Normal patrol driving requires relatively little from a tire. Most corners are taken slowly, starts and stops are usually gradual and speed is not particularly high. However, high speed pursuit and emergency driving, with many similarities to road racing, require the tire to be capable of withstanding high speeds and severe hard cornering, quick steering inputs, and both hard acceleration and braking.
It is important to know that all tires are the result of a number of compromises or trade-offs made by their manufacturers. No tire is the top performer in every category. Three of such compromises are: tread design (contact patch surface area), rubber compound and sidewall stiffness.
Traction is a direct result of friction between the surface of the tire and the road surface. The greater the tire surface area where the tire and the pavement meet— the contact patch— the more friction will be produced and ultimately the better the traction. The tire with the greatest surface area is a racing slick, which has no grooves or tread. Slicks are used in racing because they provide the greatest traction on dry pavement.
A slick tire produces almost no traction on wet, snowy or icy roads. Consequently, tire manufacturers have devised various tread designs, or grooves in the tire surface, that help to move water or snow away from the tire so it can still grip the road surface. Of course, each groove or tread cut into the tire surface reduces the surface area of the tire, thereby reducing the traction available on a dry road. In other words, some dry traction is sacrificed so the car can still be driven when it rains or snows.
Another compromise has to do with the rubber compound used to make the tire. A harder rubber compound will provide longer wear and lower operating costs but will not handle as well as a softer, stickier rubber compound. Of course, the softer compound will handle better but will not last as long. Clearly, a compromise is necessary to reach an acceptable balance of wear and handling.
A third example is related to precise handling versus ride quality. A tire with relatively stiff sidewalls will tend to react very quickly to steering inputs and will generally provide very precise handling. Such a tire will also usually produce a significantly harsher ride than one with softer sidewalls that might be a little less precise. The tire manufacturer must balance the importance of ride quality against precise handling. Ultimately, it’s a matter of balance.
Different tires produced by the same manufacturer, as well as those from competing tire manufacturers, may have significantly different performance characteristics because their designers have made different decisions regarding where the compromises should be made. This does not necessarily make any of them bad. Any given tire might be the best one to use in a specific application or under certain conditions.
The shape of the contact patch has an effect on handling and drivability. Here again, compromises or trade-offs have to be made— this time by the purchaser. For example, a wide tire will provide more resistance to lateral slippage than a narrow tire and thus should provide better dry pavement cornering. The down side, however, is that the wide tire will also be more prone to aquaplaning on wet roads than the narrow tire, which is better able to slice through water puddles. There are numerous such positives and negatives to each type of tire. Generally speaking, the police vehicle manufacturers have carefully selected the size and aspect ratio of the tires supplied as original equipment on their cars. Come tire replacement time, significant changes in either width or aspect ratio should be avoided.
The original equipment tire size currently found on all three of the police package cars offered by the American automobile manufacturers is P225/60R-16. Taken in its individual parts, this size designation can be interpreted as follows: P designates PASSENGER car (LT would designate Light Truck); 225 indicates the width of the tire in millimeters, measured at the widest point of the sidewalls, not the tread; 60 designates the aspect ratio of the tire—the ratio of the height to width— which in this case, the sidewall height is 60% of the width; R indicates that the tire is of radial design, as opposed to bias or bias-belted; 16 is the diameter of the wheel rim in inches.
The tire size designation is often followed by a number— the load index— and by a letter— the speed rating of the tire. The most common load and speed designation on current police tires is 97V. In this case, the 97 indicates that each individual tire is capable of supporting 1609 lbs. at the maximum inflation pressure indicated on the sidewall. The V shows that the tire is capable of safely sustaining speeds of up to 149 mph under perfect conditions. Speed Rated Tires for Police Cars
Some fleet buyers, and even some police administrators, ask why speed rated tires are important for their police patrol vehicles. They often cite their agencies’ no pursuit policy, or limited patrol areas and generally low speeds as reasons why they should not have to provide their officers with speed rated tires. Clearly, there are many agencies whose patrol vehicles will never approach the 130 mph limit of H rated tires, let alone the 150 mph for V rated tires. It is important to understand, however, that speed rated tires are superior in a number of ways not specifically related to the speeds to be driven. The design parameters required to construct a tire that is capable of withstanding high speeds will nearly always result in improvements in other characteristics as well. Due to the internal design of a speed rated tire, including the way the steel belts are laid in, they will generally exhibit better overall handling and cornering than a tire with a lower speed rating.
When vehicle manufacturers design patrol vehicle suspension systems, they attempt to optimize the way the suspension geometry and the tires work together to provide good, predictable handling under a variety of circumstances and road conditions. Patrol package vehicles are designed and supplied with speed rated tires as original equipment. Equipping a patrol vehicle with any tire that has a lower speed rating than was originally supplied with and recommended for the vehicle will result in less than the optimum design performance. In essence, it would be similar to purchasing high quality handguns for officers but supplying them with the least expensive ammunition that will fit in the gun, with no regard to performance.
Selecting the Best Tire
Because of the compromises discussed earlier, there is no simple, black and white answer as to which is the best tire for the patrol vehicle fleet. Any tire is a series of compromises that make it better in one aspect, and usually correspondingly worse in another. For that reason, it is critical to examine operating environment and performance needs very carefully before making a selection.
The vehicle manufacturer specifies the original equipment tires because they have proven to be the best all-around tire for the greatest variety of conditions. Obviously, if the agency is located in the Desert Southwest where rain is extremely infrequent, a tire that does best on dry pavement surfaces may be a better choice. If, however, the agency is located in the Pacific Northwest where wet roads occur much more frequently, selecting the best tire for wet pavement handling and braking may be the wisest choice.
In addition, there are differences in the way different vehicle makes and models perform with any given tire. The same tires sometimes perform differently on the Ford Police Interceptor than on the Chevrolet Impala 9C1. The key is in reviewing the test results available for the various tires offered for law enforcement service and applying them to a situation. Select the tire that performs well on the vehicle makes and models in the fleet and under the road and weather conditions most common, and for the patrol situations most often encountered.
Police Tire Tests
Over the past nine years, the National Law Enforcement and Corrections Technology Center (NLECTC), a program of the National Institute of Justice, has conducted four separate evaluations of tires specifically marketed to law enforcement. The most recent of these studies was performed in the summer of 2001. This evaluation involved a 2001 Chevrolet Impala and 2001 Ford Police Interceptor. Both were full police package vehicles, supplied by the Chevrolet Motor Division of General Motors Corporation and by Ford Motor Company, respectively.
All major tire manufacturers were invited to submit test tires for evaluation. For the most recent evaluation, four tire companies submitted samples. The following tire models, which are marketed by their respective manufacturers for use in police patrol service, were tested: BF Goodrich Touring T/A VR4, Firestone Firehawk PV41, General XP-2000 V4 and Goodyear Eagle RS-A. Each of these tires carries a V speed rating and has a load capacity of at least 97. All tires were tested at 35 psi cold.
In addition to the police tires, a generic, non-police, S speed rated tire was also evaluated to determine whether the police-spec tires were actually superior. This generic tire, identified only as Brand X, was tested in response to numerous inquiries from law enforcement agencies about the appropriateness of installing regular passenger car tires on their patrol vehicles. This non-police tire was selected at random from a local tire supplier in the area where the tests were performed.
The manufacturer of the Brand X tire does not market or make any claim that the tire is appropriate for use on police vehicles, or in police service. For this reason, the tire is not identified by brand or manufacturer name. The results presented for the Brand X tire may be used to draw generalized conclusions about the performance of tires designed for normal passenger car service when compared to tires specifically designed and engineered for use in police service. The load range of the Brand X tire was comparable to the police tires listed above. However, it carried an S speed rating, which is somewhat lower than the police tires.
The NLECTC tire tests involved the following performance tests: dry 200-foot diameter static circle, dry 100-foot spacing serpentine, dry stopping distance on asphalt, dry pavement vehicle dynamics (high-speed handling), wet static circle with a half inch of standing water, wet serpentine, wet stopping distance and tire wear (measured before and after the entire test sequence). Again, it is recommended that agencies attempt to identify those tests that most closely approximate the actual patrol situations most often encountered in their area.
Reviewing the results of the tests on the two cars shows the importance of tailoring tire selection to an agency’s needs. Looking at each individual test in each test condition it becomes obvious that each tire has some strengths and some weaknesses. With the exception of the Brand X tire, each of the tires tested either did best, or tied for best on at least one test on one car.
The raw results appear to all have equal importance or weighting. Clearly, the results of some tests may have more importance or applicability in meeting a given agency’s needs than others. For that reason, no overall winner is indicated. It is left to the individual agency to weigh the results to best apply to its local situation and vehicle. The best tire under wet conditions may not necessarily be the best tire under dry conditions. The best make and model of tire for Crown Victoria under one set of conditions may not necessarily be the best for the Impala under the same conditions.
The results from the influential high-speed handling test emphasize this point. Around the 1.43-mile road-racing course, the Crown Victoria produced the fastest lap times with the Goodyear Eagle RS-A. With the Impala, the road course results were a statistical tie between the BFG Comp T/A and the Firestone Firehawk PV41.
Wet versus dry results also emphasize the point. The best wet serpentine performance on the Crown Victoria came with the BFG Comp T/A, while the best dry serpentine performance on the Ford came from the General XP-2000 V4. With tires, the whole does not always equal the sum of the parts. Both the best dry serpentine and dry static circle results came from the General, but when serpentine and static circle are added up in the form of a road course, the Goodyear came out on top with the Ford.
Brand X Results
The performance of the Brand X tire, which represented any good quality, lower speed rated, non-police tire, could best be described as sub-par. In one test, on one car, it ranked fourth. On all of the other tests it consistently ranked fifth of the five tires tested. The best that could be said about it is that the tire didn’t fail during the testing and was able to finish the evaluation. The performance of both the Crown Victoria and Impala suffered when using the Brand X tire. It was obvious that the tire, which would probably be very well suited to normal everyday driving on a civilian car, was not up to the job for police patrol service.
The latest report is available free of charge from the NLECTC, P.O. Box 1160, Rockville, MD 20849-1160, phone 800-248-2742 or www.justnet.org.
Curt Vandenberg is the president of Independent Testing and Consulting, the subcontractor who performed the NLECTC tire testing, and a retired 1/LT with the Michigan State Police. He may be reached at firstname.lastname@example.org.