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Old 01-04-2012, 11:59 AM   #85
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Originally Posted by CrawfordGene View Post
This use of the word "compliance" is new to me. It seems to mean according to Wikipedia "the inverse of stiffness". Post 78 makes more sense to me with this definition.

Gene
There are degrees of compliance as there are degrees of stiffness. All springs are compliant to some degree, some more than others.
My def w/o looking it up: A material is compliant if it yields or deforms under a force exerted upon it.
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Old 01-04-2012, 12:30 PM   #86
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Pretty nifty drawing red'.

My take on this is that the w.d. bars relieve the weight of hitch coupler on ball, and the weight is suspended on the w.d. bars. Then it is logical for me to assume the less rigid the bars, the softer the bar suspension, and therefore less road shock is transferred from truck to trailer because flexing of the bars absorbs that shock.

I believe that is the same reasoning others (Inland Andy has taken a lot of heat on this forum for this idea) have used to prescribe lighter w.d. bars in response to heavier truck suspensions.

doug k
The ball-coupler is rigid under any and all conditions. Move one side, you move the other. The idea that weight is suspended on the bars gives the impression of compliance between the TV and trailer. That's a false intuition. The bar adds counterforce (resistance) to any flex of that joint, but it is not providing any form of compliance between them.

If you refer to my sketch, you see that with or without any bars, ALL movement of the TV is referred directly to the trailer THROUGH the solid coupling of the hitch ball-coupler. The stiffness of the bar doesn't come into play at all.

Compliance
Compliance is a spring action between two masses. The perfect example is a coil spring on a car. It provides compliance between the body and the wheel. Under many circumstances the wheel can move independently from the body because of this compliance of the coil spring which connects them. If you imaging your car with no coil springs - the wheel bolted right to the frame - that's what a ball-coupler looks like in physics. They can not move independent of each other.

The WD bars "look like" they would function as a compliance between the two platforms, but they do not, as the sketch clearly shows. They only resist bending of that joint.

The bars are doing two things mechanically. First, they are a an extension of the TV frame by which the frame can be lifted referenced to the trailer A-frame. Second, because of the first use, they create a resistance to bending of the coupler joint. Resistance is not compliance.

Not sure if that will help or not. I think the bars in a WD system are a bit confusing. They were to me. I had to carefully noodle the schematic of operation before it became clear to me what was happening. My intuition was also that the bars created compliance. But I don't think the sketch lies!
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Old 01-04-2012, 12:46 PM   #87
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The rigidity of the connection between the tow vehicle and the trailer is what transfers stresses to both the TV and trailer frames when you hit a dip in the road. A looser connection is going to bend to the contours of the road instead of transferring that bending to the frame of TV and trailer. The frame bends and transfers that load to the trailer shell. You really want the trailer frame to handle the loads the trailer puts on it not loads from load bars that are too big. A little porposing is not going to hurt anything but it can be bothersome. You are better off adding shocks to your TV instead of using really heavy load bars. You want to control oscillations in the YAW axis with sway control and limit (but not eliminate) oscillations in the pitch axis. You are trying to balance the weight between the front and back wheels of the TV. You don’t want to eliminate all movement back there. If you don’t like the up-down movement then get bigger shocks to damp out the movement.

Perry
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Old 01-04-2012, 12:52 PM   #88
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Thanks for the explanations. That makes it clear.

Well, clear to a point. All hitches seem to be compromises—balancing anti-sway, weight distribution, cost, simplicity of design, ease of use and adjustment and not being orange. The Equalizer seems like a pretty good compromise. Otherwise I'd consider the ProPride.

A lot of people report dealers installing a much heavier bar than needed. Obviously if they stock only heavier hitches they can put them on more different trailers and don't have to stock more inventory. There are also lots of reports of maladjusted hitches. I solved the maladjustment, have not solved the overhitching problem. I am still not clear whether it makes a difference to be concerned about and I'm not sure anyone can come up with a conclusive answer. I could buy lighter bars (about $166 for a pair), but didn't see replacement sockets and whether they could be adapted to the hitch head. I may still call the company after I get back home (leaving tomorrow sans trailer for Santa Fe).

I'm having brain explosions figuring this out, but I'm learning.

I think I'll go read the Internal Revenue Code to clear my head of all this physics stuff.

I thought I posted explaining front end separation, but I can't find it so I must have hit the wrong button. Briefly, the body separates from the frame and you would see torn aluminum at the bottom of the body. There have not been many reports and the solution seems to be to rivet aluminum "elephant ears" on the exterior and bolt the interior body to the frame inside. The company has fixed some for free after the warranty has run out, but don't want to do the interior fix. They may respond to firm and friendly coaxing. I haven't seen posts in a couple of years. Try searching. The cause is unclear—bad construction, bad materials, overhitching, bad driving? This is different from rear end separation which seems to be caused by a too heavy rear end—either a kitchen in the rear or a storage box mounted on the rear bumper.

Gene
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Old 01-04-2012, 12:54 PM   #89
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Originally Posted by perryg114 View Post
The rigidity of the connection between the tow vehicle and the trailer is what transfers stresses to both the TV and trailer frames when you hit a dip in the road. A looser connection is going to bend to the contours of the road instead of transferring that bending to the frame of TV and trailer. The frame bends and transfers that load to the trailer shell. You really want the trailer frame to handle the loads the trailer puts on it not loads from load bars that are too big. A little porposing is not going to hurt anything but it can be bothersome. You are better off adding shocks to your TV instead of using really heavy load bars. You want to control oscillations in the YAW axis with sway control and limit (but not eliminate) oscillations in the pitch axis. You are trying to balance the weight between the front and back wheels of the TV. You don’t want to eliminate all movement back there. If you don’t like the up-down movement then get bigger shocks to damp out the movement.

Perry
yup, but you are talking about the larger changes in angle between AS and TV, and you are absolutely correct. Red is trying to make sense out of those bumps which do not change the angle much, if at all, but transmit shock loads from the wheels throughout the entire rig.
More the "rivet popping" issue vs the "Frame stress" issue.
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Old 01-04-2012, 01:49 PM   #90
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I think the conclusion here is that the Airstream is independent of stresses caused by the tow vehicle and hitch, floating along on its own suspension without input from those items?

But Inland Andy continues to advise against that thought because he has had many trailer repairs due to "over hitching" or perhaps "overtrucking". I know he has been called ignorant in this forum, but has been in the Airstream repair business for many, many years.

Andrew Thomson also cautions against too stiff w.d. bars. He is a second generation Airstream dealer and considered by most to be a hitching expert. His expertise has routinely been ridiculed on this forum by those who know better.

My Airstream Owners Manual on tow vehicle states

NOTICE: Be realistic when ordering heavy duty springs. Only springs heavy enough to support your loaded vehicle (not including trailer) are necessary. Too harsh of spring rate will only shorten the life of the tow vehicle and trailer and will make your journeys less enjoyable.

Either I completely misunderstand the previous posts (and that may well be), or the posts are in conflict with a lot of Airstream hitching experience.

doug k
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Old 01-04-2012, 02:16 PM   #91
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"I think the conclusion here is that the Airstream is independent of stresses caused by the tow vehicle and hitch, floating along on its own suspension without input from those items?"

Nobody concluded that at all!!!! Are you reading the same thread as me? Are you just pikin' a stick at us...or trying to be a real antagonist? Just want to know the rules of engagement here.
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Old 01-04-2012, 02:21 PM   #92
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Anyone who defies conventional wisdom will attract some abuse. They may be right or not; abusers are not right about their methods of discussion. To judge what they say is best rather than jump to conclusions. This thread has remained remarkably free of abuse and I hope it stays that way.

Inland Andy has been criticized for relying on data collected 40 years ago and then for some data he collected earlier this year. Since I don't know how he tested things recently I have no idea whether he is right or wrong. CanAm Andrew (can't you guys get different names?; let's not forget Andy R.) has done suspension work to allow people to tow with much lighter vehicles than many people feel comfortable with. He has also written a number of articles for Airstream Life which explain hitching and towing. They are interesting, but I don't understand him some of the time. The result, for me, is confusion. Engineers with much more knowledge than I try to explain these things, but they are subject to jargon and then I get lost. I have my own jargon (legal) and no one understands me either; ask my wife. It is a rough world.

I have no complaint with the concept, almost universally accepted, that overhitching is bad. But how much hitching is overhitching? I disagree with Doug there is universal acceptance about the trailer being independent of the truck and/or hitch. I think (think is perhaps better replaced by "guess") there is some feedback between things, but how much is too much? That's where we get lost, or I do. As I wrote before, I don't think we will get an definite answer, but I keep hoping for getting closer to it.

dzn's comment about a difference between frame stress and rivet stress may be accurate, but only if angle only stresses the frame and road shocks only cause rivets to pop. I think angles and roads may both cause rivet popping and frame damage, but it may be true one causes frame stress more, and the other pops more rivets. Another hard thing to determine. I think we can agree neither are good things, but how much do they matter and what to do about it? This is maybe the same question as the above paragraph. We can't change the roads, but we can choose other ones and drive more attentively.

Gene
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Old 01-04-2012, 02:21 PM   #93
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Originally Posted by dznf0g View Post
yup, but you are talking about the larger changes in angle between AS and TV, and you are absolutely correct. Red is trying to make sense out of those bumps which do not change the angle much, if at all, but transmit shock loads from the wheels throughout the entire rig.
More the "rivet popping" issue vs the "Frame stress" issue.
Yes - thanks again! That's why I differentiated questions Q1 from Q2 earlier in the thread. They are different effects all together. A heavy bar, for instance, will reducing porpoising simply because it has added a lot of resistance to the flexing of the ball-coupler. The heavy bar won't change anything related to hitting a chuck hole, or a strip on the pavement though. Those events occur so fast (pulses), that the bars are out of the equation all together. That's why it is essential to note the difference between compliance and resistance.

The bars are doing a couple things mechanically, and they create a couple different kinds of effects. It's good to separate them by function in order to clear up these questions.
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Old 01-04-2012, 02:23 PM   #94
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Originally Posted by dkottum View Post
I think the conclusion here is that the Airstream is independent of stresses caused by the tow vehicle and hitch, floating along on its own suspension without input from those items?

But Inland Andy continues to advise against that thought because he has had many trailer repairs due to "over hitching" or perhaps "overtrucking". I know he has been called ignorant in this forum, but has been in the Airstream repair business for many, many years.

Andrew Thomson also cautions against too stiff w.d. bars. He is a second generation Airstream dealer and considered by most to be a hitching expert. His expertise has routinely been ridiculed on this forum by those who know better.

My Airstream Owners Manual on tow vehicle states

NOTICE: Be realistic when ordering heavy duty springs. Only springs heavy enough to support your loaded vehicle (not including trailer) are necessary. Too harsh of spring rate will only shorten the life of the tow vehicle and trailer and will make your journeys less enjoyable.

Either I completely misunderstand the previous posts (and that may well be), or the posts are in conflict with a lot of Airstream hitching experience.

doug k
Quite the opposite. The ONLY disagreement I have with Andy is, I believe he has stated that too much SPRING BAR will transmit shock loads AND pop rivets. I believe spring bars do not transmit shock loads, the ball/coupler do, therefore the sprin rating has nothing to do with shock load related failures. The big bars, IMO cause excessive stress load failures.
I've never seen Andrew's take on bar ratings. I have see his article on proper head tilt and the forces applied during turns and he is right on (I don't however, care for his advocation of underrated tow vehicles) Another thread please.....no hijacking allowed!!

You haven't said anythin in your last post which contradicts this discussion per se.
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Old 01-04-2012, 02:23 PM   #95
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Quote:
Originally Posted by dznf0g View Post
"I think the conclusion here is that the Airstream is independent of stresses caused by the tow vehicle and hitch, floating along on its own suspension without input from those items?"

Nobody concluded that at all!!!! Are you reading the same thread as me? Are you just pikin' a stick at us...or trying to be a real antagonist? Just want to know the rules of engagement here.
I misread Doug's statement the first time, but I think the "But" in the second paragraph clarifies what he is actually saying.

Gene
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Old 01-04-2012, 02:25 PM   #96
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I think the conclusion here is that the Airstream is independent of stresses caused by the tow vehicle and hitch, floating along on its own suspension without input from those items?
That is not a conclusion I have reached by any means. In a solid hitch like the EQ (or many others), the AS is very definitely subject to forces transferred from the TV directly through the coupler-ball.

If there is a conclusion on my end, it is this: The stiffness/softness of WD bars will not have more than a negligible effect on ride stiffness/harshness as felt by the trailer. A 600# bar in place of a 1000# bar will not result in less harshness at the trailer.
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Old 01-04-2012, 02:38 PM   #97
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I have no complaint with the concept, almost universally accepted, that overhitching is bad. But how much hitching is overhitching?



Gene
I also want to be clear here that my comments in this thread about the EQ hitch were not intended in any way to bear on the concept of "over-hitching."

Over-hitching is a casual term that could mean many different things under many different conditions to many different people. Unless someone wanted to define that term pretty technically, I wouldn't want to refer to it at all. And certainly not in my comments here.

My comments here are in the narrowest context possible. They relate only to the physical actions and effects of the WD bars on a typical solid WD style hitch with a solid coupler-ball interface.
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Old 01-04-2012, 02:38 PM   #98
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Originally Posted by CrawfordGene View Post
Anyone who defies conventional wisdom will attract some abuse. They may be right or not; abusers are not right about their methods of discussion. To judge what they say is best rather than jump to conclusions. This thread has remained remarkably free of abuse and I hope it stays that way.

Inland Andy has been criticized for relying on data collected 40 years ago and then for some data he collected earlier this year. Since I don't know how he tested things recently I have no idea whether he is right or wrong. CanAm Andrew (can't you guys get different names?; let's not forget Andy R.) has done suspension work to allow people to tow with much lighter vehicles than many people feel comfortable with. He has also written a number of articles for Airstream Life which explain hitching and towing. They are interesting, but I don't understand him some of the time. The result, for me, is confusion. Engineers with much more knowledge than I try to explain these things, but they are subject to jargon and then I get lost. I have my own jargon (legal) and no one understands me either; ask my wife. It is a rough world.

I have no complaint with the concept, almost universally accepted, that overhitching is bad. But how much hitching is overhitching? I disagree with Doug there is universal acceptance about the trailer being independent of the truck and/or hitch. I think (think is perhaps better replaced by "guess") there is some feedback between things, but how much is too much? That's where we get lost, or I do. As I wrote before, I don't think we will get an definite answer, but I keep hoping for getting closer to it.

dzn's comment about a difference between frame stress and rivet stress may be accurate, but only if angle only stresses the frame and road shocks only cause rivets to pop. I think angles and roads may both cause rivet popping and frame damage, but it may be true one causes frame stress more, and the other pops more rivets. Another hard thing to determine. I think we can agree neither are good things, but how much do they matter and what to do about it? This is maybe the same question as the above paragraph. We can't change the roads, but we can choose other ones and drive more attentively.

Gene
I think Red said it well in post #93. We've talked about this for a very long time under one subject heading "damage to AS while traveling" Then we continue to talk about overtrucking, overhitching, running gear balance, bad axles, etc. all in the same thread. There are so many dynamic inputs to the rig as it is going down the road that you can't combine all these thing into one coherent answer. I like this thread because it is about drilling down to ONE COMPONENT, analyzing how it works, the physics involved, how the component behaves with a particular input...then another. I think IF and when we gain consensus on understanding how this component applies forces to the AS, we can then more objectively predict what kind of damages are created by misapplication of that component and make better decisions about the component's use.

For example: I believe rivet popping from too much spring bar occurs only during high, slow stress loads during large angle deviations between trailer and TV frames. And I expect those pops to be in the very front of the AS. Perhaps only the center and side lower sections. Perhaps some front pano issues as well. Other pops, I believe to be harmonic vibration and shock load caused....running gear and bad axles respectively. I would expect those to show on the side walls, doors, rearward of the axles etc. All that is for, I assume later, discussions...this one has evolved into strictly a spring bar understanding discussion.

Am I off base?
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