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It seems like you're worrying about it more than you're thinking about it...those are not the same.
One way to think about it involves in depth engineering analysis using statics, materials science, and aerodynamics.
Another way to think about it is "hey I drove back from home depot with a refrigerator upright in my truck and a single tie down strap securing it to the bed and it stayed put just fine without ripping off the bed of the truck."
The former I can talk about a little but really you're on your own...even if you know the amount of vertical loading the Ford engineers say the bed rails can support you're not really accounting for all the forces acting upon them while driving.
The latter way of thinking involves empirical data/observations and I can give you a bunch of examples of that, but that may not be the answer you seek.
Yup, I do tend to worry when it comes to properly securing any type of load to a moving vehicle. In my mind strapping kayaks to just the cross bars is not a good idea. I would worry about buffeting, oscillating motion eventually causing metal fatigue in the rack and the bed where the rack connects to it. so I will also run a line to the front and rear of the truck.
I'd like to thank everyone for talking me off the ledge.
Would still love to see pics of rigs with racks and a tonneau cover
A buddy of mine carries a 2100- 2300lbs side by side on the bed rails.
The racked is weld steel with aluminum ramps. The rack itself is homemade, once in the bed he uses a battery operated (UTV) jack to raise the rear part of the platform and pins it in. There is some support in the front to the floor. The rear section unobstructed so he can put coolers and crap in
I can’t answer your question. But many folks put ladder racks on trucks and load them down with all kinds of things from lumber to ladders. I have seen any box on any brand of truck fail from this. I did see one fold up when a large tree fell on it.