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Old 02-18-2011, 01:11 PM   #15
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I agree with SpaceEgg. If you have a reasonable solid rear bumper, I can't see where you are going to cause any problem carrying a 50 lb bike and bumper rack on it. One of the best books I ever read was the auto biography of Bill Lear (designer of the Lear 8 track player and the Lear Jet). What drove him was designing stuff that others said was impossible to do. He did not believe what some engineers told him that could not be done, until he tried it himself. Some times he was wrong, some times he was correct!

JW- I am going to order the front Class III rack for my Tundra to carry a light dual sport motorcycle on the front of my truck. It is only $122 from Curt Manufacturint. That would be a great way to carry lots of bikes. Headlight illumination would be the only issue (new solutions have new problems ya no).

Dan
Dan 50 pounds on the bumper, "IS NOT" 50 pounds.

Your dealing with a "moment arm".

That 50 pounds is multiplied by the distance in feet from the point of last support to the ground, which is the rear axle.

For the sake of numbers, if that distance is 10 feet, then the 50 pounds, suddenly becomes 500 pounds, sitting still. When the trailer hits a bump, that weight quickly multiplies.

This is the primary reason for "rear end separation", and should not be done.

Physics plays the part.

Andy
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Old 02-18-2011, 01:55 PM   #16
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OK,I think this applies to the newer (70`s up) with the new frame,it doesn`t seem to apply to the channel iron frames.How many pictures have you seen of the Cairo caravan where they are hauling 1 or 2 spares on the back bumper,or did Wally keep breaking frames on that trip?
In the 60`s Ca. had the optional rear bumper mount,with the alum. cover. Dave
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Old 02-18-2011, 02:07 PM   #17
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.................................................. ....

My traveling water-slide idea is now totally destroyed.
Don't give up so easily. Just put a couple wheelbarrow wheels on the bottom end of the water slide to carry most of the weight.

Ken
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Old 02-18-2011, 03:06 PM   #18
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What do you guys think of the custom installations by a few people where they have placed supports in the window area to take the force off of the bumper. I am not an engineer but I would think that you would be able to transfer the force away from the week point.
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Old 02-19-2011, 11:37 AM   #19
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Last year I puchased a 2 in. hitch receiver from the front of my 2005 Dodge Ram 3500 dually. It was a stock receiver made by Reese/Cequent Towing products. Curt hitch manufacturing also manufactures a similar model. It requires the removal of 4 bolts, 2 on each side of the frame, where the bumper brackets are mounted. Put the new receiver frame back on, reinstall the bolts, and you've got a hitch receiver good for 500 lbs. on the front of your truck. This should be adequate for 4 bikes. My truck is 2WD but the clearance is OK. I used it on a caravan last summer and carried 2 bikes for over 10K miles with no problem. I used racheting nylon straps (3) to secure the bikes to the carrier and the eyes on the frame below the bumper. Hitch manufacturers make this receiver for all makes. Check out some suppliers and the price is reasonable and won't stress the frame of your Airstream trailer.
Clever. Any photos to share?

500 lbs could support a waterslide....?
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Old 02-19-2011, 08:41 PM   #20
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Affect of 50 lb weight on the bumper

Andy- You are correct that 50 lbs at the bumper is not 50 lbs at the rear wheels and that physics plays a part. However the weight at the rear bumper is not 500 lbs though. Think of the trailer as a teeter totter. The weight of 50 lbs at the rear bumper of a 30 ft long trailer is offset by a 75 lb weight at the rear wheels (with a moment arm of 20 ft). This 50 lb weight also reduces the tongue weight by 25 lbs. The same physics principle will result in a 450 lb load from carrying a motorcycle in front of my Tundra to reduce the load on the rear tires by 150 lbs. However this will also increase the weight on the front tires by 600 lbs. The weights have to balance out.

Easyride- You make a very good point about carrying a spare tire on the rear bumper. This did not seem to cause any major problems.

Dan
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Old 02-20-2011, 05:36 AM   #21
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I think andy is referring to the issue that has been historically proven as true. It is not a permanently rigid teeter totter due to force being applied back there.

Putting a 50 pound bike 10 feet away from the center of the teeter totter results in 500 foot pounds of torque applied to the future hinge point ( assuming the rear end separation takes place and it is fully separated all the way until the axles)

When you hit a bump the torque applied is much higher.

The part of this that I believe is not entirely correct is that (mathematically at least) is that with even one rivet connecting the frame to the body, the body is helping out. Additionally, if the frame/body strength is actually enough that separation never occurs you are entirely correct.

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Old 02-20-2011, 06:18 AM   #22
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We purchased one of these. Expensive and it will only carry two bikes, but it is a great system.
Arvika
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Old 02-20-2011, 07:30 AM   #23
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Jack, did you mount yours on the trailer or truck. That looks like a smart solution.

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Old 02-21-2011, 06:02 AM   #24
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Ours is mounted over the propane tanks on the trailer. Clamps to the "A" frame. The rack itself is removable and your left with two vertical tubes which are not in the way of accessing the batteries, etc. I also use a small step stool to be to lift the bikes onto the rack. Not difficult to do in my case. Jack
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Old 02-21-2011, 07:30 AM   #25
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When I was a kid...

Dad had a removable bike rack mounted to the rear bumper of our '72 31' Sovereign Twin. It was capable of holding two bikes. We used it on occasion, but eventually just placed the bikes inside leaning up against the front gaucho sofa. This was due more to convenience than to fear of rear end frame separation. It was just easier to roll the bikes into the trailer than fool around with the rack.

The brackets for the bike rack were eventually used to hold one end of the bungees used to hold the rear bath awning after the actual awning fixture wore out.

The trailer did get a pretty good case of rear end frame separation. Was that due to using the bike rack? Who knows as lots of other similar vintage Airstreams got the frame separation as well without bike rack installations.

I intend to use roof mounted bike racks on my Jeep if I take bikes along, or throw them inside the trailer like we did more often as a kid. I understand the physics that Andy and others talk about- but also like the mention of the spare tires and other gear the Caravanners loaded onto their rear bumpers on caravans.
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Old 02-21-2011, 07:58 AM   #26
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I don't think comparing what they did on the big caravans of yore to today's use is helpful. Consider that in 1959, they were not using trailers built in 1921. (which would be required to compare this to ME installing bumper mounted stuff on MY trailer in 2011.). I also don't have a mechanic following me around everywhere I go, and and entire factory at my disposal.

How do we know "they didn't have any problems?" From what I've read, they had a whole bunch of problems...but rotted floors wasn't one of them, and that is likely a key to the separation issues. No problems putting weight back there...until there's is movement possible, due to missing floor/broken bolts.
As someone else already mentioned, that can (will) happen eventually whether there's any extra weight back there, or not.

What I'd like to see as a solution is a method of attaching a rear receiver that doesn't put undue stress. In other words, what would it take to put a hitch back there, and NOT cause damage? long support arms, attached far forward on the frame? (I'm thinking of the custom receiver supports that Can-Am has done for unibody tow vehicles to distribute the stress). Beefing up the floor/frame attachments? how?
Restorers, mechanics, engineers: what exactly will it take so that I can carry bikes on the back of the trailer, without worrying about structural failures? price is no object...what'll it take?
(I'm sure that step 1 is "an eternally rot-proof floor"). what then?
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Old 02-21-2011, 10:13 AM   #27
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Not to take place of a response by someone who knows about this stuff, but would a 100 pound air-conditioning unit on the roof all the way to the back cause any concern? I would think not, because the weight would be distributed via the airstreams structure instead of it's back bumper.

If not, simply transferring some of the weight to the aluminum structure seems like it would make 100 pounds of bikes no big deal.

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Old 02-21-2011, 11:12 AM   #28
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Not to take place of a response by someone who knows about this stuff, but would a 100 pound air-conditioning unit on the roof all the way to the back cause any concern? I would think not, because the weight would be distributed via the airstreams structure instead of it's back bumper.

If not, simply transferring some of the weight to the aluminum structure seems like it would make 100 pounds of bikes no big deal.

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It's not the weight, but what matters is "WHERE" the weight is located.

Something related to this issue, is where is weight put on an aircraft?

As we all know, the tail end of any aircraft is kept as light as possible, therefore luggage or containers are never put at the rear end of any aircraft.

Yes, it could be done, but the wing would have to drastically change and be relocated. That then creates other weight and balance problems, as well as the center of gravity, within the fuselage.

Andy
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