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htc***
04-07-2010, 03:36 PM
The fast facts on run-flat tires

April 1st 2010, by Bradley Horn


If you’re checking your new car and the spare is missing, it doesn’t necessarily mean someone forgot to put it in. Rather, your vehicle may be equipped with run-flat tires.

Properly known as ‘extended mobility tires,’ run-flats can safely be driven even when they are completely out of air. Such tires must be able to run with no air pressure at 80 km/h for 80 kilometres.

They will still have to be repaired, and preferably as soon as it’s possible, but they won’t leave you stranded. In an extreme situation such as complete loss of air pressure, they’re intended to keep you safely on the road.

To achieve this motoring magic, run-flat tires are constructed differently from conventional tires. The sidewalls are much thicker, so they can hold the weight of the vehicle if necessary (a tire itself doesn’t bear the weight of your car; rather, it’s the air inside the tire that does). They have a special rim that holds tightly onto the wheel, so they won’t come off when they’re flat, as conventional tires can. They can also contain a puncture-sealing liner, so that small holes instantly seal themselves.

Run-flat technology has been around for a while, but was originally confined to ultra-premium cars, and only recently has it made its way into more mainstream vehicles. The more widespread application was partly due to the introduction of lower-cost tire pressure monitoring systems, or TPMS, which warn drivers when a tire’s pressure has fallen below a specified level. Since these tires don’t look any different when they’re flat, they can only be used with TPMS, so that drivers know if there’s a problem.

There are several advantages to run-flat tires, the chief one being that if a tire loses air, the car is still safe to drive: with many run-flat models, the tires hold up so well that it can be difficult to feel which one’s flat. There’s no danger of trying to change a tire on the side of a busy road. Eliminating the spare tire saves weight, which in turn improves fuel economy, and frees up the space normally taken by the spare. This makes run-flats especially popular on sports cars that don’t have a lot of cargo room in the first place.

However, they do have disadvantages. They’re more expensive. Their thick sidewalls can produce handling characteristics that some drivers don’t like, and they tend to give a much harder ride and are noisier than conventional tires.

The special rim that keeps them firmly attached also requires a matching wheel, which has a lip where the tire seats. You can put a conventional tire on a run-flat wheel, but you can’t put a run-flat tire on a conventional wheel.

Like any tire, run-flats can’t be repaired if the sidewall is punctured. The tread can be fixed with a patch/plug combination, but the run-flat should be removed from the wheel first, and some tire manufacturers only recommend one repair. If you need a second plug, the tire should be replaced.

http://autos.sympatico.ca/tire-guide/3570/the-fast-facts-on-run-flat-tires