X Full Timer---5th Wheel Virgin

NMDriver wrote:
Glad to see some thought put into it. You are right about the force required to move the 5er does not change and that the force is (mostly) horizontal.

That force IS horizontal as it's applied at the gooseneck ball or the kingpin, respectively, assuming the acceleration takes place on a level surface.

NMDriver wrote:
So why do you believe it makes a difference at the pin how the force is applied?

It doesn't make a difference at the gooseneck ball or the kingpin. It DOES make a difference at the pinbox to frame junction. As I pointed out in my example, the 2000 lb accelerating force is applied at the end of a moment arm (lever). A force applied at the end of a moment arm creates a moment (torque), and the torques applied to the pinbox to frame junction differ according to the length of the moment arm (again, as I demonstrated mathematically).

NMDriver wrote:
Once applied and the 5er moves in the new direction the force is reduced not increased.

If the trailer is accelerating at 0.2g (for instance), the force will remain at 2000 lbs at the gooseneck ball or kingpin as long as the acceleration continues. Once the truck/trailer reaches the desired speed and ceases to accelerate, the force (in our example) will go to zero since acceleration goes to zero (F = M x A). In actuality, at a constant cruising speed on level ground, although the trailer isn't accelerating, a force must still be applied at the gooseneck ball or kingpin to overcome aerodynamic and rolling resistance.

NMDriver wrote:
After stating "If it takes 2000 lbs of horizontal force to move the trailer at a given acceleration (regardless of the hitch type)", you then wrote "the difference in the force applied to the pinbox-to-frame junction is a function of the length of the moment arm between the hitch attachment point and the pinbox-to-frame juncture" What difference? AS YOU point out it is a constant for a given acceleration. "The force required for the truck to accelerate the 5th wheel at a given rate of acceleration doesn't change - it's always F = M x A."

Explained above. The forces discussed are at two different points - the 2000 lb horizontal force at the gooseneck ball or kingpin, and the moment (torque) applied to the pinbox to frame junction. The latter is different because the 2000 lb horizontal force is applied to the frame through moment arms of differing lengths (8" and 24" in my earlier example).

NMDriver wrote:

1) The 5er is not moved by lever action it is moved by straight line forces. Mostly horizontal. The 2000lbs at the pin is all that is felt by the pin and transferred to the frame. You cannot add force because as soon as the 2000lb is applied the trailer moves and the force goes down to an equilibrium amount to maintain the movement.

2) Even if you think of the hitch as a pivot point in a lever. The length of the lever arm is from the point on the ground where the wheels are applying power up to the pin on the frame.

It is essentially a straight line from the wheel to the pin and is pretty close to the same for any hitch. The location of a pivot point in the arm does not change the length of the lever arm nor the force it applies at the pin.

3) One last analogy. A 5er is like a string on the end of a stick. The length of the stick does not change the force the string feels.

4) IF a gooseneck applied more force for less effort by the truck then you would get better mileage with a gooseneck and could pull heavier loads with less truck. BTW: I have had both gooseneck and regular hitches and I stand by my statement that goosenecks are a better hitch, if for no other reason than they are gentler on the 5er frame to hitch and unhitch.

With all respect, your assumptions above are erroneous.

NMDriver wrote:

5) I am willing to continue this via private message but I am though on this topic in this forum.

Not necessary. I'll stand by my rigorous proof submitted earlier.

Rusty