Ideas for lightweight cab over bed board?

[cwdtmmrs]

Ntsqd, All of the campers have telescoping beds, it is no big deal.

Boonie, Here is what I do and I suspect that you will do the same, although you will have more latitude with your pass through design. My gear...backpack, fishing stuff and hunting gear is stored on the floor for travel. I keep everything low so that I can still use my rear view mirror in the cab. After I pop up, all the gear goes on the un-extended bed like an attic leaving the floor area open. I usually sleep "downstairs" on the couch bed since 95% of the time I am alone. If not, the gear gets stowed inside the truck or at the end of the couch for sleeping "upstairs". Not a major issue, and I can tell you will not be carrying tons of stuff to move about. Plus you have easier access to the cab.

More thoughts on your telescoping design: I think the slide out section on my mid 90"s FWC Ranger II is only 1/2" and not 3/4" since one mattress is 2" and the other mattress is 1 1/2" thick so that would save about 1/3 of the weight of that piece of plywood. A bonus to your design is that the foam in both mattress can be the same thickness. One thing to check for is what your maximum thickness can be and still be able to close the top. Measuring from the "C" channel , mine is no more than 5". Bed board 3/4", slide out 1/2", thick mattress 2" and thin mattress 1 1/2". Virtually anything left on the bed and my top will not close properly. It is a very snug fit on my model. If the same bodes true for you, then your mattress will be 1 1/2" to 1 3/4" thick. Not real comfortable, but doable with the right combination of foams. something to consider.

cwd

 

[Boonie]

cwd,

I should have posted this as part of my original question, but better late than never.

Actually my '81 Grandby had a 20" hinged flip-up section with cushion, so the weight in the overhead was only the bed board and corresponding cushion. The 24" overhead bed was expanded to 44" by the 20" flip-up section, so the weight of that section was behind the front wall and not in the overhead. To allow the flip-up, there was a 24" space behind the front wall/pass-thru that was a cushioned seat with storage under and in the down position the flip-up blocked the pass-thru. With my arrangement, I did not want to waste the 24" space and obviously having a flip-up blocking my walk thru would not work, but I also wanted to keep the weight in the actual overhead to something close to or lower than the original designed weight because of the added length of my overhead, thus the thought of adapting the slide out (or telescoping) bed design of the newer FWC.

So far the ideas and designs with accompanying weight calcs have not been able to come close, so I will probably need some type of support under the overhead either directly to the cab, or struts running down to the cowl. For those following my build thread the strut idea incorporating a light bar over the windshield and directly in front of the camper is a contender.

 

[ntsqd]

Our Phoenix does not have a telescoping bed, it is full queen width all of the time. With two of us in the camper traveling out of carry-on's we're moving them up to the bed to eat and down to the dinette to sleep. Shoes are also an issue for storage. Where to put them out of the way that also stays warm?

 

[cwdtmmrs]

Thom I assume that your camper bed is sleeping East/West and not North/South.

Boonie I have not seen the flip up bed arrangement but there are lots of things I haven't seen. Are you concerned about the strength of the overhang since the front wall has been removed? I think that the new FWC's have a 48" or thereabouts overhang. I am not an engineer, but I would suspect that the "C" channels carry most of that load anyway.You may not need the struts to the light bar or other external bracing. Maybe some of the smarter guys can comment here. I would use bigger/heavier tubing and inside gussets for that 69" cross piece regardless.

cwd

 

[ntsqd]

East-West sleeping it is. We're semi-actively looking for an 8' camper with North-South sleeping (ours is a 6.5'), but we have pondered remodeling one with E-W to have a permanent "pull-out" to convert it to N-S. Under the "pull-out" would be shelving designated for the the carry-ons and for the shoes.

For a depth of 20" I think telescoping is over-thinking it. I suspect that a piece of 1/4" ply can span that under the expected loading with no issue, so long as there is structure supporting the open end that spans side wall to side wall. That structure could be as simple as a 2X4 on edge that the ply is either kerfed into or cleated to. Similar support would be needed at the top of the front wall, but it wouldn't need to be another 2X on edge - it could simply be the edge of the main under-bed panel. If you want to use beam analysis for the loading consider the "2X" supported edge to be fixed and the panel supported edge to be simply supported.

 

[Boonie]

To clarify, as explained in post #1 bed was 80" and east/west. I narrowed the camper to 69" and extended the overhang 11", so I changed sleeping to north/south, therefore, the extension to the overhead by whatever means is 38" x 69".

I agree that newer campers do have a deeper overhang, but I assume they were engineered and sized accordingly. I also assume that my camper was only engineered and sized for it's original 23" overhang, therefore my concern. Also the bed is supported where the end wall was by the Jeep roll bar.

 

[Boonie]

My wife, using womanly engineering, has suggested that if I want to reduce the weight in the overhead bed, I could certainly loose 25 pounds. Thanks honey.

 

[cwdtmmrs]

I don't think it is an issue with the extended cab over. I would certainly call both FWC and ATC. They will know what changes have been made especially Jeff and Marty at ATC.

cwd

 

[Boonie]

Ok, all you engineers, you have let me down!

While pondering one of my other issues, the lift panels, it dawned on me that not only the weight of the lift panel, but also 1/2 of the weight of the roof and vinyl sides is carried at the front of the overhang. Obviously the original design included this weight load at the front of the overhang. If I changed the lift design (thanks, Livingthedream) to a side lift, I would eliminate that extra weight, which would allow me to carry the similar weight of my redesigned bed board.

How is that for an AHA moment.

 

[ntsqd]

Trying to constrain yourself to duplicating the original strains with re-arranged lengths and loads is a guessing game at best. Far easier and much more exact to just start over. Use diagrams 12 thru 14 here: http://www.awc.org/pdf/codes-standards/publications/design-aids/AWC-DA6-BeamFormulas-0710.pdf to calculate the max loading that the wall structure can support. Note that the publication is from a wood beam mfg., but that the diagrams and related formulas apply regardless of the material that the beam is made from. You will have to calculate your own values for "I", the Moment of Inertia of the beam structure. Which in this case is the whole structure in the side walls of the over-hang.
Some instruction on finding the M.o.I. of composite structures:
http://www.ce.memphis.edu/2131/PDFsF12/Moment%20of%20Inertia%20-%20Composite%20Areas.pdf
Other Ref's:
http://civilengineer.webinfolist.com/str/micalc.htm
http://engineering-references.sbainvent.com/strength_of_materials/area-moment-of-inertia.php

I would use a Factor of Safety of at least 2:1 so if the calcs say that the wall structure can support statically 1000 lbs. only ask them to support 500 lbs. [Alternately can take the wall with the smaller I value (assuming that they have been investigated and found to be different) and use it only for the max load calculations.]

For the dynamic loading an easy Rule of Thumb, that mostly gets you there (dynamics is not my strong suit), is to use a factor of 3 on the load. So for that 500 lbs. load the "dynamic load" will be 1500 lbs. (i.e. the "weight" of the load at 3G's of acceleration due to that bump you didn't see etc.) and the wall structure will need to be able to support 3000 lbs.

 

[ckent323]

NTSQD is spot on

Folks I trust rely on Roark's formulas. Do a search and you can find a PDF of the book online. Also excel spreadsheets are available online:

http://www.roarksformulas.com/References/RFE_SuperpositionWizard.pdf

I have not followed this thread very carefully but I am pretty sure that part of the load of the cantilevered over cab portion of the bed is borne by the wrap around aluminum frame. I note that the wrap around aluminum it is taller (perhaps thicker) on the newer models than on the early models. Overhang of the bed extending into the interior of the camper is borne by the side supports attached to the camper frame in my current FWC camper and by fold out legs in my old FWC camper.

Regards,

Craig

 

[ntsqd]

FWIW, I deliberately ignored any end support that the wrap-around might offer for two reasons, it is hard to estimate just how much support it offers when it is attached to the far end of the other cantilevered beam (complicates the calcs considerably), and ignoring it gives a Conservative result. That is to say that by ignoring it the design result will actually safely carry a little more weight than was calculated. Likely not a lot and that also is partly driving not venturing into trying to calculate it. Results vs. effort ratio is low.

With a partly over-hung, partly not over-hung (under-hung?) loading only the part of the loading that is on the over-hang should be applied. If it is too difficult to calculate what the percentage is then can use the full loading. You'll just need to understand how that can drive the results even more conservative than necessary. And while this doesn't need to fly or go into space, too much final Factor of Safety just makes for something that is heavy and awkward.

 

[Wallowa]

FWIW, I deliberately ignored any end support that the wrap-around might offer for two reasons, it is hard to estimate just how much support it offers when it is attached to the far end of the other cantilevered beam (complicates the calcs considerably), and ignoring it gives a Conservative result. That is to say that by ignoring it the design result will actually safely carry a little more weight than was calculated. Likely not a lot and that also is partly driving not venturing into trying to calculate it. Results vs. effort ratio is low.

With a partly over-hung, partly not over-hung (under-hung?) loading only the part of the loading that is on the over-hang should be applied. If it is too difficult to calculate what the percentage is then can use the full loading. You'll just need to understand how that can drive the results even more conservative than necessary. And while this doesn't need to fly or go into space, too much final Factor of Safety just makes for something that is heavy and awkward.

I am in awe at the thought, planning and expertise displayed in these posts...but I understand little of what is being presented... and my guiding light is "keep it light, durable and uncomplicated"...kinda a variation on "KISS"...thank goodness for us our '16 Hawk needs no major [or minor for that matter] alterations or additions to serve our off road needs...you folks obviously really enjoy customizing your campers and have the skills needed to accomplish that... At my age,it is not only 'cost vs benefit' but the temporal component is also a major factor; time is short...

Phil

 

[Boonie]

Wallowa,

If it was "durable, uncomplicated, light" it would be..... well "DUL". (Sorry, I couldn't help it.)

Boonie

 

[Boonie]

Well, I was hoping for help from some engineers, so I now have something to do during the next Colorado snowstorm.

 

[Boonie]

I want to thank everyone who has participated in this thread. After all the responses I have concluded that I was heading in the wrong direction in my original question. Since my camper is permanently mounted and I anticipated a light-bar as part of my build,

I will attach the overweight overhang to the light-bar, thus totally eliminating the cantilever issue.

This the best picture I currently have in my gallery, but you get the idea.

 

[Wallowa]

Wallowa,

If it was "durable, uncomplicated, light" it would be..... well "DUL". (Sorry, I couldn't help it.)

Boonie

Ah-ha! Good one! Good luck Boonie...wish you all the success...I am sure your rig will be anything but "DUL"....

When done, please post a description and photos...

Phil

 

[Boonie]

Thanks Phil,

I will continue posting description and photos on my build thread, ""81Grandby on "91 Jeep YJ-New Build". If you review that thread you will see that for me the build is a big part of the fun. I have taken the approach that if they don't make it at a price you can afford....Build it.

I too fall into the category of "time is short" since I am now refusing to celebrate any birthday that starts with a 7.

Boonie

 

[ntsqd]

I do not think attaching to the light bar is a good idea. Neither it nor the windshield frame are designed with that kind of loading in mind. If you are integrating a cab roll cage into the body structure and are including the windshield frame as part of the body, then so long as the cage can transmit it's loads to the frame it will work. Note that my last statement should not be construed as endorsing tying the cage directly to the frame as it does not and would be the worst possible thing to do.

For the cantilever length shown in the pic above it should not need much structure in the side wall as it is not that far. The link with the beam formulas includes the deflection formulas so you can predict just how far each proposed design would deflect under load. It may well be that deflection rather than load capacity drives the final design.

 

[Boonie]

Although I am not an engineer, I do consider myself somewhat logical. If the deflection of the cantilever is negligible (static load), then the weight transferred to and carried by the light bar through to the windshield frame hinge would be negligible, therefore, whether the windshield hinge was designed for "that kind of loading" becomes an irrelevant point. I do think the light bar connection would be more valuable in stabilizing the dynamic load introduced while four wheeling. That being said it is common in the Jeep community to not only attach light bars to the windshield hinge but entire roof racks. I will accept this as prima facie evidence that this method of attachment can carry an increased load.

This photo of a Wilderness roof rack opens the possibility that I could actually eliminate the light bar itself, install struts directly to the bottom on the bed board, and then attach lights directly to the front of the camper.


P.S. The Jeep roll cage does tie directly into the upper windshield frame.