Even way back in the 20's and 30's they were smart enough to know that the critical information in loading a truck was how much weight can the front and rear axle hold. As has been correclty pointed out, the truck + bed or hitch or whatever weights a certain amount, meaning that your axles are already partially loaded.
When calculating a max "added load", it is easiest to assume a center of gravity point where all the load is applied. You can then take the GVW of the truck, substract the "unloaded weight of the truck" to find out the max load you can add to the truck.
Now lets look at this for a minute. What if we put all this weight on the front bumper? Well then you'd be overloading the front axle and removing weight from the rear axle. What if we put all this weight on the rear bumper. Then we'd be removing weight from the front axle and possibly overloading the rear axle. (We've all seen this vehicle pulling a trailer down the highway dangerously!) So it's plain to see that the center of gravity should be placed somewher in front of the rear axle but behind the front axle.
So how you gonna figger out where to put your load without some fancy compooter back in the 1920's? Easiest way is to know how much additional weight can be applied to the front axle. This is easy to find out by knowing the max front axle weight and putting your front tires on a scale.
Lets say your front axle can take an additional 2000 lbs of weight on it, and you calculated your max added load to be 6000 lbs. It's easy to see that 1/3 of the added load should be applied to the front axle.
So measure the distance between the front and rear axle. Find the point that is 1/3 this distance in front of the rear axle. Apply all your weight at that point. You've now got a front and rear axle that are fully loaded and you are carrying your max load.
Now, we can talk about safety factors, and frame strength vs. axle vs. tire vs. brake strength, and modern semi's with dual rear axles and sliding fifth wheels and inertia concerns and all kinds of other things we have now a days. And I sure hope they are taking these things into concern with the 80,000 lb trucks cruising down the road at 70 mph next to my 3000 lb minivan with me and my kids in it. But that's NOT what they worried about back then. Heck the guy loading the truck was probably not literate and didn't know any math. But it was easy to tell him to put the center of the load 1/3 the distance forward of the front axle.
Course I wasn't alive in 1920, more or less loading trucks. I'm just explaining it the way it was explained to me. The way it was explained to me makes sense. Anybody in this forum that's got first hand experience from this era loading a heavy truck?
