) and Robert Thomson (1846) and perfected a pneumatic tire for bicycles that was adopted world-wide in a couple of years. The difference in rolling resistance is gigantic.
Just how popular are wheels, especially bicycle, today? I recently stumbled on a newsgroup exchange that discussed thread pitches (sizes). In particular, what is the world's most popular thread pitch? M6, M8, and M10 were nominations, as was 1/4-20tpi and E27 (light bulb sockets). In the end, however, the "winner" was 2mm X 56tpi. The bicycle spoke thread. With a billion+ bicycles, each with 72 spokes of nearly identical threading, you can see some substantial numbers.
That leads us directly to the second trick of the wheel: its critical role as an element in the pneumatic tire system. This first trick still begs for more attention, but we're blogging here, so let's move on!
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