## Are Tumbleweeds, vardos etc. safe for the highway?

Of course it is not quite fair to pick on tumbleweeds, but. I previously wrote just a little bit about the mechanical strength issues for tinyhouses.

By using the “drag equation” (wikipedia) you can see what the force will be on the house at whatever speed, and for whatever size of house. Ignore the car, Use meters per second and 1.2kg/m^3 and the result is in newtons. If it was e.g. 2.5 meters wide, and 4 meters high, and had a rectangular face facing in the direction of motion it would be less than 5000 newtons at 100 KPH. 9.8 newtons weight per kg, so that’s the weight of 510 kg.

So an awful lot of force. I was not able to find the strength of screws in plywood anywhere (pullout force) but I wasn’t going to spend ages on it. But you can imagine it could be 100 kg easily for the small screws jay shafer (sp?) uses, which are embedded in 1/2 inch plywood.

Then you think about the geometry and leverages involved, as I mentioned in the previous post, and the weight of the house. So if the house is say 1.5 times as high as it is long, various forces are acting on it, but you can think about the various scenarios, if it was only connected to the trailer at say 2 points, the brown dot and the red dot. Then the drag force is applied at the purple dot in the direction shown, the movement is constrained to the path perpendicular to the big diagonal line through the house, so the net force upwards, if you ignore the weight of the house, exerted on the connection at the brown dot due to translation is drag force*height of house/length of house. Obviously at the green dot halfway along the house it is twice as high.

So it is not much, and should be handled by screws easily enough, and actually just the weight of the house is strictly speaking probably enough.

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But you need a fair safety margin like 3 or more since failure would be catastrophic. But it looks like if you use a good number of screws you should be okay, anyway.

Also, during a sudden braking event the force could be, what was it, I said in the other post I think, 2 gs or something, which will be shearing the house off the trailer, so several tons worth of force. Still looks like that could be totally handled without any special building techniques.

Of course Jay and other builders and designers should still, IMO get their designs reviewed by a competent pro with the right expertise. But it looks like there is not necessarily any cause for alarm.

this is a concern of mine; I might wait until the Tumbleweed House Company gets new structural stress data supporting an improved Tumbleweed before I build mine, because their style is unbeatable! 🙂

cedarlikesMarch 26, 2011 at 08:42