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Law and Order Tests Police Tires, Part 3 of 10

Written by Law and Order Staff

Tire Testing Procedures

Tire Rack officials have the ability to precisely and completely wet their road course by the use of high-flow sprinklers. The wet surface includes pools of water on the road course. The water is not as deep as the tread grooves on a brand new tire but like a typical rain shower is deep enough to cover much of the visible road surface texture. This was not a hydroplane test; it was a wet traction test.

During wet testing, sprinklers were used around the entire course to keep the road surface uniformly wet. This was the case even though it rained almost all day during the wet braking phase. The track at Tire Rack is the standard road mix asphalt used in the area. This was not a special tire testing surface, either smooth or rough.

Prior to conducting the tests, the brakes were burnished on the Dodge Charger, following the Owner’s Manual procedures. The police Charger was run the entire time with the Electronic Stability Program (ESP) in the partial-off mode, not the default, full-on mode. The full-on ESP mode is a very good thing. Other police cars should have stability control, and will, have it by law by 2011.

Until all fleets have transitioned to vehicles with ESP, the reality is that most officers are driving without it, so it made sense to test as close to this reality as possible. Full-on ESP can mask tire traction and handling limitations somewhat, having an impact on objective measurements and often a larger impact on subjective impressions.

Using the partial-off mode allowed our drivers to get a better feel of each tire’s potential and limitations. Due to the reactionary intervention of the ESP system, the trick for the drivers was to drive the Charger right up to the point of impending partial-off ESP activation, but not so hard that it did activate. ABS braking was left fully functional throughout the test.

ESP can somewhat mask the tire’s objective potential in that it is reactionary. It simply senses a slip or slide and reduces power or realigns vehicle trajectory. The driver has the advantage of knowing what is coming next, and can anticipate the result of the slip, sometimes to his advantage in lap time. So, ESP can slow lap time somewhat. It tends to have a bigger impact on subjective feel, as it takes away the slip and slide at the point of the corner or brake zone that the driver needs the feedback the most to form his opinion.

However, before the tires get a real workout, the full-on (default) position comes on, and counter measures are activated. In a patrol setting, that is how it should be! For a tire test, however, ESP masked the potential of the tire. The partial-off mode allowed each tire to perform closer to its limit.

All the tire testing (wet, dry, braking, handling) was done with the tire pressure set at the placard level. In this case, the door frame placard on the 2009 Dodge Charger reads 35 psi, front and 35 psi, rear.

The handling tests started with 30 minutes of course familiarization using a BMW 328i. This was followed by three laps for each driver in the Charger using the “control” tires. All three drivers were competitive and auto enthusiasts by nature. They were EVOC instructors or racing competitors, or both. Yet none had any experience in the police Charger. This, again, put everyone on equal footing. It is not like one of the drivers raced, taught in or patrolled in the Charger. Excellent drivers, equally unfamiliar with the HEMI Charger.

Published in Law and Order, Aug 2009

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