Run Flat Tires: Promise vs. Reality
For anyone who relies on vehicles for work, getting stranded on a highway because of a blowout is more than a nuisance — it can mean a loss of income.
By allowing a vehicle with a punctured tire to travel 50 miles or more at up to 55 miles per hour, run-flat tires have the potential to rid us of the blowout nightmare. Though run-flats have been market tested since their introduction in 1986 on the Porsche 959, the concept still remains a major engineering challenge.
Like the fidgety kids in the backseat, fleet administrators want to know: “Are we there yet?”
Bill VandeWater, consumer products manager in sales engineering for Bridgestone Firestone, helped sort out the run-flat promise from the reality.
There are two viable run-flat technologies at present: the support ring system and the self-supporting system. Both systems typically have the same 30,000- to 40,000-mile life expectancy as regular tires, though run-flats generally cost 25-to 30-percent more than conventional tires, says VandeWater.
The support ring tire contains a separate internal ring that supports a load in a lowpressure condition.
Support ring systems offer relative ride comfort when inflated, says VandeWater.
The Michelin PAX system, the only available support ring application in North America, can be driven safely up to 125 miles on a flat. However, the PAX system tire must be replaced using a $20,000 mounting machine, meaning many mechanics don’t have the equipment to change the tire.
Michelin says the tire can be repaired or replaced at one of more than 200 Michelin dealers or 1,000 Honda dealers. In remote locations, Michelin says it guarantees installation of a new PAX System assembly within 12 hours of a service call.
The PAX tire mounts on the outside of the flange and therefore requires a new wheel when replacing the tire. VandeWater says Bridgestone is testing a support run-flat tire in Europe that does not require a new wheel.
Because the separate internal ring can support more weight than the self-support system, the support ring concept is also being looked at for taller tires with higher aspect ratios (65 – 75) for light-truck applications.
The self-supporting tire has a sidewall reinforced with extra cord inserts able to withstand the extra weight in a low-pressure condition.
Most run-flat tire models currently fall in this camp.
Though self-supporting systems use conventional wheels and mounting equipment, they are harder to mount and balance than a support ring system, VandeWater says. The ride is generally harsher as well.
Self-supporting systems work well with cars that require low aspect ratio tires (35 – 50 series) such as sports cars and small passenger cars. However, the reinforced sidewall technology is not suited to support heavier SUVs and light trucks.
The Future of Run Flats
The considerable technological and logistical barriers to implementation notwithstanding, manufacturers are continuing to engineer a better run-flat tire.
From a safety perspective, reducing the risk of losing control due to a blowout is an enormous benefit. From an auto-manufacturing standpoint, reducing the weight of the vehicle by removing the spare boosts fuel economy and creates more usable trunk space.
Though run flats have historically been used on smaller sports cars, they now come standard on such mainstream minivans as the Toyota Sienna AWD and Honda Odyssey Touring, and as an option on the Nissan Quest SL and SE.
BMW has gone even further by deciding to engineer every new vehicle for runflat tires. This is the right way to approach an old problem, according to VandeWater. “Cars up until this point have been designed all the way through and then they’ve attached run-flats at the end,” says VandeWater. “You can pull back some of that harshness when you design for runflats from the white sheet of paper on.”
VandeWater says increased demand and acceptance brings increased production, lowering prices further.