Chucking?

The bucking, or chucking is caused when you have irregular pavement like seams, pot holes, etc. When the chucking happens the truck and trailer are fighting each other-pushing and pulling so to say. When you hit a seam in the highway for instance, the trailer tires hit the seam and the trailer hooks or hesitates a split second putting drag on the truck. When it springs forward it pushes on the truck. When the seams or pot holes are in perfect harmony with the length between axles, it'll seem like your riding a wild bull. The more slop in the fifth wheel hitch the worse it will be, but even with zero slop it could still be noticeable on some highways. The idea with the king pin boxes with air bags/shocks/torsion rubber is to absorb and minimize this push pull effect.