Are 4 wheel drive and all wheel drive the same?

Four-wheel drive, All-wheel drive, AWD, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. 1234 While many people associate the term with off-road vehicles and Sport utility vehicles, powering all four wheels provides better control than normal road cars on many surfaces,citation needed and is an important part in the sport of rallying. In abbreviations such as 4×4, the first figure is normally the total number of wheels, and the second the number of powered wheels.

(The numbers actually refer to axle-ends, which may have more than one wheel.) 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive. 5 By this system, a six wheeled military transport truck would be a "6x6", while the typical American semi-truck tractor unit having two drive axles and a single unpowered steering axle would be a "6x4". Four wheel drive refers to vehicles that have a differential that may or may not be lockable, between the front and rear axles, meaning that the front and rear drive shafts will not rotate at different speeds.

This provides maximum torque transfer to the axle with the most traction, but can cause binding in high traction, tight turning situations. They are also either full-time or part-time 4WD selectable. All wheel drive refers to a drive train system that includes a differential between the front and rear drive shafts.

This is usually coupled with some sort of anti-slip technology that will allow all wheels to spin at different speeds, but still maintain the ability to transfer torque from one wheel in case of loss of traction at that wheel. All wheels are engaged to the drive full-time. The term Individual-wheel drive is coined to identify those electric vehicles whereby each wheel is driven by its own individual electric motor.

This system essentially has inherent characteristics that would be generally attributed to Four-Wheel drive systems like the distribution of the available power to the wheels. The IWD drive is not limited to 4 wheels as there is generally a motor that drives each wheel that can number upwards of 4, but could also identify a single wheeled vehicle. When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves.

This is accomplished with a differential. A differential allows one input shaft (e.g. - the driveshaft of car or truck) to drive two output shafts (e.g. - axles shafts that go from the differential to the wheel) independently 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 differential ring gear.

Each powered axle requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third or 'center' differential can be used to distribute power between the front and the rear axles. The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making slippage unlikely.

Once it does slip, however, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel will spin faster than the other wheels due to the lower traction at that wheel. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction.

This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles, because 4WD vehicles have twice as many wheels with which to lose traction, increasing the likelihood that it may happen.

4WD vehicles may also be more likely to drive on surfaces with reduced traction. However, since torque is divided amongst four wheels rather than two, each wheel receives approximately half the torque of a 2WD vehicle, reducing the potential for wheelslip. Many differentials have no way of limiting the amount of engine power that gets sent to its attached output shafts.

As a result, if a tire loses traction on acceleration, either because of a low-traction situation (e.g.