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Old 04-14-2016, 05:32 PM   #1
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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!
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Old 04-14-2016, 06:45 PM   #2
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I would greatly appreciate any thoughts from Y'all!
TMI
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Old 04-14-2016, 06:48 PM   #3
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The front axle sounds right. That's the pivot.
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Old 04-14-2016, 06:51 PM   #4
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I don't think it changes either, even when braking. The change is because of linear forces causing moments forces on the pivot.
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Old 04-14-2016, 07:04 PM   #5
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TMI
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Old 04-14-2016, 08:10 PM   #6
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Why? Just load up prudently and go camping!


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Old 04-14-2016, 08:34 PM   #7
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Why? Just load up prudently and go camping!


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Because in my particular case, I am near the limits of my TV, and I wish to know exactly what is going on so I can SAFELY load up prudently and go camping. I simply do not subscribe to the hitch manufacturer's statement that 1/3 of the tongue weight goes here, 1/3 goes there, because in the end, if there is an accident resulting in damage, injury or God forbid, death, I (or you) may need to face a court of law where it could be proven that X axle was overloaded.
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Old 04-14-2016, 08:35 PM   #8
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Interesting question! There must be an engineer out there who knows the real answer. I did a couple of thought experiments and concluded that the answer might be "it depends".

I believe that if the trailer is precisely level (defined as: equal load on each axle) then the pivot would be the mid point between the axles.

However, if the center of mass is shifted forward, the pivot point would also shift forward. Conversely, if the center of mass shifted toward the rear then the pivot point would also shift.

I've never attempted to measure the loads on each axle of my trailer separately. Maybe others have. It seems to me that if the trailer was level (as measured by a carpentry tool) that the loads would be similar. However, if weight was added in the front of the trailer and the WD was not reset then the hitch would drop, the trailer would no longer be level, tongue weight would increase, the load on the front trailer axle would increase more than the rear and the implied pivot point would be the front trailer axle. If that same load was placed behind the rear axle then the hitch would rise, etc, etc and the rear axle would look like the pivot.

The transition from front to back pivot would be when the trailer was level.

To your point, it may be that many of us run slightly nose-low and front axle heavy. That would absolutely happen if you set the hitch ball to the level height of the detached trailer then hook the trailer up and have a few inches of sink to the rear end of the TV. That condition would precisely lead to your measured observation.

I have completely convinced myself through thought alone that the above must be true... Prior experience with this feeling of confidence tells me I am likely to be informed otherwise by someone whose learning and experience is stronger than thoughts alone. So... come on engineers and physicists... What's the real answer?
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Old 04-15-2016, 07:17 AM   #9
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The only limits that matter are axle and tire rating. The rest is eyewash.

A spreadsheet won't tell anyone how good is the hitch lashup, much less the attributes of handling and braking.

The focus on "weight" as a go/no-go determinant is misplaced. Unrealistic.

I constantly see rigs out on the road that probably are fine insofar as that is concerned. But even a glance at the attitude of the rig says that real problems exist. Not addressed. Or that the heavier tow vehicle itself doesn't pose an additional risk. It does.

Playing with weight capacity numbers is a start. It sure as hell doesn't determine safety. It can work directly against that.
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Old 04-15-2016, 07:21 AM   #10
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Because in my particular case, I am near the limits of my TV, and I wish to know exactly what is going on so I can SAFELY load up prudently and go camping. I simply do not subscribe to the hitch manufacturer's statement that 1/3 of the tongue weight goes here, 1/3 goes there, because in the end, if there is an accident resulting in damage, injury or God forbid, death, I (or you) may need to face a court of law where it could be proven that X axle was overloaded.

Show us that court case.
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Old 04-15-2016, 08:49 AM   #11
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I'm confused....is OP talking about Center of Gravity or pivot point. They are two very different things.
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Old 04-15-2016, 08:54 AM   #12
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Clamb;
You have way too much time on your hands. IMHO
In almost all trailer design the distance from the axles to the hitch is greater than the distance from the axles to the rear bumper. Because of this the pivot point (fulcrum)
will always be on or near the front axle. IMHO
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Old 04-15-2016, 09:07 AM   #13
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It seems to me the pivot point to use for the calculations would be the front axle. If you tip the trailer down from level it will pivot on the front and the rear axle will eventually come off the ground. If you raise the trailer enough at the front end it will eventually pivot on the rear axle and raise the front axle off the ground.
So to me, I would think the front axle pivot point would yield the best results for what you want to do.

If you get in a wreck with a "marginal" TV I doubt if you will ever want to show your spreadsheet.
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Old 04-15-2016, 09:07 AM   #14
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That made my brain hurt.....

Get a tow vehicle with more capacity than you need and spend all that spreadsheet time going places and doing things!


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Old 04-15-2016, 09:09 AM   #15
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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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..... 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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