Wikipedia: Four wheel drive

Four wheel drive
From Wikipedia, the free encyclopedia.

Four wheel drive is a type of vehicle drive-train configuration that enables all four wheels to receive power from the engine simultaneously in order to provide maximum traction. Usually the term refers to part-time systems, designed only for use in low-traction conditions (e.g. off-road, on snow or ice). All-wheel drive (AWD, also called full-time four-wheel drive are designed to function on all types of surfaces, both on- and off-road; typically they cannot be turned off.

Part-time four wheel drive

When a part-time four wheel drive system is enabled, the transfer case connects both the front and rear drive shafts with the transmission. These drive shafts are connected to differentials which distribute torque to the wheels via the axle. The sum of the output angular velocity of the two output shafts (connected to the wheels) is linearly related to the angular velocity of the input shaft (connected to the transfer case). However, the two output shafts do not have to rotate at the same speed. During a turn, the inner wheel is spinning slower than the outer wheel. Torque is distributed evenly between the two output shafts.

However, since the front and rear differentials must spin at the same speed (due to the transfer case) the sum of the angular velocity of the front wheels must be equal to that of the rear wheels. During a turn, the front wheels travel a slightly greater distance than the rear wheels. Therefore there must be some "slip" in another part of the system. This slip occurs easily at the interface of the tire with the ground when traveling in low-traction conditions. However, when traveling on dry pavement the torque required to cause this slip causes excessive wear on the tires and the rest of the drive-train. This is why the part-time four-wheel drive system must be disabled when operating the vehicle on pavement. In this configuration, the transfer case is connected to only one of the two differentials (usually the rear).

Part-time four wheel drive systems are ideal for use in very difficult traction conditions since they allow all of the torque to be sent to the rear or front wheels. However, if one of the front tires and one of the rear tires is on a slippery surface little traction can be achieved since torque to the left and right is distributed evenly by the differentials. This problem can be addressed by using a differential locker or a limited-slip differential.

Full-time all-wheel drive

A full-time all-wheel drive system must be designed to allow the front and rear differentials to operate at slightly different speeds so that the tires do not need to "slip" during a turn on dry pavement. This can be achieved in a variety of ways. The simplest system involves an additional differential (called the center differential) to distribute torque to the front and rear differentials. The disadvantage of this system is that each wheel can only receive as much torque as the wheel with the least traction can receive. For example, if you have one tire on ice you have the same amount of traction as if all of your tires where on ice.

One method of improving the basic all-wheel-drive system is by using a viscous limited-slip center differential. This differential automatically locks the drive shafts together when an excessive difference in speed is present. Many vehicles employ this system, for example manual transmission Subarus.

Another method of improving AWD is by having an electronic system apply the brake to the wheel that is spinning. By applying the brake to the wheel that has little traction, more torque can be sent to it and therefore more torque can be sent to the other wheels. Since the other wheels do not have their brakes engaged the extra torque sent to them can be used to help propel the vehicle. This system is used in many all-wheel drive systems installed in luxury vehicles.