How Run-Flat Tires Work

Super-pneumatic strength improves safety
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Decades ago, the steel belt tire revolutionized the auto industry by making tires and cars safer than ever thanks to their increased resistance to puncture. Despite this giant leap forward, thousands of accidents still continue to occur on our roadways every year as the result of tire failure. Responding to the need for improved auto rubber, tire makers have developed the next generation of on-road safety in the form of run-flat tires

Supporting Role

A standard tire depends on internal air pressure along with sidewall construction and multiple steel belts to maintain shape while rolling down the road. The pneumatic pressure of the air inside works with the structure of the tire itself to support the weight of the vehicle. The air inside the tire also acts as a cushion against bumps and other road surface irregularities. While the sidewall is relatively strong in a normal tire, once the air leaves the containment vessel that is the tire itself, the ability of the tire to hold up the weight of the vehicle is diminished. In other words, if the tire loses air, it goes flat. A run-flat tire improves on standard tires by adding a special supporting sidewall structure that allows the tire to make it to the nearest service station completely devoid of any air pressure.

While the benefits of such tires are obvious, it is only within the last few years that they have become commercially available at a realistic price. Until recently, run-flat tires were reserved for a precious few. From Aston Martins driven by bullet-dodging secret agents, to bulletproof limousines bearing national flags, to the ultimate SUV, also known as the Popemobile, the use of run-flats was previously limited at best. Today, dropping prices have enabled wider distribution of run-flat tires, both as original equipment on many new vehicles, and as affordable replacements for older models.

Safety and Convenience

Run-flat tires are becoming more commonplace for a number of different reasons-the primary reason is the most obvious, safety. Even with no air pressure in the tire, one can travel at a safe speed to the nearest repair or replacement facility. A secondary benefit of a run-flat tire is a driver's ability to retain control of a vehicle, should a sudden flat occur. Another less obvious benefit is that hauling a spare tire is no longer as likely to have helped in a flat tire scenario. That's welcome news at least, with wheel and tire packages getting bigger and more exotic, and car interiors becoming evermore similar to rolling movie theatres; space is at a premium in today's modern automobiles. As anyone who has changed out a spare knows, weight is a concern as reduced weight results in better fuel economy and performance.

More good news about run-flat type tires is that they do not require any particularly special wheels to operate. A self-supporting tire like the Bridgestone Potenza with run-flat technology (RFT) can be mounted onto any wheel that will accept a standard tire. Bridgestone has even gone so far as to offer snow tires featuring RFT to save the trouble of changing tires during the worst possible driving conditions. Of course, the benefit here is nominal at best if you happen to have a mobile crew of hard-working, tire-changing elves at your disposal.

A Dash of Caveats

Even though there are many great advantages to a self-supporting run-flat tire, there are certain caveats to remember when considering or operating them. While run-flat tires are capable of working without the assistance of internal air pressure, there are some restrictions. Should a run-flat tire deflate, Bridgestone recommends a top speed of 50 mph, and a maximum safe distance of 50 miles.

Another important consideration deals with how well the self-supporting run-flat tire works. As the tire loses air, the sidewall structure inside the tire begins to support the entire weight of the vehicle. While a rapid tire blowout may be easily noticed in a run-flat tire, a gradual loss of tire pressure may not be noticed at all by the driver. Since this could potentially force operation of a run-flat tire beyond its safe limits, a pressure monitoring system is highly recommended. It is for this reason auto manufacturers are restricting the installations of run-flat tires only to cars equipped with this feature. In the event that a tire begins losing air, a dashboard warning light on vehicles equipped with tire pressure monitoring systems alerts drivers of the problem. Self-sealing tires work differently than run-flat tires and do not require such a pressure monitoring system. The additions of these small safety measures are a welcome trade for a new technology that won't leave you feeling flat.

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