Back in the 19th century, before carriages left their horses behind, wheels served mainly to reduce the effort required to drag a payload across often unpaved expanses. Only the horses, mules or oxen that pulled the carts actually needed any real traction, so the wooden wheels of the day were typically wrapped in a strip of iron to provide durability over whatever terrain might be encountered. Once livestock were superseded by on-board propulsion systems, the workload on the wheels changed dramatically. Driver demands for tractive effort, steering inputs and braking forces switched from reigns and whips to pedals, levers, tillers and eventually steering wheels that sent forces directly the wheels on the ground.
Just as gills and fins eventually gave way to lungs and feet on the sea creatures that crawled out onto the beach, iron rims were replaced by rubber. With a mere 2/3 horsepower coming from its single-cylinder engine, the 1886 Benz Motorwagen was able to get by with solid rubber tires. While this approach worked for the early motorcars, as engines got more powerful, tires with more compliance became necessary to handle the loads.
The tire is the first element of the vehicle suspension system that has to absorb road irregularities, and solid tires simply could not be made with enough compliance and wear resistance to be suitable for mass consumption. Before long, John Dunlop created the first practical pneumatic tire with a hollow rubber tube filled with air. And just like that, a new evolutionary sequence was underway. Pneumatic tires provided the flexibility needed to absorb impacts of both early roads and those we travel upon in the present day.
The first pneumatic tires were tall and skinny, similar to many bicycle tires still in use today. As vehicles got bigger, heavier and more powerful, tires continued to evolve, with the reinforcing plies being re-oriented from bias angles to the radials that we all drive on now. In the 1970s, Pirelli ushered in another major transition with the introduction of the P7, the first high-performance, low-profile radial. It was initially developed for the Lancia Stratos and was quickly adopted by Porsche for the original 911 Turbo. Today, even mainstream sedans like the Honda Accord and Ford Fusion regularly feature low-profile descendants of the P7 as standard or optional equipment.
Just as it sometimes seems impossible to eradicate simple organisms like bacteria and viruses, if the DNA survives, it can come back in a new tougher, drug-resistant form. The same holds true for the humble solid rubber tire. In recent years, Michelin has actively worked to revive the non-pneumatic tire in the form of the "tweel." Unlike the solid rubber tires of the 19th century, the tweel combines a thin but tough solid rubber perimeter supported by flexible polyurethane spokes and a solid central mounting hub. No car manufacturer has pursued this particular branch of the tire family tree yet, but Michelin is promoting the tweel for off-road applications like skid-steer loaders that may be more susceptible to punctures.
Will something like the tweel be the rolling stock of the future or another evolutionary dead end? Only time will tell, but history suggests that an innovative leap forward is hardly without precedent.