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Bridgestone Potenza RE960AS Pole Position (Ultra High Performance All-Season 225/40R18 88W)
Drivers who live in the snowbelt know that winter road conditions are almost always changing. Every vehicle that passes through new-fallen snow either clears a path by blowing it away, churns it into slush or packs it down into ice depending on the weather conditions. What that means is you never know what you're going to find on the road up ahead. And just as changing weather and road conditions challenge us as we drive, they also prove challenging to conduct meaningful tire comparisons out on the open road.
In an effort to better control winter's conditions, members of the Tire Rack team traveled to Northern Sweden in late January, 2009 during the height of winter season. This region near the Arctic Circle is home to a number of dedicated winter test facilities used by vehicle and tire manufacturers from around the world. Here we found consistent cold temperatures, plenty of snow and well-prepared snow and ice surfaces - ideal conditions for conducting side-by-side tire comparisons. Our evaluation was comprehensive - comparing each tire's ability to accelerate, brake and drive through the snow. We conducted both objective tests measuring each tire's performance with sensitive on-board instruments, as well as gathered subjective ratings of how each tire felt from the driver's seat.
Our third product group compared five popular Ultra High Performance All-Season tires, designed to provide year-round traction for performance coupes and sedans, including light snow.
Past experience has shown us that many Ultra High Performance All-Season tires tend to be focused primarily on the three non-winter seasons, delivering relatively high levels of dry and wet road traction and handling. But like with most things in life, there is no free lunch in tire designs, so this three-season focus usually comes at some expense in the fourth season's wintertime traction levels. Our winter test revealed just how much, or little, snow traction is provided in these "All-Season" designs.
We compared the Bridgestone Potenza RE960AS Pole Position, Continental ExtremeContact DWS, Goodyear Eagle F1 All Season, Michelin Pilot Sport A/S Plus and Yokohama Avid W4s. Our test used new 225/40R18 tires mounted on 18x8.0" wheels. All tires were broken in by driving them on clear roads for approximately 100 miles prior to testing in winter conditions. All tests were conducted on a rear-wheel drive vehicle, using a 2008 BMW E90 320i sedan and 2008 BMW E92 328i Coupe for the driving duties.
Performance Drive Ratings - Snow Handling
Our subjective snow handling test is designed to simulate what you might find out in the real world driving on unplowed roads during a moderate snow fall. The 1.1 mile, modestly uphill course was covered with several inches of groomed snow on top of a packed snow and ice base and proved to challenge each tire's ability to accelerate, brake and turn. Our evaluation was conducted with the test vehicle's traction and stability control system switched off so the driver would feel how much grip, and all of the slip, each tire had. ABS brakes were used for consistency and safety.
An All-Season tire design by its nature is a jack-of-all-trades, but master of none. So it came as no surprise that the best of the Ultra High Performance All-Season tires we tested was not able to match the subjective snow capabilities of any of the dedicated winter tires we had tested earlier in the day. But with that said, the Continental ExtremeContact DWS felt noticeably superior to the other All-Season tires in this group. Overall acceleration, braking and cornering traction was good with plenty of reserve grip to aid control and driver confidence when this tire did reach its traction limit. The Goodyear Eagle F1 All Season felt the best of the rest, doing a reasonable job at clawing its way along the course.
The Michelin Pilot Sport A/S Plus followed with less overall snow traction than the ExtremeContact DWS and Eagle F1 All Season tires. Vehicle cornering balance was somewhat less predictable, too, as moderate understeer or oversteer could occur with minimal warning. The Bridgestone Potenza RE960AS Pole Position struggled somewhat to provide acceleration traction. Overall cornering grip was close behind the Pilot Sport A/S Plus, and without that tire's unexpected understeer or oversteer.
Well back from the rest was the Yokohama Avid W4s. This tire was challenging to get -- and keep -- moving through the snow and provided limited cornering and braking traction as it made its way along the course. We were able to complete the modestly uphill course, but this tire required plenty of driver concentration to avoid getting stuck.
In an effort to quantify the subjective impressions felt by our drivers, we also conducted a number of instrumented objective tests to evaluate differences in acceleration and braking traction. These tests used sensitive, on-board accelerometers and wheel speed sensors to measure just how much acceleration and braking traction each tire had. These tests were conducted on groomed snow and clear asphalt surfaces, and averaged multiple samples to eliminate any variability in conditions.
Objective test scores are percentage based, using a popular Ultra High Performance All-Season tire as the reference (Michelin Pilot Sport A/S Plus scored as 100%). A score above 100% indicates performance better than the reference tire, while a score below 100% indicates performance below the reference.
Snow Acceleration Traction without Traction Control
This test measured each tire's wheel spin during hard acceleration without traction control or throttle modulation by the driver. An on-board accelerometer measures the longitudinal force as a wheel speed sensor measures slip at the drive wheels. Scores were generated by graphing acceleration force over wheel spin, and calculating the area under the curve for 9-60% wheel spin.
Review of the data reveals all of the Ultra High Performance All-Season tires we tested achieve peak acceleration force at relatively low wheel spin levels. The traction control system in most vehicles limits wheel spin to between 10% and 15% which works well to maximize the limited acceleration traction available from Ultra High Performance All-Season tires, but is below the point of maximum acceleration we found for dedicated winter / snow tires (see "Winter Testing at the Arctic Circle: Studless Ice & Snow" and "Winter Testing at the Arctic Circle: Studdable Winter / Snow"). Shutting off the traction control and really spinning the All-Season tires won't help, either, as the acceleration force of several of the tires tested falls off by as much as 35% as wheel spin increases beyond the peak in acceleration force.
We also noted a correlation between the data shown in the Traction Force graph and how well each tire performed during the subjective Snow Handling evaluation. In addition to having the highest peak acceleration, the graph of the Continental ExtremeContact DWS shows a well-rounded area at the peak as well as very little drop in acceleration force as wheel spin increases. And in turn, this tire proved the easiest to drive during the subjective Snow Handling evaluation. The Goodyear Eagle F1 All Season was similar, just at a lower level than the ExtremeContact DWS. The tires that were the most challenging to drive in the subjective Snow Handling evaluation show the lowest peak acceleration levels, as well as an abrupt drop in acceleration force as wheel spin increased past 10%.
Snow Acceleration with Traction Control
This test measured the time required to accelerate from a standstill to 12mph using the vehicle's traction control system to manage wheel spin.
Snow Braking with ABS
This test measured the distance to stop from 25mph on a groomed snow surface, using the vehicle's Antilock Brake System to control wheel lock up.
Snow Handling Lap
This test compared the time required to cover the 1.1mile snow handling course. The test was conducted with the vehicle's stability and traction control switched off. ABS brakes were used for consistency and safety.
Dry Road Braking with ABS
This test measured the distance to stop from 50mph on a dry asphalt surface, using the vehicle's Antilock Brake System to control wheel lock up.
Wet Road Braking with ABS
This test measured the distance to stop from 50mph on a wet asphalt surface, using the vehicle's Antilock Brake System to control wheel lock up.