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Notes From the Road

The Skinny on Run-flat Tires
By Aaron Robinson


Tough Tire Tech
Though increasingly popular, airless tires remain a huge engineering challenge. Here’s why: tires are the single most important factor in determining how a car rides and handles. After a century of designing cars around conventional pneumatic tires, engineers understand how a traditional tire grips, how it responds to acceleration and cornering forces, and how it transmits bump energy to the suspension. They know how to extract maximum performance, ride comfort, and durability by juggling tire size, tread design, rubber compound, structural stiffness, and air pressures. Run-flat tires limit what the engineers can do with some of these parameters, making their job tougher. 

The air in an inflated tire is what supports the vehicle’s weight. To make a tire that can hold up the car after it has been punctured, the tire companies have adopted three basic strategies. One is the self-sealing tire, in which the tire carcass includes a flexible liner under the tread that can plug small holes created by nails, rocks, or other sharp objects. Continental Tire currently makes a line called ContiSeal. Another strategy is a sort of tire-within-a-tire. It’s a hard rubber ring bonded to the wheel that can support the car’s weight in case the outer tire loses air. Michelin markets such a system under the PAX brand and it is standard equipment on the Honda Odyssey minivan and the Rolls-Royce Phantom (no doubt the only thing those two vehicles have in common). 

The third option is the reinforced sidewall or self-supporting tire, in which the car’s weight is carried by the tire’s beefy sidewalls – the rubber between the wheel and the tread that bears the tire brand and size info – when the air is lost. These sidewalls are comprised of thick layers of rubber reinforced with extra cord. BF Goodrich, Dunlop, and Kuhmo offer examples.

All three strategies have their pros and cons. Self-sealing tires don’t require special wheels or reinforced sidewalls, but the lining isn’t foolproof and, if it fails, the car must stop in a safe manner. The tire-within-a-tire lets engineers specify exactly the tire profile they want while assuring that you can still drive after the blowout, but requires a special wheel rim. If you had a set of such tires, those 20-inch chrome wire wheels you have been shopping for are out. The special rim also adds “un-sprung weight,” or weight not supported by the suspension. Gains in un-sprung weight are the worst kind from a vehicle dynamics standpoint, disproportionately affecting the handling and ride.

The stiff-sidewall method, the most common and what Pirelli has chosen for the Eufori@, doesn’t require a special rim and minimizes the un-sprung weight in-crease, but it limits what the engineers can do to “tune” the tire.

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