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Old 11-03-2009, 04:52 PM   #29
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Joe,

Thanks for an outstanding analysis. I guess the laws of physics prevail.
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Old 11-03-2009, 05:03 PM   #30
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Andy- I suppose one could argue that:

a)an Expedition EL really isn't that 'soft' of a suspension
or
b).2 meters/second^2 (17 lbs) is actually a very substantial force when discussing 5" metal frames.
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Old 11-03-2009, 05:21 PM   #31
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this is the sensor/software/peripheral used
with my Macbook Pro:

Vernier LabPro
Accelerometers
Logger Pro 3
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Old 11-03-2009, 06:02 PM   #32
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Quote:
Originally Posted by finalcutjoe View Post
Andy- I suppose one could argue that:

a)an Expedition EL really isn't that 'soft' of a suspension
or
b).2 meters/second^2 (17 lbs) is actually a very substantial force when discussing 5" metal frames.
Using accelerometers, I think, is a good idea. What they say then, must be converted into forces that momemtarially are applied to the A-frame.

Somewhere I think you said, something like 24000 ft/lbs of force, was applied to the A-frame.

I don't know about you, but I would not want to get hit with that kind of force.

If the forces are truly any where near that, and applied thousands of times to the A-frame, as when it's being towed, I then don't question why the fatigue failures are occuring.

Airstream says, a soft ride is necessary. I have said that for over 40 years. There are opinions that differ.

A rough ride and/or extensive impact forces, to me,will cause damages. Again, some blame the design.

I feel you are on to some answers, with more testing, and let the test results speak out.

Contrary to some, who challenge the currect designs, 30 and 40 year old Airstreams had the same fatigue problems, but were seldom towed with rugged tow vehicles, as opposed to today, where the vast majority of tow vehicles are heavy duty trucks. So the damages, obviously, are more pronounced, greater in numbers, today than in the past.

I am looking into our repair records regarding damages to the front ends, in the absence of the front hold down plate.

Perhaps, we could come up with an update kit, if indeed it would prove as an advantage.

Please keep up the excellent work, with your testing.

Andy
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Old 11-03-2009, 06:14 PM   #33
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Originally Posted by Inland RV Center, In View Post
...Airstream says, a soft ride is necessary...
what document?
where is this written or said ?
who says this?
and when?

got a reference 4 ALL 2 C ?
_____________

and the point here is IF softness even IS an issue,

it comes from the axles/wheels/tires and INHERENT trailer design and materials.

where >85% of the RIDE originates.

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Old 11-03-2009, 06:39 PM   #34
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finalcutjoe ... Thanks. This is very interesting info, and as far as I know, something that hasn't ever been done before (or at least, never published). I wish we could enable you to test additional vehicles, to broaden the data set. It might be possible to draw more conclusions in that case.

But even if you were willing to lend out your setup for testing on other vehicles, it seems we'd need to run the exact same road and have the same trailer to get meaningful comparative results. Darn.
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Old 11-03-2009, 07:47 PM   #35
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not to HJ your most excellent thread fc'

and i realize the TOPIC is force at the HITCH...

but since so little of this info exists, and the efforts to acquire such data are significant...

i thought some here might find the OTHER thread...

on 'jimmy legs' n airsteams interesting...

hiho' started posting at #60 in this axle thread....

http://www.airforums.com/forums/f437...les-39688.html

good stuff for about 20 posts, then again at #134 and BIG SURPRISE at #167.

the rest is typical axle replacement beechin'

but the graphs and shock analysis and MARGINAL improvement with new axles is good info...

cheers
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Old 11-04-2009, 08:09 AM   #36
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Again, thanks to all for continued feedback here.

A few things-

This hitch setup is about as rigid as you’re going to get. I’m a svelte 210lbs, and jumping up and down (with spring bars at MAX setting) on the orange head produces slight movement at best (but detectable on the accelerometer as slight aberrations in z-axis).

I waded into this experiment expecting to see numbers that would support the ‘overhitching’ and ‘too much vehicle’ theories. I’ve towed my 30’ Slide with a Dodge 3500 CTD, an experience that I would consider teeth rattling. My 2500 Suburban being only slightly less 'jarring', and the Expedition riding like a silver cloud (note: I intend to run our ‘track’ with the sensor in the different mules- I think quartile feedback with those numbers will give us a more understandable scale of the acceleration data)

At this point, the numbers don’t lie. The ‘harsh’ ride, that one feels inside a 'heavy-duty' tow vehicle, are NOT transferred to the trailer thru the ball and bars. Is it possible there are frequencies, oscillations, vibrations occurring that are harmful to the frame/shell of my ‘stream? Certainly. But I would wager, based on this feedback, that these forces are present whether driving a heavy-duty truck or a passenger vehicle- they’re inherent in over-the-road travel. There are those that would attempt to discredit my findings by citing my earlier misinterpretations of the data. Again, I’m not going to pretend to understand exactly ALL of what the data is trying to tell us-- It would be great if an engineer familiar with this could jump in here. (I’ve got all the data in excel, .csv, whatever is convenient for review and comment.)

But at this point- I feel very comfortable knowing that I’m towing my rig safely within manufacturer specifications- and at the same time not subjecting my 'stream to excessive abuse.
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Old 11-04-2009, 10:27 AM   #37
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This is great info! We just bought a new(used) to us TV. 2003 Dodge 3500 CTD, 4x4, dually. We have a 31ft '72 AS. What load bars should we be using? We used 1000# when towing with our Excursion. Any help would be great.

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Old 11-04-2009, 11:09 AM   #38
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Quote:
Originally Posted by finalcutjoe View Post
Again, thanks to all for continued feedback here.

A few things-
Your abbreviated tests, prove the point.

The tests show huge weight impacts on the A-frame.

Those impacts cause the A-frame to flex.

Flexing of any metal, leads to fatigue.

Fatigue leads to cracks and failures.

You also claim a rough ride.

Flexing, and to what degree of the A-frame and it's effect on the bottom part of the front shell, can very easily be demostrated to the naked eye.

Use a fork kift, as we do, to move trailers in and out of our building, and lift the front of the trailer about 18 inches or so, above level.

Then, without letting the jack post hit the floor, drop the front end quickly, and observe the flexing, in the lower part of the shell.

If that doesn't cause long term problems, then I have wasted 43 plus years of my life, working on Airstreams.

Use Duallies, use 4 X 4's, use 1000 to 1400 pound bars, and you will keep service departments busy repairing front ends, guaranteed.

How do we know? Because we do a lot of that work, and then spend the time teaching the owner, what was wrong and how to fix the problem.

A very strange and interesting observation. Many of those customers, in time, come back for other services, none of which experienced a repeat problem with the front end, in spite of the fact that the trailer and tow vehicle, are the same as before.

What changed?

The rigidity was softened. That was it.

If that's not convincing information, then so be it.

To this day, Airstream says "soft ride". Do they take out pages of ads to say so, of course not.

It's overwelming, to read the insistance owners have of damaging their Airstreams, by over rigging, yet ignoring those that made the changes, and so reported, on this Forums, the improvements.

Wow. Unbelievable.

Andy
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Old 11-04-2009, 11:29 AM   #39
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Quote:
Originally Posted by finalcutjoe View Post
... (note: I intend to run our ‘track’ with the sensor in the different mules- I think quartile feedback with those numbers will give us a more understandable scale of the acceleration data)

At this point, the numbers don’t lie...[/I]
good idea fc' and u might try the track loop with the sensor INSIDE the stream too.

doesn't need to be done at the same time as the tow vehicle and if done on the SAME loop and conditions...

each tv/trailer combo would be interesting info...
_____________

i agree there are low amplitude high frequency vibes being transmitted into the unit...

from the road via the tires/axles primarily and perhaps SOME via the hitch...

your data suggests NOT VERY MUCH via the hitch/a frame, ON YOUR 30 SLIDE...

i think ONE of the issues is this...

? is it better for a stream to move UP/DOWN somewhat level on all axles while connected (adequate w/d is needed here)...

? or is better for a stream to ROCK/roll like a teeter on soft sloppy w/d with loads shifting on the front/rear axles ??

following behind i've seen firm/flat units move very little UP/down,

while soft/sloppy units have rears that go WAY UP then way down...

where's that picture of plumber's crack?
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...What load bars should we be using?...
no offense mary but THIS thread is really about measuring forces and trying to make sense of the data.

it's NOT about how to set up any ONE rig (except maybe fc's rig)

so u might wanna START a thread on your needs and questions, or READ some of the 100s of threads on towing combos...

there are several folks here towing with 1 ton drw trucks who can offer help on tires, w/d bars, hitches and so on...

check out some of those threads in the tow vehicle/hitches section, good stuff is OUT there.
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cheers
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Old 01-15-2010, 12:14 PM   #40
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Quote:
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...
Captured 4 minutes of data at 5 samples/second, over 3.3 mile distance.
Capture started on eastbound S.Huron Drive, hard acceleration up I-275 entrance ramp, maintained 65mph southbound, exiting on Carleton Rd hard decel at stop sign.

Captured 4 minutes of data of reverse lap, northbound I-275.

I-275, both directions, was relatively smooth concrete highway (not new asphalt smooth). Carleton road was pretty rough. 10mph winds.

Ran each lap twice. Each dataset was converted independently, outputs (min, max, quartile, etc..) from each lap were averaged together.

1200 samples for each lap.
Each x,y,z acceleration sample was then translated into a vectored sum.
The 1200 vectored sum units were used for analysis.
The pounds of force unit is based on Force=mass*acceleration, assuming 850lb tongue weight.
My formal education, such as it was, was in physics.

While I agree with your conclusions generally, the forces at the hitch are much more difficult to model and are greater than what you conclude. The simplest way to model the forces is to view the trailer as pivoting over the wheels. To calculate hitch forces you would then use the moment of inertia which would be the integral of the trailer mass over the relative distance from the pivot point of the axles, and divide by the distance from the measurement point to the pivot point. You end up with, approximately, half the mass of the trailer in pounds, times the acceleration in Gs (m/s^2 divided by 9.81). Beyond some transitional point dependent on the stiffness of the axle springs, this force will be reduced slightly as the trailer pivots around its center of mass (which is up and to the rear of the axle pivot point) during application of a higher load.

This is important because it means that the typical hitch forces are about five times higher than you calculate, making the 200-400 pound difference in what the bars themselves apply insignificant.

I suspect that there's much more to the story though because driving down the road shouldn't be what stresses the rivets. It's the railroad tracks and potholes. How do the forces differ then? There are competing mechanics at work. The whole point behind WD bars is to keep the rear axle suspension of the truck in an area where the spring rate is lower. In extreme cases - pothole, railroad track - the WD bars could prevent the suspension from bottoming out by providing more distance to work with.

I can understand the impact the truck suspension has. It's obvious. Any increase in axle spring stiffness or shock absorber stiffness on the downstroke is going to translate directly into more force on the towbar. The WD bars, I still can't see it, because most of the hitch motion is vertical rather than angular. Maybe with heavy WD bars the trailer gets a little more force from the bumps the front wheels of the truck goes over but it's not much.

Nonetheless I have a great deal of respect for field experience and am inclined to believe what Andy says.
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Old 01-15-2010, 04:27 PM   #41
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My formal education, such as it was, was in physics.

While I agree with your conclusions generally, the forces at the hitch are much more difficult to model and are greater than what you conclude. The simplest way to model the forces is to view the trailer as pivoting over the wheels. To calculate hitch forces you would then use the moment of inertia which would be the integral of the trailer mass over the relative distance from the pivot point of the axles, and divide by the distance from the measurement point to the pivot point. You end up with, approximately, half the mass of the trailer in pounds, times the acceleration in Gs (m/s^2 divided by 9.81). Beyond some transitional point dependent on the stiffness of the axle springs, this force will be reduced slightly as the trailer pivots around its center of mass (which is up and to the rear of the axle pivot point) during application of a higher load.

This is important because it means that the typical hitch forces are about five times higher than you calculate, making the 200-400 pound difference in what the bars themselves apply insignificant.

I suspect that there's much more to the story though because driving down the road shouldn't be what stresses the rivets. It's the railroad tracks and potholes. How do the forces differ then? There are competing mechanics at work. The whole point behind WD bars is to keep the rear axle suspension of the truck in an area where the spring rate is lower. In extreme cases - pothole, railroad track - the WD bars could prevent the suspension from bottoming out by providing more distance to work with.

I can understand the impact the truck suspension has. It's obvious. Any increase in axle spring stiffness or shock absorber stiffness on the downstroke is going to translate directly into more force on the towbar. The WD bars, I still can't see it, because most of the hitch motion is vertical rather than angular. Maybe with heavy WD bars the trailer gets a little more force from the bumps the front wheels of the truck goes over but it's not much.

Nonetheless I have a great deal of respect for field experience and am inclined to believe what Andy says.

Jammer

We have at this time a 1972 31 foot Airstream in our shop, for holding tank repairs.

The front end has been repaired before, because of missing rivets.

About 15 rivets are missing from the front hold down plate. The rub rail below the front window is very loose. In chatting with the owner, about these issues, we talked about his hitch and it's rating. He said he had an Equalizer hitch with 600 torsion bars. We looked at the labels on the bars, that clearly said, 1000 pounds. I then asked him to jump up and down on the tow vehicle bumper, without the trailer being attached. The bumper moved maybe 1/2 of an inch, in spite of the fact that the he weighs at least 225 pounds.

As it proves over and over again, the front, if not more of the Airstream, "WILL" sustain damage, when the hitch torsion bars are excessively rated, and/or with a stiff tow vehicle suspension.

It's so sad, that so many Airstream owners waste money repairing the trailer, when the damage could have been totally avoided.

Andy
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Old 01-15-2010, 06:51 PM   #42
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Andy,

I'm listening.

I wonder, though, if perhaps the tow vehicle suspension doesn't have more to do with the problem than the spring ratings. Obviously, they both affect what happens when you jump up and down on the towbar or bumper, more or less equally. But, if the rear tires on the TV hit, say, a pothole, do the hitch springs make much of a difference? I'm trying to visualize it and it doesn't seem like the angle between the TV and the trailer would change much -- it should be just a vertical movement of the rear body of the truck and the trailer tongue moving.

Can you think back to any serious repairs, where the hitch springs were strong but the TV rear axle springs were soft?
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