2003 Chevrolet Corvette CoupeClick to enlarge picture

The Chevrolet Corvette celebrates its 50th anniversary in 2003 but also its 10th model year as run-flat tire pioneer.

These so-called run-flat, extended mobility, continuous mobility or zero-pressure tires can be driven at normal speeds, for some distance, without any air pressure in the event of a puncture, a deep cut or the ravages of just about any other road hazard.

By contrast, "self-sealing" tires use different methods to preserve inflation, but there is a definite limit to the severity of the damage they can incur. These tires simply cannot be driven without any inflation pressure.

In the Name of Safety

The original idea of an "anti-puncture" tire appeared in a patent request as early as 1892. In 1934, Goodyear introduces the Lifeguard safety inner tube that was designed to deflate progressively instead of "blowing out". After several attempts and experiments, Goodyear introduced the SST, the tire industry's first "self-supporting" tire, in 1978.

Italian tire manufacturer Pirelli teamed up with Goodyear in 1983 to show a special asymmetrical wheel designed to lock the bead in. The same principle was applied soon after to Dunlop's Denloc ultra high performance run-flat tires, created for Porsche's 959 supercar. The Denloc design was also used in endurance racing but required unique wheels that made its run-flat capability very costly.

In parallel, Michelin created the "Bib Mousse", a foam plastic insert that lets the tire roll on spite of pressure loss. It is still widely used on car and motorcycle racers in events such as the famous Paris-Dakar rally. World Rally Championship cars also use "mousse" tires on some events, but their use and effectiveness are limited, since the foam heats up quickly and degrades at sustained high speeds.

"Run-Flat" Pioneers

The first "run-flat" tire available on a regular production vehicle was offered as an option on the 1994 Chevrolet Corvette. Goodyear's EMT tires then came standard with the fifth generation Corvette, introduced as a 1997 model, an industry first. This design uses reinforced sidewalls that enable the tire to support its share of vehicle weight without collapsing measurably. The tires can be mounted on standard wheels.

Then, in 1996, radial tire pioneer Michelin unveiled a different principle that centered on a solid inner wheel "insert" on which a pressure-less tire could run safely for some distance. A year later, the French manufacturer revealed its Pneu ? accrochage vertical or PAV, which can be translated as "Vertical Anchorage Tire" for the way the tire bead "seats onto the rim vertically." The new concept made run-flat capability possible without having to contend with a large performance compromise.

The original acronym PAV was changed to PAX in 1998: "because of its universality—and for the values of peace of mind, safety and the future that it conveys."

The key elements of Michelin's PAX system are special wheels with unique tire bead locks and a solid insert that can fully support its share of the vehicle's weight and let it continue rolling even without any tire pressure.

Early PAX designs used a support ring made of rubber, but more recent versions and those in lighter-duty vehicles use a "magic ring" that combines polyurethane and rubber, the main benefit being reduced unsprung weight. And a pressure warning system is fitted, of course.

Global Joint Ventures

Michelin has since then struck alliances with major rivals Goodyear, Pirelli and Sumitomo to develop new tires and wheels around this new design in order to make it the universal standard. At the 2002 Geneva show, all three presented new run-flat tires all based on the PAX system.

The two giants of the tire industry stated: "Goodyear and Michelin are convinced, after an analysis of other tire/wheel combinations, that the PAX system is the best platform for incorporation of future tire concepts into new vehicle designs."

Both manufacturers have also worked on tires that could equip the much-publicized, ultra-frugal PNGV car project mandated by the U.S. Government. Innovative Italian manufacturer Pirelli has also joined in and announced that it was developing run-flat tires in conformity with the principles of the PAX system.

Another joint project was made public in January 2002. Bridgestone Corporation then announced that it had formed an alliance with German maker Continental AG to develop run-flat tire technology. Bridgestone agreed to share knowledge related to its SSR tires while Continental would bring its Conti Safety Ring (CSR) technology to the table.

Technology and Regulation Evolving Quickly

Modern sports cars use very low aspect ratio tires (tread width divided by sidewall height) and ride comfort is not as big a concern on these as on other vehicles. A wide tread and low sidewalls generate maximum traction and responsiveness. A short sidewall is also easier to reinforce and flexes much less to begin with.

This made the Corvette a natural choice to pioneer run-flat tires as an option on 1995 models. Several sports cars have followed suit, running on tires made by all the major manufacturers.

Because the vehicle's actual handling changes so little, the driver needs to be informed of any significant loss of pressure. Pressure monitors have always been essential with this type of tire, but they will soon become mandatory.

Carmakers will effectively need to install such systems in all new production vehicles to comply with new rules stipulated by the National Highway Transportation Agency's (NHTSA) Transportation Recall Enhancement, Accountability and Documentation (or TREAD) Act.

These new regulations will mandate low pressure warning systems for all 2004 model year vehicles, among other measures. The act was, in part, a reaction to numerous tire failures involving sport-utility vehicles that were reported in recent years.

Because of this, more run-flat tires will invariably be fitted at the factory. Many luxury and performance models will get them standard, but family-oriented vehicles such as the all-new 2004 Toyota Sienna AWD is already equipped with true run-flat tires also.

Much research and development work is being done on highly-automated, ultra-precise systems to manufacture run-flat tires, notably those which are designed from the PAX system. Because of the specially-designed wheels they use, these also require specific installation techniques and tire-changing machines.

Numerous Benefits

With run-flat tires, there is no need for a spare wheel or a car jack, for that matter. This frees up space for luggage, lowers cost and saves some weight, which makes for a small gain in fuel economy.

Several high-visibility models such as BMW's Z4 and Z8 sports cars, the aforementioned Chevrolet Corvette, the Lexus SC 430, the MINI Cooper and Cooper S, and the spectacular new Rolls-Royce Phantom are all equipped with run-flat-type tires. These have been engineered specifically in cooperation between the car and tire manufacturer along the vehicle's development process.

Some "run-flat" tires are currently available as replacement units. But they are generally more rigid, heavier and more expensive than conventional designs of the same size, especially when you factor in the cost of a pressure-monitoring system.

Run-flat tires can also substantially affect ride comfort and the durability of some vehicle components in real-world driving. Suspension parts, most notably. Ideally, a vehicle's structure and suspension components should be designed for the very different nature and characteristics of run-flat tires, and vice-versa.

A New Age Coming

According to experts, all road-going production vehicles will eventually come standard with run-flat-type tires. The rules on mandatory tire pressure monitors for all 2004 model year vehicles, brought about by the TREAD Act, will trigger substantial growth in run-flat tire installation at the factory. Such a development meshes perfectly with manufacturers' intention to eliminate the spare wheel and all related paraphernalia from their vehicles.

As OEM installation rates increase dramatically, so will the availability or fully-adapted replacement units. The cost of which should follow the reverse trend.

All this is excellent news for all of us, drivers and passengers alike. A new driving safety paradigm is coming soon, thanks to emerging and interconnected tire technologies. It's a thankless job, being round and black, but tires will indeed keep the world turning for a long time still.

Current production vehicles equipped with run-flat tires:

Audi A8 (PAX)

BMW 3-Series *

BMW Z4

BMW Z8

Cadillac CTS-V

Cadillac XLR

Chevrolet Corvette

Dodge Viper SRT-10

Hummer H1*

Lexus SC 430

MINI Cooper*

MINI Cooper S

Rolls-Royce Phantom (PAX)

Toyota Sienna AWD (2004)

*optional

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