Four wheel drive
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Four wheel drive, 4WD, 4x4, all wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people think exclusively of off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads.
Four wheel drive (4WD or 4x4 for short) was the original term, often used to describe truck-like vehicles that required the driver to manually switch between a two wheel drive mode for streets and a four wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all wheel drive" term (AWD for short) was invented to distinguish vehicles that are capable of driving all four wheels on normal roads without causing poor control and excessive tire and drivetrain wear. Most modern AWD vehicles do not continuously drive all four wheels, but instead switch from two wheel drive to four wheel drive automatically as needed.
The buyer must be wary. It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, both Quattro and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential.
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Design
When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.
One could avoid the following mechanical complexity by simply using one electric motor per wheel, with speeds under computer control. This is not normally done though, so...
A differential allows one shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requies a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles.
Such a design would handle very well. It distributes power evenly and smoothly, making it unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be... making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simplistic design works acceptably well for a 2WD vehicle. Since a 4WD is twice as likely to have a driven wheel on an icy patch, the simplistic design is usually considered unacceptable.
Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake can be automatically applied to that wheel. The torque will then be matched, causing power to be divided between the pavement (for the non-slipping wheel) and the brake. This is effective, though it does cause brake wear and a sudden jolt that can make handling less predictable. By extending traction control to act on all four wheels, the simple 4WD vehicle design based on three differentials can now recover from wheel spin. One nice feature of this design, is that it is traction control, and thus will not work against traction control. This design is commonly seen on car-like luxury SUVs.
Another way to solve the problem is to temporarily lock together the differential's output shafts, usually just for the center differential that distributes power between front and rear. Recall that a drivetrain without differentials will fight the driver, causing tire wear and handling problems. This is of little concern when the wheels are already slipping. One very common design joins the output shafts together via a multi-plate clutch under computer control. This design causes a small jolt when it activates, which can disturb the driver or cause more wheels to lose traction. Another common design uses a viscous coupling unit. A dilatant fluid inside the viscous coupling unit acts like a solid when under shear stress caused by high shaft speed differences, causing the two shafts to become connected. This design suffers from fluid degradation with age and exponential locking (joining) behavior. It can also waste fuel, because it requires that there be a slight shaft speed difference under normal driving conditions (via gearing) to prepare to fluid for operation. Older designs used manually operated locking devices.
Yet another way to solve the problem is via a Torsen differential. When a normal differential is replaced with a Torsen differential, it is possible to drive the output shafts with different amounts of torque. While this is useless in a zero-torque situation, it will help greatly when the slippage is not so extreme. As the slipping side begins to spin out of control, more power is delivered to the other side. A typical Torsen differential can deliver up to twice as much power to the non-slipping side as it delivers to the slipping side. Most Audi Quattro cars, notably excluding the A3 and TT, use a center Torsen differential. For a time, the Volkswagen Passat 4motion shared this design. The HMMWV uses front and rear Torsen differentials, but only has a normal differential in the center. Torsen differentials generally work very well, though they are expensive and heavy.
Many vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and real axles. Such systems distribute power unevenly under normal conditions, and thus do not help prevent loss of traction; they only enable recovery once traction has been lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95%/5% or 90%/10% power split. Light trucks and SUVs tend to use multi-plate clutches under computer control, often with 100% of the power going to the rear axle under normal conditions. Sports cars using this type of system always drive only the rear under normal conditions. For example, Lamborghini uses a viscous coupling unit to drive the front, and the Nissan Skyline GT-R uses a clutch. The Audi TT normally powers the front, and has a multi-plate clutch to power the rear.
History
Although the first experimental four wheel drive car was built in 1906 (by Otto Zachow and William Besserdich of Clintonville, Wisconsin), the layout was considered overly complex and unnecessary for the times. It was not until "go-anywhere" vehicles were needed for the military that four wheel drive found its place. The Jeep, developed by American Bantam, became the best-known four wheel drive vehicle in the world during World War II. Willys (owner of the Jeep name) introduced the CJ-2A in 1945 as the first full-production four wheel drive passenger vehicle.
However, it was not until Jensen applied Ferguson Formula's four wheel drive system to their 1966 Jensen FF that the system was used in a production sports car. Bugatti created a four wheel drive racer, the Type 53, in 1932, but the car was legendary for having poor handling. Niche maker Panther Westwinds holds the crown for creating the first mid-engined four wheel drive, the Panther Solo 2, in 1989. Today, sophisticated all wheel drive systems are found in many passenger vehicles and most exotic sports cars and supercars.
Terminology
Although in the strictest sense, the term "four wheel drive" refers to a capability that a vehicle may have, it is also used to denote the entire vehicle itself. In Australia, vehicles with offroad capabilities are referred to as "four wheel drives". This term is sometimes also used in North America, somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles.
The term 4x4 (read either four by four or full times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive. The term 4x4 is common in North America and is generally used when marketing a new or used vehicle, and is sometimes applied as badging on a vehicle equipped with four wheel drive. Similarly, a 4x2 would be appropriate for most two-wheel-drive vehicles, although this is rarely used in practice, as vehicles are assumed to be two-wheel-drive unless stated otherwise. A 2×4, however, is unambiguously a piece of lumber.
Oddly, large American trucks with dual tires on the real axles (also called duallys or duallies) and two driven axles are officially badged as 4x4s, despite having six driven wheels. Presumably, separate 6x6 badging would be largely inconsequential as the production of these vehicles is somewhat limited and most consumers understand that 4x4 denotes vehicles with two driven axles. It may be alternatively appropriate to consider that each group of rear tires (one each on the left side and right side of the axle) constitutes a "wheel" and that, in this sense, the truck is truly a 4x4. The famous "Deuce and a Half" truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6x6.
Another related term is 4-wheeler (or four wheeler). This generally refers to all-terrain vehicles with four wheels and does not indicate the number of driven wheels; a "four wheeler" may have two or four wheel drive.
Four wheel drives in Australia
There are two main players in the Australian market: Toyota and Nissan. Most consumers will choose one of the two brands and generally stay with it for life. The typically more massive American four wheel drive trucks and SUVs are generally not as popular among Australian consumers because they are not well suited to the Australian outback. They are often not rugged enough for the harsh conditions, and with their typically larger size they are too wide to fit on the existing wheel tracks created by previous cars (so the driver ends up attempting to carve out his or her own track). As in other countries, four wheel drives have become popular with city-dwelling people, who by and large will never actually drive "off road."