ALT-2 The Difference between all Wheel Drive and four Wheel Drive
There may be some confusion out there, but the systems variously called "All-wheel Drive" (AWD), "full-time four-wheel drive," "Symmetrical All Wheel Drive (Subarus)," and just plain Four-Wheel Drive (4WD) can be divided into two basic groups depending on their intended use. One group is best for serious off-road use, and the other primarily for aiding traction in foul weather on improved roads.
The system best suited for off-road use is 4WD. This system is sometimes called "part time" 4-wheel drive. That means there is a separate transmission called a "transfer case" that you must deliberately engage yourself to get the normally non-driven wheels to drive. The transfer case may have lower ('granny') gears available for crawling slowly up and down tight and steep terrain.
Even with 4WD, all four wheels are not necessarily driven. Without locking differentials, the wheel with the least traction will spin. I had a 4WD Toyota FJ40 in Alaska and can say from personal experience that without chains and locking differentials, four inches of wet snow and a 3% grade will stop you cold.
When considering a vehicle for genuine off-road work, don't assume that 4WD alone is enough. Other features like compact dimensions, ground clearance, articulation (ability to keep all four wheels on the ground despite extremes of terrain), approach, departure, and breakover angles, as well as undercarriage and roll-over protection, and tire size and tread pattern are important as well.
All the things that make a vehicle suitable for off-road work make it less appropriate on pavement. 4WD hardware is heavy, as are large wheels and tires. Heavy means lower performance and mileage. High ground clearance means a high center of gravity and tendency to roll over.
AWD has come to mean any number of 'full time' 4WD systems. These are really either front- or rear- wheel drive 99% of the time, but when the normally dominant end looses traction some part of the power is automatically directed to the other.
These vehicles have technologies like viscose clutches and electronic controls in a center differential to distribute engine power between the front and rear wheels depending on where it's needed. Another technology is the "Torsen" (torque-sensing) differential, a truly elegant design that uses the fact that a worm gear can only drive, and cannot be driven, to make a differential that can only drive the wheel with the most traction, providing traction that only a 4WD vehicle with locking differentials can beat.
AWD vehicles sacrifice off-road capability for on-road civility. They ride better, get better fuel economy, accelerate, brake, and handle better.
All these technologies can improve traction under certain conditions, if you recognize their limitations. The most important thing to know about AWD and 4WD is that they can get you into trouble, as well as out of it. Overconfidence in an AWD vehicle can put you off-road unintentionally, and really committed off-roaders usually do their off-roading in groups for mutual support. I once suffered the ignominy of having my Land Cruiser hauled out of a shallow ditch by a Ford Country Squire station wagon with chains.
