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Old 04-12-2013, 11:36 AM   #15
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The separation issue is not about frame strength, or strength of the body. It's about acceleration loads on the frame which tend to try to tear the frame from the body. The back of the trailer gets lots of acceleration loads when driving down the road hitting pavement irregularities. .
Hmm, so I wonder if attaching the AS bike rack to the body itself would only exacerbate this, by "pulling" on the airstream body against the weight points on the bumper/ frame? I wonder if this is why it's only offered for newer model AS's.

Also, I aren't separation and "rear end sag" issues two different problems? I thought the issue of rear end sag was insufficient strength/rigidity in the frame rails cantelevered out behind the trailer wheels, so they bent down over time, causing, among other things, the frame to seperate from the body. The poor engineering was made worse in models with heavier loads toward the back like the rear bathroom.

But I don't claim to be an expert, just a guy with time to read some forum posts.

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Old 04-12-2013, 12:45 PM   #16
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I think you are correct. The problem I see is the 28 has an awful lot of storage to the rear so adding a bike rack may add to the problem. I don't see much difference to what you designed and the AS rack I have installed on my trailer. I have a lot of items in the rear compartment which add weight to the rear and seems to take some of the heavy tongue weight off that the 28 has. Airstream is very conservative in the rating of the AS bike rack.
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Old 04-12-2013, 01:43 PM   #17
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Airstream rack attaches to the frame AND the body, which distributes the load to both, helping to eliminate frame separation.

I'm told all Airstreams use what is called a Monoque (sp?) construction where the body is as much of the strength in the structure as the frame.
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Old 04-12-2013, 02:13 PM   #18
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Originally Posted by SteveH View Post
Airstream rack attaches to the frame AND the body, which distributes the load to both, helping to eliminate frame separation.

I'm told all Airstreams use what is called a Monoque (sp?) construction where the body is as much of the strength in the structure as the frame.
I have the airstream rack on my trailer and the two supports attached to the body are free floating on the rack. You can move the support up and down on the bike rack tube by hand. In other words the supports have no bolt going thru them and the rack tube. The supports may keep the rack from tilting back but I don't think it transfers much load to the body. I will have to verify that mine is installed correctly as it seems silly not have a hard connection at the intersection of the support and rack tube.

Looks like mine is correct.

Airstream Carry-Bike by Fiamma Bike Rack for Travel Trailer RV Thule Yakima - YouTube


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Old 04-12-2013, 02:26 PM   #19
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The "moment load" often referred to is a load in an arc, not straight up and down. Watch some of the receiver mounted racks out on the road. They bounce up, the down and back. It would be like if you grabbed the top of the rack and pulled back and down hard...repeatedly. The actual force is many times the weight of the bikes and rack. It essentially torques the frame tips.

By the attachment shown above, the arc is prevented by the top arms. There is no rapid acceleration and deceleration of the load, like there is with the receiver mounted racks.
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Old 04-12-2013, 03:11 PM   #20
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The "moment load" often referred to is a load in an arc, not straight up and down. Watch some of the receiver mounted racks out on the road. They bounce up, the down and back. It would be like if you grabbed the top of the rack and pulled back and down hard...repeatedly. The actual force is many times the weight of the bikes and rack. It essentially torques the frame tips.

By the attachment shown above, the arc is prevented by the top arms. There is no rapid acceleration and deceleration of the load, like there is with the receiver mounted racks.

Rich I was thinking what I am I missing as I know the racks are designed by professionals, a light did go on and I also realized the load is also carried low on the AS rack. The fold down tray for the wheels is mounted low almost in plane with the frame mounting plates. The tray splits the load between the two attachment points almost directly on the frame. Airstream Dad would be better served to put his bikes low on a tray than hanging on the hooks of his current configuration. Am I getting some sort of understanding here?

Hanging the bikes on hooks then acts the same as using a cheater pipe on a wrench multiplying the force.
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Old 04-12-2013, 03:32 PM   #21
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Yes, distance up, and away are factors, as are weight of bikes, weight of rack, weight of receiver (to some extent). Slop in receiver to rack plays into the formula as well.
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Old 04-13-2013, 06:56 AM   #22
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Good discussion!


Maybe what I'll do is run a line from the rack and tie it off somewhere, perhaps in the rear awning hardware, or even inside the rear compartment.

The Yakima rack is pretty stiff in the hitch receiver once you tighten the pin bolt, so there isn't a lot of action to create moment load, but perhaps tying it off will minimize it even further, or at least stabilize it during a shock, like hitting a hump or pothole.

OF course, I should probably be careful that the line doesn't do something like tear off the awning! I wonder if I should have a strong attachment point installed in the body, attached directly into one of the shell ribs, for instance. Might come in handy for other in-camp niceties like a clothesline or even a hammock.

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Old 04-13-2013, 07:07 AM   #23
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Yes, distance up, and away are factors, as are weight of bikes, weight of rack, weight of receiver (to some extent). Slop in receiver to rack plays into the formula as well.
I totally agree! A receiver mounted hitch extend out almost 10" from the trailer body to frame attachment seam. My Thule T2 extended out another 3 1/2' out from the bumper. The typical non-carbon mountain bikes weigh 26 lbs. x 2 = 52 lbs. A moderate up/down 3g acceleration (multiples the weight X 3) of a dip or bump in the road you suddenly have over 150 lbs. of weight (counting the weight of the rack) flexing in an arc at the end of a 4' arm. Over time this will cause significant damage to the AS. This is why AS does not recommend a hitch receiver bike rack.

AS recommended rack:

On the hand, the AS bike rack is mounted rigidly to both frame and structural members of the trailer close in to the end of the bumper. This greatly reduces the "arm" and potential of damage.

The video below from the AS customer service site provides a detailed installation guide for mounting the rack. About 3/4 the way through you will see that the top braces are firmly attached to the AS vertical frame members by 4, #10 screws (holes drilled into the frame).

An AS bike rack that is not attached in this manner is incorrectly installed, leading to potential damage to the trailer and loss of your bikes.

http://www.airstream.com/service/customer-support/

Hope this helps! :-)
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Old 04-13-2013, 07:51 AM   #24
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We have the Fiamma rack from AS. It works well. I completed the installation in about 2 hours, being very careful.

I would guess the weight of an empty steel rack that fits a receiver, as pictured above, is probably close to the weight of the Fiamma and 2 bikes.

But, the way the Fiamma carries the weight of the bikes is completely different than a receiver mounted rack. The wheel shelf is low and near the frame attachment points.

The rack on Wally’s trailer is much the same.

The receiver mounted rack acts as a big lever, transferring all the weight to a single point.

A receiver rack has the dough-pop flop action when the rear of the trailer bounces, no matter how little it moves when one tries to shake it. All the weight of the rack and bikes is suspended a fair distance from the rear of the trailer. When the trailer hits a bump in the road the rack flops, transferring stress loads into the frame of the trailer.

How much damage could one do to the rear of a trailer with a pry bar, prying against the rear of the trailer?

The Fiamma does put most of the weight on the frame, but much closer to the rear of the trailer than a rack designed for a receiver.

It also manages the flop factor by the attachments to the back of the trailer and the low, close mounted wheel shelf.

Just my thoughts.

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Old 05-26-2013, 10:15 AM   #25
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That may cause rear separation. Look at the approved AS bike rack and take note of the additional attachment points. You can find many threads debating the issue you have to judge for yourself. I went with the approved AS bike rack because I did not want to create warranty issues on a new trailer. I do like the concept it has been around for years just never would risk the intergearty of the shell to frame joint. BTW The AS bike rack has a 75 lb weight limit which is 2 bikes which tells me AS did not want to push the envelope to much.

The Fiamma AS Bike Rack transfers all of it's vertical force to the rear chassis beams and bumper. The shell/frame attachment points provide lateral stability and some counterforce to the horizontal component of the cantilevered force. Not a bad design, but it in no way reinforces the shell to chassis attachment. The main concern with a receiver hitch mounted bike rack would be to minimize the distance between the rear of the trailer and the load, as the cantilever force increases with every inch, creating a bending moment on the chassis. Older models with less chassis strength apparently suffered when such loads were applied.
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