Mini Fithwheel Foamie for S10 pickup

[GPW]

Passing by my neighbors yesterday , I noticed he has Three fifth wheel trailers , two low boys and a huge cattle hauler... Funny I never noticed them before ... we’ll try to get some pictures ...
Looking around here, a lot of folks have them ... and big dually’s to pull them ... (this is a rural area...despite being in the city limits )

[GPW]

Forgot my camera (Duh !!!) but inspected the neighbors trailers .... Those were some pretty Massive structures on the front .. Lots of Steel ...

Just thinking that even a Foamie’ would need something substantial for the front framework... Looks like a problem for an Engineer... Much tension and compression forces in play , plus the bumping around up and down the road ... and all those Cows ...

[KCStudly]

Scale version of a cow in TTT talk is a pile of CI dutch ovens, a cooler full of beer and ice, or 15 gal water tank.

[GPW]

KC, I guess it all comes down to the math here... structure, weights, loads, dutch ovens , Beer, cows , etc. etc. etc. ... How about just “simulating" the frame structure with “scaled down " TTT material sizes , assuming ( ) the scaled down version would support scaled down loads ? ... I know it probably doesn’t work like that , hence the need for some “engineering” ... Still looks like a lot of FAB to me ... great if you have the tools ... and a shop, knowledge, skills, time ... You could do it easily KC ...

[KCStudly]

So first reason the big commercial units have big steel is due to the large weight ratings they are designed to carry. The over design is necessarily large to account for these loads and the dynamics (i.e. 1200 lbs load x 3 or 4 g dynamic loading is only 3600 or 4800 lbs, whereas for a 10k or 14k load it is 30k to 52k equivalent).

Second reason is no OEM is going to be competitive in sales if they take the time to build a monocoque or truss style frame. It's just too labor intensive to consider for profit.

The best way to figure it is with FEA, and if anyone wants to get serious about a design, show my what your thoughts are. No guarantees, but I might do some napkin scratching, just out of curiosity.

[GPW]

KC, I know this may be a poor analogy, but it makes me think of a “dragster” frame ... which must endure a lot of stress, and still be of a light enough construction style for a foamie fifth wheeler... maybe ? Still a tricky welding job , for the inexperienced...

Guess that takes us back to your original drawings? And once you add the structure , shouldn’t that stiffen up the whole frame/unit ... ?

[KCStudly]

Yes, exactly. My idea was to take advantage of the hard points at the top and bottom of the wall and spread the depth of the beam as far as possible. That way the outer fibers have the greatest mechanical advantage over the loads.

The difference is that these days dragster frames are actually designed to flex a whole lot! A 250 inch wheelbase you will see a big arch in the chassis at the big end on a hard charge. Some of them actually have slip joints that allow them to flex even more!

[GPW]

KC, all the 5 wheelers we see around here are HUGE !!! Maybe with the ultralight weight of a foamie , the stress wouldn’t be so critical ? ... Still it would be a complicated FAB for most people .. I’m guessing for the best job , you’d have to make a jig on the metal FAB table to get (and keep) all the parts aligned correctly for welding ... It does seem more possible now that we’ve discussed it ... Square steel tubing (well designed) would eliminate any laborious coping ... It seems very Strong .... my little all steel CT is built like that is a “Tank" ...and still not “that" heavy ...

[ebutler]

I have looked at several shop built (think home built) goose neck trailers and most of them rely on overbuilding to provide strength. I looked at one that was made from a three axle house trailer. It also had a home made coupler. I would not even hook up to that one. I think a frame built from square tubing would be best. The key to a straight frame is a flat and level shop floor and plenty of measuring and tack welding. Then use a well thought out plan for finish welding. Most tube aircraft are tacked in a jig and finish welded on a rotisserie to keep the "frame" straight. A jig would be out of the question for something this large for me. That said I have built several trailers the way I outlined above and every one has been good.
The big thing is too work out the design before a piece of steel is cut.

Elmo

[KCStudly]

The big thing is too work out the design before a piece of steel is cut.

Things tend to go much more smoothly that way; less lost motion and, hopefully little wasted material.

[mezmo]

'Just came across another small 5th wheel for reference:

http://www.justanswer.com/rv-motorhome/ ... glass.html

A pic is about 1/4 down the page.

[GPW]

We’ve been stopping and looking at all the 5 w trailers around here, and the more you look the more you see.... All the “rural" folks have em‘ . Every one we’ve seen has some serious Metal up front . Just thinking it may be easier to cut and weld up some proportionately sized plate , rather than all that tubing ... Easier to FAB on the floor ...if the floor’s Level

[KCStudly]

A couple sayings come to mind when talking about welding a frame like my suggestion above.

TLAR - That Looks About Right,

and,

"If I can walk across it, I can weld it." - Karl

Close enough is generally close enough. Most finished concrete floors will be close enough to use as a reference plane.

Heavy plate is just that, heavy. There are a lot of reasons not to build a mini fifth wheeler, and there are a lot of reasons one could choose to use one method over another in the frame design. Either method will still need to contend with the issues of accurate cutting, fitting, and dealing with heat effects from welding; none of which should be the deciding factor. Weight, cost, and complexity to build should factor in to the design decision, but what I have proposed should not "peg the meter" (for competent designers/fabricators) in any of these categories.

In my mind the tubular truss concept kills two birds with one stone; giving you both wall structure and unit strength for the chassis for the same, or perhaps less weight. Might be worth the added complication for someone with the knack.

[GPW]

KC, that’s true, but there’s a whole lotta’ “knack” involved (and equipment/tools)... numerous smaller pieces , more cutting and fitting .... I’m sure you or Elmo would be up to the task , but for the rest of us .... Uh, I dunno ‘ ??? Not an Easy project any way you look at it ...

(Note: ) After seeing all this , likely we’ll NEVER ever even hope to consider a 5 W trailer ... Just sayin’ ...

[KCStudly]

I won't be going there, either, but it is fun to exercise the mind!