What is all-wheel drive?
As the name implies, all-wheel-drive systems power both the front and rear wheels all the time. But in practice, there are actually two types of drivetrains that are called AWD. One does, in fact, drive all the wheels continuously, and some manufacturers refer to this as full-time AWD. The second, often called part-time AWD or automatic AWD, operates most of the time in either front-wheel drive or rear-wheel drive, depending on the vehicle's drive system. In these systems, power is delivered to all four corners only when additional traction control is needed.
How does all-wheel drive work?
Both full-time and part-time systems generally operate with no input from the driver, although some offer selectable modes that allow a degree of control over how much power goes where. All the wheels get torque through a series of differentials, viscous couplings and/or multi-plate clutches, which help distribute power to the wheels so that the car's traction is optimized. The vehicle still operates smoothly under normal conditions.
In full-time AWD, both the front and rear axles are driven all the time. On dry pavement, this kind of AWD can help the vehicle handle better and ensure that full power gets to the road. And in slippery conditions, such as ice, snow or mud, it provides always-ready traction for safer, more confident handling. Historically, a good example of full-time AWD was Audi's Quattro system, although in recent years the part-time Quattro Ultra variant has become more common in Audi's lineup because it's better on gas.
In normal operation, part-time AWD sends torque to two driven wheels, either the front or rear, depending on the make and model. The part-time system then automatically engages the other wheels when road conditions demand extra traction. Modern part-time AWD uses an array of electronic sensors that feed information to a computer, which controls the amount of power directed to each wheel. This setup is commonly found on car-based crossovers and AWD cars.
All-wheel-drive pros and cons
The best thing about AWD is that the driver doesn't have to make any decisions about engaging the system. Either all the wheels are being driven full time, or the system itself is designed to sense loss of traction and send power where it's needed. AWD is available on a wide variety of vehicles, from compact sedans to performance models to all sizes of SUVs, giving you a broad range of choices.
AWD is able to work well in a variety of conditions, from rain to snow to light off-roading, but it's considered a lesser choice by serious off-roaders. This perception is changing somewhat as modern AWD systems get better and more capable, but many drivers who like to venture far off the beaten path prefer the capability of a two-speed transfer case with low-range gearing, more on which below. AWD also increases the cost of a vehicle and, in most cases, will reduce fuel economy.
What is four-wheel drive?
This is the more traditional system that comes to mind when most people think of drivetrains that power all four of a vehicle's wheels. It isn't surprising since the principle goes back almost to the beginning of motorized transportation. The stereotypical picture of a 4WD vehicle is of a truck with high ground clearance, a shielded underbody, tow hooks and big, knobby tires. And it's true that this system is found primarily in trucks and SUVs.
But through the years, 4WD engineering has become increasingly sophisticated, and although it generally remains capable of more serious off-road use, it can now be found on a wider variety of comfortable, even luxurious, models. 4WD systems deliver torque through a series of front, rear and center differentials, transfer cases and couplings, which allow the vehicle to operate at maximum traction under a variety of conditions.
How does four-wheel drive work?
Like AWD systems, 4WD is designed to maximize traction front and rear. But 4WD systems tend to be more robust than AWD ones and can generally handle more rugged terrain. And they, too, come in two types: full-time and part-time.
Traditional 4WD systems have a two-speed transfer case with high- and low-range modes that can be selected by the driver, either with an electronic switch or a mechanical lever. The low-range setting multiplies torque to provide maximum traction in low-speed off-road environments. The high-range setting is useful for less challenging off-road scenarios as well as slippery on-road conditions, such as packed snow, ice, loose sand or gravel.
Full-time four-wheel drive
Full-time 4WD operates as a full-time AWD system does, with all four wheels receiving power on a continuous basis. Late-model Toyota Land Cruisers are a good example — they send power to both the front and the rear by default, so there is no standard two-wheel-drive mode (unlike typical 4x4 trucks with their part-time systems), but there's also a selectable low range for the really tough off-road stuff. In some designs, the driver may have the additional option of controlling how power is apportioned to the front and rear axles through selectable modes and locking differentials.
Part-time four-wheel drive
This type is the real traditionalist of four-wheel propulsion and can most often be found in trucks and SUVs that are designed to work and play in extreme conditions. In this case, the vehicle is typically rear-wheel drive by default, so the four-wheel-drive system requires the driver to opt in by either pushing a button or shifting a lever. Locking center differentials are par for the course, but many part-time 4WD systems also allow the driver to lock the vehicle's rear differential, which ensures that both rear wheels get power no matter what. Hardcore setups let you lock the front differential, too — this is the hallowed "triple-locked" configuration that means you'll only get stuck if all four wheels have no traction.
Four-wheel-drive pros and cons
There's no substitute for true 4WD in tough off-road scenarios. Even though 4WD systems are now available in well-appointed luxury trucks and SUVs, at heart they are designed for ruggedness and maximum traction when you need it most, so they are the right choice for difficult terrain.
These days, 4WD design has become increasingly refined, as has the design of the vehicles that can be ordered with it. But, depending on the make and model, 4WD vehicles still often deliver a stiffer ride than their two-wheel-drive counterparts. These systems also have a detrimental effect on fuel economy and increase the initial cost of the vehicle.