The basic idea behind powering all the four wheels is to increase traction. In two-wheel powered vehicles, all the torque has to come from the two wheels. In conditions like snow, and less torque situations, vehicle with a two wheel drive are more prone to get stuck. Thus, a four wheel drive is a good idea to maximize engine power by transferring it to all the wheels. A four-wheel drive mechanism is not completely perfect. There are a few possibilities that arise and they have to be understood and included in the vehicle either directly within the drive-system itself or by making external adjustments.
Slippage:
In addition to torque, all vehicles need traction to move. Traction is very closely associated to torque and, it can even be said that in certain conditions, traction determines the amount of torque.
4WD-more torque more slippage:
Differentials and Traction:
As powering all four wheels increases torque, it benefits the vehicle in a lot of ways. But in surfaces that offer less traction, the conditions change. In the very same snow or off-terrain surfaces, which the four-wheel drive is meant for, there is a direct disadvantage the vehicle faces. If the number of wheels gaining torque and traction are more in number, there is better movement. The draw back is, in off-terrain surfaces, there is also a possibility that this works conversely. That is, movement is negatively affected, if there are more number of driving wheels in less-traction surface positions. Let’s see how.
In case of two wheel drive, for good movement it becomes important that at least one of the two wheels receive proper traction in order to pull the other wheels out of the loose surface. If one of the rear wheels is receiving less or no traction, it will not affect the performance of the other rear wheel because of the presence of the differential. In a four wheel drive, even though differentials are present between the right and left wheels, there is none between the front and rear and thus force applied on the front and the rear axles are the same. Both the axles are connected by a drive column, but no differential is present. This scenario has certain disadvantages because the wheels turn at different speeds while taking a turn. In the case of a two-wheel drive car, this is taken care of because two wheels are ‘driven’.
While the four wheel drive offers increased torque, there is the consequence of drive train binding. This leads to the next level in the differential mechanism. That is, the introduction of a center differential. Under the four-wheel category, this is called the All-wheel-drive.