Treading More Surely

During the 25,000 miles or so of its whirling life, the automobile tire bites the road in hundreds of thousands of stops, gets scraped by sidewalks and bruised by potholes, and runs long and hard at temperatures as high as 275DEG. Through all this, the tire is generally ignored until the day it goes flat. Now tires are getting a new kind of attention.

Prodded by the press, by state governments and by Congress, which is holding hearings on a tire-safety bill, the rubber companies are rolling out a whole batch of new tires that have some of the most important changes since the introduction of rayon cord (1938), nylon cord (1947) and tubeless tires (1947). Compared with existing tires, they wear longer, are less likely to blow out, grip the road more strongly, and keep their shape better.

Warning Signs. Led by Goodrich and Goodyear, tiremakers this fall will introduce "the wear bar," a device adapted from aircraft tires, which have a red cord imbedded in the rubber to show when the tread is becoming worn. In auto tires, small portions of the tread will be cut one-sixteenth of an inch less deep than the rest of the grooves, and will show up as bald spots when the tire needs replacing.

This week in Kansas City, Goodyear also will begin test marketing "radial-ply" tires, which have been popular in Europe for years, but are new to the U.S. Normally the layers of cord in a tire are set at 30DEG angles to one another, forming a diamond pattern around the wheel, but in a radial tire the cords are at 90DEG angles, forming a pattern of crosses. With this structure, the faster a car goes, the more tread is slapped onto the ground. The result is better road grip and up to 100% longer tread life--but also a harder ride. The tires are costly: about $50 each.

Trouble Spot. The greatest changes are in the cord fibers used to make the carcass of a tire. Nylon is a particularly admirable cord, but automakers are not fond of it. Only 6% of the nation's new cars carry nylon tires as original equipment, though 80% of the tires sold in the replacement market are nylon. Detroit's resistance derives largely from the fact that nylon tires tend to make a thumping noise for the first few blocks or miles of a ride--and auto dealers can have a difficult time convincing customers that the thump comes not from the car but the tire. The noise is an embarrassing result of nylon's tendency to "flat spot," that is, to flatten slightly when the car stands still for a while. Lately, the chemical manufacturers have devised nylons that almost eliminate flat spotting. Du Pont has begun marketing its N-44 nylon cord, Chemstrand has come up with X-88, Allied Chemical with EF-121.

The tiremakers are gradually introducing these cords, but automen are even more enthused about still another new material--polyester. Goodyear has begun to produce a polyester cord that it claims does not flat spot, resists heat better than nylon, sheds water better than rayon, and wears longer than either of them. There already are 8,300,000 polyester tires on the road, of which 7,000,000 have come on cars as original equipment, but automakers have not marked them as such for fear of starting a consumer rush that factories could not accommodate. Al though many of the chemical companies are betting on their improved nylon for the short run, polyester is likely to be the fiber of the future.

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