With all eyes on paradigm-shifting automotive technologies, itis easy to forget that even the most technologically advanced motorvehicles must rely on the humble rubber tire in order to move.Sure, we may face a future of wheel-less, magnet-empowered hoveringcars, but in the meantime, there are operating facts you shouldknow about the steel-belted radial tire that will enhance travel inincreasingly sophisticated automobiles.

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The more automation separates us from the driving act, the lessattention we pay to our machinery, leaving us vulnerable tooverlooking failure signs in vehicle components that cannot beautomatically tracked by onboard microprocessors.

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Since 2008, all new vehicle models have tire pressure monitoringsensors (TPMS) as standard equipment, but that self-diagnostic tooldelivers only a partial tire health check. Beyond tire inflationissues, durability is the overarching concern because it correlatesto the miles driven, road conditions met in use, climate, storage,and tire age. The key to noticing incipient rubber oxidation andother signs of deterioration is regular visual examination.

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Tire durability hazards

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Stress, heat and oxidation are the three primary factorsaffecting tire life. Inflation pressure, load and speed are thesources of tire stress. Heat will lower, or accelerate, the pointat which these stresses cause a failure. Oxidation is a naturalprocess that affects the rubber of the tire. Virtually all of thetire's components, especially their adhesion to one another, arevulnerable to oxidation, which ultimately diminishes tirelongevity.

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What do these hazards mean for typical passenger vehicles? Forthe daily driver who maintains proper tire inflation pressure,anticipating when stresses could affect vehicle tires and makingsuitable adjustments are important. Published tire ratings provideguidance for events like transporting loads and also set the speedlimits for the make/model of the vehicle's tires under ladenconditions. These stressful driving situations increase the impactof heat and oxidation on the tires.

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An extreme example illustrates the interactions of these threetire durability challenges. Picture a laden SUV towing a trailerand running on older, underinflated tires due. Place that vehicleon a Southwestern interstate on a summer's day. A failed tire wouldnot be a surprise, but it certainly could become more dangerous ifthe driver is disengaged from active driving. Safe operation inautomated mode requires a vehicle and all its components to performflawlessly.

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

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Typical tire durability failures

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A run-soft failure can easily be confused witha road hazard failure like a puncture leading to a blowout. One isavoidable, the other is dramatic and sudden. While TPMS is intendedto reduce the number of cars operating with chronicallyunderinflated tires, the problem is still prevalent among oldermodels and in vehicles where owners ignore the TPMS. The very realproblem with a slow and steady decrease in inflation pressure(tires lose about 1 psi per month via normal permeation) is thatextreme overheating will occur inside the tires. Temperaturesinside the tire can exceed 400 degrees—the melting point of thepolyester cords typically used in the tire's radial plies. Thesidewall can experience a sudden and massive blowout, even thoughthe operating pressure of the tire at that moment might have beenextremely low. Other causes of unexpected loss of inflationpressure include: damaged or defective tires, leaky valves,and bent or corroded rims sabotaging the air seal between the tirebead and rim.

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

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

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Fatigue failures are often correlated tochemical aging, an oxidation process that occurs over time ratherthan miles of usage. The rate of the aging can vary, depending onthe tire's quality and its working environment. Actual tire age canbe determined from the numbers stamped on the sidewall. Thechallenge for the modern-day tire with high tread-life expectationsis staying intact long enough to achieve that mileage standard whennatural forces like oxidation age a tire. Lower tire pressures leadto higher internal temperatures and oxygen permeation increases asthose temperatures climb, resulting in lowered durability. Hotclimates are especially taxing to tires as a result.

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The deceptively complex tire

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

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

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