Eagle GS-C EMT tire
Randy Brayer, Goodyear Tire & Rubber
It might not rank with death and taxes, but a flat tire still earns a place on most people’s lists of dreaded events. Now Goodyear’s Eagle GS-C EMT tire promises to eliminate that fear. Even without any air in it, the EMT (or Extended Mobility Tire) can travel 200 miles without compromising car performance, handling, or speed--more than enough distance to reach the next gas station with ease and dispatch.
Since 1989, Chevrolet’s Corvettes have had low-tire-pressure warning systems. In 1991, a team at Goodyear set out to design a run-flat tire that would debut as an option on the 1994 Corvette. The team had to overcome two technological hurdles. Historically, when you thicken a tire’s sidewall you also compromise its ride, says senior tire engineer Randy Brayer, the team’s leader. First, we had to attain the manufacturer’s run-flat distance requirement of 200 miles without performance trade-offs. Second, we had to get the tire to stay on the rim.
To devise sidewalls strong enough and thick enough to keep an airless tire from collapsing, the team modeled different cross-sectional tire profiles on a computer. We simulated their interactions with the road under load, Brayer says. Optimizing our design by computer saved us years of building and testing different tires. In addition to the right shape, the team also needed the right material. Tire fabric is made to perform under tension, with internal air pressure pushing out. As a tire loses air, it runs under increasing compression, with the car’s weight bearing down. Internal temperatures soar, and the tire begins to degrade and flatten. To forestall that, Goodyear makes its sidewall reinforcements from a proprietary new polymer that dissipates heat and lengthens the tire’s run- flat life.
The second challenge--getting an airless tire to hold a rim--was no less a puzzle. Normally a tire needs both air pressure and a slight lip along the tire’s bead (its inner edge) to guarantee a seal between the tire and the metal rim that bolts to the wheel. When a tire’s air pressure falls below 10 or 12 pounds per square inch, the extra force generated in cornering can break the seal, and the tire then comes off the rim. We needed to work with standard auto rims, Brayer says. Automakers resist any kind of special rims or wheels with a vengeance. After more than a year of trying, the team combined a toughened bead material with a new bead contour that solved the problem. The engineers also corseted the EMT with a synthetic fabric beneath the tread to keep the tire from growing in circumference under the extra heat and centrifugal force created by running at high speed.
The result is a tire that, even with chunks ripped from the tread, can carry a Corvette occupied by two people and a trunk filled with luggage 200 miles at a smooth and steady 55 miles an hour. (Brayer and a passenger drove on an EMT with its air valve torn off for 400 miles at speeds above 55.) An airless EMT can go about 50 miles before it begins to sustain damage to its sidewall reinforcements. As the damaged tire approaches its 200-mile limit, it notifies the driver by an increasingly pronounced vibration. Goodyear expects the EMT’s retail price will be about $400, just 20 percent more than top-of-the-line high-performance tires.
Now that EMTs will be available on 1994 Corvettes, Goodyear is designing a version for luxury touring sedans--the next car class expected to add low-tire-pressure warning systems as standard gear. Eventually, Brayer expects EMTs will become standard issue on a broad range of car models and may eventually be as common as anti-lock brakes. The EMT isn’t a tire. It’s a technology. And everyone’s going to want it.
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