The four patches where a car's tires meet the road, each just slightly larger than your hand, govern handling and performance. They also affect the environment because of their impact on fuel economy due to rolling resistance (friction between the tire and the road). While engineers previously could increase economy by boosting tire inflation pressure, it would adversely influence handling and safety. But now such design conflicts may be resolved.
E-metric tires for GM's AUTOnomy fuel-cell powered car use low hysteresis compounds and high inflation pressure to reduce rolling resistance.
Recently General Motors introduced its AUTOnomy concept car that will be powered by hydrogen-based fuel cells (see DN 3.25.02, p. 27). Low-rolling-resistance tires are critical for GM engineers to achieve the desired 300- mile range.
AUTOnomy's Goodyear Tire & Rubber Co. E-metric tires (designated for use on electric or hybrid vehicles) use higher inflation pressures, which cut flexing of the tread as it contacts and then pulls away from the road during rolling—thus less energy is lost. A tire can also support a greater load at higher pressure (termed reduced load percentage). The fact that the AUTOnomy tires run on large diameter 22-inch wheels (compared to the typical 15-16 inches) also cuts resistance from flexing.
But higher pressures in ordinary tires would result in a rock-hard ride with potentially poor handling. "You can't just jack up the pressure, you get too small a footprint," says Bill Egan, chief engineer of advanced tire technology at Goodyear. "We developed a new tire profile. A narrower rim width and rounder sidewalls make the tire more shock absorbent." The E-metrics are max rated to 51 psi, but most electric and hybrid uses will be under that number. Pinching-in the rim width (the "thickness" distance from one side of the wheel to the other) and rounding (ballooning) the shape of the sidewall spreads the stresses out on the sidewall, he adds.
Another key technology noted by Egan is the low-rolling-resistance material compounds that form the tire and its smaller tread elements, which flex more easily when they touch the road. Goodyear engineers formed these into a "leaf" pattern that helps with flexing—and looks environmentally friendly. "The whole tire has reduced hysteresis, which reduces internal stresses and strains as it cycles," he says.
"In developing a low-roll for a specific car, you have to first size the tire correctly, with higher load capacity than with an ordinary tire. Use that in conjunction with high pressure to better performance," Egan summarizes.
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