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Old 12-10-2010, 10:05 AM   #57
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Ok, Bart & Jammer
I think you two are the best ones to run this by. Please understand I'm not trying to make any particular point or argue. I have done a really crappy job of explaining my points in the above posts. What I am trying to do is set a baseline for when Andy's data comes in. (Thanks again, Andy!). How are we going to apply it as we all introduce all sorts of variables and concerns? Those variables being the topics of many threads and posts, some of which got heated. Some of those concerns surround potential trailer damage, rig handling, etc.

As I understand it, a spring bar is is a cantilevered beam, with a load applied at it's tips only, right? (I guess I'm taking the TV suspension out of the equation here and assuming a rigid mount for the front end of the bar)

A spring bar with the same cross section throughout its entire length will:
-deflect in a symmetrical arc
-will be relatively consistent in its tip's vertical (upward load applied at chain attachment/L-bracket, and force borne equally throughout the length of the beam) travel when equal and incremental load is applied (ie: as a random example, 100 pounds of load deflects the tip 1", the second 100 pounds applied moves the tip close to an additional 1", and so on, up to the beam's yield point) Right? (I get shakey here on my knowlege of beam behvior, particularly as it approaches failure point loads)

In the case of a tapered spring bar, we have a tapered cantilevered beam with a continuously varible cross section. Right?
This beam will:
-not deflect in a symmetrical arc
-not be relatively consistent in its tip's vertical (upward force applied at chain attachment/L-bracket, and force borne equally throughout the length of the beam) travel when equal and incremental load is applied (ie: 100 pounds of load deflects the tip considerably more than 1", the second 100 pounds applied moves the tip less than the first 100 pound application, but greater than the first beam above, and so on, up to the beam's yield point) In other words, the cantilevered beam is more "compliant" or "flexible", at least in the lower loads of it's rating. I am assuming same materials, and load rating specifications for both bars. This is apparenty where we have a disconnect...I refer to this as a variable rate spring. Not so, in the engineering world, I gather.

All this discussion is not , for me, just an exercise in physics. Here is what I hope to find out. When setting up my weight distribution, It doesn't matter which MFR's bar, which beam style, or what rating I use setting my rig on level ground in a static attitude. I'm going to move X amount of weight with X amount of force applied to the bars.(within reason here , guys I am not suggesting 500 # or 1400# bar for 900# of tongue weight, at this point anyway. Andy's data may suprise us.)

However, my concerns come in when introducing dynamic loads driving down the road.

Where am I on the two bars' deflection vs. load applied curves am I while sitting still?
How does that position on the curves change as I encounter:

Variations in pavement which set up the porpose effect? (I think I'd like a stiff bar in this situation. One which would limit the amplitude of the oscillations)

Vibrations which expansion joint and potholes, etc. cause? (I'd like to have a compliant tip to help protect the AS from rivet popping, etc)

And the BIG one. (for me) When approaching steep angles of entry at campsites, gas stations, etc., where will I be on the curve and what kind of extraordinary forces are being applied to the trailer tongue.

I like the way my Equal-i-zer setup handles every aspect of road travel....except that last one. I cringe every time I approach a steep entry angle.

Do either of you see how we can plot the anticipated forthcoming data to make each of us individually be able to make logical choices as to setup?

What will the curves look like when plotting load vs. deflection of these two beams?

At what point on the curves of the two beams do they closely behave the same...or is there one?

How will I be able to detrmine, If at all, an anticipated position on the graph for a given situation? (ie, steep entry angle).
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Old 12-10-2010, 08:12 PM   #58
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This doesn't apply to our situation, and only matters where the spring bar itself is a significant contribution to the overall mass of the system.



Modulus of elasticity for various alloys:



At 100F, less than 5% difference between carbon, cr-mo and nickel alloys.

- Bart
Does this chart include hardened and tempered carbon steels? I don't see a reference or distinction within that alloy. I guess I am assuming that hitch spring bars are hardened and tempered like suspension spring steel.
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Old 12-10-2010, 11:10 PM   #59
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Hi, next we need to know how much pressure on the frame, from ball to spring bar perch, will it take before the frame buckles. Test this on 2" X 4", 2" X 5", or whatever the most common frame sizes are.
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Old 12-10-2010, 11:10 PM   #60
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Does this chart include hardened and tempered carbon steels? I don't see a reference or distinction within that alloy. I guess I am assuming that hitch spring bars are hardened and tempered like suspension spring steel.
From Dieter, "Mechanical Metallurgy", page 281 :

Mechanical Metallurgy - Dieter (Si Edition)

"The modulus of elasticity is determined by the binding forces between atoms. Since these forces cannot be changed without changing the basic nature of the material, it follows that the modulus of elasticity is one of the most structure-insensitive of the mechanical properties. It is only slightly affected by allowing additions, heat treatment or cold work."

- Bart
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Old 12-10-2010, 11:15 PM   #61
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From Dieter, "Mechanical Metallurgy", page 281 :

Mechanical Metallurgy - Dieter (Si Edition)

"The modulus of elasticity is determined by the binding forces between atoms. Since these forces cannot be changed without changing the basic nature of the material, it follows that the modulus of elasticity is one of the most structure-insensitive of the mechanical properties. It is only slightly affected by allowing additions, heat treatment or cold work."

- Bart
How, or does, it matter with the differences between low carbon steel < .3% carbon and high carbon steel (which springs are typically made of) at .6 - .99% carbon?
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Old 12-10-2010, 11:17 PM   #62
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Originally Posted by dznf0g View Post
In the case of a tapered spring bar, we have a tapered cantilevered beam with a continuously varible cross section. Right?
This beam will:
-not deflect in a symmetrical arc
-not be relatively consistent in its tip's vertical (upward force applied at chain attachment/L-bracket, and force borne equally throughout the length of the beam) travel when equal and incremental load is applied (ie: 100 pounds of load deflects the tip considerably more than 1", the second 100 pounds applied moves the tip less than the first 100 pound application, but greater than the first beam above, and so on, up to the beam's yield point)
A constant spring rate can be expressed as follows:

Force = displacement * spring constant

Where displacement is how much the spring bar is deflected, and spring constant depends on size and length of the (steel) bar.

What is meant by a variable spring rate is where the spring constant isn't constant, but is also a function of X.

This doesn't happen for simple steel bars, regardless of shape, heat treatment, alloying materials, color or advertising budget.

If you want a variable spring rate, then you need to change the geometry of the spring or the amount of spring in play.

- Bart
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Old 12-10-2010, 11:19 PM   #63
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Hi, next we need to know how much pressure on the frame, from ball to spring bar perch, will it take before the frame buckles. Test this on 2" X 4", 2" X 5", or whatever the most common frame sizes are.
Exactly my concern.
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Old 12-10-2010, 11:20 PM   #64
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How, or does, it matter with the differences between low carbon steel < .3% carbon and high carbon steel (which springs are typically made of) at .6 - .99% carbon?
Modulus of Elasticity, Strength Properties of Metals - Iron and Steel - Engineers Edge

contains rather an exhaustive list.

- Bart
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Old 12-10-2010, 11:41 PM   #65
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A constant spring rate can be expressed as follows:

Force = displacement * spring constant

Where displacement is how much the spring bar is deflected, and spring constant depends on size and length of the (steel) bar.

What is meant by a variable spring rate is where the spring constant isn't constant, but is also a function of X.

This doesn't happen for simple steel bars, regardless of shape, heat treatment, alloying materials, color or advertising budget.

If you want a variable spring rate, then you need to change the geometry of the spring or the amount of spring in play.

- Bart
So, are you saying that two bars with the same rating (whatever that means), one which has uniform cross section through it's entire length will behave (deflect) the same as a tapered (continuously variable cross section) throughout it's operating range with the same loads applied? This makes my head spin!!!
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Old 12-10-2010, 11:50 PM   #66
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So, are you saying that two bars with the same rating (whatever that means), one which has uniform cross section through it's entire length will behave (deflect) the same as a tapered (continuously variable cross section) throughout it's operating range with the same loads applied? This makes my head spin!!!
The tapered bar will be more flexible; in other words, the spring constant will be lower. But doubling the amount of bend in each bar will always double the amount of force it exerts. Once that is no longer true, the bar will have taken a permanent set.

- Bart
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Old 12-11-2010, 12:15 AM   #67
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The tapered bar will be more flexible; in other words, the spring constant will be lower. But doubling the amount of bend in each bar will always double the amount of force it exerts. Once that is no longer true, the bar will have taken a permanent set.

- Bart
So, if Andy only test to deflection at increasing loads we may not get many answers? (assuming no MFRs lie about their ratings) WAIT, I mean no MFRs marketing folks embellish in a positive fashion, as to the attributes of their particular product.

Seriously, though, I'm still struggling how force relates to flexibility when we imaging the motions going on back there during various geographic undulations.

There, I've used up all my big words with just a half a beer left. How's that for timing?
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Old 12-11-2010, 10:47 AM   #68
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The tapered bar will be more flexible; in other words, the spring constant will be lower. But doubling the amount of bend in each bar will always double the amount of force it exerts. Once that is no longer true, the bar will have taken a permanent set.

- Bart
Bart,
You're confusing me again with the "more flexible" statement. All I am interested in (I think) is tip travel at varying loads applied at the tip. When measuring from the ground to tip of spring bar and adding the additional tip travel, won't the graph look like the attached? X axis is load in pounds x 100, and the y axis is inches of vertical movement from an unloaded bar as load is applied.

Won't the graph look like the attached?

I'm beginning to wonder if we're not communicating again. I get that the two systems are going to have the same strength. I am not plotting strength, just tip deflection at various loads.

What am I missing with the "flexibility" phrase?
Attached Files
File Type: xls spring bar.xls (31.0 KB, 42 views)
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Old 12-11-2010, 10:50 AM   #69
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Oh, yeah, don't get hung up on the values....totally bogus numbers, for illustrative purposes only.
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Old 12-12-2010, 12:39 AM   #70
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A simple spring bar will produce force in proportion to the displacement from the rest position:

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, but in each case the line will pass through zero, and the plot of force vs displacement will be a straight line so long as the bar isn't bent so far that it takes a permanent set.

- Bart
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