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Old 04-15-2016, 09:09 AM   #15
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Originally Posted by dznf0g View Post
I'm confused....is OP talking about Center of Gravity or pivot point. They are two very different things.
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.
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Old 04-15-2016, 09:44 AM   #16
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Originally Posted by clamb View Post
Tandem axle- where exactly is the tongue weight pivot point?
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!
My thoughts:

The tongue weight pivot point is centered on the hitch ball.(answering your question in the title of the post)

The fulcrum point of the load on the axles (I think this is what you are asking about) is not a fixed point when the rig is in motion. It's constantly shifting as the rig moves, since both of the vehicles have flexible suspension and the road is uneven. I see no practical reason to try to pin point where this pivot (or fulcrum) point occurs while the vehicles are parked.

Weighing and calculating various loads from the weighing results is what is important.
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Old 04-15-2016, 10:04 AM   #17
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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 raise 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.
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Old 04-15-2016, 10:14 AM   #18
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First, ignore the comments like TMI and "my brain hurts" and eyewash.
I agree with dzfn0z - the pivot point and the center of gravity are two separate things. Start your work with center of mass calculations.

Have you looked at the work that nickcrowhurst did on this several years ago?
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Old 04-15-2016, 10:15 AM   #19
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Quote:
Originally Posted by dznf0g View Post
..... snip...As you lower the front of the trailer, does the TW increase.....? yes...... snip.....
Actually no.... not on a tandem axle.

When the trailer tongue is lowered the fulcrum point shifts forward, toward the front axle. There would be more weight on the front axle, less on the rear. In this case more weight would be behind the fulcrum point, lessening the tongue weight.
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Old 04-15-2016, 10:18 AM   #20
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Actually no.... not on a tandem axle.

When the trailer tongue is lowered the fulcrum point shifts forward, toward the front axle. In this case more weight would be behind the fulcrum point, lessening the tongue weight.
Duh, correct...fingers ahead of brain again. I'll see if I can still edit above.
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Old 04-15-2016, 10:24 AM   #21
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Quote:
Originally Posted by dznf0g View Post
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.
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Old 04-15-2016, 10:42 AM   #22
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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.
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Old 04-15-2016, 10:49 AM   #23
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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.
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Old 04-15-2016, 11:18 AM   #24
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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
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Old 04-15-2016, 11:18 AM   #25
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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.
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Old 04-15-2016, 11:26 AM   #26
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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!

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Old 04-15-2016, 12:36 PM   #27
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To much analysis

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.
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Old 04-15-2016, 12:43 PM   #28
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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.
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