Underbelly project: tanks hang below frame members . . .

[profdant139]

Other folks must have encountered this problem, but I have not found any discussions of it. My tanks (fresh/black/gray) all hang down about six inches below the bottom of the steel beams that make up the frame of my trailer's chassis.

I had intended to use coroplast to enclose the underbelly and the insulation that I will be adding. But although coroplast is semi-flexible, it is not infinitely flexible -- I doubt it would clamp to the exterior beam flanges and still bend down around the tanks without breaking.

I see two possibilities: use a heavy plastic fabric instead of coroplast. That would be pretty simple -- it would just bend around the bottoms of the tanks. But I would not be able to glue the insulating foam panels to the top side of the fabric because the fabric would not be flat.

Or a more expensive and tricky solution: bolt some sort of downward extension to the beams -- maybe aluminum I beams -- so that the bottom of the "add-on" beams would be below the level of the tanks, so that the coroplast would not have to bend. I would also add thin aluminum cross-bars under the coroplast to support it. (And I would probably glue the waterproof styrofoam insulating panels to the top side of the coroplast.)

Fortunately, I have a lot of extra ground clearance, so I have plenty of room to add additional "sidewalls" underneath the trailer.

Your thoughts and advice, as always, will be greatly appreciated!

[profdant139]

Good tip on the self tapping screws. I am a through-bolt fanatic, though -- I like to over-engineer my project. I will try both methods.

And I just came across a great tip for drilling upside down -- use a scissor jack under the drill to push it upwards:

An easier way than what I have been doing

[Lantley]

Use self tapping screws and a large drill. I was able to drive self tapping screws into the flange of my beam. It was much easier than I imagined. It is certainly worth a try.

[profdant139]

Well, I took the plunge and just bought some used aluminum beams at a metal recycling plant -- they are C channel shape. (Actually more like the letter E without the middle prong.) . The flanges are 1.75 inches, and the center (the web? not sure what that is called) is 4 inches. They are heavy, but not nearly as heavy as steel.

I was tempted to go with the PVC lumber -- probably lighter and cheaper. But I want something that will hold up to a lot of abuse, and I don't know if PVC would have been able to take the stresses underneath a travel trailer bouncing down the highway.

Still debating about using beam clamps to fasten the aluminum beams to the bottoms of the chassis members. I will probably chicken out and use through-bolts -- I really do not want this assembly to come adrift at highway speeds, and the beam clamps are not designed to withstand high vibration situations.

And yes, I will put a gasket between the steel and the aluminum -- never would have thought of that on my own.

The hardest and most miserable part will be drilling holes in the flanges of the chassis members, laying upside down under the trailer with the drill motor in my tired arms. And that is a slow process -- the drill bit has to turn slowly, unlike going through wood. I wish there were an upside down drill press for those underneath projects. Of course, I will wear a face shield -- those hot metal shards will never get near my eyes.

I will use nylon core nuts on the bolts -- those are used in aerospace manufacturing, and they stay put.

Obviously, I will document the whole project carefully and will post a report when it is done. Which could be a while.

Thanks again for all of your tips and advice!

[profdant139]

SD, thanks for that tip -- I have never heard of this stuff! I work with real wood, but PVC is far superior for this application, I think. (I would never use wood for structural stuff underneath a trailer.) Although I have never seen this in my local big box stores, I have never looked for it, either.

[SDcampowneroper]

profdant139 wrote:
SD, that is a very interesting idea -- do you mean Trex lumber?

Dan, no, not Trex. Cellular core solid PVC dimensional lumber. Its used for trim on buildings. Ive used it as brick mould, as trim around garage door openings, to wrap steel posts, in dimensions as big as a 2x 6. it works just as easy as wood, never rots. No structural strength. Big box lumber stores have it in their millwork dept.
Max

[westend]

profdant139 wrote:
westend, is the gasket to prevent electrolysis?

Exactly! It's heartbreaking to accomplish a complex project only to find a few years later that metals are reacting and there is slight chance of recovery.

[profdant139]

SD, that is a very interesting idea -- do you mean Trex lumber?

[SDcampowneroper]

OP, Ok, I get your no sag with foam concept.
another idea instead of unistrut which is heat, corrosion, conductive we used pvc 'lumber' on a tt we modified for wintering over in Canada. Solid PVC , such as 'brick mold' fastened to the frame with sdst screws, 2" foam then the coroplast with wood screws to the pvc.
Pvc can be bought anywhere as dimensional lumber, shaped like wood has insulation value, without rot.
For heat tracing we used cut to length ss 5w' cable, old waterbed heaters under the tanks , all routed up to where we could plug them into the inverter when traveling or into a shore power plug .

[profdant139]

SDcamper, a rubber sheet or heavy plastic tarp would work, but it would not support the styrofoam. And a hole in the floor won't do me much good, since we so rarely heat the inside of the trailer (especially while on the road). I am going to install some heat tape or tank pads, for use when we have hookups.

kevden, I like the idea of unistrut -- very beefy stuff! That'll hold up the corplast and the foam, for sure.

I'm still trying to get a flat smooth bottom, rather than one that sags. So I am hoping to extend the beams in a downward direction.

[SDcampowneroper]

OP.
Excellent blog in your profile! You are quite resourceful.
How about a rubber sheet, like roofing rubber for your underbelly? perhaps cut a couple of 3" holes strategically through the floor, place a 12v computer fan in one to circulate warm air into the belly?

[kevden]

Beam clamps are available at Lowe’s or Home Depot along with uninstrut channel. I would sandwich a section of coroplast between the sagging tank and at least 2 equally spaced pieces of unistrut between the frame rails. You could fasten the unistrut to the frame rails with unistrut beam clamps( may vibrate loose on a trailer) , or drill and bolt the unistrut to the I beam flange. The coroplast between the tank and unistrut would insulate the tank from chafing on the unistrut (unlikely because unistrut is smooth with radius corners), and if you cut and install the coroplast it as a separate section (overlap the edges) it will be easier to service the tank later if needed. It should install relatively easy with empty tanks.
After posting, I re-read your first post to make sure that I understood. If insulating the tanks is the objective you could still use the unistrut to support the sagging tank, and sandwich closed cell foam board between the tank and the coroplast. You would just need longer bolts to fasten the unistrut to the i beam flanges (beam clamps would not work in this case). Also, this would be a good opportunity to install an electric tank heater pad.

[profdant139]

westend, is the gasket to prevent electrolysis?

[westend]

You need some Beam clamps and a material to make up the drop of the tanks relating to the frame. Your description as to what you would do with the area beneath the tanks is exactly how I'd handle it. I'd use aluminum too. I'd also attach a piece of flat rubber as a gasket between steel and aluminum mating surfaces.

Most larger cities have a steel and metals distributor. I use Discount Steel , as an example. DS will cut and ship if that's a necessity.

[kerrlakeRoo]

As kt said, cut the coroplast around the tanks and insulate above it, and then you could either insulate the tanks with something like a spray on foam insulation or glue on panels and then use some sort of fabric to close them in for waterproofing and road hazard protection. Rubber roof membrane comes to mind as a possibility.