Run-flat Tires: A Primer


  • Michelin PAX

    Michelin PAX

    This cutaway of a Honda Odyssey's Michelin PAX wheel and tire assembly clearly shows the semi-rigid support ring mounted to the unique wheel. | March 18, 2010

2 Photos

Most everyone dreads a flat tire. Not only are they inconvenient, but drivers who haven't experienced a blowout at speed wonder how they'd react. And no one likes the idea of being stuck at the side of a busy road in nice clothes with an unplanned dirty job to do.

Run-flat or zero-pressure tires are intended to support the weight of the car for a short time, providing the driver with 100 or so miles of range to get off the highway and find a repair shop. Sounds like a slam-dunk no-brainer, right? But is it really that simple?

The players

Two kinds of zero-pressure tires exist in the market today. Both types still require the usual amount of air to provide day-to-day performance.

Self-supporting tires (SSTs) are the original and most common run-flat type. Heavily reinforced sidewalls support the vehicle after air departs the scene. This sort of run-flat is designed to fit on normal wheels with no modifications.

Michelin's PAX, a patented auxiliary support run-flat system, is a relative newcomer. PAX sidewalls, while still stiffer than normal tires, are not as rigid as SSTs. Instead Michelin designed a unique wheel that positions a semi-rigid "support ring" inside the tire to hold the car up when the air goes bye-bye. A non-standard bead design is necessary where wheel and tire meet.

Enter TPMS

A side effect of the stiff sidewalls found on run-flats is that they never look flat. As a result, the danger of driving on underinflated tires is even greater, as many people don't check their tire pressures until they "look" low.

To counter this problem, the use of tire-pressure monitoring systems (TPMS) has become mandatory in run-flat applications. Since run-flats only provide a limited zero-pressure driving range, TPMS is critical to help the driver know when the mileage clock starts ticking, and more importantly, when time is up.

The good

  • You can drive on a flat tire — The chief benefit of a run-flat tire is the ability to drive 100 or so miles after all air has gone. Drivers usually have to reduce speed to about 50 mph to get that range. The owner's manual will have exact figures for each tire/vehicle application.
  • Better stability after a blowout — Because the tire can support the vehicle without air, a sudden deflation results in less weight transfer and tread destabilization. Steering and handling remain near normal.
  • Lower vehicle weight — With the spare eliminated, and sometimes the jack and tools as well, vehicle weight should theoretically go down. It's not as much as expected, because run-flat tires weigh more, owing to those reinforcements. In fact, a Honda Odyssey with PAX actually gains weight.
  • Repairability — SST run-flat repair guidelines are nearly similar to those for standard tires. Michelin's PAX has more stringent repair procedures, including a warning that repairs can only be carried out at a "Michelin PAX System authorized servicing dealer." In either case, if the zero-pressure driving distance or speed is exceeded, the tire might need outright replacement. Furthermore, tire sealant-in-a-can leak repair products shouldn't be used, because they can foul many types of TPMS air-pressure sensors.

The bad

  • No spare — Run-flat-equipped vehicles carry no spare, and sometimes the jack and tools are omitted as well. In fact, eliminating the spare and reallocating that space to some other purpose (styling, third-row seat, interior room, etc.) is a big reason why run-flats are offered.
  • Harder ride — The stiff sidewalls that make a run-flat work also result in a harder ride. PAX sidewalls are not as stiff as SST sidewalls, lessening the effect somewhat. Yes, the suspension tuning can be adjusted to compensate, but the loss of tire compliance is never fully recovered.
  • Tread wear — While this is still a point of contention, and possible litigation, reports that their run-flats have a shorter lifespan than standard tires have been made by consumers. As you might expect, tire company representatives are mum on the subject. Past experience tells me that the temptation to put a soft tread compound on a tire to counter a hard ride is strong. The downside with this approach is that softer compounds tend to wear more quickly.
  • Blowouts are still possible — If a driver fails to heed or notice the run-flat warning and drives beyond the zero-pressure range or above the speed limitation, tire disintegration can still ultimately occur, with the same destabilizing effects.
  • Heavier tire weight — Run-flat reinforcements add weight. And it's all unsprung weight, the bad cholesterol of vehicle mass that degrades ride and handling. In SSTs, this may amount to a few pounds per tire. However, I measured a single 2006 Honda Odyssey PAX assembly at 75 pounds, compared to 50 pounds for a standard Odyssey EX wheel and tire. That's a huge 25 pounds per corner, 100 pounds per car of added unsprung weight.
  • Cost — Run-flats cost more money to replace. Compared to standard tires, expect to pay double or more. At the time of this writing, Internet prices for Toyota Sienna OEM run-flats were $152-$176 each, while the equivalent standard tire cost $71. Honda Odyssey PAX tires, not available online, were $200 each or more, while the standard ones cost $81.
  • Lack of choice — PAX tires are made only by Michelin, so no price competition exists. Sure, Michelin has gifted the manufacturing rights to other tire makers, but as this is written, none have decided to make them. PAX wheels are an oddball metric diameter, so nothing else will fit. And the metric rim with its unique bead requires special tire-mounting equipment, so many tire stores cannot mount or even repair a PAX tire. The SST situation is better, as they fit on standard wheels and are made, in some sizes, by more than one manufacturer. Still, run-flat users don't enjoy the shopper's carte blanche that ordinary tires allow.
  • Less on-shelf availability — Because run-flats are presently a low-volume class of tire, drivers shouldn't expect to roll into just any tire store and buy one. That's OK when replacing worn tires, when time isn't critical. But if a family is traveling and needs a new run-flat en route, they'll probably have to make a detour to find a suitable tire dealer. An overnight stay is not out of the question while waiting for a tire to be shipped in. Early adopters are advised to read their owner's manual and tire warranty carefully for details.

The irony

TPMS is such a good idea that the federal government has made it mandatory for all cars, not just those with run-flats. Twenty percent of 2006 vehicles have it, increasing to 70 percent in 2007 and 100 percent in 2008.

Ironically, TPMS makes the case for run-flats less compelling. Since these systems excel at alerting drivers to underinflation and slow leaks before they can fester and weaken a tire, the likelihood of certain types of blowouts and flat tires is reduced. In making the case for mandatory TPMS, the NHTSA cited tire industry data claiming that 85 percent of tire deflations are slow leaks — some of which go unnoticed and end up as blowouts. The remaining 15 percent are due to sudden ruptures or large punctures. Other industry sources put the sudden rupture percentage even lower than 15 percent.

You have to decide

Run-flats work as advertised, but they have unspoken downsides that everyone needs to be aware of. Cost and availability may improve over time, but that depends on how customers react to the prospect of no spare, a potentially harder ride and reduced replacement choices. In the short term, higher replacement costs and supply issues are the reality.

TPMS is soon to be a standard-equipment fact of life on cars without run-flats. These systems are being implemented to reduce the likelihood of the very thing that run-flat tires were designed to address: the blowout and roadside stranding.

Your own personal feelings, budget, driving patterns and geographical circumstances need to be figured in the decision to go with run-flats or not. For some of us, the decision is not as easy as it first sounds.

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