Today's tire is a sophisticated, thoroughly reliable vehicle component offering a host of user benefits such as excellent tread wear, tear resistance and long life. But tires are not eternal. Even when the tread is not completely worn out, they should be replaced after a given period of time for safety reasons.
While examining a failed retreaded tire recently, I thought about the need for putting a time limit on tire life. The sidewall DOT coding showed the tire to be pretty old. It was more than four years old when retreaded. Then it remained on a storage rack for two more years-exposed to sunlight, ozone and weather-before being placed into service on the vehicle.
The tire's failure was tragic because it resulted in a loss of life. The diagnosis of its failure was straightforward-the tread separated from the casing because of extreme rusting in the steel belts, causing a blowout while the vehicle was traveling on the highway.
In certain areas of the U.S., tires are exposed to roads wet from rain, snow, fog or humidity much of the time-not to mention the adverse effects of increased salt use during cold weather.
Any tire with cuts in the tread-and that means most of them-will ingest a lot of water over its lifetime. And once this moisture starts working on the wire filaments of the steel belts, they quickly disintegrate into patches of brown, powdery rust.
Often, this rust goes undetected until the tire blows out and the tread flies off along with the remains of the belts.
Since bare steel won't adhere to rubber, steel cords must be given a brass coating containing 65 to 75 percent copper.
This coating should not be removed when retreading or repairing steel-cord tires. Otherwise, those shiny exposed steel cords are an invitation to disaster because adhesion will be almost non-existent.
We all know the lack of attention given tires by the average motorist. And since motoring safety seems to be paramount these days, perhaps it's time to establish a ``sell by'' date for tires similar to those used for other products.
Most manufacturers feel eight years is a realistic life expectancy for the average tire. In that case, why not advise motorists that an 8-year-old tire is a potential hazard waiting to nail some unsuspecting user?
I am well aware that tires can remain in stock for a long time-sometimes even several years-before placed into use. This may be more of a problem today with the ever-increasing range of sizes and types.
Still, I think the vast majority of tires will be in service on the road within two years of manufacture.
So wouldn't it be a good idea to advise no more than six years use from then on-regardless of how many miles the tire has covered?
John Q. Public may set up a howl if told to throw away a tire that still has obvious service potential-that is unless convinced he's jeopardizing his own life by not doing so.
To be realistic about improving safety, no tire should be expected to last more than eight years.
Unfortunately, people are using their cars longer and driving fewer miles each year. According to government statistics, 35 percent of the cars in use today are more than 8.1 years old 26.4 million cars or 22 percent are older than 12 years. Think about how many are still riding on the original tires-accidents waiting to happen.
Like humans, tires start to die the day they're born. Sunlight produces ``crazing'' in rubber. Ozone produces cracks developing at right angles to the stress of the material. (Minute ozone cracking is referred to as ``checking.'')
Fortunately, sidewall materials today are much more resistant to fatigue, oxidative degradation and ozone attack than those of only a decade ago.
This is a far cry from the early days of motoring when it was recommended that any tire known to be old should not be exposed to heat vulcanization. The already deteriorated materials would not stand treatment by heat.
An ``old tire,'' incidentally, was one that had been manufactured three or more years before or carried as a spare for more than a year before being placed into use.
Nevertheless, steel-belted casings stored in unprotected areas and those having tread cuts and nail hole punctures are not good candidates for retreading. Section analyses show the heaviest rust is found on the innermost belt-indicating that the offending moisture came from inside the tire.
In the last several decades, tire service requirements have become more and more demanding. In the case of truck tires, this has resulted from higher running speeds and greater tractor horsepower, both of which can cause major casing fatigue during the tire's first tread life.
Passenger car tires, meanwhile, have experienced a tremendous increase in tread life expectancy (more than four times that of bias ply tires). And while tread life has been increasing, the average miles driven annually by U.S. motorists have been decreasing.
The result is that tire use may continue far beyond the product's service capability. Setting a realistic time limit on its use could be the answer. What do you think?