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Old 08-16-2018, 08:39 PM   #1
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Bumper Bike Rack as part of rear frame upgrade

We do carry our bikes in the tow vehicle, it keeps them dry and secure and does not lengthen the tow. To do so we need to take the front rim off. We have found when traveling between camps it is very handy to use a bike rack. As a note about the rear frame strength: during the trailer restoration the rear three feet of the frame was strengthened with 3/16 plate the only sections not address was the inside the rear storage.

This project accomplished that part of the upgrade along with adding a receiver hitch. The receiver hitch was located within the curve of the the rear bumper maintaining the size of storage area. The result is the receiver section is proud of the bumper; if the receiver portion of the hitch had been placed flush with the bumper the storage space would have been reduced to half it original size.


images: storage frame interior sistered with 3/16" plate, hitch mounted to new plate, bumper installed showing bike rack
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Old 08-16-2018, 09:15 PM   #2
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Nicely done! Looks like it boxes in the frame rails in the rear which should strengthen them nicely.

I'm actually in the midst of doing the same thing and have my bumper off. Slightly different approach. Hoping to finish it next weekend. Thanks for sharing.
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Old 08-17-2018, 05:05 AM   #3
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Adding that much weight to the rear may run afoul of Airstream’s design parameters, which generally limit a bike rack to a Fiamma holding two regular bikes. More weight than this could introduce serious sway issues IMO.

Fiamma weight: https://www.google.com/search?q=Fiam...com&gws_rd=ssl
Out of control sway: http://www.airforums.com/forums/f238...ay-152451.html
Quebec rollover: http://www.airforums.com/forums/f48/...ec-153984.html
http://www.airforums.com/forums/atta...5&d=1469187521

The life you save may be your own.

Good luck,

Peter
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Old 08-17-2018, 07:46 AM   #4
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Quote:
Originally Posted by OTRA15 View Post
Adding that much weight to the rear may run afoul of Airstream’s design parameters, which generally limit a bike rack to a Fiamma holding two regular bikes. More weight than this could introduce serious sway issues IMO.

Fiamma weight: https://www.google.com/search?q=Fiam...com&gws_rd=ssl
Out of control sway: http://www.airforums.com/forums/f238...ay-152451.html
Quebec rollover: http://www.airforums.com/forums/f48/...ec-153984.html
http://www.airforums.com/forums/atta...5&d=1469187521

The life you save may be your own.

Good luck,

Peter


It will be more than fine. It’s one steel tube and this set up has to be only a few more pounds than the Fiamma rack.

Thank you for sharing. This is an excellent design! I was going to begin working on something similar this weekend but now may reevaluate to incorporate more of your approach.
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Old 08-17-2018, 08:57 AM   #5
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I did fabricate the hitch, I will get the weight for you-all. The 2" square tubing lands on 3" square 1/4" plate to make it easy to align the hitch to the strengthened frame. The bumper curve requires the lower corner of the 3" square plate to be trimmed. Next time I would shift the square plate so the bumper side would be flush.

The 3/16" plates were 5X16" which comes to just over 4 pounds each. I had already added the steel on the outside in a previous project going inside the trailer using a stitch welding method to minimize distortion.

I also used compresses air to cool the welds to reduce the fire risk.

As a side note I believe towing causes abrasion on the metal frame. I have added the same type of metal to the wheel wells, then applied the classic automotive undercoating to reduce the future abrasion.

Thanks you-all for contributing
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Old 08-17-2018, 10:09 AM   #6
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Thanks for the update. In addition to the extra weight, another factor to be considered [vs. the Fiamma set up which is located entirely over the rear bumper storage area], is that the new center of gravity of the hitch/bike rack/bicycles mass will be much farther back [behind the rear axle] than the Fiamma set up, which is tucked in nice and tight to the Airstream's back window.

The increase in the radius of this extra weight must be squared, thus increasing the moment of its force on the trailer. Just a lay understanding of this, but it is nothing to sneeze at. That "Out of control sway" thread linked earlier has lots of comments on this but it is indeed a very long thread.

Good luck towing the new rig loaded with bikes. I hope you have good sway control.

Peter

PS -- Here is the image which was linked earlier from the Quebec rollover thread, from Post #99 there. The storage box on the back of that AS was much larger than your rack/bikes will be, but the image is a good wake-up call that this is a serious matter IMO. BTW that storage box was thrown clear of the wreck, but if you read the posts after #99, there are some pretty good informed guesses about what the box looked like.


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Old 08-17-2018, 12:20 PM   #7
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Originally Posted by OTRA15 View Post
Thanks for the update. In addition to the extra weight, another factor to be considered [vs. the Fiamma set up which is located entirely over the rear bumper storage area], is that the new center of gravity of the hitch/bike rack/bicycles mass will be much farther back [behind the rear axle] than the Fiamma set up, which is tucked in nice and tight to the Airstream's back window.

The increase in the radius of this extra weight must be squared, thus increasing the moment of its force on the trailer. Just a lay understanding of this, but it is nothing to sneeze at. That "Out of control sway" thread linked earlier has lots of comments on this but it is indeed a very long thread.

Good luck towing the new rig loaded with bikes. I hope you have good sway control.

Peter

PS -- Here is the image which was linked earlier from the Quebec rollover thread, from Post #99 there. The storage box on the back of that AS was much larger than your rack/bikes will be, but the image is a good wake-up call that this is a serious matter IMO. BTW that storage box was thrown clear of the wreck, but if you read the posts after #99, there are some pretty good informed guesses about what the box looked like.


Posting this image in response to the OP is like shooting a flea with an elephant gun. Assumes that everyone in the world is outright stupid and suicidal.
Please!
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Old 08-17-2018, 01:13 PM   #8
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As an engineer.....

I couldn't resist and I was curious myself so I did a CG analysis. I recognize one must also consider the polar moment, but I couldn't do that analysis quickly. For my 9000# Classic 30, taking the location of the axles and overall length from memory of a previous measurement, adding 60# of bicycles and rack centered 2' aft of the bumper moves the CG aft 1.25". I would hope the Airstream's stability is sufficient to tolerate that small a shift in CG.


With apologies for plagiarism, "I'm not a mechanical engineer, but I play one on TV", so take this for what it's worth.


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Old 08-17-2018, 01:15 PM   #9
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Quote:
Originally Posted by OTRA15 View Post
Adding that much weight to the rear may run afoul of Airstream’s design parameters, which generally limit a bike rack to a Fiamma holding two regular bikes. More weight than this could introduce serious sway issues IMO.
Would appreciate the help by posting a link to the actual Airstream design or rear loading parameters that you mention. What document contains these?

Our manual did not have any load restrictions for the rear storage area under our bed or the pull-out storage compartment in rear bumper. Both are within a couple feet of where a rear-mounted bike rack would be.

73/gus
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Old 08-17-2018, 03:42 PM   #10
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Yes; please ...
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Old 08-17-2018, 05:27 PM   #11
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I would not be concerned about the rearward shift of centre of gravity. Instead, I would be concerned about stressing the connection between the frame and the shell.

As anyone who has dealt with tail droop on a 1970s Airstream knows, the frames are light and relatively flexible, and the shell actually holds the frame up. The connection takes the form of small bolts connecting the shell to the perimeter of the plywood floor, and bolts attaching the plywood floor to the frame.

That is the reason the custom Can Am bike racks and the Fiamma bike racks are attached to the shell. The shell carries the weight of the bikes, and the connection to the bumper is primarily for stability.
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Old 08-17-2018, 05:28 PM   #12
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FYI, I measured the hitch to be 20 pounds. I am sure I could have made it with the thin wall square pipe and reduced the weight in half. It is certainly stronger than necessary.

I would guess that AS did their calculations with the rear bath design. I would think the full holding tank(s) would dominate the stability calculations requiring them to increase their guard band to account for the added weight and the fact that it is a liquid which means it will also be moving inside the tank.
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Old 08-17-2018, 06:57 PM   #13
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Quote:
Originally Posted by gklott View Post
Would appreciate the help by posting a link to the actual Airstream design or rear loading parameters that you mention. What document contains these?

Our manual did not have any load restrictions for the rear storage area under our bed or the pull-out storage compartment in rear bumper. Both are within a couple feet of where a rear-mounted bike rack would be.

73/gus
This page shows the Fiamma bike rack designed to hold 75 lbs., and located over the rear storage area:

https://store.airstream.com/product/...kes-by-fiamma/

The Fiamma search results linked in Post #3 contain many threads/posts, some of which have more specific info from AS as I recall, and I will look more carefully tomorrow. The weight of the Fiamma rack itself is also stated someplace. I think it is about 30 lbs., for a total weight of 105 lbs. +/- allowed back there. More importantly, the CG of the allowed weight is forward of the back of the OEM bumper, not aft of it.

Peter
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Old 08-17-2018, 07:07 PM   #14
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. . .
. . . one must also consider the polar moment, but I couldn't do that analysis quickly.
. . .
This is the calculation I was suggesting needs to be considered, not just the linear change in the CG. If you put 100 extra lbs. of weight at the very of the bike rack assembly here, and accelerate that weight during an evasive road maneuver, the angular momentum of that weight so far behind the rear axle, and also further back than the Fiamma CG, will be like the tail wagging the dog IMO.

Peter
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Old 08-17-2018, 08:34 PM   #15
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I misspoke. It is not the polar moment, but the moment of inertia. 60# of rack and bikes does in fact produce a significant, albeit small, reduction in the moment of inertia, about 10%.

I suspect the more significant effect is, as AlbertF noted, the stresses induced on the structure by the g-forces when going over bumps. Attaching the bike rack to the shell distributes these forces between the shell and the frame. Until we recently bought some folding bikes I can easily put in the bed of the truck I was going to add a receiver to the rear of the trailer and modify my bike rack to attach it to the shell along the lines of the Fiamma rack. I got tired of them getting dirty while they were bouncing around on the rack in my front receiver.

Al
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Old 08-18-2018, 06:50 AM   #16
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Please remember I stiffened my rear frame including the area inside the trailer to provide more static and dynamic resistance to movement. We don't know how each trailer was made and the potential "preload" of separation forces between the frame and shell due to its construction. I am familiar with frame welding and that process will bend the frame; which could help or hurt this type of trailer.

In my view, Rear or Front separation occurs because there is a difference in the motion of the frame and shell. The resistance to movement is based on the stiffness of the materials along that axis. Ideally they would deviated the same amount at the same rate therefore resulting is no opposing forces to cause the attachment system to fail. With all that said, I think you really want to see the engineering measurements of the reaction of the trailer to forces at the hitch and axles, along with the strain on the shell skin. Unfortunately even if this work has been done it won't include: old frames, rusted out frame members, modifications, bad shock absorbers, and axles that have lost there responsiveness.

Adding weight to the skin for a bike rake doesn't help frame separation it hurts it.


Thanks for all the contributions.
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Old 08-18-2018, 07:47 AM   #17
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This is the calculation I was suggesting needs to be considered, not just the linear change in the CG. If you put 100 extra lbs. of weight at the very of the bike rack assembly here, and accelerate that weight during an evasive road maneuver, the angular momentum of that weight so far behind the rear axle, and also further back than the Fiamma CG, will be like the tail wagging the dog IMO.

Peter
Three years ago I fabricated my own rear hitch mount that I bolted to the skid channels welded to the rear end of the frame. Purchased a Sari bike rack. The hitch, bike rack and our two bikes weigh a total of 137 lbs. It's riding on the back end of our 30' Classic and before that the 30' International. Last fall I had to do a sudden crash avoidance at 70 miles per our, full lane change and back. No the rear end of the trailer did not split off, I did not sway out of control and did not roll on my side. Both sides of the trailer wheels lifted of the pavement back and forth but settled down immediately.
If an Air Stream is that badly designed and so flimsy that 200 lbs in back throws it of balance rips apart and rolls over they shouldn't be on the road.
And to suggest that only safe way to do a bike rack is to install a Fiama or have Can Am install one is down right ridiculous.
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Old 08-18-2018, 09:57 AM   #18
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I like the previous 2 posts and agree.
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Old 08-18-2018, 10:02 AM   #19
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The basic point all along has been that exceeding Airstream's design parameters is a risk which should be undertaken intentionally and consciously, that's all.

Can someone push the envelope and succeed? Of course. Until there is an accident. Even a broken clock is correct twice a day.

Newcomers, however, often do not appreciate the risks, and are especially susceptible to apparently-expert advice on these matters, which is basically "go for it" -- without any caveats or disclaimers about the limitations of the advice, or the inherent risks.

Read the Out of Control Sway thread for the extended learning curve, of an owner who finally "got it" that he was risking his family's life and limb with his initial approach. [link in Post #3]

"Eyes wide open basically."

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Old 08-18-2018, 10:11 AM   #20
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Bumper Bike Rack as part of rear frame upgrade

I think the OP appears to have his eyes wide open.

When I located 300lbs of lithium batts and related gear under my rear queen bed in my 2017 30’ international I did extensive homework to include securing airstream frame layout design schematics... and part of my due diligence including speaking with airstream and one of the most experienced airstream commercial uplifters in the country who often change out frames for commercial applications....

I had several dialogue with both parties and asked if they could produce airstream “design parameters”. They could not produce them for me and airstream also indicated that airstream engineering have not done any formal frame / shell load testing of said design parameters.

So all the conjecture about weight aft of the rear axles is hearsay without data or real world test results. Putting a bike rack on a 30’ trailer is not the same as loading 300lbs of gear on the back of a 25’ trailer like my fellow Quebecer we did so one can not compare the two. Each instance of loading a particular trailer with any kind of weight anywhere in or on the trailer should be treated as an individual test case.
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