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Four-by-four (4x4) refers to the general class of vehicles. The first figure is normally the total wheels (more precisely, axle ends, which may have multiple wheels), and the second, the number that are powered. Syntactically, 4x2 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.


Four wheel drive (4WD) refers to vehicles that have a transfer case, not a differential, between the front and rear axles, meaning that the front and rear drive shafts will be locked together when engaged. This provides maximum torque transfer to the axle with the most traction, but can cause binding in high traction turning situations. They are also either full-time or part-time 4WD selectable. 4WD is not intended for high speeds without a limited-slip mechanism.


All wheel drive (AWD) or "permanent multiple-wheel drive" refers to a drive train system that includes a differential between the front and rear drive shafts. This is normally coupled with some sort of anti-slip technology that will allow differentials 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. Typical AWD systems work well on all surfaces, but are not intended for all consumers.


Individual-wheel drive (IWD) was 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. However, because of the inherent characteristics of electric motors, torque can be negative, as seen in the Rimac Concept One and SLS AMG Electric. The can have drastic effects as in better handing in tight corners.




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 a car or truck) to drive two output shafts (e.g. - axle 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 right sides. When all four wheels are driven, a third or 'centre' differential can be used to distribute power between the front and 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 wheel slip.


Limiting slippage

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. - driving on gravel or ice) or the engine power overcomes available traction, the tire that is not slipping receives little or no power from the engine. In very low traction situations, this can prevent the vehicle from moving at all. To overcome this, there are several designs of differentials that can either limit the amount of slip (these are called 'limited-slip' differentials) or temporarily lock the two output shafts together to ensure that engine power reaches all driven wheels equally.

Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, providing torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.

The two most common factory-installed locking differentials use either a computer-controlled multi-plate clutch or viscous coupling unit to join the shafts, while other differentials more commonly used on off-road vehicles generally use manually operated locking devices. In the multi-plate clutch the vehicle's computer senses slippage and locks the shafts, causing a small jolt when it activates, which can disturb the driver or cause additional traction loss. In the viscous coupling differentials the shear stress of high shaft speed differences causes a dilatants fluid in the differential to become solid, linking the two shafts. This design suffers from fluid degradation with age and from exponential locking behavior. Some designs use gearing to create a small rotational difference that hastens torque transfer. A third approach to limiting slippage is taken by a Torsen differential. A Torsen differential allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where there is no torque, but provides excellent handling in less extreme situations. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower traction side. A fairly recent innovation in automobiles is electronic traction control. Traction control typically uses a vehicle's braking system to slow a spinning wheel. This forced slowing emulates the function of a limited-slip differential, and, by using the brakes more aggressively to ensure wheels are being driven at the same speed, can also emulate a locking differential. It should be noted that this technique normally requires wheel sensors to detect when a wheel is slipping, and only activates when wheel slip is detected. Therefore, there is typically no mechanism to actively prevent wheel slip (i.e. you can't "lock the differential" in advance of wheel slip), rather the system is designed to expressly permit wheel slip to occur, and then attempt to send torque to the wheels with the best traction. If preventing all-wheel slip is a requirement, this is a limiting design.

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