Conventional wisdom about broken glass and other kinds of hazards encountered on the road abounds.
The sight of smashed glass on the highway prompts most motorists to swerve to avoid damaging their tires. Potholes are generally viewed as obstacles, sort of a test of one's driving skills. Rubbing a tire against the curb is considered unfortunately clumsy, but nothing to worry about.
Such thinking is correct in some cases and entirely wrong in others. The current controversy over tire safety and recalls raises questions about just what kinds of road hazards can inflict serious exterior or internal damage to tires that can later cause blowouts.
With their substantial polyester cord plies (about 1 pound in a typical passenger tire) and steel belts (another pound) and their heavy cushion of synthetic rubber (about 6 pounds), modern tires represent a formidable piece of armor for running across pavement at high speeds.
In general, experts agree that glass is not one of the most serious threats to a tire. This is particularly true of glass from broken windshields or car windows, which use tempered safety glass composed of a sandwich of plastic film between two layers of glass.
You can instantly recognize broken safety glass because it shatters into small, almost perfect cubes. This glass typically lacks the sharp edges that could cut deeply into tires, according to Ed Wagner, a tire safety consultant who operates a laboratory in Louisville, Ky.
"It has to be a pretty heavy piece of glass" to work its way through the rubber, Wagner said.
A bigger threat to tires is broken glass bottles, the shards of which can have sharp edges, especially around the bottle necks. Such fragments seldom slice through a tire, but they can become embedded in the rubber and work their way slowly through the various layers. Wet rubber cuts more easily than dry rubber, so hitting glass on a rainy day carries extra risks, Wagner said.
Even so, picking up a piece of glass in a tire is seldom enough to cause a catastrophic blowout. That typically occurs when the cords or belts of a tire are damaged or when separation occurs.
That kind of damage to belts and cords typically results from a high-speed impact with debris or a pothole. The impact often weakens, stretches or breaks the tire's cords and belts, according to Keith Baumgardner, general manager of Tire Consultants Inc. of Georgia. An obstacle the size of a 4-by-4 piece of lumber on the roadway is a definite risk, he says.
Striking such an object can cause the steel belts to snap or stretch in an X-shaped pattern, Baumgardner said. It can also break the adhesion of the steel belts to the surrounding rubber. Then, hundreds or even thousands of miles later, the tire could blow out.
Damage to a tire's sidewall from scraping against curbs is another serious risk, since the sidewalls typically are only about a quarter of an inch thick and are easily damaged.
"If you are rubbing against a curb, you are asking for trouble," Baumgardner said.
The risk of internal tire damage is that it's usually impossible to recognize. And a tire that has been compromised can fail without warning, according to Steve Mazor, the Southern California Auto Club's safety and engineering expert.
"With internal damage, there is not anything to see," Mazor said.
Nonetheless, if you hit something substantial with your tires, such as a curb, a deep pothole or a piece of lumber at high speed, it makes sense to have the tire inspected by a reputable tire dealer. Such damage sometimes can be spotted by inspecting the inner surfaces of the tire.
A different kind of tire damage comes from pinholes that do not cause air leaks but allow water to penetrate into the steel belts. The water causes corrosion of the belts, which both weakens the belts and undermines their adhesion to the rubber. Parking in puddles is a particular problem, but even rain can cause water to penetrate through such pinholes.
The most important part of tire maintenance remains proper inflation and regular visual inspection to check for things such as cracks, cuts, embedded glass or nails and sidewall blisters or bubbles that would indicate internal weakness.
Remember, too, that a tire typically loses about 1% of its inflation every month. A perfectly good tire thus can become under-inflated in just a few months.
Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail: firstname.lastname@example.org.