The Precision Driving Unit of the Michigan State Police recently teamed up with the National Institute of Justice to conduct an exhaustive test of police-oriented tires. The test involved tires from Goodyear, Firestone, Nokian, Cooper, Nitto and Pirelli. The test vehicles included the Dodge Charger; Chevrolet Caprice, Tahoe and Impala; and Ford CVPI.
The MSP developed a list of tires to test based on input from automotive engineers from Chrysler, Ford and General Motors, in addition to their own years of experience. Of the selected brands, only Goodyear, Firestone and Pirelli make tires used by these automakers as “Original Equipment” on their police package vehicles. Cooper, Nitto and Nokian indicated to the MSP that their tires were “suitable” for police applications. However, these last three do not conduct police-specific durability testing. 12-Stage Testing
The tire testing program was a 12-stage process conducted at the MSP Precision Driving Facility in Lansing, Mich. The tire testing program was conducted using a series of vehicle tests to assess straight-line stopping distance, maximum lateral acceleration in a steady state turn, wet Jennite® stopping distance and braking in a turn on wet asphalt.
The tires were tested in both new condition and in half-worn condition. Half-worn, in this case, means after being driven 100 laps around a one-mile road course simulating pursuit or emergency driving conditions. All vehicle tests were also conducted with the original equipment (OE) tires for comparison purposes. A Ford CVPI with new tires was used as a “control vehicle” for the purpose of monitoring any changes in the test surfaces over the course of the two-week testing program.
The dry asphalt stopping distance testing evaluates the compatibility of various replacement tires offered for police service with each vehicle’s brake and chassis control systems. Likewise, the wet Jennite stopping distance tests simulate the compatibility of the replacement tires in lower coefficient road conditions. The braking-in-a-turn (BIT) testing on wet asphalt provides the basis for evaluating the compatibility of the various replacement tires with the vehicle chassis control systems including anti-lock brake system (ABS) and stability control.
Finally, the steady-state, static circle, turning test addresses the ultimate, lateral traction and stability of the vehicles fitted with the various replacement tires. This same set of four testing protocols was also conducted on the tires after 100 laps of simulated pursuit or emergency driving to evaluate these same tires in a worn condition.
This testing protocol permits comparative assessments of various replacement tires offered for police service in each vehicle configuration evaluated, and also permits similar assessments of these same tires in a new versus worn condition. Therefore, relevant comparisons of performance between tires offered for the same vehicle will be addressed. In addition, comparisons of performance of the same tire in a new versus worn condition were compiled to assess the significance of this aspect of tire performance.
Measurements of tire wear at each vehicle wheel position were also quantified. Although not specifically relevant to ‘normal’ tire-wear life estimates, a directly comparable measure of tire wear by tire position and vehicle model was compiled from the testing program results. Results, Not Recommendations
The MSP does not make recommendations, or even offer an assessment of the results for any of the tire, brake pad or patrol vehicle testing they do in conjunction with the NIJ. Instead, they publish all of the results of all their testing in exhaustive detail. It is up to the individual police and sheriff’s departments across the U.S. and Canada to review the results, make an evaluation as it pertains to their unique needs, and then make decisions based on that agency-specific evaluation.
The tires from Cooper, for example, were run on three different police sedans – and they were disqualified in each case. They simply could not complete the test sequence. They wore past the trend blocks on the Impala and wore all the way into the cords on the Charger and CVPI. The Cooper tires may be available in a size that fits the police vehicle, but it failed the tests that more police-oriented tires easily passed.
Two very different tires from Pirelli performed quite well. The Pirelli P6 Four Seasons beat the Goodyear tire in every new tire test category (dry, wet, wear) on the Impala. In the half-worn condition, the Pirelli continued to either beat or tie with the Goodyear. It makes you wonder about the decision by GM to switch from the Pirelli P6 Four Seasons to the Goodyear Eagle RS-A on their police package Impala.
The other Pirelli tire, the PZero Nero, did well on the Ford CVPI. It performed midway between the Firestone and the Goodyear in nearly every test phase. On the other hand, the PZero Nero is a much more expensive tire than the comparable police tires from Firestone and Goodyear, and arguably less available. Nokian – More Than Snow
Nokian is better known by fleet managers in the snowy, northern states and in Canada than the southern part of the US. In snowy regions, Nokian is downright famous for their Hakkapeliitta winter tires. Nokian tires hold the world’s top speed record on ice. The best kept secret is that Nokian also makes All-Season tires – they list them as “summer” tires.
Even to fleet managers familiar with Nokian winter tires, it may come as a surprise that their “summer” tires were so competitive during the MSP tests. On both the Impala and the Caprice, the new Nokians out-performed the new Goodyears in four out of six categories. On the Ford CVPI, it was an even split. One of the great outcomes of the 2011 MSP/NIJ tire testing is to give well deserved exposure to the Nokian brand of tires. The tire experts in Finland clearly do summer tires as well as they do winter tires. Firestone Versus Goodyear
The big showdown, of course, was Firestone versus Goodyear. Direct, heads-up, no-holds-barred comparisons are possible from averaging the results from the Charger and the Ford CVPI. First of all, the Firestone Firehawk GT Pursuit and Goodyear Eagle RS-A are quite comparable. The Firestone has better dry braking and better steady state dry cornering. The Firestone has a lot better wet braking under a variety of conditions. (Virtually every new tire test shows this same wet traction result.)
On the other hand, during MSP testing, the Goodyear turned in slightly better lap times. However, the big deal was the tire wear. No one is saying the tire wear during this kind of abusive track testing is comparable to the tire wear under normal patrol conditions. However, it may be worth noting that the Goodyear had much less wear during the various test stages than the Firestone, like half the wear.
All that said, the Firestone versus Goodyear results from the Tahoe must be noted. The Firestone turned in better dry braking, static cornering and wet braking, and much better wet braking while turning. The lap time advantage with the Goodyear tires hides the fact that the MSP were unable to complete the Goodyear tests on the Tahoe due to excess tire wear. In comparison, the Firestone on the Tahoe had almost the least wear of the entire MSP test. Goodyear is obviously aggressively trying to resolve the wear issue. New Versus Worn
The MSP took an entirely new and novel direction with their 2011 tire testing. They tested the new, properly scrubbed-in tires, and then repeated all of the tests with 100 miles of track testing on the tires. In almost all cases, the tires after 100 miles were not half-worn. In fact, in 13 cases out of 17, the tires were less than 70 percent worn (average of four tires) after completing the entire series of tests. Of course, the testing on four of the tires was halted by the MSP due to excessive wear – all three Cooper tires and the Goodyear on the Tahoe.
Testing to failure is one thing – the tire either wore to the cords or started to chunk tread blocks. But how about performance changes on the way to being totally worn out? The real significance of these MSP tests is how the relative performance between tires may change as each wears.
To see examples of the performance change as it wears, the Cooper tire is clearly not the best example. Neither is Pirelli – one is too expensive to be relevant, and the other has been essentially made obsolete by GM. The Nitto and Nokian are just not familiar enough to be good examples. The comparison that is the easiest to relate is the Firestone versus Goodyear on the Charger and CVPI – the two most popular police sedans and the two cars running both Firestone and Goodyear for which the MSP has the full test results. Wear Affects Wet More Than Dry
In the dry braking stage, the Firestone had a slight advantage over the Goodyear when new. The Firestone kept that slight advantage when tested again in the worn condition. In each case, the stopping distances were about 10 feet longer for the worn tire, compared to the new tire.
In the wet braking stage, the Firestone had an advantage over the Goodyear when new. The Goodyear turned the tables for an advantage went tested worn in the wet. The same reversal happened during the wet brake-and-turn stage. The Firestone started off with a solid advantage when new. When worn, the Goodyear took over the same solid advantage.
The final test of half-worn tires was the steady state circle, which shows the maximum cornering force the tire is able to give under ideal test conditions. In this maximum g-force test, the Firestone held a clear and solid advantage over the Goodyear when both were new. The Firestone held that same clear and solid advantage when both were half-worn.
The key to understanding the new versus worn comparisons is remaining tread depth, and its effect of performance. Remember, the tires had the same test track miles on them, but in some cases very different tread wear. For example, the entire testing process wore an average of 60 percent of the Firestone away, but only 32 percent of the Goodyear.
That means that performance not related to tread depth (dry braking, dry cornering) would not change much. It also means performance directly related to tread depth (wet braking, wet brake and turn) could change a great deal. The Firestone gave up a lot of wet braking performance as it wore because it was more worn as-tested. The Goodyear changed much less in performance, new versus worn, simply because it wore less on the Charger and CVPI during the track testing.
This raises the obvious request for apples-to-apples testing: both tires with exactly the same percent of treat left, say 50 percent, or perhaps, both tires with the tread depth exactly touching the wear bars. The “take-away” lesson from this MSP test stage is really that tire wear definitely affects wet braking and wet braking while turning. Of course, that is why tires have tread in the first place. Tires Are Not All Equal
Every major police tire test conducted in the past 20 years, including tests conducted by Police Fleet Manager with Tire Rack, and including this latest MSP test, have shown the same results: Just because the tire fits the rim does not mean it should be used on an emergency vehicle. The brands that are used as Original Equipment are better in every way than most aftermarket brands – those generic, low cost, “whatever” tires from the local discount tire store.
Just because the tire has the same speed rating as the OE tire does not mean it should be used on a pursuit-rated vehicle. Certainly, tires of lower speed ratings should never be used on police vehicles. The speed rating means more than top speed – it specifically relates to the internal stability, durability and integrity of the tire.
Finally, come replacement tire, you are looking for “High Performance All-Season.” While the All-Season label might mean all four seasons (but probably not winter), the Touring label really means summer-only.
Best bet? Put the OE tires back on. Second best bet? Select a tire that has done well in police-specific testing. The entire, 352-page report is available online at the MSP website. Just use “tire” as the search word. The Police Fleet Manager test results are also online. Go to www.hendonpub.com, click Resources, click Article Archives and search for “tire tests.” (Ed. Note: The complete 350-page MSP 2011 Police Vehicle Tire Evaluation is available online at www.michigan.gov/msp – search for “tire.” A summary of this MSP tire test is in the May-June 2012 issue of
Police Fleet Manager, which is also online at www.hendonpub.com, click Resources, click Article Archives and search for “tire.”)
Test Methodology Stage 1
- Record pertinent sidewall information and the measurement of new tire tread depth before testing began. Stage 2
- Condition the tires and brakes prior to the start of testing. Each candidate tire, and vehicle brakes were burnished during 10 60-0 mph full ABS stops. Each stopping distance is corrected to reflect 60 mph as the initial speed, thus providing a more accurate comparison of each stop. The dry asphalt coefficient of friction 0.85 was used. Stage 3
- Each vehicle and candidate tire conducted five 60-0 mph full ABS stops. In an attempt to eliminate brake temperature as a performance factor, a one-mile cool down was conducted between stops. Speeds at trigger of the measurement were adjusted to 60 mph. Stopping distances were adjusted to reflect changes in track conditions using the track index established by the control tire. Stage 4
- Each vehicle performed 10 35-0 mph full ABS stops on a wet Jennite (non-abrasive asphalt sealant) having a coefficient of friction of 0.35. Water is applied to the Jennite surface using large commercial irrigation sprinklers. In an attempt to eliminate brake temperature as a performance factor, a one-mile cool down was conducted between stops. Speeds at trigger of the measurement were adjusted to 35 mph. Stage 5
- Each vehicle and candidate tire performed 10 40-0 mph stops in a turn on wet asphalt. Water was applied to the road surface by a large irrigation sprinkler. Five stops were conducted traveling clockwise and five stops were conducted traveling counter clockwise. In an attempt to eliminate brake temperature as a performance factor, a one-mile cool down was conducted between stops. Speeds at trigger of the measurement were adjusted to 40 mph. Stage 6
- Each vehicle and candidate tire performed a steady state turn around a 300-foot diameter circle both clockwise and counter clockwise. The vehicle was driven to the point where it was unable to maintain its turn radius. The dual-axis optical sensor measured speed and lateral acceleration in relation to the force of gravity noted as “G” at point of departure. Vehicles having Electronic Stability Control (ESC) were run with ESC on, ESC partial off or ESC off depending on system functionality. Lateral acceleration figures adjusted to reflect track changes using the track index. Stage 7
- To complete the tire endurance and wear test, each vehicle and candidate tire was driven 100 laps on a one-mile road course: 50 laps clockwise and 50 laps counter clockwise. Each lap was driven in a manner to simulate emergency or pursuit driving. Every 10 laps the vehicle stopped in order to obtain temperatures of rotor and tires. Stage 8
- Repeat Stage 3 (dry braking) with worn tires. Stage 9
- Repeat Stage 4 (wet braking) with worn tires. Stage 10
- Repeat Stage 5 (wet brake and turn) with worn tires. Stage 11
- Repeat Stage 6 (constant circle) with worn tires. Stage 12
- Each candidate tire tread depth was measured at the conclusion of Stage 11. Measurements were initiated at the valve stem (considered Top Dead Center) moving clockwise to the 90-degree, 180-degree and 270-degree locations. Using the beginning and ending measurements, average percentage of tread consumed during the test was calculated.