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Old 04-15-2016, 01:11 PM   #29
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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?
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Old 04-15-2016, 01:12 PM   #30
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And before I'm accused of trolling, I am not
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Old 04-15-2016, 01:17 PM   #31
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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
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Old 04-15-2016, 01:34 PM   #32
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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.
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Old 04-15-2016, 02:52 PM   #33
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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.
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Old 04-15-2016, 02:57 PM   #34
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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.
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Old 04-15-2016, 03:08 PM   #35
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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)
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Old 04-15-2016, 03:11 PM   #36
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"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."
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Old 04-15-2016, 03:24 PM   #37
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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.

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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.
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Old 04-15-2016, 03:31 PM   #38
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"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.
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Old 04-15-2016, 03:32 PM   #39
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Lots of great discussion of Physics and moments and stuff. I think perhaps going back to the original post:

Quote:
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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.
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Old 04-15-2016, 03:34 PM   #40
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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.
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Old 04-15-2016, 03:59 PM   #41
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I am not confusing the center of gravity with fulcrum point.

On a tandem axle trailer there are two bearing 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.

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 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.
Yeah, I was trying to keep the three dimensionality of the CoG out of this conversation, since we are only concerned with TW and fore and aft shifting of weight. But OPs last concern is where to place weight so there is NO EFFECT on TW. Since we have tandems (especially since we don't have equalizer equipped tandems) we have an average fulcrum located equidistant between the axles, statically, and fluctuating back and forth between the axles, dynamically. Obviously, the CoG is forward of the axles, by design, for the trailer to be nose heavy by 10% minimum.
In order to have no effect on TW, any weight added must be aft of the CoG and forward of the average fulcrum. If the added weight is on the CoG, 50% of its weight would be added to TW. If it were on the average fulcrum, there would be a reduction in TW, as the weight is aft of CoG (although CoG will have moved somewhat rearward due to added weight aft of it's previous location).

So after all that, what practically do we have to work with for any significant weight storage? In my case, not much...a narrow isle-way, an under dinette bench drawer and a drawer under the counter. So my ceramic dinnerware are in the drawer and the pots and pans are under the dinette seat. I'm done....no more room for that "perfect pivot/CoG/no effect on TW" nonsense.

Again, occasionally weigh your rig, weigh your tongue and move stuff around until you get 10 - 15% of total on the tongue. The engineers already cast our die, fabricated our destiny.....when they designed the thing.
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Old 04-15-2016, 04:09 PM   #42
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I think I will stop commenting in this thread and go grease the extreme fulcrum points on my trailer and check out the brakes
I'm trying to get ready for summer travel out west.

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