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Old 01-06-2012, 05:07 PM   #127
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doznf0g---

Ok, I think I understand your comments and experience. It makes sense and I think follows the expectations of the principles that we've been talking about here. I don't think there are any major contradictions.

dozn:"In my case, I had 1000# EQ bars and 900lb tongue weight. At a given driveway angle, there was not much additional deflection of the EQ bars and the front suspension compressed some and the rear suspension extended quite a lot....disturbingly so."

This conforms directly to the principle shown in the last sketch. When the bars are stiff it will cause the rear suspension to extend down, the front suspension to compress, and the weight of the TV will transfer back onto the A-frame.

What I should add here is that the action of the bars is the direct cause of the rear suspension extending. So, those two elements are directly related and inversely proportional. Less bar flex = more suspension extension. More bar flex = less suspension extension. Agreed?

dozn: "With the 800# Reese bars and 900# tongue weight, I see about an additional 2" of spring deflection, minor front suspension compression and a little rear suspension extension."

Do you mean the bar is deflected 2"? That seems like a lot of deflection to me. As predicted - more bending of the bars will give less suspension extension at the rear. Direct trade-off. Is there is limit to how far they can bend before failure? Do we have any data about that? That much bending will bring all the bar materials and heat treating into play. I do not know what they will take.

I think this begins to inform the compromises involved in building a WD hitch. Stiffer bars are nice for lifting the TV in a static mode. That achieves a nice way to distribute weight on the TV axles. But stiffer bars are not desirable in the dynamic mode of passing over a gully, because they will cause a huge weight shift BACKWARDS onto the A-frame! Typical engineering problem of carefully balancing priorities.

  • Good robust A-frames are a good insurance against damage.
  • A TV with some suspension travel would seem better than one without
  • How much bending is acceptable for the bars? Big question.
  • Get the softest bars that will still lift the TV into proper weight distribution, but not fail when bent!
  • If you have a TV that is rear heavy at the start, it will be hard to get a satisfactory solution here because it will need very stiff bars.
  • To have true confidence in the bar selection, you would have to have engineering data on the bar's failure points and failure modes. probably won't be easy to get.
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Old 01-06-2012, 05:24 PM   #128
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Gene, I dunno!
I HAVE seen a number of trailers both in person and in pictures, AS and non-AS with bent tongues....how they got that way is up for speculation....but it happens.

At what point do they flex and cause damage to the front end (wall). Again, I dunno.
At what point do they permanently deform? We would need to know RBM and Tensile strength of the material, the forces applied AND....this is really, really important....somebody a lot smarter than I to do the calculations, so we could bump them against Andy's numbers.

Anybody smart and know the AS frame specs?
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Old 01-06-2012, 05:33 PM   #129
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doznf0g---

Ok, I think I understand your comments and experience. It makes sense and I think follows the expectations of the principles that we've been talking about here. I don't think there are any major contradictions.

dozn:"In my case, I had 1000# EQ bars and 900lb tongue weight. At a given driveway angle, there was not much additional deflection of the EQ bars and the front suspension compressed some and the rear suspension extended quite a lot....disturbingly so."

This conforms directly to the principle shown in the last sketch. When the bars are stiff it will cause the rear suspension to extend down, the front suspension to compress, and the weight of the TV will transfer back onto the A-frame.

What I should add here is that the action of the bars is the direct cause of the rear suspension extending. So, those two elements are directly related and inversely proportional. Less bar flex = more suspension extension. More bar flex = less suspension extension. Agreed?

Yes, more TV rear suspension extension, more TV front and AS suspension compression

dozn: "With the 800# Reese bars and 900# tongue weight, I see about an additional 2" of spring deflection, minor front suspension compression and a little rear suspension extension."

Do you mean the bar is deflected 2"? That seems like a lot of deflection to me. As predicted - more bending of the bars will give less suspension extension at the rear. Direct trade-off. Is there is limit to how far they can bend before failure? Do we have any data about that? That much bending will bring all the bar materials and heat treating into play. I do not know what they will take.

As I tried to say, it's hard when everything is in that bind to tell how much additional bar flex there is, but it was pretty darned curvy. (that's a technical term) I'd guess this entry was a good 20* incline plus the crown of the road pitching down to the gutter.

I think this begins to inform the compromises involved in building a WD hitch. Stiffer bars are nice for lifting the TV in a static mode. That achieves a nice way to distribute weight on the TV axles. But stiffer bars are not desirable in the dynamic mode of passing over a gully, because they will cause a huge weight shift BACKWARDS onto the A-frame! Typical engineering problem of carefully balancing priorities.

  • Good robust A-frames are a good insurance against damage.
  • A TV with some suspension travel would seem better than one without
  • How much bending is acceptable for the bars? Big question.
  • Get the softest bars that will still lift the TV into proper weight distribution, but not fail when bent!
  • If you have a TV that is rear heavy at the start, it will be hard to get a satisfactory solution here because it will need very stiff bars.
  • To have true confidence in the bar selection, you would have to have engineering data on the bar's failure points and failure modes. probably won't be easy to get.
Are we there yet, to make those conclusions?
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Old 01-06-2012, 05:34 PM   #130
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Originally Posted by CrawfordGene View Post
Redwood,

I'm seeing, maybe, what you are saying and the drawing in post 123 helps a lot to visualize it.

I wonder if it makes a difference?—if this were such a problem, wouldn't there be lots of reports of bent tongues? With thousands of RV's, many with stiff hitch systems, driving over dips every day, I would think we'd hear about a problem. Some cheap trailers probably have weaker tongues than Airstreams and it would seem they would be having even more problems. I realize over time the tongue could be stressed so many times that eventually it would deform, and it would not be obvious what caused it, but even then, wouldn't there be reports of that? I also wonder if in the early days of trailering bent tongues were common before manufacturers leaned how to make them stronger.


Gene
Some clarifications:
I don't see damage to the tongue as the issue. What I am projecting is that the entire horizontal frame of the trailer gets bent downward under these heavy load shifts. Imagine the chassis of the trailer being held at the two ends and pushing (bending) the chassis at the middle by say 1/2" or so, just to visualize it. THAT is the kind of flex in the chassis that I think will cause problems. Not simply a bent tongue.

Now, when I say "load shift" up there, I mean the "physics" action that occurs when a very stiff WD bar causes the rear TV suspension to extend over a gully. You can't see it, but a huge weight transfer takes place from the TV directly onto the A-frame at the exact point that the rear of the bar mounts to the A-frame. In fact, what is happening is that the WD bar is "pulling down" on the A-frame with all the weight that the TV is unloading as the coil spring extends down.

More exactly - the horizontal frame is literally hanging between the ball and the rear trailer axle and the WD bar is then pulling straight down at its connection point with all the weight offloaded from the TV. It's really quite an impressive action! If that rear wheel lifts, you have 6,000# or so hanging directly on the A-frame! And if you are accelerating, it might quickly become 12,000#!

As to how real it is, ask Andy. If I understand his point clearly, he is doing repairs to trailers all the time because of this frame deforming damage.

Now - for clarity, and to understand the principles - I am of course exaggerating the effect in order to clarify it. I am sure rear wheels don't often lift off the ground over a driveway. I am using it illustratively. But weight shift does happen and A-frames do deflect.
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Old 01-06-2012, 05:37 PM   #131
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dzn', now I'll have to start looking at tongues for bending. I didn't expect answers to my questions. I'm much better at questions than answers myself.

What is "RBM"?—Resisting Bending Moment was one of many I found for the acronym, but what does it mean? I liked RBM = Rainbow Butt Monkeys, however, I don't think that is what you meant.

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Old 01-06-2012, 05:48 PM   #132
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Originally Posted by redwoodguy View Post
I don't see damage to the tongue as the issue. What I am projecting is that the entire horizontal frame of the trailer gets bent downward under these heavy load shifts.
The connection between the body and the tongue/frame is one issue, leading to possible front end separation. dzn' seems more concerned with tongue deformation. Trailers built at different times with different materials may have a propensity towards one or the other. Neither is good.

How often these things happen is unclear. Andy's statement about repairing this issue and dzn's about seeing a lot of bent tongues are generalizations and none of us knows how any of it happened or exactly how often. But you have to start somewhere when you see some evidence of a problem and then go back and figure out the frequency of the problem.

Gene
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Old 01-06-2012, 05:50 PM   #133
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Quote:
Originally Posted by CrawfordGene
dzn', now I'll have to start looking at tongues for bending. I didn't expect answers to my questions. I'm much better at questions than answers myself.

What is "RBM"?—Resisting Bending Moment was one of many I found for the acronym, but what does it mean? I liked RBM = Rainbow Butt Monkeys, however, I don't think that is what you meant.

Gene
Yeah......not so much the second one.....funny though. An engineer will have to define it precisely but Resisting bending moment refers to the strength of a beam (in this case) where it permanently deforms. Tensile strength refers to the strength of the material itself. I THINK to both. Some engineer jump in and help out.

The bending I have seen is not at the mounting point of the bar ends. It seems always to be at or rearward of the front wall. Perhaps where the a-frame is connected to the frame rails. That may be more of a common weak point.
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Old 01-06-2012, 06:12 PM   #134
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I'm looking for pics, but here's one bent at the snap ups:

RV.Net Open Roads Forum: Travel Trailers: Bent TT A-frame

Our own Robertsunus:

http://www.airforums.com/forums/f36/...mal-65927.html

RV.Net Open Roads Forum: Travel Trailers: Tongue Bent?

Good Sam Club Open Roads Forum: Travel Trailers: Bent TT A-frame

2006 wildcat with bent frame and cracks on the pin - Forest River Forums
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Old 01-06-2012, 07:08 PM   #135
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Originally Posted by dznf0g

Yeah......not so much the second one.....funny though. An engineer will have to define it precisely but Resisting bending moment refers to the strength of a beam (in this case) where it permanently deforms. Tensile strength refers to the strength of the material itself. I THINK to both. Some engineer jump in and help out.

The bending I have seen is not at the mounting point of the bar ends. It seems always to be at or rearward of the front wall. Perhaps where the a-frame is connected to the frame rails. That may be more of a common weak point.
Gene, strike the RBM definition....I was thinking of yield strength.
RBM is the force countering the bending of the beam (in this case) at any given point on the beam...I think...still need an ENGINEER HERE PLEASE!
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Old 01-06-2012, 07:35 PM   #136
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Gene, strike the RBM definition....I was thinking of yield strength.
RBM is the force countering the bending of the beam (in this case) at any given point on the beam...I think...still need an ENGINEER HERE PLEASE!
ha ha - - Well, I'm an engineer, but not a mechanical engineer. A bending moment occurs when a bending point on a material is in a spot other than where the force is being applied. A resisting bending moment is just whatever force(s) exist to resist bending.

My guess here is that analyzing all the bars for actual mechanical parameters is beyond the scope we could cover here. It would take a lot of instrumentation to do it empirically, or a lot of material specifications do do it statistically. I have neither.
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Old 01-06-2012, 07:45 PM   #137
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"Since the question involves ride harshness, ....snip.....
(At the moment I realize that the two bars are not available for the same head, but that's a practical thing to be solved for the test)....."

Actually, they are. Blue Ox uses the same head, and you buy the correct bars based on the tongue weight of your trailer. They have bars rated at 550, 750, 1000, 1500 lbs.

I'm no engineer, but one of the questions I would want answered is this: does the OE mfr of the frame ( in this discussion, Airstream ) intend the product to be used with a weight distribution hitch ? In other words, are the stresses imposed by a WD hitch considered in the design of the A-frame and perhaps other parts of the frame/suspension ? If the frame was never designed for the stresses transmitted by the WD hitch and it's bars, then it might make some sense the A-frame is bending. On the other hand, if the frame is considered to be strong enough to absorb ( and transmit to the trailer suspension ) these forces, then I would be questioning why the frame is bending. I have to wonder if, over the years, someone has added gussets/plates to the A-frame to stiffen it. And if so, did that solve the problem, or did it transmit more stress loads to the riveted body structure ?

My hunch is there are some engineers at Airstream that have done all the tests, using strain gauges and all the other tools to measure these parameters.....and they know the answers to all this. And they ain't talkin'. And probably won't be saying anything either, under strict guidelines set out by the corporate legal department.

I would guess the way to test this would be to use something like the BlueOx system, and simply measure suspension travel at all axles, and use strain gauges at a few places on the A-frame, and do the tests a few times with each different weight bar. I'd want to do some measuring of the frame as well throughout the testing sequence to make sure that it hasn't bent during the previous test.

So I guess my take on it is that the best we out here in the real world can do, is rely on "facts".....as in "data".....and it sure seems that Andy has more data across a broad spectrum of trailers, tow vehicles and years than many of us. He may not be an engineer ( or maybe he is ??? ) but he has obviously seen the results from being in the field.

So that's my two cents....which is coming from a guy that does not even yet own an A/S ! But I do have one of those "other brand white boxes"....and quite a few years of pulling various trailers, from pop-ups and motorbike trailers to 36 foot gooseneck horse haulers. For what it's worth ( I know....not much ), in the horse world which I am heavily into, I can tell you that a LOT of trailers ( stock and horse trailers, as well as our always present, and usually abused flatbeds ) get overloaded very often. It's rare to see these kinds of trailers suffer damage from overloading. I guess the manufacturers "know" this stuff is gonna get abused, so they engineer in a pretty good safety margin.

Does not look like there is as much "margin" in the construction of RV's to my eye.

.....okay....crawling back into lurk mode here....great discussion, love to see folks thinking about and discussing these issues.

george
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Old 01-06-2012, 08:11 PM   #138
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Really good thoughts there George -- thanks for chiming in.

Quote:
Originally Posted by gmw photos View Post
"Since the question involves ride harshness, ....snip.....
(At the moment I realize that the two bars are not available for the same head, but that's a practical thing to be solved for the test)....."

Actually, they are. Blue Ox uses the same head, and you buy the correct bars based on the tongue weight of your trailer. They have bars rated at 550, 750, 1000, 1500 lbs.

That's a cool design. Unfortunately, not the one I own. But yes, that would make the ideal testing platform wouldn't it?

I'm no engineer, but one of the questions I would want answered is this: does the OE mfr of the frame ( in this discussion, Airstream ) intend the product to be used with a weight distribution hitch ? In other words, are the stresses imposed by a WD hitch considered in the design of the A-frame and perhaps other parts of the frame/suspension ? If the frame was never designed for the stresses transmitted by the WD hitch and it's bars, then it might make some sense the A-frame is bending. On the other hand, if the frame is considered to be strong enough to absorb ( and transmit to the trailer suspension ) these forces, then I would be questioning why the frame is bending. I have to wonder if, over the years, someone has added gussets/plates to the A-frame to stiffen it. And if so, did that solve the problem, or did it transmit more stress loads to the riveted body structure ?

I was hinting at this question earlier when I wondered why mfrs didn't build WD into their frames from the start. I have no clue what they know or don't know. I have few insights into the RV manufacturing industry. I'm a bit surprised some engineer at AS doesn't pop in from time to time to give an opinion.

My hunch is there are some engineers at Airstream that have done all the tests, using strain gauges and all the other tools to measure these parameters.....and they know the answers to all this. And they ain't talkin'. And probably won't be saying anything either, under strict guidelines set out by the corporate legal department.

I would guess the way to test this would be to use something like the BlueOx system, and simply measure suspension travel at all axles, and use strain gauges at a few places on the A-frame, and do the tests a few times with each different weight bar. I'd want to do some measuring of the frame as well throughout the testing sequence to make sure that it hasn't bent during the previous test.

Yup.

So I guess my take on it is that the best we out here in the real world can do, is rely on "facts".....as in "data".....and it sure seems that Andy has more data across a broad spectrum of trailers, tow vehicles and years than many of us. He may not be an engineer ( or maybe he is ??? ) but he has obviously seen the results from being in the field.

So that's my two cents....which is coming from a guy that does not even yet own an A/S ! But I do have one of those "other brand white boxes"....and quite a few years of pulling various trailers, from pop-ups and motorbike trailers to 36 foot gooseneck horse haulers. For what it's worth ( I know....not much ), in the horse world which I am heavily into, I can tell you that a LOT of trailers ( stock and horse trailers, as well as our always present, and usually abused flatbeds ) get overloaded very often. It's rare to see these kinds of trailers suffer damage from overloading. I guess the manufacturers "know" this stuff is gonna get abused, so they engineer in a pretty good safety margin.

Does not look like there is as much "margin" in the construction of RV's to my eye.

One of the few insights I have into RV industry is that they are extremely cost conscious. My new AS has a lot of MDF in it. It bugs the heck out of me, and I think it is the worst possible material to put in a trailer. As they say, "It's heavy, but it's weak!" BUT, it is cheap, and it can be disguised in a way that people won't find objectionable. It holds down costs. I don't know enough about the chassis and frame to intelligently comment. But if the MDF is any kind of signal.....

.....okay....crawling back into lurk mode here....great discussion, love to see folks thinking about and discussing these issues.

No reason to lurk - the more the comments the better!


george
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Old 01-06-2012, 10:57 PM   #139
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Airstream is a small company and can't afford to do a lot of tests or engineering. If they did, they wouldn't have built trailers with rear end separation (to heavy a kitchen in the rear) or OSB subfloors. I expect that if there's a problem they guess their way through it. We won't be getting information from them and not because they even have a legal department (which I don't believe they do).

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Old 01-06-2012, 11:21 PM   #140
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doznf0g---
Good robust A-frames are a good insurance against damage.
  • A TV with some suspension travel would seem better than one without
  • How much bending is acceptable for the bars? Big question.
  • Get the softest bars that will still lift the TV into proper weight distribution, but not fail when bent!
  • If you have a TV that is rear heavy at the start, it will be hard to get a satisfactory solution here because it will need very stiff bars.
  • To have true confidence in the bar selection, you would have to have engineering data on the bar's failure points and failure modes. probably won't be easy to get.
Are we there yet, to make those conclusions?
ha ha -- no, I suppose not. However, it is a good way point. For starters I feel I have achieved my first goal to educate myself on the first principles of the WD hitch. What I refer to as the "what causes what and why?"

My next goal would be to get the current hitch & rig dialed in. As Rednax has chided me - get the scale numbers. Yes, I must get that done. Then I want to get a good video of my rig traversing these gas station gullies and see how much bar deflection I get, and how much suspension extension I get. Strictly an empirical approach there.

Beyond that - what would I do if not satisfied? I can't afford to trade hitches right now, so that's out. EQ doesn't make the optional bars, so that's out. I'll most likely dial it in as best I can and work with it for a year and re-evaluate. I like the hitch well enough and would only consider changing if the empirical study looks "alarming" to use a word from doznf0g.

One last choice might be to design, and then have fabricated, a "800# bar" that fits the 1000# EQ head socket. I doubt very much that it would be a difficult project to design - a beam is a beam is a beam. I don't know if manufacturing one pair would make any economic sense at all. I'd probably be able to buy a whole new hitch cheaper than making one pair of custom spec bars.

Oh yeah, I also want to try my new accelerometer app on my Galaxy Tab. I don't want to waste that $1.50 I spent. I'll try it inside the trailer for fun, just to learn what level of g-forces exist inside over a typical road.

If anyone has videos of their rig traversing these GSGs - by all means post them!
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