Lou, that is an excellent diagram…..even “diagram man” moves his arms !!!
Like Lou said, we have been having a friendly little “convince me” debate in between working on our home obligations. Already, I have discovered that the fulcrum point moves to the front axle when using a WDH….I had never thought about that before.
When someone places a trailer tongue on a hitch, that is a Class 1 lever……….the effort (trailer tongue) is trying to lift the front axle (resistance), and the back axle act like a fulcrum. See the 3rd diagram down on the right here: Lever - Wikipedia, the free encyclopedia
The front axle gets lighter, and the rear axle gets the weight lost from the front axle, plus the weight of the trailer tongue. Using my combination as an example (rounded off for simplicity), putting 700 lbs. on my hitch (effort) takes 300 lbs. off my front axle (resistance), and puts 1000 lbs. on my rear axle (fulcrum).
But when a WDH is used, the front axle becomes the fulcrum point like in a Class 2 lever……the rear axle is the resistance, and the WDH is the effort. The wheel barrow in Lou’s diagram is an excellent example.
Comparing the wheelbarrow to our rigs:
Wheelbarrow front axle = tow vehicle front axle
Wheelbarrow legs = tow vehicle rear axle
The Wheelbarrow handles = hitch
“Diagram Man” arms = both trailer tongue and WDH
“Diagram Man” feet = trailer axles
If the dude pushes down on the wheelbarrow handles, that effort is going to try to lift the wheel off of the ground. Because he is pushing down, his feet will get lighter and that difference will be transferred to the wheel barrow, putting that over the 100 lb. weight limit. Our trailer axles do not get lighter when we hook up, because the weight is already on the tongue, and supported by the jack or hitch ball.
But if Diagram Man picks up on the handles, his arms now acts like a WDH….some of that 100 lbs. of weight is transferred to the wheel (tow vehicle front axle), and I think (hence some of the discussion and debate) some of the weight goes to the dudes feet….the effort to raise handles. Again to simplify, since there are some lever length issues (wheelbase), if the dude lifts the legs off of the ground, then I think ~25 lbs. will be added to the wheel, and ~25 lbs. to the guy’s feet. If you set just the wheelbarrow on a set of scales, you would have 75 lbs. on the front wheel, plus 0 lbs. on the legs, resulting in a 25 lb. lose of weight. You could now add another 25 lbs. to the wheelbarrow without exceeding the GVWR.
Now we come back to Lou’s diagram and question……there is 100 lbs. of load in the wheelbarrow, but a set of scales will only show the wheelbarrow weighing 75 lbs. If the 100 lb. load limit is the frame itself, then that wheelbarrow is at its limit. But if the load limit is the wheel (say 100 lbs. GAWR) then you should be able to add 25 lbs more weight to the load. But that is a Class 2 lever.
With our vehicles, we are using 2 forces….a Class 1 lever when we put the trailer tongue on the hitch, and an Class 2 lever when we use a WDH. I am thinking because the WDH takes the weight off of the tow vehicle and putting it back on the trailer axles, then that frees up some extra weight capacity on the tow vehicle. In using a WDH, the truck frame does not "see" the tongue weight....it only "sees" the tongue weight minus the weight being relieved by the WDH. I think that is what Lou and I have been trying to figure out.
Hope all of that convoluted explanations, out loud thinking, and questions make sense, because I think I confused myself.
If there is an engineer in the house, you are welcome to join in.
I think my head hurts.