How Does Moving a Gooseneck Trailer Connection Rearward Affect Hitch Load?

[Lnewqban]

After further thought, the weight placed on the chassis itself doesn't change, because the tongue weight of the trailer doesn't change. i.e. the trailer's downward force is 2,500 pounds no matter where it is placed on the vehicle.

Not much perhaps, but it changes.
More or less of the total weight can be moved onto the tires.

Two extreme cases:
If all the weight (trailer plus usefull load) could be relocated on those wheels, no weight would be felt by the truck.
If it could be located aft the wheels, the truck would feel a lifting force.

Welcome!

 

[jbriggs444]

Not much perhaps, but it changes.

As I understand the class of scenarios being considered, the single variable is the fore-and-aft placement of the hitch on the towing vehicle. A fixed trailer with a fixed tongue arrangement is hitched at the chosen hitch point.

Edit: This understanding is not correct. We are not talking about moving the hitch around within the bed of the truck.

With this understanding in mind, for a fixed trailer the tongue weight would obviously be fixed. So the total weight supported by the towing vehicle tires would be fixed regardless of hitch placement. The ratio of front tire force versus rear tire force would vary, of course.

An equalizing hitch could change things. We had one of those on our family's Shasta trailer back in the day. The torque at the hitch tended to off-load the towing vehicle rear wheels while loading up the trailer wheels and the towing vehicle front wheels.

 

[Baluncore]

At least you now know the difference between a gooseneck hitch and a fifth-wheel hitch. I am glad I could help you.

My experience is with fifth wheels on semitrailers. It does not matter what type of hitch is employed, the hitch is the hinge point of the combination of tractor and trailer.

The gross weight of the trailer and load can be seen as two parts. The first part is carried by, and balanced about, the trailer axle set. That offers no static load to the tractor hitch point.
The second part is the unbalanced mass that provides a moment, that rests effectively on the hitch. That moment is 2500 lbs at the old hitch position, 318.5" from the axle-set, which works out at;
318.5" * 2,500 lbs = 796,250 inch⋅pounds
If you extend the trailer axle-set to hitch point length by 20", the length becomes 338.5", so the vertical load on the tractor hitch will become;
796,250 / 338.5 = 2,352 lbs, which is 148 lbs less than the original.

But I can have no confidence in the numbers, because posts like these contradict an extension of 20" between the axle-set and the modified hitch point.

I assume the 20" is a rigid extension of the trailer gooseneck.
What is the distance between the hitch point now, and the mid-point of the tandem axle set?

Length from the trailer connection point to the center of the axles does not change in either configuration. That length is 318.5 inches.

 

[jeredpilot]

Not much perhaps, but it changes.
More or less of the total weight can be moved onto the tires.

Two extreme cases:
If all the weight (trailer plus usefull load) could be relocated on those wheels, no weight would be felt by the truck.
If it could be located aft the wheels, the truck would feel a lifting force.

Welcome!

"Two extreme cases:
If all the weight (trailer plus usefull load) could be relocated on those wheels, no weight would be felt by the truck.
If it could be located aft the wheels, the truck would feel a lifting force."

Yes, 100% - Exactly correct.

As far as "but it changes" I think it will not be an appreciable amount. - it will be negligible.

 

[jeredpilot]

As I understand the class of scenarios being considered, the single variable is the fore-and-aft placement of the hitch on the towing vehicle. A fixed trailer with a fixed tongue arrangement is hitched at the chosen hitch point.

With this understanding in mind, for a fixed trailer the tongue weight would obviously be fixed. So the total weight supported by the towing vehicle tires would be fixed regardless of hitch placement. The ratio of front tire force versus rear tire force would vary, of course.

An equalizing hitch could change things. We had one of those on our family's Shasta trailer back in the day. The torque at the hitch tended to off-load the towing vehicle rear wheels while loading up the trailer wheels and the towing vehicle front wheels.

Yes, 100%

The only change is the torque applied to the hitch itself due to the 20 inch lever. But the pounds of weight placed on the pickup chassis does not change.

 

[jeredpilot]

But I can have no confidence in the numbers, because posts like these contradict an extension of 20" between the axle-set and the modified hitch point.

If any one has not used words correctly, sir, it is you. I mentioned one such instance last evening from post number 18 and it was a bit more than you could stand.

I do appreciate your effort though.

But the answer is very simple. My configuration does not change the weight placed on the vehicle. Think about and you'll realize your error. But I am just a dumb layman with no degree in physics. What do I know? ;-)

 

[jbriggs444]

Yes, 100%

But you go on to contradict this understanding

The only change is the torque applied to the hitch itself due to the 20 inch lever. But the pounds of weight placed on the pickup chassis does not change.

A change to the moment arm does change the weight applied to the pickup chassis. Extending the moment arm allows the same torque to be provided by a different force at the hitch.

There are two changes. One is extending the moment arm. This tends to reduce the required support force. The other is the weight of the extension. This tends to increase the required support force. Which effect dominates depends on the weight and imbalance in the trailer and the weight and length of the extension.

 

[jeredpilot]

But you go on to contradict this understanding

A change to the torque does change the weight applied to the pickup chassis. Extending the moment arm allows the same torque to be provided by a different force at the hitch.

I completely disagree. The torque is a twisting force applied to the hitch itself.. The weight on the chassis remains constant. (this is assuming a parked, non-moving position)

Part of the whole problem with this discussion is "vocabulary" which I conceded from the first post I was not fluent in. As well as the imperfections of conveying thought through words, which is inherent in language itself.

 

[jbriggs444]

I completely disagree. The torque is a twisting force applied to the hitch itself.. The weight on the chassis remains constant.

This is not correct.

 

[jeredpilot]

This is not correct.

Okay. Please help me understand.

 

[jbriggs444]

Okay. Please help me understand.

The torque that I have in mind is from the chassis support force applied to the tongue of the trailer and assessed about a rotational axis at the trailer axle. Torque is computed as force times distance from the axis of rotation. The torque can be reduced if the force is not at right angles to that distance, but the two are perpendicular here, so no reduction is needed.

For the trailer to be supported without tipping (i.e. in equilibrium), this torque is fixed. At least until we contemplate any additional weight from the tongue extension.

If the torque on the trailer is fixed, the required support force will decrease when the distance from trailer hitch to trailer axle is increased.

 

[Baluncore]

But the answer is very simple. My configuration does not change the weight placed on the vehicle. Think about and you'll realize your error. But I am just a dumb layman with no degree in physics. What do I know? ;-)

You have too much confidence for a beginner.
As time moves on, maybe you will realize I was right, that the weight applied to the tractor hitch, will fall by almost 150 pounds.

Vocabulary: A "torque" in physics, is called a "moment" in engineering.

 

[jeredpilot]

You have too much confidence for a beginner.
As time moves on, maybe you will realize I was right, that the weight applied to the tractor hitch, will fall by almost 150 pounds.

Vocabulary: A "torque" in physics, is called a "moment" in engineering.

I am indeed a 'beginner'. But I have quite a lot of years of actual real-world experience in working with such things. So if I am incorrect, please accept my apologies.

I clearly understand I do not have enough knowledge to unequivocally solve this problem. That is why I am asking questions.

But yesterday at least, my questions offended you. That was not my intent.

 

[jeredpilot]

The torque that I have in mind is from the chassis support force applied to the tongue of the trailer and assessed about a rotational axis at the trailer axle. Torque is computed as force times distance from the axis of rotation. The torque can be reduced if the force is not at right angles to that distance, but the two are perpendicular here, so no reduction is needed.

For the trailer to be supported without tipping (i.e. in equilibrium), this torque is fixed. At least until we contemplate any additional weight from the tongue extension.

If the torque on the trailer is fixed, the required support force will decrease when the distance from trailer hitch to trailer axle is increased.

So by that, it appears you are in agreement with @Baluncore ?

You seem to be saying, the fixed 2,500 lbs will lessen due to an effective lengthening of the overall wheelbase? That is what I understood @Baluncore to be saying last evening.

 

[jeredpilot]

You have too much confidence for a beginner.
As time moves on, maybe you will realize I was right, that the weight applied to the tractor hitch, will fall by almost 150 pounds.

Vocabulary: A "torque" in physics, is called a "moment" in engineering.

And- My plan is to test our answers by using an actual certified truck scale.

I am serious about understanding this. And I am open and willing to be wrong; but I am not willing to misunderstand it.

 

[jrmichler]

My understanding of the problem. The trailer wheels are not moved. The only change is a 20" extension that moves the hitch point 20" farther from the wheels. The load is not changed between the two cases. The total weight of the trailer does not change, except for a small additional weight from the welded extension.

If my understanding listed and shown above is correct, then @Baluncore's calculation in Post #38 and quoted below, is completely correct. Since the total weight does not change, the weight on the wheels will increase by 148 lbs with the 20" extension.

The second part is the unbalanced mass that provides a moment, that rests effectively on the hitch. That moment is 2500 lbs at the old hitch position, 318.5" from the axle-set, which works out at;
318.5" * 2,500 lbs = 796,250 inch⋅pounds
If you extend the trailer axle-set to hitch point length by 20", the length becomes 338.5", so the vertical load on the tractor hitch will become;
796,250 / 338.5 = 2,352 lbs, which is 148 lbs less than the original.

He calculated the tongue load moment in his first calculation. A moment is a force times a distance. In this case, the force is the tongue load in Case 1 and the distance is the distance from the center of the pin to the center of the axle. When you change the distance from the tongue force to the axle, the moment does not change. In his second calculation he takes the same moment and the longer distance, and finds the new tongue load.

I am serious about understanding this. And I am open and willing to be wrong; but I am not willing to misunderstand it.

You have the right attitude. Do not give up, we will get you there. A good experiment to show the concept is to clamp a board to a toy wagon, load the wagon with something heavy, then lift one end of the wagon by lifting the board using a spring scale. The farther from the wagon that you lift, the less the force to lift one end.

My plan is to test our answers by using an actual certified truck scale.

Good plan. Just be aware of the weight of the extension. If the extension adds 150 lbs weight, the new tongue weight will still be 2500 lbs.

 

[A.T.]

Seems like two different things are being discussed:

Current Configuration

Proposed Configuration

For the above modification, the vertical force at the arrow location doesn't change. But you get a moment transmitted where the dark connector attaches to the rest of the truck.

For this modification, the vertical force at the vertical arrow location does change (decreases). But you get a moment transmitted by the extension where it attaches to the trailer (thick front wall).

 

[Lnewqban]

"Two extreme cases:
If all the weight (trailer plus usefull load) could be relocated on those wheels, no weight would be felt by the truck.
If it could be located aft the wheels, the truck would feel a lifting force."

Yes, 100% - Exactly correct.

As far as "but it changes" I think it will not be an appreciable amount. - it will be negligible.

Is it what you have in mind similar to this?

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