Tires That Won't Go Flat Wait for Market : Manufacturing: Producers hope to expand sales beyond presidential limousines, off-road racers and specialty vehicles.

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Thumpeta, thumpeta, thumpeta. . . .

The familiar, sickening sound of a flat tire. Or maybe it's alarming, depending on the weather or the neighborhood. It promises skinned knuckles, run pantyhose, intemperate language. At best, a long wait for road service.

You'd think somebody would invent tires that won't go flat.

Well, they did--more than 100 years ago. And they've kept reinventing them ever since. You might even know someone who drives them, someone like Mary Anne Backscheider of Junction City, Kan.

"I love 'em," she said. "I really feel comfortable knowing I have them."

Backscheider, a budget analyst at a U.S. Army hospital, is one of about 2,000 Chevrolet Corvette owners whose cars are shod with Goodyear Eagle GS-C extended mobility tires, known in the trade as run-flats.

Earlier this year, Backscheider put off a tire repair for weeks, adding air when the Corvette's electronic sensor told her to. She couldn't feel a difference in the ride or handling.

"I finally took it in to have it checked. They took it off and there was a screw in it," Backscheider said. The Kansas City Star featured her tire repair in a story--she may have been the first motorist to have an official "flat" on the Goodyears.

"They had never seen it before" at the Goodyear store, she said.

Pneumatic tires--those with air inside--were invented in the mid-1800s, and the idea of preventing flats has been around almost from the start. A Goodyear history says J.B. McCune patented a tire in 1891 that had several air chambers to keep it from going flat.

But until fairly recently, run-flats required too many compromises to make them practical for the masses. A major drawback to the run-flat tires used on presidential limousines, off-road racers and other specialty vehicles is that they generally can be used only once in the run-flat mode.

"For the general mass market . . . if you use it once and have to throw it away and buy a whole new tire, that's a disadvantage," said Mike Wischhusen, product engineering manager at Michelin North America, the U.S. arm of the French tire maker.

The specialty application run-flats have rigid inserts or plastic foam inside the tire to give support when the air doesn't.

The Goodyears on Backscheider's Corvette and the run-flat tires that Michelin says will become an option on a luxury performance sedan in the coming year have different construction. They rely on rigid sidewalls, not inserts, to maintain the tire shape. And they can be repaired after they're used.

Goodyear says the tires it puts on the Corvettes can go 200 miles at 55 m.p.h. with zero air pressure and still be repairable.

This type of tire "is something we developed back in the early '80s," said Goodyear Tire & Rubber Co.'s product design chief, Bill Egan.

"We demonstrated them to a couple of the auto (manufacturers) but they weren't ready. We kept pitching it at them, and they finally stepped up to the table."

Auto makers are reluctant to add complexity and cost to new cars and trucks for features customers aren't demanding, and run-flat tires aren't yet on motorists' lists of must-have options. The tire makers say it may be years before consumers want them and are willing to pay extra for them.

There are other considerations besides demand:

* Ride. A tire with a sidewall thick enough and stiff enough for it to keep its shape, stay on the wheel, and start, stop and maneuver safely can make a boulevard feel like a battlefield. That isn't so much a problem with high-performance cars like the Corvette, which already has a stiff ride. Other types of cars would have to be designed with special suspensions that compensate for the ride qualities of run-flat tires.

* Sensors. If the tire doesn't go flat, how do you know it's picked up a tenpenny nail? Backscheider's Corvette has electronic sensors in each wheel that detect low air pressure. It's an option offered with or without the run-flats, but it's required to make them practical and safe, manufacturers say.

* Cost. Tire makers expect run-flats to be about 20% more expensive than conventional premium tires. Add to that the cost of the sensor system. On a 1995 Corvette, the sensor system option is $325. To upgrade to the Goodyear run-flats from the standard high-performance tires on Corvette costs $70.

* Service. Run-flats would have to be usable on conventional wheels and repairable by the same folks who already fix your flats. "In order for this technology to be successful in the mass market, we can't be requiring service stations to invest thousands of dollars in new equipment," Michelin's Wischhusen said.

Down the road, run-flats have the potential to let car makers eliminate spares from the standard equipment in new vehicles, but tire makers aren't ready to predict that will happen. They say motorists like the feeling of security they get from a spare in the trunk, even if they never touch it.

And there are plenty of places where 200 miles might not be enough to get to a garage for a tire repair.

Auto makers don't like to tip their hands to competitors by talking about plans for future features on new cars and trucks. But it seems likely that run-flats will turn up on other vehicles in the next several years.

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