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Weighing Up Run-Flat Tires

Everyone hates a flat tire, they are, to say the least, inconvenient and no-one likes being stuck on a busy road with an unplanned dirty job. Run-flat or zero-pressure tires are made to carry the car for a short time, providing 100 or so miles to get off the highway and find a safe and relatively comfortable place to deal with the problem.

Run-flat tires still run when they are flat. A simple enough concept to grasp, I guess? They have been offered for the last few years and can be driven short distances after losing their air. This allows drivers to find a safe place to stop and change their tire. Although, some manufacturers have seen it as an opportunity to completely remove the spare from new models and save space, which means you have to find a dealer and buy a new run-flat tire.

zero-pressure tires are made to carry the car for a short time

This might be a case of car makers having their cake and eating it, too. The development of these tires is only in its infancy, but there will be more to come as car and tire manufacturers look for maximum efficiency from weight and space savings.

Zero-pressure tires suffer a few negatives. Replacement costs are perhaps higher than consumers expect and wear rates have been another negative. However, the wear rates could be attributed to complacency over the perceived indestructibility of this type of tire.

… offered on 3 percent of vehicles in North America

All new technology seems to suffer teething problems.

Run-flats have stronger sidewalls to support the tire if it loses air. They are offered on roughly 3 percent of all vehicles in North America now. Major companies in the zero-pressure tire game, like Michelin and BMW, are not getting ahead of themselves -- modest forecasts consider an increase to 4% by 2011.

Run-flats are mostly found on luxury vehicles. BMW has been enthusiastic and these tires are also showing up on cars like the Mini-Cooper S and mini-vans like the Honda Odyssey.

The obvious appeal with zero-pressure tires is the added safety. Tire makers claim that a vehicle with them will be easier to control if a tire fails. There is a trade off in comfort though, as run-flats have a harsher ride.

Instead of having stronger sidewalls, there's a variety with an internal supportive ring made of polyurethane. The thinking behind this is that the polyurethane allows a more comfortable ride and better fuel economy because the sidewalls do not need to be so stiff.

One of the setbacks to this is that stores and car dealers must have special equipment to service this type of tire. The equipment is expensive, so this can create supply and service gaps.

If there is no spare, you need to travel to a dealer to have it repaired or purchase a new one

Customers who find zero-pressure tires appealing are looking for convenience, added safety and peace of mind. It’s comforting to know that if you have a flat tire on a busy highway, you will be able to travel a short distance until it’s safe to change that flat, rather than try and change it in a dangerous highway setting.

However, you cannot drive a flat run-flat forever, so you will still have to put on the spare. If there is no spare, you need to travel to a dealer to have it repaired or purchase a new one. The supply and service gap could come into play here.

Two kinds of zero-pressure tires

Two kinds of zero-pressure tires exist. Both still require air to provide day-to-day performance. Self-supporting tires (SSTs) are the most common run-flat. Heavily reinforced sidewalls support the vehicle after air is lost and this type of run-flat fits on normal wheels. Michelin's PAX, is a newcomer. It positions a support ring inside the tire and a non-standard bead design is necessary. As previously mentioned service and sales gaps exist.

The biggest quandary of all is that zero-pressure tires never look flat. So how do you know when to change them? To solve this problem, tire-pressure monitoring systems (TPMS) have become compulsory in run-flat applications. TPMS lets the driver know when time is up for the run-flats they are … running flat. (TPMS systems are becomming standard equipment. They are now compulsory on new passenger cars sold in the United States.)

Only time will tell if run-flat tires are a passing fad or the future of tires.

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