Dear Reader,
Of the two objects pictured here, which one shares an indispensable characteristic with the modern bicycle wheel?
It’s a trick question — and a silly one at that. But there is one way in which a bicycle wheel has more in common with the Golden Gate Bridge than it does with a wagon wheel. I’ll explain………
A modern bicycle wheel consists of a design where a hub and rim are joined by spokes that are tensioned, or put another way, are in a constant state of being stretched. The spoke is retained at the hub, usually by means of an enlarged head which is unable to pass through a smaller diameter hole in the hub flange; the other end of the spoke is threaded and passes through a hole drilled in the rim. It is then captivated by a threaded nipple that advances from the back side of the rim. Lateral trueness, radial trueness (roundness) and dish (correct positioning of the rim relative to the hub) is attained by applying tension to the spokes by turning the threaded nipples.
Wagon wheel, late 1800’s
Golden Gate Bridge, San Francisco
A wagon wheel is a different beast. Since the end of the 18th century its design has been characterized by spokes that are compressed, or put another way, are in a constant state of being squashed.
From the end of the eighteenth century wagon wheels comprised of a hub, a number of wooden spokes , a rim consisting of sections known as felloes and an iron tire that was applied while red hot . the tire contracted as it cooled, exerting a compressive force that bound all parts of the wheel together.
When a wagon is loaded with weight, the spokes stretching from the bottom of the hub to the part of the rim that touches the ground are the ones supporting the load. Well, duh, you might think ; of course the weight is being supported in that way. Is this the way bicycle wheels work as well? Not so much. Because the spokes of a bicycle wheel are in tension (being stretched) the bike and rider actually HANG from the spokes at the top of each wheel. Spokes at the bottom of the wheel become slightly shorter under the weight of the rider; spokes at the top of the wheel become slightly longer. Wheels rotate and spokes change position, resulting in a dynamic process of constantly changing spoke tension. To digress for a second; in order for a wheel to deliver reliable predictable performance, spoke tension must be uniform and as high as the components (the rim in particular) will allow. Spokes break more often due to low tension rather than high; insufficient tension promotes excessive movement at the spoke head/ hub flange interface: this in turn hastens spoke failure.
Anyhow; back to my seemingly silly comparison between a bicycle wheel and The Golden Gate Bridge; just as the roadway of a suspension bridge hangs from tensioned vertical suspender wires, the bicycle rider hangs from the spokes at the top of the wheel. As a bonus, suspension bridges have a tendency to be lighter structures than more conventional bridges; the support towers can be fairly thin, since the forces at work on the bridge are carefully balanced on each side of the towers. The modern, tensioned spoke bicycle wheel is also lighter than it’s “ velocipede” granddaddy with it’s wooden spokes, rim and steel hoop tire; is this proof that lightness (in all things) equals progress……?
A bicycle wheel rotates approximately 500 times over the course of a kilometer and the spokes undergo a sequence of loading and unloading every time the wheel rotates. If you put 25, 000 miles on a wheel set, your spokes have loaded and unloaded 20 million times. Built correctly, starting with appropriate component choice and finishing with careful assembly, the bicycle wheel is a beautiful, functional work of art.