Every Vehicle’s Bottom Line: the Tires

Steve Mazor was driving home from Burbank on the Santa Ana Freeway three weeks ago when the right-rear tire on his company car blew out at 70 mph.

He backed off the gas, made his way from the fast lane to the right shoulder while decelerating and gently braked to a stop--the proper procedure to follow to avoid loss of control when a tire suddenly loses pressure.

Mazor--chief automotive engineer for the Automobile Club of Southern California--has had a bit of practice at it.

Less than a month earlier, the left-front tire on the same car blew out as it bumped over a Botts dot lane marker as Mazor drove in the fast lane on the Glendale Freeway.

In both cases, the tires were nearly new, mid-priced brand-name replacements for the original-equipment rubber. There was no apparent reason for either blowout.

The car Mazor was driving, a 1981 Toyota Cressida with more than 120,000 miles on the odometer, is serviced at the Auto Club’s technical center. The vehicle “is maintained as good as you will ever find a car being maintained, and that includes frequent and regular tire maintenance and inspection,” he said.

Still, the tires blew.

“It just happened,” Mazor says.

Blowouts, regular flats--even catastrophic tread separations such as those that have plunged Bridgestone/Firestone Inc. and Ford Motor Co. into turmoil recently and made tire safety a topic of dinner-table conversation across the country--do, indeed, just happen with no warning and no identifiable cause.

But many more tire failures are caused by events that car owners and drivers can control, experts say.

And if any public good comes from the Firestone crisis, it’s likely to be that it has raised awareness of tires and the importance of regular inspection and proper maintenance.

In the Firestone case, more than 1,000 tires--most of them mounted on Ford sport-utility vehicles and pickup trucks--have disintegrated at high speeds in recent years, leading to several hundred accidents linked to several dozen deaths in the U.S.

Using Your Tires to Read the Road

To really appreciate how important tires are requires an understanding of what they do and how they are constructed.

Simply put, tires are the most critical operating component of a vehicle. Without them, the car or truck--no matter how powerful its engine, how sophisticated its suspension or how luxurious its interior--has to sit, no more useful for transportation than the shade tree in the front yard.

But there’s more to the tire than giving mobility to a vehicle.

“It is the tire that gives you all the information you get behind the wheel, the feeling of what the car is doing,” says Jacques Couture, chief instructor for the Jim Russell Driving School at Sears Point Raceway in Sonoma, Calif.

“The contact patch--where the rubber meets the road--tells you everything. And when you have to swerve to avoid hitting a child playing in the street, it is the tires that make or break the maneuver. Or when you are driving at 70 miles an hour in a rainstorm, it is the tires--if you have the right ones--that keep you on the road,” Couture says.

That contact patch, no more than a few square inches per tire, also is all that sticks the car to the road, all that keeps it upright during cornering and turning maneuvers that create tremendous centrifugal forces that want to pull vehicles off the asphalt and fling them--and their occupants--like a stone shot from a whirling sling.

“The tire is the only part of the car that touches the ground,” notes Rick Brennan, director of strategic planning for Yokohama Tire Corp. “Your acceleration, your braking, your steering, your control of the car is only as good as the tires’ capabilities.”

That, Couture says, is why wheel and tire widths, tire pressures, sidewall stiffness and tread design are so important in the world of high-speed performance driving.

And it is why for most of us, the simple act of walking around our car once or twice a month--visually inspecting the tires for cuts and gouges, checking their pressure with an accurate tire gauge and replacing them before they wear out--is as important as buckling our seat belts.

Tire Pressure Often Neglected

Of all the problems that can affect tire performance and lead to flats and other kinds of tire failure, under-inflation is the most common, tire specialists say.

All tires are marked on the sidewall with a maximum inflation figure, shown in pounds per square inch, or psi. But that’s just a guide to keep you and the tire installer from exploding the tire.

Although Ford’s recommended inflation for Firestone tires on its Explorer SUVs has raised questions because it is 25% lower than Firestone’s own recommendation, tire specialists continue to insist that the vehicle manufacturer’s recommended pressure is best.

“That’s because the car is designed with a whole interactive package of suspension components including the tires,” Mazor says. “A few pounds of variance up or down from the [vehicle] manufacturer’s recommendation isn’t all that bad, but if you want to change more than that, seek the advice of a professional tire dealer.”

Mazor says studies by the Auto Club have found that tire pressure is the most neglected automotive maintenance item.

“Since the demise of the full-service gas station, most people never think to check pressure until they see a tire that looks low--and by then it is probably too low,” he says.

The problem is that a too-soft tire can create dangerous heat buildup that can cause internal damage and lead to tire failure.

Another thing to check for in a visual inspection is uneven tire wear. Any sign that one part of a tire’s tread is wearing out faster than others, or that one tire is wearing faster than the rest, can be an indication of improper inflation or misalignment or even internal tire damage.

Tires should also be inspected, usually by running your hands over the inner and outer surfaces, for cuts, gouges and bumps that could indicate an internal weakness allowing air to push out on the sidewall--the automotive equivalent of an aneurysm.

To maintain even wear on otherwise good tires, owners should rotate them every 6,000 miles or so.

Tires should always be replaced when the tread depth is worn down to between 3/32 and 1/16 of an inch, and probably sooner if they are subject to a lot of high-speed driving, heavy loads or wet weather.

The tire industry has spent billions of dollars developing rubber compounds, tread designs, belting materials and manufacturing techniques to make tires perform better and last longer--and to create scores of aftermarket tires that improve on the performance of the original-equipment tires that come on new vehicles.

Those high-end tires, whether built by a Pirelli or a Goodyear, generally offer big improvements in steering response and wet traction. But the so-called OEM tires--or replacements that hew to the specifications of the originals--are usually all the typical motorist will ever need.

Tire Design Is Series of Compromises

A passenger vehicle tire is basically a balloon--a synthetic-rubber bladder rarely more than three-fourths of an inch thick exclusive of the treads, made of layers of chemically bonded material forced into shape by bands and belts of steel and fabric, stiffened on the sidewalls by extra layers of rubber and steel cord.

The treads are designed to let the tire shed things--water, snow, mud, sand--that get between the tread surface and the road. In dry conditions on a smooth road, the best traction is actually achieved with a tread-less tire--a “slick.”

But few motorists can afford to keep several sets of tires and to continually change them to account for road and weather conditions, so most tires today are designed in a series of compromises to balance factors such as traction, ride comfort, tread life, heat and speed capacity, responsiveness and noise (the knobbier the tread, the noisier the tire).

Virtually all tires today are radials--meaning that the steel and fabric cords, or body plies, that give a tire its flattened U shape run crossways, under the tread, from one of the edges, or beads, that seal against the wheel to the opposite edge. The old bias-ply tires used cords that wrapped around the tires’ O-shaped circumference.

Radial construction makes a tire more flexible and better able to conform to the road surface, Mazor says. Most are bolstered by belts of steel cord (hence “steel-belted radials”) running under the tread around the circumference.

A tire’s sidewall--the part that carries the manufacturer’s name and tire style in big raised letters, and information including size, load rating, speed rating, maximum inflation pressure, and classifications for tread wear and heat resistance in smaller letters--provides vertical support and is the surface that transmits steering input.

A tall sidewall is more flexible than a short one, so it gives a softer or more comfortable ride but takes a bit longer--fractions of seconds--to transmit steering information to the contact patch on the road. Sidewall height is called the aspect ratio and is measured as a percentage of the tread width. It shows up in the tire size as the second number.

So a tire designated P245/75R16--the normal size for a Chevrolet Suburban--is a passenger tire 245 millimeters wide (about 9.6 inches) with an aspect ratio of 75%--or 183.75 millimeters (7.2 inches). The R designates radial construction, and the 16 is the inside or bead diameter of the tire--the diameter of the wheel it fits. Overall, that tire and wheel combination would have a diameter of 30.4 inches (16 plus 7.2 plus 7.2).

An oversize replacement would be a P285/45R20. The wheel would be 20 inches in diameter, the sidewall height would be 45% of the width, or 128.25 millimeters, and the overall diameter would be 30.1 inches, almost identical to the 16-inch tire even though mounted on a wheel that is 4 inches bigger.

Riding on a Quartet of Rubber Balloons

But sidewall stiffness, tread design and steel belts aside, a tire, like a child’s party balloon, is fundamentally useless without air to inflate it and give it the strength to hold aloft anywhere from 3,000 to 7,000 pounds of vehicle, driver and passengers. Trailers, bags of cement in the trunk, sheets of plywood in the cargo box--all add to the tremendous load on these four rubber balloons.

And those balloons aren’t static. They rotate hundreds of times a minute, striking objects as fine as grains of sand and big as bricks and 2-by-4s, sometimes sharp as tacks, sometimes blunt and immovable as a concrete curb.

A tire’s rotation causes friction as the rubber hits the road and flexes, and that friction creates heat. Normally dissipated by air flow and tire design, heat can build up if the tire is too soft--under-inflated--and cook the rubber, breaking down the chemical bonds that tie the various layers together.

But pump them up too much and you start breaking down the contact patch, diminishing its size and the tire’s ability to stick to the road, says Yokohama’s Brennan.

An over-inflated tire also loses flexibility, giving cars and trucks a harsher ride and creating excessive internal pressures that can cause ruptures if the tire hits a solid object--like a large stone in the roadway or a curb or even the edge of a pothole.

In the end, it’s a question of picking the correct tires for your vehicle and driving needs and then treating them like the balloons they really are: Just enough air, please, and you’ll go a long way toward heading off trouble.

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Times staff writer John O’Dell covers the auto industry for Highway 1 and the Business section. He can be reached at john.odell@latimes.com.

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Tire Care--to Go

A well-stocked tire maintenance kit doesn’t have to eat up much room--or money. Basic ingredients should include:

* A portable tire inflater that can be powered from the vehicle’s accessory port or cigarette lighter socket.

* A flashing red warning light--or a combination light and pump, like the one pictured here.

* Gloves for those who don’t like dirty hands.

* A top-quality tire pressure gauge with a dial readout.

* A small, thin ruler--or homemade measuring stick--for checking tread depth.

* Waterless hand cleaner and a few clean rags.

Stow the kit in your car or truck so it is handy when you are on the road.

And, of course, always carry a properly inflated spare, emergency flares or glow sticks and the correct type of jack for your vehicle.

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Know Your Tire

Where the rubber meets the road is only one part, albeit an important one, of a automobile tire. As illustrated, tires are complex assemblies, made of multiple layers of rubber, nylon or other fabrics and steel cords, each with a specific purpose.

* Treads are designed to pluck water, dirt and other substances from between the tire and the road surface and channel them away from the contact patch, where the tread hits the asphalt.

* The belts, usually of steel cable, are designed to hold the tire’s round shape and give it extra strength.

* The body plies--of fabric, steel or both--wrap the tire from bead to bead to provide lateral strength.

* The bead provides an airtight seal when the tire is mounted on the wheel.

* The sidewall gives the tire flexibility when cornering.

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Source: Yokohama Tire

Deciphering the Sidewall

Everything you always wanted to know, and probably more, is revealed on the sidewall of a tire. Here’s a translation:

Source: Yokohama Tire