The rear side of truck is provided with double wheels.
Buses and trucks have double wheels at the back because these are heavy vehicles with most of the loads resting towards the back. Also, the engine that drives the vehicle is attached to the back wheel.
broad tyres have large area of contact and less pressure on the ground. Because of this, heavy vehicles can move even on soft roads and fields without sinking.
Double wheels are not used in the front since that would make the steering uncomfortable.