Critique my chassis

[StanDahl]

Working closely with the generous support of eamarquardt, I've come up with this chassis design. Since my design is non-traditional, I think it requires a somewhat non-traditional frame. At points marked "A", there is a half-lap joint, at points marked "B", there is a chunk cut out and a bend in the frame piece with a crosspiece inserted. This design seems strong and doesn't have the dropped V leading to the hitch, which leaves a big part of the floor unsupported. A real-life specimen is here: http://www.tnttt.com/gallery/image_page.php?album_id=1588&image_id=34299

I'm not satisfied with the back end yet. The "B" points may be too radical a bend, but Gus keeps spanking me for putting butt joints there. There are still some unsupported parts, could angle iron be put in place there and nipped and bent to conform to the curve? I think that would keep the weight down. I'm thinking that the floor will be two sheets of 1/4" plywood with 1/2" of styrofoam in between with plywood or pine sandwiched in the edges so the foam isn't exposed on the outside. The whole thing will be wrapped in aluminum, with 1/8" baltic birch under that.

Picture 7mod.jpg (62.39 KiB) Viewed 519 times

Here's a side view. I'm still learning to use SketchUp, and making it show the actual bends and laps is way more labor intensive than its worth.

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Whaddaya think?

[parnold]

It's a very pretty blue!

I'm no engineer, but it looks more than sufficient to me!

I give it

[halfdome, Danny]

Pretty nice looking lay out .

At your "B" points I'd go with mitered butt joints, welded all around, ground flush to accept a sistered/welded gusset on the underside to give additional strength.
Any way that's how I do my chassis.
I also weld a 3/16" plate across where both side pieces meet the draw bar, top and bottom, after initial welding, as mentioned above.
Just my opinion.
Danny

[angib]

Yeah, those front B points are critical and making a good joint between the A-frame and the main rails is all that matters - the cross member is secondary. So get the long joints fitting right and 100% welded before trying to fit the cross-member in.

At the back I'd leave out the last cross-member altogether as it isn't doing anything that the two angled cross-members (the 'rear A-frame', so to speak) can't do. I presume you want a rear receiver - if not, I'd drop the rear centreline piece too.

[KCStudly]

+1 what Angib and Danny said.

As far as pie cutting and welding angle to form the rear curve, you would be better off following Angib’s suggestion to eliminate extra steel, and then reduce the ply thickness on your floor (maybe use 1/8 inch or 5mm, at least on the bottom), but go thicker on the foam and wooden sub frame. The further apart you put the stressed skin panels the more rigid the floor and the more capable it will be to suitably cantilever out past the steel without additional support. The amount of cantilever that you are depicting appears to be modest. At the front it is probably not as big an issue because you will not likely have big point loads right at the edge, but in the back where it is easily conceivable that you could be dragging a fully loaded cooler off the edge, stepping up or sitting on the edge it would be more important (assuming that your rounded rear opens like a traditional hatch).

If it were me, I would go at least 1 inch thk on the foam core, if not 1-1/2 (in fact, I will be using 1-1/2 thk foam with 5 mm ply top and bottom, and I have no cantilever to my floor at all, but am using relatively light angle intermediate xmbrs in my frame and don't want any sag under my 240+ lb. butt). Perhaps others can advise on the amount of cantilever vs. stressed panel height that has worked in the past.

[StanDahl]

Okay, how's this?

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I like the idea of trimming that last crosspiece, it seemed like there was just too much in there somewhere, but I couldn't put my finger on where. I will keep the centerpiece in the rear, it would be nice to be able to put a bike rack in there.

I would like to support the floor edges better, especially on the galley section where there could be some force (mis)applied at some point. As for 1/8" ply on the bottom, I know it's done, but it just seems sooo thin. It would have thicker wood on the edges, and I suppose over the chassis bars, so... maybe I should just trust you guys and go with it.

[Fishingtomatoseed]

If you are worried about the outer radius. You take your radius measurement to a metal shop and have them bend a pice of angle or square tubing to fit. Just another option. But like everyone else it will work the way it is.

[linuxmanxxx]

I build my floors with 3/4" foam 1x2 and 12" and 18" alternating centers and cross members matching my trailer right around 18" as well and I can walk on the 3/4" with very little deflection so anything over this is overkill and not needed. I use HD 5mm underlayment on both sides of the floor and its all glued solid with 3M 30NF water based contact cement and just a squirt of tightbond3 where wood meets wood after the 30NF is sprayed on and after all walls are up its strong as a tank pretty much. I go with the 1x2 flipped vertical for the roof to prevent sag and it works perfectly just double the 3/4" foam there. If you use the torsion box sandwich methods with the correct adhesives, you can build light and very strong and waterproof qualities built into the glue you use. I have hung wall out past the end of the trailer 6" so what you are laying out in the last would work perfectly and provide plenty strength to support your structure.

[KCStudly]

With double 1/2 inch in your floor, Linux, you probably didn't need the wooden sub frame and foam at all. I had suggested a thinner membrane, and was therefore thinking that more separation than the 1/2 inch that Stan (the OP) had proposed would be better, especially with a cantilever situation. Surely you did not use 1/2 inch ply in your roof?

More than one way to, er, well I like cats, so let's just say there are lots of correct ways to do a thing. Thinner skin with more separation equals light weight with high strength (but less puncture resistance). Thicker skin with less separation equals same strength but more weight with higher point load and/or puncture resistance.

I think we can all at least agree on that.

The 1-1/2 inch thick foam I intend to use in my floor is not only for strength, but for insulation and acoustic value, too. May be over kill for those purposes as well, but that's how I have decided to do it.

Nice to have so many here doing so many different things that we can pick and choose which is the best for our own.

[linuxmanxxx]

I use just simple 5mm underlayment (luan) on top and bottom with 3/4" foam inbetween that matches the thickness of the 1 x 2 runners laid flat. In the roof I just simply turn the 1 x 2s vertical and use double cuts of the 3/4" foam. There isn't any 2" foam available in Texas plus that would just take up too much cabin space loss to the thickness of the floors and walls. I can walk on the roof of my camper after completed and walk on the floor nonstop finishing the interior until I close the roof in so believe me when I say 3/4" is plenty strong enough in a torsion build. The wall thickness runs right at 1 1/8" overall but my last build I went with 1/8" panelling inside so it's a shade thinner than the 5mm luan and no finishing required just throw in trim. It's white oak and was pretty cheap and even more weight loss in the build. Got it at mccoys and almost all their panelling was 1/8" type so lots of choices there.