Tandem axle- where exactly is the tongue weight pivot point?

[clamb]

Originally Posted by dznf0g

I think the exercise for a "pivot point" is fruitless.

If you had no tire flex (solid steel wheels, and no spring flex (hubs welded in place) then your predicted change in TW as you raise and lower the tongue would be as predicted. But as you lower the front of the trailer the spring action of the rubber rods and the tire sidewall will not change TW as your math may indicate. You would have to know the exact spring rates of both the tire and the axle rods to get mathematically accurate.

As you lower the front of the trailer, does the TW increase.....? yes, but not at the same rate that as if you had no "springs" compression in the mix.

Assuming you set the trailer dead level in a static condition (perfect world) then the "pivot point" should be calculated equidistant between the axles. As the trailer tongue moves up and down from level, the "pivot point" will be in flux toward the front or back axle until such a time that one or the other tire/axle is off the ground. Then the pivot point will be at the axle which is still on the ground.

Warn has that part right....pivot point and TW is in constant flux as you move down the road.

The change in the two figures is less with a dual leaf spring/equalizer setup....but still valid.

All true- I think what needs to be stressed here is that the spreadsheet is intended to predict axle weights based on real measurements and while at rest, just as the system is at rest when weighing the axles at a scale.

When I load up for a trip, anticipating how much weight I will be adding during the trip, I don't want to make a 35 mile round trip to the nearest scale before beginning the trip.

The ball to trailer axle is a variable in the spreadsheet, and as I stated in the OP, when I used the front axle distance, the results came closest to my observed weights. Just trying to be sure there isn't a problem elsewhere in the calculations, that's all.

[clamb]

Originally Posted by markdoane

Have you looked at the work that nickcrowhurst did on this several years ago?

Was not aware of that- thank you- I'll go through it.

[A W Warn]

An example, of how the fulcrum point and center of gravity shift, can be visually demonstrated using a pendulum (or plumb bob).

If a weight is hung by a string from the ceiling of the trailer, hanging almost to the floor: Raising the hitch would swing the weight toward the rear (fulcrum moving toward the rear). Lowering the tongue will swing the weight toward the front(fulcrum moving forward). This weight will also move left or right toward the lowest side of the trailer.

Imagine how this weight hanging on a string (a pendulum) would move while driving down the road. That is how dynamic these points are.

[dznf0g]

Originally Posted by clamb

All true- I think what needs to be stressed here is that the spreadsheet is intended to predict axle weights based on real measurements and while at rest, just as the system is at rest when weighing the axles at a scale.

When I load up for a trip, anticipating how much weight I will be adding during the trip, I don't want to make a 35 mile round trip to the nearest scale before beginning the trip.

The ball to trailer axle is a variable in the spreadsheet, and as I stated in the OP, when I used the front axle distance, the results came closest to my observed weights. Just trying to be sure there isn't a problem elsewhere in the calculations, that's all.

I guess I am fairly accurate with a level trailer sitting on my pad and my Sherline scale.

I only drive to the scales when a significant change has occurred or every couple of years.

Every couple years for: added stuff (creeping weight increase) or stuff moved to more convenient locations (creeping weight shift)

Significant change is: New TV, new significant addition to TV, gear, or trailer.

In between, and in absence of these points, nothing changes much that a guesstimate and a Sherline won't handle.

This spring will be a re-calculate and scale weigh. Over winter we: got a new TV, added a modest solar package, got a new screen house (stored in the trunk), and I have relocated a lot of gear to make more efficient use of space. The below sheet is my TW and receiver weight calculations (done this morning) with tanks empty, propane full, but no groceries, clothes nor bikes loaded (front Arvika rack). Next time everything is loaded, I'll update this sheet and run to the scales. If my predictions are within a hundred pounds or so on all figures, I'm good with not needing a more detailed "pivot point" sheet. For it to be accurate anyway, you'd have to weight everything before you load it and do a bunch of math for each item. More trouble than going to the scale occasionally, IMPO.

Hensley TW calculator.xlsx

[franklyfrank]

Originally Posted by clamb

I have been working with a member of the F150 forum to arrive at a spreadsheet that will result in theoretical axle weights using a WDH, based on all the usual suspects...tongue weight, cargo weight (not JUST cargo weight, but exactly where, and how much). The spreadsheet is based mainly on the information developed by Ron Gratz, which addresses forces and fulcrums, ball- to axle distances, etc.

This spreadsheet will allow entry of specific weights at specific locations to arrive at the axle weights, and compare to axle ratings to determine a safe setup. When it is finalized, I will post a link to it here, or at least in a new thread. You should even be able to determine approx. how much tension is required on your WDH bars. Good stuff!

My reason for this thread is, in doing the preliminary trial to prove the values generated by the spreadsheet, I used the distance from the ball coupler to the midpoint of the trailer axles as the "ball to axle distance", and I couldn't get the spreadsheet to agree with actual CAT scale (hitched) axle weights, but when I shifted toward the front axle, it improved, and when I used the actual distance from front axle to ball coupler, everything fell into place pretty well.

I did an experiment- I measured the height of the rub strip on the side of the trailer, centered at the front axle. I then jacked the tongue up a ways, and down. The measurement didn't change. All tire pressures were equal, BTW.

So, I am thinking that this is the correct pivot point to use, but I do realize that the pivot point does change somewhat, constantly in flux, if you will. Braking and accelerating alone will change tongue weight.

I would greatly appreciate any thoughts from Y'all!

Sounds like the global warming conundrum.
Why don't you make it easy on yourself and give your brain a rest.
Go to your nearest cat scale and weight your TV empty. Go back home load it for your trip, go back and reweigh the axles. Go back home again load your trailer, hook up with your WD hitch, go back to the cat scale and weigh all the axles with WD engaged and without. This will give you the real live information.
I have done this every time I changed hitches or loading for different trips.

[Larry C]

Clamb, yer reading way too much into a problem that really doesn't exist....the fulcrum (you call pivot point) is somewhere in front of the front axle, regardless if it's a twin axle or triple axle. It cannot be behind the front axle, or you will have a negative tongue weight. (Yes that is possible, especially on heavy equipment trailers)

The further the coupler is located from the fulcrum, the more tongue weight you will have...as long as your tongue weight is somewhere in the "sweet spot" of 10 to 15% of the actual trailer weight, get the WD and sway control adjusted where you are comfortable, with the truck and the trailer level, and enjoy the trip...
Nuff said!

Larry C

[Nmengineer]

My Civil Engineering background tells me that is way to much analysis. Heck you could brake hard and have your load shift and all you calculations be for nothing. Paul and Gina said it best get a better TV so you have a little factor of safety in your equipment.

[dznf0g]

Originally Posted by Larry C

Clamb, yer reading way too much into a problem that really doesn't exist....the fulcrum (you call pivot point) is somewhere in front of the front axle, regardless if it's a twin axle or triple axle. It cannot be behind the front axle, or you will have a negative tongue weight. (Yes that is possible, especially on heavy equipment trailers)

The further the coupler is located from the fulcrum, the more tongue weight you will have...as long as your tongue weight is somewhere in the "sweet spot" of 10 to 15% of the actual trailer weight, get the WD and sway control adjusted where you are comfortable, with the truck and the trailer level, and enjoy the trip...
Nuff said!

Larry C

Larry, Don't confuse CoG with pivot point (fulcrum). CoG is forward of the center of the axle(s). Not necessarily the fulcrum.

[clamb]

OK I better rephrase this. At what location, relative to tandem axles, and when the trailer is level and not in motion, is tongue weight NOT influenced by placing weight at said location?

[clamb]

And before I'm accused of trolling, I am not

[dznf0g]

Originally Posted by clamb

OK I better rephrase this. At what location, relative to tandem axles, and when the trailer is level and not in motion, is tongue weight NOT influenced by placing weight at said location?

On the center line midpoint between the axles on a tandem....theoretically

EDIT: Wait, I need to ponder the "no effect" statement

[dznf0g]

If the weight is exactly on the longitudinal CoG, it will have equal effect on the system fore and aft of the original CoG. Each "end" of the system will have an increase in weight of 1/2 of the total added.

You would need to know the exact longitudinal CoG of your particular loaded rig on any given day and any given attitude to determine where the point is at which you would have neither an increase nor a decrease in TW with a weight added. It would be slightly aft of the CoG and forward of the center of axle(s).

As we've said before, due to available placement of storage locations and the variable distances of those locations from CoG and fulcrum, weighing your rig and moving objects is the best way of balancing your rig.

As soon as you get it right, you'll go buy groceries and beer and water, or fill your propane tanks, or fill the water tank, or don't dump your poop tank, etc. .....and it all changes again anyway.

[Jacob D]

Originally Posted by clamb

I am referring to pivot point. I am also asking for information, not opinions, insults or attacks, or how I should really be living my life. I always felt that this was a congenial forum.

I am a 30 year machinist, engineer wannabe. That means I like to figure things out and play with numbers. It's a fun hobby for me and gives my brain some exercise as it wastes away.

You can treat your AS as a simple beam on paper. Measure each axle weight separately and the total trailer weight apart from the TV. Sum the moments of forces about the ball hitch as follows: Multiply each axle weight by its center distance from the ball and add the results together. Set that sum equal to the total trailer weight times X. X will be the distance from the ball to the center of gravity which for this example is the fulcrum and is equal to the sum of the two axle weights times their distances from the ball divided by the total trailer weight. This procedure is called summing the moments and using the known fact that a system in equilibrium will have a zero sum of its moments. This also assumes no up/down, side to side or to and fro motion, equal weight distribution side to side and no WD hitch. A WD hitch properly adjusted might cause a slight variation in X but probably not much.

[A W Warn]

Originally Posted by clamb

OK I better rephrase this. At what location, relative to tandem axles, and when the trailer is level and not in motion, is tongue weight NOT influenced by placing weight at said location?

The location to place weight inside the trailer would not be a fixed point relative to the axles. Though, generally it would be at some point between the axles.

That exact point where tongue weight would not be affected by added weight would be exactly balanced over the fulcrum point. That exact fulcrum point is infinitesimal, a pin point.

[dznf0g]

Originally Posted by A W Warn

The location to place weight inside the trailer would not be a fixed point relative to the axles. Generally it would be at somewhere between the axles.

That exact point where tongue weight would not be affected by added weight would be exactly balanced over the fulcrum point. That exact fulcrum point is infinitesimal, a pin point.

AW, you're still describing the CoG, not the fulcrum. Imagine this:
Trailer is on a pair of jack stands whereby it is perfectly balanced in the air with the front frame the same distance off the ground as the rear. That point on the trailer is the Longitudinal CoG point AND the jack stands are the fulcrum at the CoG point That's why it is perfectly balanced.
Now move the jack stands to the "jack here" points aft of the tires. The jack stands are still the fulcrum, but the nose crashes to the ground. Why? because the fulcrum changed but the CoG of the trailer didn't. (no mass was relocated)

[dznf0g]

"ful·crum
ˈfo͝olkrəm,ˈfəlkrəm/Submit
noun
the point on which a lever rests or is supported and on which it pivots."

"Centre of gravity, in physics, an imaginary point in a body of matter where, for convenience in certain calculations, the total weight of the body may be thought to be concentrated. The concept is sometimes useful in designing static structures (e.g., buildings and bridges) or in predicting the behaviour of a moving body when it is acted on by gravity."

[ROBERT CROSS]

Originally Posted by dznf0g

I guess I am fairly accurate with a level trailer sitting on my pad and my Sherline scale.

I only drive to the scales when a significant change has occurred or every couple of years.

Every couple years for: added stuff (creeping weight increase) or stuff moved to more convenient locations (creeping weight shift)

Significant change is: New TV, new significant addition to TV, gear, or trailer.

In between, and in absence of these points, nothing changes much that a guesstimate and a Sherline won't handle.

This spring will be a re-calculate and scale weigh. Over winter we: got a new TV, added a modest solar package, got a new screen house (stored in the trunk), and I have relocated a lot of gear to make more efficient use of space. The below sheet is my TW and receiver weight calculations (done this morning) with tanks empty, propane full, but no groceries, clothes nor bikes loaded (front Arvika rack). Next time everything is loaded, I'll update this sheet and run to the scales. If my predictions are within a hundred pounds or so on all figures, I'm good with not needing a more detailed "pivot point" sheet. For it to be accurate anyway, you'd have to weight everything before you load it and do a bunch of math for each item. More trouble than going to the scale occasionally, IMPO.

Attachment 260775

Originally Posted by Nmengineer

My Civil Engineering background tells me that is way to much analysis. Heck you could brake hard and have your load shift and all you calculations be for nothing. Paul and Gina said it best get a better TV so you have a little factor of safety in your equipment.

^
X2

I weigh our TW with the Sherline only when our baseline load has changed markedly....as in more Beer in the rear.

The simple fix needed.....re-adjustment of the WD bars to level the rig.
If a major load increase has occurred a trip to the scales to confirm everything is within load parameters.

My principal engineering principle....
“Its better to have what you don't need when you need it, than not to have it when you don’t.”

Disclaimer.....

I've forgotten more than I'm currently aware of.

[dznf0g]

"I weigh our TW with the Sherline only when our baseline load has changed markedly....as in more Beer in the rear."

Funny you should mention that! One of the things I am doing in my re-evaluating storage efficiency and balance is to move my BEER (and water/pop) from the sofa arm compartments to the triangular cubbies at the foot of the bed in the rear. Each one will hold 15 cans! The soft items I used to keep in there now go forward. One of about a dozen relocation so far.

[markdoane]

Lots of great discussion of Physics and moments and stuff. I think perhaps going back to the original post:

Originally Posted by clamb

My reason for this thread is, in doing the preliminary trial to prove the values generated by the spreadsheet, I used the distance from the ball coupler to the midpoint of the trailer axles as the "ball to axle distance", and I couldn't get the spreadsheet to agree with actual CAT scale (hitched) axle weights, but when I shifted toward the front axle, it improved, and when I used the actual distance from front axle to ball coupler, everything fell into place pretty well.

I think your dilemma is that the Center of Gravity just happens to be co-located at the front axle. Not at all impractical as that would place the CoG roughly 18" in front of the midpoint of the axles.

[A W Warn]

Originally Posted by dznf0g

AW, you're still describing the CoG, not the fulcrum. Imagine this:
Trailer is on a pair of jack stands whereby it is perfectly balanced in the air with the front frame the same distance off the ground as the rear. That point on the trailer is the Longitudinal CoG point AND the jack stand are the fulcrum at the Cog point That's why it is perfectly balanced.
Now move the jack stands to the "jack here" points aft of the tires. The jack stands are still the fulcrum, but the nose crashes to the ground. Why? because the fulcrum changed but the CoG of the trailer didn't. (no mass was relocated)

I am not confusing the center of gravity with fulcrum point.

On a tandem axle trailer there are two points (not considering the tongue) that can be the fulcrum. If you lift the tongue high enough the front wheels will lift off of the ground, the rear axle is the fulcrum. If you lower the tongue low enough the rear wheels will lift off of the ground, the front axle is the fulcrum. In a normal towing position the fulcrum point shifts somewhere between these two extremes when both axles have load/weight applied. That point is not the center of gravity.

The center of gravity is not specifically over the axle or the fulcrum point. The center of gravity is the exact point where the entire weight of the trailer would balance in every direction. Imagine, if the trailer could spin in every direction, flipping end over end and rolling side to side, bottom over top, balanced like a gyroscope, if there were some way to do that. The point at the center of the balancing spin is the center of gravity. Thought I do not have the data or the knowledge to calculate it, I suppose the center of gravity point would be in the air, somewhere above the trailer's floor level.