Materials run tires flat out

Technology turns in strange ways. And sometimes impetus comes out of tragedy--on a deserted freeway exit ramp early one morning in January of last year, Ennis Cosby was killed while changing a tire. "This prompted many inquiries to the company as to what could be done to avoid having to pull over to fix a flat," according to Robert Hall, Goodyear (Akron, OH) manager of advanced product and process technology. Surveys by the firm showed that 80% of those polled experienced a flat (20% in the last six months), of which 96% felt it produced a hazardous situation. Only 11% of the flats happened more than 25 miles from home. Interestingly, while not a common flat problem, 80% feared a blowout at high speed.

While flat-tire remedies are nothing new (see sidebar), none have had all the capabilities of Goodyear run-flat tires' Extended Mobility Technology(TM) (EMT): the ability to go 50 miles at 55 mph, high reliability, and reasonable cost.

Fighting fatigue. A tire running flat has its sidewalls bent to tight radii. The problem is the extreme tension in the outer plies, according to Bob Toth, marketing manager for Goodyear performance tires. The extreme flexing while rolling also causes rapid heat buildup in the tire. To combat these conditions, and fatigue loading, EMT hinges on an ultra-tensile steel body ply (carcass) laid up from what the company says is the strongest tire cord available (see graph). Made of 0.007-inch filaments using unique chemistry, die drawing, and heat treatment technologies, the wire gives high dimensional stability and stiffness to the body ply, along with cut resistance. By providing about 50% of the area of a fully inflated tire's contact patch, the wire maintains handling performance. The ultra-tensile steel replaces the two-ply polyester carcass used in many tires and removes stress in the steel tread-belt edges.

Achieving the run-flat contact patch is a source of pride with Goodyear engineers, who note when development started, all support was on the sidewalls. They claim that other flat-running tires are either sealant based, and only 75% effective in stopping leaks, or use non-steel cord reinforcement that cannot travel as far or handle as well. "We can't wait to see a Consumer Reports comparison--our 50 miles is longer than their 50 miles," one Goodyear rep says.

Another chemistry key to running flat is the twin extended-mobility inserts in the sidewall. Complementing the strong body ply, they offer high-temperature strength and heat dissipation. Finally, two steel belts add further strength and a modified bead keeps the tire attached to the rim during low-pressure running.

Goodyear's run-flat technology has been available for several years, but only on specialized vehicles and for specific reasons. First introduced as an option on the 1994 Corvette, it became standard on the 1997 C-5 Corvette, which had no spare tire and no tire-changing equipment. And without a place to put it, similar to the problem of where do you keep a wallet in a bikini, the 1997 Plymouth Prowler was designed without a spare as well. But the tires on these vehicles have low aspect ratios with short sidewalls that are easier to support without air. Goodyear needed the new material developments to make EMT practical for higher sidewall mainstream tires.

Run-flat performance is not without penalty. The tires are 30-35% heavier than conventional tires, but that should come down to about 20% as new sidewall extended-mobility insert materials are developed. Rolling resistance, now 15-20% higher, should drop by a similar factor with newer inserts.

On the road with the King. Goodyear introduced the run-flat tires to the press at the Walt Disney World Speedway this past February. Driving a course laid out around the one-mile track, in minivans to performance coupes, this editor was able to compare the same vehicle with and without the right front tire deflated--and Goodyear seems to have gotten it right.

While tire drag was noticeable in most cases, driving within the 55 limit gave no handling problems. Compared to running with a conventional tire flattened, the feel was more of an alignment problem than the usual heavy hand on the steering wheel. For cars without very sensitive or responsive steering, it's easy to see that a driver making only moderate turns, or with the radio on or passengers demanding attention, may not notice a flat. And a rear flat would be even less discernable. Interestingly, driving a Dodge Intrepid, the tire dynamics gave rise to a noticeable low-pitched warbling sound from standing waves set up in the tire.

NASCAR legend Richard Petty, who was on hand for the occasion, then posed the obvious question, "How do you know when your tire is flat?" While a professional such as Petty would likely notice a low tire, in the absence of the Intrepid's singular audio cue, for the rest of us Goodyear is requiring some type of pressure warning or monitoring system (DN 10/6/97, p. 124). The tires themselves are embossed with the proviso that such a system must be installed. And, although simple to mount and dismount, certified Goodyear dealers are required to install the tires in order to guarantee bead integrity and proper low-pressure warning system operation.

Pressure monitoring is done mainly by direct sensing with piezo elements, radio or induction linked to a control panel display. Other systems measure tire motion to indirectly determine pressure. Future developments may include monitoring tire temperature as well. Bob Toth mentions great interest in these systems by suppliers of other components. Specifically, ABS brake manufacturers are looking into multiplexing tire data into the ABS receiver and sensor electronics already at the wheels. He adds, "The tire data receiver is a major expense but (ABS) electronics are already in the dash."

And finally, what about "intelligent tires?" Robert Hall notes that chip-in-tire technology (DN 2/17/97, p. 78) is not ready for the mass market yet "since the increased original-equipment base infrastructure isn't there to support it." But with inroads from run-flat tires, it will probably be in place sooner rather than later.