Stress, heat and oxidation are the three primary factors affecting tire life. Inflation pressure, load and speed are the sources of tire stress. Heat will lower, or accelerate, the point at which these stresses cause a failure. Oxidation is a natural process that affects the rubber of the tire. Virtually all of the tire's components, especially their adhesion to one another, are vulnerable to oxidation, which ultimately diminishes tire longevity.

What do these hazards mean for typical passenger vehicles? For the daily driver who maintains proper tire inflation pressure, anticipating when stresses could affect vehicle tires and making suitable adjustments are important. Published tire ratings provide guidance for events like transporting loads and also set the speed limits for the make/model of the vehicle's tires under laden conditions. These stressful driving situations increase the impact of heat and oxidation on the tires.

An extreme example illustrates the interactions of these three tire durability challenges. Picture a laden SUV towing a trailer and running on older, underinflated tires due. Place that vehicle on a Southwestern interstate on a summer's day. A failed tire would not be a surprise, but it certainly could become more dangerous if the driver is disengaged from active driving. Safe operation in automated mode requires a vehicle and all its components to perform flawlessly.

Photo courtesy of Peter R. Thom & Associates, Inc.

Typical tire durability failures

A run-soft failure can easily be confused with a road hazard failure like a puncture leading to a blowout. One is avoidable, the other is dramatic and sudden. While TPMS is intended to reduce the number of cars operating with chronically underinflated tires, the problem is still prevalent among older models and in vehicles where owners ignore the TPMS. The very real problem with a slow and steady decrease in inflation pressure (tires lose about 1 psi per month via normal permeation) is that extreme overheating will occur inside the tires. Temperatures inside the tire can exceed 400 degrees—the melting point of the polyester cords typically used in the tire's radial plies. The sidewall can experience a sudden and massive blowout, even though the operating pressure of the tire at that moment might have been extremely low. Other causes of unexpected loss of inflation pressure include: damaged or defective tires, leaky valves, and bent or corroded rims sabotaging the air seal between the tire bead and rim.

Highway-speed failure. Under normal loads, inflation pressures, and conservative use—i.e., self-imposed operating speed limits as well as avoidance of roadway hazards—a tire is expected to hold together until the tread wears out. Nowadays that target is 100,000 miles. Should the vehicle driver exceed the tire's speed limit, that tire will most probably fail via a belt separation. Likewise, should a tire's wear limit exceed its fatigue endurance limit, that tire will probably fail via a belt separation.

As the temperature at the belt edges increases with excessive speed, a point will be reached when the combination of centrifugal and other forces will cause the tire's rubber to tear. This is called delamination. Typically, in a few moments and in the space of several tire revolutions, a bulge followed by cracks in the tire will lead to the layers peeling away. Remarkably, this dramatic event can shed the entire tread from a tire without causing any air loss, but more typically, the air loss is instantaneous.

Fatigue failures are often correlated to chemical aging, an oxidation process that occurs over time rather than miles of usage. The rate of the aging can vary, depending on the tire's quality and its working environment. Actual tire age can be determined from the numbers stamped on the sidewall. The challenge for the modern-day tire with high tread-life expectations is staying intact long enough to achieve that mileage standard when natural forces like oxidation age a tire. Lower tire pressures lead to higher internal temperatures and oxygen permeation increases as those temperatures climb, resulting in lowered durability. Hot climates are especially taxing to tires as a result.

The deceptively complex tire

By appearance a radial tire looks to be a simple automotive component, but that black rubbery skin presents a deceptive face. Today's steel-belted radial ply highway tire is a highly engineered vehicle component. It is also a precision product that is unforgiving of what might otherwise seem to be the smallest of mistakes or anomalies. These are operating realities that need to be considered as automated technologies take over our vehicles.

William J. Woehrle is the tire group leader at Peter R. Thom and Associates Inc., a national firm of consulting automotive engineers. He can be reached at [email protected].