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Pirelli Snow Tires

Written by PFM Staff

At the request of Ford Motor Company, prior to their annual vehicle testing, the Los Angeles County Sheriff’s Department tested a Pirelli V-rated, 17-inch snow tire on a Ford CVPI. This test was part of Ford’s ongoing brake pad development. The snow tire was Pirelli’s Sottozero Winter 240, a Severe Snow Service-rated tire, and embossed as such with the snowflake-on-the-mountain symbol.

Few would doubt the ability of a Severe Snow-rated tire to work in the snow. The real challenge, and the reason for the test in the heat of southern California, is how well the true snow tire performs as a pursuit tire in the hot and dry.

A day after a snowfall, the roads are dry and occasionally warm. Likewise, snow tires put on early in the season, or left on late in the season are driven on dry and warm roads. It is also common for new snow tires to be put on at the beginning of the season and have them remain on the car until they are down to the wear bars. In some cases, this is well into spring.

Can a Severe Snow-rated tire withstand lengthy pursuit conditions on warm, dry pavement? How does a V-rated Severe Snow tire handle under pursuit conditions compared to a regular V-rated police tire? Two concerns exist. Will the snow tire “chunk” or break off pieces of the tread block during hard driving? Will the snow tire be so different in handling that it causes the officer driving problems?

The LASD’s standard tire test is 32 laps, or 64 miles, around the road racing course used during their annual testing. This is followed by their standard brake test and the two-lap city pursuit course. Unlike a traditional race track with banked or off-camber turns, the LASD course is flat, laid out on a massive parking lot at the L.A. County Fairgrounds in Pomona.

Of the vehicle tests annually conducted by major police departments, this is the hardest track on tires. Just surviving the 64-mile run, with the tire still fully intact, is considered a “passing” score.

The car is driven in four 16-mile segments, pausing between runs just long enough to change drivers. Two EVOC instructors from the Los Angeles County Sheriff and two EVOC instructors from the Los Angeles Police do the driving.

The Winter 240 Sottozero, a Severe Snow-rated tire, is designed specifically for large sedans. The tire is an asymmetric design with two off-center and prominent grooves to minimize aquaplane in water or slush. The outside shoulder blocks are fairly large for good cornering in the dry. The tread blocks are filled with sips for grip on ice and packed snow. The tread blocks are not as open as some lower speed-rated snow tires, thus its performance in deep snow may not be as good, but it is Severe Snow-rated and it is V-rated.

The Pirelli Sottozero survived the 64-mile pursuit under Los Angeles conditions.

After 16 miles, the tires showed no significant wear. If a tire is going to lose a chunk of the tread block, it will be in the first few miles while the blocks are tall. After 32 miles, the outside front tire showed some feathering on the outside shoulder tread blocks. This feathered trailing edge is common on all tires...the front edge of the block is simply being scrubbed away.

After 48 miles, again on the outside front tire, the shoulder tread blocks and the blocks on the outer edge of both aqua grooves showed feathering. Finally, after 64 miles, but only the outside front tire, all of the tread blocks showed significant wear and feathering. However, there was no chunking whatsoever, and plenty of tread depth remained. The other three tires on the car showed minimal wear, and virtually no feathering at all.

Mechanically, the Pirelli Sottozero passed the test, a test that some all-season tires cannot pass.

From a handling standpoint, the tires were predictable but slippery. Some may think the soft, snow-oriented tread compound would be stickier and give better traction. While the compound is soft, it is also designed to be sticky on cold roads, i.e., less than 32 degrees F.

In the 90-degrees F heat of Los Angeles, the snow compound did not have the same dry traction as the factory original Goodyear Eagle RS-A. However, it was close. Two different Ford CVPIs, one with the RS-A tires and one with the Sottozero snow tires, were driven by the same instructor. On an 85-second course, the Severe Snow tire was about 1.2 seconds slower than the All-Season tire.

However, the EVOC instructors were quick to point out factors other than the tires that were part of this 1.2-second difference. First, the transmission shift points in the two Ford CVPIs were slightly different, i.e., the one with the RS-As kicked down to a lower gear at an especially important part of the course.

Second, due to driver rotation, the car with the Severe Snow tires always had 16 miles more on the brakes than the All-Season tire cars, i.e., the brakes kept getting hotter and hotter throughout the 64-mile test. So the combination of snow tires, different shift points and hotter brakes all added up to the 1.2-second difference.

The Sottozero snow tires were a little less responsive than the All-Season tires on the initial turn-in for the corners. The drivers had to turn in a bit sooner because the turn-in took a bit longer. However, the Sottozero tires were stable and predictable while cornering, according to both LAPD and LASD drivers.

The snows had slightly less overall traction and had significantly more tire squeal under hard cornering. The tires produced both slightly more understeer and slightly more oversteer. The EVOC instructors compared the snow tire to a tire with a hard compound...just not quite as much traction. The Sottozero showed virtually no sidewall rollover, i.e., the tread remained flat on the pavement. Feathering across the entire tread, rather than just the outside tread blocks, was more evidence that the sidewall kept the tread on the road.

A new design of Goodyear Eagle Ultra Grip GW2 is also scheduled for a similar test. The GW2 is a directional snow tire but does not have as aggressive a V-groove pattern as the Ultra Grip GW3. The V-rated Ultra Grip snows are, in fact, based on the RS-A tires. The basic carcass construction is virtually identical. This is already a police-proven tire. The GW2, then, is basically soft and high silica snow compounds and open snow tread patterns added to the RS-A tire.

New for late-2005, Goodyear is changing the tread design of the GW2. Instead of an openly grooved centerline, they are using a solid center rib. This helps turn-in responsiveness and helps to stabilize the rest of the aggressive tread blocks. It also helps to break into snow and move snow away better.

To smooth the transition to Pirelli tires, especially for police replacement tires, Pirelli will handle state bids and GSA bids, and they will also do stand-alone contracts for cities and counties. They will ship factory-direct to 1) a police central distribution warehouse, 2) a local Pirelli dealer, 3) a regional Pirelli warehouse, and 4) direct to the police garage, post or barracks. With a police account, they will guarantee shipment of the police tire to whatever location within 36 hours.


Published in Police Fleet Manager, Mar/Apr 2006

Rating : 10.0


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Ford CVPI (Crown Victoria Police Interceptor)Goodyear Eagle RS-AGoodyear Ultra Grip GW3 TiresPirelli Sotto Zero Winter 240 TiresSevere Snow-rated TiresTire TestingTires
 

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