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Old 04-21-2019, 05:57 AM   #21
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Remember how very early on I said this conversation would happen and itís based on a lot of math I donít understand......well there it is....
....and normally followed by a link to the neat little video clip of the model trailer on a roling road which induces sway when loaded behind the rear axle!

I have an easy solution for folks worried about sway when adding bikes on the back of their Airstream...swap to a motorhome.

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You can even hang a small motorbike off the back then!
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Old 04-21-2019, 06:41 AM   #22
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Too expensive to swap to a motor home. It would be a lot cheaper to just weld the trailer frame to the tow vehicle frame.
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Old 04-21-2019, 06:49 AM   #23
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Originally Posted by martin300662 View Post
....and normally followed by a link to the neat little video clip of the model trailer on a roling road which induces sway when loaded behind the rear axle!



I have an easy solution for folks worried about sway when adding bikes on the back of their Airstream...swap to a motorhome.



Attachment 338689



You can even hang a small motorbike off the back then!




Such a wise guy!

Love your moho! Hope allís well!
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Old 04-21-2019, 09:55 AM   #24
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You have a good argument up until your PS. The total weight of the trailer has to be resisted. If less weight is on the tongue, then MORE normal force is on the tires. It is not the weight on the tires that is most important in sway, it is the angular momentum around the CG, which needs to be forward of the axles and as far forward as tongue weight will allow. Putting the CG forward has a major damping effect on the oscillatory pendulum (the trailer) because it must also swing (or push sideways) the tow vehicle.

Yes, there is some resistive force created by the short moment arm between the tires on the front and rear axles (pity the poor single axles?), but it is small compared to the force created by the moment arm from the tow vehicle front and rear tires, multiplied by the distance from the tow vehicle rear axle to the hitch. If this wasn't true, the single axle Airstreams would have serious, probably uncontrollable, sway problems. This is also why you see more sway accidents when a light SUV is the tow vehicle than when a heavier truck is in the lead.

When the numbers come in, I think I'll still be more worried about bounce than sway.

Zep

PS: I leave the code exercise for building the simulation to others. It would be easy to do it with an analog computer. Anyone got some spare op amps and couple of capacitors?
I apologize! Sorry for raining on your parade!

Having gone through a loss of control sway event myself, I guess I am more cautious than others. I'll try to refrain from further comments, but I'll still be following.

Hope you continue to share information/results because I'm curious how you will arrive at your numbers, measuring the horizontal forces.
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Old 04-22-2019, 05:55 AM   #25
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Thanks for all the comments.

The bottom line remains -- that any weight in excess of Airstream's design parameters, plus its increased distance behind the trailer's rear axle(s), exposes you to possible liability for damages in any accident involving sway or roll-over IMO.

I can hear the injured party's attorneys in court now:

"So you were aware that Airstream did not approve of this rear hitch, and yet you still went ahead and towed with that much weight located in an unapproved location?"

Eyes wide open, that's all . . .



Happy trails,

Peter

PS -- See Post #3 also.
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Old 04-22-2019, 07:27 AM   #26
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Such a wise guy!

Love your moho! Hope allís well!


All good this side of the pond....just about 70oF with a slight breezy off the sea (we are only 1/2mile from the beach!)
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Old 04-22-2019, 10:38 AM   #27
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Originally Posted by A W Warn View Post
Hope you continue to share information/results because I'm curious how you will arrive at your numbers, measuring the horizontal forces.
Good point. I guess I could instrument the axles for side loads, but I hadn't planned on it. You could put stress tabs on the frame at the axle attachment points and get bending stress, but I'd rather be camping... I'm going for the easy data first -- total weight and tongue weight.

It wasn't five minutes after I posted the "mass/spring/damper" example that I realized that it was a fun thought experiment, but somewhat wrong. It is a major simplification not to consider the coupling between the Airstream's natural frequency in roll and coupling between that and the pendulum frequency.

Second point: The Airstream "allowed" weights a something of a red herring. I do a lot of work on '70s models and it doesn't matter how long they are, the frame is the same C-channel steel. Even without any bikes on the back, the longer Airstreams, eg 31', often get rear end separation. And to think that the "allowed" added tail end weight is similar for 18' and 31' Airstreams just doesn't pass the smell test. Tail end weight is structurally more important in a longer Airstream.

Overall, the issue is achieving the proper balance of weight distribution, eg tongue weight and total weight. What about black tank water? In my '77 Safari the tank (13 gallons?) is as far aft as it can be, and that's a 104 lb variable in the worst spot. What about the fresh tank. This is a 250-350 lb variable, but it's in a good spot, generally with a cg slightly forward of the axles. But is a full tank (added mass lowers the rolling natural frequency) the best? What about sloshing in a half-full tank?

The questions that occur to me (after seeing all your concerns, valid concerns, by the way) are:
1. What's the percentage of sway accidents with a tail rack versus without?
2. What's the ratio of sway accidents when the tow vehicle weighs less than the Airstream versus more?
3. Do sway accident rates increase with longer Airstreams?
4. Are there cautions and warnings listed anywhere about the fill levels in the holding and fresh tanks?
5. Is there some critical loading/tank fill that puts the Airstream close to some dangerous frequency "cliff" so that adding 100 lbs to the tail end pushes the total vehicle into oscillation? If so, what is it?
6. Why is it that the "approved" bike rack is safe, but the same amount of weight approximately 12-18" further aft is not? I mean, how close to unsafe is the "approved" rack?

The point is, we ought to be questioning a lot more than just bumper bike racks. When I was using a V-8 Ford Explorer to tow a '70 Caravel, pretty much at the light end of the Airstream scale, sway was a problem above 55 mph. Adding a sway damper improved things a little, like may 65 mph. The Caravel was not modified, nor was it oddly loaded. The engineers clearly did not have that figured out. Towing with a 7.3 diesel Ford completely eliminated the sway. All I'm saying is that the bike rack argument completely fails in this case.

Zep
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Old 04-22-2019, 02:14 PM   #28
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All good points, thanks.

Peter
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Old 04-22-2019, 03:02 PM   #29
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Thanks for all the comments.

The bottom line remains

"So you were aware that Airstream did not approve of this rear hitch, and yet you still went ahead and towed with that much weight located in an unapproved location?"





Happy trails,

Peter

PS -- See Post #3 also.
Airstream Adventures NW, a five rivet dealer with dealerships in Idaho, Washington, Oregon, And California, who also own and run the Airstream customization shop Ultimate Airstreams, is selling custom reciever hitches attached to the frame of AS trailers for this very purpose. They have done a bunch of them, and I think if it was a issue or not allowed by the mothership they would have huge liability issues.

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Old 04-22-2019, 03:08 PM   #30
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I’ll take that bet.



Doubt if Airstream has signed on for any such modifications IMO. It would open them up to incredible liability issues, and the parent company Thor would not sign off on it either IMO.

Please post something in writing from AS to that effect.

Anyone who wants to create additional risk in their life is welcome to open the door.

“Eyes wide open” has been the only point all along.

Peter

PS, Mike, you misquoted me . . . that post was about what an attorney might say in court . . . Changes the whole context!
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Old 04-22-2019, 03:26 PM   #31
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I’ll take that bet.



Doubt if Airstream has signed on for any such modifications IMO. It would open them up to incredible liability issues, and the parent company Thor would not sign off on it either IMO.

Please post something in writing from AS to that effect.

Anyone who wants to create additional risk in their life is welcome to open the door.

“Eyes wide open” has been the only point all along.

Peter

PS, Mike, you misquoted me . . . that post was about what an attorney might say in court . . . Changes the whole context!
I just talked with Kyle at the home office in Jackson; he said they do not authorize adding a hitch to the rear of the Airstream. I mentioned about AS Adventures, and he said that would be on them, but had not heard they were installing hitches..."it would be a new one on him." I called AS Adventures; I purchased my last 2 from AS Adventures...the parts manager is calling me back shortly.....stay tuned! Foot note; he said they do install hitches on their MH's...could that be the mistake...a MH not a TT?
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Old 04-22-2019, 03:42 PM   #32
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Thank you for that direct feedback!

Peter
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Old 04-22-2019, 04:59 PM   #33
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I just talked with Kyle at the home office in Jackson; he said they do not authorize adding a hitch to the rear of the Airstream. I mentioned about AS Adventures, and he said that would be on them, but had not heard they were installing hitches..."it would be a new one on him." I called AS Adventures; I purchased my last 2 from AS Adventures...the parts manager is calling me back shortly.....stay tuned! Foot note; he said they do install hitches on their MH's...could that be the mistake...a MH not a TT?
OK; AS Adventures Portland called me back. They said it's a "gray area", but they do install receiver hitches. He said he wasn't aware if it could/would void the warranty; they do it at customer request. He said their only concern is not knowing what an owner ends up putting in the receiver, beyond a bike rack...and that could be an "issue". Each one is custom to the trailer, and sometimes they need to beef up the installation with more support. He said their weight loading recommendation is up to 150lbs and it "should be fine", assuming your front hitch weight is adequate and you don't have a lot of weight in the rear compartment.

So, sounds like there is not an issue, as long as there are no "issue's" causing an "issue"!
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Old 04-22-2019, 05:12 PM   #34
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I think that the focus on any receiver rack being a liability risk and verboten is misguided. A few things are important to consider.

First is the weight carried. To maintain the same stability, all else being equal, weight carried inside the trailer should be reduced by an equivalent amount. Not the same amount, but an equivalent amount considering the relative distances from the axles.

Second is whether the load can move around, increasing dynamic loads and introducing the risk of fatigue. The Airstream rack, and the other racks installed by dealers such as Can Am, have a horizontal longitudinal strut at the top, running to the trailer. Something like an awning strut. This reduces the waving around effect, and thus the leveraging applied at the bottom of the shell. It would be straightforward to have a quick release strut for a receiver rack, and then the trailer would see very similar forces to the Airstream rack.

Third, a real risk with a receiver is that an owner or subsequent owner will use something like a luggage carrier, and significantly overload the receiver.

With those risks kept in mind and managed, I don’t see why a receiver rack needs to be vilified.
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Old 04-22-2019, 05:30 PM   #35
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OK; AS Adventures Portland called me back. They said it's a "gray area", but.... said their weight loading recommendation is up to 150lbs and it "should be fine".
I'm amazed at how many times I've heard "should be fine" from Airstream dealers in regards to towing, tow vehicles, weight capacities etc.

My advice would be to do your own research using multiple credible sources. Better yet, defer to the Airstream engineers. They know much more about the design, capabilities and capacities of their products than most of us here on the forums.
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Old 04-22-2019, 05:51 PM   #36
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I just talked with Kyle at the home office in Jackson; he said they do not authorize adding a hitch to the rear of the Airstream. I mentioned about AS Adventures, and he said that would be on them . . .
Please follow up with Kyle about Airstream’s position on that dealer installing rear hitches. Thanks.

Highly unlikely that AS will sign off on this dealer wandering off-course like this IMO.

If any manufacturer says “No” to a particular practice like this, owners who then proceed with the practice are going way out on a limb IMO.



“Eyes wide open,” that’s all . . .



Peter
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Old 04-22-2019, 06:02 PM   #37
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OK; AS Adventures Portland called me back. They said it's a "gray area", but they do install receiver hitches. He said he wasn't aware if it could/would void the warranty . . .
Can’t believe he was honest enough to admit that . . .

Please ask Kyle from AS about this also, and whether AS is gong to contact the dealer and straighten them out.

Thanks,

Peter
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Old 04-23-2019, 12:49 AM   #38
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You guys keep debating your little hitch.

I've got a beefy tow vehicle that can handle anything. And I've beefed up everything even more so I can carry more beef in my newly installed beefy caboose.

What's sway?

Thinking ahead, I even added a hitch to my caboose. You know, for dem bikes.

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Old 04-23-2019, 04:11 AM   #39
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We've decided to go that route also. Our Honda 5000 gen and fuel all fit inside nicely!

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Old 04-23-2019, 07:29 AM   #40
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One item not mentioned thus far is the structure of the Airstream. Zep's picture appears to be one of his older trailers. The frame rails are both of heavier gauge and larger dimensions (stiffer) than newer airstreams. The difference in my 1970 vs the 1997 are huge! Also the 1970 has a gusset plate in the front and rear reinforcing the frame/body connection. These two details make the older Airstreams much more accommodating to modifications such as this as opposed to the newer (weaker) trailer frames.
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