Spring bar selection

owenssailor wrote:
What is your source for using 600 lb WD bars for a 1200 lb tongue when doing restore of 50% of weight removed from front end of truck? My understanding is that you size the WD bars by tongue weight and then adjust properly to get front end of TV to the right spot.

First, we need to understand what is meant when a WD bar has a rating of, say, 1000#. Here, we need to make some assumptions because I've never seen a manufacturer's definition of bar rating.

We know that a WDH traditionally is called an "equalizer". And we know that a WDH traditionally was used to achieve equal load being added to the TV's front and rear axles.

Some WDH manufacturers still adhere to the "equal squat" approach when telling customers how to adjust the WDH.
Therefore, I believe it is safe to assume that you can use a WDH to achieve "equal squat" without exceeding a bar's working stress limit.

A WDH is a relatively simple mechanical device, and it can be shown using basic physics how much bar loading is required to achieve a certain amount of load transfer.
For typical TV/TT/bar dimensions (TV ball overhang = 60", TT coupler to axles' midpoint = 240", effective WD bar length =30"), it can be shown that each WD bar must be loaded to about 110% of tongue weight to achieve equal squat.
If the above assumptions are reasonable (including the "equal squat" design loading condition), it is safe to assume a bar rated for X# can handle an uplift force of 1.1X#.
Therefore, it is reasonable to assume that an uplift force equal to 110% of a bar's TW rating will not overload the bar.

If your objective is to achieve equal squat, you should choose a bar with a rating equal to or greater than the TW.
If your objective is to restore the front axle load to its unhitched value (100% Front Axle Load Restoration), the load on each bar only needs to be equal to about 80% of TW -- which we can assume would load the bar to about 80/110 = about 73% of its working stress limit.
For 50% FALR, each bar would need an uplift force equal to about 40% of TW.

For a rule of thumb with an added safety margin of about 25%, we can say that:
Minimum WD Bar Rating = FALR times Tongue Weight

Again, this all is based on the assumption that, with WD bars having a TW rating equal to the TT's TW, and with both bars subjected to an uplift force equal to their TW rating, the WDH would generate enough load transfer to achieve "equal squat" without overloading the bars.

Ron