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Old 12-09-2010, 05:20 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, 05:45 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, 07:41 AM   #45
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I luv this....SECGURU.quote...
Dunning-Kruger effect, a catch-22 seemingly designed by Satan himself that makes incompetent people unable to judge their own incompetence because measuring competence is the thing they're least competent at.

But...we promised not to speak politics here or about the Politicians.
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Old 12-09-2010, 08:07 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, 08:07 AM   #47
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Could not resist the engineering humor:
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Old 12-09-2010, 08:26 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, 08:35 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, 09:13 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, 02:25 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, 02:37 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, 02:40 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, 05:14 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, 06:42 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, 10:02 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.

- Bart
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