E&J Pushnwind:
I am not sure why you think there is no lateral force on a fifth wheel. There certainly are..in fact because there is a round pin that fits into the fifth wheel you can say that there are forces at any given time in almost any direction of the compass.
Your point about car carriers is not really complete. The STINGER type of fifth wheel arrangement you see on Auto carriers has all the same forces as any other. However it does exert different leverage upon the Tractor.
As for wind and auto carriers I am afraid that you have overstated the case somewhat. When an auto carrier is loaded about 75-80% of the area on the side is blocked by cars and trailer structure meaning that side winds can affect them quite a bit. Add to that the fact that the cars on top will sway on their own due to being on their suspensions means there is an added lever arm way up high oscillating at a different rate than the trailer making things even worse.
But to cut to the chase here. The ball type hitch is a proven design that has worked well for many many years. There is nothing wrong with it. A 5th wheel has some advantages but only if the design of the hitch and the plate and pin on the trailer are up to the job (not always the case) The same forces are present no matter which hitch you are using. HOW those forces are transmitted to the TV is different.
The distance from the average p/u truck rear axle to the ball hitch pivot point is so close as to be mathematically insignificant in thinking about which forces are the most likely to cause sway. SIDE WINDS in and of themselves are not the cause of sway...it is the driver CORRECTION that causes the sway.
As for passing vehicles. Tractor trailers pass me all the time when I am towing my 30ft tt. It doesnt move.
My guess is that some drivers, slightly spooked by that huge truck and its Air Wake zooming by them instinctively let off the accelerator pressure and/or make a sudden move to the right side. THIS is SURE to cause sway as the Big Truck goes by.
As for your analogy of the boat wake...It's not completely accurate. If you look at what really happens to the air there is a period of force as the air wake hits and then a SUCTION as the truck goes by.
When your trailer is going down the road there is a similar wake coming off the front as it passes thru the air. this air pushes out from the side of the trailer near the front and comes back at some point to slide along the side of the trailer. For all intents and purposes this air flow is even on both sides. therefore it isnt really a cause of sway.
However when you have a dramatic rear overhang as some trailers and RV's do this does present a lever arm for a greater amount of air to hit behind the trailer axles on one side or the other.
The best way to avoid this unequal force is to keep a steady acceleration on when being passed by any larger vehicle.
