4x4 / 4WD / AWD / IWD Information
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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.
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.
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