Underbelly project: tanks hang below frame members . . .

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

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.

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!

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.

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

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.

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

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 .

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.

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?