I’m trying to determine tongue weight using my 3D model. I’m manufacturing a model of a vehicle, and the vehicle is actually the trailer as well. The model/trailer has two axles mounted in the typical locations you'd expect to see them on any given trailer. Here’s what I know:

- Total mass of the trailer (with and without the axles, tires, and wheels)
- Center of mass of the vehicle
- Distance of the hitch to the center mass
- Distance of the center of mass to the center-line of the dual axles.

Wayne Tiffany made this statement:

*Use SW to check where the CG is of your trailer and any load on it. If the CG is in front of the axle centerline, then the force on the tongue will be the percentage of that load that the CG is in front of the axle vs. the distance from the axle to the hitch ball. For example, if the distance from the ball to the axle is 50" and the CG is 10" in front of the axle, and the mass of the trailer is 1000#, then the tongue load will be (10/50)*1000, which is 200#.*

My question*:* Should I include the mass of the two axles and tires/wheels in the calculation? (Ignore the front axle in my image, as this axle is not load bearing and will be lifted out of the way during tow mode. The only load bearing axles are the two axles toward the rear.)

I’m thinking that the axles/tires/wheels don’t actually apply any load, and that only the masses above the axles/tires/wheels would apply to the calculation. Any thoughts?

Likewise, am I making the correct assumption to use the center-line distance between the two axles in the formula that Wayne provided? It seems like a no-brainer, but I wanted to get some input from this well respected community.

Here is how I would approach this.

Like Josh Brady suggested use a free body diagram. First in the greater scheme of things it doesn't matter much if you include or exclude the axles and tires but if they are included when determining the center of gravity you need to include them in the calculations as they will affect the CG.

Once you have the Total Weight and the Center of Gravity determined draw a horizontal line from the center between the axles to the location of the hitch (or pin). Now draw an arrow pointed up with the point at where the hitch is and another one at the center of the axles. These are the "support points" for the trailer. Now at the location of the center of gravity draw an arrow pointed down with the point on the line. This is the Load and the drawing is a "free body diagram" that represents the structure.

What you now do is assign the weights to the arrows. To determine the weight on the hitch assign a 0 to that arrow and assign X for the weight at the support for the axles. Now at the center of gravity down arrow assign the Total Weight to that arrow as well. If you have a trailer and can position the load (or Loads) at different locations on the trailer you can determine the desired locations by separating the center of gravity for each of them and having a different down arrow representing each item's weight at it's center of gravity. (This is why I suggest something more complicated than Wyane's correct but simpler method). Now you sum the weights around the free body.

0=23000 pounds*(380.54"-54.93")-X pounds*380.54" giving you 19680 pounds supported at the axles. Subtracting that from total weight of 23000 gives the tongue weight of 3320 which is 14.43% of the total weight and is acceptable for tongue weight % but on the high side. It also agrees with your numbers using the much simpler method but now you may understand why it works.

(TL/DR)

If the axles and tires are included in finding the C of G then they need to be included it the over all weight calculations. And yes you calculate the load from the center between the axles.for simplicity. You might be able to find the weight at each axle but it is more difficult and the axle center is accurate enough.