The standard automotive powertrain for the majority of the 20th century was the front engine, rear-wheel-drive (RWD) design. The rear axle assembly housed the differential and individual axles, and it is through this assembly that power was transferred to the wheels.
Even though both front-wheel-drive (FWD) and four-wheel-drive (4WD) cars were also designed and manufactured during the early years of the automobile, they didn’t flourish and the durability and simplicity of the typical RWD design made it the sole choice of automobile platforms for many manufacturers.
In a typical RWD vehicle, the power generated by the engine is transferred through the transmission to the driveshaft, differential and axles to the rear wheels. In a typical 4WD vehicle, a differential/axle assembly is located at the front of the vehicle, and to transfer power to the front, a transfer case is also installed after the transmission and a short driveshaft is installed between the transfer case and front axle.
You will also notice that the front differential/axle assembly is different in two ways. One, the differential location is offset for clearance since the engines were always mounted in the center and, two, since the front wheels must turn to steer the vehicle, the axles must have some type of articulating joint at the end, the most common of which is the traditional Universal Joint (U-Joint.)
The transfer case transfers the power that exits the transmission to either the rear wheels (RWD), or the front and rear wheels at the same time (4WD.) Another feature of a traditional transfer case is that it offers both high and low ranges in either RWD or 4WD positions, as well as a neutral position. This is so that if the vehicle must overcome particularly difficult terrain, it can be placed in the low range so the engine will operate at a higher RPM to provide additional torque to the wheels. The high range is 1:1, which means the output speed of the transfer case is the output speed of the transmission. The low range ratio varies depending on manufacturer.
An important aspect of all this is differential operation. The differential itself transfers the power from the driveshaft to the axles, and it is necessary because it allows power to be transferred to the wheels, but also allows them to travel at different speeds when turning a corner. A conventional differential is considered an “open” design. An operating characteristic of an open differential is that it transfers power to the wheel that spins the easiest.
As an example, if one wheel is on ice, that wheel will spin, resulting in minimal traction. The same affect is what causes a car under heavy acceleration to “burn rubber” with only one wheel. To combat this problem, there is another type of differential that is referred to as “limited slip.” There are many different names for this type of differential depending on the manufacturer, but their operation is the same.
A limited slip differential contains clutch packs built in between the side gears and the differential case. When one wheel begins to spin from loss of traction, the clutches will grab and transfer power to the other wheel. The same clutches will slip just enough to allow the wheel speeds to differ when going around a corner, so the normal differential action is still available.
The majority of cars and trucks on the road come standard with open differentials, due to the additional cost of limited slip. Limited slip differentials have always been an option, just not standard. So, on a four-wheel-drive vehicle equipped with open differentials, technically speaking, the maximum number of wheels that can put power to the ground at any given time is two…kind of funny on something known as a 4×4, but it’s still twice as much traction as RWD only, and for the most part it got the job done. Most people who were really going to be in some serious off-road situations would be sure they were equipped with limited-slip differentials.
4WD, as it was originally developed, was a rather primitive system that required input from the driver, from engaging to transfer case to engaging hubs on the front wheels in many cases. Technology was the eventual downfall of rudimentary 4WD systems as we know them, but the drive to utilize this technology came from the safety benefits of AWD.
The ability to transfer power to all four wheels has incomparable benefits for traction, vehicle stability and handling. Not only does this translate to the safety of daily driven vehicles, but it translates to performance, as well.
With the advancement of computer and electronic technology, antilock braking systems (ABS) and traction control systems (TCS) all of a sudden knew exactly what was happening at each wheel at all times. Was it losing traction, was it locking up under braking? All this data was now available, and engineers knew that the key to vehicle performance, safety and handling all together, was in the ability to precisely control what happened at each wheel at any given point in time.
Traditional differentials, even limited slip, were mechanical devices. There was no external control of how they operated. With electronics and computer control, the traditional differential became a technologically advanced unit containing not only gearsets, but clutch packs like those in an automatic transmission, and their own pumps to pressurize the fluid.
The same technology is present in both front and rear differentials, as well as center differentials/transfer cases. AWD systems have the ability to precisely control the amount of torque that is transferred to any given wheel at any point in time, providing absolute control of the vehicle.
In conclusion, 4WD is functional, durable, rough and tough, but not user friendly. AWD, the product of technology, computers and electronics, is technologically superior, and provides the safety feature we rely on in today’s vehicles.
