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Old 07-01-2015, 08:08 PM   #1
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Math Question on Calculating Tongue Weight

I'm thinking there must be a formula for determining how much increase in tongue weight will result if a certain weight is added a certain distance between the axles and the coupler. For example, let's say it is 200" from the axles to the couple and I add 300 lbs. of water 60" from the coupler. What would be the increase in weight at the coupler? Anyone know??
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Old 07-01-2015, 08:18 PM   #2
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Not really too many variables all trailers are going to be different.

Scale weight will be only accurate method


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Old 07-01-2015, 08:43 PM   #3
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Typically anything forward of the axles increases your tongue weight. Ideal tongue weight is 13-15% of loaded TT weight. Positioning between the axle and actual tongue, I'm sure there some law of gravity or whatever that affects it differently but I never learned it. I always just hit the scales and calculated my tongue weight and now pin weight.
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Old 07-01-2015, 08:52 PM   #4
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Theoretically, the 300lbs placed 60" from pivot would put 90# at the end of a 200" tongue.

BTW, mathematically it is a class 2 lever

(f1)(d1)=(f2)(d2)
300x60=f2x200
18000=200f2
18000/200=90
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Old 07-01-2015, 09:09 PM   #5
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But of course, that depends on whether you are right handed or left handed and what you had for breakfast.

Seriously though.. that would be dependent on (d1) and (d2) having a consistent weight and pitch along the axis, which they never will. It will get you in the ballpark, but the flux of variables make it such an engineering feat to the point that it's not worth the effort when you can just stick'r on a scale.

Don't get me wrong. It would probably get you within +/- 15-20%, but it's just oh so much easier to hit the scales and move your weights forward or back to fine tune your pin weight.
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Old 07-01-2015, 09:19 PM   #6
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Originally Posted by Yarome View Post
But of course, that depends on whether you are right handed or left handed and what you had for breakfast.

Seriously though.. that would be dependent on (d1) and (d2) having a consistent weight and pitch along the axis, which they never will. It will get you in the ballpark, but the flux of variables make it such an engineering feat to the point that it's not worth the effort when you can just stick'r on a scale.

Don't get me wrong. It would probably get you within +/- 15-20%, but it's just oh so much easier to hit the scales and move your weights forward or back to fine tune your pin weight.
The OP ask a simple question without variables and moving weights or distances. I just answered his question.
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Old 07-01-2015, 09:35 PM   #7
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Originally Posted by OldCoot View Post
Theoretically, the 300lbs placed 60" from pivot would put 90# at the end of a 200" tongue.

BTW, mathematically it is a class 2 lever

(f1)(d1)=(f2)(d2)
300x60=f2x200
18000=200f2
18000/200=90
Thanks OldCoot. Just what I was looking for. Just wanted an idea how much more tongue weight I will have when I fill the fresh water tank before I get to my dry camp location.
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Old 07-01-2015, 09:38 PM   #8
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The OP ask a simple question without variables and moving weights or distances. I just answered his question.
Just joshin around. I hope it didn't come across as flippant.
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Old 07-01-2015, 09:53 PM   #9
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Just joshin around. I hope it didn't come across as flippant.
Seriously though.. that would be dependent on (d1) and (d2) having a consistent weight and pitch along the axis,

BTW, how can 300# not be consistent and what is pitch along the axis?
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Old 07-01-2015, 10:43 PM   #10
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BTW, how can 300# not be consistent and what is pitch along the axis?
Because along a 200" length (perfectly level) the weight distribution is not the same along the entire length. You may have 5#'s per inch for a certain length while further down the line that may increase to 20#'s. So if you place a determined weight 60"'s from one end, either in the former or the later length the lever weight may be significantly different and impact the weight differential on one end or the other.

The pitch or degree of incline/decline will be affected by gravity. So when you are trying to calculate weight increase at point A, you would also have to account for degree and gravimetric shift on that specific point.

IOW, there is no simple calculation that can be done to determine a specific weights impact on your pin weight. You might get close enough for hand grenades or horseshoes, but if you're down to the wire on you max gross limits, find yourself an MIT grad or just weight it up on the scales.

OC's response was spot on for what you asked, but may not necessarily translate to real world applications for what you are actually trying to accomplish.
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Old 07-02-2015, 06:22 AM   #11
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It can all be mathematically calculated.
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Old 07-02-2015, 06:33 AM   #12
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It can all be mathematically calculated.
Of course it can. I just said, " there is no simple calculation". But to DO those type of calculations you would need additional weight measures at various points of your rig. Instead of taking it to the scales to get the weight measures you need to do the math for the tongue weight, why not just weight the tongue and be done with it.

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Old 07-02-2015, 06:51 AM   #13
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Originally Posted by OldCoot View Post
Theoretically, the 300lbs placed 60" from pivot would put 90# at the end of a 200" tongue.

BTW, mathematically it is a class 2 lever

(f1)(d1)=(f2)(d2)
300x60=f2x200
18000=200f2
18000/200=90
90 pounds would be the additional weight to the axles. The walking beam pivot would be the fulcrum. The distance to the load is 200" minus the 60". The correct additional tongue weight would be 210 pounds. 300x140/200
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Old 07-02-2015, 07:57 AM   #14
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90 pounds would be the additional weight to the axles. The walking beam pivot would be the fulcrum. The distance to the load is 200" minus the 60". The correct additional tongue weight would be 210 pounds. 300x140/200
Now that is funny, but the amount of additional load on the end of the tongue is 90#. The additional load on the walking beam is 210#.
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Old 07-02-2015, 08:02 AM   #15
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Oh for crying outside, here we go......
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Old 07-02-2015, 12:17 PM   #16
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the 300 pound load....is it closer to the coupler or the axle??
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Old 07-02-2015, 12:25 PM   #17
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the 300 pound load....is it closer to the coupler or the axle??
MY BAD, OP did say it is 60" from coupler, I need to read a little closer, so it would add 210# to tongue wt.
I apologize to you Mr. Crockett!
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Old 07-02-2015, 12:48 PM   #18
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all done in the interest of just trying to get at the facts...my deepest respect for you Coot.
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Old 07-02-2015, 01:56 PM   #19
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probably a pertinent time for this post. I never considered myself a type "A" personality type. But I now believe those are the type of people that take time to post here..and I may be correct this time. I could imagine, whether a board of directors or a rock band, differences arise all the time. Doesnt mean were argumentative or "A" holes. Its ok to keep presenting your case if you still have pertinent points...if you still are on task even if the thread evolves a different direction IMO. Repeating yourself or resorting to insults is just self debasing. Ive learned a lot here...even a little about RVs..actually a great deal.. Ive learned to appreciate every personality here as well. I love it here, these boards are alive!! Now lets DO have a good day gentlemen...
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Old 07-02-2015, 10:53 PM   #20
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But would we have to trade the 3/4 ton SRW for a 1 ton DRW, if we added 300lbs? Seriously, the best I recollect from physics class, you would have the fulcrum 60 inch in on a 200 inch run (the midpoint of the add'l weight). Hence 70% of the mechanical advantage is between the fulcrum and wheels, meaning the wheels are carrying 30% of the weight with 70% borne by the hitch.
Therefore 300lbs x (100%-70%) = 90 lbs to the wheels and 210lbs to the hitch. What the heck. Old Coots formula may be easier to understand.
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