Rodar; It looks like you got a great start on your recumbent. I'm eager to see its completion.
Yes, your observations make sense. Some of it is just personal preference. I would build a cargo carrier differently, so I didn't care about the long chain.
If you understand the benefits of keeping most of the weight below the line, then you understand what I mean by 'pendulum effect'. If the trailer can balance upright on its own due to the high hitch and low center of gravity, it will lean with the bike around a turn and won't even need a u-joint.
In actuality my u-joint merely suppresses swaying motions that arise at certain speeds probably due to resonance amongst steering inputs, imbalance due to the heavier off axis flywheel, possible slight misalignment of tire to hitch (dogwalking) and low resistance to twisting torque in my flexy tongue etc. In my case it required only a lightweight, sloppy u-joint to tame the thing down completely. My u-joint is made from cheap, loose tolerance gate hinges. It takes a beefier, tight-tolerance hitch and twist-resistant tongue to keep a topheavy trailer upright.
My preference is minimalism. Keep it simple, as lightweight and as easy to build as possible so more people can make one that works well. My build requires no welding.
In my experience, whenever I had the trailer at a high angle to the bike, I was going very slowly and making such a tight u-turn that I just used the pedals instead of the engine to make the maneuver. Even though it's 4 HP, I have a 40 mph ratio so it doesn't have enough torque at low speeds to interfere with my balance when the trailer is at a high angle to the bike.
When I initially had problems with topheaviness I solved it by extending the trailer instead of making a heavier, close-tolerance hitch and torque resistant tongue. Extra chain is inexpensive and added no complexity.
Maybe I would have been satisfied with a shorter trailer and a stronger u-joint, but I appreciated the suspension effect I obtained with the longer trailer. The extra length reduces bumpiness due to a little flex, but also due to the fact that it doesn't lift the engine as high going over a bump as it does with a closer wheel. If the engine is directly over the wheel, the engine lifts 2 inches over a 2 inch bump. If the engine is in the middle between the hitch and tire, it lifts only 1 inch over a 2 inch bump. This may seem a minor point, but it resulted in great improvement and there is still very little tongue weight. I can ride much faster when I'm not hindered by a bouncy trailer. I didn't miss having engine weight close to the wheel for traction. It still spins rarely and only in loose gravel.
The most stress in the 'gooseneck' part of my trailer is where the 3/4" conduit tongue pieces are bolted to the angle iron frame. A bolt hole at this stress point would reduce tubing strength too much, so use a u-bolt or j-bolt here and it will be plenty strong.
I might not be pleased with a seatpost hitch if I had lots of low speed torque and did lots of low speed turns, but in my case it's ideal. It seems to me most are overbuilding their tongues & hitches. Due to making use of the pendulum effect I don't have to fight any significant torsion in the tongue or hitch, so two light pieces of cheap conduit work just fine and nothing needs to be welded.
I might be forced to beef things up and use a lower hitch position if I used a torque converter but I have no use for one here in rural Illinois flatland. I feel this design could handle a 5 or even 6 HP engine as is, or at most with a couple of braces (or stouter tubing) at the lower bends in the tongue.
Nearly everything would change if I was building a cargo carrier.
Did you have a lot of low speed torque with your pusher?
I just realized this is a departure from the original topic. I hope we can steer this back closer to the issue of one pusher, different bikes.
