Fuel Cans

I would expect that the likely failure mode to be fatigue failure of the aluminum jack bracket or the lag bolts holding the steel jack bracket will fatigue fail the aluminum camper frame where they screw into it.

I'm going to call it "The Engineer's Dilemma." An item that will work fine for decades for someone unaware of it's design deficiencies will fail soon for someone who is aware of them. It's probably one of Mr. Murphy's corollary laws.
 
craig333 said:
Don't use studs. Mine connected to the jack mounts and underneath where the two bars are bolted through the underside of the camper (inside the sink compartment on mine) and to the can carrier. No through bolts through the siding and into whatever (yes, it would be hard to hit the stud in the center).

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Our Grandby has an AT can holder mounted like craig333's. I did the mounting just before we left for Baja in January where we drove hundreds of miles on washboard. The NATO can was full the whole time and we experienced no problem whatsoever with the can mount. Ours is supported underneath by two pieces of 3/16" U channel that are bolted underneath the propane compartment, and on the sides with a piece of angle that is mounted to the jack brackets. It seems really bombproof.
 
One other mounting spot, preferably on the opposite side as the jack bracket but even if only 1/2 way across, is huge towards longevity of the whole thing. More spots than that is even better.
 
Hey Guys, I searched but don't see an answer. I am adding an Aluniness rear bumper with swing outs for Spare and a box. I don't think I will have room for Jerry Can(s) so was planning on RotoPax. I have unused rear steps and have seen how some mount the Rotopax there.
I have emailed but no reply yet from Rotopax. It looks like they are out of 2 gal Diesel RotoPax cans and I don't even see a 4 gal option. I'm open to any suggestions of another like product or how I may carry a Jerry can.
 
I think a jerry can holder like the AT one pictured above would be your best bet to carry fuel cans. Can you have it span from the steps to your jack bracket? The nice thing about these is that when you don't need spare fuel, you can use it for firewood, or trash or other stuff you may not want to carry inside. A gallon jerry can is also far more space efficient - the 4 gallon rotopax is huge!

If you are moving your spare out from under the vehicle - any chance you could add an auxiliary fuel tank in this space? This would be the best option as you are carrying the heavy stuff down low and closer to the axel. One big draw back with the rear mounted fuel cans (and spare tires for that matter) is that it is about the worst place on a vehicle to carry extra weight - up high and way out back.
 
XJINTX said:
Hey Guys, I searched but don't see an answer. I am adding an Aluniness rear bumper with swing outs for Spare and a box. I don't think I will have room for Jerry Can(s) so was planning on RotoPax. I have unused rear steps and have seen how some mount the Rotopax there.
I have emailed but no reply yet from Rotopax. It looks like they are out of 2 gal Diesel RotoPax cans and I don't even see a 4 gal option. I'm open to any suggestions of another like product or how I may carry a Jerry can.
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Mount it on top of your Aluminess storage box. Ron
 
I don't travel rough roads much so I think I will be fine with 3 gallon Rotopax on jack mounts. Here is my prototype bracket out of steel with 1/4 spacer behind to space bracket away from camper body. When Rotopax is locked on it blocks access to bracket bolts and nuts on back side will be welded to strip of metal for theft protection. I have now ordered two sets of brackets in stainless steel.

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TacoTruck said:
I don't travel rough roads much so I think I will be fine with 3 gallon Rotopax on jack mounts. Here is my prototype bracket out of steel with 1/4 spacer behind to space bracket away from camper body. When Rotopax is locked on it blocks access to bracket bolts and nuts on back side will be welded to strip of metal for theft protection. I have now ordered two sets of brackets in stainless steel.

20170530_110851-L.jpg


20170523_085250-L.jpg
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Nice work!
 
ntsqd said:
I get roughly 23.8 lbs of liquid (6.8 lbs/gal for most liquid fuels), so guessing about 25-26 lbs for the fuel and the can. Really, really rough Rule of Thumb for Dynamic Loadings is 3 times the static weight, so around about 77 lbs swinging up and down and fore to aft and back on every bump. My NATO cans measure 13" wide by 6.375" deep, so that puts the centroid of the weight (mass actually), in the horizontal plane, ~14.5" from any corner.

That makes the Moment on the jack bracket something like: 77 lbs X 14.5" = 93 lbs-ft (can think of a Moment as a torque). What this means is that every bump that equals or exceeds 3 times the pull of Gravity is exerting at least 93 lbs-ft of torque on the jack bracket. This doesn't include any Impulse loading from the can not being completely full. That can drive the actual, extremely brief duration loading significantly higher (think: like hitting the end of that 14.5" long lever with a 3.5 lbs or less hammer).

Because I know all of this I know it would fail for me. It is the curse of an Engineering education. Hopefully my outlining it all doesn't mean it will fail for you too.

EDIT: DOH! moment (no pun intended). I calc'd the diagonal of the whole can, not the centroid location. I kept looking at that number thinking it was too big. So, the actual leverage length is roughly 7.25" which makes the Moment 46.5 lbs-ft instead of 93 lbs-ft. Still quite a large number, but it will increase the fatigue life over the 93 lbs-ft number.
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The Engineering head scratching part...and I'm not an engineer, but as a pilot my entire adult life, I have fairly good background knowledge of stress and fatigue as it relates to aircraft design and flight.

So, my two cents (and worth what you paid...nada) on jack brackets and attaching fuel cans/holders. The jack brackets are designed solely to carry the static load of the camper along a vertical axis. The brackets will likely fail once corroded to the point they can no longer hold the weight of the camper.

When we begin using the jack brackets for different purposes then designed, such as mounting jerry cans/holders, we introduce new variables into the equation. Loading is predictable and the stress may be fairly predictable as well as long as you travel down a smooth road. Induced stress become unpredictable when driving off-road, on a rutted, wash board road, or rough track/trail. Will the jack bracket fail and if so when? Absolutely, but its anyone's guess due to the randomness of stress from such off-road travel.

Many things, such as airplanes, are designed and built to flex. Watch the wing tips of a wide-body airplane flex upward, quite dramatically as the airplane rolls down the runway and into the air and the wing and fuselage are now carrying the full load of the airplane at liftoff; the amount of wing tip flex is dramatic on some models. Materials can bend and flex all day long up to their elastic limit. When you exceed the elastic design limit of a material, you introduce fatigue. Generally, materials will not fail at the first introduction of fatigue, unless the induced stress is so severe as to cause material failure. But rather, fatigue is cumulative to the point of material failure.

Without performing any analysis, and just looking at carrying a fuel can on a moment arm attached to the jack bracket, one can surmise the bracket or internal camper framing which the bracket is attached to, will fail at some point in the future. The bracket will likely fail due to repetitive, random stress from amplitude loading and perhaps due to some corrosion thereby reducing the life of the material.

In other words....Murphy is indeed along for the ride...every ride when carry a fuel can in a holder attached to the jack bracket.

Somewhere, buried deep within WTW or his blog is Jonathan Hansen's post describing swerving to miss a car that pulled out in front of him while driving his TACO/FWC rig. The maneuver to avoid the vehicle placed so much stress on the forward bulkhead of his camper that stress cracks developed. Yes, the elastic design limit was exceeded and the material failed in one stressful event. FWC analyzed the issue and resigned the forward framing to support the stress introduced from this type of maneuver as a result of this incident; and repaired Jonathan's FWC.

I don't know if FWC has analyzed the framing and jack brackets for carrying fuel cans/holders. But since the factory installs AT fuel can holders, I suspect the engineers have looked at the jack design. At least lets hope so!
 
I hated Fatigue Analysis. I came away from that part of that class knowing that mild steel is usually considered to have an infinite fatigue life and aluminum structures are designed to live 500,000 cycles at most. If you're vibrating that structure at 5 Hz (i.e going down the road) that is roughly 1700 hours of life-span. If you vibrating that structure at 50 Hz (Maybe a washboard road?) that fatigue life is only 167 hours. All of that assumes that the structure was designed for such a loading. If it is not then there is no way to easily know what the fatigue life will be.
 
I love the discussion about the potential of dynamic stresses and understand its relevance. My simplistic mind makes me think that although the jack brackets were designed for a static load, they must be designed to hold a minimum of 500 pounds. I think for the amount of time I will have a full can on the road, and the amount of success others have had in the past, I think I will give it a shot.

Now the bolts I use may be a whole other discussion :).

In my line of work there were all these B.T.U.
formulas. Fortunately we were able to break it down to a simple solution. "Put the wet stuff on the red stuff".
 
Has anyone ever heard of FWC jack brackets failing under any condition? :unsure:
Does fluid in a gas container provide a dampening effect on wash board road as opposed to a solid weight? Engineers?
I'm just a redneck fabricator, LOL.
 
This fatigue thing has me worried. The mounting plate I fabricated for my 5 gallon Jerry can bolts to the jack bracket and the lower rear wall step (basically a triangular bolt pattern). Do I need to worry about fatigue with this set-up? I used 1/4" 6061 aluminum. It seems really solid.
 
I doubt the AT fuel can holder, or for that matter, the jacket bracket will fail. I think the weakest and likely point failure will be in the AL framework under the camper skin. If the engineer(s) at FWC have taken into consideration mounting a fuel can/holder on the bracket, then failure is probably a non-issue, or very remote.

I've seen some corroded carbon steel jack brackets on older FWCs. Why I opted for the SS jack bracket.
 
Once the Moment is removed by bolting on both sides of the plate a significant portion of the potential for fatigue goes away because the leverage against the jack bracket is effectively gone.

Something suspended in the fuel would see some damping, but the fuel won't damp itself. (Experienced Baja race crew guys leave their coolers full of water so their beer doesn't get shaken as badly.)

When considering corrosion issues look up the galvanic potential between the two proposed metals. Stainless in direct contact with aluminum is one of the highest likely to occur in a camper while galvanized to aluminum is one of the lowest. If you're in Sedonna it's likely large of no consequence. If you're in Baton Rouge it very likely will be important.
 
ntsqd said:
When considering corrosion issues look up the galvanic potential between the two proposed metals. Stainless in direct contact with aluminum is one of the highest likely to occur in a camper while galvanized to aluminum is one of the lowest. If you're in Sedonna it's likely large of no consequence. If you're in Baton Rouge it very likely will be important.
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Don't you need an electrolyte present for galvanic corrosion to occur between two dissimilar metals such as SS and AL? I'm aware rainwater can be either a strong or weak electrolyte due to major/minor ions. Perhaps why you referenced Sedona v Baton Rouge.

I have had my Hawk camper with SS jack brackets on the NC Outer Banks several times. I'd be interested to see if I have a galvanic corrosion under the skin between the AL framework and SS brackets.
 
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