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Old 12-12-2010, 08:00 AM   #71
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Quote:
Originally Posted by barts View Post
f = kx,

where f is in lbs, x is in inches, and k, the spring constant, is in lbs/inch.

the value of k will depend on the geometry of the bar. Different thickness of bar will produce different slopes,
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BINGO!!!!! By gosh, I think you've got it!
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Old 12-12-2010, 09:06 AM   #72
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Today, we started a research program on load equalizing hitches.

We are measuring the bend in the load equalizing bars, at 100 pound increments, with different brands as well as square and round bars.

The range of the tests start at a low level and up to and including a 20 to 25 percent overload.

We will publish the results as soon as the tests are completed, hopefully by the end of this year.

Andy
Looking forward to it!

.
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Old 12-12-2010, 06:47 PM   #73
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Andy's work ties in precisely with my Load Distribution Hitch Calculator at:

http://www.airforums.com/forums/f464...sis-19236.html

At post 9, markdoane points out that what is required to complement the calculator is precisely the work Andy is doing.
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Old 12-12-2010, 09:31 PM   #74
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Nick,

Since then I have measured a couple of sets of bars to determine the spring rates. The bars I measured were a set of Reese 600# bars, and a set of Draw-Tite 800# bars.

The Reese bars developed a spring deflection rate of 166 lbs/inch, and the Draw-Tite bars measured 183 lbs/inch. Both sets were 1.125 inches wide.

If we assume a 2" bend for optimal deflection, the 600 lb bars measured 664 lbs, and the 800 lb bars measured 732 lbs. I think that is a reasonable variation, considering the bars were the same width.
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Old 12-12-2010, 10:29 PM   #75
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Nick,

Since then I have measured a couple of sets of bars to determine the spring rates. The bars I measured were a set of Reese 600# bars, and a set of Draw-Tite 800# bars.

The Reese bars developed a spring deflection rate of 166 lbs/inch, and the Draw-Tite bars measured 183 lbs/inch. Both sets were 1.125 inches wide.

If we assume a 2" bend for optimal deflection, the 600 lb bars measured 664 lbs, and the 800 lb bars measured 732 lbs. I think that is a reasonable variation, considering the bars were the same width.
Mark, are you assuming the second inch of travel is double the load of the first inch, or linear, or did you actually measure the load at two inches of travel? And, I have to assume these were both sets of trunion type bars?

Did you measure any round bars of simular advertised weight?
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Old 12-12-2010, 10:35 PM   #76
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I think he's assuming linear. 2" X 166 lbs X 2 bars= 664 lbs?????
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Old 12-12-2010, 10:38 PM   #77
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I think he's assuming linear. 2" X 166 lbs X 2 bars= 664 lbs?????
Yes, I did the math also, but personally, I don't think the bars load linear.
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Old 12-12-2010, 10:46 PM   #78
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Yes, I did the math also, but personally, I don't think the bars load linear.
Yeah, I'm still having trouble reconciling the above posts between Bart and me. My head has started hurting trying to figger it out. I'm waiting till the numbers roll in. I'm also trying to figure out how we can make a spreadsheet to understand how deflection and associated force rises as the angle between the TV and trailer frame changes.... ie, the steep approach angle thing.
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Old 12-12-2010, 11:52 PM   #79
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Originally Posted by dznf0g View Post
I'm also trying to figure out how we can make a spreadsheet to understand how deflection and associated force rises as the angle between the TV and trailer frame changes.... ie, the steep approach angle thing.
If we assume bars have length L and spring constant k, and we
denote the truck beginning to climb a hill as a positive angle theta, we
can say (assuming small angles of deflection, blah blah):

force due to hill = L * sin(theta) * k

or for a 24" bar, 10% grade (5.7 degree slope) and 500 lb/inch spring constant:

24 * sin(5.7) * 500 = 1190 lbs....

So climbing a steep hill effectively bends the bars a bit more than 2" extra....

Those w/ 1000 lb bars can wince now...

This is why I feel a more constant force hitch would be a good thing for
one's trailer if one needs a WD hitch... and why those headed off road may wish to relax their WD bars somewhat...

- Bart
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Old 12-13-2010, 06:24 AM   #80
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Yes, I did the math also, but personally, I don't think the bars load linear.
That is correct. The bars were loaded at 260 lbs and 420 lbs. The spring rate increases as the load increases.

Here are the spring rates at 260 lbs load and 420 lbs load.

600# Reese bar
166 lbs/inch at 260 lb load
179 lbs/inch at 420 lb load

800# Draw-Tite bar
173 lbs/inch at 260 lb load
195 lbs/inch at 420 lb load

The spring rates are the average for two bars at each treatment. All were trunnion bars.
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Old 12-13-2010, 07:08 AM   #81
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Time For a New Hitch for me

Great timing Andy, I need a new hitch.
Looking forward to seeing your results.
In response to some posts, with all that math, tapered bars and anti sway de-vices, it is easy to forget that; the load needs to be balanced in the trailer and tow vehicle, tires inflated to proper level and most importantly the driver should have some experience, common sense and be wearing the thinking cap.
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Old 12-13-2010, 07:48 AM   #82
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Great timing Andy, I need a new hitch.
Looking forward to seeing your results.
In response to some posts, with all that math, tapered bars and anti sway de-vices, it is easy to forget that; the load needs to be balanced in the trailer and tow vehicle, tires inflated to proper level and most importantly the driver should have some experience, common sense and be wearing the thinking cap.
Very true, and I am assuming a proper static setup, per scales and vehicle load specs for all conversations. That should be anyone's startin g point before discussing bar load increases during travel, etc.
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Old 12-13-2010, 07:58 AM   #83
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Quote:
Originally Posted by barts View Post
If we assume bars have length L and spring constant k, and we
denote the truck beginning to climb a hill as a positive angle theta, we
can say (assuming small angles of deflection, blah blah):

force due to hill = L * sin(theta) * k

or for a 24" bar, 10% grade (5.7 degree slope) and 500 lb/inch spring constant:

24 * sin(5.7) * 500 = 1190 lbs....

So climbing a steep hill effectively bends the bars a bit more than 2" extra....

Those w/ 1000 lb bars can wince now...

This is why I feel a more constant force hitch would be a good thing for
one's trailer if one needs a WD hitch... and why those headed off road may wish to relax their WD bars somewhat...

- Bart
Yup, I get that. What I wold like to see eventually, is a spreadsheet with various slopes and loads. I would like to get to the place where I can eyeball a steep entry and say "that rise is about 24" over the length of my TV wheelbase, so I am weel within the load limits of my AS frame, hitch, etc." Or, conversely, "I gotta get out and unload the bars..."

Does anyone have any idea on the physical property specs of AS farmes for the various build configurations?
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Old 12-13-2010, 08:06 AM   #84
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Quote:
Originally Posted by fastrob View Post
Great timing Andy, I need a new hitch.
Looking forward to seeing your results.
In response to some posts, with all that math, tapered bars and anti sway de-vices, it is easy to forget that; the load needs to be balanced in the trailer and tow vehicle, tires inflated to proper level and most importantly the driver should have some experience, common sense and be wearing the thinking cap.
All of that if fine and dandy, and I will not disagree with any of it, but keep in mind Andy is only testing bars amount of bend vs load in these tests:

"We are measuring the bend in the load equalizing bars, at 100 pound increments, with different brands as well as square and round bars.

The range of the tests start at a low level and up to and including a 20 to 25 percent overload."

Personally, I think he needs to measure the amount of weight needed to bend each bar 1", and in additional 1" increments, rather than measure the bend at each 100 pound of weight loading. I think he will end up with so much data the way he says he is going to test that it will simply be confusing.

As an example, if testing a 1000 pound bar, he will have no less than 12 measurements for each bar if he goes to 20% overload.
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