DETROIT—The radial tire changed the course of tire technology, converting what had been a cranky, failure-prone, 15,000-mile part into a reliable, 40,000-mile component that many consumers ignore.
But Jacques Bajer knows what a complicated piece of engineering a modern radial tire is.
He was a principal engineer in Ford Motor Co.'s introduction of steel-belted radial tires to North America in 1967. Now, as president of Tire Systems Engineering, a Grosse Pointe, Mich., consulting firm, he has documented the complex factors that can create tire trouble.
"In the production of a radial-ply tire, there is no one part of the tire that needs less attention than any other," he said. "Everything is important, from the macro to the micro."
Nobody has cited publicly an exact cause for the failures that have led to Bridgestone/Firestone Inc.'s voluntary recall of 6.5 million Firestone tires.
The National Highway Traffic Safety Administration's continuing investigation focuses on catastrophic failures, in which the steel belts and tread appear to have separated from the tire's radial carcass.
Speed a factor
NHTSA reports show that in at least 28 cases, the tire remained inflated after the apparent separation. Most failures were reported at speeds of 55 to 70 mph, and some tires failed after fewer than 2,000 miles.
Ford and BFS would not grant interviews to discuss the construction of auto tires. But Mr. Bajer and other experts note that radial tires are complex, involving up to 20 components and up to 10 rubber compounds.
The various rubbers, chemically linked by the vulcanizing heat and pressure in manufacturing, hold everything together without glues, binders or mechanical fastenings.
But that rubber can fail if internal stresses or external factors cause excessive heat buildup, which breaks the rubber's strong chemical bonds, or if flaws in tire manufacturing leave weaknesses that amplify under stress.
Three stress points
Radial tires create stress points in three places. The polyester radial plies that make up the tire carcass allow the tire to flex in order to lay the tread flat on the pavement, but that flexibility puts a hot spot at the outer bulge of the tire sidewall.
The shoulders of the tread, where reinforcing belt edges end, also are a stress and heat point. And the inner radius where the stiff tire bead meets the wheel rim is a third stress area.
Inside the vulcanized rubber, rings of steel cable, layers of calendered steel wire, and polyester body plies must work with perfect adhesion as they flex, bend and spin at tremendous speed. Correctly inflated, the tire functions like a pre-stressed concrete form capable of bearing a strong load. As tire air pressure drops, so does its designed-in strength. Heat, the worst enemy of tires, builds quickly.
Air pressure is a key factor. Ford specifies a pressure range of 26 to 30 psi on the recalled tires; Firestone recommends 30 psi. In light of the recall, General Motors Corp. said it recommended 35 psi for its light trucks using tires similar to those Ford uses.
Mr. Bajer said proper inflation is the key to tire function: "The efficiency with which the tire-elastomer-cord composite structure operates, pre-stressed by the inflation pressure, is the key."
Heat is the enemy
Joseph Grant, an engineer at Continental General Tire Inc., said drivers rarely realize how quickly heat can build at high speeds. "Truck tires get the hottest. The shoulder temperature of a truck tire running at 60 mph gets up around 170 degrees Fahrenheit," he said.
A passenger car with a severely underinflated tire can travel only about 70 miles at 60 mph or more before the rubber gets too hot to touch. When the rubber temperature passes the boiling point of water (212 degrees F.), serious damage can occur. At that point, the tire begins to degrade.
Mr. Bajer said tire designers control heat by limiting the tire's tendency to change shape while moving, partly through using the correct pressure and partly by designing a tire with some user neglect in mind. Stop-action photos of tires running at high speed show that a standing wave wrinkle can form in the tire sidewall and in the tread as it reaches down to the road, while other parts of the tire stretch away from the wheel in a centrifugal effect.
Under such enormous forces, a manufacturing defect could cause a tire to fail. The most common problems for tire makers, according to a 1994 paper published by Mr. Bajer, include unevenly tensioned or distributed radial cords; wavy, off-center or off-stepped belting; air inclusions; blisters; porosity; and ripples in the rubber and the many splices that join the wires and belting within the tire.
This article originally appeared in the Aug. 14 issue of Automotive News, a sister publication of Tire Business.