It’s winter. Are you driving on winter tires?

It’s winter.  Are you driving on winter tires?

Ok, so the Southern Californians, Floridians and other sun-belt heliophiles probably can skip this one (unless they drive in snow… ever).  But for the rest of us locked in winter’s cold embrace, the proper choice of tire for the season can make the difference between safe cruising and a mid-road triple lutz.

What is a Winter Tire?

A Winter Tire is one designed to provide uniquely better traction in cold, snow and ice conditions than summer tires or all season tires.  Winter tires incorporate elements designed to provide higher traction AND to remain more pliable (less stiff) under severe cold conditions.  As a result, winter tires should (as a rule) never be driven consistently in temperatures above 40 degrees F.

Visibly, a winter tire is designed with wider grooves that are mandated to be a percentage of overall tread area.  Counter intuitively, these “void spaces” hold snow which actually grips better to itself than to the rubber creating a high level of friction. More importantly, the tire is specially constructed to keep its grip when the mercury drops.  In fact, this difference in compounds makes the most difference; the added traction from the more aggressive tread is a bonus to the tire working at all.

For added traction, some extreme winter tires are equipped with rubber or even metal studs.  Be extremely careful if you opt for these tires!  Due to the potential road damage of a studded tire on a dry road they are illegal in many states and in many others have seasonal limitations.  The Rubber Manufacturers of America has an excellent list of the state by state regulations concerning studs.

How do I tell if it’s a winter tire?

First off, it will often have a name implying cold weather such as “arctic”.  Snow tires will be visibly more aggressive than summer or all-season models with a minimum amount or snow void space in the tread, and like All Season tires are generally meet “M&S” (for mud and snow) rating requirements.  Many winter tires meet the industry requirements for the “Severe Snow Service” rating which mandates that a tire outperform a sample tire on packed snow by a minimum of 10% – these tires will be marked with the ASTM “Mountain and Snowflake” symbol.  (A very few All Season tires carry the severe snow service rating).   The aggressive appearance is due to the tread voids and a higher number of “sipes”.  Sipes are the little cuts that are molded in to the tread block elements.  The best way to think of sipes is that they are the “teeth” of the tire.   The more sipes you have, generally the better the “bite” of the tire.  Sipes do have a downside:  more sipes generally create quicker and more uneven wear.  This is why new snow tires can become noticeably louder after a few thousand miles have been put on them.

So how does that “grippy” thing work?

It might be helpful if you stop thinking of your tires as rubber and start thinking of them as what they are:  oil processed with other chemicals (you call that a polymer) and reinforced by metals and other polymers to maintain the tires shape and performance.  Just like oil, the tires will become less viscous (viscosity is essentially how “slippery” a fluid is or how much friction it creates against itself) as the temperature drops.  High performance oil can take on the consistency of jello when cold; similarly, at around 40 degrees F the soft polymers that make a race tire grippy and a supreme joy on a corner will begin to tighten and lose performance.    As the temperature continues to drop the tire will become noticeably harder until it reaches what polymer chemists call the “glass transition point” where it essentially becomes a crystallized solid with virtually no give or traction- it might as well be one of the stones they use in Olympic curling, you know, the ones designed to slide with no friction?  The temperature at which this occurs depends entirely on the construction of the tire in use but in general, the “better” the tire (as in high performance and grip) the more cold-sensitive the tire will be.  As an illustration of the effect of cold on rubber, back in 1986 something similar occurred with the tragic loss of the Space Shuttle Challenger- a rubber o-ring, designed to be flexible at one temperature, shattered and broke at a lower temperature.  The sudden hardening of your tires on an icy road could be no less dramatic to you or your passengers.

Winter tires have wider tread voids, often “over the shoulder” lugs, special tread elements, and tread compounds which ofter incorporate silica at a higher percentage for better traction.  Silica (as a polymer chemist would tell you) decreases the glass transition point of the tire and keeps the surface pliable in even extreme conditions.  This isn’t to say that a winter tire can’t harden to the consistency (and grip) of a hockey puck just that it takes much lower temperatures to do so.  The downside of all of this pliability is that a winter tire used in the summer will get even softer.  Lacking the content that allows their summer tire siblings to maintain strength at high temperatures, the winter tires will wear extraordinarily quickly and shed surprising amounts of tread.  Unfortunately, this softness does not equate to “sticky” performance.  In fact, a hot winter tire in a performance application will provide unpredictable cornering results and may actually feel slippery or “greasy.”  Sometimes a single summer of driving on winter tires is enough to make the tires useless for the following winter!  Given the price premium of a good winter tire you really don’t want to have to buy a new set just because you didn’t get around to changing them out…

All season tires have to take a balanced approach to construction offering a larger window of operating temperatures and a more durable tread but compromise on performance both in the summer and winter.  Further, an all season tire has the same “hockey puck” transition as a summer tire just at a lower temperature.   That said, all-season tires are an excellent choice for drivers that don’t operate in deep snow or in extremely cold temperatures.  Remember, similar to summer tires, the highest performance all season tires generally sacrifice cold tolerance for warm weather grip and wear resistance.

Important!

Keep in mind that the conventional wisdom of a few years ago suggested that you could “get away” with putting snow tires on only the rear axle of a rear wheel drive vehicle.  Side by side testing shows that this is absolutely not the case- your braking and cornering performance will suffer.  Also never mount snow tires on only the front axle of the vehicle.  Such a configuration won’t provide significant improvements in traction and will create a potentially life-threatening lack of stability and control.

Also, as in any tire choice pay attention to the speed rating of the vehicle.  This is often posted on the inside of the driver’s door or can be found as a letter on the OEM tire sidewall.  Choose a winter tire that has an equivalent or greater speed rating for your vehicle.  If a equivalent or higher rating isn’t available (sometimes the case if you are using ultra-high performance tires) you may use a lower speed rating only if you limit your speed to that rating!

When to make the switch

Keeping in mind the that winter tires disintegrate in hot temperatures and that summer tires are skates in freezing temperatures it shouldn’t be hard to guess when you should change- before the first snowstorm of the year or before the average temperature drops below freezing.  Some companies (including TireVan) are quite willing to store your off season tires for a nominal fee removing some of the headache of the switch.  As an added bonus, miles not driven on your ultra high performance tire will translate into a longer life and more summers of hard-cornering bliss.

Comments?

At TireVan, we always appreciate your feedback and try our best to respond promptly to any questions. So, please free to share your thoughts on this article, or ask us questions you may have on this or other topics.

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