To get better gas mileage, engineers are fussing with the structure and composition of tires. The latest advance comes from a material at the beach.
Of all the technology that goes into new cars, tire technology may be the least appreciated. Tires are certainly not the first thing consumers think about when shopping for a car. Indeed, we tend to notice them only when they go flat or wear out. But tires are not just hunks of rubber--they’re a complex blend of materials--and their design is increasingly sophisticated, as automakers push for every edge in fuel economy.
A tire is a horribly complex thing to design, says Dieter Overhoff, an engineer with the Pirelli Armstrong Tire Corporation in Breuberg, Germany. From a physicist’s point of view, we are dealing with a very strange material. The compounds we use don’t behave in a linear fashion. And the interaction of these various compounds is such that you can’t completely describe a tire mathematically. We are in a field in which book law is never valid. And every day we discover that something we knew to be the truth yesterday has been proved wrong today.
Tires used to be a whole lot simpler. The first ones, on London carriages in the early 1880s, were solid rubber. But by the end of that decade the twin demands of comfort and increased speed led to the invention of the now-familiar pneumatic tire--hollow rubber filled with compressed air--and the wheeled world hasn’t been the same since.
A tire’s role in life is to create friction between the tread and the road surface, allowing the tire to transmit driving, braking, and cornering forces. The higher the friction, the more control the driver has.
But here’s the rub: while friction holds the car to the pavement, it also resists the tire as it rolls; that forces a car’s engine to consume more fuel as it works harder to overcome the resistance. And that, of course, contributes to global environmental problems.
In this country alone cars generate a billion tons of the planet- warming gas carbon dioxide annually. Over the past 25 years, automotive engineers have reduced exhaust emissions and increased fuel economy in order to meet federally mandated gas-mileage and air-quality requirements. This was achieved primarily by designing car bodies that were more aerodynamic and reducing the internal friction of engine and transmission parts.
But further gains in these areas were increasingly difficult and costly to manage. That’s why engineers began looking closely at improving a tire’s rolling resistance. Throughout the 1980s they did this by changing the mechanical and architectural design of the tire. In the 1990s they’ve turned their attention to composite materials that can cut rolling resistance while maintaining traction and wear.
Many people assume that tires are simply made from rubber and that rubber grows on trees (okay, inside trees). But in fact a variety of natural and synthetic rubbers are used, along with other materials. Some parts of a tire have as many as a dozen layers of material sandwiched together--steel on rubber, rubber on textile, rubber on rubber, and so on.
A tire’s basic shape and strength are derived from its skeletal structure, called the carcass, which is made of two or more layers, called cords. Typically, cords are made of nylon, rayon, or polyester, then coated with rubber to form a tire’s ply. An early tire design was the bias ply, so named because the cords extend across the tire’s width at an angle, or bias, of roughly 30 to 38 degrees. In each successive ply the cords run opposite to the previous layer. That creates a symmetrical crisscross pattern that gives a bias-ply tire its strength.
Additional reinforcing strips called belts, made of rubber-coated steel, are laid over the plies. On top of that is the tread, the wear- resistant component of a tire, which physically comes in contact with the road. Finally, on each side of the tread, an additional layer of rubber is bonded to the plies to form the sidewall. The end of the sidewall terminates in the bead, the part of the tire in contact with the wheel rim. The bead consists of steel wire encased within a rubber compound that’s held against the lip of the rim by the tire’s air pressure.
As much as 90 percent of a tire’s rolling resistance can be attributed to hysteresis--the dissipation of energy that occurs when the tread, sidewalls, and carcass of a tire are deformed as the tire rolls. The remaining 10 percent of resistance results from aerodynamic drag and the friction between the tire and the ground and between the tire and the rim.
As a tire is deformed, it builds up energy-losing heat. How much energy a tire consumes and converts into heat is a function of what engineers call the viscous-elastic properties of the materials that make up the tire. Think of Silly Putty and a steel spring as being on opposite ends of the viscous-elastic spectrum. A material that returns to its original shape, giving back all the energy initially applied to it (the steel spring, in this example), is said to be elastic. One that absorbs energy and assumes a new shape when deformed--the Silly Putty--is highly viscous.
Dropping two kinds of balls from the same height--a Superball, say, and a squash ball--illustrates the concept of hysteresis. The Superball will bounce back almost to the height from which it was dropped because it gives back most of the energy imparted to it when it hits the floor; it has low hysteresis. The squash ball, conversely, absorbs more energy on impact and can’t bounce as high; its hysteretic losses are higher. A tire made from squash balls would exhibit more rolling resistance than one made from Superballs. Industry experts have estimated that a 20 percent reduction in rolling resistance would yield a 4 percent savings in fuel, a significant decrease when most gains these days are measured in mere tenths of a percent.
In the late 1960s, radial tires--in which the cords run not at a bias but parallel to the wheel’s axle--began to replace bias tires and significantly reduced fuel consumption. As a radial rolls down the road, it flexes less, so the tire is deformed less. That gives a radial tire roughly 20 percent less rolling resistance than a bias-ply tire. From 1970 to 1992 radial tires have saved approximately 77 billion gallons of gasoline while slashing carbon dioxide emissions by some 1.1 billion tons.
Throughout the 1980s engineers further decreased rolling resistance by trimming the mass of the typical car tire by 15 percent. We optimized the shape of the tire and eliminated the cosmetic fat--features like big blocky, macho-looking shoulders that don’t technically do anything, says Mike Wischhusen, an engineer at the Michelin Tire Corporation in Greenville, South Carolina.
Eventually, though, the search for greater efficiency forced designers back to the basics: the chemical reactions and interactions among the various rubber compounds that are the building blocks of a tire. One of these building blocks is carbon black, a sooty by-product of the burning of oil that gives tires their blackness. If rubber were not mixed with an additive like carbon black, a tire’s tread would be too soft and have no resistance to abrasion. But carbon black has the disadvantage of making the tire behave more like a squash ball than a Superball.
A decade ago engineers discovered that rolling resistance could be reduced by replacing carbon black with silica, the basic component of ordinary beach sand. At first silica seemed to be the magic elixir tire engineers longed for, but they soon realized it too offered no free ride: its lower rolling resistance came at the expense of lower durability, decreased traction in wet weather, higher cost, and a more complicated manufacturing process. So silica quickly went back to the beach, and the engineers and chemists went back to their drawing boards and mixing pots.
Now, however, Michelin seems to be in the driver’s seat with a closely guarded proprietary silica blend that lowers a tire’s rolling resistance below that of carbon black but doesn’t sacrifice the favorable tread characteristics that carbon black provides and ordinary silica doesn’t. Just as important, Michelin’s silica-blend tires are only slightly more expensive.
Several current car models have been fitted with radial tires that have incorporated the silica blend. The cars are 5 percent more fuel efficient than cars with comparable tires without the silica blend. If every car in America was fitted with these tires, 2.4 billion gallons of fuel would be saved each year, and carbon dioxide emissions would be reduced by 22 million tons.
But Michelin and other tire companies are not content to rest on their laurels. Federal and state standards for fuel economy may get even tougher, so the search for new blends continues. Tire companies have spent millions of dollars on reducing rolling resistance. Yet ironically, the salutary effects of a new high-tech tire can be entirely undone by a driver who doesn’t maintain it properly. A driver can achieve better performance with a poor tire that’s inflated properly, says Goodyear engineer Sam Landers, than he can with a great tire that’s inflated incorrectly.