Some praise the advantages of all-wheel drive (AWD) or four-wheel drive (4WD or 4×4) vehicle over the typical front-wheel drive (FWD) or rear-wheel drive (RWD) vehicle. Four wheels driving the vehicle has some major benefits when it comes to traction. On loose road surfaces, such as mud, gravel, sand, or rain, it is easier to maintain traction and forward momentum, as well as directional stability, with four wheels pulling. Even if one or two tires lose traction, the other tires can still pull. On the highway, four wheels driving makes cars especially maneuverable, and even sports cars use this technology to give their cars that extra boost in driving performance.
Generally-speaking, AWD, 4×4, or 4WD all refer to the drivetrain being set up to transfer engine power to all four wheels, but these terms are not interchangeable. Engine power goes through the transmission to a transfer case, which then transfers power to the front and rear axles, that is, all four wheels. Actually, 4×4 refers to four-wheeled vehicle with four wheels powered, so a 6×6 Unimog would be a six-wheeled vehicle with six wheels powered, while a 6×4 Unimog would be a six-wheeled vehicle with only four wheels powered. Keep this in mind when you see a 4×2 pickup truck, which is a four-wheeled vehicle with just two wheels powered, typically the rear, which would make it a rear-wheel drive (RWD) pickup truck. The best way to describe the difference between AWD and 4WD is the center differential, or lack thereof, but lets start with the differential.
What's the Differential?
In each driven axle, a set of differential gears allows for the wheels to turn at different speeds. Driving straight ahead, the differential gears drive both the left and right wheels at the same speed. When going around a turn, however, the wheel on the outside of the turn will turn faster than the wheel on the inside of the turn. The differential gear allows for this. Otherwise, the tires would squeal around the turn, since one of them would have to skid to go the same speed as the other.
There are three kinds of differentials, open, limited-slip, and locking. Open differentials will allow one wheel to spin free if that tire loses traction, so no engine power will go to the opposite wheel, even if that tire as traction. A limited-slip differential is designed to prevent this from occurring, but not 100%. It is still open enough to allow for regular road travel. A locking differential is kind of like eliminating the differential altogether. In low-traction conditions, locking the differential sends the same amount of engine power to both wheels. Driving on the road with the differential locked, however, would lead to binding and scuffing around corners.
AWD – Modern Technology
All-wheel drive or full-time four-wheel drive refers to a 4×4 drivetrain that cannot be disengaged. It is always working, but how? We already know that there are two differentials in a 4×4, one in each axle, both front and rear. When going around a turn, the front wheels go faster than the rear wheels, which is why, in an AWD transfer case, there is a third differential. As we learned, the differential allows for speed and power differences, in this case, between the front and rear axles. The center differential is typically of the limited-slip type, which helps to maintain traction in very bad conditions, while allowing for everyday driving on normal road surfaces.
The front and rear differentials in an AWD vehicle may be open, limited-slip, or locking, depending on vehicle type and options. Some AWD vehicles, designed for off-road use, feature locking front, rear, and center differentials for maximum off-road traction, which is typically engaged electrically.
4WD – Back to Basics
This type of 4×4 drivetrain, 4WD, does not have a center differential, but it can also be engaged or disengaged, via shift lever or buttons. The transfer case in a 4WD vehicle does not allow for differences in speed or engine power between the front and rear axles, which can lead to binding and scuffing when on good road surfaces.
Off-roading, however, is where you really appreciate no slippage in the 4WD drivetrain, as long as you have the right differentials in the front and rear axles. Differentials in 4WD vehicles can also be of the open, limited-slip, and locking type. A locking differential offers maximum traction, while an open differential can result in zero traction, depending on how bad the road is. A limited-slip differential offers a good balance between the two, making for smooth on-road handling and acceptable off-road traction.
What Was All That About?
When comparing 4×4 vehicles, perhaps the best thing to keep in mind is what you will be using the vehicle for. If you plan on doing serious off-roading, or the roads you regularly travel are barely recognizable as roads, then you could opt for a 4WD vehicle, as long as you learn how to use it. Make sure to look for a model with at least limited-slip differentials, but a better option will be locking differentials. Some 4WD vehicles come fully equipped with locking differentials, but there are aftermarket options, as well, for the serious off-road enthusiast.
On the other hand, for most drivers, commuting to work and school or taking the family to market, AWD offers traction and safety that you don't have to think about. You don't have to remember to engage the system to make it work, because it's always engaged. If you need more traction, some models offer locking differentials, which require nothing more than pressing a button. Just remember to press the button again to disengage the system when you get back onto a good road.