The science of tires

It happens to all of us sooner or later. We’re in a hurry, we head off in our car–and discover we have a flat tire.

This happened to me twice in December, and got me thinking about tires, which is unusual, because usually we don’t think much about tires at all. After all, they’re pretty simple devices, aren’t they? They’re just round rubber doughnuts pumped full of air, right?

Wrong. The modern automobile tire is really as much a technological marvel as the car itself, a complicated device made of thousands of individual pieces created from dozens of materials.

Of course, they weren’t always so complicated. The first pneumatic tire, or “elastic bearing,” patented in 1845 by Robert W. Thomson, was made from several layers of rubber-saturated canvas. John Dunlop, who patented a pneumatic tire for bicycles in 1888 and formed the first tire company.

By 1900, tires had round cross sections, separate inner tubes inflated to a pressure of 70 pounds per square inch, and rubber-coated, cotton-cloth covers. Treads were introduced 10 years later and layers of reinforcing cord in the 1920s. Whitewalls, which ran at a lower inflation pressure and gave a smoother ride, followed in 1929. Synthetic materials began to be used in the 1930s, and inner tubes were eliminated in 1954.

The visible parts of a tire are the tread and the sidewalls. The tread actually reduces grip on a smooth, dry surface because it reduces the amount of rubber is in contact with the road–that’s why racing tires are slick. But in the wet, the forward portion of the treaded tire’s “contact patch”–the part of the tire in contact with the ground–routes water outward through continuous channels, allowing the rest of the contact patch to grip much dryer pavement. Snow tires and off-highway tires have deeper treads or even cleats to bite through snow, slush, dirt or mud.

Sidewalls have two conflicting purposes: they must flex up and down to cushion the ride, and still be rigid enough to bear up under the loads imposed by steering, braking and acceleration.

Beneath the tire’s surface are reinforcing cords that give the tire its strength. Bias-ply tires were the first to use reinforcing cords. A bias-ply tire has two or more plies of cord running across the tire at an angle, or “bias,” forming a criss-cross pattern from bead to bead (“beads” are the rubber-coated steel hoops that are held against the wheel by air pressure, keeping the tire in place). Next came the belted-bias tire, in which reinforcing belts are placed between the various plies of cord.

More recently, the radial tire added reinforcing belts running hoop-fashion from bead to bead to cords angled closer to the tire’s centerline. This make’s a radial’s sidewalls more flexible, making the tread grip better and last longer.

Anyone who has bought tires has probably been confused by all the letters and numbers associated with them. What, for heaven’s sake, is a P165/75R13? But it’s not as complicated as it sounds: that’s just a passenger-car (P) radial (R), 165 mm wide, 75 percent as tall as it is wide, designed to fit a 13-inch wheel.

Tire technology never stands still. Today, a major focus is on improving mileage, for environmental and economic reasons. One way to do that is reduce the energy wasted by the tires. Most of that–90 percent–is a result of the deformation of a tire as it rolls, which turns energy that could be used to move forward into heat. The rest of a tire’s rolling resistance is a combination of aerodynamic drag and friction.

Radial tires already have roughly 20 percent less rolling resistance than a bias-ply tire. Today’s tires also contain less mass. Now the search is on for new materials: for example, whereas in most tires, the rubber is mixed with carbon black, which gives tread its resistance to abrasion (and also makes tires black), Michelin has developed a silica blend that lowers a tire’s rolling resistance below that of carbon black and is only slightly more expensive.

“Run-flat” tires are now available on some cars, too. Stiffer sidewalls allow these tires to be run without air for 100 kilometres or more without being damaged.

Expect many more innovations in the years ahead. Supercomputers, virtual reality, infrared thermography, holography, X-rays and other high-tech tools are constantly being applied to the process of making tires better than they’ve ever been before.

It’s a shame they have to go flat before we fully appreciate them.

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