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How do i calculate tongue weight?

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Old 06-07-2010, 01:15 PM   #1
tellico [OP] tellico is offline
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Question How do i calculate tongue weight?

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?
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Old 06-07-2010, 01:20 PM   #3
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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?
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Old 06-07-2010, 01:22 PM   #4
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opps! just googled it. hooke's law only applies to linear springs..i guess it is linear like that.
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Old 06-07-2010, 01:53 PM   #5
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Quote:
Originally Posted by Brunes View Post
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??

Trying to figure out the tongue weight by measuring the sag.
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Old 06-07-2010, 01:54 PM   #6
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Jane Doe sounds like a real big woman.
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Old 06-07-2010, 02:02 PM   #7
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Quote:
Originally Posted by tellico View Post
Trying to figure out the tongue weight by measuring the sag.

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!!!
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Old 06-07-2010, 02:03 PM   #8
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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.
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Old 06-07-2010, 02:19 PM   #9
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Quote:
Originally Posted by tacomaprerunner View Post
Jane Doe sounds like a real big woman.
The girls get better lookin' at closing time.
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Old 06-07-2010, 02:29 PM   #10
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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?
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Old 06-07-2010, 06:07 PM   #11
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Quote:
Originally Posted by aficianado View Post
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?
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.
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Old 06-07-2010, 06:12 PM   #12
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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.
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Old 06-07-2010, 06:23 PM   #13
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Quote:
Originally Posted by tellico View Post
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?
I think you need to factor in age too. Seems the older things get, the more they sag.
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Old 06-07-2010, 06:29 PM   #14
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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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