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Law and Order Tests Police Tires, Part 1 of 10

Written by Law and Order Staff

It has been eight years since the NIJ-NLECTC tested police tires. That was long before any 17-inch (Ford) or 18-inch (Dodge) police tire. In fact, it was long before the Dodge Charger. It was also long before most police fleet managers ever heard of Continental Tire or Pirelli Tire. While Pirelli tires were original equipment (OE) on the then-new Chevrolet Impala, and the Impala was one of its test vehicles, Pirelli tires were not tested. This was back when General and Firestone were OE police tires, and BFGoodrich was a major player.

This was also long before the 12-week strike at Goodyear in 2006, which left many fleet managers and police vehicle manufacturers alike wondering what tire to use. Ford had no other tire approved for the CVPI, and many fleet managers had never used anything other than Goodyear.

Funding issues and body armor priorities kept the NIJ-NLECTC from updating the police tire tests. So, LAW and ORDER stepped up to the plate. We teamed up with Tire Rack, the same tire experts that enthusiast car magazines like Car & Driver turn to for tire testing.

We selected three OE police tires from Goodyear (Ford), Pirelli (Chevrolet) and Continental (Dodge); an aftermarket tire from Firestone designed for police use; and a low-cost, aftermarket replacement tire from General.

From Goodyear, the benchmark of law enforcement, the Eagle RS-A. The Eagle RS-A is clearly the standard by which all police tires are measured. It is OE on the Ford CVPI and Chevrolet Tahoe PPV, and it is available for the Chevy Impala and Dodge Charger. The Eagle RS-A is the known commodity in policing with a long and proven track record.

From Pirelli, the P6 Four Seasons. This is OE in the Chevy Impala and is available for the Ford CVPI.

From Continental, the ContiProContact. This is OE on the Dodge Charger and is available for the Ford CVPI. From Firestone, the Firehawk GT Pursuit. This is an aftermarket tire developed specifically for police use on the Ford CVPI and the Dodge Charger. From General, the AltiMAX HP. This is a lowest cost, lower speed rated aftermarket tire selected as an example of what happens when a tire not developed for one of the police platforms is pressed into police service.

We performed four very different tire tests: ABS braking on dry pavement, ABS braking on wet pavement, vehicle dynamics on a dry road course and vehicle dynamics on a wet road course. On the road course, we also measured transient response through a cone slalom (wet and dry) and total corning g-force around a static circle (wet and dry). For each of these objective, instrument-measured areas of performance, we also asked each of the test drivers to fill out a subjective driver impression assessment for each tire. That gave us 16 data points for each of the five tires.

The test vehicles were 2009 Dodge Chargers powered by the 5.7L HEMI® V-8. One was used for wet braking and dry handling, the other for dry braking and wet handling. However, this is not a test of replacement tires for the Charger! This is a test of OE and aftermarket tires used on the Ford CVPI, Chevy Impala and Dodge Charger. The Charger was just the test vehicle. The results will help you make better tire selection decisions for all police sedans.

Braking Tests

The ABS brakes and electronic stability control are only as good as the tire. ABS modulates the brake line pressure to prevent the tire from skidding on the pavement. But it is the traction between the tire and the pavement that gives the feedback to the ABS system. ABS does not give the tire more braking power; it just makes the best use of the tire’s traction. Tires with better traction stop the car sooner.

To warm up the brakes in a consistent way and to conduct the stopping distance tests in a consistent way, each set of tires made nine braking events, counting the warm-ups, i.e., nine laps around the course. The driver exited the outside oval, stabilized the speed at just above 50 mph, got the suspension leveled out from the curve and applied the brakes.

On the first run, the brakes were given about half pedal effort. The car continued around the oval and about one minute later, the second braking was done. This was a full ABS-activated stop. This was repeated again to complete the warm up process. The car continued around the course for the next run, which would be a full ABS stop and the distance recorded. The stopping distances were measured by a V-box (Vericom). The driver got the car up to a speed over 50 mph and simply braked hard. The final result was an average of six ABS-activated stops from 50 mph at about 1-minute intervals.

As the braking tests were under way, each of the stopping distances was compared to the one before it and the one after it. This was intended to be a test of the tires, not the brakes. We wanted to be certain the brakes were not building up heat, i.e., a gradual increase in stopping distances. This was not the case. The 1-minute cool down and lap around the track between measured braking was enough to cool and stabilize the Charger’s brakes.

The Charger’s OE brakes kept up with the 50 mph to 0, full ABS stops, separated by 1 minute. The average g-force deceleration from the first stop to the sixth stop for each tire was consistent. The slightest increase in stopping distances during the fifth and sixth stops were due to the all-season tires heating up, not Charger brake fade, according to both Tire Rack and Firestone officials.

Why not do the test from 60 mph? Two reasons. First, the braking distances from some lower speed-rated tires under wet conditions at 60 mph were longer than the straight section of the Tire Rack course. Second, every brake test done by anyone differs in the brake warm-up-heat soak procedure and road surface coefficient of friction, even if the initial stopping speed is the same, i.e., 60 mph. Due to these test differences, you could not compare these results to brake tests annually performed by the Michigan State Police or the Los Angeles County Sheriff, even if the initial stop speed was 60 mph.

Second, the results are not accurate when you attempt to extrapolate the stopping distance from the 50 mph actual initiation speed to a 60 mph theoretical speed. Also of note, due to test condition and surface coefficient of friction differences from one test to another, it is not possible to compare the absolute values of one stop test to another, such as those performed annually by the Michigan State Police or the Los Angeles County Sheriff, even if the initiation speed of this test was at 60 mph.

All the braking tests were done on the same road surface, by the same driver and vehicle, following the same warm up-braking sequence and braking from the same suburban highway speed. Brake force tests were conducted first on wet pavement and then later on dry pavement. What makes these results valid and allows these tires to be directly compared is the consistent test procedure. Standard test methodology was employed during our test to account for the small variables in temperature and surface during the course of the testing.

Handling Tests

The Tire Rack road course is designed to test tires, of course. Tires are only tested when entering and exiting curves and during the curve—not on straights. So Tire Rack’s road course is all about turning. Even the extremely short straight used for the braking test was converted to a cone slalom during the handling tests.

The course driven for the dry handling was exactly the same as for the wet handling. The only difference was the number of laps. In the wet handling, each driver made three laps for the average. For the dry handling, each to minimize the effects of brutal tire wear, each driver made two laps through the course for the average.

The total average time around the road course gives us the best overall measure of the tire’s performance. Obviously, the driver is a factor in lap times. The skill (consistency) of the drivers, the blind test protocol, the use of two laps around the course, warm-up laps to get familiar with the course and the progressive up and down sequence of wet testing and the repeated control used during dry testing all minimize the human factor.

In addition to the total lap time, the time through just the cone slalom was also recorded. This gives us the transient response of the tire, i.e., a sudden evasive movement or accident avoidance maneuver. The time around just the static circle was also recorded. With a standard calculation we can calculate the maximum lateral g-force the tire can generate before losing traction. This is the maximum cornering power of the tire, which is not dependent on driver skill.

What is a Tie?

If one tire is 1/10th second faster around a 31-second road course, is that a “real” difference? In any ranking, we want to know if we have a true 1st Place and 2nd Place. Or, realistically, do we have a two-way tie for 1st Place? Or even a three-way tie?

Math wizards look for “statistical significance.” We could have used a statistical calculation with an associated confidence level, which virtually no one without a college degree in math would understand. Instead, we opted for a simple percentage difference to separate places and determine ties.

On one hand, the extreme spread in the dry testing was a little less than 3% on the handling course and about 5% on braking. The extreme spread in the wet testing was a little more than 3% on the handling course and a little less than 7% on braking. This means, all the tires were within 3% of one another on overall handling, and within 6% on braking, wet and dry.

On the other hand, selecting just a 1% difference in the results to determine separate ranks seemed a bit tight. In reality, two tires, just 1% different, will seem identical to even a very skilled driver.

So, we selected a 2% difference to be the deciding factor. At 1% difference, the tires seem the same. At 3% difference, all the tires are in a tie for 1st place. At 2%, we could draw a distinction between what both the stopwatch showed and what the drivers felt. And 2% is, in factor, more than half of the extreme spread. For this test then, a “tie” means the differences in performance between any two tires is less than 2%. Or, the different ranks (1st, 2nd, and 3rd) are separated by more than 2%.

Most people would expect bigger differences among the tires in wet performance and smaller differences in dry performance. That is just not true. The road course times, slalom speeds, cornering force and braking distances in the wet had an extreme spread between 3% and 7%, depending on the event being timed. The most variation was in the g-force around the static circle. In the dry, the extreme spread ranged from 3% to 12%. The most variation was in responsiveness through the slalom.

Results

Blind tests. Door placard inflated, properly broken-in tires. Experienced EVOC instructors. Precision measurement equipment. Standard asphalt road surface. Wet track testing. Dry track testing. Three different drivers. Four objective test parameters. Four subjective test parameters. Industry-accepted subjective test methods. Widely respected test facility and staff. Tests monitored by officials from three major tire companies. So, what are the results? Was there a “winning” tire?

When the wet and dry, objective and subjective results are all factored together, giving equal weight to all four test protocols, one tire did win. The clear, stand-alone overall winner of the 2009 police tire tests was the Firestone Firehawk GT Pursuit. This is available in sizes to fit the Ford CVPI and the Dodge Charger.

The second place overall tire was just as clear, the Continental ContiProContact. The ContiPro-Contact is not as widely or easily available as the Firestone tire, but the ContiProContact gives very good performance, wet and especially dry.

The third place tire was the Goodyear Eagle RS-A. The RS-A is the standard by which other police tires are put into perspective, and two other police tires solidly bested that standard. The Eagle RS-A is an aging tire in a game where the most recent technology, i.e., the newest tire, usually wins. The younger ContiProContact is better, and the almost new Firehawk GT Pursuit is better yet.

The fourth place tire was the Pirelli P6 Four Seasons…and not because we tested the H-rated version. Pirelli makes some true high-performance, all-season tires. However, these tests indicate the P6 Four Seasons, which is more of a touring, all-season tire, is simply not up to the performance of these Firestone, Continental and Goodyear tires.

The fifth place tire was the General AltiMAX HP. We selected this tire to show what may happen if just any ol’, low-cost retail tire is put on a police vehicle. The “generic” tire tested by the NIJ-NLECTC in 2001 came in dead last. Our tests confirmed the organization’s bleak results with an entirely different retail-oriented tire, which came in dead last.

What if your jurisdiction has an extremely wet climate? The best tires for the Seattle kind of wet weather happen to be exactly the same as the best overall tires: first, and by a wide margin, the Firehawk, then the ContiProContact, then the Eagle RS-A. The best wet weather tire may be one of the better melted snow tires. Yes, temperature will also have an effect on traction.

What if your jurisdiction has an extremely dry climate? The best tires for the Phoenix kind of dry weather are a tie between the ContiProContact and the Eagle RS-A, with the Firehawk dropping back to third place for a dry-only patrol area.

The overall placings were equally weighted between wet and dry results, and between subjective and objective results. The placings were also equally weighed between the various sub-categories of performance, i.e., tire response in slalom, tire cornering around static circle, braking distances, and total lap times. The reason? Panic stops, for example, may not necessarily be covered by just looking at overall lap times.

Making Decisions

Tire tests are all about making decisions. The tire is one of the most complex compromises on the police vehicle. You cannot gain in one area without giving up in another. And as these tests proved, all tires perform differently. They are not all the same. A tire is not a tire. Some tires perform BETTER than others, even though they don’t “feel” like it. Some tires perform WORSE than others, even though they don’t “feel” like it.

If you take one tire off a patrol car and replace it with a different tire, you WILL change the braking and handling characteristics both in the wet and in the dry. By switching to some tires you will extend panic stopping distances and make the car harder to handle under emergency driving.

You may not care that one tire is one second faster around a road course than another. However, in an ABS-activated panic stop from just 50 mph, there is a 5-foot difference between tires. That is the length of a hood, clearly the difference between a crash and not, just by selecting a low-performance tire.

Put another way, that is about the average difference between stopping on dry pavement and stopping on wet pavement. Pick a low-performance tires, and it is like panic stops on wet pavement all the time.

Are all V-rated tires about the same? No. A higher performance tire, V-rated designed for the police platform does better than a touring class, V-rated tire. But even among high performance, police-oriented tires, differences exist. Are H-rated tires about the same as V-rated and W-rated tires? No. Does the tire technology required to make 18-inch tires make all 18-inch tires equally good? No.

Can you replace the OE tire on the Ford CVPI with another tire and improve performance? Probably. The Firehawk GT Pursuit and ContiProContact both offer more performance. Departments using the Ford CVPI should seriously consider the Firehawk GT Pursuit as a replacement tire. At least get a couple of sets to begin wear testing.

Can you replace the OE tire on the Charger with any other tire and realistically improve performance? Not really, and definitely not unless you select the Firehawk HP Pursuit. If you select something else, you are likely to hurt vehicle performance. Those departments with the Dodge Charger would do just as well to continue to use the ContiProContact. However, if for local availability or bid price reasons another tire is needed, begin with the Firehawk GT Pursuit. Again, get a couple of sets for tire wear testing. After the Firehawk GT Pursuit, consider the Eagle RS-A.

Can you replace the OE tire on the Chevy Impala with another V-rated tire and improve performance? Yes. The ContiProContact and Eagle RS-A both appear to be better choices. The Firehawk HP Pursuit is tire is not available for the Chevy Impala. Only a H-rated Firehawk GT (not Firehawk GT Pursuit) is available for the police Impala that requires a V-rated tire.

If in doubt, should you put the OE tire back on? Probably. However, this test shows improvements for the Ford CVPI, and definitely the Chevy Impala, are possible with prudent tire selection. The purposes of this test are to point toward some tires as good options to start your own in-service wear tests. And to point away from other tires. And to say, in some cases, leave well enough alone. Each of the test tires, will be on display at TrackTech during the 2009 Police Fleet Expo. (www.policefleetexpo. com)

Disclaimer: Tire Rack provided all the objective test data and moderated all of the subjective driver impression rankings. However, all of the conclusions, rankings and methods of ranking and rating are the opinions and decisions of LAW and ORDER, and not necessarily the position of Tire Rack Inc.

Published in Law and Order, Aug 2009

Rating : 10.0


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