Why is it sometimes impossible to shut the trailer doors on sloped ground?

My first car was a unibody. I remember putting my winter tires on the first time and only having one corner jacked up at a time. Imagine how easy it was to open and close the doors with one corner lifted 8" higher than the rest lmfao.

 

If that's true you might be better off seeking your answers somewhere else. You're beating your head against the wall because it feels good when you quit.
Plus, it gets you a lot of attention. You seem to like that.

 

He wants to know the physics, huh? Feeling like Don Quixote, so here we go!

On flat ground, all the forces flex the trailer down in the center. Modern box trailer is a monocoque structure, meaning all the parts of the trailer carry some of the structural load, the roof and sides included. But on flat ground, the rear frame of the trailer isn't carrying any torsion load to deform its shape, provided the load in the trailer is properly distributed from side to side.
As the structure deforms under load, the rear frame will tip forward as the trailer sags, but the uniform loads mean no twist in structure. The rear frame could be mounted with horizontal transverse pins with substantial free travel, and there would zero effect on trailer as all the loads it is carrying are in a fore and aft direction.

Now if you place the truck such that drives and tandems are on ground with different side slopes, everything changes. Each end of trailer wants to be parallel to ground, and the trailer structure twists. The front and rear frames of the box try to resist that twist by holding the walls, roof, and floor square, but the torsion tries to twist them out of square.

The front frame is much stronger because it's not an open frame like the rear frame. The skin being attached to the frame and stringers strengthens the front of the trailer so it acts much more like a steel plate, with minimal deflection from square.

The rear frame on the other hand, only has the strength of its corners to resist deflection. And the fact that the dimensions of the corner structure is constricted by the need for a clear door opening limits its ability to resist the torsion, and it flexes out of square accordingly. One could build an equal weight structure from larger dimensionally but thinner walled tubing that could be much stronger, but the door opening would be reduced. (Of course a stronger rear frame would mean higher loads into the skin of the trailer, that could lead to issues there that requires stronger material and/or fasteners.) But unless the rear frame is made as resistant to deformation as the nose, there will be always deflection from square at the open rear frame, and making the rear doors difficult or impossible to close

And typing all this is giving me flashbacks to trying to close the doors on 80's Utility reefers. Those twisted so easily that parking on a penny made the doors hard to latch!

 

You answered everything here in simple language, with no jargon.

See? It's not hard, guys.

 

It would start another thread asking if it can call a lawyer like the Hammer to Sue the bridge as it has broken the law of gravity then the Troll would retreat under a law abiding bridge that it understood from a physics point of view and spank his monkey over all the attention he’s getting. At least he’s not whining like Hopeful Eyes used too

 

Does the trailer flex more in hot weather than cold weather? Temperature is another variable that I don't feel has been addressed here.

 

I think 9/11 answered questions about metal strength and temperature, didn't it?