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Old 04-20-2019, 03:34 AM   #1
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Bumper bikes

I'm betting this simple modification is already a product on the market...

The bike rack designed for Airstream seems fragile to me. I've talked to a couple owners and I don't think it is up to the task and it's expensive. I like my Kuat bike rack, but I think any high quality hitch rack will work with this modification.

1. Get a piece of 4-1/2" x 1" rectangular steel tube. Weld 6" flanges at each end. Buy a hitch receiver and weld it at the bottom side in the center. Total cost $40 ($29 steel, $11 receiver, Airstream friend who is a welder). Paint it, if you like (it will be hidden). Sorry for the rotated photo.
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2. Remove the bumper and slide the new hitch support in place. Drill two 1/2" holes through the frame at each end and bolt it in. You could do without the flanges and just weld the rectangular tube to the Airstream frame.
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3. Cut a small relief (about 1/2" high) in the bumper for the receiver. You can use short machine screws and the original holes for the top attachment and either ignore the bottom attach points or drill and tap into the rectangular steel tube. (This all depends on the vertical placement of the rectangular tube. Mine is about 3/8" up from the bottom flange of the Airstream frame beams, but it could be flush with the bottom flanges.) You can see my Kuat bike rack is installed in this photo.
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4. An almost invisible installation and plenty strong for supporting a couple of bikes.
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Old 04-20-2019, 05:31 AM   #2
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It certainly looks tidy. I think every time this type of solution is discussed in these forums, the general consensus is that the Airstreamís construction doesnít tolerate such a hitch well (the amplified bouncing of the bikes on the hitch leads to the frame separating from the shell) and that the weight of the bikes and their distances behind the axles significantly increases the likelihood of sway based on mathematical formulas I donít understand but are meant to describe levers and moments and basically, itís apparently pretty problematic.

Some who have done such a mod say they have no problem with it. Others have tried it with less than happy results and come up with other solutions.

I think you do good work and wish you well. The purpose of this reply is to prepare you for the string of responses that may follow if history is any indicator of what to expect.
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Old 04-20-2019, 05:51 AM   #3
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Originally Posted by SteveSueMac View Post
It certainly looks tidy. I think every time this type of solution is discussed in these forums, the general consensus is that the Airstream’s construction doesn’t tolerate such a hitch well (the amplified bouncing of the bikes on the hitch leads to the frame separating from the shell) and that the weight of the bikes and their distances behind the axles significantly increases the likelihood of sway based on mathematical formulas I don’t understand but are meant to describe levers and moments and basically, it’s apparently pretty problematic.

Some who have done such a mod say they have no problem with it. Others have tried it with less than happy results and come up with other solutions.

I think you do good work and wish you well. The purpose of this reply is to prepare you for the string of responses that may follow if history is any indicator of what to expect.
Well said . . . thank you SSM for a cogent summary!

Zepp, FYI here are the search results for "rear hitch bike rack" --

https://www.google.com/search?q=rear...com&gws_rd=ssl
"rear hitch" -- https://www.google.com/search?q=rear...=airforums.com

And here is one of the main threads:

http://www.airforums.com/forums/f464...er-174590.html

If you want to expose your family and the motoring public to unnecessary risk, by adding excessive weight [beyond Airstream's design parameters] that far behind the axles, feel free . . . IMO.

Good luck,

Peter

PS -- To go into the proposed modification with eyes wide open, you might want to read the many Sway/Roll-over threads like:
-- Quebec roll-over:http://www.airforums.com/forums/f48/...ec-153984.html
-- Out of Control Sway: http://www.airforums.com/forums/f238...ay-152451.html

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Old 04-20-2019, 09:44 AM   #4
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I made a similar hitch. It hung below the tray but attached to the side rails like yours.

I did not like the way it performed (too much bounce), And, because of the increased chance of sway, I took it off. It's still laying around somewhere. I need to repurpose it for some other use.
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Old 04-20-2019, 09:59 AM   #5
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Not that anyone asked, but my assessment is that it is not so much the weight as it is the distance behind the bumper and the lack of support to prevent bouncing. I think a bike rack that emulated the design of the Airstream-approved rack, i.e. bikes above the bumper instead of behind, and support attachment to the ribs and skin at the top, would work much better.

I put a front receiver on my truck and used a good quality bike rack in it but I was not happy with the amount of bounce I saw. It would be worse on the rear of the trailer due to the porpoising and the distance to the wheels. There are videos on the web showing the bouncing of bikes on a rack behind a white box. The amount of bounce depends, obviously, on the stiffness of the mount and rack, but my rack was a fold-down version that had quite a bit of slop in the pivot point.

We gave up and bought folding bikes that ride in the back of the TV.

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Old 04-20-2019, 11:00 AM   #6
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I agree with you that much of the stresses is due to the punishing bouncing that could occur. It is as important of a factor in the design as anything.

Factors to ride quality at the rear of the trailer include the tow vehicles ride quality itself and what forces it projects into the tongue, that are further magnified at the rear position). The quality and condition of the trailers axles. And additional play in the receiver, bike rack, and bike mounting system could further exacerbate it.

In my particular setup, my tow vehicle is plush riding. I added a hitch tightening bolt within the bike rack receiver. And utilize a bike rack that is strong and rigid to minimize play. My rear facing camera on the AS shows very little movement of my bikes under tow.

To your point, many standard hitch bike racks are not rated for use at the back of the trailer.
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Old 04-20-2019, 12:16 PM   #7
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Quote:
Originally Posted by SteveSueMac View Post
...the weight of the bikes and their distances behind the axles significantly increases the likelihood of sway ...
I see lots of people worried about sway. I've never seen it, even when maneuvering aggressively. But the two vehicle is a diesel and I think that makes a big difference. Back in the day when we were towing with a Ford Explorer, anti-sway bar and max of 55 mph was the order of the day.

The bikes are light, but the Kuat bike rack is heavier than they are and it has an articulated joint for folding up. I would guess the whole thing will weigh just a few pounds more than a second battery, but unfortunately is further from the axles.

In one of the recommended threads, I saw Barts say that a Sherline tongue weight gauge is an essential tool. I just ordered one.

Bounce is another issue. I guess I'll get a camera on it. But I have a front hitch on the diesel and the bounce up there was pretty terrible (and the bikes totally blocked the view forward at night). I don't think it'll be any worse in the back, but a test tow will tell the tale.

I'm thinking the best answer is to stay away from that horrible section of the 405, just as you're leaving the Valley heading south.

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Old 04-20-2019, 12:54 PM   #8
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If you try it and it works for you it's good👍...If someone else tries the same thing and it's not good for them👎...it's also good.👍 🤔
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Old 04-20-2019, 01:03 PM   #9
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Sway should not be a problem. If you compare the inertia of the bikes and rack with the inertia of the trailer as a whole you will see that it will not make a big difference. However if you are concerned then you could move some other cargo in the rear the trailer to a point closer to the axles to compensate.
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Old 04-20-2019, 01:29 PM   #10
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The point is to not get complacent with load on the rear of a trailer. Even if you have an HD truck. It's that attitude that will lead to an incident.

I regularly camp with a group where we take turns towing a communal trailer. It's a small to medium size utility trailer with cooking/bbq and camp equipment. Probably 2-3k lbs at most.

Even this little trailer when loaded out wrong and the heavy stuff behind the rear axle of the trailer WILL cause a newer F250 to sway. It's not a joke and an HD truck has never been a silver bullet in this regard to compensate for a badly loaded trailer.
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Old 04-20-2019, 02:39 PM   #11
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I've had the Fiamma/Airstream bike carrier on three Airstreams. Install them correctly and they work well. Never had a failure. Weigh only 25 pounds.

Pay attention to the 75 pound combined bike weight limit. Don't carry your heavy e-bike.

I've seen two rear receiver type carriers, installed by Airstream dealers, fail from cantilever torque induced weld failures shortly after installation. This is a cost saving DIY project that could cost a lot more in the end.
They may also contribute to sway because of the weight concentrated further to the rear than the engineers intended.
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Old 04-20-2019, 04:22 PM   #12
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We've towed over 20K miles with two bikes on the back of our 26' Overlander without an issue. Our receiver was installed differently, but I'm sure it was welded as solid as Zepp's. As an avid fan and copier of Zepp's designs, I would not be worried. We've never had sway issues with two older mountain bikes on the rear hitch. We use a Reese Dual Cam High-Performance Sway Control.

There are other options if you don't want to expose the bikes to the weather:

If you are concerned and/or able to carry your bikes inside the trailer, search for lcurrie's post. She installed a fork mount to hold the bike inside the trailer.

Now that I carry a fancy carbon bike, I had a plate made that I bolted into the back of my Crew Max. It features a fork mount that holds the bike inside the truck. We just fold the seats up for transporting the bike and fold them back over the fork mount when we need the seats for passengers.
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Old 04-20-2019, 04:59 PM   #13
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Now that I carry a fancy carbon bike, I had a plate made that I bolted into the back of my Crew Max. It features a fork mount that holds the bike inside the truck. We just fold the seats up for transporting the bike and fold them back over the fork mount when we need the seats for passengers.
I like! But I only have an X-cab, not a crew cab. Is that big enough or do I need to do the experiment myself?

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Old 04-20-2019, 05:04 PM   #14
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If you try it and it works for you it's good👍...If someone else tries the same thing and it's not good for them👎...it's also good.👍 🤔
Bob, I am entertained. Or as Calvin would say, "let's go exploring!" I'll get back to this thread once I get my tongue scale and some actual numbers...

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Old 04-20-2019, 05:42 PM   #15
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Very nice work ... consider doing the same thing for your TV ... added bonus is that the bike(s) will not load up with road debris from the low pressure area behind the AS. Hard to get even a light coating of grit off of the lubricated parts of your bike ...
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Old 04-20-2019, 07:45 PM   #16
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Bob, I am entertained. Or as Calvin would say, "let's go exploring!" I'll get back to this thread once I get my tongue scale and some actual numbers...

Zep
IMO, your tow vehicle will go a long way to counteract sway.

Trailer balance ratio, tongue weight equaling 10 to 15% of gross trailer weight on the ball, is important. So, weighing is important. However, it is not the determining factor of safety in the case of adding bikes and hitch in the rear.

As we all know, the trailer's load should be concentrated over and or just in front of the balance point (axles) to be correct.

Sway is a movement in a horizontal arc ( or yaw). Any additional weight at the rear (behind the axle) is not helpful in resisting sway, in fact it will contribute to sway. The further back weight is placed behind the axles the more it contributes to sway.

Example - try this: Hold your hands down by your hips and rotate your hips. Your hands will hardly move. Next hold your arms straight out from your sides, then rotate quickly at the hips. When your hips reach that stopping point your arms move slightly further around the arc. Now do the same thing but hold a weight in each hand. Arms will move further after hips stop, and with more force. In this example your body is the fulcrum, your arms are acting as a levers, resisting the rotational forces of the weights/your hands.

Imagine: Your bikes and hitch hanging on the rear. Your trailer frame rails are the lever arm, your axle/wheels are the fulcrum point, the bikes and hitch are the weight. The distance a lever arm extends from the fulcrum affects the factor of the weight being amplified. If the bike and hitch weigh 80 lbs and they are 8' back from the center of the axle, during a quick left then right avoidance maneuver the rotational force of the 80 lbs could be amplified to greater than 600 lbs. This is in addition the entire weight of the trailer and cargo that is already behind the axle which is also being amplified by the lever affect. This amplification of weight by the lever effect is how/why weight on the back end of a trailer is a problem, not the up down movement or gravity weight.

ps:
The main thing resisting this horizontal motion of the trailer are those two patches of tires on the rear axle of the tow vehicle touching the road. Adding weight in the rear of the trailer will lift some weight off of the tongue which will decrease friction/resistance of the tires on the road surface. In this case less weight on the tongue does affect the resistance of that horizontal sway movement.
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Old 04-20-2019, 08:06 PM   #17
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Everybody does the bike thing their own way. My experience with sand, grit, and rust on the chain, screws, sprockets, etc. propelled me to purchase a cap for the truck, and place the bikes in there.

I wouldn't attach bikes to the rear of my airstream, or the front of my truck.
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Old 04-20-2019, 11:59 PM   #18
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Quote:
Originally Posted by A W Warn View Post
ps:
The main thing resisting this horizontal motion of the trailer are those two patches of tires on the rear axle of the tow vehicle touching the road. Adding weight in the rear of the trailer will lift some weight off of the tongue which will decrease friction/resistance of the tires on the road surface. In this case less weight on the tongue does affect the resistance of that horizontal sway movement.
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?
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Old 04-21-2019, 12:07 AM   #19
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Here's a simple thought experiment. Image that the Airstream is a long rod with a mass located about 1/3 of the way along the rod. The tow vehicle is tied to the front end of the rod, so you have a pendulum behind the tow vehicle. Add a damper to the the system (tires on the road), which would be a force applied at 90 degrees to the direction of travel at some point along the rod. If the mass is forward and the damper is well aft, the pendulum can't swing. As you either move the mass aft or the damper forward, the pendulum becomes less restrained. At some point as the mass and damper move towards each other, or heaven forbid, reverse their relative positions to the tow vehicle, oscillation becomes possible if there is an excitation force of some kind (jerky steering, wind, pothole on one side).

Yay for the good ole' mass-spring-damper differential equation you all studied in high school...

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Old 04-21-2019, 05:28 AM   #20
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...snip....

Yay for the good ole' mass-spring-damper differential equation you all studied in high school...



Zep




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....
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