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.

[rodneym]

So, are you at least going to use the quickjacks as intended?

[patrickt]

Originally Posted by rodneym

So, are you at least going to use the quickjacks as intended?

No, I am going to dangle my car from a crane, high in the air, and video tape it for everyone here.

[1795]

Originally Posted by patrickt

No, I am going to dangle my car from a crane, high in the air, and video tape it for everyone here.

Come on Patrick, we are all in need of something to keep us busy while we are all cooped up

[ERA 626]

From what I can remember my ERA quick jacks were for show and not very structural in nature. They may be able to support the 2400 lb car if pulled STRAIGHT but at the angle you have in the photo I would believe they will bend and break the fiberglass. why even try? just attach to the frame member below...
On the CSX cars, kirkhams, hi-techs etc... the quick jacks are structural and often used on a frame rotisserie as well as jacking up the car.

[twobjshelbys]

Quickjacks are not structural. Even on the originals they bent and you would see stress bends in the aluminum when the cars were lifted.

[Karl Bebout]

After I'd taken a good look at my quick jacks, I thought they should be ample to hold the car and I've used them with my floor jack once or twice.
Patrickt, thanks for the math that pretty well confirms my opinion.

[patrickt]

In my picture, with my car weighing 2,500 lbs., and the sling angles at 60 degrees, the horizontal force pulling sideways on each quick jack is 722 lbs. when the car is dangling from a crane. I think the threaded rods would handle that value even at their crappy low strength rating. Now, if we use the 750 lb. value to pull the car up the incline, the horizontal force on each quick jack is only 217 lbs. https://www.engineersedge.com/mechan...-equations.htm

[xb-60]

Originally Posted by patrickt

I’m no engineer....

I am. Here's some free advice....don't even consider doing a pull (or a lift!) using the sling and the two quick jacks.

Use one quick jack and only do a direct pull.


Stay safe!!!
Cheers,
Glen

[patrickt]

Originally Posted by xb-60

Use one quick jack and only do a direct pull.

OK, but if the crane won't hold my car from just one quick jack I'm holding you responsible.

[xb-60]

Originally Posted by patrickt

OK, but if the crane won't hold my car from just one quick jack I'm holding you responsible.

OK. I'm across the Pacific from you though

[kevins2]

Patrick, if you're looking for a secure way to tow the car onto a tow truck, take a look at the photos below. I use two 6' straps with one end of each secured to an A arm. From there it's a straight shot out each side of the oil cooler so no stress on the body.

[patrickt]

Originally Posted by kevins2

Patrick, if you're looking for a secure way to tow the car onto a tow truck, take a look at the photos below. I use two 6' straps with one end of each secured to an A arm. From there it's a straight shot out each side of the oil cooler so no stress on the body.

Well that is quite clever. That would clearly pass the crane test.

[xb-60]

Originally Posted by patrickt

Well that is quite clever. That would clearly pass the crane test.

ERAs are built strong. There's clearly one way to prove it...."The crane test."
Keep a camera on hand. I look forward to seeing the results

Stay safe!
Glen

[FUNFER2]

My buddy tried it with mine, and bent my damn rod.

[ACHiPo]

Originally Posted by xb-60

ERAs are built strong. There's clearly one way to prove it...."The crane test."
Keep a camera on hand. I look forward to seeing the results

Stay safe!
Glen

"Here, hold my beer!"

[DanEC]

Originally Posted by patrickt

Well that is quite clever. That would clearly pass the crane test.

Hey Bubba - hold my beer and watch this.

[ACHiPo]

Originally Posted by DanEC

Hey Bubba - hold my beer and watch this.

Damn, hate it when my smart-ass remark is just 20 minutes after somebody elses!

[1795]

As I have said in another post, and Glen confirmed, horizontal pull on one jack post will work without deflection to get your car towed up onto a tow truck, if the bolts are of sufficient strength and the quick jack mount is made of sufficient strength steel and welded well. Done it over a 100 times to get the race car on and off the trailer with no deflection in the mount.

Patrick, I did think that your wrapped strap was a good idea until others pointed out the bending force that would pull the quick jacks together.

[patrickt]

Originally Posted by 1795

Patrick, I did think that your wrapped strap was a good idea until others pointed out the bending force that would pull the quick jacks together.

Actually, I plan on fabbing up a beautiful looking folding tow hook on the front quick jack, with the tow hook receiver mounted behind the quick jack itself, the quick jack threaded rods and bolts all upgraded to Grade 8, and a new 5/8" hole going through the middle of the quick jack in to the receiver, so the tension from the hook is straight on the bolts and threads and nothing is in shear. The hook unscrews and can be moved from front to rear. But it doesn't hurt to work the crowd up every once and a while. This is what it will look like: