Bicycle Trailer Hitch Design Considerations
The requirements for hitches for single wheel and two wheel trailers are different.
3 types of rotation through this joint: Pitch, yaw, roll.
Pitch: as the bike transitions up to a ramp,
Yaw: to allow a flat turn,
Roll: A two wheel trailer will stay flat while the bike leans into corners. A single wheel trailer will fall over if it's not constrained.
The Surly hitch has the X-shaped component providing yaw and roll, and the axle ends have bearings in them to provide freedom to pitch.
The BOB tailer allows pitch at the axle, but the joint before the platform only has 1 degree of freedom, allowing yaw. The trailer can't roll independently of the bike.
The mounting point differences change the impact of the tongue weight on the bike dynamics. Seatpost clamps are simpler to design and use and allow tight turns without bike-trailer contact, but increase the torque created by the tongue weight. Mounting at the rear axle minimises the torque, but all the load will be added to the rear wheel, and the trailer has to be further from the bike to avoid contact with the bike or your heels in sharp turns.