The Evolution of the Wheel
The wheel went through several incarnations to get to the level of technology we are familiar with today.
As I mentioned in a previous article, wheels likely developed from log rollers used to move heavy loads.
Wear on the center of log rollers made a difference in diameter, and led men to understand that this difference made their work easier, and thus led to the development of the axel. Once the concept of the axel was understood and one piece wheel and axels worn or hacked from a single log were surpassed, wheels were generally a solid disc of wood fixed solidly to an axel.
By Persian times wheels were fabricated from multiple pieces of wood, and required a skilled craftsman to produce them. The Persian wheel is a complex multi-piece design, and isdefinitely an advance, but the Greek "H-spoked" wheel was much lighter and used less material. As the wheel continued to evolve it took on the radiating spoked pattern we generally think of in realtion to wooden wheels.
In the Iron Age, iron "tires" were sometimes added to a wheel to protect the wood and increase the servicable life of the wheel. The earliest such "tires" were iron plates nailed around the circumference of the wheel, but men soon learned to work iron into a hoop which could be heated and expanded. Such a hoop will shrink around the wheel and fit tightly around it when it cools providing the advantage of not needing to be nailed on. As arts such as this progressed, wheel wrights eventually realized that a properly fitting iron hoop could not only save nails to hold the "tire", but could also hold the spokes in place as the key structural binding component of the wheel itself. This is very near the peak of wooden wheel technology.
Iron wheels could have been made for wagons, but were generally too heavy and expensive for the average horse drawn wagon, but once again the steam engine makes the impractical, practical by providing additional horsepower, and iron wheels on locomotives are the next large step forward. Then, of course comes the spoked metal bicycle wheel, the metal automobile wheel, and the rubber tire which not only protects the wheel, but cushions the ride of vehicles that do not run on rails. And finally, in the 20th Century, plastic wheels began to replace wood or metal wheels for some applications.
