- The Washington Times - Friday, February 15, 2002

In the popular sport utility vehicle trend, one major issue always seems to come to the forefront.

No, I'm not talking about size, price, color or engine size I'm referring to the method by which the vehicle in question transfers power from the engine to the pavement, or unpaved surface in the case of off-road adventuring.

There are two basic systems to consider with variances specific to each. The first system is all-wheel drive (AWD). AWD is generally a full-time system that requires no intervention from the operator of the vehicle it is automatic and operates by measuring and distributing the power proportionately to the wheels that most need it.

In situations where traction is minimal for instance, power may be equally rationed between front and rear wheels. On ideal road surfaces, under normal-traction conditions, the traditional power split would provide up to 90 percent of the vehicle's power to the rear wheels as a driving force and only 10 percent to the front wheels.

In more sophisticated AWD systems each individual wheel electronically reports slippage with the others compensating power transfer to obtain optimum traction. Most AWD-system-equipped vehicles employ automatic transmissions.

An alternative system is four-wheel drive (4WD). This system may incorporate either the use of a manual transmission or an automatic shifter and operates in the following modes: 2 high (rear wheels only for high speed normal surface driving); 4 high (equal distribution of power to both front and rear wheels where added traction is needed; and 4 low (with exceptionally low gearing where maximum traction is an important consideration). Many automatics provide a separate shift lever to enter low, low range. Shifting may, in many cases, be accomplished "on-the-fly" (without stopping) when moving from 2 high to 4 high or vice versa (this is with automatic-locking hubs). Most systems require stopping or at least slowing to 3 mph or less when shifting into or out of low, low range.

When a vehicle is equipped with manual hubs, it is always necessary to stop and exit the vehicle to physically rotate the hubs in order to engage the locking mechanism.

Without going into a detailed operational explanation of each system, which can become technically cumbersome, it should suffice to point out the predominant vehicle applications. AWD may be found on several automobiles, some minivans and many SUVs where off-road operation is not really a consideration.

4WD is the system of choice for those who intend to operate their vehicles off-road under more rugged applications and where a low, low range is a definite necessity. 4WD is more apt to be found on serious SUVs and trucks where maximum torque must be used to ensure needed pulling power, and positive traction at all four wheels.

There are merits to both systems. Be sure to decide how you will use your vehicle and select the most economical and most effective system to suit your needs. If you have no plan of going off into the wilderness to blaze new trails in the name of adventure, you certainly don't need 4WD with a super-low range.

AWD will do you proud for those family ski trips and you probably won't have to chain up to get there.


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