The invisible danger of aging tires: the U.S. tire industry is aware of the dangers posed by age degradation but has failed to alert consumers.

Tire manufacturers have long known that tires more than six years old regardless of tread depth, pose a substantial safety hazard to consumers. Tire age degradation has been an "open secret" within the industry, but the public has only recently started to take notice as the number of crashes caused by "aged" tires has grown. Prompted by consumer advocates and a growing body of research on the effects of tire aging, the government and manufacturers are beginning to address the issue.

Tires, like other rubber products, have a limited service life. Over time, a tire's internal structure degrades, reducing adhesion between the belts, which in turn facilitates tread separation. This internal degradation occurs regardless of tread use and wear. It is invisible and cannot be discerned, even by tire experts, without destructive testing.

Car crashes related to tread separation caused Ford to recall millions of Firestone tires in 2000. (1) After investigating the accidents, the National Highway Traffic Safety Administration (NHTSA) began examining tire aging as a factor in tread separations. In 2002, it proposed a tire-aging test that tire manufacturers would be required to perform on their products. (2)

The proposal recognized auto-safety advocates' long-held concern that government test requirements ensured only short-term durability mad did not protect against catastrophic tire failures from long-term chemical degradation. Unfortunately, overwhelming industry opposition and the industry's lack of consensus on an aging standard led NHTSA to shelve the proposed rule-making until it completed research on potential test protocols. (3) The agency is expected to reexamine tire-age test requirements and publish the results of its ongoing research in June 2005.

Ford Motor Co. also began sponsoring studies on tire aging after the 2000 recall of Firestone tires on Ford Explorers. Their findings, which are being released publicly and presented at tire industry technical conferences, lay an important foundation documenting the mechanisms of tire age degradation. (4)

Tire construction

To understand tire aging and what tread separation is, you need to know how a tire is made. A steel-belted radial tire has several components, including an inner liner, two polyester body plies, two steel belts, two bead reinforcing strips, the sidewall rubber, and a tread.

After these components are assembled in their "green" (uncured) state, the tire is loaded into a tire press for vulcanization. In this process, the tire is subjected to extreme heat and pressure, which cause the components to fuse into a single structure--a completed tire. (5) In a typical tread separation, the top steel belt and tread become detached from the rest of the tire.

With age, tire components dry out, causing the adhesion between them to break down. The process of oxidation hastens the deterioration: It occurs as air permeates the inner liner, a thin rubber lining that coats the inside of the tire and acts as a robe. The internal oxidative degradation caused by air permeation occurs over time, regardless of whether the tire is used. Current studies suggest that in-use tires age no more than those stored as spares on the same vehicle. In other words, age degradation occurs regardless of the mechanical fatigue that a tire undergoes. (6)

That rubber deteriorates with age has been recognized since the …