The real problem happens when you drive across a swale or depression. Halfway through the crossing, the tow vehicle has crossed the swale and is headed up the other side, but the trailer is still headed down into the swale. In other words, there is an angle between the tow vehicle and the trailer. This is where the "spring" in springbar is important. The springbar needs to bend in order to accommodate this angle. If it doesn't bend, it puts a tremendous stress on the both the A-frame of the trailer and the tow vehicle. The tow vehicle is trying to drive the trailer's A-frame into the ground, while the trailer is trying to lift the tow vehicle's rear end off the ground. This will be especially problematic on unibody vehicles. Fortunately, the suspension of the tow vehicle allows the rear end to be lifted somewhat.
A sketch can be helpful, and I'll post one if I can find it. Meanwhile, though, the owner's manual for many weight distribution hitches warns (this one is from Equalizer)
Do not tow your trailer through profound ditches, dips, or swales. Excessive strain on the spring arms and hitch head may cause hitch fatigue or failure.
As with many things in the world of hitches, it is easy to go all Chicken-Little about this. I think that as long as you drive on mostly level roads, you will be OK. Guys who go off-road a lot in steep country, and encounter a lot of up and down, may not think so.
The easy solution is just to buy some lighter springbars. Unfortunately, they seem to be incredibly expensive.
Bill
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