In February 2004, United States and Canadian agencies participated in winter field tests at the Royal Canadian Mounted Police (RCMP) Depot to evaluate pursuit-rated winter tires recommended for use on the Ford Crown Victoria Police Interceptor.
The National Institute of Justice’s
National Law Enforcement and Corrections Technology Center (NLECTC)–Northwest organized the tests in collaboration with the Canadian Police Research Center
. The tests were conducted in February 2004 on snow-covered test-driving tracks at the RCMP Depot in Regina, Saskatchewan, Canada.
Participants also included Ford Motor Company, the US Army Cold Regions Research and Engineering Laboratory, and The Tekne Group, Inc. The RCMP Depot drivers training unit provided test vehicles and trained law enforcement driving instructors to operate the test vehicles. The RCMP Depot also provided logistical, mechanical, and test track preparation support.
Manufacturers were asked to participate and submit samples of tires for evaluation. The winter tires tested were all commercially available, nonstudded tires sized for the Ford Crown Victoria Police Interceptor (CVPI). All tires were certified as meeting the North American Rubber Manufacturers’
“severe snow standard” rating. Both H-rated and V-rated tires were used in the testing.
Five brands of winter tires were supplied by the manufacturers for testing: 1) Continental ContiWinterContact TS790, 2) Firestone Firehawk PVS, 3) Goodyear Eagle Ultra Grip GW2, 4) Michelin Pilot Alpin, and 5) Pirelli Winter 210 Snowsport. The Goodyear Eagle RS-A Plus, the All-Season, V-rated tire that comes with Police Interceptors as original equipment, was used as the control tire.
Temperature, wind, fresh snow, humidity, and solar load can all affect the track surface. Given the nature of winter testing a precise set of conditions is hard to predict and hold, so a control test was required. The control tires were run periodically throughout the test series to evaluate whether testing conditions had changed substantially. Using this tire as a baseline also allowed the evaluation of all-season versus winter tire performance.
From a tire manufacturer’s standpoint winter tires are designed with a given set of conditions representative of the tire’s optimum performance. Typically, the tires are designed for temperatures in the 5 deg F to 25 deg F range. It is important to note that the temperature during this test series was quite low, at times reaching –40 deg F. Tests conducted under warmer conditions may produce differing results.All-Season v Severe Snow
All-Season tires are a design compromise. As good as they are in most conditions, All-Season tires are not optimized for winter conditions in the same way that Severe Snow tires are optimized. Severe Snow tires have cold weather rubber compounds, channeling tread patterns, a large number of tread sipes for wet surface control, and an open tread block pattern for better deep snow traction.Test Methods and Results
Two 2003 Ford CVPIs from the RCMP fleet were used in testing. Each test run was conducted on a fresh portion of snow, moving across the test track. The test vehicles were equipped with data acquisition systems that recorded time, distance, and acceleration. Optical speed sensors were used instead of the traditional mechanical fifth wheel. Measurements made during the test series include acceleration time, braking distance, and maximum lateral acceleration.Straight Line Acceleration/ Deceleration Test
The objective was to determine acceleration time and braking distance. Acceleration time indicates the longitudinal acceleration capacity of the vehicle and is reported as the average time of 10 recorded values. Braking distance indicates the maximum stopping effectiveness of the vehicle and is reported as the average distance of 10 recorded values.
In this test, the driver accelerated from a standing start in a straight path as quickly as possible to just above 31mph, modulating the throttle to control wheel spin. When the vehicle reached that speed, the driver applies the brakes to decelerate as quickly as possible in a straight path down to 2mph, modulating the brake pedal to minimize wheel lock up or hard enough to engage the antilock brake system (ABS) at all four wheels. The test is repeated 10 times.
In this test, the Continental ContiWinterContact TS790 clearly produced the best combination of straightline acceleration followed by straightline braking. Conti’s Severe Snow tire was about 16% better than the All-Season control tire. The other four winter tires produced similar performance to one another, all about 10% better than the control tire. In this test, the clearest differences between a Severe Snow and All-Season tire can be seen.Turn-In or Slalom Test
The objective was to determine maximum lateral acceleration, which indicates the cornering capability of the vehicle. Results are reported as the peak lateral acceleration for each run and the average for the peaks for 10 recorded values.
In this test, while traveling at 31mph, the driver aligns the vehicle at approximately 45 degrees to the test surface. The steering wheel is straightened so the vehicle is following a straight path. The speed is kept at a constant 31mph throughout the procedure except when turning the wheel, during which the throttle should be held constant.
The steering wheel is turned approximately 90 degrees in approximately one second so that the vehicle starts to turn away from the side of the road to which it was initially pointing. If the front tires are not saturated (fully loaded with snow and no longer offer any additional traction), the driver increases the steering angle in small increments until they are at the saturation point.
The final steering angle (to be used on all subsequent test runs) should be high enough to saturate the front tires. The steering wheel is held steady. As the vehicle approaches the center of the track, the test continues with a turn in the other direction. The procedure is repeated five times for a total of 10 runs.
In this test, the Goodyear Eagle Ultra Grip GW2 clearly performed better than the rest of the tires, producing about 10% greater lateral acceleration than the control tire. The Ultra Grip was followed by the Continental ContiWinterContact TS790, which had about 5% greater lateral acceleration. In this test, the other three winter tires did not perform significantly better than the All-Season control tire.Significant Test Findings
All-Season tires are not a substitute for winter tires. Use a set of four, matched, winter tires for winter driving. As a cost-saving measure, some law enforcement agencies purchase only two winter tires for the rear of a rear-wheel drive vehicle. This mismatch of tires between front and rear can cause unstable vehicle handling and should be avoided.
This set of four matching tires is even more critical for those who put winter tires on the front of a front-wheel drive car, since less traction at the rear of any vehicle results in dangerous oversteer.
Another concern is the use of All-Season tires instead of Severe Snow winter tires, i.e., the belief that All-Season tires are a substitute for true winter tires. Tests determined a set of four, matched, Severe Snow-rated winter tires are best for winter driving.
Winter tires are specially designed with tread patterns and rubber compounds to deliver superior performance in cold weather or snow. In our test conditions, every brand of winter tire tested performed significantly better then the all-season tire supplied with the police sedan.
For further information on this testing, contact Bruce Richter, Deputy Director, Winter Tire Test Program Manager, NLECTC–Northwest, 3000 C Street, Suite 304, Anchorage, AK 99503–3975, Phone 866–569–2969 and email firstname.lastname@example.org.Changes Since The Tests
Since the NLECTC’s February 2004 testing, much has taken place. First, the Los Angeles County Sheriff’s Department has conducted high-speed, warm temperature tests on some Severe Snow-rated tires. This testing was performed to see how well a Severe Snow tire might handle and hold up under extended pursuit conditions. This is important since most police departments continue to run their winter tires well into early spring, when pavement temperatures can rise well above the ideal range for the Severe Snow compounds.
Second, Goodyear has redesigned the tread pattern for their Ultra Grip GW2 winter tire. The V-rated Ultra Grip winter tire are, in fact, based on the Eagle RS-A tires. The basic carcass construction is virtually identical. The GW2, then, is basically soft and high silica snow compounds and open snow tread patterns added to the RS-A tire. The new tread design was introduced in late-2005, and was obviously not tested by the NLECTC.
Third, Pirelli has introduce a new Severe Snow tire for police use, their Sottozero Winter 240. The Sottozero is an asymmetric, directional V-rated winter tire. This particular tire was tested by the Los Angles County Sheriff and, in fact, survived the 32-lap, 64-mile pursuit test. That is an achievement that some All-Season tires cannot do. The LASD and LAPD test drivers indicated the Sottozero had stable and predictable handling.