Sizing jack stands for wind load?

DrewE, the van does move in the wind. It is not static. It flexes, rather than falls, so far.

The calculations for this are beyond my knowledge, but I am hoping one of the engineers here will weigh in.

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.

Wow, Mex, that is amazing! I would never think the dirt would hold like that.

Probably not relevant to me, in temporary camps, but fascinating.

B. O., I never said anything about the van blowing onto its side, never thought it would. However, if it came off the jack stands while I was asleep that would kill my back. THAT is the problem.

If the wind is strong enough to blow your van off the stands that is the least of your problems.

Will they allow you to bury two small junk car tires? Not a joke. If you can do that you can do the same thing as I have on Quicksilver. Bury tires or even 2 feet square 1-1/8" sub floor grade plywood.

Drill two holes 18" apart in the center of each. Half inch NYLON rope will hold your van.

Bury squares under 18" of dirt or sand. With sand pour water on top to compact the sand.

Sticking out of the ground will be two loops you tied into the above-ground NYLON rope.

Attach one cargo ratcheting load strap to each loop and the other end to the frame of your rig.




Keep pulling the lever ratchet-ratchet-ratchet, until the strap tightens.

https://www.harborfreight.com/catalogsearch/result/index/?dir=asc&order=EAScore%2Cf%2CEAFeatured+Weight%2Cf%2CSale+Rank%2Cf&q=cargo+ratchet+strap

Quicksilver laughed at a 150 mph broadside hurricane. It didn't even quiver.

Overthinking this. If it falls off a stand where is it going, it will just land on the wheels. Never saw a van blown over by anything less than a tornado.

B.O.

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.

Naio wrote:

DrewE wrote:

Do bear in mind that jack stands are often sold in pairs and rated for load for the pair, with each of the stands actually rated individually for only half that weight. It's sometimes a bit misleading, with the small print taking away what the big print giveth.

Oh, dang, I did not know that!

So the 3ton stand I have are really 3000lbs each... that still might be ok for the front, where gaw is around 3000lbs? And for the back, where gaw is around 6000, are 5 ton / 5000 jacks ok?

I do know to go with steel. I was exited over the lightness and high weight rating of those screw top aluminum jack stands -- until I saw a couple break in two when a jack slipped. Not good for sudden shear forces.

You really have to look at the stands or their instructions or packaging to know for sure. Some are rated by pairs, some individually; I've seen both. I think usually if they're sold as a set of two they rate them by the pair, but if they're sold individually the rating is for the individual unit; however, I wouldn't treat that as anything more than a rule of thumb.

Realistically, so long as you have some reasonable margin over the axle weight divided by two, you should be OK. 5000 pounds is in my opinion quite sufficient for a 3000 pound load, even taking into account wind forces. If the wind is blowing enough that you have 5000 ponds on the one side and 1000 pounds on the other, you probably don't want to be anywhere near the area, particularly if there are trees or sand or what have you around. I'm assuming you aren't intending to camp on the summit of Mt Washington, for instance, where the old observatory/stage road office building is literally chained to the rock to keep it from blowing away (and where the highest recorded wind speed was observed, at 231 mph).

Naio wrote:
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 the increased load with wind gusts. 45 mph gusts are common. (When they reached 60mph this winter I went to a motel for 2 nights, but my cat hated that. I thought my little van might blow off the jack stands!)

I guess my current ones will be ok for the front of my new van, but I will need a stronger pair for the rear.

Is gvw a good guide? Or how SHOULD I estimate wind forces and strength?


Are HF stands ok?

Thanks, always.

Your problem is similar to what a ship-of-the-line had-- to keep the lower gun ports aimed at the dastardly French ship downwind of her. (The good guys--us--always had the windward gauge)

(Think camping neighbour with outside speakers, playing Hip-Hop.)

A "topsail wind" is 2 lb per sq ft, so your van will heel 7 degrees. This puts that much extra pressure on the jacks to keep it level, so your lower gun ports are not pointing at the ground next to your van, and you want to use them to add to your fire on that annoying French guy down wind in the next camp site.

So now it is all math-- how much extra weight is on the "down" vector from your van NOT heeling 7 degrees in a topsail wind? (The simple solution is left to the student.)

DrewE wrote:
Wind isn't going to put more than the full vehicle weight on any one jack stand, since the applied force on the van is in the horizontal rather than the vertical plane. Your approach is safely conservative.

Do bear in mind that jack stands are often sold in pairs and rated for load for the pair, with each of the stands actually rated individually for only half that weight. It's sometimes a bit misleading, with the small print taking away what the big print giveth.

Oh, dang, I did not know that!

So the 3ton stand I have are really 3000lbs each... that still might be ok for the front, where gaw is around 3000lbs? And for the back, where gaw is around 6000, are 5 ton / 5000 jacks ok?

I do know to go with steel. I was exited over the lightness and high weight rating of those screw top aluminum jack stands -- until I saw a couple break in two when a jack slipped. Not good for sudden shear forces.