In part time four wheel drive vehicles, differentials are fitted to both front and rear axle assemblies.
When a two-wheel drive range is selected, the drive is transferred through the rear final drive and the differential gears to the rear axle shafts and road wheels.
The differential gears allow the rear wheels to rotate at different speeds when the vehicle is turning, while continuing to transmit an equal turning effort to each wheel.
When four-wheel drive is engaged, the drive is transmitted through both front and rear axle assemblies, and differential action occurs in both.
However, in a turn, side-swiveling of the front wheels for steering makes the front wheels travel a greater distance than the rear wheels. This causes a difference in the rotational speeds of the front and rear wheels. Since there is also a difference between inner and outer wheels, each axle shaft now turns at a different speed.
Differences in speed can also arise from differences in tread wear between front and rear, or in tire inflation pressures.
Since front and rear propeller shafts are locked together at the transfer case, the difference in speed cannot be absorbed in the transmission, and the transmission drive line can be subjected to torsional stress.
In off-road conditions, differences in speed can be absorbed through slippage of the tires on the ground’s surface.
On firm road surfaces, only two-wheel drive should be engaged. This is to prevent excessive tire wear and possible damage to the transmission drive line.
