Bicycle Trailer Hitch Design Considerations

Question

I have been pulling a bicycle trailer with my Surly Karate Monkey for approximately 3-4 years. I have been using cheaply obtained used kiddie bike trailers, usually removing the fabric and adding a simple, lightweight deck to support the loads I carry. Works great! The trailer is attached to the bicycle with the standard left side quick hitch. Its a simple design and more than adequate for light duty use. I have been progressively increasing the loads on my trailer and demanding more utility from the system.
I am in the process of designing a new hitch system and looking for design considerations.Surly makes a nice hitch system for their trailers. BOB also makes a nice trailer design. There are also trailers that attach to the seat post or upper frame area.
There's also this setup
What are the considerations for where to attach a bicycle trailer hitch? What is best for heavy utility trailer use versus pulling a kid to the park?
I am considering something of a hybrid trailer hitch that attaches to both sides of the axle and has a frame incorporating features of both the surly/bob going back to a single bicycle wheel. This single bike wheel would have a frame around it including an attachment point for the simple single point hitch to pull kiddie bikes. The idea is to reduce tongue weight of the trailer acting on the frame of my bicycle and increase the draft load I can pull. I would also be adding trailer brakes to the system.
Thanks for any input.

Emyr · Accepted Answer

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.

Bicycle Trailer Hitch Design Considerations

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