This past weekend, a friend of mine was showing me pictures of his new 2 post car lift he put in his garage. He knows I’ve had a 4 post lift and we went back and forth with the pro’s and con’s of each. Two things we disagreed upon, and I was wondering if others could weigh in, were these: First, I told him that one of the con’s of a 2 post lift was long term storage of a vehicle on it, because the suspension components could develop a “negative set” from hanging down. Am I correct on this or not ?? Also, the other point I strongly disagreed with, is that you should pour 5000 pound (strength) concrete for 2 post lifts to be mounted on (versus typical 3000 lb concrete for a 4 post lift). He told me that the lift company told him that ordinary redi-mix (3000 lb) concrete was fine to bolt the 2 post lift to. I would think that with a vehicle being balanced on a 2 post lift, that there’s more stress on where it’s bolted into the floor. Who’s right on these 2 points ???
What’s a wheel weigh…15 lbs? With the axle, 25 lbs?
Test question: if a spring will support 800 pouds of compression for years on end without deforming, will it deform under 25 pounds of distention?
No, the suspension will not take a “negative set”.
Two pose lifts generally better underchassis access. The downside is that they may require more “site prep”. Many home-based 4-post lifts stand on the garage floor like a coffee table, and only compression forces are exerted. Two post lifts have to actually be properly mounted on a footing sufficient to hold the system stable.
All manufacturers of lifts have installations. These recommendations ahould be followed. The specific lift manufacturer’s website should have this information.
In short, for a four post lift the ability of the pads or floor upon whhich it sits needs to be sufficient. A two post lift may require that proper footings be sunk, like a large lightpost for a streetlight. Knowing the compression rating of the concrete is unsufficient to deterrmine if the lift is stable.
We had a Suburban up on a two post lift and one of the posts pulled fully out of the concrete and the other partialy. For vehicles as large as the Suburban I see little to no extra safety margin, I might add that working on Suburbans on two post lifts is done everday,does not mean you must like it.
The concrete requirements can easily be ascertained by going to the lift mfr website and see what they recommend. IIRC, the PSI rating is the same but the thickness is different for 2 vs 4 post lifts. Chances are, you could get by with a lower PSI rating for the 4 poster simply because the weight is distributed across 2x as many points (and it’s specified to be thicker) but for safety margin and less confusion, they probably will specify the worst case for both.
People may argue this point but I’ve had it happen so it’s my story and I’m stickin’ to it! Long term storage should be on the suspension, the way the frame was designed to handle the weight. If you suspend by the frame, over a fairly long time, the frame will be distorted by the weight. I had a fully assembled car on jackstands for a few years. The frame bowed from both ends being cantilevered. The engine side was the worst, naturally.
Ouch! Someone didn’t provide proper footings…
not necessarily…Suburban’s and Expeditions, and the like are far heavier than normal vehicles. A lot of shops won’t lift them because the lifts aren’t rated high enough to support the load. They can weigh in at up to 7200 Lbs, more when loaded. That’s about 1000 lbs more than your standard pick-up, and 3K more than your standard car. He also didn’t say whether it was a 1500 2x4 or 2500 4x4 model (or any variant in between). That matters. 
I think TT nailed it below. Just my .02.
Chase
I too agree with TT, and attempted to say in my earlier post that it’d critical to follow the lift mnufacturer’s instructions for installation. Generally when a post pulls up I’d suspect an insufficient mounting rather than an overload of the lift, but it’s really just a guess without looking.
Failure to provide sufficient footings for a 4-poster can result in cracked and uneven concrete, but failure to provide sufficient footings in a 2-poster can result in Oldschool’s experience.
The investigation did blame incorrect concrete mixing and the lift had just been put into service that day. There was a wall close enough so as that the Suburban (4X4) did not hit the ground.It was professionally installed at our local GMC Cadillac Dealer.
I was under the impression that 4 post lifts generally exert LESS pounds per square foot than a parked car under the same circumstances ?? I can tell you that the “shoes” (pieces that rest on the floor) on the lift that I have, certainly exert less pounds per square foot than any of the vehicles that I’ve placed on the lift. That’s because they’re probably around 1 square foot (each) which rests on the floor. I don’t know of any standard car tires that would exert that much on a surface. I could be wrong here ?? In any event, the manufacturer suggested a minimum of 4" of 3000# concrete and I had it poured around 5"-6" in the area of the posts.
The difference is that the lift sits in the same spot day in, day out, 24/7.
Again, with a sufficient floor under the lift, and base under the floor, it isn’t a problem. But garage slabs aren’t always poured over the best of bases.
My house was made in 1940. It didn’t even have a basement floor, nevermind a floor in the garage. Somewhere over the years a floor was poured in the basement (poorly) and a floor was poured in the garage (who knows). It works, but I’m not sure I’d want to put a lift on it. And I definitely would not want to put a 2-post lift there! There are countless houses like mine across the country.
Ouch! Thank God nobody was hurt.
I use a 9000 pound two post lift daily. We have four others of the same make and model in the shop. We have an old weaver 9000 pound two poster, and a 12000 pound four post lift. We have an alignment rack as well. One asymetric. There is a lack of working room under the four poster. Makes work very frustrating. In my 13 years here nobody has dropped a car/truck off a lift and it’s pretty common to lift loaded 3500 model trucks. I know for a fact every piece of concrete in my shop floor is at least a foot thick. The only problem I have seen is the whole 10 foot by 10 foot slab moving. As far as the suspension goes the only problem I could see is the shocks/struts being constantly extended. Ought to be good on the springs though.
The main thing with hanging suspension is that it can kill shock absorbers or struts due to the extension. Anything else should be fine.
I can’t speak with any authority on concrete but other than preferring the 4 post over the 2, I’d say the lift company guys don’t have their vehicles and lives on and under that lift.
It would seem to me that any price differential on the 5000 Lb. concrete would be money well spent.
There’s also some pics floating around on the net where some lifts have nosed over and caused all kinds of damage. Better to use overkill with the concrete is the way I look at it.
Two post lifts properly installed are perfectly safe. But it’s more than just the concrete that determines that. Concrete by itself has poor tensile strength, and proper reinforcement in the footings and proper embedding of the correct mounting studs is as important as the proper aggregate. Four posts lifts create only conpression forces, but two post lift also create substantial tensile forces. Improperly installed, a two-poster can pull its moorings right out of the concrete.
It sounds like there’re contractors out there that don’t know how to properly install these things.