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armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

I always wanted to do something with the wasted space behind the rear axle, and I finally found a rather unique use for it. Here's what I came up with...



It's just a sheet of 3/16" stainless cut and bent to shape, and it forms a great stash spot for a 4gal (or two 2gal) RotoPaX fuel (or water) can. It uses factory threaded holes in the frame rails to bolt into place (no drilling even!), and just a pair of Voile straps to hold the RotoPaX in place. It mounts totally flush to the frame rails so nothing hangs low to get hung up on things on steep departures, and of course the can is armored by 3/16" thick steel even so. The fuel/water can is admittedly a bit of a PITA to get in and out of there (just 'cuz you have to crawl under the truck to get at it), but it works: Just undo the two orange Voile straps and slide the can forward to pull it out. And it's low (for good weight balance) and totally out of the way (so it's there when you need it, and not in your way when you don't), so good from those perspectives.

FYI it looks closer to the exhaust than it is. But just to be safe I ran with a water RotoPAX down there first, tested in the heat of summer on a couple long slow wheeling outings, and the RotoPaX never even got warm to the touch. That said, it's certainly not compatible with any "high-tuck" exhaust kits.

To fab the thing, I bought the sheet of stainless from a local steel yard (Queen City Steel), made use of a CNC plasma cutter at our awesome local maker-space (Generator) to cut the shape, and then brought it back to the steel yard who also has an enormous press brake that easily put the 90° bend in it for me. Easy peasy!

Here's a dimensioned drawing and some more photos (click any one for higher res). PM me if you want to re-create this and I can provide a DXF file.

















Hope this proves useful to somebody else out there too!

EDIT to add link to my build thread: RavenFJ
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

two questions:

Rotopax need to be vented here and there as they get hot and expand. Are you able to do that easily? Will it spew out contents onto hot exhaust if/when you do that?

How about a higher speed rear collision....what happens to the Rotopax? Does it crush like a Ford Pinto and spray fuel all over?
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

Rotopax need to be vented here and there as they get hot and expand. Are you able to do that easily? Will it spew out contents onto hot exhaust if/when you do that?
Good question. RotoPaX suggest that you "burp" their gas cans periodically to account for changes in temperature/elevation/etc that cause the contents to expand & contract. And no, it's not really practical to do that with this setup: You'd have to remove the can from under the truck and place it vertically to "burp" it, then put it back. So no, I don't do that, and I initially had some issues as a result. But I've found what seems to be a pretty good alternative solution (so far):

When I fill up my RotoPaX tank, I forcefully compress it using a couple big C-clamps to reduce the volume inside. Then I fill to the very top (i.e. beyond the "fill line", right to the cap) to ensure minimal air inside (air expands and contracts in response to temperature and elevation much more than fuel or water does). Then I put the cap on good and tight before removing the C-clamps. With this approach, the RotoPaX contents have room to expand and contract without blowing the can up like a marshmallow and eventually pushing fuel through the cap. So far so good; I'll report if I run into any problems going forward.
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

How about a higher speed rear collision....what happens to the Rotopax? Does it crush like a Ford Pinto and spray fuel all over?
I won't claim to know for certain whether this is worthy of concern. But the RotoPaX is fully inside the frame rail, and it'd take quite a lot to crush that thing. The Pinto had a lot more going on than just a rear-positioned fuel tank:



In comparison, my RotoPaX can is tucked fully inside the FJ's beefy-ass frame-rails and beefy rear frame member (plus burly aftermarket rear bumper in my case), and further buffered by 3/16" thick stainless on the under side, with minimal nearby "puncture sources". Seems pretty darn well protected to me.

I guess I'd just say that given the choice between having a RotoPaX hanging off my spare tire (like many aftermarket mounts I've seen), versus having one tucked fully inside my frame rails and further shielded by a sheet of 3/16" thick stainless, my gut tells me the latter is likely the safer option.
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

I would like to think this would hold up better than a typical Jerry can mounted on the back of a bumper in the event of a rear end collision . Better yet some of these Bumper even have mounts for Propane tanks .
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

This is very cool. I'm tempted to make one. Grayson is there room for two 4 gallon cans? One lay on top of the other? Or is the body floor too close?
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

Grayson is there room for two 4 gallon cans? One lay on top of the other? Or is the body floor too close?
No room for anything more than one can as I built it. If you were to get a little more creative with additional bends in the steel plate, you could in theory drop the center plate down further to make room for two cans there. But you can't do the same simply by adding spacers because the tail pipe would interfere, so it'd involve more bends (like 5 bends versus 1), which would significantly complicate the fabrication process. Plus of course it'd no longer be flush with the framerails, and so you'd (in theory) lose a little bit of clearance or departure angle. So yes, I did give this some thought. In the end I opted for simplicity and clearance over capacity.
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

I was afraid things would be getting in the way. Very cool though. I still may build one, just have hard time pulling the trigger on a $100 gas can I won't use but twice a year. When my current cans still work. Thanks for sharing.
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

Good question. RotoPaX suggest that you "burp" their gas cans periodically to account for changes in temperature/elevation/etc that cause the contents to expand & contract. And no, it's not really practical to do that with this setup: You'd have to remove the can from under the truck and place it vertically to "burp" it, then put it back. So no, I don't do that, and I initially had some issues as a result. But I've found what seems to be a pretty good alternative solution (so far):

When I fill up my RotoPaX tank, I forcefully compress it using a couple big C-clamps to reduce the volume inside. Then I fill to the very top (i.e. beyond the "fill line", right to the cap) to ensure minimal air inside (air expands and contracts in response to temperature and elevation much more than fuel or water does). Then I put the cap on good and tight before removing the C-clamps. With this approach, the RotoPaX contents have room to expand and contract without blowing the can up like a marshmallow and eventually pushing fuel through the cap. So far so good; I'll report if I run into any problems going forward.
Very helpful idea on how to fill the Rotopax for Moab heat - thanks, and keep us posted on how it works!
 

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Re: armored RotoPaX "belly stash plate" — use that empty space behind the rear axle!

Good use of space. And thanks for offering the drawings up if someone wants to use your ideas.
 
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