Driving rear-wheel drive vehicles in wintry conditions is an often unavoidable and sometimes hazardous requirement for those in law enforcement, and proper winter tire selection is an important tool in improving vehicle handling performance. In February 2004, U.S. 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 Centre (CPRC). The tests were conducted Feb. 2-6, 2004, on snow-covered test-driving tracks at the RCMP Depot in Regina, Saskatchewan, Canada. Participants also included Ford Motor Company, the U.S. 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. Winter tires tested were all commercially available, non-studded tires sized for the Ford Crown Victoria Police Interceptor. All tires were certified as meeting the North American Rubber Manufacturers' "severe snow standard" rating and also were rated for law enforcement driving speeds by the manufacturer. H-rated and V-rated tires were used in the testing.
The five brands of winter tires supplied by the manufacturers for testing were the Continental ContiWinterContact TS790, the Firestone Firehawk PVS, the Goodyear Eagle Ultra Grip GW2, the Michelin Pilot Alpin and the Pirelli Winter 210 Snowsport. The Goodyear Eagle RS-A Plus, an all-season pursuit speed-rated tire supplied on the Crown Victoria 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 manufacturers' standpoint, winter tires are designed for a given set of conditions representative of the tires' optimum performance. Typically, the tires are designed for temperatures in the -15 to -4 degrees Celsius range. It is important to note that the temperature during this test series was quite low, at times reaching –40 degrees Celsius. Tests conducted under warmer conditions may produce differing results.
All-Season Versus Severe Snow Standard Rated Winter Tires
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. Winter 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 Crown Victoria Police Interceptors 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 included acceleration time, braking distance and maximum lateral acceleration.
Straight Line Acceleration/Deceleration Test
Objective: 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.
Methodology: The driver accelerates from a standing start in a straight path as quickly as possible to just above 50 kilometers per hour (km/h) (31 mph*), modulating the throttle to control wheel spin. When the vehicle reaches a speed of just above 50 km/h (31 mph*), the driver applies the brakes to decelerate as quickly as possible in a straight path to 3 km/h (2 mph*), 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. Exhibit 1 shows the results of these tests.
* - rounded to the nearest whole mile per hour.
Turn-In or Slalom Test
Objective: 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.
Methodology: While traveling at 50 km/h (31 mph*), 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 50 km/h (31 mph*) 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. Exhibit 2 shows the results of these tests.
* - rounded to the nearest whole mile per hour.
Significant Test Findings
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-drive wheels of the vehicle. This mismatch of tires between front and rear can cause unstable vehicle handling and should be avoided.
All-season tires are not a substitute for winter tires. Another concern is the use of "all-season" tires as a replacement for winter tires. Tests determined a set of four matched "severe snow condition" 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 Police Interceptors.
Note: Equipment Performance Report: 2011 Patrol Vehicle Tires is available for summer tire evaluation.
For further information on winter tire testing of patrol vehicle tires, contact:
700 N. Frederick Ave.
Bldg. 181, Rm 1L30
Gaithersburg, MD 20879
Phone: (800) 248-2742
Fax: (301) 240-6730