AWD Leaves 4x4s Spinning Their Wheels

It must be hip to have four-wheel drive. How else to explain the herds of trendy sport-utility vehicles on the road, when most drivers admit they never take these rugged vehicles off road?

The companies that make SUVs know that the people who drive them, especially in the Snow Belt, mostly want to make it home safely, loaded with kids, when weather stands in the way.

Car buyers may think they want four-wheel-drive, but car engineers know better.

Enter a new trend - full-time all-wheel-drive. The advantage of these AWD systems over the more traditional four-wheel-drive (4WD) is simple. "It's what the driver has to do," says Marc Bartalini, a salesman at Grava Jeep in Medford, Mass. "Nothing."

In return, the driver gets better stability and confidence on slippery surfaces.

Four-wheel-drive.... All-wheel-drive. What's the difference?

It boils down to this: With all-wheel-drive, all four wheels receive power from the engine all the time. With four-wheel-drive, only two wheels - usually in the rear - receive power until the driver flips a lever or pushes a button to power up the other two.

Technically, the difference is slightly more complicated. Most four-wheel drive - or part-time - systems effectively lock the front wheels to those in the rear. But when the vehicle turns a corner, the front wheels travel farther than the rear ones. So they need to be able to rotate faster.

With 4WD, they can't. The front wheels are effectively locked to the same speed as those in the rear. This doesn't matter much on a slippery surface - snow, ice, mud, sand, even dirt, where tires can slip to take up the slack. But on dry pavement, the front wheels must stay disengaged, so they can spin faster.

AWD systems are designed so both sets of wheels can receive power, even going around a turn on a dry road.

In snow, both systems work equally well. Where AWD really helps is a few hours later, after the plows have scraped most of the roads dry, but temperatures stay buried below freezing.

With most of the roads dry, 4WD systems should be disengaged to prevent damage to the vehicle. But AWD systems are still working, taking occasional patches of slush or ice in stride.

The best system for light snow is all-wheel-drive, second best is ordinary front-wheel-drive, followed by either four-wheel-drive or rear-wheel drive, depending on the vehicle. (But weight is a big factor in two-wheel-drive vehicles: Many heavy rear-wheel-drive vehicles go better than small, lightweight front-drive cars, despite the former's poor stopping and turning abilities.)

In light snow, 4WD systems are worse than front-wheel-drive, because the 4WD system actually causes tires to slip in turns - and once a wheel is sliding it will tend to keep sliding and not contribute to keeping the car out of a ditch.

Jeep pioneered AWD trucks in the late 1970s and bolted the system on some of its American Motors cars. It was primarily Audi, with its Quattro sports car, that popularized on-road AWD in the early 1980s.

The recent craze in sport-utility vehicles, however, sprang from trucks that were designed to go off-road, where part-time 4WD systems are more durable.

Many sport-utility makers now realize that competent, roomy snow cars are what buyers really want, evidenced by the popularity of Subaru's AWD Outback station wagon.

At the same time, many carmakers that used to offer AWD passenger cars - Ford, Toyota, Volkswagen, and Nissan, for example - have abandoned that market to the two traditional players: Audi and Subaru. Those two carmakers have seen dramatic sales increases in the past year.

All-Wheel-Drive VehicLEs Cars

Audi A4, A6, and A8 Quattro

Subaru Impreza, Legacy, SVX

Mitsubishi 3000GT VR4

Eclipse GSX

Eagle Talon (optional)

Sport-utility vehicles

Chevrolet Blazer (optional)

GMC Jimmy (optional)

Oldsmobile Bravada

Ford Explorer (optional)

Expedition (optional)

Jeep Cherokee (optional)

Grand Cherokee (optional)

Toyota Land Cruiser

Previa and RAV4 (optional)

Lexus LX 450

Infinity QX4

Mazda MPV

Honda CR-V

Land Rover Defender

Range Rover 4.0 SE and Discovery

Dodge Caravan (optional)

Plymouth Voyager (optional)

Chrysler Town & Country (optional)

Ford Aerostar (optional)

Chevy Astro/GMC Safari (optional)

Due out next year:

Mercedes-Benz E300 wagon

M-class sport-utility

Volkswagen Passat wagon

Subaru Forester sport-utility