Four wheel drive or 4x4, is a type of four wheeled vehicle drivetrain configuration that enables all four wheels to receive power from the engine simultaneously in order to provide maximum traction. Usually the term refers to part-time systems, designed only for use in low-traction conditions (e.g. off-road, on snow or ice). All-wheel drive (AWD, also called full-time four-wheel drive) is designed to function on all types of surfaces, both on- and off-road; typically it cannot be turned off.

Part-time four wheel drive

When a part-time four wheel drive system is enabled, the transfer case connects both the front and rear drive shafts with the transmission. These drive shafts are connected to differentials which distribute torque to the wheels via the axle. The sum of the output angular velocity of the two output shafts (connected to the wheels) is linearly related to the angular velocity of the input shaft (connected to the transfer case). However, the two output shafts do not have to rotate at the same speed. During a turn, the inner wheel is spinning slower than the outer wheel. Torque is distributed evenly between the two output shafts.

However, since the front and rear differentials must spin at the same speed (due to the transfer case) the sum of the angular velocity of the front wheels must be equal to that of the rear wheels. During a turn, the front wheels travel a slightly shorter distance than the rear wheels. Therefore there must be some "slip" in another part of the system. This slip occurs easily at the interface of the tire with the ground when traveling in low-traction conditions. However, when traveling on dry pavement the torque required to cause this slip causes excessive wear on the tyres and the rest of the drive-train. This is why the part-time four-wheel drive system must be disabled when operating the vehicle on pavement. In this configuration, the transfer case is connected to only one of the two differentials (usually the rear).

Part-time four wheel drive systems are ideal for use in very difficult traction conditions since they allow all of the torque to be sent to the rear or front wheels. However, if one of the front tires and one of the rear tires is on a slippery surface little traction can be achieved since torque to the left and right is distributed evenly by the differentials. This problem can be addressed by using a locking differential or a limited slip differential.

Full-time all-wheel drive

A full-time all-wheel drive system must be designed to allow the front and rear differentials to operate at slightly different speeds so that the tires do not need to "slip" during a turn on dry pavement. This can be achieved in a variety of ways. The simplest system involves an additional differential (called the center differential) to distribute torque to the front and rear differentials. The disadvantage of this system is that each wheel can only receive as much torque as the wheel with the least traction can receive. For example, if you have one tire on ice you have the same amount of traction as if all of your tires were on ice. While this system is not ideal for maintaining traction on poor surfaces, it does improve handling in all situations. If one tire slips, the torque applied to the other tires is reduced, preventing them from slipping. When all wheel drive is used in sports cars designed for on-road use, the typical goal is to improve the handling, so a center differential system works well.

One method of improving the traction of the basic center differential all-wheel-drive system is by using a viscous limited-slip center differential. This differential automatically locks the drive shafts together when an excessive difference in speed is present. Many vehicles employ this system, for example manual transmission Subaru and Volkswagen vehicles.

Another method of improving AWD is by having an electronic system apply the brake to the wheel that is spinning. By applying the brake to the wheel that has little traction, more torque can be sent to it and therefore more torque can be sent to the other wheels. Since the other wheels do not have their brakes engaged the extra torque sent to them can be used to help propel the vehicle. This system is used in many all-wheel drive systems installed in luxury vehicles, LUVs.

A different type of system often called "full time all-wheel-drive" is more correctly termed "automatically engaging part-time all-wheel-drive". In these systems, power is normally applied to only one pair of wheels. When wheel slip is detected, a computer or a hydraulic pump progressively engages a clutch that transfers power to the other pair of wheels; one such car that uses such a system is the Nissan Skyline GT-R, which would otherwise be RWD under casual circumstances. Alternatively, the power transfer can be accomplished with something like a viscous coupling (without a differential). The viscous couplings in this case use a dilatant material that undergoes an extreme increase in viscosity with slippage, thus providing rapidly increasing torque transfer to the secondary drive when the primary tires slip. These systems are often installed to aid traction in slippery conditions such as wet or snow, not as a means to travel off-road. The confusion between the two different methods is usually due to marketing materials that do not distinguish them. In fact, there have been occasions when the same brand name has been given to different types of systems (for example, Audi's quattro refers to a full-time system available in most of their models, except for the the A3).

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 both North America and Australia, vehicles with offroad capabilities are sometimes referred to as "four wheel drives." This term is used somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles.

The term 4x4 (read four by four) is used to denote the total number of wheels on a vehicle and the number of driven wheels. This term 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.

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."

See also

• Category:All wheel drive vehicles