Floor failure!!!

Your 10,000# number is stated as breaking. Do you consider Breaking and fracturing to be the same?

A past colleague of mine is also a Mechanical Engineer, yet you have so much more knowledge of the trailer than even what I could pass along to him.

He defines break and fracture differently.. His break number alines with yours, fracture is lower..

I'm going to come out of all this smarter than when I started. I really liked not knowing all this (not yet ready to laugh about that)...

But I've wound up doing a lot of mechanical, electrical, and thermal engineering in order to design and manage engineering teams designing solar projects. 

I've always seen structural systems designed using allowable load (eg, based on the 36ksi number for mild steel). Above that and you can get plastic deformation, meaning that the metal bends and stays bent. Ultimate yield is the point at which the steel will break after having been stressed beyond the allowable design point. It deforms first way before breaking, so that's not the number to be using for our frame and axle designs. 

I think your friend raises an interesting point. He is probably referring to fatigue fractures. You get fatigue fractures starting where there are tiny discontinuities in the steel, which there always are. These can then propagate slowly over thousands or tens of thousands of load cycles, eventually resulting in breakage failures. They can occur at stress levels well below ultimate.  For steels the endurance limit threshold below which cyclic loads can continue essentially indefinitely without fracture propagation is generally taken as about half ultimate yield, so around 30ksi or 1.31g which is a little less than allowable design stress. 

If our frames were designed more conservatively, say around a 2.5-3g allowable load, we would get very few if any cycles above the threshold so no opportunity for propagation of fractures to occur. But at about 1.3g we are likely getting some significant number cycles above the threshold and so there is the possibility for fractures to occur even if we're not quite reaching the point where we are bending the frames. Are we getting tens of thousands of cycles at that level? Who knows? 

Bottom line IMHO is we are under designed and so subject to a variety of failure modes in our axles and frames that we really shouldn't be having to deal with. 

The journey begins! 

There is yet another force that will push upwards on the frame at that same "weak point". That is braking. If you were to brake aggressively you would get a lot of torque acting on a moment arm extending from the tire contact patch up to the frame attachment, trying to twist the frame. That moment arm is pretty big especially if you have a riser kit. My moment arm for that braking force is about 14 inches, twice the 7 inch arm of the torsion axle itself (I have risers and 15 inch tires). That means that if the braking force was about the same as the down force the effect is twice as large at that "weak point". I'm sure that none of us are able to get that much stopping power out of our trailer brakes but even so, aggressive braking is going to add significant stress at that point on the frame rail. 

It's rumored that the HRE's have a 5200# axle. If true, are their frame's also of thicker steel?

I met a HRE owner that claimed his axle was a 5200#...