Tractors, earth mover and other heavy commercial vehicles do not have suspension for the following reasons :-
First its required to get know what are things affected by a suspension system.
If a suspension is installed in a vehicle , then the vehicle will not have constant motion body. The body tend to oscillate in the direction of axis of the spring.This oscillation can change ride height while operating . The condition may be while working or moving.
The body will start to roll , take this example
Consider the above system. Assume as if the brown mark is the support attached to a vehicle.
Ideally we say that the spring oscilates up and down. Practically a spring can also bend causing irregular motions.
Under varied driving the spring will oscillate in multiple axis . The mass will start to bounce in all possible directions . Also the distance between the mass and the support will be changing constantly.
If the car accelerates faster the mass will fall back . If it brake faster then the mass will take a nose dive (or moves forward).
Now the answer:
- In a tractor mostly used to pull loads or equipped in farming conditions , if there is a suspension installed , then the ride height will be changing. So it may affect the mass being pulled.
- Consider the tractor is ploughing , there will be load trying to make the tractor stop as its inserted in the ground. This may cause the body of the tractor to tilt backwards
(assumed the plough is attched to rear axle).
This reaction may cause the spring to bend .
Also the steering heavily will affect the steering characteristics.
This also the same reason why earth movers.
If the back bucket or hoe pitches(yaws) at vertical axis the body will roll to the direction faced by the hoe.Also the depth of excavation will change under these circumstances.
These equipment ( tractors and back hoes) will work in sloppy hills. Under these position the body will roll along the slope.