Will It Grip or Will It Slip?It's a chilly February morning in northern Minnesota, and a broad expanse of groomed snow stretches before our thermally booted feet. We're standing on the 54-acre snow VDA (Vehicle Dynamics Area) at Automotive Enviro Testing (AET), a company that builds and maintains a sprawling, snowy proving ground in Baudette, Minnesota, each winter for those bent on testing the performance of cars and tires in winter conditions.
And testing tires on snow is precisely why we're here. It's Step One of our quest to quantify the differences among three types of tires (summer, all-season and winter) in three types of driving conditions (dry pavement, wet pavement and snow).
When we're done playing on the white stuff, we'll ship these tires to a specially built facility for wet-asphalt tire testing in Arizona (of all places) for Step Two and then haul them back to California to finish off the rubber with our usual testing regimen on dry pavement. This test sequence — snow before wet before dry — is critical because it ensures that the tread stays in top condition for each successive test.
When we're done, we hope to answer some important questions. Do snow tires represent a big advantage in winter driving? What's the best type of tire for wet pavement? Are all-season tires really a helpful compromise? Are the differences really all that significant?
The Tires
You've heard it before: Tires are much more than black, air-filled cylinders that provide protection for your shiny alloy rims. The four fist-size contact points that exist where the tires meet the ground represent the only link between the multiple thousands of man-hours of complex engineering in the car above and the wide range of road and weather conditions that can exist below.
Tires matter — a lot. The variety of possible road conditions is so vast that you can significantly alter the performance of your vehicle with a simple change of tires.
Let's meet the players in our little experiment:
Summer tires — known simply as "tires" before all-season tire use became widespread — are currently marketed as "performance" or "high-performance" tires. The tread is generally comprised of large tread blocks with high lateral stiffness, while wide circumferential grooves expel and sequester water. The rubber formulation is calibrated for warm weather and pays little heed to snow and ice.
Snow tires, on the other hand, utilize rubber compounds specifically formulated to produce grip at low temperatures and a tread that features a pattern of elements that are oriented to physically bite into snow. A network of hair-thin cuts — known as "sipes" — crisscrosses the tread blocks, subdividing them into numerous edges for added grip. The result is a squishier, less stable tread than you'd see on any summer tire.
All-season tires can't be as singled-minded as these others. Their tread pattern seeks to serve both dry-weather grip and snowstorm bite, so we see medium-size tread blocks that provide some lateral stiffness along with an increased number of biting edges. Sipes are present, but not in great numbers. An intermediate rubber compound with a wider temperature range is often employed.
And because all-season tires are the default tire on many new cars today, they have to somehow simultaneously minimize noise levels, maximize tread life and promote fuel economy with low rolling resistance. In short, all-season tires are asked to do everything.
Selection Criteria
As you might imagine, this test represents a big undertaking, with many potential pitfalls. We took pains to eliminate as many variables as possible before we booked hotel rooms in the wilds of northern Minnesota.
For starters, we would test same-size tires to isolate the effects of the various tread patterns and rubber compounds. We also sought a trio of test tires that were produced by a single tiremaker — it didn't matter which — to ensure that they were developed with a shared engineering philosophy. Beyond that, we wanted original equipment tires (OE) that were developed specifically for a particular car and installed on the assembly line. Finally, we planned to test the tires using the very car for which they were developed.
The ideal tire candidates would therefore come from a vehicle that offered same-size, same-brand summer and all-season OE alternatives, straight from the factory. Winter tires, of course, are always aftermarket replacements.
Only one car satisfied all of these specific requirements: the Honda Civic Si. It can be bought with your choice of Michelin Pilot HX MXM4 all-season tires or, for $200 more, Michelin Pilot Exalto PE2 high-performance summer tires, both in size 215/45R17. And Michelin builds an aftermarket snow tire in that very size, the Michelin Primacy Alpin PA3.
The Tests
Online tire seller Tirerack.com agreed to provide new mounted and balanced test tires for our effort and it shipped them straight to AET's door, ready to go. They're sitting outdoors in the snow, acclimating to the temperature as we survey the test course and install our VBOX data logger in the bright red 2009 Civic Si four-door we drove up from Minneapolis last night.
Typical tire validation tests in snow focus on a tire's performance in city and suburban traffic situations, so 0-40-mph acceleration and 40-0-mph braking tests are the norm. We're adding these tests to our usual 60-mph evaluations and will continue to use them on all three surfaces.
But we're not running our usual 600-foot slalom because the multiple runs required to get good data create deep ruts in the snow that skew the results. But we can still use a skid pad for lateral grip measurement because we can get representative data with no more than a single pass in each direction on AET's snow circle.
But we're not running our usual 600-foot slalom because the multiple runs required to get good data create deep ruts in the snow that skew the results. But we can still use a skid pad for lateral grip measurement because we can get representative data with no more than a single pass in each direction on AET's snow circle.
Lest you think that performance testing is inconsistent with the careful driving necessary to survive on slippery roads, consider this: Reduced traction on slick surfaces makes at-the-limit situations likely in everyday driving. And along these same lines, we'll be keeping our Civic's traction control system up and running during all of our tests because, if not now, when? As for stability control, our 2009 Civic doesn't have it, so our test driver will have to stay alert.
Snow Test Results
No one expects the snow tires to come in anywhere but 1st place on this wintry surface at AET. The point here is to see how big their advantage really is over all-season and summer tires.
It takes 11.7 seconds for our Civic Si to accelerate to 40 mph on snow tires, and 14.5 seconds to get there on all-season rubber — nearly 3 seconds and 24 percent slower. As for the summer tires, well, they require, ahem, 41.7 seconds as they struggle to 40 mph. That's no typo; it takes a half-minute longer — 257 percent more time — for the summer tires to reach this modest speed.
What about our traditional 0-60-mph test? Well, snow tires get to 60 mph in 19.1 seconds, while the all-season treads arrive in 22.9 seconds, nearly 4 seconds later. Forget the summer tires, however. The available 3,650 feet of snow — seven-tenths of a mile — isn't enough. We figure 67 seconds and 3,100 feet are needed to get there, and then there's the small matter of needing to stop again.
And that brings us to our next test: full stops with ABS engaged. Here again the snow tires dominate, stopping from 40 mph in 156 feet, some 28 feet shorter than the all-season tires' 184-foot performance. Meanwhile, our summer tires skate to an ultimate distance of 351 feet, the ABS actuator rattling for all it's worth the whole way.
Increase the starting speed to 60 mph and these distances more than double. It takes 362 feet for the snow tires to stop and 421 feet for the all-season donuts. The summer tires sit this one out because they can't manage to get themselves to 60 mph in the first place. (We do the math instead and come up with an estimate north of 800 feet.)
Skid pad results follow the same now-predictable pattern. Our snow tires pull 0.30 lateral g, the all-seasons manage 0.28g and the summer tires produce a pitiful 0.15g despite a heroic effort by our shivering hot shoe.
Wet Test Results
As with the snow test, it takes a purpose-built test surface to get meaningful and accurate wet test results. What's needed is a large flat VDA with a special asphalt formulation and a water-delivery system that can maintain a uniform depth of water (the approximate thickness of a quarter), so there's a consistent coefficient of friction across the entire surface.
We find it at Arizona Proving Ground (APG) near Phoenix, formerly a Volvo facility and now branded as Ford. Its pristine micro-pool looks gorgeous on this May afternoon as the surrounding mountain peaks, little fluffy clouds and spring sunlight reflect in the glassy surface.
It turns out we can test everything to and from 60 mph here, so for brevity's sake we'll stick to a discussion of those results. Those who want to can find the 40-mph results on the accompanying charts.
Acceleration testing provides the first surprise, as the all-season tire trails the pack with a 15.4-second 0-60 run. The snow tire's 12.7-second effort for 2nd place is significantly better, but the summer tire tops them all with an 11.9-second performance, over 20 percent quicker than the all-season tire. In fact, the all-season tire began encountering trouble near 40 mph, where it had been only 0.4 second behind the summer tire's performance when hydroplaning and wheelspin paid a visit.
Things are much the same when braking from 60 mph. The summer tire's 157-foot stop is the shortest, the snow tires come up 2nd at 181 feet and the all-season tires lag further behind in a flurry of ABS activity on the way to a distance of 215 feet, some 58 feet longer than the summer tire.
On the wet skid pad the summer tire smokes them once more, even delivering a little tire squeal as it churns out 0.81g — a figure many car-tire combinations can't match on dry pavement. The winter tire trails with a 0.71g run characterized by noticeable squirm, presumably from the side loads acting on the numerous sipes in its snow-biased tread pattern. That said, it still bests our all-season tire, which once again brings up the rear with a disappointing 0.65g showing.
Dry Test Results
Home sweet home. We're back at our usual test ground at Auto Club Speedway in Fontana, California, for some good old dry asphalt. This is the surface we're all familiar with, so we figure we can predict the finishing order with ease: summer, all-season, then snow.
And so it is. The summer tires (actually you could also think of them as three-season tires) top the charts in acceleration (8.7 seconds), stopping distance (120 feet) and lateral acceleration (0.86g). OK, this acceleration figure isn't the best we've ever achieved with a Civic Si, but for the sake of consistency we're still using traction control for these launches.
Our all-season tire ties for top honors in the 0-60 test with another 8.7-second run, but its stopping distance and lateral grip figures sag in comparison to the summer tire, with marks of 131 feet and 0.84g, respectively. Not bad, but still second-best.
Pity the poor snow tires, as they are well and truly out of their element here. They manage a competitive 8.9-second acceleration pass to 60 mph, but the 155-foot braking runs and 0.81g lateral acceleration laps take a visible toll on the tread, which might not make it to the car's next oil change if we keep this up. And the noise they make when cruising straight and level reminds us of a lifted off-road pickup.
Summing Up
What can we make of all this? For one, low-friction surfaces demand respect. Our stopping distances in the wet range 30-40 percent longer than those recorded on dry pavement. Meanwhile, stops on snow consume at least three times the distance as they do on dry asphalt, even with the use of best-case tires in each situation.
Second, no single tire type excels on all surfaces, and the differences between each are sometimes striking. These differences are so massive, in fact, that we feel that certain generalizations can be extrapolated from our small trio of carefully selected test tires.
To the surprise of exactly no one, our winter tires dominate in snow and the summer tires dominate in the dry. The eye-opener here relates to wet performance, where a well-developed summer tire embarrasses an all-season tire made for the same car by the same folks. Anyone who never sees or visits snow would be very well served by summer tires for year-round use.
Another key take-away from this exercise is the utter worthlessness of those same summer tires on snow. Anyone who uses snow tires in winter and summer tires the rest of the year — a good strategy to maximize performance and control all year — needs to time the switch-over carefully to avoid getting caught out by the first rogue snow accumulation of the season.
And the lameness of summer tires on snow makes it easy to see why the California Highway Patrol and other local authorities can have a hair-trigger when it comes to requiring snow chains. It also explains why so many carmakers spend a lot of energy on all-season tires; they don't know where you live or where you'll drive, so they want to make sure you've got passable winter rubber.
But in delivering this capability, all-season tires sacrifice a noticeable bit of dry and wet performance. Meanwhile, snow and summer tires provide clear benefits to those who can use them. In this particular test, at least, all-season tires live up to the old figure of speech our old dad used to trot out on occasion: "jack of all trades, master of none."
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