From the Gallery of Good Ideas: Pneumatic tires

We moderns are accustomed to rapid turnover in technology. Each year’s crop of smartphones is smarter than the previous year’s. Monophonic vinyl records gave way to stereophonic in my childhood, to cassette tapes in my youth, to digital CDs in my young adulthood, to mp3 files on portable players, to music streamed from an infinite music library in the cloud. New medications and medical technologies push the outer bounds of normal, healthy lifespans ever farther into the future.

Sometimes, though, a technology is so good it defies replacement. The billion-and-a-half automobiles on the world’s roads today, and the billion bicycles, roll on tires scarcely changed in more than a century.

Sumerians seem to have invented the wheel around six thousand years ago. These were disks of wood with a hole bored to accept an axle. The oldest surviving wheel yet discovered was found in a bog in Slovenia. Carbon-dating puts its age at more than five thousand years. The wood came from an ash tree, and the wheel has a diameter of about 30 inches.

That original technology was sturdy and economical. Solid wood wheels were still in use in certain countries in the nineteenth century. By then wheel-making had become a specialty, and wheelwrights contrived to lighten wheels by replacing the solid disks with spoked wheels. Some of the wheels had rims of iron to improve longevity.

Railroads, popularized in the mid-nineteenth century, required stronger wheels than wood could provide. Steel wheels did the job. But these were heavy, expensive and needed steel rails to roll on. Vehicles besides rail cars continued to rely on wooden wheels. Occasionally leather was affixed to the rims to soften the ride.

Europeans first encountered rubber when Cortes saw Aztecs playing a game with bouncy balls. Not until the eighteenth century did European explorers locate the trees from which the latex sap came. Its first use in Europe was rubbing out pencil marks, which gave it the name rubber.

Other uses were limited by the sensitivity of latex to changes in temperature. When heated it turned gooey; when chilled, brittle. But an American named Charles Goodyear discovered that adding sulfur to latex widened the temperature range in which it retained its rubbery usefulness.

Wheelwrights tried adding vulcanized rubber (as the sulfured version was called) to their rims. It wasn’t very durable. A Scotsman named Robert Thomson figured out how to fill a toroidal rubber tube with pressurized air. Sheathed in leather and mounted on a wheel rim, Thomson’s patented device was somewhat more durable than previous rubber tires. But it was expensive. Passenger coaches and freight wagons continued to roll on wooden wheels with rims of iron.

The pneumatic tire awaited a transportation technology better suited to its strengths. The first such technology was the bicycle. Durability wasn’t a problem because the bikes were much lighter than coaches and wagons, and expense mattered less because the bike tires required less rubber. Pneumatic tires for bicycles sometimes came in two pieces: the air-filled tube and a separate sheath of rubber. Sometimes the tube and sheath were combined in a single, tubeless tire.

Pneumatic tires made bicycles the signal advance in personal transportation since the domestication of the horse. In fact, the central appeal of bicycles was that they let people dispense with horses. Bicyclists took to the roads of America and other countries by the millions in the 1890s; they became a major force for building better roads. Mud was a greater impediment to bicyclists than to horses; and cyclists voted, while horses did not.

In a feedback loop typical of new technologies, the popularity of bicycles made possible by pneumatic tires created demand for better and cheaper tires. The combination of separate inner tubes and tires became the industry standard, allowing riders to replace either a tube or a tire without having to replace the other.

The success of the pneumatic tire on bicycles encouraged developers of automobiles, which lagged about two decades behind the bikes, to put similar tires on their cars. Early automobile tires were the combination of inner tube and tire, with the latter, especially, being much thicker and heavier than on bikes. Tire-makers added layers of cloth and eventually woven steel to the part of the tire that met the road.

The secret of the success of the pneumatic tires, on both bikes and cars, was the air cushioning they provided. The compressed air allowed the tires to deform as they ran over stones or into potholes, absorbing energy that otherwise would have been absorbed by the frame of the bike or car and the body of the rider or driver. This saved wear on the tires and on what the tires supported. Moreover, the ride characteristics of the bike or car—harder and more efficient, or softer and more comfortable—could be modified merely by adjusting the air pressure in the tires.

During their century-and-a-third on the road, pneumatic tires have changed remarkably little. Bicycles today use both tubed and tubeless tires; automobiles are exclusively tubeless. Tinkerers have tried to remedy the weakest point of pneumatic tires: the possibility of puncture and loss of air. Steel belts in the treads of car tires have dramatically reduced the incidence of flats. Tubeless bike tires have liquid sealant inside that congeals in punctures and prevents the loss of air. Run-flat cars tires have an inner structure that supports the car long enough to reach a place where the flat can be repaired or replaced.

Alternatives to pneumatic tires have occasionally been tried. These commonly consist of spongy material that mimics rubber but doesn’t contain compressed air and so can’t be punctured. But they’ve never taken hold. They’re too mushy or too hard, not durable enough or too expensive.

Pneumatic tires still rule the roads. It’s not hard to tell why. Whether on two wheels or four, what could be better than rolling along on a cushion of air?