Of all the major automotive safety developments of the past decade, run-flat tires might not immediately come to mind, but they're one of the most important. Over time, they'll make being stranded along the side of the highway with a flat a thing of the past - welcome words to anyone who's ever tried to figure out the instructions for their jack and install their spare in the pouring rain on the side of a congested highway with a narrow shoulder!

Run-flat tires have extremely strong sidewalls, so strong that they're capable of supporting the tire in an "inflated" position for extended periods - within a limited speed and distance. With no air, it's important that the driver corners gently, but we've been told that run-flats with no air still perform better than the skinny, hard-compound temporary spares typically in new cars.

But if the tires perform nearly as well with air in them as without, how do you know when their inflation pressure is normal, dangerously low, or even when they're flat? The only solution is some type of pressure-monitoring system that will alert the driver.

There are two distinct types of pressure-monitoring systems being used in new vehicles:

Direct systems use a band that clings to the inside center of the wheel rim. A small, lightweight sensor is located along the band (and usually installed opposite the valve stem for best balancing). Via a low-wattage radio-frequency transmitter, the sensor sends a central unit its reading, then the central unit either sends the reading to an instrument panel display or waits for a low pressure to trigger a warning light/chime. The small battery in each wheel lasts up to five years and is generally replaced with each set of tires. Direct systems like this are your only choice if you want to install run-flats on a vehicle that didn't originally have them.

Indirect systems use the wheel-speed sensors that are already present for the anti-lock braking system (if so equipped). A "smart" feature simply determines if there's a difference in the effective rolling radius between wheels, and if so alerts the driver that one of the tires is low. Since run-flats have nearly the same profile when properly inflated as when not, the difference is very subtle and the system has to be tuned to be very sensitive. This method, however, is the least expensive for automakers to implement on new cars.

Run-flats were available in the Chevrolet Corvette in 1993, the first application in a normal production vehicle. At the time, they were cutting-edge in tire technology and the replacement cost was so tremendous that some owners ended up retrofitting their vehicles with regular tires. 

Less than ten years ago, both the tire business and those within the auto industry were saying that run-flats would be very slow to catch on for normal production vehicles, as the cost of special rims and the requisite pressure-monitoring system-not to mention the extra cost of the tires themselves-was just too high.

But several developments have changed that and made run-flats more accessible, faster than the industry had anticipated. The federal TREAD Act enacted in November 2000-a direct result of hearings related to the Firestone tire-recall controversy-includes a requirement that all new vehicles have some type of low-pressure warning system by November 2003. And if all vehicles must have such a device, it then lowers the automakers' extra cost of adding run-flats to new vehicles.

Another significant development is that run-flats no longer need to be installed on specially designed rims. According to Goodyear, their run-flats can be installed on any "high-tolerance" rim, and the requirements are now similar with the other tire brands.

Additionally, there have been some significant advances in "rubber" compounds over the past few years that have allowed tiremakers to offer run-flats with a softer, more shock-absorbent outer compound in a design that doesn't compromise wear. Run-flats typically have the same 30,000- to 40,000-mile life as ordinary tires.

There's one especially important reason automakers have begun to offer run-flat systems on many new vehicles with all-wheel-drive. For most vehicles with AWD, a temporary-type spare can't be used, as depending on the system's design having two wheels of different effective radii might overheat the differential, cause other damage to clutches or couplings, or temporarily render an all-electronic ABS-based system useless.

Automakers are also finding other benefits of offering run-flats that will ensure they see increased installation in the next few model years. In using them, there's no longer the need for any spare tire, or even a place for one. This saves weight and also allows better packaging, in turn cutting assembly costs and aiding fuel economy.

One disadvantage of run-flats - and it is a minor one - is that the mounting procedure is a little different due to the inflexible sidewall. A tire store inexperienced with run-flat tires could damage the tire or the rim. "Make sure the tires are installed by a factory-authorized dealer," warned Tricia Ingraham, a Goodyear spokesperson.

Also, there are all-season and high-performance run-flats, but no run-flat winter tires as of yet. A tire expert suggested that this might be because winter tires require a much softer, stickier compound that may be difficult to match with the hard sidewall reinforcement.

It is possible to install run-flat tires on a If you want to upgrade to run flats in a late-model or older vehicle, there are a few things you should be aware of:

Expect harder impact harshness and more road noise. New vehicles that offer run-flats take into account the different characteristics of the tire in their suspension design, but retrofitted to vehicles that originally had ordinary tires, run-flats will give your car a harder ride and you'll likely notice more tire and road noise making its way inside. The car might also track differently.

The cost of the pressure-monitoring system. "You can add run-flats to your current car, but you have to add on a pressure monitoring system," said Ingraham, who added that Goodyear requires their dealers to make sure the vehicle has a proper pressure-monitoring system if run-flats are installed. For a reliable monitoring system with a dash-mounted display, plan to pay $250 to $300, including installation.

The tires themselves are pricey! Currently, the cost of run-flats runs from about $175 to $350 per tire - 25 to 50 percent above that of comparable premium tires.

You might need to upgrade rims. Many early run-flats - and even those of just a few years ago - required mounting on costly special rims. Now, most run-flats mount on conventional wheel designs. The requirements of each tire model and maker are different, but the general rule is that if you have a premium alloy-type rim on your vehicle you probably won't need new wheels for run-flats.

They require extra curb care. Run-flats don't bulge outward as much as normal tires, so the wheels are left more vulnerable to gouging from curbs.

If you can wait, prices will come down! The cost of run-flats - and the requisite pressure-monitoring systems - will no doubt go down as the number of vehicles with them installs skyrockets over the next few years.

Don't forget, the biggest benefit of run-flats is safety. They make the highways safer for both you and other motorists - by greatly cutting the possibilities of losing control in a "blowout" situation, and allowing you to drive slowly and carefully to a service station instead of having to stop in a hazardous place to try to change the tire or signal for help.

As with just about any new, "high-tech" automotive development, prices will fall over time for run-flats, and they'll soon make their way into nearly every new vehicle as a "must-have" safety item. Until then, check your tire pressures!

 

The following tire brands currently sell run-flats in the U.S.:

Firestone (www.bridgestone-firestone.com)

Goodyear (www.goodyear.com)

Michelin (www.michelin.com)

Yokohama (www.yokohamatire.com)

The Tire Rack (www.tirerack.com) stocks most run-flat tires, and can provide advice on compatible wheels and proper installation.

 

 

©2007 by The Car Connection™ All Rights Reserved - The Car Connection is a Trademark of DA Acquisition

Bengt Halvorson

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