This principle is very useful in loading a truck:
If the driver loads the cargo in such a way that the center of mass (center of gravity) is directly above the drive axle (or halfway between the tandem drive axles), then the entire weight of the cargo bears directly onto the drive axle(s).
If the driver loads the cargo in such a way that the center of mass is somewhere forward of the drive axle(s), then some of the weight bears onto the steering axle, and less of the weight bears onto the drive axle(s).
The farther the center of mass of the cargo is forward of the drive axle(s), then the more of the weight bears onto the steering axle.
Conversely, if the driver loads the cargo in such a way that the center of mass is behind the drive axle(s), then all of the weight of the cargo bears onto the drive axle(s) -- and some of the weight on the steering axle shifts onto the drive axle(s)!
The farther the center of mass of the cargo behind the drive axle(s), the more of the weight on the steering axle shifts onto the drive axle(s).
How much weight goes on or comes off the steering axle?
That's easy -- with a simple calculation:
If the center of mass of the cargo is 5 feet forward of the drive axle(s) or behind the drive axle(s), and if the wheelbase is 25 feet, then 5/25 of the weight moves.
5/25 = 1/5 = 0.20 = 20%.
In this same example, if the cargo weighs 5,000 pounds, then 1,000 pounds (1/5 of 5,000 pounds) shifts onto the steering axle or off it -- depending on whether the center of mass of the cargo is 5 feet forward of the drive axle(s) or 5 feet behind the drive axle(s).
If the center of mass of the cargo is 5 feet forward of the drive axle(s), then 1,000 pounds of the cargo bears onto the steering axle, and 4,000 pounds bears onto the drive axle(s).
In the other direction, if the center of mass of the cargo is 5 feet behind the drive axle(s), then all 5,000 pounds comes to bear on the drive axle(s) -- AND 1,000 pounds come off the steering axle and shift onto the drive axle(s), so that 6,000 pounds altogether come to bear onto the drive axle(s).
To picture this, imagine a seesaw on a playground.
Imagine what happens -- imagine the action of the seesaw -- as two or more children take different positions on the seesaw.
Now imagine that your truck is a seesaw with its fulcrum at the drive axle(s).
Then imagine what happens as your cargo occupies different positions -- fore and aft -- inside your cargo box.
This is not empty speculation; it really works.
Note this well:
If a driver wishes to avoid adding more weight to the steering axle, load the cargo in such a way that the center of mass of the cargo is nowhere forward of the drive axle(s) -- either directly above the drive axle(s) or behind the drive axle(s).
How can we determine where the center of mass is?
A careful eyeball estimate often works fairly well.
Maybe we could even measure the cargo.
Try this sometime!
By the way, this principle is one very good reason to avoid loading cargo all the way into the nose of the cargo box unless necessary.
Many loaders tend to shove the cargo all the way into the nose -- even if the cargo is rather small.
Many drivers also tend to do so.
By controlling the position of the cargo, we can control not only the loading of the axles but also the quality of the ride and the handling of the truck.
As always, best wishes to all,
Doc.
PS. For those who dislike math and physics, please feel free to disregard this whole thing.
