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A conventional tire relies on the air pressure inside it to support the weight of the vehicle. Without enough air pressure its relatively thin sidewall collapses on itself as the weight of the vehicle above crushes it against the pavement, forcing the sidewall to bend well beyond what it's designed to withstand. It doesn't take very long for the over-stressed sidewall of an underinflated or flat tire to fail, leaving the driver stuck on the side of the road.
For those drivers who are worried about being stranded, the attraction of a run-flat tire is its extended mobility, with the ability to drive up to 50 miles at 50 miles per hour even with no air in the tire. To accomplish this, the sidewall of a run-flat tire must be thicker and use harder rubber than a conventional tire to support the weight of the vehicle when the tire is flat or significantly underinflated. That thicker and stiffer sidewall is with you every mile you drive, creating a firmer ride even when the tire is properly inflated.
Recognizing the inherent trade-off in ride quality and the less-than-universal appeal among consumers, tire engineers have been trying to minimize the negative impact run-flat tires have on ride quality. Bridgestone is a long-time supplier of run-flat tires as Original Equipment on a variety of vehicles, giving them plenty of opportunity to tune and develop the performance characteristics of their run-flat tire designs.
Unique cooling fin design* helps the tire run cooler, allowing for a thinner sidewall and more comfortable ride.
Bridgestone's new third generation (3G) run-flat features a sidewall design that helps minimize the run-flat tire's negative influence on ride comfort. While still thicker than a conventional tire, the signature features of the 3G run-flat sidewall are a unique exterior cooling fin design and special rubber compound, both designed to reduce the heat buildup that occurs while driving on a tire that is no longer inflated. Reduced heat buildup during run-flat operation allows for the thinner sidewall design, ultimately improving ride comfort.
The Potenza RE960AS Pole Position RFT is the first non-O.E.M. tire line to feature Bridgestone's 3G run-flat design, and also uses the same tread pattern and compound as the conventional (non-run-flat) Potenza RE960AS Pole Position tire. This presented a unique opportunity for the Tire Rack team to compare these two side by side to evaluate what changes adding run-flat capability brings to ride comfort and handling. To gauge just how far the 3G design has evolved we also included an earlier-generation Bridgestone Turanza EL42 RFT run-flat tire that is used as Original Equipment on 3 Series BMWs. Our evaluation used 2011 BMW E92 328i coupes fitted with new, full tread depth 225/45R17 tires mounted on 17x8.0" wheels.
What We Learned on the Road
Our 6.6-mile loop of expressway, state highway and county roads provides a great variety of road conditions that include city and highway speeds, smooth and coarse concrete, as well as new and patched asphalt. This route allows our team to experience noise comfort, ride quality and everyday handling, just as you would during your drive to school or work.
Out on the road our team found the ride quality of the Potenza RE960AS Pole Position RFT run-flat tire to be very similar to the conventional Potenza RE960AS Pole Position, with a small advantage to the conventional version. The older-generation run-flat design in the Turanza EL42 RFT felt noticeably harsher when it encountered sharp-edged impacts both small and large, such as expansion joints, poorly patched pavement or minor potholes.
The conventional Potenza RE960AS also produced slightly less impact noise, with the 3G run-flat version very close behind. Again the older generation run-flat design of the Turanza EL42 RFT produced some additional booming as it hit bumps in the road.
The handling of the Potenza RE960AS 3G RFT was actually a little crisper than the non-run-flat version, taking advantage of its inherently stiffer sidewalls to help the tire change direction more briskly. The conventional Potenza RE960 was close behind, also feeling nimble and responsive. The Turanza EL42 RFT trailed the two Ultra High Performance All-Season tires, feeling somewhat less responsive as you would expect from a Standard Touring All-Season tire.
Many of our team commented on how similar the run-flat tire behaved to the conventional Potenza RE960AS Pole Position. They felt very little trade-off in everyday ride comfort and road manners when choosing the run-flat tire over the non run-flat version.
What We Learned on the Test Track
Our 1/3-mile per lap test track course includes 90-degree street corners, 5-cone slalom and simulated expressway ramps. Run in both dry and wet conditions, the test track allows our team to experience the traction, responsiveness, handling and drivability normally only encountered during abrupt emergency avoidance maneuvers or competition events.
Out on the track in dry conditions we found all three tires performed well. As we found on the road, the run-flat Potenza RE960AS Pole Position RFT felt just a little more responsive than its conventional counterpart. Overall grip of both tires felt similar and braking traction was sure-footed. The Turanza EL42 RFT was able to produce a lap time equivalent to both versions of the Potenza RE960, but felt a little less responsive or composed in rapid transitions. The Turanza EL42 RFT is an Original Equipment tire on our BMW 3 Series test cars, and shows how it has been highly tuned by BMW and Bridgestone engineers to handle well on this platform.
In the wet, the non-run-flat version showed a minor advantage in ultimate traction over the 3G run-flat. Both tires felt predictable and stable, with good cornering and braking traction. The Turanza EL42 RFT struggled somewhat to find traction and did not have the stability or confidence of the conventional or fun-flat Potenza RE960AS Pole Position tires.
We find many Original Equipment tires (like the Turanza EL42 RFT) do not offer the wet traction of most replacement tire lines (like the Potenza RE960AS Pole Position) due to criteria placed on the O.E tire's design by the vehicle engineers. So in the case of the Turanza EL42 RFT we don't think it is the previous-generation run-flat design that is the primary cause of its reduced level of wet traction.
Fuel Consumption Results
Our Real World Road Ride features a relatively flat 6.6-mile loop of 65 mph expressway, 55 mph state highway and 40 mph county roads along with two stop signs and one traffic light every lap. Our team drove approximately 500 miles over the course of several days. Since we wanted to compare fuel consumption results that typical drivers would experience, our drivers were instructed to maintain the flow of traffic by running at the posted speed limits and sustain the vehicle's speed using cruise control whenever possible. They did not use hypermiling techniques to influence vehicle fuel economy.
We observed a small difference in fuel consumption during our test. Based on our results the 0.4 mile per gallon difference between our lowest and highest observed fuel economy would result in an annual difference of only about 8.5 gallons of premium gasoline. At the current cost of $4.00/gallon, it would amount to about $34 for drivers traveling 15,000 miles a year.
It's important to note our test's fuel consumption measurements follow consistent procedures designed to minimize variables that could influence the results, however they do not represent an exhaustive long-range fuel consumption study. While our procedures require the test vehicles in each convoy to run under the same prevailing conditions, the week-to-week differences in ambient temperatures, barometric pressures and wind speeds that we experience over a season of testing can influence vehicle fuel consumption and prevent the absolute mpg values of this test from being compared directly against those of others.
Larger differences in consumption between tires may indicate a difference that might be experienced on the road, while smaller differences should be considered equivalent. As they say, your mileage may vary.
Driving on a Flat Tire
We also took the opportunity to drive on the Potenza RE960AS Pole Position RFT 3G run-flat with the right front tire at zero inflation pressure.
Under our driving conditions the extended mobility of the run-flat tire easily delivered on Bridgestone's promise of 50 miles at up to 50 miles per hour when driven without air. We can see why all tire manufacturers require the use of a tire pressure monitoring system (TPMS) when using run-flat tires, as it's possible to not know you have a flat. At low speeds a subtle but discernable growl could be heard coming from the flat 3G run-flat tire, but as speed increased above 20 mph the wind and vehicle noise quickly drown out the flat tire's tell-tale sound. Even though it was a front tire that was flat, directional control was easy to maintain even at higher speeds. There was only a slight pull in the steering wheel towards the side with the flat tire. The flat tire was most noticeable during slow, tight turns to the left (with the flat tire on the outboard side), where the audible growl was very noticeable and a rumble could be felt through the steering wheel. Noticeably absent at all speeds was the traditional flap-flap-flap sound of a flat tire, likely due to the combination of the run-flat tire's stiffer sidewall design and the low profile of our test size. During our 50-0 mph panic stop test, the car with one flat RFT tire took an additional 10 feet to stop than the car fitted with all four tires properly inflated, and displayed a noticeable but very controllable pull in the steering wheel.
This is what happens to a conventional tire driven a short distance without air.
The outside of a run-flat tire driven without air may not show any damage…
But the inside of that same run-flat tire shows stress damage and must be replaced.
Under the same conditions, driving on a conventional tire without air is very noticeable. The flapping and growling sound coming from the tire without air is appreciably louder, and the vehicle pulls harder towards the flat tire. Where the run-flat tire was able to deliver 50 miles at 50 mph, it doesn't take long for the sidewall of the conventional tire driving without air pressure to fail as its sidewall shreds from the excessive deflection under the weight of the vehicle.
It's also important to remember that a run-flat tire that is driven on with low or no inflation pressure will need to be replaced (just as a conventional tire would). It's not that you can simply repair the puncture, reinflate and be on your way. When asked to provide extended mobility, the run-flat tire sacrifices itself in the process. The damage may not be apparent from the outside of the tire, but a visual inspection of the inside will reveal the tell-tale signs of stress damage left on the surface of the innerliner. The difference is the run-flat tire will allow you to continue to travel safely for some miles at near-normal road speeds to seek protection from inclement weather or precarious situations, whereas the conventional tire that is flat will require you to stop wherever you are.
We find the number one reason drivers choose to take off run-flat tires is because of an uncomfortable ride. But since many vehicles originally equipped with run-flat tires are not designed to carry a spare tire or changing tools, driving on conventional tires brings the risk of being stranded if a puncture occurs.
Bridgestone's 3G run-flat technology in the Potenza RE960AS Pole Position RFT helps give drivers a reason to stay with run-flat tires and keep their extended mobility capability without paying a significant penalty in ride comfort or handling.
Bridgestone Potenza RE960AS Pole Position RFT (Ultra High Performance All-Season): The Potenza RE960AS Pole Position RFT is an Ultra High Performance All-Season self-supporting replacement run-flat tire developed for drivers looking for enhanced ride quality and all-season traction for sports cars, sports coupes and performance sedans originally equipped with O.E. run-flat tires. The Potenza RE960AS Pole Position RFT features Bridgestone 3G RFT Technology designed to offer almost the same riding comfort as conventional tires while providing temporary extended mobility for a distance of 50 miles at up to 50 mph even after a puncture has allowed complete air pressure loss. Potenza RE960AS Pole Position RFT tires are also designed to provide year-round driving flexibility by offering predictable handling, traction and control on dry and wet roads, as well as in light snow. Read more.
Bridgestone Potenza RE960AS Pole Position (Ultra High Performance All-Season): The Potenza RE960AS Pole Position with UNI-T AQ II Technology is the Ultra High Performance All-Season member of Bridgestone's Potenza family of performance tires for the drivers of sports cars, sports coupes and performance sedans. The Potenza RE960AS Pole Position is designed to provide year-round driving flexibility by offering predictable handling, traction and control on dry and wet roads, as well as in light snow. Read more.
Bridgestone Turanza EL42 RFT (Standard Touring All-Season): The Turanza EL42 RFT is a Standard Touring All-Season run-flat tire designed for Original Equipment use exclusively on BMW 1, 3 and 7 Series cars, as well as Infiniti Q45 sedans and Lexus GS and SC sedans and convertibles. The Turanza EL42 RFT is designed to provide temporary extended mobility for a distance of 50 miles at up to 50 mph even after a puncture has allowed complete air pressure loss. The Turanza EL42 RFT was developed to combine the assurance of driving on a run-flat tire with the year-round versatility of an all-season tire that provides traction in dry, wet and wintry conditions, even in light snow. Read more.