here is the physics 101 from the NASA website... in our case only the example 1 is applicable:
A force
F is a
vector quantity, which means that it has both a magnitude and a direction associated with it. The
direction of the force is important because the resulting motion of the object is in the same direction as the force. The product of the force and the
perpendicular distance to the center of gravity for an unconfined object, or to the pivot for a confined object, is^M called the
torque or the
moment. A torque is also a vector quantity and produces a rotation in the same way that a force produces a translation. Namely, an object at rest, or rotating at a constant angular velocity, will continue to do so until it is subject to an external torque. A torque produces a change in angular velocity which is called an angular acceleration.
The distance
L used to determine the torque
T is the distance from the pivot
p to the force, but measured perpendicular to the direction of the force. On the figure, we show four examples of torques to illustrate the basic principles governing torques. In each example a blue weight
W is acting on a red bar, which is called an arm.
In Example 1, the force (weight) is applied perpendicular to the arm. In this case, the perpendicular distance is the length of the bar and the torque is equal to the product of the length and the force.
T = F * L
In our case, the Force is stationary for the sake of the argument and is
F = m x a (mass multiplied by acceleration).
the mass is known, 240 pounds (108.5 kg),
the acceleration is 9.8 m/s*s (earth gravity),
so our force is 1063 kg*m/s*s,
and the length of towbar is 18 inches (0.4572 m) = 486 kg*m*m/s*s = 486 N*m (not to confuse with 486 joules, which is the energy resulted from the same calculations)...
naturally, the longer the towbar, the greater the "lever" effect...
1 Nm = 0.7375621 lbs*foot
486 Nm = 358.46 lbs * foot (or, more common term, "foot-pound)
so, in the stationary condition, we are applying a torque of roughly 350 pound foot onto the hitch bar and the hitch assembly.
Once we introduce the additional forces of driving forward and bouncing the load vertically, the numbers will decrease on "up" and increase on "down" move, also, as the vehicle is moving on the decline, the torque will decrease, as if the vehicle is moving on the incline, up the hill, the center of bike's gravity will move further away from the 18" mark and the torque will increase...
I am just too lazy to venture into these calculations...
The main point is, will the mounting points sustain the load, and more importantly, will the places where they are mounted to, sustain the load... those items can be calculated, based on the type of metal used, the thickness of the metal, the type of the attachment of different materials, the ability of the metal to take the repeating bending stress without losing the resistance property and the metal fatigue that will eventually settle in the material before anything would break.
These are the calculations taken into the account by the engineers who design these things...
Remember the Fast 5 movie - when Dominique Torreto and Co. ripped the "unbreakable" safe from the enclosure, as the enclosure was not reenforced enough to protect the vault... so, it did not matter that the vault was good, the surroundings weere not up to par...
oh, and by the way, I am not here to rain on your parade - I am in engineering myself, and can only appreciate a well executed mechanical (or electrical/electronic) masterpiece... And I think, the OE tow hitch is an engineering masterpiece...