Frames

[rabbit]

Originally Posted by ThomasG

And I've got no idea what this means:
Quote:
Originally Posted by rabbit
. . . .......... 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?

jack

[rabbit]

Excerpt from Andrew Gibbons post #4 on thread "How to beef up Trillium 1300 frame?"

Originally Posted by Andrew Gibbons

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.

Jack

[Lora]

I've thought that it would make fg trailers much lighter if they had all aluminum frames. Pros & Cons?

[Donna D.]

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.

[Andrew Gibbens]

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".

[rabbit]

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.

jack

[Donna D.]

Originally Posted by rabbit

I haven't seen or read much on Burro frame failures. I should do.

jack

Here's a good read for you Jack: http://www.fiberglassrv.com/forums/f...one-44445.html

[Thomas G.]

Originally Posted by Donna D.

Here's a good read for you Jack: http://www.fiberglassrv.com/forums/f...one-44445.html

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

[rabbit]

Thread referenced by Donna reveals Sac City operator used aluminum frames for a short time in the 80s.

Wonderful book about an aircraft with a structural flaw that fails and a highly-stressed engineer who doesn't: Nevil Schute's No Highway adapted for the silver screen as No Highway In the Sky with Jimmy Stewart.

Jack

[Thomas G.]

Originally Posted by Andrew Gibbens

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..........

It would seem that you'd see the greatest deflection due to torsion when you are camping and using stabilizers on all four corners.

I'd agree that the torsional deflections while towing are small, but to improve durability of the frame and body, I think that a torsionally stiff frame would be preferable to one less stiff, especially when it can be built at the same weight and cost.

[rabbit]

I agree, Tom. With jacks and stabilizers deployed on the "four corners" and extended/elevated equally, the deflection would be seen as longitudinal bowing in side elevation. Given two jack stands on diagonally opposite corners or only one on one corner, a bit of elbow grease and a closed mind, I could produce the major deflection usually termed frame twist. Static forces can produce a lot of damage and are very accessible to simple observation by very simple minds.

jack

[Andrew Gibbens]

Originally Posted by Thomas G.

It would seem that you'd see the greatest deflection due to torsion when you are camping and using stabilizers on all four corners.

Yep, that's the perfect time to do some serious damage!

[Brian B-P]

Originally Posted by Thomas G.

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

I get the sarcasm, but seriously...
Aluminum can produce substantial weight savings in some cases (aircraft skins, for instance), and real corrosion advantages in others. A trailer frame isn't either of those. I've seen aluminum used for exposed boat trailer frames to impress people (and maybe for the corrosion benefit), but I haven't it used for rational benefit in a travel trailer frame... but perhaps it could be done.

My 17-foot widebody Boler B1700 has a 2"x4"x1/8" box steel frame. That's what, 40 kg (90 pounds) of steel? A really well-executed aluminum design might save what, 10 kg (22 lb)? That can't be worth the material expense, the design effort (especially on the connections to the suspension and coupler), or the risk of getting it wrong.

For anyone checking my rough calculations (and please do if you are interested), note that while aluminum is much less dense than steel you need more volume of it for the same strength. This works out well in panels, not so well in tubes.

The typical travel trailer manufacturer has approximately zero structural design capability (not an insult, just reality). They are not aircraft manufacturers, who probably spend more on getting a single aircraft type approved than the typical RV manufacturer can afford in the entire life of the company, on all development efforts. A single aircraft type - once developed and approved - can stay in production for decades; it is too expensive to change the fundamental design more frequently than that. A travel trailer manufacturer can come and go in less time than an aircraft type certificate application takes to process!

RV manufacturers use well-established design practices and off-the-shelf chassis components to make trailer chassis, like any one of us using common lumber sizes and rules-of-thumb building a deck on our house. The trailers are not structurally optimal, but the they're workable, like most decks. I don't want to tow someone's essentially amateur structural experiment down a highway.

That's my view, anyway.

[Thomas G.]

Originally Posted by Brian B-P

I get the sarcasm, but seriously...
.......

Yes, I'm not advocating that trailer frames be made of aluminum, just making the case that when properly engineered it could be done well.

Poor engineering is poor engineering, no matter what material is used.

[Brian B-P]

Originally Posted by Thomas G.

... when properly engineered it could be done well.

Poor engineering is poor engineering, no matter what material is used.

[Jared J]

I'm with brian and thomas. It can be done, I just question the feasibility of it. Look at the cost of a steel vs aluminum car trailer, and the weight savings. If you're cutting things that close, it's time to rethink what you're doing anyway, IMHO.

Also, any half-wit can repair a steel frame with a basic welder, aluminum requires a little more skill.

[Thinh]

...Here is my personal opinion ab RV frame respect to materials used as steel, aluminum or combination of both...In the case of an engineer who works for a big corporation, such as automobile/transport truck manufacturers, he got the job to do with high salary into the products. These products after all kinds of pilot/checks/tests and approvals before mass production, the costs would be at relative factor during their designs (I think, could be wrong though). The engineering teams would have all kinds of resources to assure final products which are market-able and profittable. And when it comes to the better product from competitors, the cost of product would play more important role in their designs. To a...backyard DIY amarteur, he would modify, alter his..toys with his limited and available resources in term of costs...So, in my own opinion, an engineer's work and a home maker/self DIY's work are not the same. Like someone said, people use what are available to them. That's why they have to work themselves besides their passion of the work. Otherwise, one would walk in a dealer or sit at home with his/her fingers and...order a brand-new toy. To back up for what I wrote, here is ..my photo, my frame of my trillium and the left side of it is a....throw-away aluminum ladder( already dismantled a half) which will be used into my project of reinforce frame/rebuild a stronger bunk bed. Thanks for all readers.

[stevebaz]

Dont forget significant regidity and strength added to the frame from the floor being bolted on. This turns the frame to a semi box beam at least until the screws rust out.

[Francesca Knowles]

Originally Posted by Thomas G.

Yes, I'm not advocating that trailer frames be made of aluminum, just making the case that when properly engineered it could be done well.

.

As it (not that uncommonly) is! Here's one example:




Here's what the manufacturer of these aluminum (including frame) trailers has to say about design- principles that apply regardless of material used:
Quoting from : Floe Aluminum Trailers

STRONG RECTANGULAR FRAME
. There are three key factors that make up 95% of a trailer’s structural integrity.

The first is the tongue mounting system. To ensure safety and security, it is imperative that the tongue system will perform without twisting, cracking or bending.
The second is the design and strength of the main structural beams that the axle(s) are bolted to. These are what the entire load of the trailer rides on. If these bend the rest of the trailer will too.
The third key factor is the strength of the cross members. If these are not strong enough, they will twist and cause corner-to-corner flex in the the trailer’s bed. This will ultimately result in cracked welds.

And let's not forget that Camplite is building some really nice camping trailers about the size of ours that are nearly 100% aluminum, including the frame.

Francesca

[Thinh]

..Thanks for the inputs, Steve and also Frankcesca for the research with photos. BTW, by chance, I only welded extra " triangle pieces" on the bottom rails of the frame before painting it. One mind at work would never be better than a bunch when it is open to the whole group. In my opinion, the modification only has it's true meaning when it's application benefits more with minimum costs... The learning trend would have never been stopped...Learning not only enrich one's knowledges but also encourages his/her own creation in the....proper way. I will keep posting in photos of what I am about to do in the future. Thanks again for all. Cheer!