The seesaw analogy works to a point, but it is too simple. Both steering and lateral stability are affected by the ballast of the truck. Any change in the ballast will alter vehicle stability in four directions and will make steering unpredictable. Sometimes the change in ballast causes the vehicle to understeer. In other situations, a change in ballast will cause it to oversteer.
All modifications require the written approval of the manufacturer.
Many modifications to the ballast are unintentional. The installation of a taller or heavier mast, for example, alters the balance of the forklift. Some operators add ballast to the back of the truck to increase its lifting capacity. This practice is always dangerous. Users of electric forklifts face a specific temptation.