For more than a decade, car companies have put run-flat tires on some of their most exclusive models. But now, more auto makers are offering them on a wider range of vehicles, including sports and luxury vehicles, family minivans, and subcompact cars like the MINI Cooper.
It's easy to understand why. Tires that go flat but keep rolling serve as saviors to any motorist who dreads pulling to the side of the road to change a flat. Auto makers like them because run-flat tires help reduce the weight of the vehicle and thus boost fuel economy. Run-flat tires also create more usable trunk space where the spare tire once resided.
CORVETTE'S CHOICE. By 2010, about 10% of all new cars are expected to have run-flats as either standard equipment or an option, according to tire makers. Some auto manufacturers, such as BMW, are more aggressive in their efforts. "We see run-flats as a major consumer advantage," says BMW spokesman David Buchko.
Because of the company's historical focus on ride quality and driving experience, BMW's embrace of the technology is going a long way toward spreading acceptance of run-flats. Already standard on the Z4 sports car, 5 Series, 6 Series, and 3 Series, run-flats will come as either standard equipment or part of an option package for all its model lines by 2007, BMW says.
Once the domain of luxury and sports cars such as the Chevy Corvette, which has had run-flat tires for a decade, the wheels are going more mainstream. They're now available on the Honda Odyssey (HMC), Toyota Sienna (TM), and Nissan Quest minivans (NSANY), for example.
LEGAL TROUBLE? But the switch to run-flats presents challenges to carmakers. The tires tend to have stiff, thick sidewalls or an inner core that makes the tire tougher and thus able to keep rolling for awhile without air. Indeed, BMW says a driver can go 150 miles at 50 mph on four punctured tires. But that means a harsher ride, making the driver and passengers more likely to feel road imperfections.
To overcome the harshness, auto makers tend to design ride comfort into the vehicle -- with softer shock absorption to make the ride smooth, for example. Problems can arise depending on how well the vehicles are tuned to support the tires and how hard the owners drive their cars.
Last fall, several owners of Toyota Sienna minivans with run-flat tires filed a class-action lawsuit against Toyota and tire-manufacturer Dunlop, complaining the tread wears out too soon. The suit claims Sienna minivan tires last only 10,000 to 20,000 miles, about one-third the life of most other types, and need annual replacement at a cost of $1,300 or more -- at least $500 above the cost of conventional tires.
MUCH IN DEMAND. Auto makers aren't alone in pushing the technology. Indeed, they listened to consumers -- who wanted it, too -- and began making plans to offer run-flats five years ago. When J.D. Power & Assoc. asked thousands of car buyers what they most desired in a new vehicle for a 2001 study, 87% said run-flat tires, making these the most sought-after feature by far.
The tire brands offering run-flats include: Bridgestone, BF Goodrich, Goodyear (GT), Pirelli, Dunlop, and Michelin. Among the models offering run-flats are: Toyota Sienna; Mercedes Benz E and S class; Lexus SC430; BMW 3-, 5-, 6-, and 7-Series; BMW Z4 Roadster; Infiniti Q45; and G35; Ferrari 612 Scaglietti; Honda Odyssey; Nissan Quest; Cadillac CTS; Cadillac XLR; Mazda MX-5 Miata; Mazda RX-8; Mini Cooper; Acura RL; Hummer H1; and Audi A8.
Most tire makers use a run-flat technology called self-supporting, which relies more on stiffer sidewalls than conventional tires. Bridgestone has served as a leader in the technology and, in the past 10 years, has supplied or sold more than 1 million run-flat tires to auto makers for use as original equipment on vehicles. "We're now making the benefits and convenience of run-flats available to a much broader consumer base," says Michael Martini, president of the North American Consumer Original Equipment division of Bridgestone.
OLD IDEA, NEW BUZZ. Rival tire maker Michelin features a technology it calls PAX, which uses an inner support ring to keep the tire rolling after it loses air. Michelin supplies the PAX run-flat tire to Honda for the Odyssey minivan, to Nissan for the Quest minivan, and to Rolls Royce for its Phantom. The key elements of the PAX system are special wheels with unique tire-bead locks and a solid insert that supports a vehicle's weight so it can keep rolling without any air pressure.
Complicating matters -- for everyone but the Rolls-Royce crowd for whom money is no object -- is that replacing the tires requires a special rim and special tools. For some time to come, that could mean tire servicing would need to be performed by the car dealer or a branded tire dealer.
The idea of a run-flat tire has been around for more than a century. The original "anti-puncture" tire showed up in a patent request in 1892. In 1934, Goodyear introduced the Lifeguard safety inner tube, designed to deflate progressively instead of "blowing out." Then in 1978, the company introduced the SST, the tire industry's first "self-supporting" tire.
NEEDS MONITORING. Safety and security drive consumer interest. Not only do drivers frequently lose control of a vehicle during a single-tire blowout, but drivers, especially women, wish to avoid the vulnerability of finding themselves stuck on an isolated road with a flat tire.
Run-flat tires aren't cheap. Four Bridgestone Blizzak Run-Flats cost about $1,200, compared with perhaps $500 to $800 for comparable conventional tires. Run-flats still need servicing when they lose air. And they aren't always practical for regular driving, because they can't go forever. In fact, auto makers recommend motorists restrict their vehicle speed to no more than 55 mph and limit their distance to between 50 and 150 miles, depending on how much weight the vehicle is carrying.
Because these tires can run out of air without the driver realizing it, many auto makers supply vehicles with tire-pressure monitors to warn the driver if air pressure has dropped. Indeed, in a demonstration of a BMW 5-Series equipped with Bridgestone run-flat tires by BusinessWeek at Pocono Raceway last fall, our test driver couldn't tell which tire was airless.
NO-RUSH SCENARIO. More vehicles will have tire-pressure monitoring systems thanks to a law enacted after the Firestone/Ford Explorer tire-failure problems. Under the law, auto makers must phase tire-pressure monitoring systems into their new models beginning with the 2007 model year.
Indeed, many of the models shown at the recent 2006 North American International Auto Show boasted tire-pressure warning systems. Carmakers pitch run-flats as convenient. They say a motorist can have a flat tire while out on the town Saturday night and wait until Monday to have it serviced without damaging his or her good clothes or waiting for emergency road service.
"Consumers like run-flats as standard equipment, but we're still in a period of sticker shock for them as options and replacements until prices come down," says Tucker Lennon, a Detroit-based tire and wheel engineer who has them on his MINI Cooper.
Now, if we could just design a car that can't run out of gas.