The air suspension springs of trailers /semi-trailers are preferred and used to prevent loaded cargo and ensure driving safety by preventing adverse effects to arise from structure of the road such as libration, tilting, jumping, roaming. In addition, the air suspension springs are proven to protect the vehicle and load better than the steel leaf spring systems.
In general, while there are two air suspension springs in each axle, there is lifting spring in one axle in vehicles with multiple axles in order to prevent the tires when the vehicle is empty.