Since its invention many thousands of years ago, the wheel has come a long way. For one thing, wheels are no longer made of wood, so there's no doubt the ride is much smoother these days. What hasn't changed is the fact that the wheel is still one of mankind's greatest inventions. Can you even imagine modern life without it?

The early wheel was very simple: a solid curved piece of wood. Leather was eventually added to the edge to soften the ride. As time progressed, wheels became solid rubber, which led to today's technology - the pneumatic, or air-inflated, radial tire.

Vulcanization and Charles Goodyear

Rubber was not always as useful as it is today. Early rubber did not hold its shape. It would be sticky in hot weather and become inflexible in the cold. In 1839 Charles Goodyear was credited with the development of the vulcanization, the process of heating rubber with sulfur. This transforms sticky raw rubber into a firm but pliable material that makes it the perfect material for tires.

Although Goodyear dedicated his life to making the most out of rubber, he would never profit from his efforts. Charles Goodyear died bankrupt. Forty years later, though, a rubber company would honor his hard work by using his name for their new tire company.

Solid Rubber Tires

Soon, after the development of vulcanization, tires were made out of solid rubber. These tires were strong, absorbed shocks well, and resisted cuts and abrasions. Although they were a vast improvement over earlier versions, these tires were very heavy and did not provide a smooth ride.

Pneumatic Tires

The pneumatic rubber tire uses rubber and enclosed air to reduce vibration and improve traction. Robert W. Thomson, a Scottish engineer, first patented the air-filled tire. Unfortunately, his idea was ahead of its time and was not a commercial success.

In 1888, John Boyd Dunlop of Belfast, Ireland became the "second inventor" of the pneumatic tire. Dunlop claimed to have no knowledge of Thomson's earlier invention. The second time around, the pneumatic tire caught the public's attention. And the timing was perfect because bicycles were becoming extremely popular and the lighter tire provided a much better ride.

Bias Ply Tires

For the next fifty years, vehicle tires were constructed of an inner tube that contained compressed air and an outer casing. This casing protected the inner tube and provided the tire with traction. Layers called plys reinforced the casing. The plys were made of rubberized fabric cords that were embedded in the rubber. These tires were known as bias-ply tires because the cords in a single ply run diagonally from the beads on one inner rim to the beads on the other. However, the orientation of the cords is reversed from ply to ply so that the cords crisscross each other.

Today you can still find bias-ply tires as authentic equipment for antique and collector cars, as well as for certain types of off-the-road tractors.

Radial Tires

Michelin invented radial tires in 1948. They were first introduced in Europe that year. Radial tires are so named because the ply cords radiate at a 90 degree angle from the wheel rim. Also, the casing is strengthened by a belt of steel fabric that runs around the circumference of the tire.

Radial tire ply cords are made of nylon, rayon, or polyester. The advantages of radial tires include longer tread life, better steering, and less rolling resistance, which increases gas mileage. On the other hand, radials have a harder riding quality, and are about twice as expensive to make.