Bridgestone Blizzak LM-25; photo courtesy Bridgestone. Click image to enlarge
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By Paul Williams
Ottawa, Ontario – Browse through popular automotive magazines of years past, and you’ll see that run-flat tires were a continuing subject of fascination. “One day,” you can imagine an engineer of the 1950s reflecting, “people will fly to the moon, phones won’t need wires, and you’ll never have a flat tire!”
One day indeed. That time has come and gone, and many cars are now fitted with run-flat tires as original equipment, including models from BMW, Mini, Rolls Royce, Ferrari, Acura (in the U.S.) and Lexus (in Canada, the SC 430). According to Mark Kuykendall of the Bridgestone Engineering Group in Nashville, Tennessee, the first application of run-flat tires was on a Porsche 959 rally car in 1985. It had no room for a spare, was used on and off the highway in punishing conditions, and the Bridgestone run-flats were viewed as a competitive advantage.
Run-flats were also used on the 1990 Callaway Corvette, and if you’re detecting an evolving connection between run-flat tires and high-performance vehicles, you’d be right. Run-flats are still comparatively expensive, and the higher-priced performance and luxury market is where you see most original equipment (OE) applications of run-flat tires on vehicles today.
The benefits for consumers? They save space, may save weight, improve safety and provide an enhanced level of security (with a flat tire, you’d have to pull over, maybe in a remote or dangerous area; with a run-flat, you can keep going).
Mr. Kuykendall explains that there are currently two run-flat systems (tires, rims, tire pressure monitoring system, or TPMS) on the market. The Pax system, developed by Michelin, uses a support ring attached to the interior face of the wheel that supports the vehicle if the tire is punctured. The wheels are specific to Pax applications.
The SSR system (Self Supporting Run-flat) is the more widely-used run-flat technology offered by Bridgestone and other manufacturers. The SSR tire uses a hard rubber insert along the sidewall that supports the load of the vehicle if the tire loses pressure. Again, a special rim is used, although it is less specialized than a Pax rim, and will accept any brand of SSR run-flat tires.
However, as of April, 2008, Michelin discontinued and is no longer marketing the Pax system to car manufacturers as original equipment. The company still supports dealers and vehicle owners using the Pax system, however.
Note that you can’t install run-flat tires on a non run-flat rim, but you can mount conventional tires on SSR run-flat rims, if you so choose. More on that below.
Note also, that vehicles with OE run-flat tires will have TPMS installed to monitor tire condition. The main reason is that after a puncture, the Bridgestone run-flat, for example, will continue to give normal service for up to 80 kilometers at 80 km/h. In that event, a TPMS may be the only way to learn that you have a problem.
You might think that with a stiff sidewall, the ride would be compromised when using run-flat tires, and to a certain extent, you’d be right. However, handling is a big part of the performance market, and vehicles in this segment already feature low-profile tires and a firm ride.
Nonetheless, some early versions of run-flat tires were known to give a harsh ride, while subsequent generations are considerably improved. Explains Mr. Kuykendall: “The early tires were built very robust. At that time, we didn’t have a lot of on-the-road experience, but the tires get better in terms of ride with each generation. The more fitments we get, the more we learn.”
My experience confirms this. Driving the run-flat equipped BMW 335i, for instance, you’d never know you weren’t riding on standard issue performance rubber.