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GM Tire Testing
In the late-1960s, in GM’s Milford Proving Grounds began testing tires well beyond that routinely done by the tire companies. By 1972, this extensive battery of tests grew to a 2-inch thick, Tire Performance Criteria (TPC) manual. Any tire on a GM passenger vehicle had to meet its TPC specs, in addition to the individual standards set by the major tire companies and the industry standards established by the Tire and Rim Association.
Tires are a complex compromise of wet and dry performance, ride and comfort, noise, fuel economy and wear. With a retail tire, a great deal of emphasis is placed on ride and comfort, noise and fuel consumption. With a police tire, the focus is on braking, high-speed operation and cornering with a special emphasis on shoulder wear. In fact, surviving 32 laps at Grattan Raceway (MSP) and a similar test on the Pomona Fairgrounds (LASD-LAPD) is so difficult that other aspects of police tire performance are traded off to get better outside shoulder wear.
The tire approved for police use on General Motors vehicles is not just any all-season radial with a suitable speed rating. Compared with a retail-oriented all-season tire, the police package all season has a little better dry traction, better wet traction and much better cornering ability. Likewise, compared with a typical all-season tire, the police package tire has less initial tread depth, greater rolling resistance (less fuel economy), heavier weight (more unsprung mass), a stiffer sidewall (stiffer ride) and much less snow traction.
In the case of a police tire on a FWD car, i.e., the Pirelli P6 Four Season on the Chevrolet Impala, the tire must have even more abrasion-resistant tread with even more solid tread blocks. The tire must do both the most of the cornering and also provide the most traction during both acceleration and braking. The average, retail, speed-rated, all-season tire will definitely not perform as well on the police Impala as the Pirelli P6.
While a tire developed for a RWD police sedan (Eagle RS-A Plus) may physically fit on the Impala, it was not designed for these dual duties demanded of an FWD police sedan. A tire designed for an FWD police sedan has a different Tire Performance Criteria than one designed for a RWD police sedan.
When the Tire-Wheel System group is done, the tire that meets TPC specs will operate at the top speed of the vehicle for more than a tank of gas. Such a tire at recommended loads and tire pressures, operated under normal ambient conditions, will last at top speeds longer than any pursuit.
Catastrophic tire failure? This occurs at LOW tire pressures, not high ones. Tires suffer a sudden “blow out” or carcass failure when driven at high speeds after neglect, i.e., overloaded and under pressured. The failure mode at HIGH pressures is the tread “chunking,” i.e., throwing off pieces of the tread block, especially new tires with deep tread.
The GM battery of tire tests includes dozens of both industry-standard and GM-proprietary tests...force and moment testing (handling and cornering), residual aligning torque (pulls and leads), rolling resistance (this is 30% of fuel economy), braking and acceleration traction, snow traction, wet traction, uniformity, noise, load carrying, weight (mass of tire, where less unsprung weight is better), revolutions per mile, high speed capability, air retention properties, electrical resistance (the tire is supposed to conduct electricity to ground the static charge), tread wear and grooved road wandering (on roads with ¾-inch grooves to lessen aquaplane and reduce noise).
On-car testing includes extensive ride and handling tests and also snow and wet handling on special test tracks with controlled road surfaces and water depths. The centerpiece of the GM tire testing program, however, is their proprietary Accelerated Tire Endurance (ATE) test.
Unique to GM, the tire is tested at maximum load, at certain tire pressures, at various speeds and road surfaces for a high number of miles. It must complete this harshest of all tests without a tire failure. The ATE simulates a “life of tire” test. A tire that earns the GM “TPC” number molded into the sidewall may have gone through the industry’s most severe and complete testing.
The validation testing on the Pirelli P6 Four Season used on the police package Impala resulted in a 1-inch thick binder of documents and test results. The real question is not, “How does GM test police tires?” Instead, “How does the police department test tires before putting something different than the OE tire back in the car!” Keep that in mind when you order a set of cheap retail all-season tires from the local Tires “R” Us.
Bruce Wiley is the product and marketing manager, Law Enforcement Programs, General Motors Corp. Fleet & Commercial Operations. He may be reached via firstname.lastname@example.org.
Published in Police Fleet Manager, May/Jun 2006
Rating : 8.0
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