I recently analyzed the failure of *an old tire involved in a fatal accident, and the diagnosis wasn't difficult: The tread had separated from the casing due to rusting of the steel belts. Even though the tire's tread depth was a bit above the legal 2/32-inch limit, it was the original equipment on a 1986 vehicle. Such failures are not unusual, and I wonder why we don't scrap tires after they reach a certain age.
Tires are not everlasting. Even when the tread is not worn out, they should be removed for safety reasons, if nothing else.
When I was a retreader, tires that were four or more years old were summarily rejected for retreading on the belief they would surely fail.
Today, many technicians believe a tire should be used for no more than six to eight years at the most, irrespective of whether the mileage is 20,000 or 60,000 miles.
Considering that the average life of vehicles running on our highways is eight years, it's a sure bet that many are running on tires eight or more years old.
We must also consider the fact that some tires are warehoused for a period of time, so their age may even be greater.
Thus, some experts say to allow six years from the date of installation, then scrap it. Realistically, no tire should be expected to last more than six to eight years.
There also are other factors that can lead to tire failure. As any tire rolls down the highway, it is subjected to strong forces that literally try to tear it apart.
With radials, these forces are most pronounced along the belt edges-the areas where the stiff belts meet the flexible sidewalls. It is here that most failures occur, and the cause usually can be traced to incorrect tire pressure, neglected tread cuts and improper alignment.
Slight underinflation is relatively slow in bringing about the disintegration of a tire, but it is very sure in its action.
The exact air pressure in a tire can never be judged by appearance alone. Severe underinflation is quite obvious, but most consumers will confuse the appearance of an underinflated tire with the notorious ``radial bulge'' and assume that this is the normal profile.
Made from a few pounds of rubber, cord and a small amount of steel, tires are attached to the vehicle only by a few square inches of contact between the tire beads and the wheel rim.
And tires hold air so well, that most motorists forget to inflate them. This disregard most certainly is not very sensible, as it not only reduces tread life, it also increases fuel consumption.
Running a tire 20 percent underinflated, which in most cases is only 4 to 5 psi, will reduce tire life by as much as 15 percent to 20 percent and increase fuel consumption by as much as 10 percent.
Another point to keep in mind is that the customer cannot always rely on the tire fitter to inflate tires to the proper pressure because they often underinflate or sometimes overinflate tires due to carelessness or inaccurate gauges.
Thus it is good practice to make an add-on sale of a pressure gauge to the motorist so that he or she can check tire pressures on a regular basis to save on fuel consumption and tire wear. It also adds a bit of profit to the sale.
Tread cuts also can adversely affect tire life. Most tires will suffer tread cuts during a long service life, and in doing so, they will ingest quite a bit of moisture.
Once this starts working on the steel cords in the belts, the moisture will cause oxidation and the cords will quickly crumble into patches of rusty powder. Due to capillary action, water can spread a half-inch from a penetration within 24 hours.
Strangely enough, rust often affects the innermost ply more often than it does the outermost ply in the belt. Moisture often may penetrate into the interior of a tubeless tire by excessive amounts of mounting lubricants and moisture in air lines. Once inside, the moist air will heat up and percolate through the liner into the body of the tire and cause rusting of the steel belts.
No one can accurately predict the life span of any individual tire-the variables are much too great.
A broken bottle, a piece of metal or other road hazards have a way of suddenly ending a tire's life. There is no way to prevent a hard, sharp object from penetrating the tread or sidewall of a tire.
And when you are asked to repair a penetration, there are three words to keep in mind if you are thinking about plug repairs-Don't allow them!
Air can often seep along the plug partway through the puncture and create an air pocket between the tread plies leading to separations and subsequent failure.
Repairs to tubeless tires must always be made from inside the tire by removing it from the wheel.