How Run-flat Tires Work

Michelin Pilot Sport Cup Zero Pressure tires are included in the optional Z07 performance package on the 2013 Corvette Z06 and a PDE performance package on the 2013 Corvette ZR1. The tires are essentially street-legal versions of a racing tire. Want to learn more? Check out these car safety pictures.

It happens a lot more than we think: Race car technology inspires a useful development for an everyday car. Unfortunately (though not surprisingly) these innovations tend to be expensive (at least at first) and limited in availability. But as the technology proves itself, such components are easier to find on everyday cars.

Run-flat tires are one such example; though the limited availability issue is improving somewhat. Consumer Reports says that as the technology behind run-flats get better, they're becoming more mainstream and easier to find -- and the prices are dropping, too. Believe it or not, run-flats are becoming almost common as they're now standard equipment on a lot of mid-range and luxury cars. But anyone shopping for a new car should be aware of the benefits and drawbacks of run-flats before buying.

Run-flats are designed to hold out for about 50 miles (80.5 kilometers) after suffering a puncture wound. This grace period, so to speak, enables drivers to cope with a blowout without resorting to panicked and potentially dangerous tactics, like cutting across several lanes of traffic to reach the side of the road, swapping to the spare on a narrow shoulder with traffic whizzing by, or losing control of the car entirely in a blowout situation. One last roadside scenario that run-flats may save you from: We've all seen a car sitting pathetically on its brake rotor on the side of the road after its owner lost control of the flimsy jack. With run-flats, you won't be "that guy."

The Art of Running on Empty

NASCAR run-flats are designed with multiple air chambers and liners -- if the tire is punctured and an air chamber fails, another air chamber takes over the space to maintain contact with the road and prevent the catastrophic loss of control that would otherwise result from a high-speed blowout. Goodyear has been manufacturing tires like this since 1966 [source: Goddard]. The public version of run-flat tires is not quite as complicated, but they don't need to be. Though pro drivers only need to make it back to the pit crew, the conditions on the track make the stakes pretty high. A regular car and driver can get by with a simpler design.

Tires arguably take more of a beating than any other part of the car. As the tire experts at Tire Rack explain, it's the air inside the tire that bears the weight of our car, not the tire itself. This distinction is important to understanding how tires really work -- they impose shape and structure on the air contained within. If that air is able to escape, the entire purpose is defeated.

Contrary to popular belief, most blowouts aren't the direct result of a puncture on a previously pristine tire [source: Tire Rack]. These dramatic and dangerous events are more likely to occur when a tire with a previously unnoticed slow leak (and resulting low tire pressure) suffers some sort of puncture damage or simply can't bear any more load. A tire's capacity is determined by the amount of air it's designed to hold in a chamber around the wheel, the strength of the air chamber and the rest of the tire's structure and the air pressure that's actually in the tire. If the air pressure is incorrect, it forces the other two factors to compromise, which weakens the entire tire and makes it more susceptible to damage. Think about a huge yoga ball that's lost a little bit of air -- it doesn't bounce or roll smoothly, and can't support your weight as well as one that's properly filled.

The development of run-flat tires is essentially a way to intervene between a relatively normal occurrence -- a blowout -- and its potential consequences -- a minor inconvenience at best, a fatal crash at worst. Run-flats provide a valuable service. They lessen the immediate, dangerous effects of a blowout, essentially buying the driver a grace period.

Run-flats work because the tire's structure was re-evaluated and redesigned so that the tire can bear the car's load even after the loss of air pressure. A very stiff sidewall allows the tire to hold its shape and continue to protect the rim from making contact with the road. Run-flats also have specially designed beads to maintain a proper seal between the tire and the wheel, even after air pressure is lost. If a regular tire goes flat, the seal isn't under as much pressure and the bead seal often fails. But the stiff structure of a run-flat means any weaknesses are forced elsewhere. Typically, a seal between two different materials is a weak point in almost any construction, and so the intersection of rubber and metal is reinforced in run-flat applications.

It's often said that a damaged run-flat has a range of about 50 or 55 miles (80.5 or 88.5 kilometers), which is true for most brands on the market. However, some types of run-flats can reportedly go as far as 100 or 200 miles (160.9 or 321.9 kilometers) after sustaining damage [source: State Farm].

And while run-flats will allow you to continue on your merry way, a tire with low air pressure might not be able to glide over potholes or road debris the way a nice, firm tire can. That's why tire pressure monitoring systems are a useful ally in helping to prevent blowouts. Cars with run-flats should be equipped with tire pressure monitoring systems because a slow leak in a run-flat won't be detected by simply eyeballing each corner before hopping in and driving off. This, of course, makes tire maintenance a bit more expensive, but the safety benefits more than outweigh the extra cost.

What's wrong with a blowout every now and then?

A Goodwrench technician checks the wheels and tires with a Tire Pressure Monitoring Systems (TPMS) tool.
A Goodwrench technician checks the wheels and tires with a Tire Pressure Monitoring Systems (TPMS) tool.

Despite run-flats' obvious benefits, they also have a few serious compromises that should be considered before buying a new car.

Since the structure of run-flats is a little different than other types of tires, they might not provide the kind of driving experience you're used to (especially if you take your car's speed or brawn seriously). Because a very stiff sidewall is essential to their function, run-flats tend to provide a rather harsh ride because they provide less cushion from the road. Run-flats can't provide a wide selection of specialized tread types either, like those popular for sport driving or occasional off-roading; Consumer Reports says that even drivers who are okay with the compromise have said that run-flats' tread sometimes lacks in basic capabilities [source: Consumer Reports]. They're also heavier than regular tires, which reduces your car's overall efficiency.

After a run-flat is punctured, its performance potential is further compromised. Of course, its primary purpose is to get your car to safety, so as long as you aren't faced with a dangerous situation, the tire did its job. A damaged run-flat isn't going to be exactly comfortable, but it's designed that way. It's important to understand that a run-flat tire isn't designed to be driven like that forever -- it needs to be replaced as soon as possible.

When you finally make it to your mechanic or tire shop to replace your blown-out run-flat, be prepared for potential sticker shock: Run-flats typically cost considerably more than their traditional counterparts. And since the tread tends to wear down faster, you might be faced with the wallet-busting plight of replacing all four corners long before you'd expected. Another potential roadside shock for car owner's with run-flat tires: Most cars originally equipped with run-flats don't come with a spare in the trunk -- a can of liquid fix-a-flat and a small air compressor is a common substitute.

And one final consideration if you're making a decision about investing in run-flats: A puncture in a run-flat tire generally can't be repaired, whereas a regular tire can often be saved with an inexpensive plug. However, after run-flats become the norm, most people will probably never learn how to change a tire anyway.

Author's Note: How Run-flat Tires Work

I had my laptop open to write this article and had the TV on, too, which is unusual. I never watch TV. I never change tires, either.

But "A Christmas Story" was on, so I decided it was time for my annual indulgence. And I'd forgotten about the "f-bomb" scene -- Ralphie's ill-fated attempt to help the old man change a flat. It was a minor inconvenience, like running low on gas, so I guess people must have gotten good at it. When Ralphie was describing his father's tire-changing prowess, I realized, no one ever changes tires anymore, do they? I can't remember the last time I was in a car that got a flat, or even saw someone with a car on a jack stand on the side of the road. Tires must have gone flat a lot more often back then, and not just because we have run-flats now. Wheels aren't rounder now, right? And I'd find it hard to believe that roads are smoother. Maybe tires aren't universally better, though. Maybe everyone just uses their cellphones to call for roadside assistance -- something Ralphie and his father couldn't do in 1940s Indiana.

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  • Consumer Reports. "Tire Buying Guide." Oct. 15, 2012. (Dec. 19, 2012)
  • Goddard, Nick. "How Tony Stewart's NASCAR Tech Trickles Down to Your Car." Popular Mechanics. July 2, 2012. (Dec.19, 2012)
  • State Farm. "Run Flat Tires; Pros and Cons." June 27, 2011. (Dec. 19, 2012)
  • Tire Rack. "Tire Tech Information -- Run-Flat Tires." (Dec. 19, 2012)