Running Flat, Without Harshness

In That Defining Moment of Motoring, it's bad if too much rubber meets the road.

That's why Bridgestone Corp. went easy on the rubber in developing its third-generation run-flat tire that offers a smoother ride than its predecessors.

Bridgestone, the world's largest tire maker, expects the new product will boost sales and expand the use of run-flats as more auto makers see fit to equip their vehicles with them.

The new run-flats can be done in a wider range of sizes, including those previously difficult to develop, such as tires for big SUVs, says Yoshihiko Ichikawa, a tire-development division director for Bridgestone.

There are limits. At 6,614-lbs. (3,000-kg), the Hummer H2 “would be very difficult,” he says.

Most run-flats rely on reinforced sidewall rubber that temporarily allows the tire to be driven if there's a loss of air pressure from a puncture, even to the sidewall itself.

But it's not enough just to slab on the rubber. That creates inflexibility, causing a harsh ride, even if the tire is fully inflated, as evidenced by early run-flat performance.

The first ones from the 1980s suffered from “far from ideal” ride quality due to thick sidewalls, says Ichikawa. “Original run-flats and comfort were incompatible.”

Debuting in 2005, Bridgestone's second-generation version offered improved sidewall-reinforced rubber for a better ride, but still left room for improvement.

The new generation run-flats perform better because of improved polymers that advance riding comfort.

Bridgestone says the new tires achieve a 105 vertical stiffness deflection index — a measure of a tire's ability to absorb the shock between a vehicle and road surface — compared with 100 for conventional tires, 115 for second-generation run-flats and 120 for the originals.

The new ones rely on heat control. The simplest way to smooth out run-flat ride is to make the sidewalls thinner and softer. But that can create problems when driving without air pressure, because excessive heat builds from the up-and-down springing of the deformed sidewall as the deflated tire rotates.

How to deal with that heat generation while maintaining and increasing the durability of run-flats lacking air pressure “became a major challenge in efforts to improve riding comfort,” Ichikawa says.

The solution is a new sidewall-reinforced rubber that retards heat by using NanoPro-Tech polymers. They reduce friction that occurs between carbon molecules in motion.

Compared with second-generation run-flats, the rubber in the new ones cut by half the heat built up in sidewalls when deflated tires are driven on, Bridgestone says.

The Japanese tire maker has grand plans to expand its run-flat business because of the latest advancements. Bridgestone is pitching the new tires to auto makers, particularly those that have not offered run-flats on many models.

“We're approaching OEMs,” says Takashi Tomioka, Bridgestone's director-tire products strategy division. “And we're increasing production capacity, first in Japan, then in Europe, the U.S. and South Africa.”

Bridgestone hasn't announced any OEM customers for the new tires, but BMW AG appears to be a prospect. The BMW 3-Series was one of the first cars equipped with Bridgestone's second-generation run-flats.

After introducing run-flats on production vehicles in 1987, Bridgestone broke the 5 million-unit cumulative sales mark in 2006. It took only two more years to hit 10 million.

Last year, the company supplied about 3 million run-flats to the auto industry. “Our aim is to increase sales seven fold in 10 years,” Tomioka says.

Currently, run-flats are on mostly premium vehicles. Bridgestone expects the market to open up. “Otherwise, we can't achieve the high numbers we are anticipating,” Tomioka says.

Bridgestone's very first run-flats of the early 1980s were on special vehicles adapted for disabled people who were unable to kneel down and change a flat tire. In 1987, the selection of Bridgestone run-flats as standard equipment for the Porsche 959 marked the company's onset of supplying the tires for conventionally produced vehicles.

Tomioka foresees a day when run-flats are on virtually all passenger vehicles, eliminating the need for the once-ubiquitous spare tire, which currently holds the dubious distinction of being the only standby car part carried in vehicles.

Ridding vehicles of spares — and for that matter jacks and related tools — would reduce vehicle weight and improve fuel efficiency. It also would allow for a wider range of chassis-design options.

Eliminating the spare tire becomes essential as auto makers develop alternative-fuel cars with lighter curb weights to extend driving range, Tomioka says.

If all vehicles used run-flats, it would end the need for about 59 million spare tires annually and reduce 2 million tons (1,814,360 t) of carbon emissions associated with tire production and disposal every year, Tomioka says.

Environmental benefits aside, the practical advantages of run-flats are safety and convenience. “We'll market run-flats more actively as the best way to achieve safety and overall comfort,” Tomioka says.

Run-flats provide greater stability if there's a sudden loss of air pressure, as evidenced by extreme-driving demonstrations during a media event at Bridgestone's European test track 34 miles (55 km) south of Italy's capital.

In one demonstration, a fast-moving vehicle making a wide turn swerves out of control when its conventional left-rear tire instantly loses air pressure. When the same thing happens to a vehicle equipped with run-flats, the driver maintains control and comes to a stable stop.

Other tests at the 368-acre (148-ha) proving grounds include driving comparisons showing varying ride quality of regular tires, second-generation run-flats and the latest generation.

Cars equipped with each set of tires are driven on various road surfaces, including uneven concrete, broken concrete, so-called Scottish gleneagles basalt rough surface, double granitic chips and U-profiles/cleats.

Other comparison tests are done on the winding and sometimes hilly roads of the Italian countryside.

The difference in ride quality between conventional tires and second-generation run-flats is noticeable; much less so when the latest-generation run-flats go up against regular tires.

Narrowing those differences is important, because many consumers had rough-ride issues with previous run-flats. “We were aware of that,” Tomioka says, citing internal market research.

Depending on the vehicle, some owners are more tolerant of hard-ride quality, says Robert Saul, a product planning manager for Bridgestone Firestone North American Tire LLC.

“A Chevrolet Corvette owner expects and prefers a firm ride, but a luxury-sedan owner, even a Mini owner, doesn't,” he says.

Run-flats aren't cheap. Like the previous version, Bridgestone's new ones cost 120%-130% more than standard tires.

But Bridgestone says run-flats give peace of mind to drivers who needn't worry about changing a flat tire, nor fear doing so in risky spots, such as a narrow shoulder of a busy roadway.

Motorists can drive up to 50 miles (80 km) at 50 mph (80 km/h) on a deflated run-flat without damaging it. Punctures usually are repairable, if the driver does not push past those limits.

Bridgestone says about 60,000 motorists in the U.S., 10,000 in Japan and 8,000 in Europe get flat tires every day.

Eventually, Bridgestone also expects to sell its new run-flats to individual consumers at tire dealerships. “It really opens it up,” Saul says of prospective retail sales.

Unlike before, the new run-flats can be put on cars that originally had conventional tires, as long as the vehicles are equipped with tire-pressure monitoring systems. Those have been mandated on light-passenger vehicles produced since September of 2007.

Meanwhile, Bridgestone is developing two other run-flat technologies that it ultimately plans to take to market.

One is a new ply that uses heat to curb tire deformation. When a puncture and air loss occur, the ensuing friction heat shrinks the sidewall. That shrinkage creates a material density that limits, rather than contributes to, sidewall deformation.

In contrast, the other technology reduces heat from a puncture-induced loss of air pressure. That's done by putting a series of small cooling fins — hash marks of sorts — along the exterior sidewall.

The fins redirect airflow, sucking air towards the sidewall surface and cooling it.

“The fin is in the concept stage,” Ichikawa says. “We see it as being an SUV application.”

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