July 20, 2012
Run-flat tires have evolved for the North American market and promise year-round performance. Do they hold true to their promise?
While most of today's run-flat tire options are three-season summer tires developed for use as Original Equipment on global vehicle makes and models, several tire manufacturers have responded to the needs of North American drivers by developing all-season performance run-flat tires that promise the year-round traction needed to drive in America's wide variety of weather conditions, including winter's cold, slush and light snow.
Yokohama joins Bridgestone and Michelin in producing a non-O.E. all-season run-flat performance tire line developed specifically for the U.S. replacement market. Yokohama's AVID ENVigor ZPS (Zero Pressure System) tires combine the performance of their popular AVID ENVigor all-season line with the peace of mind provided by their ZPS self-supporting run-flat technology.
Since the variety of all-season performance run-flat tires is rather limited, we combined the V-, W, and Y-speed rated tires available in the 245/40R18 size we've been using on our test cars. While this also combined tires from two tire performance categories (High Performance All-Season and Ultra High Performance All-Season), it allowed us to compare the AVID ENVigor ZPS with the popular Bridgestone Potenza RE960AS Pole Position RFT, as well as the previously untested Pilot Sport A/S Plus ZP and Pirelli P Zero Nero All Season Run Flat tires. Our evaluation used 2012 BMW F30 328i sedans fitted with new, full tread depth 245/40R18 tires mounted on 18x8.0" wheels.
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.
In everyday driving the single biggest area of concern for most drivers riding on run-flat tires is ride quality. Older and many Original Equipment run-flat tire designs weren't known for providing a comfortable ride. Thankfully many of the newer designs have made big gains in this area. The Bridgestone Potenza RE960AS Pole Position RFT's 3G sidewall design sets the standard for run-flat ride quality, doing the best job at softening the ride over all sizes of bumps along our drive. The Michelin Pilot Sport A/S Plus ZP was close behind, feeling a little firmer than the Bridgestone when encountering the larger impacts. The ride of the Pirelli P Zero Nero All Season Run Flat wasn't objectionable, but did feel a little stiffer than the Bridgestone and Michelin tires over most impacts. The Yokohama AVID ENVigor ZPS rounded out the group, riding like a traditional run-flat tire and feeling noticeably stiffer than the others when encountering medium and larger bumps.
The Pilot Sport A/S Plus ZP and Potenza RE960AS Pole Position RFT both generated minimal tread noise and did a good job muffling the booming sounds that often come from larger impacts. The P Zero Nero All Season Run Flat produced minimal tread noise, but bigger impacts and expansion joints could be heard as well as felt. The AVID ENVigor ZPS seemed to transmit a bit more impact and tread pattern noise into the cabin than the others.
All four tires handled well with an advantage in crisp steering response going to the Pilot Sport A/S Plus ZP. The Potenza RE960AS Pole Position RFT also handled very well, with steering response that was linear, but a little slower than the Michelin. The P Zero Nero All Season Run Flat followed and handled nicely, but without the sense of urgency of the Michelin and Bridgestone tires. Rounding out the group was the AVID ENVigor ZPS which delivered stable handling, but just wasn't quite as responsive as the others.
Our 1/3-mile per lap test track course includes 90-degree street corners, a five-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.
During the abrupt maneuvers on the track in dry conditions, the Potenza RE960AS Pole Position RFT held a small subjective advantage over the others, displaying predictable handling and feeling easy to control. The Pilot Sport A/ S Plus ZP was very close behind with somewhat more responsive steering, but not quite the Bridgestone's ease to drive at the limit. The P Zero Nero All Season Run Flat felt lively and responsive, but didn't have quite the predictability of the Bridgestone and Michelin tires. The AVID ENVigor ZPS delivered balanced handling, but didn't have the peak grip of the others.
In wet conditions, the Pilot Sport A/S Plus ZP led the way feeling composed and having the highest overall grip level. The Potenza RE960AS Pole Position RFT was close in overall traction, but displayed more understeer limiting its speed through the corners. The P Zero Nero All Season Run Flat and AVID ENVigor ZPS both displayed noticeably lower overall traction, feeling somewhat slippery in comparison to the Michelin and Bridgestone tires.
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 to sustain the vehicle's speed using cruise control whenever possible. They did not use hypermiling techniques to influence vehicle fuel economy.
@ 15,000 Miles
|% vs. Most Efficient|
|Bridgestone Potenza RE960AS Pole Position RFT||30.6||490.2||-0.98%|
|Michelin Pilot Sport A/S Plus ZP||30.3||495.0||-1.98%|
|Pirelli P Zero Nero All Season Run Flat||30.9||485.4||--|
|Yokohama AVID ENVigor ZPS||29.7||505.1||-4.04%|
While none of the tires in this test were designed with low rolling resistance as a high priority, we did find a modest difference in observed vehicle fuel economy. Based on our results the 1.2 mile per gallon difference between our lowest and highest observed fuel economy would result in an annual difference of almost 20 gallons of premium gasoline. At the current cost of $4.00/gallon, it would amount to an annual difference of about $78 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.
Winter weather is often unpredictable, and road conditions can change so fast that it's hard to know what waits for you around the next corner. For consistency, our snow testing is done at a dedicated winter test facility in Northern Sweden, and ice testing is done at a local hockey rink to simulate the glare ice often found at intersections and in other high-traffic areas. We measure each tire's ability to accelerate and brake in both conditions, as well as gather subjective ratings of how each tire feels from the driver's seat while driving through several inches of groomed snow on a handling course.
Snow Handling Subjective Impressions: The Pirelli P Zero Nero All Season Run Flat was easy to control and provided good snow traction. The Michelin Pilot Sport A/S Plus ZP was adequate, but wasn't able to start, stop or turn as well as the Pirelli. Another big step down in traction was the Bridgestone Potenza RE960AS Pole Position RFT which was hard to get and keep moving. The Yokohama AVID ENVigor ZPS rounded out the group and struggled to find traction.
It's important to remember that a run-flat tire that's driven 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.
While not repeated for this test, the last time we tested the Potenza RE960AS Pole Position RFT we took the opportunity to drive a car 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. And we expect the other tires in this test would also meet their respective manufacturer's extended mobility rating.
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 RE960AS Pole Position RFT run-flat tire, but as speed increased above 20 mph the wind and vehicle noise quickly drowned 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 a 50-0 mph panic stop, 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.
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.
Drivers who want to maintain run-flat capability and need an all-season performance tire to cope with winter weather now have a variety of good replacement tires to choose from.
The Michelin Pilot Sport A/S Plus ZP puts the performance in performance run-flat, with a blend of sporty handling, good traction and reasonable road manners. The Bridgestone Potenza RE960AS Pole Position RFT shows how close the best run-flat design comes to matching the ride quality of conventional tires, while still delivering good handling and dry/wet traction. Winter traction is not one of this tire's strong suits. Out on the road in everyday driving the Pirelli P Zero Nero All Season Run Flat isn't far behind, and in the snow this tire delivers good start, stop and turning traction. It would benefit from an increase in wet grip, though. And rounding out the test group is the Yokohama AVID ENVigor ZPS, which can't match the ride quality, wet traction or winter weather capability of the others.
Bridgestone Potenza RE960AS Pole Position RFT (Run-Flat 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. It 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.
Michelin Pilot Sport A/S Plus ZP (Run-Flat Ultra High Performance All-Season): The Pilot Sport A/S Plus ZP (Zero Pressure) is the Ultra High Performance All-Season run-flat tire member of Michelin's Pilot family of low profile, high-speed tires. Developed in the Original Equipment (O.E.) sizes for Chevrolet Corvette C-5 and C-6 coupe and convertible sports cars, the Pilot Sport A/S Plus ZP is designed to temporarily support the weight of the car even after a loss of air pressure, as well as combine enhanced all-weather performance, wet grip and treadwear along with year-round traction, including in light snow. Read more.
Pirelli P Zero Nero All Season Run Flat (Run-Flat Ultra High Performance All-Season): The P Zero Nero All Season Run Flat is Pirelli's Ultra High Performance All-Season run-flat tire developed for sports coupes and sedans. P Zero Nero All Season Run Flat tires feature Pirelli Self-Supporting Run Flat Technology that enhances safety and convenience by providing temporary extended mobility in the event a puncture allows complete loss of air pressure. Pirelli Self-Supporting Run Flat Technology allows tires to literally "run flat" for up to 50 miles at 50 mph (80 km at 80 km/h) unless otherwise specified by the vehicle manufacturer for Original Equipment tires. The P Zero Nero All Season Run Flat is designed to deliver all-season traction and handling for drivers who operate their vehicles in America's various weather conditions, including in light snow. Read more.
Yokohama AVID ENVigor ZPS (Run-Flat High Performance All-Season): The AVID ENVigor ZPS (Zero Pressure System) is Yokohama's High Performance All-Season run-flat radial developed to meet the year-round driving needs of sports car, coupe and sedan drivers. With the ability to provide temporary extended mobility for a distance of up to 50 miles at 50 mph in the event a puncture allows complete air pressure loss, AVID ENVigor ZPS tires are designed to combine performance, comfort and treadlife along with year-round traction, even on light snow-covered roads. Read more.
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