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How do i calculate tongue weight?

Discussion in 'Towing' started by tellico, Jun 7, 2010.

  1. Jun 7, 2010 at 1:15 PM
    #1
    tellico

    tellico [OP] Member

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    Is there any way to calculate the tongue weight based upon the sag of the rear? As an example, if John Doe weights 200lbs, and he stands upon the trailer hitch causing the rear to sag 2 inches. Then, Jane Doe, who weighs 300lbs, stands upon the hitch and the rear sags 3 inches. Would Jack Sprat, who weights 400lbs, cause it to sag 4 inches? And, would a tongue weight of 350 lbs cause it to sag 3.5 inches?
     
  2. Jun 7, 2010 at 1:19 PM
    #2
    Brunes

    Brunes abides. Staff Member

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    I don't think there is a direct correlation between distance of sag and weight.

    Do you have a set weight you are trying to find out how far it will sag or trying to figure out what weight you have by measuring sag??
     
  3. Jun 7, 2010 at 1:20 PM
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    aficianado

    aficianado Well-Known Member

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    damn. it's been a long time for me. it's called "hooke's law". you take two known loads and deflections and calc the "spring constant"..then you can us that "k" to determine all sorts of stuff. i do remember is isnt linear like your example. you have any real #'s?

    cant you use a bathroom scale like everyone else? :)
     
  4. Jun 7, 2010 at 1:22 PM
    #4
    aficianado

    aficianado Well-Known Member

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    opps! just googled it. hooke's law only applies to linear springs..i guess it is linear like that.
     
  5. Jun 7, 2010 at 1:53 PM
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    tellico

    tellico [OP] Member

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    Trying to figure out the tongue weight by measuring the sag.
     
  6. Jun 7, 2010 at 1:54 PM
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    tacomaprerunner

    tacomaprerunner Dang liberals.

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    Jane Doe sounds like a real big woman.
     
  7. Jun 7, 2010 at 2:02 PM
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    Brunes

    Brunes abides. Staff Member

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    That's what I guessed. There is probably some arcane math with some structures engineering that you would need to do. Get a cheap bathroom scale and a put your trailer tongue on that and call it close enuf.

    Once you get a couple of loads on there you can create a comparison for your truck...LIke this time you got a 500lbs load and it drops so many inches, next time you put 480 and it drops a different amount. You can guessimate after a while- but with all the variables...a scale is the way to go!!!
     
  8. Jun 7, 2010 at 2:03 PM
    #8
    clarkie152

    clarkie152 Well-Known Member

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    it depends on the trailer. some trailers for some reason are lighter on the tounge than others. i can lift up my uncles dump run trailer when it is full, but i struggle with a buddy empty van trailer.
     
  9. Jun 7, 2010 at 2:19 PM
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    tellico

    tellico [OP] Member

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    The girls get better lookin' at closing time.
     
  10. Jun 7, 2010 at 2:29 PM
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    aficianado

    aficianado Well-Known Member

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    ok,. using your numbers..and trying to remember physics...beer is bad.

    your 200 lbs person getting 2" of displacement gives you a spring constant of 17800N/m..
    taking the 3.5 displacement..you have a tongue weight of roughly 1582.42lbs. sounds like a big trailer!

    are these real numbers? i think the data is bad, because you need to measure at the spring?
     
  11. Jun 7, 2010 at 6:07 PM
    #11
    tellico

    tellico [OP] Member

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    The data is made up. I am looking for a graph that gives me sag versus tongue weight for a 2008 Tacoma Reg Cab 2.7 liter with Toyota dealer springs. Reason? I think my springs are bad because the sag is around 4 inches with a boat trailer, which with boat, weighs approximately 1500 pounds. Needless to say, the nose is pointed up.
     
  12. Jun 7, 2010 at 6:12 PM
    #12
    05 TRD Sport

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    It's easy. Just weigh your head and then weigh your head with someone holding your tounge. Subtract second number from the first and that's how much your tounge weighs.
     
  13. Jun 7, 2010 at 6:23 PM
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    MurphMan

    MurphMan Senility Rocks!

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    I think you need to factor in age too. Seems the older things get, the more they sag.
     
  14. Jun 7, 2010 at 6:29 PM
    #14
    Juggernaut

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    Rear springs are not linear, so F=-kx will not work. As others have said get a scale, or stop thinking so hard and get tsb/aal/bags and don't worry about it again.
     
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