Running flat out: BMW is a leader in pushing run-flat tires

BMW is getting ready to launch its next-generation compact SUV, the X3, and it will ride--as most BMWs and Minis sold today--on run-flat tires. Of all the companies producing cars today, BWM has embraced run-flat technology like no other.

Chevrolet with the Corvette, and some Lexus models, among others, use run-flats in certain applications. But BMW is moving, or has moved, the majority of its lineup to ride on run-flat tires.

The 2003 models of the Z4 and the 5-series were the first to be shod with run-flats, and since then, as new models were developed, the spare tire in a BMW became an endangered species.

Tom Baloga, BMW's vice president of engineering, said the trend will continue. It's a matter of safety, he said, as changing a tire along the side of the road is a dangerous proposition. Plus, Baloga said the space where the spare tire used to be was quickly consumed in the new models for some other task. As automakers work to reduce the weights of their vehicles, eliminating the spare tire--even if it's a space-saver--helps cut curb weight.

The only real exception to BMW's use of run-flats has been with the M cars. The company's sport models continue to be fitted with traditional performance rubber.

Better for the environment

There are good reasons to embrace run-flats. Not only is it a safety issue, as Baloga pointed out, but when you think about the millions of cars sold annually throughout the world, reducing tire usage by 20 percent means the savings of millions of tires and the natural resources and energy needed to make them. Plus, you save the steel or aluminum wheels on which they are mounted.

From a pure conservation point of view, run-flats are a green choice.

But, like most things in life, run-flats are a compromise. While they offer the ability to continue to drive--albeit at a slower speed--if the tire loses air pressure, because of the nature of their construction, run-flats don't perform like "normal" radial tires. They can't. What makes them run-flats--stiff sidewalls that stay up even when the tire loses air pressure--adds not only unsprung weight to the vehicle, something enthusiasts abhor, but they deliver a harsher ride than conventional tires do.

They're also expensive--20 percent to 30 percent or more than a conventional tire.

The ribs in the sidewall of this Bridgestone Turanza run-flat tire helps keep the tire cooler if air pressure is lost. That lets Bridgestone make the sidewall thinner, which contributes to a softer ride.

Keeping cool

Bridgestone is selling its third generation of run-flats, and while they have a better ride and handling feel than any run-flat before them, you don't have to be Mario Andretti to tell the difference when riding on them. This new tire is fitted with ribs, or fins, on the sidewall that help cool the tire so it keeps its shape when running in the "flat" mode. Heat is the enemy in run-flat operations, and keeping the temperature down allows for the sidewalls to be made thinner than those in the past, allowing the ride to be less harsh.

"People are sometimes surprised about the poor ride of run-flats," said Matt Edmonds, vice president of the Tire Rack in South Bend, Ind., one of the nation's largest tire retailers. "It's not uncommon for people to switch from run-flats to snow tires and comment that the ride is so much better.

"Run-flats certainly have a place in the market, and the technology is getting better, and it will continue to improve. But it's not quite yet the same as radial tires," Edmonds said.

"We have sold a lot of radials to folks who had run-flats on their car. They are running without a spare . . . most of them buy an AAA membership to get the roadside assistance," he said.

Edmonds noted in situations where the sidewall is severely damaged or punctured--something he recently experienced himself--can render a run-flat useless. So run-flats are not a panacea.

But with companies such as BMW pushing the notion forward with the tire companies, technology improvements will make run-flats better, ultimately making the driving experience better, too. And hopefully, less expensive.

By Roger Hart