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发帖数: 4944 | 1 "Sure-footed all-wheel-drive handling." That's the kind of lingo you're
likely to hear in car ads and marketing material, and it has prompted me to
shout at televisions, print ads, and the occasional car-company rep: All-
wheel drive doesn't help handling!
It's disingenuous to say or infer that AWD enhances cornering prowess, or
that it'll help a driver avoid a fallen tree or dodge Bambi. When it comes
to handling, all-wheel drive is overrated (not to mention heavy and gas-
sucking), especially in foul weather.
"I'd rather have a Toyota Camry on four new snow tires than the best all-
wheel-drive vehicle on all-season tires," said one tester who requested
anonymity.
Before you start flaming, I'm not anti-AWD. Rather, I'm just incensed by
those who fudge its ability beyond all recognition. AWD is great at aiding
accelerating on slick surfaces and keeping a vehicle moving on snowy roads.
Rally racers like AWD because it helps their over-powered cars accelerate on
gravel and dirt paths. I co-drove an AWD car to victory in a 24-hour race,
and in the rain I enjoyed how the car accelerated off the corners.
However, my experience—hard-earned from wrecking more than one AWD vehicle
during snow-handling tests for a tire company—is that AWD is counter-
productive when the roads are slick. At the same time AWD doesn't improve
your handling, it does offer an overly optimistic sense of available
traction, and it provides the potential to be going so much faster when you
need to stop. (Note to those from warm climes: Snowbanks are not puffy and
cushiony.) The laws of physics mean a vehicle's cornering power is the job
of the tires and suspension.
"In the snow, it is all about the tires," says automotive engineer Neil
Hannemann, whose resume includes helping to develop the original Dodge Viper
, creating a proof-of-concept vehicle for the original all-wheel-drive
Chrysler minivan, and driving ice racers on frozen lakes. Having power to
four wheels rather than two sounds like it would help the car handle, which
is why you see those ads that infuriate me. But good tires beat AWD.
Some disagree, saying AWD helps bad-weather handling because it quells power
on oversteer, the fishtailing rear-drive cars experience when a ham-footed
driver is too rough on the accelerator. It is true that AWD is excellent at
preventing the tail from stepping out under power. But this is not "
improving handling." It's really aiding acceleration.
And it's true that some advanced AWD systems now on the market help the car
turn a little bit if the driver is assertively pushing the accelerator; they
do it by dragging the inside wheel and diverting more force to the outside
wheel. But my experience, and that of the test drivers I consulted, found
little more than a small benefit. Once the tires' grip limit is reached, no
more can be created. (For nitpickers and engineers: Yes, more aerodynamic
downforce will increase grip, but I'm talking about road-going vehicles at
highway-legal speeds, where that kind of performance edge isn't really
applicable.)
There are more advanced AWD systems on the horizon. These torque-vectoring
differentials are advanced versions of the current systems that cause one or
more tires to turn faster or slower. The goal of these systems is to
harness the grip all four tires have to offer.
Even so, we're talking about minor improvements. If you're looking for the
peace of mind in knowing that you'll be able to get home if an unexpected
snowstorm hits, AWD may be a good choice for you. However, if you think that
AWD will help your car better grip slippery corners or dodge an indecisive
squirrel, you're sadly mistaken. A good set of snow tires is a better
investment if you live where it snows frequently or if the highway
department is poor at plowing roads.
"All the best [AWD systems and electronic-stability control] will still get
beat by a good set of snow tires," Hannemann says.
http://autos.yahoo.com/news/the-myth-of-the-all-powerful-all-wh |
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