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Old 04-13-2013, 09:58 AM   #15
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All you have to do is get your 1000lb bars ground down in the middle to the same cross section as the 600lb bars, and then you will have 600lb bars that work with your 10,000lb capacity head. Others on here have done it.

Just measure a set of 600lb bars for their height and width. Take your 1000lb bars to a machine shop and have them grind the middle down to the same dimension. Taper it back to full size over about 8" either direction.

Problem solved, and it won't cost that much to get them ground. You could do it yourself if you have disk grinder. The key is to get it a smooth hour glass shape with no sharp transitions.

Best of luck,
Jim.

I disagree.

Cutting the entire bar down, would be OK.

Cutting a section out of it, creates a huge weak spot, that will snap when enough stress is applied.

As an example, take a 2 x 4 board, and cut out a section in the middle, and then stress it.

Guess where it will break.

Cutting that board so that it's a 2 x 3 as an example, while making the board weaker, it will not have a weak spot, assuming no knots in it.

Andy

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Old 04-13-2013, 10:12 AM   #16
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That jump test would produce different results if the jumper weighed 110 pounds or crossed the scales at 275 pounds....
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Old 04-13-2013, 11:32 AM   #17
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Originally Posted by Inland RV Center, In View Post
Jim.

I disagree.

Cutting the entire bar down, would be OK.

Cutting a section out of it, creates a huge weak spot, that will snap when enough stress is applied.

As an example, take a 2 x 4 board, and cut out a section in the middle, and then stress it.

Guess where it will break.

Cutting that board so that it's a 2 x 3 as an example, while making the board weaker, it will not have a weak spot, assuming no knots in it.

Andy

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Respectfully disagree ... not an accurate analogy - boards do not flex as do most metals ... too many years since my engineering classes, but I am sure others here are more current on the technical aspects.

What I do recall is that the transitions from the full size dimension of the load bar to the lesser dimension must be a gradual transition or the base structure of the metal will be impacted upon flexing. The "bar reduction" procedure has worked quite well for us. Full size is needed on the bar end to mate with the head unit.

The key point here is to not exceed the limits of the bar and induce stress in the middle of it ... those archers among us will understand the basics of the reduction as it has been done for centuries on bows ... wood, metal, and composite.
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Old 04-13-2013, 11:39 AM   #18
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Hey Andy,

I agree with you if you were to just make it a notch or a rough blockout. It would need to be blended in smoothly over a fair distance to not get a stress riser.

It is crummy that EQ makes different attachment types for the different capacity bars. They should do one hitch head swivel design that all bars fit.

I wouldn't grind both dimensions of the bar, personally. I'd only modify the height. I would get hold of a 600lb bar and mic it's width and depth. I don't know what they are, but for the sake of argument, just assume the 600lb bars are 1" tall and .75" wide. Now assume your 1000lb bars are 1.25" tall and .8" wide.

You have to calculate the stiffness, which is very simple. The formula is I=(bh^3)/12 where I is called the Moment of Inertia and relates to bending stiffness. B is the base width, and H is the Height. So for our 600lb bars, we have I = (B * H^3)/12 --> I= 0.75 * 1^3/12 = .0625 in^4.

So now, you simply have to make your 1000lb bars equal that value.
Stock, they are: I = (0.8 * 1.25^3 )/ 12 = .130 in^4. You can see that the stiffness here goes up with the cube of the height, so making the bar a little deeper makes a huge difference in stiffness.

But anyway, what I would do is set my I = to 0.0625in^4, and then keep the width the same on the 1000lb bars, just reduce the height. So if you solve it that way, you have: 0.0625 in^4 = (0.8 * H^3)/12. You solve for H, and you get H=0.98".

So, I would mark my bars about 15" aft of the pivot point, and grind off an equal amount top and bottom. Slightly over 1/8" in this example. I would taper that out over a nice long distance, at least 8" either side.

It's the vertical bending stiffness that you're trying to match up. You could do this with a disk grinder and a belt sander at home easily enough. Just make it smooth.

You could also do like Andy suggested and just grind the whole bar down, though you'd have to leave it thicker at the head stock end where it goes into the hitch head.

You certainly do not want an abrupt transition anywhere, as that would lead to stress risers and you could fail the piece.


It would be easier to just buy another hitch. But if money is tight, this could be done at home.

My buddy has a Bridgeport mill, so I kind of forgot about having to pay a machine shop But yeah, that would alter the equation a lot. But no more material than would probably need to be removed, I'd probably just do it with a grinder, a file, and some emery paper. Don't forget to paint it...

Take care,
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Old 04-13-2013, 12:00 PM   #19
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Equalizer will tell you that even if you have oversize bars, it doesn't matter—you use only what you need. My brief experience with engineering as a major only lasted a few months, so I don't understand the equations, but 45,000 miles of towing with oversize bars (the dealer selected them before I knew better) and no problems is good enough proof for me.

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Old 04-13-2013, 08:48 PM   #20
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That jump test would produce different results if the jumper weighed 110 pounds or crossed the scales at 275 pounds....
Also depends on how high you jump.

The real purpose is to do something, instead of assuming.

Answers are always better than guesses.

Using the "KISS" strategies most always provides meaningful information.

Andy
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