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A glimpse into the complex task of choosing automobile tires.

Charles Goodyear knew he was onto something when, in 1839, he tossed a wad of raw rubber against a hot woodstove and discovered vulcanization. But, visionary though he may have been, the then-destitute inventor could hardly have imagined the capabilities of today's automobile tire.

Carcass Design

It almost goes without saying that you're in the market for a radial-ply tire. Almost, because the country dweller may be one of the few people who still have a reason other than plain stinginess to buy a bias-ply tire.

In every way save one, the radial is technically superior to the bias-ply. It lasts up to twice as long; it offers more traction—wet or dry; it runs quieter; it has as much as 20% less rolling resistance, which gives better fuel mileage. The one thing it doesn't do as well as the sturdy bygone is withstand heavy impacts—say, from a nasty pothole on a country lane or a rock on a logging road. Let's look at construction methods to see why.

A radial tire is built to flex. Its thin sidewalls, with plies laid perpendicular to the tire's circumference, allow the tread area to deflect easily as it rotates, which lowers rolling resistance. This sidewall flex, combined with the belt(s) wrapped around the circumference of the tire to hold shape and make the tread stiff side to side, helps the tread stay flat on the road in turns. The only significant penalty for the radial's flexibility is that a severe bump can collapse the sidewall, pinching it between the rim and belt and rupturing the plies.

The bias-ply, on the other hand, has several crisscrossed plies running from one side of the rim to the other at a 25° to 40° angle to each other. A circumferential belt is found on a derivative—the belted-bias tire—which may improve durability and tread wear but has little effect on handling. In either case, the combination of more material and triangulated plies dragging across each other makes the bias-ply's sidewalls stiffer than the radial's, producing a tire that's less compliant but better able to withstand rough treatment.

Carcass Materials

Below the tread, the rubber compounds used in the carcass of one tire differ little from those of another, generally being a mix of styrene-butadiene, polybutylene and perhaps some natural rubber. Add about 10 pounds of carbon black, oil extenders and wire in the bead (where the tire attaches to the rim), and you've got about 20 pounds of the typical 25-pound tire. None of these items are likely to receive much attention in a typical product brochure, so you'll probably have to take their quality on faith.

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