Applications A part-time four-wheel-drive system that will not be used on pavement needs only two differentials, one for each axle. In theory, a full-time four-wheel- or all-wheel-drive vehicle will need three differentials, one for each axle and one between the front and rear axles. However, as a wider variety of drive systems make their way into trucks and crossovers, a center differential is not always necessary.
Some AWD and full-time 4WD vehicles use torque split to maintain the driving dynamics of a 2WD. In vehicles derived from front drives, many are front drive normally and add rear drive as required: The amount of output transferred to the rear is often a function of strength--the Saturn VUE four-cylinder can direct 69-percent power to the rear, while the V-6 sends a maximum of 57 percent. Conversely, vehicles like the Escalade and X5 run a rear-biased torque split, usually near 1/3 or 2/3 percentage, which maintains the dynamics of a rear-drive system, while adding some front drive for directional stability and low-traction steering improvement. Systems like Infiniti/Nissan's All-Mode are designed to work in "auto" for best handling and performance and offer the "2WD" position primarily for dynamometer and emission testing.
A front axle disconnect system slides a collar (arrow) over two parts of the axle shaft so that both front wheels are driven. The collar may be powered by heat, electricity, vacuum, or hydraulics. This is a Toyota ADD system.
In vehicles derived from most two-wheel-drive trucks and sport/utilities, primary drive is to the rear. When four-wheel drive is selected, drive is added to the front axle from the transfer case. The connection between front and rear driveshafts in the transfer case is mechanically locked in part-time systems, which is why they can't be used on pavement where there is no tire slip. In full-time and AWD systems, the connection must have some means of allowing a small difference in front and rear driveshaft speeds so the truck can get around a corner without binding.
This can be accomplished by using another (center) differential in the transfer case, so front and rear drive can operate with some independence. In many cases this center differential may be locked (sometimes only in low range) for off-pavement use, as with the Hummer, earlier Range Rovers and Discoverys, and Chrysler's Selec-Trac.
Making Connections The connection may also be made by a viscous-coupling unit (VCU), a sealed canister or barrel-shaped device that's filled with thixotropic silicone fluid and a series of interwoven plates resembling CDs in a changer magazine. One set of these plates is attached to the drum shell, and the others are attached to a driveshaft through the middle. As the speed varies between the two sets of plates, because one driveshaft is rotating faster than the other (either by turning or by wheels slipping), the silicone reacts to the heat being generated by thickening, which in turn starts to limit the speed differential between the two sides of the VCU. This allows the slip necessary to avoid binding, without allowing one axle to spin while the other does nothing. Some VCUs have a locking function for off-road use (Montero, Range Rover) while others do not (Escalade).
Another alternative is an electro-magnetic clutch, initially used on Ford's 90's Aerostar AWD and now found in its ControlTrac (Expedition, Explorer, Navigator). This system uses an electromagnetic mechanism controlled by a computer receiving input from driveshaft speed sensors to variably apply pressure to a clutch pack in the transfer case. In "auto" this system defaults torque split 98 percent to the rear, adding front drive only as slip is detected. While the system can vary up to 100 percent output to the front, complete lack of traction at the back would imply at least partial loss at the front, so the front-axle assembly will not have to cope with all the engine's output. This is an important consideration--even a small V-8, run through low-range first gear at full throttle, could generate 10,000 lb-ft of torque to one axle, and many front axles simply won't take it.