Snow Tires for Police Vehicles

Snow Tires for Police Vehicles

Not all severe snow-rated tires perform the same.

www.bridgestonetire.com

www.goodyear.com

www.tirerack.com

By Police Fleet Manager Staff

 

If you are going to the expense and hassle of a separate set of tires for winter operations, you want to get the best tires for the task. You do that with the pursuit-rated tires, the tires specifically built for high-speed police work. Do the same for Winter/Snow tires.

There are big differences between retail All-Season tires and police All-Season tires. It is the same with Winter/Snow tires. Some Winter/Snow tires perform much better than others, even though they all have the same severe snow, Snowflake in Mountain embossment.

We want All-Season police tires to be a balance of tire life, dry traction, wet traction, and cost. Not so much for Winter/Snow tires. We change to a completely different tire for one reason only: winter road conditions. Tire life is less of a consideration and to a lesser degree, so is initial cost. We want performance in light snow, heavy snow, slush, packed snow, and ice. In a winter snow tire, traction is all that matters…or we would leave the All-Season tires on.

 

Winter/Snow Tire Facts

Winter/snow tires are much safer than All-Season tires during the winter season even for police departments that do not deal with heavy snow or where the snowfall is quickly removed. At temperatures under 45 deg F, the Winter/Snow tires remain softer and more pliable and grip cold/dry pavement better than All-Season tires. The result is much shorter stopping distances than All-Season tires even on dry pavement.

By design, All-Season tires give up some performance under both scorching summer conditions and frigid winter conditions to give moderate performance at both extremes. So, in police use, All-Season really only means three-season. All-Season tires are really good for only spring, summer and fall. In tire test after tire test, and in accident investigation after accident investigation, All-Season tires are poor choices for winter driving.

Winter/snow tires should be put on in sets of four, not just on the drive wheels. Under all road conditions—from dry pavement to glare ice—you want the front and rear tires to have exactly equal traction. For example, winter tires on the rear will have better traction in snow, but worse traction on dry pavement than All-Season tires on the front. Totally different traction between the front tires and the rear tires confuses electronic stability control, traction control, and anti-lock braking systems.

Why do you even need Winter/Snow tires if you have traction control, stability control, and antilock brakes? These safety systems are only as good as the traction where the rubber meets the road. Traction comes from tires, not electronics. These systems don’t provide more traction; they just make the most of the available traction.

The true Winter/Snow tires, the ones with the Snowflake in Mountain embossed in the sidewall, get that ‘severe snow’ rating by passing a performance test. However, that simply means the tire has 10 percent better traction that a standard reference test tire. Huge snow performance differences still exist among tires with a severe snow rating.

 

Two Kinds of Winter/Snow Tires

As defined by Tire Rack, the ‘neutral and detached’ authority on tires, there are two basic categories of Winter/Snow tires: Performance Winter/Snow tires and Studless Ice & Snow tires. A third category exists, studdable Winter/Snow tires. However, if you are even considering studded snow tires, you don’t need any persuasion to remove All-Season tires for winter. We covered studded tires in a two-part series. See www.hendonpub.com, click Resources, click Article Archives, and search for ‘studded tires.’

There are a dozen makes of Winter/Snow tire that fit the police package and the special-service package sedans, crossover/SUVs and trucks. However, most police fleet managers are conservative enough, cautious enough, concerned enough to stick with the two makes that come from the factory on pursuit-rated vehicles: Goodyear and Bridgestone-Firestone.

These are also the two likely brands to be on most state or regional cooperative bid. From here, the decision-making process is extremely simple. Find the tire size and speed rating you are looking for and check the Tire Rack ratings for those tires. The Performance Winter/Snow category of tires typically has higher speed ratings than Studless Ice & Snow. The Studless Ice & Snow has better…ice and snow…traction than Performance Winter/Snow.

As an example, the Dodge Charger Pursuit V8 AWD and the Ford PI Sedan turbo EcoBoost V6 used as traffic enforcement on the interstate should probably get one of the Performance Winter/Snow tires. Speed ratings up to V (149 mph) are available. All the rest of the police and special service vehicles should probably get one of the Studless Ice & Snow tires. Speed ratings up to H (130 mph) are available.

Of course, all of the special service vehicles are speed limited to under 112 mph. That means any R (106 mph) or S (112 mph) Winter/Snow tire will work. If the vehicle is speed-limited to under 130 mph, or is never driven over 130 mph, the better winter weather performance of the H-rated Studless Ice & Snow tires makes this category of tire the obvious choice.

 

Performance Winter/Snow is the tire for departments wanting enhanced dry road handling from their Winter/Snow tires. These departments are willing to trade off some snow, packed snow, and ice traction to get it. Studless Ice & Snow is the tire for departments wanting the maximum snow, packed snow, and ice traction from their Winter/Snow tires without taking the next step to winter tire studs.

 

SIDEBAR 1:

Winter/Snow Versus All-Season

Think of ‘snow’ tires by their proper term, ‘winter’ tires. A winter tire has three major advantages over an All-Season tire: rubber compound, tread sipes, and tread pattern.

 

Rubber Compound

The most important aspect of a winter tire is a tread compound that remains soft at lower temperatures. As the temps approach freezing, the rubber in an All-Season tire begins to get hard. This stiffer tread compound does not conform to irregular road texture as well, and as a result has less actual contact with the road surface. A winter tire compound remains soft and pliable at lower temperatures.

Winter tires and All-Season tires ‘break even’ in terms of tire performance at 45 deg F. That is not traction on snow or ice. Instead, it is acceleration, braking, and cornering on DRY pavement. At temps below 45 deg F, the All-Season tire gets steadily worse and the winter tire gets steadily better.

At 32 deg F, if the performance level of a winter tire is referenced at 75 percent of maximum, the All-Season tire is performing at 45 percent of maximum. At 5 deg F, if the performance level of a winter tire is referenced at 90 percent of maximum, the All-Season tire is performing at 30 percent of maximum. That difference in dry traction is due strictly to the cold-tolerant rubber compounds used in winter tires.

On packed snow and wet snow, the adhesion from the winter compound itself affects performance exactly as much as the design of the tread grooves themselves. On icy snow and mirror ice, the open tread pattern has zero effect on performance. Control of the car comes strictly from equal parts of the compound adhesion and the effects from the tread sipes.

 

Tread Sipes

The second major difference between a winter tire and an All-Season tire is the sipes, or tiny slits in the tread blocks. These are not the grooves or gaps or channels in the tread. Instead, they are cuts or slices in the tread blocks. An All-Season tire may have a few sipes in the tread. However, the winter tire has so many sipes, it is this feature alone that visually defines truest high-speed winter tires.

The sipes play a major role under three conditions: wet snow, icy snow, and mirror ice. In fact, 40 to 50 percent of tire performance under these three common winter conditions comes strictly from the sipes. They play a minor but still significant role in the other adverse conditions of fresh, deep snow and packed snow.

Sipes are designed to open, fill with snow, and then trap that snow. Tire tests have proven that under some conditions, i.e., wet snow, the snow-packed sipes offer more traction than the grooves in the tread. In other words, snow-on-snow produces more traction than rubber on snow.

At the higher speed ratings, which cannot be achieved with aggressively spaced tread blocks, the winter tire performance comes almost exclusively from the sipes and rubber compounds. They are also designed to close and turn their tread block solid during cornering. Don’t discount the profound affect the sipes alone have on winter tire performance.

 

Tread Pattern

The third and most obvious design difference of a winter tire is the tread block design, i.e., open grooves and aggressive tread patterns. In fresh-fallen snow, the tread grooves are responsible for about 50 percent of the traction performance. Under these same road conditions, the compound adhesion plays a 40 percent role while the sipes effect has a 10 percent role.

On packed snow, the effect of the tread pattern is 40 percent, the compound adhesion is 40 percent, while the sipes contribute 20 percent. Here is the point, as soon as the tire rolls over the fresh snow, the surface quickly changes from fresh fallen snow to packed snow. Under wet snow conditions, the tread grooves, compound adhesion, and sipes effect all contribute to 33 percent of the resulting traction.

All-Season tires are good for only occasional and only light snow and then only with new tread. As the tread wears, as the temps get colder, or as the snow deepens, these All-Season tires simply won’t work. All winter tires have a substantial advantage over All-Season tires on icy snow and glare ice. With the average winter tire performance referenced at 100 percent, during Tire Rack testing in a skating rink, the average All-Season tire had a traction index of 66 percent. Winter tires are the cheapest cold weather insurance the department can buy.



Published in Police Fleet Manager, Nov/Dec 2017

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