What are run-flat tires?

“Now,” says the engineer, “go take a lap.”

What? Here we are in the pits of the famous Circuit Paul Ricard Grand Prix race course, a ribbon of asphalt set amid the green hills and rocky outcrops of southern France with a view that almost cannot be believed – and this guy wants me to drive a lap with a flat?

Ah, but this MINI is wearing Pirelli’s new line of [email protected]™ run-flat tires, and as the description suggests, they are engineered to run without air at speeds up to 50 mph for distances up to 100 miles.

I take the MINI out, and except for some buzzing from the flat tire and looseness in the steering, it laps the track safely and without incident.

Run-flat fever
The nuisance of slow leaks and the danger of high-speed blowouts are what self-sealing and run-flat tires have been fighting for more than two decades.

As far back as the late 1970s, Pirelli was equipping the Lamborghini LM002, a monster sport-utility with the V-12 engine, with a giant self-sealing tire called the Scorpion BK. In recent years, the race to develop better and more affordable run-flat tires has intensified.

All of the major tire companies, including Michelin, Bridgestone/Firestone, Goodyear, and of course, Pirelli, offer some kind of tire that can run without air for a limited distance. No spare tire necessary. If you suffer a puncture, these tires won’t leave you stranded on the freeway shoulder. They can get you a bit farther down the road – up to 100 miles in some cases.

The automakers think this is a dandy idea and have included run-flat tires on their option sheets. On some models, including the all-wheel-drive version of the Toyota Sienna minivan, the MINI Cooper S, Lexus SC430 and the Chevrolet Corvette, run-flat tires are standard equipment.

One big reason for the trend is the danger of freeway-speed blowouts. Another less apparent but very important reason: The ongoing war for space under the car. Increasingly popular amenities such as fold-flat seats and all-wheel drive are causing a crowding problem below the car’s floor. Extra driveshafts, wells for folded seats, the fuel tank – there isn’t room for everything.

Engineers are salivating over the huge hole consumed by the spare tire. Get rid of the spare, jack and associated tools and you not only free up a ton of space, you also trim both the vehicle’s weight and its cost.

With such incentives, the spare tire could be an endangered species. You must have noticed how the spare has already shrunk on most passenger vehicles from a full-size copy of the road tires to a tiny “doughnut” – usually a high-pressure pneumatic or a solid-rubber ring, almost small enough to slide down the slot of a vending machine.

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.

Types of run-flat tires
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:

1. The self-sealing tire. 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.

2. 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).

3. Reinforced sidewall or self-supporting tires. 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.

The good and the bad
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 it 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 [email protected], doesn’t require a special rim and minimizes the un-sprung weight increase, but it limits what the engineers can do to “tune” the tire.

The black art of black rubber
Tuning tires comes down to forging a series of compromises between conflicting forces to get the ride and performance balance ideal for your new model.

Here are some examples: Stiffer sidewalls create better performance because the tire deforms less in corners and under acceleration and braking. That creates the kind of crisp control responses you expect in a sports car.

Stiffer tires also transmit more bump energy into the chassis, which makes for a harsher ride. Larger tires have a bigger “contact patch,” the vital link between the car and Mother Earth. The bigger the contact patch, the more grip you have and the better the car turns, accelerates and brakes. But it also generates more noise, cuts fuel economy and has worse sloppy-weather traction. Softer rubber compounds in the tread provide better grip, but they wear out more quickly.

Car engineers are accustomed to juggling these trade-offs to get the ride and handling they want.

With run-flats, it’s a whole new world.

The stiffer sidewalls of a run-flat mean smaller contact patches and more bump-energy transmission. Indeed, the first run-flat tires were cited for their low grip and harsh ride. As demand for run-flats has increased, the car industry is becoming smarter about how to adapt the technology.

For starters, the tire makers are devising better rubber formulas that reduce the ride penalty caused by the reinforced sidewalls. Driving a 2005 Ford Mustang GMT equipped with Pirelli’s [email protected] tires through the villages of southern France turned a lot of heads but didn’t bobble ours over rough pavement. The automakers have also started tuning their suspensions specifically for run-flats. One tweak is relaxing the “durometer,” or stiffness of the suspension bushings, to help cushion impacts that would be better absorbed by a conventional tire.

What does it all mean?
If your car didn’t come with run-flats originally, proceed with caution. Run-flats work best with suspensions that are designed for them from the outset.

You may find the ride too stiff for your liking. If the new car you are shopping for has run-flats, forget the negative press you have read in the past. Run-flat technology has rolled a long way since the early days, and it’s going to continue.

Part of Pirelli’s display included a prototype wheel rim cast with a hollow chamber that is pumped with air. If the tire develops a leak, a warning signal lights on the dash while a valve allows air to pass from the chamber to the tire, maintaining tire pressure until you can safely pull over. Another system in development constantly monitors air pressures, calling the driver on his or her cell phone if the pressures drop below normal.

Wait. Tires that call you on your cell phone? It’s not just dumb rubber any more.