Why do run-flat tires suck? Science, and tradeoffs
Most of the bigger tire brands offer some form of run-flat tires, and most of those come fitted straight from the factory.
Those of you who own a car with run-flat tires — tires that can go about 50 miles at 50 mph once punctured — know that ride comfort is the main sacrifice. God help you if you get an already-sporty car and add run-flat tires to the option list. Your ride will go from barely tolerable as an enthusiast to not-at-all tolerable. We talked to friend of Autoweek and tire guru Woody Rogers from Tire Rack to get the details of this devil’s bargain.
Almost all of the mainstream tire brands now sell some form of run-flat, including Pirelli, Bridgestone, Continental, Dunlop, Michelin and Goodyear. There are two types: self-supporting, with a reinforced sidewall that can support the weight of the car, and “support ring” tires, which employ a ring of hard rubber around the rim that supports the weight of the car. Today we’re mainly concerned with self-supporting.
Why do automakers use run-flats?
Mostly, they are an extra layer of safety when on the road. Fifty miles after a puncture is usually plenty of range to get home or to a tire shop rather than be stranded on the side a road with a questionable scissor jack and a rusty tire iron. Independent of the neighborhood, the side of the road or highway is a dangerous place to be. It also relieves the manufacturer of putting a spare tire, or even a can of sealant, in the trunk. That cuts weight and frees up more space in the trunk for cargo.
It does seem that manufacturers are moving away from run-flats in favor of a conventional tire plus sealant kit, he also told us.
Another problem to keep in mind is that run-flat tires can’t be repaired and reinflated. That means an expensive tire also needs to be replaced by another expensive tire. Additionally, as tire sidewalls get smaller in general it becomes a bigger challenge for tire designers to get to an acceptable ride with run-flats. See Graham Kozak’s drive of the Mercedes-Benz C300 for evidence. Other drawbacks include the fact that they must be used with tire pressure monitoring systems, because they don’t look different when they’re low on air.
The moral of the story? One, learn to change a tire. Two, don’t spec run-flats on your next car, especially if it’s a performance car. Three, if you already have run-flat tires, remember that they can’t be repaired. And if you get lucky enough to have them all wear out at the same time, get some standard-construction tires. Or look into the new self-sealing tires, but that’s another story, for another day.
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