4 Wheel Drive Zone

Most early motor vehicles had a basic set up with the engine mounted in the front with a gearbox attached to it driving the rear wheels only and this became the normal design of most vehicles for many years. As design progressed engine position and transmission (gearbox) position was experimented with to help with traction and handling. Rear engine and mid-engine vehicles became available as did front and all wheel drive. Four wheel drive vehicles began life early in the twentieth century and a number of companies had their own designs for supplying off road trucks for business and commercial use. The importance of having load carrying vehicles that could transverese harsh terrain was important due to the lack of roads. Both the British and American armies during World War One had four wheel drive trucks for heavy terrain use for carrying troops and equipment. However the need for a true multi purpose on and off road vehicle became apparent.

With the onset of World War Two the need for off road vehicles that could literally drive over any terrain became a necessity of the military and with any technology war tends to drive designs forward quickly. In the early days of the war the United States Military required a light-weight four wheel drive vehicle that could transverese almost any terrain. A number of companies came forward with designs including Bantam, Ford and Willys. The result of this was the world famous Willys Jeep. The jeep proved to be both very rugged and dependable through out the war and by the close it had become well known for its on and off road ability and its durability.

After the war the potential within the civilian market for jeeps was clearly apparent and Willys filed for the trademark registration of the name Jeep and began production for the public sector. The first models named Civillian Jeeps (CJ) began production in 1945 and the name Jeep became an icon of tough go anywhere vehicles soon to become a legend to future four wheel drive owners. The history of the Jeep and production models are as follows:

1945 - 1949 CJ-2A
1946 - 1953 CJ-3A
1947 - 1965 Willys Jeep Truck
1948 - 1950 Willys Jeepster
1952 - 1968 CJ-3B
1954 - 1983 CJ-5
1955 - 1981 CJ-6
1976 - 1986 CJ-7
1981 - 1986 CJ-8
1953 Kaiser buys Willys Overland.
1970 American Motors Corporation AMC takes over Kaiser-Jeep.
1987 Chrysler Corporation buys American Motors Corporation.
1998 Daimler-Benz merges with Chrysler Corporation.

The modern day Chrysler Jeep has many models and incorporates all the benefits of modern technology and comfort but it has tried to retain some of the old willys features as the front grill shows.
Back in the United Kingdom the Land Rover project was first born and the legend began.

The Lamborghini Murciélago is a 4WD/AWD that powers the front via a viscous coupling unit if the rear slips.  The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a manually lockable center differential, with Torsen differentials for both front and rear.  A Subaru Impreza rally car uses AWD for traction on loose dirt.When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. The problem is even more complicated when driving all four wheels. 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.

A differential allows one input 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 requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third differential can be used to distribute power between the front and the rear axles.

Such a design handles well. It distributes power evenly and smoothly, and makes 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. Although the amount of torque applied to each wheel will be identical, the amount of traction at each driven wheel will be limited to that of the wheel with the least traction (all four wheels on ice in this case). 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 but, since 4WD vehicles are more likely to be driven on surfaces with reduced traction, the differential design is less acceptable.

Limiting slippage

Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control the brake is automatically applied to that wheel. By preventing one wheel from spinning freely power is divided between the pavement for the non-slipping wheel and the brake for the slipping wheel. This is an effective solution, although it causes additional brake wear and may cause a sudden jolt that affects handling. By extending traction control to act on all four wheels the simple three-differential 4WD design will see limited wheel spin. This design is commonly seen on luxury crossover SUVs.

Locking differentials temporarily lock 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 dilatant 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 which hastens torque transfer.

A third approach to limiting slippage is the 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. It 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 tractive side.

Finally, many lower-cost 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 rear axles. Such systems distribute power unevenly under normal conditions and thus do not help prevent the loss of traction, instead only enabling recovery once traction is 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.