A run-flat tire is specifically engineered to withstand a puncture so that a car can still be driven, even if air pressure is reduced to zero. Its reinforced sidewall ensures that a vehicle can travel for up to 80 kilometres at a maximum speed of 80 km/h.
There's a compelling safety argument to be made for run-flat tires: Blow a tire on a highway on a cold, rainy night and, instead of digging out the spare tire and jacking up the car to replace the flat – or waiting for roadside service with cars and big rigs steaming by – run flats allow you to keep driving.
But there have also been justified driver complaints about run-flat tires: they're expensive, rough-riding, have a shorter life span and are not as "fun to drive" as regular tires. Bridgestone addressed these criticisms at the launch of its DriveGuard line of tires, the first run-flat tires without an original equipment (OE) fitment, and which use Bridgestone's RFT technology. To do so, it put its tires up against common and premium all-season tires at the Circuit of the Americas Formula One track near Austin, to showcase the capabilities of its new DriveGuard line.
Many companies make run-flat tires, including Michelin, Continental, Goodyear, Kumho and Pirelli. Bridgestone says it was the first to use them on a production sports car – the 1989 Porsche 959 – though such tires didn't appear in North America until General Motors introduced run-flat Goodyears as an option on the 1994 Corvette.
DriveGuards are Bridgestone's fourth generation of run flats, designed with extra-strong sidewalls to support the weight of the car even if a blowout leaves zero air pressure in the tire – allowing the car to continue driving at 80 km/h for up to 80 kilometres. Yes, faster speeds and slightly longer distances may be possible, said Robert Saul, Bridgestone's performance and run-flat tire manager, but the chances of repair will decrease.
Just like regular tires, once run flats are punctured or driven too long on excessively low pressure, they must be replaced. Bridgestone says that minor damage to the DriveGuard can be patched, an advantage over run flats from Yokohama or various European tire makers, which don't endorse any type of repair, according to tirerack.com, an online tire dealer. DriveGuards use a unique cooling square pattern on the sidewall, which Saul said aerodynamically channels air to the deflated tire, which also allows for thinner, lighter and more ride-friendly sidewall construction.
"Heat buildup is the enemy – as the tire heats up (when driven flat), it degrades," he said. Bridgestone arranged various low-speed, real-world-condition tests of the DriveGuard. The first was a drive on a flat DriveGuard tire at speeds up to 60 km/h in a Camry, where the tire pressure monitoring system's (TPMS) yellow light was the most tell-tale sign that something was amiss. You could feel a grumbly mushiness whenever one turned left and threw the weight balance toward the flat right front tire, but it was largely unnoticeable otherwise.
The biggest eye-opener was the dry pavement ride-and-handling test at speeds up to 80 km/h. A DriveGuard-equipped Nissan Altima on a slow-speed course was pitted against one riding on its conventional stock Michelin Primacy MXV4s, and a third with Bridgestone's Turanza Serenity Plus premium all-season touring tire. The test included a ride over bumpy corner rumble strips to see how the tires absorbed the rough ride. End result: The DriveGuard was not as cushy as the touring tire, but the stiffer sidewall helped it turn quicker, with noticeably sharper steering response than the stock tires, and equal in comfort to the Michelins.
While the on-road performance aspect of run flats is impressive, longevity is a question mark. A J.D. Power tire study of OE run-flat tires released last year found that run-flat tire owners were nearly twice as likely to have replaced the tire in the first two years, and that run flats wore out on average about 10,000 km sooner than conventional tires. Bridgestone counters that DriveGuard tires are rated for 80,000 or 100,000 km, depending on speed rating, among the highest in their segment.
Consumer Reports, in its annual tire-guide ratings last year, concluded that despite some reader complaints about the stiffer ride, quicker wear, and higher prices of run flats, their potential safety advantages can outweigh their disadvantages, especially as prices come down.
Pricing for the DriveGuard has not been announced, but Bridgestone said it will be in line with the Turanza Serenity Plus, which lists between $149 and $483 on 1010tires.com. A common P215/55HR16 Serenity Plus fitment costs about $154, as per the same Canadian site. That's not bargain basement, but d in the same ballpark as its premium all-season rivals.
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