DIY Welded Tube Steel Subfloor

[perryg114]

Yes it happens all the time. It is one reason the bottom skins fall off the trailer. Water and different metals = corrosion. There is no such thing as a totally dry Airstream. The aluminum acts as an anode and corrodes. The is especially a problem at the steel hold down plate at the back of the trailer that is riveted to the aluminum skin.

Perry

[Kemblkr]

Originally Posted by Jacob D

This reminds me of a question that pops into my head when we talk about rotten floors etc. What do aircraft manufacturers use for floor boards. Isn't is some kind of sandwich board with aluminum sheet top and bottom? Wonder why Airstream doesn't do something like that to permanently do away with the rotten floor problem and maybe lose some weight in the process.

Many smaller airplanes actually are made almost completely of wood (wings, ribs, fuselage, etc). The trick is to coat these woods with a moisture barrier and they look like new after 50 years. Spar urethane works great and it is very disheartening that anyone builds an RV floor without protection. If you notice Airstream is now coating all edges and the tops and bottom of the subfloor perimeter with a sealant. Simple? Hell yes and should be done from the get go.

[Boothy]

Originally Posted by kevinkatz

I think that this is a good idea to get the plywood inside the body.
How did you bend the metal to get such a smooth and accurate curve?
How difficult was it?
Did you double it up to get the extra width to match the channel? Would it have been too difficult to bend larger material.

I made a template out of ply based on the interior curves of the trailer and then mounted that template to a full sheet of 4x8 ply to create a rudimentary bending jig. Cut a few of the scraps up and make stops with them and you've got a pretty configurable setup. start with bending each corner from the center line of the trailer and brace it then weld it across and tack it together at center. You then have a completed single layer and basis to expand the jig and add the second/outer layer of square tubing.

Bending was very easy, I laid the setup on the flattest section of floor I could find and put my weight on the jig to hold it in place. Just wear kneepads. Surprisingly there was little to no twisting or distortion, if one uses care when welding.

doubling the material up achieved two things- strength, and ease of machining. When you double up the two squares you end up with a 1/8th I-beam in the center of subframe framing. I feel that it was the only way to get adequate strength. It is VERY difficult to bend rectangular material, and I could not find anywhere in the bay area with tooling for 5/8 square anything. Not to mention all 4 curves are different!

[Boothy]

Originally Posted by wdgwdg

Boothy, Perryg114 brings up a good issue. Galvanic corrosion. It's much worse in a marine setting, but must still applies to airstreams somewhat. I don't know if it has been a problem with the long term experience of steel base frames and aluminium skins.
1. Do you plan on any isolation of steel from aluminium? 2. Does anybody know of these issues in a trailer setting , rather than on boats?

1. Yes. I pre-coated the entire subframe with por15 which will stand up to just about anything. Then I wrapped it in Poly insulating tape. If the metals cant touch, a corrosion circuit cannot occur.

2. Galvanic Corrosion depends on a lot of factors from humidity, salinity, and the amount and thickness of dissimilar metals in contact. While there are common practices each application is different. Im considering placing sacrificial zinc anodes along the bottom of the belly skin and frame rails so that the trailer frame doesn't turn the belly skin into swiss cheese.

I am a failed Mech Engineer and made it through material sciences so am familiar with galvanic issues. The airstream is parked about 100' from the wrath of the pacific near SF so this will come to play.

I've done some rough back of the envelope calcs and the corrosion cell could generate between 1-2V. The Al being the cathode and the steel being the anode in the varying thicknesses it would take a VERY VERY long time for substantial corrosion to occur IF the trailer is kept sealed and maintained. This frame is not an excuse to not maintain, seal, etc.

[Boothy]

Originally Posted by Kemblkr

Many smaller airplanes actually are made almost completely of wood (wings, ribs, fuselage, etc). The trick is to coat these woods with a moisture barrier and they look like new after 50 years. Spar urethane works great and it is very disheartening that anyone builds an RV floor without protection. If you notice Airstream is now coating all edges and the tops and bottom of the subfloor perimeter with a sealant. Simple? Hell yes and should be done from the get go.

Fascinating stuff! Amazing the things straight grain wood can do!

[Boothy]

Update-

Trailer frame has been completed.

Added extra outriggers and bracing to maintain rigidity offered by ply tied into trailer frame.

Center section of trailer has no subframe cross supports so it will be easier to deck out in one run.

Everything worked out awesome. The added cost for the steel and consumables was about 120$ and about 20 hours of additional time.

[kevinkatz]

Thanks for sharing and your detailed reply. You have done a great job and I may very well take inspiration when I build the chassis on my 1949 Curtis Wright.