by KCStudly » Sun Sep 20, 2015 7:40 pm
When you try to bend something it may fail in shear (by breaking clean thru at a support point or where loads oppose, like scissors cutting) or in bending at mid span when the outer fibers (the ones that have the most "leverage" applied to them) fail.
A materials ability to resist shearing forces is directly proportional to its cross sectional area; so is easy enough to have enough material present. Whereas a given member's ability to resist bending is dependent on the materials inherent strength, the span, the load, and the moment of inertia. Moment of inertia is a ratio (expressed in inches to the quadruple power) that relates to how much of the cross sectional area is located further away from the sections center. What this means is that a shape like an I-beam with a lot of its mass far away from the center (the top and bottom flanges), yet still rigidly separated by the middle web, can support loads equal to a much heavier solid rectangle shape. In turn, if the same weight of material was reformed to place the outer fibers further apart, even though these fibers are not individually any stronger, they have more leverage to resist the bending forces because the are further out.
Bear with me, I'm getting there.
The floor and trailer chassis are, in structural terms, a relatively thin "membrane". Because they don't have much depth, the outer fibers aren't very far apart and therefore they don't resist bending that well. Now the walls and bulkheads of your camper's cabin have the top and bottom fibers very far apart. In effect they are very tall I-beams and if properly supported from bowing, will be very rigid indeed.
So now consider the box. You will have a front wall and a bulkhead (... and maybe a short rear wall, if your feet will extend under the galley counter) spanning across the trailer frame extending out on ether side. If you were to only place this, standing next to the trailer, if you reached up and grabbed the top of either, you would likely not be able to flex it downward, even if hanging your whole weight on it. Sure, the suspension would flex and the the chassis, at this point, might even twist like a potato chip, but the bulkhead itself would be very rigid in this mode.
Now add a side wall supported by the bulkhead and front wall. So now you have a beam that is about 4 ft deep in section and only about 7 ft in span. That's a short stiff beam. Sure, if you push on the side of the wall at this point you might be able to get it to bow in or out (thin membrane), but if you were able to stand on its top edge and balance yourself, it would surely not bend downward.
Let's complete the puzzle. Add the floor (not necessarily the build order, just for the sake of analysis). If we don't attach the floor well to the side walls and go stand inside near the wall; sure, that thin membrane would flex and bend down; but if we do attach it well to the walls, that same rigidity in the walls will stiffen the edge of the floor. Our weight will be carried back thru the side wall, into the bulkhead and front wall, where it will be carried to the chassis.
Now add the roof. The tops of all of the walls are now well supported against bowing in or out, further stiffening them and allowing them to handle even more vertical loading. Add cabinet frames that are tied into the walls like bridge trusses and things get even stiffer.
So your 1 ft overhang will be supported by the trailer frame along one edge, and by the wall along the other edge. 1 ft is a short span.
The bigger concern is the reduction in footprint of the tires relative to the sail area and height of the side walls. As the sail area increases and the center of that area gets farther from the ground, the force from a side wind has less of a brace to oppose it. It is easier to knock over a skinny person on stilts than it is a lineman in a three point stance.
So like Bob Henry's Chubby example, building fat, low, and heavy is not a big concern for a relatively narrow axle. But like a certain vardo being built now, and Penguino I, building tall and large on a narrow axle can be a real concern, especially if built lightly.
KC
My Build:
The Poet Creek Express Hybrid Foamie
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"
TnTTT ORIGINAL 200A LANTERN CLUB = "The 200A Gang"Green Lantern Corpsmen
