Can I Dangle My Car in the Air from the Quick Jacks Alone?

[patrickt]

I’m no engineer, but… my six foot synthetic web sling pictured below is rated at 3,100 lbs. in vertical load. As fashioned in the pic, as a basket sling hitch snaked behind the quick jacks, that load limit is doubled and then reduced by the sling angle of the legs which, in this case, is about 60 degrees. That reduces the effective load limit to .866 of the doubled value. So, that 3,100 lb. synthetic web sling can comfortably hold over 5,000 lbs. when used as a basket sling, as pictured below.

The two quick jacks on each end of my ERA Cobra are bolted to the frame via four 3/8” threaded rods of unknown grade. The lowest grade steel threaded rods, bolts and washers available from either Grainger or McMaster-Carr show the tensile strength of their crappiest low strength steel to still be 40,000 psi or higher (in comparison, their grade 8 rods are 150,000 psi). If we take that 40,000 psi number, and multiply it by 0.0775 (the tensile stress area for a 3/8-16 rod), and then take that value down to 60%, for a hefty safety margin, we get 1,860 lbs. per rod. Four rods gives us over 7,000 lbs. of safely supported load even with the crappiest low strength steel.

So, I say I can take my 2,500 lb. ERA Cobra and easily dangle it in the air from a synthetic web sling attached to the quick jacks alone. I could even jump up and down on it while it was dangling. Mostly, I just wanted to figure out whether I needed to upgrade the hardware to be safely pulled up on to a flatbed via the quick jacks, which I clearly don’t, as it only takes about 750 lbs. of force to pull a 2,500 lb. car up a 15 degree load angle at 1ft/sec^2. So, there you have it.