Can winter tires on a front-driver be as good as a
4x4? We find out
There’s probably not a driver who has been out and about in
the past two weeks in a conventional two-wheel-drive car on normal tires and
not felt that sickening slip as adhesion is lost on ice, snow or slush. It
feels like you’re actually accelerating. And although in the vast majority of
cases a combination of your actions and on board safety systems will keep you
out of the hedge, the next time you see an SUV sailing serenely on its way,
apparently oblivious to the prevailing conditions, you’d not be human if you
didn’t turn just a faint shade of green with envy.
So you decide to do something about it and arrive at the
question we’re asked more than any other at this time of year. Do you have to
splurge thousands on a car with four-wheel drive, or can you actually get away
with just fitting a set of winter tires to your existing wheels?
Do you have to
splurge thousands on a car with four-wheel drive, or can you actually get away
with just fitting a set of winter tires to your existing wheels?
Instinctively, we thought we knew the answer but were
unaware of any testing that had been done to find out for sure. So we thought
we’d better do some ourselves.
The trick was to find two cars that are essentially
identical save for one being fitted with four-wheel drive and the other not.
It’s not as easy as you think. But Skoda provided the answer in the form of two
Yetis, both fitted with the same 108bhp diesel motor, both wearing 225/45 R17
tires but one driving its front wheels alone, the other driving all four. We
then asked Skoda to equip the front-driver with winter tires, leave the
all-wheel drive car on conventional rubber and deliver both to a snowy test track
for us to find out more.
We were there with two distinct goals in mind. Firstly, we
wanted to measure as objectively as possible the actual difference in
performance of each configuration. Then we wanted to see for ourselves which
version was not merely gripper in these conditions but also gave the driver
more confidence. This latter component is important: we’d happily sacrifice a
small amount of grip in exchange for a more trustworthy feel at the wheel. A
car that has lost o grip but behaves unpredictably or fails to communicate when
the limit has been reached is always going to be more likely to throw you of
the road than one that keeps you constantly informed of conditions underfoot.
225/45 R17 tires
For the purposes of this exercise, a tire has essentially
three jobs to do: it must help the car to accelerate, to brake and to corner.
So we devised the very simplest of tests and measured each one.
Acceleration
This was a race from rest to 30mph and aimed purely at
measuring traction. We left the electronics turned on because that’s what
normal people do and it helped eliminate variances in driving style from one
run to the next. The same timing equipment used for our road tests provided the
data.
Quite unpredictably, it was the front-drive Yeti that leapt
most smartly from the line, beating the four-wheel-drive to 5mph. Thereafter,
the car able to divvy up its engine’s workload to all four corners of the car
swiftly took over. The all-wheel-drive Yeti was a couple of tenths ahead at
10mph, almost a second clear by 20mph and a substantial 4.4sec quicker to
40mph.
The off-the-line anomaly is explained by the fact that even
a four-wheel-drive Yeti spends most of its time driving only its front wheels.
Only when its Haldex system detects a loss of grip at the front does it spring
into action and start shuttling power rearward – a process that takes a small
but apparently measurable period of time. Thereafter, and with four-wheel drive
fully operational, even on conventional tires it blitzes its front-wheel-drive
sister. Which is hardly surprising: a winter tire will provide more traction in
snow, but not twice as much.
Braking
This test was conducted from just 20mph, a speed that might
seem barely fast enough for a discernible difference to become apparent. But
you’d be surprised – maybe even as much as we were. We started to brake at
22mph so the brakes were fully engaged by the time the timing gear triggered at
20mph. we made no attempt to cadence brake, steer or in any way attempt to
modify the car’s natural behavior – it was just ABS all the way until the cars
eventually stopped.
On regular tires,
the 4x4 took far longer to stop
Were the inferences not so serious, what happened next would
have been comical. The Yeti on winter tires slithered smartly to a halt, while
the 4x4 just kept going and going. Its best run saw it come a halt 4.5 meters
further up the track than the front-drive Yeti’s worst. The front-driver’s best
stop was 6.8 meters shorter. And that could be the difference between a mildly
scary moment and quite a big accident.
Why such a disparity? Simply because the moment the Yetis
started to brake, both became effectively zero-wheel-drive vehicles. Robbed of
its one and only advantage, the 4x4 was on a hiding to nothing as the contest
became a simple comparison of winter versus standard tires on snow. It never
stood a chance.
Cornering
The winners of the first two tests were not hard to predict,
even if the margin of victory in the braking test was greater than we imagined.
The cornering test would be more interesting. You might think this was already
in the bag for the winter tires, but it might not be quite that simple.
A tire lives inside a circle of adhesion in which
longitudinal and lateral acceleration forces compete for its grip. Step outside
the circle and the tire loses grip by spinning on acceleration, or locking
under braking, or sliding under cornering force. But if you reduce a tire’s
workload in once direction, say by halving its traction requirement by using
four-wheel drive, you bring the tire back from the edge of the longitudinal
limit of the circle. This spare capacity can then be used to transfer more power
to the road, or generate extra cornering force, or any combination of the two
that still keeps the tire inside the circle. That is why powerful
four-wheel-drive cars are so phenomenally good at exiting wet roundabouts.
But our test, which involved driving around a constant
radius circle and measuring the lateral g-force developed by each car, told a
different story. Neither developed much grip: the 4x4 generated 0.17g, the 2WD
car on winter tires 0.23g. This may not seem like much, but in percentage terms
it’s massive: the Yeti on winter tires generated fully 35 per cent more grip
than the 4x4.
The explanation is that in maximum steady-state cornering,
the tires are already at the limit of the circle of adhesion. Only if the cars
slowed down to provide some room inside the circle and then tried to accelerate
would four-wheel drive provide any kind of advantage.
The subjective test
This only really applies to cornering because in the braking
and acceleration tests, with stability control and ABS working overtime, both
cars tracked straight and true. Through the corners, however, the Yeti on
winter tires executed instructions with unfailingly greater accuracy. Moreover,
in steady-state cornering, while the 4x4 Yeti was constantly varying its
front-to-rear power distribution, resulting in many shades of understeer,
driving the same circle in the Yeti on winter tires required little more effort
than dialing in the appropriate lock and watching as the car described a beat
and consistent arc.
Conclusion
We need to be careful about how much is read into these
results. The conditions clearly favored winter tires and, despite how it has
seemed of late, such weather remains rare in the UK. Had it been merely wet,
the results could have been very different.
The Yeti on winter
tires generated 35 per cent more grip than the 4x4
Bear in mind, too, that the operating temperature band of
winter tires is quite narrow, and when used outside their preferred environment
they will perform worse while wearing faster, using more fuel and, to some
extent, compromising ride, refinement and steering response. Then again,
four-wheel drive adds weight, mechanical complexity and also uses more fuel:
46.3mpg versus 52.3mpg for these Yetis, with a commensurate effect on CO2
and therefore tax bills. And you can’t nip to your local tire bay and get the
four-wheel drive removed.
We came to gather evidence to help decide if you need
four-wheel drive when the weather gets bad, or whether you can save yourself
thousands by simply keeping a set of winter tires on standby for when
conditions suit.
The answer could scarcely be clearer: four-wheel drive gives
you a clear traction advantage so will be better at climbing slippery slopes
than two-wheel drive and winter tires. But as soon as you have to come back
down again, or go around any kind of corner, winter tires are clearly and
significantly superior.
Of course, the ideal scenario is to fit your 4x4 with winter
tires and enjoy the best of all worlds, but faced with a simple choice between
the two, our tests indicate strongly that most people most of the time would be
better off, both literally and figuratively, keeping their current front-drive
car and investing in a set of winter boots.
