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Old 04-15-2016, 04:16 PM   #43
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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.
Clamb your engineering knowledge is way beyond me. But hopefully you will find kindred spirits here. I use the old trial and error approach.

As for the forums, there is a plethora of opinions and semi-educated guesswork around here as well as certain keywords that will get you enough contradictory advice for a lifetime. At that point congeniality tends to have a meltdown. Keywords include (but are not limited to) weight distributing hitches, sway control, tires, rodents, and etiquette. Non illigetimus carborundum!
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Old 04-15-2016, 04:58 PM   #44
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OMG, four pages of back and forth stuff, all to figure out how to make an inadequate TV adequate (barely). You makes your choices and you live your consequences. Happy motoring��
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Old 04-15-2016, 05:15 PM   #45
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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).
Fully agree with keeping the three dimensional aspect out of it.

Agree that the fulcrum will be between the axles, but it will depend on whether the trailer is flat or not. It will be equidistant between the axles in a perfect (flat) world, but it will shift fore and aft, even when static, depending on whether the trailer is nose down or nose up, IMO.

Here is where I differ. In order to have no effect on tongue weight (in lbs) the added weight must be placed between the trailer axles, not at the CoG. If it is added at the CoG the tongue weight will go up, in lbs, but not by 50% of the added weight. The tongue weight will remain at the same % of total weight as prior to additional load being added (ie 10% or 15% as the case may be).

Maybe some of the confusion comes down to whether we are talking about the same tongue weight in lbs, or as a % of total weight.

If the weight is added behind the axles, then the tongue weight will go down (in lbs).

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Old 04-15-2016, 06:22 PM   #46
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Jcl,

Yes, I was assuming dead flat level in all the discussions of "static".

I'm with you...and ran some numbers as far as % vs. pounds...my bad. But my interpretation of no effect was that regardless of how many pounds OP was adding to the trailer, he wanted the same pounds on the ball. Maybe I'm misunderstanding that point.

I guess the bottom line here is...again....with the dynamics of levelness, suspension squat and the constantly moving CoG and Fulcrum point (on tandems)..combined with the limitations of 1.25" drawbar adjustments and WD limitations...all one can do is weigh and experiment. That's why the kind engineers give us a RANGE for TW and the ability to change it by moving stuff around.
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Old 04-15-2016, 06:47 PM   #47
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Well, looks like I found what I was looking for. I set up the bathroom scale arrangement, and added 250 lbs of water to the water tank, which sits squarely between the axles, and the scale did not change one iota, so I will use that as a data in the spreadsheet, which I will post here when the bugs are worked out (if there is any interest).

ChuckFeldt, the purpose of all this is not to make my "inadequate TV adequate", it is to ensure that it is safe to take on the road, and the purpose of the spreadsheet is to have a good idea of how added weight comes into play without having to make unnecessary (and long) trips to the scale My TV has a GVWR of 7050 lbs. My max tow rating is 10,600 lbs, and a CVWR of 16,100 lbs. My available payload is 1634 lbs. I did extensive research before purchasing it, including reading posts by other owners here who are very happy towing with it.
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Old 04-15-2016, 07:10 PM   #48
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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.
I base my loadings on what is legal and published; in accordance with the mantra: "C.Y.A."
Then I'll try it on the road, and if it feels good, I use it.
If it doesn't, I go to the "Experts".

But; If the questions arise in a court of law, I can point to the published info and say: "Ask them! I'm only doing what I'm told to by the Experts".
Going outside the published info; I'm on my own, and I had better have good answers.
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Old 04-15-2016, 07:18 PM   #49
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Well, looks like I found what I was looking for. I set up the bathroom scale arrangement, and added 250 lbs of water to the water tank, which sits squarely between the axles, and the scale did not change one iota, so I will use that as a data in the spreadsheet, which I will post here when the bugs are worked out (if there is any interest).

ChuckFeldt, the purpose of all this is not to make my "inadequate TV adequate", it is to ensure that it is safe to take on the road, and the purpose of the spreadsheet is to have a good idea of how added weight comes into play without having to make unnecessary (and long) trips to the scale My TV has a GVWR of 7050 lbs. My max tow rating is 10,600 lbs, and a CVWR of 16,100 lbs. My available payload is 1634 lbs. I did extensive research before purchasing it, including reading posts by other owners here who are very happy towing with it.
But your TW as a percentage of total weight changed. Again, the TW RANGE comes into play. And I'll bet the weight did change. ....albeit by a small iota....probably not measurable with the setup used.
Another reason, like I stated above, this exercise is fruitless. But at least I got some mental exercise.
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Old 04-15-2016, 07:19 PM   #50
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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, 07:30 PM   #51
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Level the trailer, remove any weight distribution in the hitch, and for simplicity assume a single axle.

Add 100 lbs directly over the hitch ball. The tongue weight goes up 100 lbs, and the weight on the axle does not change.

Place the 100 lbs halfway between the hitch ball and the axle centerline. 50 lbs will be added to the the tongue weight, 50 lbs to the axle weight.

Place the 100 lbs 3/4 the way from the hitch ball to the axle centerline. 25 lbs will be added to the tongue weight, 75 lbs to the axle weight.

Move the 100 lbs directly over the axle centerline. No weight will be added to the tongue and 100 lbs will be added to the axle weight.

For multi axle trailers with all identical spring rates, the zero tongue weight case requires the 100 lbs to be placed between the front axle and the point midway between the front and rear axle.

The closer the hitch ball is to the front axle, the closer to the front axle the weight must be placed. The farther the hitch ball from the front axle, the closer to the midway point the weight must be added.
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Old 04-15-2016, 07:39 PM   #52
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Clamb your engineering knowledge is way beyond me. But hopefully you will find kindred spirits here. I use the old trial and error approach.

As for the forums, there is a plethora of opinions and semi-educated guesswork around here as well as certain keywords that will get you enough contradictory advice for a lifetime. At that point congeniality tends to have a meltdown. Keywords include (but are not limited to) weight distributing hitches, sway control, tires, rodents, and etiquette. Non illigetimus carborundum!
"ILLEGITIMUS NON CARBORUNDUMOUS"
I haven't heard that one in a long time, and used to use it myself.
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Old 04-16-2016, 12:27 AM   #53
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Try this:
Put a scale under each tire with the trailer level and loaded for the road. Record the reading of each scale. The readings of each scale will not be the same. I am certain of that.
Lower the tongue jack to it's absolute extreme. No blocks no foot plate, no wheel. The hitch ball receiver should be almost on the ground if you are loaded tongue heavy.
IMHO you will see weight taken from the rear scales as the tongue gets lower and the rear bumper gets higher.
Extreme example:
If you were to take the tires off of the rear axle and lower the tongue the front axle would be the fulcrum point.
If you were to take the front axle tires off rear tires on and lowered the tongue the rear axle would be the fulcrum point.
What I am saying is there is no constant when you are traveling down the road.
Not sure the COG has anything to do with it. If the COG is lower there is less chance of tipping the coach on it's side.
The higher the COG the greater the chance of tipping over. I have never heard of and can't imagine a trailer flipping end to end because of a high COG. Side to side YES.
Don't get too far into the woods on this. To me it is like the weather models. There are four from around the world and they never agree.
IMHO every time you hit a dip or bump in the road with either the TV or coach you change the fulcrum point. This does not affect the towing characteristic dramatically.
What I believe effects the handle more than anything is the porpoise affect of a maladjusted weight distribution system, head, tail and side winds etc
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Old 04-16-2016, 12:50 AM   #54
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With my 3/4 ton Chevy PU and 31' Airstream and all tires on scales, if I put 100lbs. on top of the ball I would expect the rear axle weight of the truck to go up about 85lbs. and the front axle to get 1lb. lighter; the front trailer axle weight to get about 15lbs. heavier and the rear axel about 1lb heavier. The rig would still be sitting about dead level. If I put the WD bars on then I have no idea what would happen because I'd have a couple of beers while sitting in the sun and then go lay on the bed in the back of the trailer for a while. I keep my beer just ahead of the front axle and it's a mid bath trailer and I really should go on a diet. As far as what the OP was thinking to start the post I'd guess the fulcrum was the front axle for all practical purposes if I understand his concerns. The situation and forces will vary for different wheelbase TV's and length trailers. If anybody ever gets in a bad wreck and gets blamed for causing it I'd bet that the cops write the driver a ticket for "driving too fast for conditions" no matter what the load distribution was.
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Old 04-16-2016, 12:50 AM   #55
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Tandem axle- where exactly is the tongue weight pivot point?

I'm going to agree with TG Twinkie.

With my nice ProPride hitch setup, a difference in trailer loading can introduce very noticeable porpoising. This is real obvious going uphill with a lot of engine torque being put on the TV rear axle.

The cure is a slightly heavier WD setting to eliminate it. About 1/2 to one inch tighter on the bars. It's very obvious when I have it right, and real uncomfortable until I get the TV front end properly planted with the bar adjustment jacks.

Being aware of the motion, and knowing how to tune it our is the main thing. You gotta pay attention to what the rig is telling you as you drive.

Ditto to adjusting brake controller to prevent the "Hensley Bump" when stopping hard. AS loading also has a big influence on where the gain has to be set on the controller.

I find that going a short distance at highway speed tells me a lot about how the whole rig is acting, and what needs adjusting and in what direction for a good, safe ride.


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Old 04-16-2016, 01:21 AM   #56
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TG Twinkie is right. Think of the "Narrows Bridge" a few years ago, it is a perfect example of an object being out of balance, if you will, being dynamically unstable. Instead of returning to a stable state it got worse and worse until it flew apart. The same thing with towing. Stay within the recommended 10-15% tongue weight and you will be safe. I would venture to say that 15% feels better than 10%. But, the point remains that if you do not know the facts (weight and balance calculated) you may have to learn the hard way. Just like airplanes a CG too far aft equals instability (sway) and/or jack knife. I have been experimenting with my 34' Slide. I removed the batteries and boxes and placed Lithiums under the bed. I can tell the difference, I am now experimenting with the tension on the draw bars to see where the sweet spot will be. At least I don't have 1,750# on the hitch anymore. Both Hensley and ProPride say they have a lot of trailers out there heavier than that with no reported problems. Other than hitch parts wear, my only bad problem was GM's receiver being so flexible it moved up instead of the FV when tightening the bars. After market Class Vs won't do that.
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