Frames - Page 2 - Fiberglass RV


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Old 03-16-2013, 08:32 AM   #15
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Of course, in the auto business the whole car is modeled using finite element analysis, so it can be crash tested, durability tested and even tuned for noise, vibration and handling before a single physical prototype is created.

Getting back to the frame, the displacements that we are talking about are indeed small and the mode shape is dependent on the frequency of the excitation. The excitation can come from road irregularities or tire unbalance, for instance. Road impacts tend to contain a wide spectrum of frequencies and the primary mode shape will just be the structure's first natural frequency. In the bad old days before CAD, we put accelerometers on the structure and whacked it with a big hammer.

Never the less, the laws of physics that finite analysis uses on a micro level are the same ones that we are familiar with on a macro level. Tubes are good in torsion, I beams are good in bending, etc.
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Old 03-16-2013, 09:28 AM   #16
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. . . and channel iron is just an I-beam with the web offset to the extreme case and angle iron is sort of spaghetti every way unless the legs are restrained. The evidence is less accessible to the layman at scales much taller than your head and much shorter than there it is again.

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Old 03-16-2013, 02:50 PM   #17
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I know that Tom and Jack are talking because I can see their lips moving but ................
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Old 03-16-2013, 03:33 PM   #18
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I know that Tom and Jack are talking because I can see their lips moving but ................
And I've got no idea what this means:
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. . . .......... The evidence is less accessible to the layman at scales much taller than your head and much shorter than there it is again.

jack
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Old 03-16-2013, 03:36 PM   #19
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My practical experience with FEA is about zero. AutoCAD is an industry standard, but it sucks for most things 3D. Solidworks, a rising standard, has several FEA plug in's, in fact, there are so many extra cost, third party add on modules, that you can spend loads more on the extra's then the Solidworks software.

3D CAD Design Software SolidWorks
FEA software (finite element analysis) - Simulation Premium - SOLIDWORKS - Videos

Jack, the stuff is way easier to use then reading that link, sorry. My understanding is that using FEA is kinda a fusion of art and science. Getting the most out of the mesh kinda requires quite a bit of intuition.
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Old 03-16-2013, 03:37 PM   #20
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I get some of it, but this is WAY overboard for a little trailer frame. Backwoods farmer engineering will get you there easily enough.
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Old 03-16-2013, 04:04 PM   #21
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And I've got no idea what this means:
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. . . .......... The evidence is less accessible to the layman at scales much taller than your head and much shorter than there it is again.

jack
And I do?

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Old 03-16-2013, 04:53 PM   #22
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Excerpt from Andrew Gibbons post #4 on thread "How to beef up Trillium 1300 frame?"

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But I don't think the triangular brackets add any strength to the frame, so I don't see the benefit of adding them. A trailer is effectively a three-wheel vehicle (one at the front and two in the middle), so there isn't nearly so much torsion as you will find in a four-wheeler, which those brackets would help (a little).
This my testament almost verbatim. At least I'm not wandering alone in outer darkness.

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Old 03-16-2013, 08:06 PM   #23
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Aluminum Frames?

I've thought that it would make fg trailers much lighter if they had all aluminum frames. Pros & Cons?
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Old 03-16-2013, 08:08 PM   #24
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Aluminum frames don't save that much in weight. If the frame breaks, and we had threads that show that they do, and you're out in the boondocks or near a small town... good luck finding someone that can weld aluminum.

I wouldn't have one.
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Old 03-17-2013, 07:09 AM   #25
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I believe you can get some torsion in a trailer, but not much - when one wheel hits a bump it transfers the load pretty directly into the body.

In contrast, a four-wheeled vehicle can get some very impressive torsion going - one design case might be 4g diagonal bump loading (that's a visit-the-dentist level of acceleration) on only two diagonally-opposite wheels. Now that is serious torsion.

Aluminium frames sound wonderful but aluminium does not have a fatigue limit, so eventually it will crack - as Burro (wasn't it Burro?) proved. In contrast steel does have a fatigue limit, so provided you keep the stresses below that level, it won't crack. For a fiberglass egg which may have a much longer life than other types of trailer, this is important.

Of course, aluminium frames can work for trailer, but you need a lot of experience to decide exactly how strong they need to be - and again Burro proved that getting that right isn't easy. Trailer manufacturers are not at all like auto/truck manufacturers, as they can't afford to spend a quarter of a million dollars to find out if a new model is strong enough. In their position, my attitude would be "if it has worked so far, stick with it".
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Old 03-17-2013, 08:12 AM   #26
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Yes, aluminum has no predictable fatigue limit. Bending-induced fracture is common and early. Cold-setting (correcting distortion by removing accidental bends) is not advisable.

Frame rails under my latter day Burro are 2"x4" rectangular steel tube; cross members are steel angle. I haven't seen or read much on Burro frame failures. I should do.

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Old 03-17-2013, 08:22 AM   #27
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I haven't seen or read much on Burro frame failures. I should do.

jack
Here's a good read for you Jack: Aluminum Frame - Anyone have one?
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Old 03-17-2013, 08:26 AM   #28
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Here's a good read for you Jack: Aluminum Frame - Anyone have one?

Building anytime out of aluminum is ridiculous. I can remember when some idiot at Boeing actually suggested making an airplane out of aluminum.
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