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4WD and part time 4WD

Most Sports Utility Vehicles (SUV) come equipped with four wheel drives in India. Actually they are available on all types of cars in mature economies. Some of the four-wheel drive cars in India are Mitsubishi Pajero, Mahindra Bolero, Tata Safari and others.

But all the 4WDs are governed by a basic principle. The 4WD can be or two kinds - permanent and part time. The difference is in speed momentum of wheels when encountered with a rough ground. It is called centre differential.

When a four wheel drive is driving on a highway, all four wheels are moving at the same speed. However, when negotiating a curve, each wheel attains different speed depending upon the radius of turn. The permanent 4WD have a central differential fitted to gain harmony between the wheels while negotiating a curve. The same is not installed in part time 4WD.

The central differential is controlled by transmission which help the permanently 4WD to over come the problem. When the 4WD hits the rough ground, a single wheel can slow down the raging vehicle. This is why such vehicles are equipped with central differential lock that stops their action and converts them into part time 4WD mode. The centre differential lock should not be activated when driving on smooth road, as it could lock the wheel movement.

Technically, a 4WD is actually a two wheel drive with one front wheel and one rear wheel spinning when they loose traction. One wheel keeps rotating while pushing traction to higher level.

To achieve maximum stability, limited slip or locking differential is installed. With a limited slip differential, the wheel with traction gets limited drive before the other wheel on the same axle rotates. Locking differential allows both wheels on the same axle turn at the same speed, regardless of the amount of traction.

When you are driving a part time 4WD (which is not equipped with centre differential) do select 4WD drive on smooth roads. This is because a centre differential helps attain harmony between the four wheels which are spinning at different speeds. In the absence of centre differential, there could arise a scenario called ‘axle windup’ or ‘transmission windup’ when 4WD is selected, the drive shafts and transmission undergo high stress, leading to two things- one of the wheels could slip or spin faster or the drive shaft and transmission could break down. This could initially cause instability and eventually lead to car breakdown.

It is important to understand the functioning of 4WD–permanent and part-time–to enjoy a smooth drive. The two functions are drastically different though they sound the same initially.

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