Higher Speeds, Other Road Trends Can Strain Tires to Breaking Point
As experts and safety advocates search for answers to the rash of deadly tire-related crashes, a general observation has emerged: Tires that may have been barely good enough in the past are being strained beyond their limits under today’s harsher driving conditions.
The explanation does not exclude possible defects unique to the design or manufacture of specific tires. But it reflects the reality that tires today are exposed to more punishment, and some may not be up to the task. Added stresses on tires include:
* Higher speeds. With repeal in 1995 of the federally mandated 55-mph speed limit, most states quickly increased highway speed limits to 65, 70 or even 75 mph. Actual traffic flows were faster than that, including in some wide-open stretches where blistering heat makes the risk of tire failures inherently greater. In Texas, for example, where about 15% of vehicles exceeded 70 mph in the last days of the 55 mph limit, the proportion quickly rose to 50%, according to a study by the Insurance Institute for Highway Safety.
During the 1970s and ‘80s, when radials were replacing bias ply tires, the federal limit may have enhanced their durability. Of course, the lower limit was widely violated, too, but data suggest average speeds were considerably lower than today.
To be sure, all tires are designed to travel at 70 mph with ease. But higher speeds mean greater heat, which can weaken the adhesion between a tire’s steel belts--particularly in tires already compromised by poor workmanship or road damage.
Higher speeds also add to the mechanical stress on tires. As tires revolve, the tread pulls outward on the upper of the two steel belts, straining the bond with the bottom belt. At higher speeds, this force increases exponentially, experts say.
* The boom in SUVs and other light trucks. Sport utility vehicles run on larger tires that experts say have always been prone to belt and tread separation. And the top-heavy vehicles are harder to control--and more likely to flip over--when a blowout occurs, increasing the risk of tire failure ending in tragedy.
Before the SUV boom, blowouts were more apt to be “innocuous” events, raising heartbeats but sending no one to the morgue, said Harold Herzlich, a tire expert in Las Vegas. “They’d leave a piece of tread on the side of the road, and that’s all you’d see.”
With tire failures today, “the scale of injuries is more than it used to be,” said Alan Milner, a metallurgist who works for plaintiffs in tire litigation. The consequences have “been exacerbated since the lifting of those speed limits, and the proliferation of these SUVs.”
* The longevity of radial tires, which gives marginal tires more time to fail.
Motorists in the past were lucky to get 25,000 miles from their bias ply tires. With longer-lasting radials, the standard tread life warranty is for 40,000 miles. And to woo consumers, warranties for many models cover 50,000, 60,000, or even more than 80,000 miles. Consumers may be forgiven for imagining that their tires are immortal.
Yet, because rubber deteriorates with age, the adhesion between the belts weakens over time, whatever the appearance of the tread. Acknowledging the effects of age, Bridgestone/Firestone Inc., recently shortened the warranty for factory defects in its tires from five years to three.
The aging of rubber may also explain reports of blowouts of nearly “new” spare tires. Experts warn that consumers are largely unaware that the structural integrity of new-looking but old spare tires may have deteriorated over time.
