Spring bar selection

JBarca wrote:
Now thinking present day, 50% FALR is becoming a topic of discussion. If one does use 600# TW rated WD bars on a 1,200# TW, to achieve 50% FALR, what happens to the WD bar during dynamic situations where heavy back flex in the hitch under of some level of speed occurs? Like coming off a high up RR track or dropping out of a high up gas stations curb, pot hole etc.

John, the "back flex" angle will be determined almost entirely by the geometry of the "dip" which the TV/TT passes through. When the Front of TV and/or rear of TT are elevated relative to the ball coupler, the WD bars will experience additional deflection. The additional force exerted on a bar will be directly proportional to the stiffness of the bar. If a 1200# bar is twice as stiff as a 600# bar, the additional force on the 1200# bar will be twice as great as the force on the 600# bar.
If the "back flex" added loading on a bar were the same for all bar ratings (or greater for the bars with lesser ratings), I would expect that people with 400 and 600# bars would report much more bar damage when going through dips than people with stiffer bars would report.

The lighter bar will try and deliver all it can and the loaded TW has the ability to be 2 times the weight pushing down on the ball then the WD bar may have been tested for. Will the WD bar permanently deform? This goes directly against the Reese warranty.

Since the "back flex" induced added deflection will be essentially the same, the lighter and more flexible bar will experience less added load than will the heavier bar.

If one uses a 1,200# WD bar on a 1,200# TW and adjusts the WD hitch to only deliver the force needed for 50% FALR, then the back flex situation should not affect the WD bar as it has been fully tested to comply with this situation. Or has at least been proven by consumers to not bend in most cases.

I think it also is safe to say that a 1200# bar on a 1200# TW at 100% FALR has been proven by consumers to not bend in most cases.
A 1200# bar generating 100% FALR with 1200# TW will be loaded to about 960#.
A 600# bar generating 50% FALR with 1200# TW will be loaded to about 480#.
If "back flex" results in a doubling of the load on both bars, each bar will end up at 160% of its assumed static working load limit.
The load on a bar is not determined by TW -- it is determined by how much load it is asked to transfer.

I have not yet found the WD hitch mfgs to change their method of how to size a WD bar per loaded TW. Yet some hitch mfg's now declare less than 100% FALR in their instructions. I may have missed a few which state it is OK to down size a WD bar to a lower FALR which is why I was asking.

It was a huge step for Progress Mfg and Reese to move away from the "equal squat" approach and move toward a FALR-based approach. However, I think they don't have sufficient incentive to try to connect bar rating with FALR. The ones who do have the incentive are the customers who want to avoid the problems of excessive bar stiffness. There are many reports in the Airstream community about damage due to excessive stiffness. And, there are many Airstream owners who are using WD bars rated well below the trailer's tongue weight.
I've not seen any reports of bar damage from Airstream owners who are using bars rated for less than the tongue weight.

Ron