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Old 09-24-2010, 01:22 AM   #21
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We are in the design stages of our own lightweight chassis. One of our concerns is the durability of the aluminum and the joining systems. As an engineering professor once told me, "With aluminum it is not a matter of IF it will fail but WHEN it will fail." This is one reason why pressurized aircraft have a specific number of flight cycles between inspections and rebuilds. There are many all-aluminum chassis trailers on the road from snowmobile trailers up to and including most semi trailers. The key element in these vehicles is that flexing of the chassis does not matter a great deal. In a travel trailer flexing of the chassis can cause significant damage to the interior finish and potentially the body. Torsion axles can add to this flexing as they depend on the rigidity of the frame to work against to absorb shocks from the road.
I'm getting that the coach needs to be isolated from the aluminum chassis in a way to isolate the chassis flexing. Perhaps part of this isolation could be accomplished with an insulating structural material under the marine grade structural plywood flooring. I've seen the all aluminum low-boys, and they sure do have lot of built in camber.
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Old 09-24-2010, 06:03 AM   #22
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now my turn to step in and voice my two cents...

The frame is designed to flex as the trailer goes down the road. That is part of the semi monoquot construction. Many rebuilders restorers beef up the frame to remove this flex. That turns them into aluminum boxes on a steel frame.

Most trailers I work on are at least 45 years old. Once repaired, encapsulated, and painted, the frames will last even longer due to more than just a quick slap of paint being applied. How long are you expecting this to last? Will you be alive in 45 years?

All the power to Vintstream for trying this. Just because it failed in the past does not mean it will today. Any idea how many tries it took to get a man on the moon?

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Old 09-24-2010, 06:46 AM   #23
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Aluminum trailers have the advantage of lighter weight and corrosion resistance, thats it. In that trailer the problem is attaching all the pieces together. Riviting the crossmembers is the way to go, using cast aluminum brackets at the connections would be best. Generally aluminum welds are half the strength of the parent metal. Now with steel, the welds are generally stronger than the parent metal.
Just look at this pic of theirs.
http://www.vinstream.com/images/frame_2.jpg
I see they are using an inadequate blue welder as can be seen in the pic. I would say these guy's know just enough to get themselves in trouble.
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Old 09-24-2010, 01:38 PM   #24
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I have solved all of these problems with my design for a carbon composite frame, which incorporates the floor and frame in a single piece. Unfortunately, this puddle of perfection costs $200,000, but hey!

Joking aside, steel frames work well, and have known flaws that we are able to cheaply and easily address with ready available equipment and labor. Aluminum frames solve those problems entirely, but introduce new and possibly unknown problems just because of our lack of experience with the specific design involved.

Ideally, they could put two or three frames out there in the hands of people who will give them a LOT of road-time, and facing regular inspections. Once we see these frames with maybe a combined million miles of travel, we'll have some real world data.

That's the advantage of the current steel frames - hundreds of millions of miles of experience with them, so they have been down-engineered just enough to know where the limits are, and avoid them.
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Old 09-24-2010, 01:43 PM   #25
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I have solved all of these problems with my design for a carbon composite frame, which incorporates the floor and frame in a single piece. Unfortunately, this puddle of perfection costs $200,000, but hey!

Joking aside, steel frames work well, and have known flaws that we are able to cheaply and easily address with ready available equipment and labor. Aluminum frames solve those problems entirely, but introduce new and possibly unknown problems just because of our lack of experience with the specific design involved.

Ideally, they could put two or three frames out there in the hands of people who will give them a LOT of road-time, and facing regular inspections. Once we see these frames with maybe a combined million miles of travel, we'll have some real world data.

That's the advantage of the current steel frames - hundreds of millions of miles of experience with them, so they have been down-engineered just enough to know where the limits are, and avoid them.


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Old 09-24-2010, 02:22 PM   #26
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. o O (what if...?)

Could part of the frame rusting & rotting be due to the fact that the plywood holds water? I mean, there it is (the frame), all snug inside a fairly-well closed chamber with the belly pan underneath it, and the the absorbent plywood above it. And I almost forgot: throw in some varmint feces and insulation to the mix.

Come back in thirty to fifty years, and we're surprised it's gone south?

But, it the Mother Ship decided to change the "sponge" (plywood) for something like nyloboard, and used the same steel frame, but painted it at the factory in POR15, what would we then be finding when we open theses old tombs?

Welllll, different story, me hearties...

This is something that AS could do now, today, and add real value to their trailers.
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Old 09-24-2010, 02:33 PM   #27
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Hmmm, I have a different theory. I am sure your theory is a component of the corrosion, but I think the corrosion pattern says something else is a far bigger factor.

The corrosion almost always starts at the bottom of the steel in the belly pan. This is where condensation collects, and creates a wet contact between the frame and the belly pan. The rust seems to start at the rivet holes and works out under the coating It seems to spread until the steel has eroded enough that it is no longer resting in the condensation puddle, and is no longer in contact with the aluminum.

Also, I have seen this corrosion the same even when there's a rubber mat separating the wood from the metal on to of the frame.

Just my observation from a dozen trailers in person, and the many many photos on this fine forum.

My solution?

I would use POR15, and a tape to prevent contact between the frame and belly pan.

IMHO.
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Old 09-24-2010, 03:44 PM   #28
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Hmmm, I have a different theory. I am sure your theory is a component of the corrosion, but I think the corrosion pattern says something else is a far bigger factor.

The corrosion almost always starts at the bottom of the steel in the belly pan. This is where condensation collects, and creates a wet contact between the frame and the belly pan. The rust seems to start at the rivet holes and works out under the coating It seems to spread until the steel has eroded enough that it is no longer resting in the condensation puddle, and is no longer in contact with the aluminum.

Also, I have seen this corrosion the same even when there's a rubber mat separating the wood from the metal on to of the frame.

Just my observation from a dozen trailers in person, and the many many photos on this fine forum.

My solution?

I would use POR15, and a tape to prevent contact between the frame and belly pan.

IMHO.
Hi Dave Park;
Frame rust! It seems that I am not the only one that has it right. Condensation inside the belly pan is the # 1 destructive factor. The ever changing ambient temperatures from night to day and the other way around are quickly absorbed by belly pan skins as temperatures rises or falls. The temperature inside the well insulated trailer changes at much slower rate. Poorly insulated floors promote condensation as the two air masses of different temperature collide. The condensation as a rule will first try to bridge minute gaps in contact areas. Plywood absorbs the moisture as the condensation continues to linger in those areas. Liners tape in 0.060" laid on top of the frame makes a great moisture barrier. It is soft enough to compress and fill all voids. My frame is Stainless Steel and floor is aluminum clad plywood on both sides. Because of dissimilar metal composition I have the liners tape separating aluminum from Stainless. Half inch of cork floor with 3/4" of air space under the ply and bubble foil makes for exceptional temperature barrier. Thanks, "Boatdoc"
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Old 09-25-2010, 12:35 PM   #29
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Most of the corrosion issues we have seen are related to moisture infiltration through the belly pan acting as a water scoop, leaking windows (particularly in the '66 - '68 model years), and leaking plumbing (espically in above deck water and waste tanks and bath/shower drains. Once the moisture is into the fiberglass insulation it take a very long time to dry out especially in humid climates. The units which were insulated at the factory of by an owner with spray foam are particular nightmares for holding water against the steel chassis.

When we upfit a chassis for heavier GVWR applications such as a kitchen or even a heavier camping buildout (i.e. additional water capacity or more features) we design in a certain amount of allowable flex. It is more a factor of keeping the weight down through the use of lighter, thinner wall material than being concerned with flex. Vintage chassis were built with lighter gauge materails than the current models and were much more of a monoquot design than current builds from the factory. Modern Dexter chassis are quite stiff even without the body on them.
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Old 09-28-2010, 09:25 PM   #30
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Quote:
Originally Posted by Dave Park View Post
I have solved all of these problems with my design for a carbon composite frame, which incorporates the floor and frame in a single piece. Unfortunately, this puddle of perfection costs $200,000, but hey!

Joking aside, steel frames work well, and have known flaws that we are able to cheaply and easily address with ready available equipment and labor. Aluminum frames solve those problems entirely, but introduce new and possibly unknown problems just because of our lack of experience with the specific design involved.

Ideally, they could put two or three frames out there in the hands of people who will give them a LOT of road-time, and facing regular inspections. Once we see these frames with maybe a combined million miles of travel, we'll have some real world data.

That's the advantage of the current steel frames - hundreds of millions of miles of experience with them, so they have been down-engineered just enough to know where the limits are, and avoid them.
Dave,

Why stop with a composite frame and floor? What about a carbon composite frame, floor, and exterior panels- heck, you can even do carbon wheels, cabinets, shower/toilet. Kevlar could also be added to the mix for repelling small arms fire.
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Old 09-29-2010, 07:50 AM   #31
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I believe that if the insulation is attached to the floor and is ~ 1/2 inch above the belly pan AND the potential of water entering the bellypan......with a properly painted frame rust should be kept to a minimum. I think the fiberglass insulation is like a sponge & hold the water and if against the frame = R U S T....A/S hopefully has corrected a error that has been used ( 1973 ) where the belly pan is on top of the side skin and any water can run down the trailer side and possibly enter under the belly skin, get the insulation wet and again R U S T. And water can for sure enter from other openings under the trailer. Keep the insulation hi & dri may let the water escape and cause no harm...Enjoy JC
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Old 10-01-2010, 01:48 AM   #32
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We had a trailer with an aluminum chassis come into the Timeless shop today. Upon inspection we found this failed weld. The vehicle showed no signs of overloading. This appears to be a failure related to normal torquing and/or vibration. The trailer is equipped with a steel tongue as can be partially seen in the photo. There is a C-clamp installed to stablize the chassis to allow us to move it with out risking further damage.

The thickness of this material will require a heavy capacity TIG machine and a very experienced welding operator to correctly repair. The balance of the welds in the trailer will be inspected. Where we find one failure we have to assume all of the welds were made in the same manner by the same operator until proven sound.




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Old 10-01-2010, 06:40 AM   #33
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I've had a lot of experience with both steel and aluminum frame structures. Steel is the best material in this aplication.
It would be posible to build an aluminum frame for a trailer but you would have to rivet the thing togther to allow for flex. This would call for a lot of custom machined parts and many man hours of labour. It would just not be cost effective. Then you would still have issues with galvanic corrosion everywhere steel would touch aluminum.
Trailer manufacturers have delt with this by using light weight steel frames, plywood decks and light weight flexable bodies. This gives the best balance of strength, flex, cost and durability.
If cost was no object, then a carbon fiber trailer body with a steel tongue and steel wheel carriage bonded to the body would be the best. You just could not afford it.
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