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Cooley On Cars
Smarter Driver, Understanding run-flat tiresYou may have heard about run-flat tires but unless you drive certain cars, odds are you probably don't have them. CNET's Brian Cooley explains why this forward thinking tire technology just couldn't become the standard for all tires.
Now run-flat tires are kind of amazing. They can be driven under-inflated or un-inflated and up to freeway speeds 55 miles an hour or so is recommended up to 200 miles in some cases, depending on the tire. How they do that's interesting. Here's a conventional tire and if I get on the sidewalls here, I can deflect them using-- probably done this with an old tire laying around even on a swing when you were a kid. Come over here to a run-flat though, try and deflect that sidewall, you'll break your thumb trying to do it. Extremely strong sidewall structure this way as well as laterally. This thing has sidewalls of steel, quite literally and can support the weight of the car without any air inside helping out. That's what makes it work. Roadside-- when you get a flat with a run-flat tire, you won't be out there on the side of the road trying to change a wheel which you may have never done before. The side of the highway is no place to learn a new mechanical skill. Blowouts-- run-flats are pretty much immune to sudden pressure loss, even without air they pretty much maintain their shape. In fact, the first indication anything is wrong may come from the tire pressure alert on the dash, not the feel of the steering wheel. Space and weight-- run-flats allow car makers to use the space formerly taken by the spare for something better like the battery in the case of this BMW and loses the weight of that spare wheel, tire, and jack. Now the cons, run-flat tires tend to cost more and weigh more. Two things you don't want in a tire. First of all the cost, easily 20, 25 percent more than a conventional tire about the same size. I'm ball parking there but that's not unusual. That's a big price difference. Secondly, the weight, you can't feel this but if I pick up this run-flat, it's really dense. This similar tire that is conventional and about the same size is actually, dramatically lighter and that will change the dynamics of how your vehicle handles and rides. Less weight is always better in any part of your wheels, brakes, tires, or suspension arms. Harder to find, you may not find a run-flat in your size sitting on the shelf when you need one. So, while the run-flat may get you to the nearest service station easily, you might spend a long time waiting there. The CNET Car Tech team has reservations about being stuck with the limited tire selection when you have run-flats. Honda's and Acura's that came with Michelin PAX run-flats for example, can only use those tires unless you do a complete change that could include wheels, tires, and tire pressure sensors. At a recent JD power survey found that owners of performance oriented cars with run-flats are only has as likely to recommend their tire brand to someone else. The weight and feel tend to deaden the cars' handling. Honda was an early user of run-flat tires, but they used a different system than this, something called the Michelin PAX technology. It was a hard plastic shell, but lived inside the tire and supported it. Same basic idea, but they bailed out in 2009. Don't put those on cars anymore. That leaves BMW, Mini, and some Corvettes as the main users today of these stiff sidewall technology like I've showed you. That actually is not enough to make an industry wide revolution. What did replace the run-flat or keep it from really getting to fruition, were better technologies for traditional spares, which we've covered-- actually if you go look up maintain.