TABLE OF CONTENTS Jan 2013 - 3 comments

Separating Fact from Friction

Winter weather is here and so is the debate over whether or not a lack of winter tire use is causing collisions. To go beyond generalizations, detailed information is essential to properly understand the effect that winter tire use would have had in a given collision.

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By: Mark Fabbroni, Forensic Engineer, Giffin Koerth
2013-01-01

Winter driving season is back — and with it, the winter tire debate is again in full swing.

Advertisers are promoting the benefits of winter tires; media coverage is feeding the perception that not using winter tires is the sole cause of the latest multi-vehicle pileup; and good old “Uncle John” is repeating his now-familiar caution that he has used all-season tires for years during the winter and winter tires are just a waste of money.

With so much misinformation out there these days, it can be difficult to distinguish the root cause of a collision from things that may be contributing factors or, worse yet, simply unrelated circumstances. There seems to be a lack of understanding of what makes winter tires different from all-season tires, what those differences mean in terms of vehicle performance, and how winter tires should be used.

Exacerbating the problem is the Internet, which — for better or worse — provides a forum for anyone proclaiming to be a “car expert” to offer advice to the driving population. Contradictory information is abundant and confusing, but there is little disagreement among engineers as to the truth of the matter.

WINTER V. ALL-SEASON

The objective of winter tire designers is straight-forward: design a tire that can readily move loose snow and ice out of the way so the tire can make contact with the hard packed surface below and, subsequently, adhere to that surface even if it is ice-covered and the temperature is very low.

There are two primary differences between winter tires and their all-season cousins, those being the tread patterns used and the materials from which the tires are constructed. For obvious reasons, winter tire designs often include an intricate network of large channels to assist with the bulk movement of snow and ice from beneath the tire.

Often overlooked, however, are the finer details. These include greater use of “sipes” — thin slits cut into the rubber — that significantly increase traction in wet and icy conditions.

As for tire materials, the compounds used in winter tires are generally “softer” than those used in all-season tires and are formulated to better adhere to ice and snow.

In general, tire “softness” decreases as the temperature drops. Softness, however, carries with it a trade-off in terms of vehicle performance: a tire that is too soft will not be able to generate the loads required for good handling and braking performance and will wear prematurely. Conversely, if a tire is too hard, then traction suffers.

There is an optimal level of “softness” for any given temperature. At low temperatures, the softer winter tires provide better performance than all-season tires, and at high temperatures, the situation is reversed. The transition temperature at which most winter tires begin outperforming most all-season tires on dry asphalt is generally accepted to be approximately 7 degrees Celsius. The difference in performance increases sharply at sub-zero temperatures.

When temperatures drop well below zero and there is snow on the roadway, winter tires in good condition will significantly outperform their all-season counterparts; when temperatures are above zero and roads are clear, the differences in performance between the tires is marginal; and when temperatures creep into the teens during a thaw period, a vehicle may actually perform better if equipped with all-season tires (regardless of what the calendar says).

The last point is a contentious one that sparked some controversy in Quebec several years ago when it became mandatory to use winter tires during the winter. Concern was expressed that the provincial government was mandating the use of a technology that may, in some conditions, make the vehicle less “safe.”

Limited testing has been done to compare the performance of winter tires in dry conditions above freezing to all-season alternatives, but this seems to suggest the difference in performance may be small at the temperature highs that Quebec could realistically see during winter months.

The net result is that mandating winter tire use may have negligibly reduced vehicle performance a small percentage of the time while overwhelmingly improving vehicle performance the majority of the time. Not surprisingly, the result has been a reduction in the number of winter collisions.

COLLIDE OR AVOID

To determine whether or not improvements in tire performance would make the difference between colliding with or avoiding another vehicle in a particular instance, it is necessary to assess how much traction is being demanded of the tires during a particular vehicle manoeuvre. Driving on tires that perform worse than others in the same conditions is not ideal — but it may be more than enough to be safe under the circumstances.

The differences in performance only become an issue when a driver demands something of the tire that it is incapable of providing at a given moment. This is a key point that must be considered in any assessment of causation.

Beyond these issues, the variety of configurations in which winter tires are mounted on vehicles has further clouded the debate.

In practice, it is not uncommon to see vehicles equipped with a mix of all-season and winter tires. Conventional wisdom generally results in a configuration in which two winter tires are mounted on the drive axle while all-season tires are left on the undriven axle. The oft-cited reason for this type of setup is the need for good traction on the drive axle to be able to accelerate out of deep snow.

However, this reasoning is wrong, since there is much more to vehicle performance than just acceleration.

Vehicle stability, particularly during heavy steering or braking in slippery conditions, is arguably more important from a safety standpoint and it is significantly compromised when a vehicle’s four tires do not have the same performance capabilities.

While “snow tires on the drive wheels only” configuration was never optimal for vehicle performance, it was less dangerous in the days of rear-wheel drive vehicles. With the domination of front-wheel drive vehicles in the marketplace today, however, the conventional wisdom can have catastrophic results.

As a collision reconstructionist, I have seen many examples of vehicles that have lost control in winter conditions, causing them to veer into oncoming traffic or off the road in part because they were equipped with winter tires on only the front axle. The simple explanation for this is that vehicles are less stable under braking and steering when the tires with the most traction are located on the front axle, than when the situation is reversed.

That said, most tire manufacturers correctly point out that the maximum benefit derived from the use of winter tires occurs when they are placed on all four wheels of a vehicle. This is not only optimal for stability reasons, but for performance as well.

Testing has demonstrated that vehicles equipped with four winter tires in winter conditions will stop shorter than vehicles with various configurations of winter tires mixed with all-season tires.

All of the foregoing is true regardless of the type of technology with which the vehicle in question is equipped. Anti-lock brake, traction control, stability control and all-wheel drive systems are great assets to have in winter driving conditions; however, these systems merely take advantage of the tractive capabilities of a given tire, rather than changing what those capabilities are. As such, they, too, are entirely dependent on good tire performance to be of any assistance.

Establishing whether or not use of a specific type of tire had any effect on a given collision is highly dependent on the specifics of the weather conditions at the time, the condition and type of tires involved, and where on the vehicle the tires were installed. All of these factors must be assessed within the context of the driving manoeuvres attempted and the vehicle speeds involved to identify if tire type or use was a contributory cause.

The winter tire debate is far more complex than is typically reported or advertised.

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Mark Fabbroni, Forensic Engineer, Giffin Koerth
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Caption: Mark Fabbroni, Forensic Engineer, Giffin Koerth


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