Hey, guys, I'm a noob here, and this is my first post, but I've been following discussions on WTW over the past several months as I figured out what I want in a camper. Right now I'm waiting on a Hawk which should be delivered sometime next month. I've learned a lot from all of the discussions here.
The posts about using climbing slings to tie down the camper unlocked some old memories. When I was in college, I did a lot of caving (the geology of the area I live in is karst and like swiss cheese). At first, I had to learn how to rig ropes for rapelling into deeper parts of the cave. Often that involved putting a sling around a column or knob, and then hanging the rope from the sling. When a load is put on the rope, it makes the junction between the rope and the sling look like the letter Y (where the two arms at the top of the Y are the sling arms, and the vertical line of the Y is the rope). The older, wiser cavers always taught that the angle between the arms of the Y should never be greater than 120 degrees. Above that, the tension on the arms of the sling will exceed the weight of the caver hanging on the rope.
Shortly after that, I wound up taking a class in statics, where you learn to analyze the force on a structure using trig and vectors. Applying what I learned confirmed what the older cavers taught. In fact, the analysis shows that if the angle between the sling arms is 150 degrees, then the tension on the sling arms is about double that on the rope.
The same principle applies to turnbuckle tie-downs. If the turnbuckles are mostly vertical, then a horizontal force applied to the camper can generate some pretty large tensile forces in the turnbuckle. For example, if some motion results in a horizontal acceleration of 0.1 g of a 1500 lb camper relative to the truck bed, then a lateral force of F=ma = 149 lbf is generated. If the turnbuckle is inclined 30 degrees from the vertical, a tensile force of 149/sin30 = 298 lbf is felt by the turnbuckle - not too far from the 350 lbf rating of the FWC turnbuckle. Seems to me that it wouldn't be hard to see more than 0.1 g acceleration when off-roading.
The turnbuckles won't necessarily take the full acceleration load, though. If the camper is resting against the side of the truck bed in the direction of the acceleration, then the bed side will share some of the force. If there's a rubber bed mat and the camper doesn't slide over it, then some of the force will be taken up by the bed mat.
Anyway, check my numbers and my reasoning. Structural mechanics is not my field.
I tend to agree with the opinion expressed in posts here that you'd rather have the turnbuckle fail than the truck bed. I'll be interested to see how they mount my Hawk to my aluminum bed 2015 F150. When I stand or walk in the bed of that truck, there's a slight amount of give - doesn't feel as solid as a steel bed.
If I find some time after getting my camper, I might put some accelerometers on the truck and camper and some strain gages on the turnbuckles and run those into an Arduino datalogger just to see what kind of forces they see in use.
