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