The Allwood - it's all wood

Design & Construction of anything that's not a teardrop e.g. Grasshoppers or Sunspots

The Allwood - it's all wood

Postby angib » Fri Nov 20, 2015 4:59 pm

OK, it's about time we had some heresy around here. I've said before that I think it's possible to build an all-wood teardrop without any metal. Well, I admit making the suspension and coupler in wood would be difficult.

The Allwood

So I've been drawing up what I think would work. This is effectively a boat on land and needs similar skills to build it. Doing it without epoxy might be possible, but I doubt it. This is not some simple, cheap alternative to buying a steel frame - it would be both harder and more expensive.

Image

The fundamental thing is the body design has to change to eliminate the thin, flexible tongue or A-frame that cannot be made easily in wood. Instead the whole body extends down to the coupler. The Wanderbug in the Vintage Plans section has some diagonal braces from the body to the tongue that do a similar job:

diag-tongue-braces.jpg
diag-tongue-braces.jpg (77.62 KiB) Viewed 2443 times

A similar monocoque/unibody design could be used for an all-composite teardrop.

I don't claim this is a proven design - it's just a starting point. I don't mind if you don't believe it's wise, or even possible, but if so I hope you won't mind if I don't believe you. Anyone who has experienced the forces that go through a large sailboat when it gybes won't think a few bumps are a serious problem, and there are almost no sailboats built with a metal chassis inside them!
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Re: The Allwood - it's all wood

Postby Tom Kurth » Fri Nov 20, 2015 8:50 pm

Not to criticize the triangular nose, I'm sure it provides the necessary strength for tongue support. But, is there any reason one could not just make wooden I-beams to run back to the running gear?

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Re: The Allwood - it's all wood

Postby angib » Sat Nov 21, 2015 5:07 am

Tom Kurth wrote:But, is there any reason one could not just make wooden I-beams to run back to the running gear?

Like this, but in plywood:

Image

They would need to be a substantial depth, maybe 8" or 10", to have the same exposed length as a conventional A-frame and then they need to continue back under the body, since it would be very hard to connect them into a 'normal' teardrop body. So that body would have to sit 8" or 10" higher to fit them under.

The logical thing to do would be to make them a shallow angle and run them straight from the coupler to the axle. The axle could run through the I-beams, to save height.
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Re: The Allwood - it's all wood

Postby pohukai » Sat Nov 21, 2015 12:18 pm

I do like the design shape. It really uses the frame to maximize storage.

I don't know what the conditions of the roads are where you live nor how it will be used, but for my type of camping and the long freeway drives concern me if I were to build a 100% wood trailer. My concern is not so much the occasional big bump (although it is even a concern in my steel frame), it is the fatigue ... the small but constant impacts that scare me.

I wish you well and I look forward to reading about your build.

Jim
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Re: The Allwood - it's all wood

Postby angib » Sun Nov 22, 2015 4:31 am

pohukai wrote:...it is the fatigue ... the small but constant impacts that scare me.

I couldn't agree more - though I'm uncertain if the word fatigue can only be used about metals. But, yeah, low stress cyclic loading, repeated maybe 100 million times, and the creation of cracks that can grow, is a big concern - just as it is for metal frames.

This is the reason why I made the comments about cabinetmaking and housebuilding skills not being enough, as their jointing techniques rarely achieve full strength of the parent material. Things like biscuits, splines and pocket screws may make decent glued joints but they are terrible stress raisers.

Most of a conventional teardrop body could be made from 1/8" ply but it would have poor knock resistance - turn over in the night and you would risk cracking the sidewall with your knee. So a lot of the joints only need to be strong enough to join 1/8" ply. This certainly wouldn't be true for an all-wood monocoque and I did give quite a lot of thought to this. It's why there are ply brackets fitted underneath all the floor framing so that those joints cannot open up even a little under cyclic load. Similarly where extra material is added near the coupler, all pieces have tapers on them to avoid a stress-raiser where they end.

Using epoxy, one of the important things is to design joints to avoid peeling stresses as that is where it is weakest. In a few places such as fixing the floor ply to the underside of the two bulkheads, I would happily add some screws to prevent peeling, but only because the peeling loads aren't great there.
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Re: The Allwood - it's all wood

Postby Tomterrific » Sun Nov 22, 2015 8:20 am

Wood is strong in compression but weak in tension. Steel is strong in tension.
You can use wood for the frame but the tongue should be steel or a wood/steel structure. Consider treated pine for the frame assembled with bolted plates as well as glue.

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Re: The Allwood - it's all wood

Postby Tom Kurth » Sun Nov 22, 2015 4:56 pm

So if one was to make his own I-beam, could he make a curved I-beam that would eliminate the depth concern by curving it from the hitch to align with and become part of the base of the wall? As a woodworker of long experience, making such a beast wouldn't phase me at all, but not being an engineer like you, I would have no faith in it.

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Re: The Allwood - it's all wood

Postby pohukai » Sun Nov 22, 2015 5:03 pm

Here's a good link/reference for wood compression vs tension.

http://www.fpl.fs.fed.us/documnts/pdf2001/green01d.pdf

Even with this information, knowing the loads is something most, if not all of us will not have available. Designing with natural materials will always be more challenging since the material is not 100% uniform.
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Re: The Allwood - it's all wood

Postby angib » Mon Nov 23, 2015 2:07 pm

I think Tomterrific must have got timber mixed up with concrete, which does have poor tensile strength.

Anyone who thinks timber has poor tensile strength needs to look at timber roof trusses where timber in tension holds up most modern roofs. Or covered bridges, for that matter, some of which have lasted 200 years with timber in substantial tension.
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Re: The Allwood - it's all wood

Postby rowerwet » Wed Nov 25, 2015 10:32 am

Add epoxy and fiberglass or carbon fiber to wood and you can do just about anything to it. God's composite is still the best.
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Re: The Allwood - it's all wood

Postby rowerwet » Wed Nov 25, 2015 10:52 am

Tom Kurth wrote:Not to criticize the triangular nose, I'm sure it provides the necessary strength for tongue support. But, is there any reason one could not just make wooden I-beams to run back to the running gear?

Best,
Tom

The beauty of this nose/shape is that you make the walls the structure that carries the whole stress. No weight wasted on members only there to contain.
That nose could be easily made into a tongue box by adding a door to one side, with proper reinforcement to carry the stresses.
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