In addition to constantly supporting the vehicle’s weight, even when it’s parked, tires will be stretched about 32 million times as they roll through a typical 40,000-mile lifetime. Tires will also experience subfreezing-cold through scorching-hot temperatures, as well as be exposed to direct sunlight, ultraviolet rays, harsh chemicals, acid rain, snow, slush and ice.
In order to be up to these challenges, tires must be complex products featuring hundreds of raw materials formed into a few dozen major components that are assembled and cured into one. While defects in materials or manufacturing workmanship can contribute to tire trouble, catastrophic failures are more often the result of severe service conditions, inadequate maintenance and road hazards that trigger separations and/or blowouts.
Pneumatic Tires: Air-pressurized tires are elastic laminated products. They consist of layers of fabric or steel internal reinforcing casing cords coated with rubber, cured in molds with heat to vulcanize the rubber.
It’s pressurized air, elastic rubber and reinforcing cords that allow tires to continuously change shape as they roll into and out of contact with the road. Tire sidewalls stretch from neutral to bulging loaded profiles while their treads transition from being round to flat footprints. This engineered stretching and compressing is called deflection, which contributes to pneumatic tires ability to combine load carrying capacity with ride comfort, traction, handling and wear.
While designed to deliver years of service, here are some of the key in-service factors that influence the durability of pneumatic tires.
Over-Deflection: Tire deflection is the result of maintained tire inflation pressures and the loads carried. While properly inflated and correctly loaded tires deflect as designed, operating tires with too little air pressure (under inflation) or forcing them to carrying too much weight (overloading) results in excessive deflection that causes severe casing stresses.
Regardless of the cause, over-deflection exceeding the tires’ engineered limits is likely to permanently reduce the rubber’s bond to the reinforcing plies and weaken the casing cords. Over-deflection at highway speeds can result in a tire’s abrupt catastrophic failure, often called a “blowout”.
A vehicle’s recommended cold tire inflation pressures and maximum permitted weight of passengers and/or cargo are listed on the Tire and Loading Information placard on the driver’s doorjamb and in its owner’s manual.
Driving Speed: At 60 mph, typically sized tires roll about 800 times per mile every minute (or 13.3 times per second). Increasing driving speeds to 80 mph increases the tires’ deflection frequency to 1,067 times per minute (or 17.8 times per second). In addition to intensifying the centrifugal force experienced by rolling tires, driving at higher speeds further increases the number of deflection cycles per second and reduces the time available for them to return to their neutral shape between cycles.
Combining these factors makes it important to select tires engineered for speeds equal to or greater than those at which the vehicle will be driven.
Previous Damage: Tires are exposed to accident damage and road hazards, such as impacts with potholes and curbs, as well as cuts and punctures caused by road debris, screws and nails. While major impacts will often result in immediately some visible external damage, lesser impacts may cause invisible internal damage and only be distinguished by a scuff or minor cut. However as the tire continues to roll during the following days, weeks and months, normal deflection during use may further weaken adjoining bonds, causing the damaged area to gradually spread until the tire eventually fails.
Tires experiencing severe impacts should be dismounted and carefully inspected inside and out to confirm if they must be replaced. Cut or punctured tires can only be repaired if their location and size, as well as repair procedures, meet industry/manufacturer guidelines.
Temperature: Tire operating temperatures rise as ambient air temperatures climb, tires are exposed to external sources of heat or driving speeds are increased. Sweltering air temperatures, searing-hot roads and high-speed driving can all challenge a tire’s ability to dissipate heat. Because rubber is an elastomer, tires are susceptible to breakdown when they are subjected to temperature extremes beyond their designed capabilities.
Tires are engineered to perform in naturally occurring temperatures, however hotter ambient temperatures further challenge the tires’ ability to radiate heat and can stimulate existing weaknesses from previous service.
Blowouts: Contrary to myth, tires correctly inflated to the vehicle manufacturer’s recommended cold inflation pressures don’t blowout due to excessive air pressure. Tire inflation pressure increases resulting from changing ambient conditions and/or tire-operating temperatures are not sufficient to cause properly maintained tires to burst or blowout.
Tires fail when forced to operate with too little inflation pressure, too much load or at speeds exceeding their design limits. Catastrophic failures are usually the result of a combination of the above factors acting together. While the initial cause may have been any one of them, it is typically their combined influences over time. And because previously damaged tires may have been permanently weakened, they should be periodically inspected to reduce the possibility of them contributing to a future failure.
More Frequently, Farther and Faster: With vacation travel and more hours of daylight during summertime months, drivers tend to be on the road more often, traveling greater distances and driving at higher average speeds than the rest of the year. Because heat is often a significant contributor to tire failures, blowouts tend to occur more frequent in hot climates or during summertime driving as higher ambient temperatures further challenge tire durability.
What’s a driver to do?
Replace worn-out tires with a new set featuring equivalent load capacity along with tread designs and rubber compounds that match expected driving conditions. Specify tires with speed ratings that meet or exceed the speeds at which the vehicle will be driven.
Check and set the tires’ recommended cold inflation pressures every month and before long trips.
Never overload the vehicle as that may contribute to catastrophic tire failure or permanently weaken the vehicle’s tires. Consider the weight of any items being transported and never carry more passengers than there are seatbelts.
Obey speed limits and avoid potholes and accident debris, as well as have tires professionally inspected after experiencing impacts and road hazards.
If the vehicle’s driving characteristics unexpectedly change (suddenly developing a vibration, odd noise, sloppy handling or begins to pull to the side), the vehicle should immediately be driven to a safe location and inspected. If nothing is visually apparent and the condition persists, the vehicle should be taken to a professional service provider to have it and its tires inspected to confirm correct inflation pressure and the absence of damage.
Drivers can control many of the challenges their tires face, as today’s tires typically only require basic periodic care to deliver years of wear. A few minutes a month and occasional visits to an automotive service provider will help assure years of satisfying service.
It is important to remember that tires will never regain their original strength anytime after their structural integrity has been compromised by under-inflating, overloading, road hazards or due to accident damage. Replacement, rather than retention, is the more reliable choice for tires that may have been permanently damaged.