X Full Timer---5th Wheel Virgin

RetiredSquid wrote:
Seems like i asked a good question or controversial one! Like i said in original post was a full timer 15 years ago. Have had three 3/4 ton trucks and one 1 ton. The GVWR **** is all old school, and I purposely bought the 3/4 because I dont want a big 5er......the wife does...
..I guess size does matter to a woman :-)

:B

Seems like i asked a good question or controversial one! Like i said in original post was a full timer 15 years ago. Have had three 3/4 ton trucks and one 1 ton. The GVWR **** is all old school, and I purposely bought the 3/4 because I dont want a big 5er......the wife does...
..I guess size does matter to a woman :-)

Why do people use "dry" weights when picking out an RV that they'll be towing? Its not a realistic weight and most times, is wrong. Same with "dry" pin weight. I would think when one matches a 5er up with what they're going to tow it with, they would use a more realistic weight somewhere between the 5ers dry weight and its GVW, and would think they should use 20% of the 5ers GVW to get a more accurate idea of how much pin weight they'll be putting in the bed of their truck.

Just seems it would be a more logical way of matching a TV with what one is going to tow. I see more and more post on quiet a few RV forums as well as truck forums with people using non realistic "dry" weights when matching what they're going to tow with their TV. Doesn't make a lot of sense to me.

I have a F250 diesel short bed crew cab, and I limited myself to a 11K lbs. dry weight trailer with 2K lbs. dry weight kingpin weight.

My trailer is a 2015 Grand Design Reflection 323BHS bunk house trailer. And my truck tows it just fine. I added AirRide 5000 airbags, however they were not really needed since I have no squat.

We absolutely love the bunkhouse, as I use it for my man cave to avoid having to watch my wife's television shows. The grandkids can also watch 'toons there.

In a nut shell, forget the 42' with your F250. You may be able to pull a 32 to 33' 5er provided you do not exceed the trucks GVWR or GAWR,

You need to weigh your truck and do some simple math to see what you can pull and carry.

And Rusty is correct on the stress on the pin box area for a 5er with a goose neck adapter or even a Goose Box. Some folks cannot see the forest for the trees.

Ken

Retired squid....
Take a look at the 2015 Jayco 29.5 BHDS.
It's a Bunk House.... weighs 9950 pin weight 1515.

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

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.

So why do you believe it makes a difference at the pin how the force is applied? Once applied and the 5er moves in the new direction the force is reduced not increased.

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."

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.

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

RustyJC wrote:
You know, my dear old schoolteacher Mom used to tell me, "If you find yourself driving down the road and everyone else is coming at you, at some point you have to consider that you might be driving the wrong way."

Your hammer analogy isn't all that bad - EXCEPT the constant is the force transferred from the tow vehicle at its attachment point at a given rate of acceleration. If it takes 2000 lbs of horizontal force to move the trailer at a given acceleration (regardless of the hitch type), 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. If that distance is 24" in the case of the gooseneck ball and 8" in the case of the 5th wheel kingpin, then the resultant moment will be 3 times greater with the gooseneck (2000 lbs x 2 ft = 4000 lb-ft) than it will with the 5th wheel hitch (2000 lbs x .66667 ft = 1333.3 lb-ft). 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.

If you don't believe me, then how about the following excerpt from the instructions of one gooseneck adapter manufacturer (emphasis mine):

A word of caution is in order about the use of the Cody Coupler! Because of its design, additional pressure may be placed on the "King Pin" of the trailer it is mounted on. This is a result of the leveraging action involved. Because of this, we feel it is wise to mention several steps that should be taken by the owner of such a rig. These are "common sense" suggestions that could and should apply to any type of trailer hook-up.

REGULARLY INSPECT THE INSTALLATION OF THE COUPLER
Make certain the "King Pin" is securely attached to the Pin Box. Occasionally, additional reinforcement might be advisable.

AVOID THE "POP-A-WHEELY SYNDROME"
Smooth, even starts and stops are always better than the jerky motion.

TRAILER BRAKES SHOULD ALWAYS BE IN GOOD WORKING ORDER!
And properly hooked up.

In actuality, since the load plates of the 5th wheel hitch and pinbox prevent rotation, the force transmitted to the trailer with a 5th wheel hitch isn't a torque but a shear force of 2000 lbs in the example above. For simplification, I treated the 5th wheel hitch arrangement the same as the gooseneck adapter.

Rusty

Easier to say that you're adding an 18" cheater that's trying to rotate the king pin and/or pin box and transferring that force to the frame. :-)

Lyle

You know, my dear old schoolteacher Mom used to tell me, "If you find yourself driving down the road and everyone else is coming at you, at some point you have to consider that you might be driving the wrong way."

Your hammer analogy isn't all that bad - EXCEPT the constant is the force transferred from the tow vehicle at its attachment point at a given rate of acceleration. If it takes 2000 lbs of horizontal force to move the trailer at a given acceleration (regardless of the hitch type), 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. If that distance is 24" in the case of the gooseneck ball and 8" in the case of the 5th wheel kingpin, then the resultant moment will be 3 times greater with the gooseneck (2000 lbs x 2 ft = 4000 lb-ft) than it will with the 5th wheel hitch (2000 lbs x .66667 ft = 1333.3 lb-ft). 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.

If you don't believe me, then how about the following excerpt from the instructions of one gooseneck adapter manufacturer (emphasis mine):

A word of caution is in order about the use of the Cody Coupler! Because of its design, additional pressure may be placed on the "King Pin" of the trailer it is mounted on. This is a result of the leveraging action involved. Because of this, we feel it is wise to mention several steps that should be taken by the owner of such a rig. These are "common sense" suggestions that could and should apply to any type of trailer hook-up.

REGULARLY INSPECT THE INSTALLATION OF THE COUPLER
Make certain the "King Pin" is securely attached to the Pin Box. Occasionally, additional reinforcement might be advisable.

AVOID THE "POP-A-WHEELY SYNDROME"
Smooth, even starts and stops are always better than the jerky motion.

TRAILER BRAKES SHOULD ALWAYS BE IN GOOD WORKING ORDER!
And properly hooked up.

In actuality, since the load plates of the 5th wheel hitch and pinbox prevent rotation, the force transmitted to the trailer with a 5th wheel hitch isn't a torque but a shear force of 2000 lbs in the example above. For simplification, I treated the 5th wheel hitch arrangement the same as the gooseneck adapter.

Rusty