The wheel is one of man's oldest inventions. Traditionally, wheels such as those used on an automobile are constructed of a rim and hub which are used to support an inflated tire which is subsequently fastened to a rotatable axle on a vehicle. Bicycle rims have utilized a slightly different approach towards rim construction. The bicycle wheel design is aimed at minimizing weight and maximizing strength. This allows human power to drive the bicycle in an energy efficient manner.
Generally, a bicycle wheel consists of a rim and hub which are strung together with connecting spokes that serve to balance and center the hub within the circumference of the rim. The connecting spokes are generally fabricated from steel or some other metal alloy.
In the past, the composition of spokes has been dictated by the fact that spokes represent stress concentration points along the circumference of the rim when used to construct a wheel. Generally, the number of metallic spokes used to construct a conventional wheel is deliberately minimized to reduce weight while maintaining adequate structural strength. This approach increases the distance between adjacent spokes and creates a high stress along the axis of the spoke. As a result the spokes may break or have to be strengthened by adding any variety of mechanisms to the wheel. The latter alternative frustrates the initial objective of weight reduction.
However the present technology of wheel spokes has certain drawbacks based largely on the design and composition of the spokes. For example, by subjecting metallic spokes to high stress levels, the usable stress range--for shock absorption purposes--within the spoke is decreased. This situation is compounded by elastic hysteresis under the imposed tensile stress which seriously minimizes the long term durability of the spoke. The yielding or creep behavior of metal alloys is an inherent stress relieving mechanism and a principal cause of the hysteresis. As a result, most metal spokes are compositionally incapable of damping vibrations unless they fail or irreversibly stretch.
The phenomenon of irreversibly stretching is generally referred to as lack of memory. Lack of memory in context means that the specific metal article will tend to permanently stretch or distort when subjected to stress and strain during use. In short, when subjected to forces of constant magnitude, metals will tend to remember the stress and strain they encounter and flex and yield in the direction of these forces. This memory phenomenon is a principal stress relieving mechanism. As a result, permanent disfigurement occurs and, metal spokes become weak and harder to tighten and, in turn, true over time.
Another problem of metal alloy spokes is that under too great an amount of stress they may fail. Failure of metal spokes is routinely unpredictable and generally complete, potentially resulting in the complete failure of the wheel.
Furthermore, this failure profile is often exacerbated by the inflexibility of metal spokes. Rigid metal spokes must often be stressed or strained to be installed. Due to the basic design of the wheel and the inherent character of metals to remember stress and strain forces, this initial bending or disfigurement may result in fatigue, yielding and failure after installation, and during use.
Accordingly, a need exists for a light weight wheel spoke or spoke material which is strong enough to provide a durable support element for hub and rim wheels.