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Understanding Run-flat Tires

Understanding Run-flat Tires

By Jeff Youngs, February 24, 2012
Run Flat TireOne of the first practical applications of run-flat tires appeared in the late 1980s on Porsche's 959 sports car. The limited-edition exotic could top 200 mph under the right conditions, such as a test track, so the automaker requested a tire that wouldn't disintegrate if it lost air pressure at high speeds. The tire worked as engineered, but it required a special wheel design to keep the tire in place when deflated, thus preventing it from being used outside specialized applications.

The engineering marvel that is today's modern vehicle tires is credited back to 1846, when a European inventor wrapped rubberized canvas around a wheel and filled it with air. It took another 50 years for tire technology-including the crucial valve stems-to be perfected. Interestingly enough, for the next 50 years, the basic design of a tire-a flexible sealed vessel to hold pressurized air-has changed very little.

A traditional pneumatic (air-filled) tire offers countless benefits. These include a comfortable ride, road grip under a variety of driving conditions, and long tread life. However, the advantages are quickly forgotten when the tread or sidewall becomes punctured and loses air-a flat tire. Engineers have been trying to perfect a tire design that can still be "driven on" after suffering a complete loss of air pressure for decades (some of the earliest were invented for disabled drivers). The name of this new tire would eventually be called a "run-flat" design (they are also called "zero-pressure" or "extended mobility" tires).

Less than 10 years ago, tire manufacturers began to introduce the next generation of run-flat tires that could be fitted to standard vehicle wheels without modification. The only stipulation was that the vehicle had to be fitted with a tire-pressure-monitoring (TPM) system so the driver could tell when the tire was flat.

Unique by construction, a run-flat tire looks much like a conventional tire from the outside. On the inside, however, it is very different. The air in a conventional (pneumatic) tire is designed to hold the shape of the tire under various loads. Without the air, the tire collapses on its sidewalls and becomes impossible to use safely. A run-flat tire overcomes these obstacles by having very strong reinforced sidewalls. When the run-flat tire loses pressure, the sidewalls are substantial enough to hold the tire's shape. The vehicle does not drastically "drop" onto a deflated run-flat tire, so directional control is retained. So subtle may be the loss of pressure that to the naked eye the tire looks normal. As a result, the aforementioned TPM systems are required on all new light motor vehicles-with or without run-flat tires-sold after September 1, 2007.

Run Flat TireA run-flat tire is designed to offer extended mobility, so the driver can proceed to the nearest safe spot to have the tire repaired or replaced. They are not designed for long-distance driving or to transport heavy loads when they have no air pressure. In addition, most tire manufacturers recommend an estimated 50-mile range on a run-flat tire at zero pressure under optimal conditions (actual safe mileage is based on many parameters, including vehicle load, speed and ambient temperature).

Like any emerging technology, run-flat tires still have a few drawbacks. This is one reason why most automakers are not offering them as standard equipment on new vehicles. The most obvious drawback is the expense. The additional engineering and development, coupled with a unique design, means the average run-flat tire will carry a 25-percent price premium over its conventional counterpart. In addition, the run-flat tire's construction means it will be heavier, with thicker sidewalls. This will not only affect ride quality, but fuel economy as well. Some automakers tune the suspensions of their models specifically for the unique characteristics of run-flat tires.

Minor drawbacks aside, today's run-flat tire technology offers an excellent safety cushion when installed on a properly equipped vehicle with a TPM. Rather than risking personal safety by changing a flat tire on the side of a busy road or freeway, run-flats allow drivers to continue their journey or to drive safely to the nearest tire facility.

The latest-generation tires, introduced in 2009, are just now arriving at local tire dealers. New construction promises a lighter tire that offers a much better ride than its predecessors. Furthermore, the manufacturers have attempted to address the cost issue to make them more competitive in the marketplace. Without a doubt, run-flat tires are one safety-related automotive technology that is unlikely to find itself stuck on the side of the road.

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