Mon09012014

On The Road

Differences between all-wheel and 4-wheel drives

Over the past few decades, more vehicles have been produced with power being transmitted to all four wheels.

Where four-wheel drive (4WD) was once the only method, and the sole province of truck-based vehicles with a military heritage, some variation of all-wheel drive (AWD) is now available on many cars and crossovers.

But we would wager most drivers of these vehicles don’t know how 4WD is different from AWD, or how either system operates.

Four-wheel drive

Let’s start with the acronyms. A four-wheel-drive vehicle – typically a full-size body-on-frame truck or sport-utility vehicle like the Jeep Grand Cherokee – uses a system that is a direct descendant of systems originally designed for military vehicles intended to operate off-road and at low speeds.

Not too long ago, the four-wheel-drive systems had to be manually engaged for low-speed, off-road use, locking the wheels to the axles and drivetrain. Then the system had to be disengaged once back on the pavement, to allow the wheels to turn at different speeds when negotiating curves at normal speeds.

In today’s vehicles, engagement and disengagement is electronically controlled, typically with a system of buttons on the console near the gearshift, but the principle of locking the differentials and changing the gearing for off-road and on-road use remains the same.

The advantage of these 4WD systems is that if the driver has the necessary skills, developed through practice and training, there are few places the vehicle cannot go. The disadvantage is that these systems are mechanically complicated, add to the cost of buying and maintaining the vehicle, reduce potential fuel economy and are almost never used by most 4WD vehicle owners.

All-wheel drive

By contrast, a vehicle with all-wheel drive – usually a standard unibody sedan or coupe like the Subaru Impreza – uses a less-complicated system developed for rally competition to improve control and handling on graded but unpaved road systems. These systems are engaged at all times, relying on automatic mechanical and electronic systems to actuate clutches on the differential that transmits power to the front and rear wheels and brakes on individual wheels.

The advantages of AWD systems are that they provide power to all four wheels, reducing the chance of sliding off the road or spinning out, especially in slippery conditions. They vary the amount of power to individual wheels to maintain vehicle control when traction is lost at one or another wheel due to varying surfaces. New systems add little weight to the vehicle, do not diminish potential fuel economy and add little, if any, cost to the price.

Tips on the two systems

• If an AWD system is standard or optional on the car you’re considering, we recommend it. You’ll be safer in all conditions and much less likely to have an accident due to slippery pavement.

• Unless you regularly plow through snow on unpaved roads or drive through mud or gravel on a farm or construction site, you probably don’t need 4WD.

• Keep in mind that for general use, to get the maximum value out of either 4WD or AWD systems in slick conditions, you must have all-weather or mud and snow (M&S) tires. Summer or high-performance tires negate nearly all the benefits of the systems.

If you do need to drive where chain-controls are in force, such as in the mountains during winter, the laws are specific. Whether the vehicle is 4WD or AWD, you can only drive without chains if the car is equipped with four matched winter or M&S tires, and you must also carry chains should conditions worsen.

• Just because your vehicle has one of these systems driving all the wheels, it does not make you immune to the laws of physics. Once all four wheels are sliding or spinning, the kind of drive system or tires no longer matters. At that point, you will be just along for the ride until you hit something or get stuck in the ditch.

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