Bumper Bike Rack as part of rear frame upgrade

[franklyfrank]

Originally Posted by waninae39

FYI

just like the FIAMMA rack, it best to have some upper support on the back ( ie close to the rear lights where the vertical AS internal frames are)

this helps resisting twisting and torque movement as your are driving.
the tongue moment will try to lift the front end as the bike moves back and forth

my 2 cent as a long term Professional Engineer

So, let me see if I understand this correctly.
There is in most cases 1,000 lbs sitting on the trailer tongue.
3 to 4 tons sitting on the axles
Therefore 125 lbs of hitch and bike combo bolted solidly to the rear end of the frame will whipsaw all of that mass back and forth.
Got it !

[pteck]

Originally Posted by franklyfrank

So, let me see if I understand this correctly.
There is in most cases 1,000 lbs sitting on the trailer tongue.
3 to 4 tons sitting on the axles
Therefore 125 lbs of hitch and bike combo bolted solidly to the rear end of the frame will whipsaw all of that mass back and forth.
Got it !

He's not wrong.

Geometry of structures and projection of forces, are completely different between what you point out. May be worth keeping an open mind.

[franklyfrank]

[QUOTE=pteck;2440663]He's not wrong.

Geometry of structures and projection of forces, are completely different between what you point out. May be worth keeping an open mind.[/QUOTE

Please !

I have spent 40 years of my life managing my own Infrtructure construction business.
Utilizing 3 heavy/wide load lowboys moving excavators weighing 165 k lbs to shovels and everyting else inbetween, building roadways to wastewater treatment plants. I also owned 25 semidumps so my daily life was a constant concern with loading and weights.
I have been towing my Airtsream all over the country with 2 bikes on a bike rack on back end for the last five years without any problems whatsoever.
What you are implying is complete nonsense scaring inexperienced newbies away from viable and much better options than the Fiamma bike carrier.
BTW for what its worth I am a civil engineer . What I learned about engineers along the way is that most are very rigid and loth being challenged.

[panamerican]

At the very least, it will promote rear end separation. For those that are not familiar with this phenomenon. Anything placed on the back of the trailer that hangs out will exhibit a cantilever type situation.

If you look at most rear Airstream frames, you would see reinforcement tabs under the frame where the shell meets. If you place load in the manner that was suggested, regardless if you loose control or not, you WILL begin at some point to see the body separate from the frame. There are only a few bolts holding it on back there and the cantilever effect will add unplanned stresses to that section of the trailer. This is why a special designed bike rack is sold.

I have seen others place a hitch but it's attachment point to the frame was further in from the reinforcement tab, negating the issue.

One other thing to point out, heavy machinery is not the same as a semi-mono alum shell with a few bolts holding it to a frame. Distortion will happen much sooner with alum than with heavy steel of construction equip.

If you contact the factory and ask them if this is recommended, the factory will give you a flat out no, don't do it, for the reasons outline here and possibly more.

[waninae39]

I am an engineer also. I agree with @franklyfrank

adding a weight at the back simply moves the horizontal centre of gravity. This can be offset by adding a similar weight an equal distance to the front. This is how a lever work

Add 1/2 the weigh double the distance is the same.

Its best to go to a SCALE and get 2 runs, with and without WD and sway.

then, do the same again with whatever load you add.

then you can make a decision with FACTS as to what weight needs to be added and where

always measure BEFore and After. Then you can make a real decision, not some NAG or estimate

[Shiny16]

Originally Posted by waninae39

I am an engineer also. I agree with @franklyfrank

adding a weight at the back simply moves the horizontal centre of gravity. This can be offset by adding a similar weight an equal distance to the front. This is how a lever work

Add 1/2 the weigh double the distance is the same.

Its best to go to a SCALE and get 2 runs, with and without WD and sway.

then, do the same again with whatever load you add.

then you can make a decision with FACTS as to what weight needs to be added and where

always measure BEFore and After. Then you can make a real decision, not some NAG or estimate

So I guess it would be ok to put my small 250 pound dirt bike on a rear rack as long as I move 250 pounds of gear in cabin to the the front.

I think there might be a bit more to this.

[Mollysdad]

I've been following with some amusement. The 'engineers' point out that the Airstream frame is designed to work with the shell attached.
So, I think, won't the shell be attached when I'm adding my bike rack?

[panamerican]

There is a reason Airstream condones the Flamma carrier. Not at anytime will you find Airstream recommending a hitch be installed on the back of the trailer that does not connect north of the support tabs.

We can go round and round, but the proof is, I would encourage anyone having done this, or considering it to contact and open a support ticket with Airstream and you will quickly find out that attaching a connection in the methods that have been suggested, with all due respect to the engineers participating on this thread, will eventually lead to frame separation, where the shell slowly but surely begins to detach from the frame....again, contact Airstream technical support for the engineering details as to why it's not considered a best practice to do what I have seen here via the pictures. You may have several years where this does not seem to be an issue, however, over time, you may see that you have weakened the connection between the shell and the frame over time.

In the end, it's your trailer, and you can and will do what you feel is best, but when the body does eventually separate, you cannot say you were not warned- but please, don't take my word for it, contact Airstream directly and share what they suggest.

Unless Airstream changed something between all the conversations I've read on this subject, on this forum, I don't see any change, unless I see something in writing from Airstream saying you are good to go.

[bweybright]

Originally Posted by waninae39

I am an engineer also. I agree with @franklyfrank

adding a weight at the back simply moves the horizontal centre of gravity. This can be offset by adding a similar weight an equal distance to the front. This is how a lever work

Add 1/2 the weigh double the distance is the same.

Its best to go to a SCALE and get 2 runs, with and without WD and sway.

then, do the same again with whatever load you add.

then you can make a decision with FACTS as to what weight needs to be added and where

always measure BEFore and After. Then you can make a real decision, not some NAG or estimate

So you as an engineer are willing to help people like me who are not engineers figure this out on a forum? That is an amazing gift you are offering. Thank you....

[Gigler]

When will Airstream provide a hitch mount as an alternative to the Fiama bike rack?

[Flht2k]

This is a great topic. Airstreams in the past had hitches attached to the frame. Not sure why they stopped, but I have seen many hitches attached to the frame and the hitch end protrudes out below the bumper. Airstreams of portland installs these all day and all night for 750. I would rather have this system than the fiamma system anyway.

Just my pennies opinion

[ricjones]

Been reading a lot about this. Finally found a person who got a custom welded fabrication welded right into the frame. And he has been full timing for 2+ years with no added sway and no frame separtion.

https://www.youtube.com/watch?v=Zzpwm7fVZts&t=706s

[franklyfrank]

Originally Posted by Al and Missy

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.


Al

My sentiments exactly.
There is a very simple and sure fire way to fabricate a 2" receiver that slides into the two skid u channels welded to the end of the frames from the factory.
I made one , having extensive experience in fabricating and welding.
Light weight high strength tubing the weight is 22 lbs simply bolted to the Skid U channels. No need to weld anything to the frame.
Two bike and a bike rack came to a total of 134 lbs.
Been using it on three different AS the current one being a 30' Classic since 2012 and I am here alive and well writing this today.
I looked at the FIAMA and simply didn't like its looks and fact that the struts had to be screwed into the Aluminum skin .

[foobar]

Torklift makes a receiver/hitch that is designed specifically for airstream trailers.

https://torkliftcentral.com/airstrea...ry-hitch-x8000

You drill 4 holes in total in the skid plates under left/right frames under the bumper (2 holes in each skid plates), then bolt the torklift hitch to the skid plates.

Easy-peasy

I plan to purchase and install one of these. I will buy a lightweight bike rack to place in the receiver and will place two ultra-light mountain bikes on the rack. Hitch tighteners (or equivalent) will be used to ensure that the rack does not rattle and bounce around in the receiver.

I don't believe there will be any perceivable impact to trailer handling or stability if I properly balance the loads in the trailer and keep the tongue weight at an acceptable level vs the total weight of the trailer (unfortunately, I too am an engineer and understand statics and dynamics).

As to the risk of frame distortion or frame to shell separation? I accept that risk and will make any necessary repairs/strengthening efforts if/when it occurs. I will monitor that area closely as time passes.

[foobar]

Thought I would add that Ultimate Airstream frequently custom fabricates and installs receivers below the bumpers. Their ultimate implementation ends up very similar to what is available off the shelf from Torklift (Torklift is cheaper though).

[foobar]

Attached two photos that I downloaded from the torklift website. Thought a couple of photos might help us understand the design a little better.

[foobar]

Better photo. Took me a minute to figure out how to convert it from webp format into jpg format so that it could be uploaded.

[OrangeCrush]

Originally Posted by foobar

I will buy a lightweight bike rack to place in the receiver and will place two ultra-light mountain bikes on the rack.

Shop carefully for the rack. 1UP only approves Super/Heavy duty rack 2 bike max for RV use. Kuat has a special model. Neither is light. Call 1UP and they will explain why which may help you understand the concerns.

[Shiny16]

Originally Posted by foobar

Torklift makes a receiver/hitch that is designed specifically for airstream trailers.

https://torkliftcentral.com/airstrea...ry-hitch-x8000

You drill 4 holes in total in the skid plates under left/right frames under the bumper (2 holes in each skid plates), then bolt the torklift hitch to the skid plates.

Easy-peasy

I plan to purchase and install one of these. I will buy a lightweight bike rack to place in the receiver and will place two ultra-light mountain bikes on the rack. Hitch tighteners (or equivalent) will be used to ensure that the rack does not rattle and bounce around in the receiver.

I don't believe there will be any perceivable impact to trailer handling or stability if I properly balance the loads in the trailer and keep the tongue weight at an acceptable level vs the total weight of the trailer (unfortunately, I too am an engineer and understand statics and dynamics).

As to the risk of frame distortion or frame to shell separation? I accept that risk and will make any necessary repairs/strengthening efforts if/when it occurs. I will monitor that area closely as time passes.

Why worry about a light weight bike rack when its “approved” for 450pounds.

[foobar]

Originally Posted by Shiny16

Why worry about a light weight bike rack when its “approved” for 450pounds.

Hi Shiny16.

The weight rating is for the torklift hitch by itself. Basically, it simply means that the torklift hitch isn't the weakest link in the overall structure. The 450lb hitch rating doesn't tell you anything about the strength of the structure you are mounting it to.

I'll exaggerate intentionally in order to make the point as clear as possible.

If I mount the 450lb rated torklift hitch to a couple of panes of glass and then put a bike rack on the torklift hitch, which do you think will fail first? The torklift hitch? Or the panes of glass?

So even if the torklift hitch is rated at 450lbs, it doesn't really tell you anything about the ability of the Airstream structure to actually support 450lbs of loading ... especially when dynamic forces are involved. Picture mounting a tennis ball on the end of a thin wooden rod. Then hold the opposite end of the wooden rod and shake your hand up and down. The tennis ball will bounce around wildly until the rod breaks. This is what everyone is worried about when they discuss mounting bike racks at the end of an airstream.

Real world example. A friend of mine bought a lightweight SOB trailer to pull behind a Nissan pickup truck. Then he decided to mount a bike rack on the back of the lightweight trailer and take the trailer to Alaska. During the trip to Alaska, he ended up breaking the frame and wall structure of the trailer due to the extra weight of the hitch+bicycles on the end of the frame and the extra dynamic torque events it placed on the trailer structure. The trailer manufacturer would not honor the structural warranty because the trailer was not designed to handle the dynamic loads of a bike rack on the rear of the trailer (exacerbated by the rougher dirt roads in Alaska). This type of failure with respect to carrying bikes on the rear of the Airstream is what some folks on this thread are worried about.