In winter, what's best:
Two wheel drive on snow tires,
or four wheel drive on all season tires?
It's a good question, and one
that perhaps far too few folks are asking before they plunk down thousands
of dollars for four-wheel-drive
hardware that adds power-robbing weight and friction and complicates maintenance and repair. We set up a test to see if snow tires and two-wheel drive could match the sure-footedness of all-wheel drive using stock tires in wintery conditions.
The first thing we learned was
not to call them snow tires. Snow tires were the low-tech, knobby-looking
things your dad mounted on the back
of the family wagon in the good old days. These new ones feature space-age rubber compounds and computer-designed tread patterns, and they're to be fitted in sets of four. All the big tire makers offer them and three winters is the approximate life expectancy of these soft tires.
To answer the question thoroughly, we rounded up two Audi A6s, one of which was front-wheel drive (fwd) and the other four-wheel drive (4wd). We also got a pair of Mercedes-Benz E320s in rear-wheel drive (rwd) and 4wd. Bridgestone Blizzak tires were used for the two German sedans.
We started in the predawn chill by trying to climb snow-covered traction grades measuring 10, 15, and 30 percent, or 6, 9, and 17 degrees.
Each ascent was launched from a level standstill just below the grade.
Four-wheel drive won this first
round of testing. Both four-wheel-drivers, even shod with stock all-season
tires, were able to ascend the
30-percent grade, whereas the 2wd cars could only conquer the 10- and 15-percent grades.
As our braking tests would later
confirm, winter tires can improve straight-line grip by as much as a third
relative to all-season tires.
But four-wheel drive doubles the grip of a stock two-wheel-drive car when accelerating or climbing a grade.
On snow, we accelerated to 50 mph as quickly as possible, and we conducted panic stops from 50 mph. Top speed on ice was 30 mph.
The Audi and Benz traction and stability control systems varied widely in their strategies, so we switched these systems off to eliminate this variable from our testing. Anti-lock systems were used for all brake tests.
Acceleration: As with
our experience on the traction grades, we found that having twice the number
of all-season tires pulling together
improves acceleration more than does adding the latest traction technology to a 2wd car. In the snow, winter tires allowed our fwd Audi to
reach 50 mph in 9.7 seconds (30 percent quicker than the same car on all-season tires), but the A6 Quattro managed the trick in 7.6 seconds
on stock tires (a 45-percent improvement over the stock front-driver). Similar results were observed on the Benz, with winter tires boosting snow performance by 36 percent over the stock rwd Mercedes E320 (to 10.5 seconds) and the 4MATIC yielding a 51-percent improvement over the stock rwd E320 (to 8.1 seconds). Winter tires shaved another 0.7 second off the E320 4MATIC's 0-to-50-mph time.
Score one more for 4wd.
On ice, with the traction-control
system switched on, the rwd Benz could barely move on stock tires, but it
launched easily on winter tires
(we still managed our best times with the traction-control systems off). The Audi's fwd traction advantage made this phenomenon less
Braking: The picture
changes when you stand on the whoa pedal, because with four-wheel anti-lock
braking, all four contact patches
are working regardless of the driveline setup. Better-biting tires generate more stopping force, and the weight of a 4wd system simply
adds to the momentum that has to be stopped. So it comes as no surprise that 4wd tended to lengthen stops from 50 mph
(by as much as 12 feet on the Audi and 18 feet on the Benz relative to the stock 2wd setup).
Fitting winter tires shaved stopping distances substantially (by 44 to 64 feet in the case of the A6s, and by 22 to 37 feet on the E320s).
Winter tires win this one handily.
To measure which setup offered the best handling, we first used a snow-packed 425-foot-diameter skidpad to measure steady-state
lateral acceleration. We moved on to an increasing-and-decreasing slalom maneuver on the snow field to assess dynamic behavior.
(All timed handling maneuvers were conducted with the traction and stability systems switched off.)
The surface of a snow-covered skid pad is degraded as much by traffic as
it is by changing weather conditions.
To account for this, we ran each car around the 425-foot-diameter circle once, reversed the batting order, and ran them again.
The first and second runs for each car were then averaged.
Winter tires won again at the
snow circle, providing twice the improvement in lateral grip that 4wd on
stock tires could offer.
Predictably, winter tires improved the fwd Audi's performance most dramatically, as its front tires are burdened with the tasks of propelling
and steering the car. On stock tires, terminal under steer set in at just 0.28 g. Quattro upped that figure to 0.31 g, but winter tires boosted
grip to 0.34 g. The rwd Mercedes, whose rear tires assist with steering duties when urged on by the throttle, managed 0.30 g on stock tires.
The 4MATIC scored 0.32 g, but rwd with Blizzaks managed 0.34 g. On winter tires, the more understeer-oriented A6 Quattro scored the same 0.34 g as its fwd counterpart, but the more neutral, tail-out E320 4MATIC on winter tires managed the top score of the day at 0.37 g.
Even without the hard data,
the subjective difference in vehicle behavior is dramatic. Using winter tires,
each of the four cars responded
far more faithfully to the helm. The Audis would still under steer, and the Mercedes-Benzes would still oversteer if provoked, but the level of provocation required and the speed at which the tires broke away was noticeably higher.
Slalom course: As we
found on the skid pad, winter tires again showed roughly double the dynamic
handling advantage that four-wheel
drive offers. On stock tires, the Audi Quattro was just 5.5 percent quicker accelerating through the cones than the stock fwd A6, but the combination of fwd and winter tires boosted performance by 12.6 percent. Slowing through the cones, the 4wd A6 on stock tires was
slower and more difficult to control than the fwd A6 on winter rubber. It was more likely to slide sideways, perhaps due to its added mass.
The 4MATIC system upped the E320's average slalom speed by 2.5 percent, but mounting winter tires on the rwd E320 yielded a
4.7-percent boost over the stock rwd E320.
Again, the numbers don't begin
to describe the difference felt at the helm. Our best run in the A6 Quattro
on stock tires was accomplished
by flicking and sawing heroically at the wheel and creating quite a spectacle. By contrast, the fwd A6 on winter rubber ran 3.5 mph faster
on its very first easy-does-it run by merely steering around the cones. The E320s both tend toward throttle-on oversteer, even on winter tires,
so the subjective difference the tires make is somewhat less dramatic than in the Audi. It's also interesting to note that winter tires helped the E320s slightly more in the decelerating slalom tests and they helped the Audis more in the accelerating slalom.
What About When the
Most places that get enough snow and ice to warrant buying winter tires also experience frequent thaws.
That means folks end up driving
winter tires on dry or damp pavement a lot of the time. In those conditions,
the winter tires provide
noticeably less grip for stopping and turning than do all-season tires. Their tall, soft tread blocks feel squirmy and imprecise during transient maneuvers. They also tend to wander more in crosswinds and on crowned pavement.
And most winter tires carry a speed rating of "Q" (160kph) or lower.
So What's the Bottom Line?
Four-wheel drive helps get cars going.
When it comes time to brake or
change direction on low-traction surfaces, the extra mass of the driveline
becomes more of a detriment.
Folks who live in hilly places
that get snow may need the climbing capability of four-wheel drive.
If it snows a lot in those hilly
places, they should definitely invest in winter tires, too.
Even flat-landers who happen
to have steep driveways may wish to consider a four-wheel-driver.
Almost everyone else will most
likely be better served by using winter tires rather than investing in four
Acceleration takes longer, but
in an emergency, the handling behavior and improved lateral grip of two-wheel
drive and winter tires
-- in the slippery stuff -- are the safer bets.
Winter tires boosted the rwd
Benz's acceleration times more than they did the fwd Audi's, but in almost
every other test, the inherently
front-heavy Audis derived more benefit from the winter rubber than did the more evenly balanced Benzes.
This finding certainly suggests that front-drive cars benefit from winter tires as much or more than rear-drivers do.
And finally, unless snow or
ice covers your roads many times in a winter, the snow benefits of winter
tires may not outweigh their
drawbacks on dry pavement.