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Old 12-09-2010, 03:13 AM   #41
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Originally Posted by Journalist View Post
This should be interesting. I'm really interested to see some comparisons with the tow vehicle taken out of the equation. Folks can argue about hitch setups on here until their fingers bleed from typing, but it's all a moot point as long as the nearly infinite combinations of options and tow vehicles are thrown into the mix. It can only be a great addition to the wealth of information archived here. Thanks Andy!
Most of the forum members would have a stroke if I told them I tow my Airstream with nothing but a ball and two safety chains. That is, until you realize the truck weighs twice as much as the trailer, is only a couple of feet shorter than the trailer and still carries considerably more weight on the front axle than the rear while hitched up.
You have plenty weight on your front tires when your trailer is hooked up im sure. Passengers and gear add even more.
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Old 12-09-2010, 03:26 AM   #42
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I apologize for the disruption my question has caused. It was not my intent to hijack the thread. I thought the question was closely related to the subject. But clearly a lot of people are mad.

So, Andy and everyone, please accept my apology. Andy, I am always impressed with your devotion to the Airstream community and I am very grateful for the contribution you are making with this research. I look forward to the results, as I'm sure do many other readers.
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Old 12-09-2010, 04:20 AM   #43
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I apologize for the disruption my question has caused. It was not my intent to hijack the thread. I thought the question was closely related to the subject. But clearly a lot of people are mad.

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Oooooooh, we are just some kids talking while the teacher is out of the classroom.
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Old 12-09-2010, 04:45 AM   #44
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Oooooooh, we are just some kids talking while the teacher is out of the classroom.
Thats funny................but true
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Old 12-09-2010, 06:41 AM   #45
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Old 12-09-2010, 07:07 AM   #46
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Hi, what I would like to see is: How far does a 1,000 lb spring bar have to bend, to have, or hold, 1,000 lbs. And what is the difference in inches, or some other measurement, comparing same rating spring bars from different companies. Also since my set-up has 1,000 lb spring bars, I would assume that, that means each bar is actually 500 lbs.
My assumption too...but you can never seem to get these kinds of questions answered from the hitch MFRs.
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Old 12-09-2010, 07:07 AM   #47
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Could not resist the engineering humor:
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Old 12-09-2010, 07:26 AM   #48
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I apologize for the disruption my question has caused. It was not my intent to hijack the thread. I thought the question was closely related to the subject. But clearly a lot of people are mad.

So, Andy and everyone, please accept my apology. Andy, I am always impressed with your devotion to the Airstream community and I am very grateful for the contribution you are making with this research. I look forward to the results, as I'm sure do many other readers.
No worries, my heinie is still stinging from the last time I got off topic (hijacked was used to describe it). I just didn't want to get yelled at again!

Start anothe thread with your questions and we'll pick up where we left off. Does anybody know how to get posts moved to a new thread?
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Old 12-09-2010, 07:35 AM   #49
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So, if we assume that the truck has a 140" wheel base and the ball is 48" behind the rear axle: 48/140*500 = 171 lbs. I think this is a bit more than 100 lbs. What happens if I am not carrying a passenger?
In Journalist's case, he's always going to have a 1100lb diesel motor sitting on the front axle, so don't panic just yet.
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Old 12-09-2010, 08:13 AM   #50
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This cannot be; deflection of a beam made of a material that obeys Hooke's law is always linear with respect to a defined loading condition. Progressive springs as used in vehicle suspensions typically rely on some of the coils compressing 100% in the case of coil springs in motorcycles, or leaf spring leaves that don't engage until the load reaches a certain point (truck overload springs), or linkages that deflect significantly and change their leverage (mono-shock motorcycle suspensions).




All steel alloys have very similar modulus of elasticity, which is what determines deflection per unit of stress.

- Bart
Bart,

I don't want to try and argue, because I am not an engineer, but I have just checked with one (no, not at Holiday Inn Express!). Todays leaf and coil suspensions are very different than just a matter of adding leafs which engage with compression. Some MFRs main leaf are progressive rate and have a different cross section as you move from center to ends. There is some discussion of an actual composition difference as you move from center to end also. They won't discuss that, as it is propritary.

That cross section also applies to coils which actually have a progressive variation in cross section area as you move long the length of the coil.

This applies to Andy's tests in that, I would expect that some of the tapered spring bars will not have a linear rate of flex with a linear rate of load increase.

In the case of uniform cross sectioned spring bars (like Equal-i-zer), I would expect that the rate of flex relative to load will be more constant, until you get closer to it's yield point. Of course, it's alloy is probably not the same s some of the tapered bars MFRs. It will be very interesting.

I am also told that various alloys of spring steel have very different properties as to flexibility or deflection rate at a given cross section.
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Old 12-09-2010, 01:25 PM   #51
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In Journalist's case, he's always going to have a 1100lb diesel motor sitting on the front axle, so don't panic just yet.
I also have a diesel and I am assuming that the truck suspension and tire pressure were designed for the diesel engine. I also have a quad cab with a long box and an automatic transmission giving me a weight carrying capacity of 1830 lbs in my 3/4 ton truck. My truck also has a canopy which weighs 200 lbs, a trailer with a tongue weight of 790 lbs empty, and a 47 lb generator. This reduces my carrying capacity to less than 800 lbs. By the time I add in the dogs and my wife and I we are getting close to the weight carrying capacity of the truck.

Since I have a 160'" wheel base and the tow ball is 48" behind the rear axle, 30% of the tongue weight will be removed from the front axle and added to the rear axle. Since the tongue wieght of the trailer (when empty) is 790 lbs, more than 240 lbs (48/160*790) will be transfered to the rear axle if I do not use a weight distrubution hitch. This does not include any additional wieght I have behind the rear axle such as the generator and the weight of the hitch. Therefore, if I do not use a weight distrubution hitch, I will not only be unloading the front axle, making the steering less reliable, but I will also be overloading the rear axle, potentially causing a blow out or break down.

I wonder how many people actually spend the time to figure out if they are within the manufacturers specifications. I wonder how many people have taken their rigs to a certified scale to find out if they are unloading the front axle and/or overloading the rear axle.
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Old 12-09-2010, 01:37 PM   #52
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I wonder how many people have taken their rigs to a certified scale to find out if they are unloading the front axle and/or overloading the rear axle.
So what was your last scale weights loaded like that?
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Old 12-09-2010, 01:40 PM   #53
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Bart,

I don't want to try and argue, because I am not an engineer, but I have just checked with one (no, not at Holiday Inn Express!). Todays leaf and coil suspensions are very different than just a matter of adding leafs which engage with compression. Some MFRs main leaf are progressive rate and have a different cross section as you move from center to ends. There is some discussion of an actual composition difference as you move from center to end also. They won't discuss that, as it is propritary.
Bart is still correct.

In some cases, as a leaf spring compresses, different parts of the spring flex. A leaf spring nearing full compression flexes mainly at the ends since the rest of the spring is supported by the lower leaves in the spring pack. The lever arm is small and so the apparent spring rate is high.

At maximum height the spring flexes closer to the center since it receives relatively less support from the lower springs in the pack. This leads to a longer lever arm and a lower spring rate. By tapering the springs it is possible to engineer the desired changes in the spring rate.

Conversely, WD spring bars are, mathematically, single beams, and their geometry does not change appreciably as they flex, that is, the moment arm of applied force remains nearly constant. While tapering the bars makes sense since there is more stress closer to the head, it does not affect the linearity of the relationship between force and flexion.
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Old 12-09-2010, 04:14 PM   #54
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Bart is still correct.

In some cases, as a leaf spring compresses, different parts of the spring flex. A leaf spring nearing full compression flexes mainly at the ends since the rest of the spring is supported by the lower leaves in the spring pack. The lever arm is small and so the apparent spring rate is high.

At maximum height the spring flexes closer to the center since it receives relatively less support from the lower springs in the pack. This leads to a longer lever arm and a lower spring rate. By tapering the springs it is possible to engineer the desired changes in the spring rate.

Conversely, WD spring bars are, mathematically, single beams, and their geometry does not change appreciably as they flex, that is, the moment arm of applied force remains nearly constant. While tapering the bars makes sense since there is more stress closer to the head, it does not affect the linearity of the relationship between force and flexion.
OK! I just called a second engineer. He said we have a failure to communicate and some engineering terminologies mean different things than applied technology "street talk" does...apparently.

We'll have to wait for Andy, but I believe we'll see some notable differences in how a tapered (street talk inserted here) "progressive rate" spring bar behaves and a square bar of uniform cross section behaves with the same "max tongue load rating" as they are loaded progressively through their operating range.

This second engineer said that the differences between tapered and uniform spring bars are more noteable during dynamic loading rather than static loading.

Oh and he also said spring bar alloy will make a significant difference between bar's behaviors and will introduce a large unknown in the findings.
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Old 12-09-2010, 05:42 PM   #55
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To answer all the debate about my towing setup, see the attached photo. Unless your rig is roughly the same, I don't recommend running without a WD hitch. Mine is a rather different case than the vast majority of Airstream owners. The combination of a truck that is really way too much and the super light weight of a mid-size vintage Airstream come together to make it all work. Very few smaller Airstreams will ever be towed with a huge dually pickup, and it's not good for the trailer to beat it to death with super stiff springs, rock hard tires, and all the negatives associated with a heavy duty truck, but I don't really have much of a choice until I can afford a second tow vehicle. Now, can we please get this thread back on topic.
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Old 12-09-2010, 09:02 PM   #56
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This second engineer said that the differences between tapered and uniform spring bars are more noteable during dynamic loading rather than static loading.
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.

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Oh and he also said spring bar alloy will make a significant difference between bar's behaviors and will introduce a large unknown in the findings.
Modulus of elasticity for various alloys:



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

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Old 12-10-2010, 09:05 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, 07:12 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, 10:10 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, 10:10 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."

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