Sizing jack stands for wind load?
I often camp in windy places. In my current, small van, I sometimes have to lift the tires off the ground to get stability. I use jack stands that are each rated for my whole gvw, to allow for th...
Forum Discussion
Naio wrote:
I'm thinking it not just the weight of the rig. Force is mass times acceleration. I don't know what the acceleration is when a 60mph gust hits my van broadside, but it's -something-, and that's the bit I am thinking about.
These are pretty much textbook problems in statics. (Statics is the analysis of forces on things that are not moving, and is a basic part of mechanical engineering. The companion study of dynamics involves the analysis of forces on things that move.)
F=ma is a description of how a body reacts to the net forces on it. The word "net" is key; a force may be applied, but if counteracted by an opposing force, it doesn't cause any acceleration.
We have many, many cases where forces do not result in acceleration in everyday life, though we don't think of them as such usually. DrewE right now is sitting in a chair and has a force of about 150 pounds exerted on him by gravity. This is not causing any acceleration downward because the floor (via a chair and DrewE's feet) is exerting an opposite force of about 150 pounds upwards. If DrewE were in an elevator starting upwards, the force from the floor would be something greater than 150 pounds, and DrewE (and the elevator car and whatever else is in it) would accelerate upwards. Once the destination was reached, the force from the elevator floor would be less than 150 pounds, and DrewE would accelerate downwards, or equivalently decelerate upwards.
For your van, the basic assumption and goal is to have no acceleration, or to exactly counteract the forces of gravity and from the wind with opposite forces applied through the jack stands. If the wind force becomes great enough, the jack stands might not be able to apply an opposite force, either because it fails (and crumples) or because its base is too small for the horizontal force required (and the jack stand falls over sideways) or because the force required from one jack stand to maintain stability ends up being in the downwards direction (and the van falls over off the jack stands). We could do analyses and math and such to figure out when that would be, but suffice to say that for realistic situations your setup is OK. I'm assuming you wouldn't put yourself in the situation of being parked broadside to hurricane winds but would instead use the wheels and engine of the van to get out of Dodge.
