Myth of the all-powerful all-wheel drive (ZT)

"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.

所有跟帖： 

• 不敢苟同。有時 ”專家“ 把簡單的事情想得太複雜了。 -southmountainer- ♂ (282 bytes) () 03/24/2013 postreply 14:04:59

• 4wd/awd在陷住脫困時有幫助 -dola- ♂ (111 bytes) () 03/24/2013 postreply 14:43:58

• 這個也難說，我以前的老Volvo 850 雪地裏不比4驅奧迪差 -主教- ♂ (167 bytes) () 03/25/2013 postreply 08:35:35

• all-wheel drive的關鍵是中央差速器和各輪獨立刹車的電子穩定係統 -追風大老虎- ♀ (0 bytes) () 03/25/2013 postreply 15:34:57

• 這個磚家這種類比就有問題 -expertofdeduction- ♂ (348 bytes) () 03/25/2013 postreply 15:50:21

• 我想他這篇的主要目的，是讓人不要因為有全驅就盲目地大膽妄為 -色大膽小- ♂ (0 bytes) () 03/26/2013 postreply 05:43:32

• 目前開過的AWD，Saab,Subaru的比較可靠。CR-V的是最不靠譜的。Jeep 算4x4吧，就不摻合了。 -palmdodo- ♂ (0 bytes) () 03/26/2013 postreply 08:27:01