Automobile Engineering in ME
The basic purpose of a four wheel drive (4WD) system is that it

Ensures that effective braking can be performed, even on slippery surfaces
Ensures effective transmission of engine torque to all four wheels, even on slippery road surfaces
Eliminates the need of snow tyres, tyre chains, etc.
Delivers improved cornering on dry road surfaces

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Automobile Engineering in ME
The drive-shafts are connected to the differential and wheel hubs through universal joints because the universal joints

Compensate for variations in the relative positions of the differential and the wheels which result from bumpy road surfaces or other similar driving conditions.
Absorb any difference in speed between the left and right wheels when the vehicle is turning
None of these
Absorb the vibrations transferred from the surface of the road

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