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Kirkham Motorsports

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  #1 (permalink)  
Old 04-04-2020, 01:06 PM
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Default Can I Dangle My Car in the Air from the Quick Jacks Alone?

Iím no engineer, butÖ 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 3/8Ē 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 donít, 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.

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Old 04-04-2020, 01:36 PM
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So, are you at least going to use the quickjacks as intended?
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Old 04-04-2020, 02:05 PM
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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.
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Old 04-04-2020, 02:18 PM
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Quickjacks are not structural. Even on the originals they bent and you would see stress bends in the aluminum when the cars were lifted.
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Old 04-04-2020, 02:20 PM
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Quote:
Originally Posted by rodneym View Post
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.
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Old 04-04-2020, 02:30 PM
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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
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Old 04-04-2020, 02:40 PM
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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.
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Old 04-04-2020, 04:54 PM
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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
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Old 04-04-2020, 06:38 PM
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Quote:
Originally Posted by patrickt View Post
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!!!
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Old 04-04-2020, 07:01 PM
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Quote:
Originally Posted by xb-60 View Post
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.
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Old 04-04-2020, 07:30 PM
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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.



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Old 04-04-2020, 08:53 PM
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Originally Posted by patrickt View Post
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

Last edited by xb-60; 04-04-2020 at 09:18 PM..
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Old 04-05-2020, 12:12 AM
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Patrick, it doesn't look to me like your math matches your intent. Yes, hanging in the air, the side force on the quickjacks is about 722 lbs for a 2500 lb car. but the sling tension of 1443 lbs needs to be added as you've looped the sling rather than using two straps and tying them off seperately. Neglecting friction, the tension in the sling is constant all the way around. So you may be looking at about 2165 lbs pulling the quickjacks together.

Your quickjack rods may not fail in tension or shear, but what you have here is a bending moment. If the rod length is 8" long, that's 1443 ft-lbs of torque. They will bend.

Might be a good idea to recalculate for that towing forced of 750 lbs. and look into the bending force for those rods.

Of course I could be wrong. It's 4:00 am here and I can't sleep for some reason.
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Old 04-05-2020, 04:52 AM
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Quote:
Originally Posted by kevins2 View Post
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.
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Old 04-05-2020, 05:01 AM
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Quote:
Originally Posted by patrickt View Post
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!
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Old 04-05-2020, 05:15 AM
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My buddy tried it with mine, and bent my damn rod.
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Old 04-05-2020, 05:57 AM
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I think the threaded rod was a B7 alloy. About 125 kpsi. I can't guarantee that we still get that same stuff.

HOWEVER! Bending stresses are much larger than linear stresses if you pull perpendicular to the shaft, and calculations get much more complicated (and depend on the tubing/housing size). We've always cautioned against using the jackpads as originally intended.
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Old 04-05-2020, 06:26 AM
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Quote:
Originally Posted by Argess View Post
Patrick, it doesn't look to me like your math matches your intent. Yes, hanging in the air, the side force on the quickjacks is about 722 lbs for a 2500 lb car. but the sling tension of 1443 lbs needs to be added as you've looped the sling rather than using two straps and tying them off seperately. Neglecting friction, the tension in the sling is constant all the way around. So you may be looking at about 2165 lbs pulling the quickjacks together.

Your quickjack rods may not fail in tension or shear, but what you have here is a bending moment. If the rod length is 8" long, that's 1443 ft-lbs of torque. They will bend.

Might be a good idea to recalculate for that towing forced of 750 lbs. and look into the bending force for those rods.

Of course I could be wrong. It's 4:00 am here and I can't sleep for some reason.
You are correct I think. They bend like a garden hose.
This video of a homemade bender, bending 3/8" all thread rod shows it.
https://www.youtube.com/watch?v=cNToITxxVp0
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Old 04-05-2020, 08:14 AM
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You are correct I think. They bend like a garden hose.
This video of a homemade bender, bending 3/8" all thread rod shows it.
https://www.youtube.com/watch?v=cNToITxxVp0
Hmmmmm, that was compelling.
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Old 04-05-2020, 08:38 AM
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Well that is quite clever. That would clearly pass the crane test.
Hey Bubba - hold my beer and watch this.
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