How to purchase a good fence semi trailer ? Main beam:The fence cargo semi trailer main beam should pay attention to whether the thickness is an international standard. The international standard for the main beam of a cargo trailer is 14/8/16 mm. The fence truck trailer steel plate in the middle of the beam uses Q345B manganese steel, the thickness is 8mm. Many unreliable suppliers will use 6mm main beam to save costs. But the carrying capacity was reduced by 26%.