Making a Flatter/Lower Flatbed, ?'s


In my quest to keep things as low as possible in my F450+Sunrader build, I have dreamt up a plan to build my flatbed so as to keep things as low as possible.
My flatbed will be spring mounted, similar to a M35's system:
army duece dot five box_mounts 03.jpg

My flatbed will be similar to Boblynch's in the F550 Flatbed build thread:

Boblynch went with the 2x4 steel main beams, that rest on the frame rails, usually with wood strips between them. He did this in order to keep things low, some larger trucks have 6" tall beams. In his illustration the darker color indicates 2x4, the rest is 2x2.

I want to keep things even lower, if possible. To do this I came up with the idea of two X's within the flatbed framework. All of the steel is 2x2 in my concept and the "outriggers" are welded to the side of the main beams, except for the front-most and rear-most that span the entire width of the flatbed. (sorry for the lousy sketch, not to scale, 1:12 scale approx):

The main purpose of the main beams being larger than the rest is to stiffen the flatbed. The X's will stiffen the flatbed, the question is if it is enough.

If you want to know more about this, I found this link helpful. It covers chassis design and the link takes you specifically to the torsional rigidity pages.

To those with real world, fabrication or engineering knowledge, please let me know your thoughts. Can I get the stiffness I need with this design? Because the flatbed will be spring mounted it will not have as much twist put into it, but it will receive stress. I also wonder about the number of X's, I suppose it could be anywhere from 1 to 3. Thanks a lot.


Not Lost, Finding Another Way
I have a few years in the shipyards as a ship fitter, so my experience isn't all encompassing, but I do know a little about fabrication.

I would think that your overall design is fine, but unless you are worried about weight versus strength, I would add a some more support in the outer sections in a "Z" or "zig-zag" pattern. Using 2" square tube should be sufficiently strong to support most loads that your truck will encounter. If there is an issue with ridigy and you are still wanting to keep a low profile, trussing the underside would be a possibility. Obviously trussing would have to be sectional in order to keep the deck height down. I would think that you could either literally have a ~21" truss or a truss that ties into the frame rails with the use a rubber or poly mount.

Just some ideas. I'm sure someone with more knowledge will correct any ****** ups in my thought process, but it's a start.


Kodiak Wrangler
I think it looks great.
Since you are putting the Sunrader on, the load should be spread out pretty well.
Just take a look at were water tank(s) might be if they are outside of the center frame.
Wood is fine for the 'cushion' but I would use something like UHMWPE that has more 'slide' (less friction).


From a packaging standpoint it really makes mounting anything under there a severe PITA when you're dealing with out of square members.

Also, 2x2....?.. What's the wall thickness? What kind of gusseting are you planning?


Quick clarification - our 2x4s are spring mounted to the frame rails and we do not use wood spacers. The setup is very strong and has served us well. Good luck with the project.



Any progress on this project?

You're on the same track that I'm thinking of taking for a planned build.

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I'm not sure I visualize properly, but Xs really are good for keeping things square corner to corner, which in a flatbed does not see any stress . Does your design still sit on top of the truck frame rails?, if so I would run more 2x2 cross wise like in the design you show above. This won't add any height, but will make the parts of the bed outside the frame rails stronger.
It's possible I have the wrong mental picture :)


Expedition Leader
If you plan to remove the Sunrader camper from time to time, then trying to lower the flatbed a couple of inches might be worth the effort. If the camper will be more or less permanently attached to the flatbed or subframe, then you can accomplish your goal of lowering the center of gravity by locating heavy items like the house batteries and water storage beneath the subframe. There's quite a bit of room up inside the vehicle frame rails.