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

[Buzz]

Originally Posted by danec

hey bubba - hold my beer and watch this.

:lol: :lol: :lol:

[patrickt]

Originally Posted by Buzz

:lol: :lol: :lol:

O ye of little faith. And on Good Friday of all days.


Here are some progress pics. As you can see, I drilled the hole and have it put together. Here's how it will appear after I mount it back on the car. I still have to cut, debur and chamfer that hefty bolt that is about an inch too long. Remember, it just unscrews, so it's there when you want it and gone when you don't. The trick to getting that hole cut perfectly was to use a cobalt bit and cutting fluid. Shortening the Grade 8 bolt shouldn't take long, but that can wait 'til tomorrow.

[Argess]

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.

[spdbrake]

Originally Posted by Argess

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

[patrickt]

Originally Posted by spdbrake

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.

[strictlypersonl]

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.

[t walgamuth]

Looks nice!

[razerwire]

I have jacked up car with rod between quick jacks and one floor jack in center of rod, I then put jack stands close to quick jacks but there is extra reinforcing from frame on my car. Only continuation competition and challenge cars are reinforced to my knowledge. Would not dare to even try on Bennett car.

[patrickt]

Well this should do the trick.

[awfink55]

I thought April Fools Day was a few days ago????

[patrickt]

Originally Posted by awfink55

I thought April Fools Day was a few days ago????

Hardly. This weekend I'll fab the back mounting for that directly behind and through the quick jack plate and mount it so that it's easily removable for placement on the rear quick jack or in the glove box. Now, there are a lot of cheap "cosmetic" tow hooks out there for as little as ten bucks. If you buy one, make sure you buy quality. There are a number of test videos on YouTube where they destroy the cheap knock offs. That little gem in the pic goes for $125.

[FredG]

Yes absolutely. Go for it. Please post the video. LOL

[patrickt]

Originally Posted by FredG

Yes absolutely. Go for it. Please post the video. LOL

Now, you know I'm not going to actually dangle my car from a crane, but this installation should be able to easily handle it, if I so chose. The installation is directly through the face of the quick jack, which is 1/4" thick steel. The tow hook is fastened with a 5/8-11 Grade 8 flange bolt. The bolt is welded to the back of the quick jack so that the tow hook is easily unscrewable and mountable to the rear quick jack as well or for storage in the glove box. The hardware fastening the quick jack to the frame is 3/8" B7 Alloy, which is roughly equivalent to Grade 5. That's easily upgradeable to Grade 8.

[rodneym]

....or, you can mount it to the top of your passenger roll bar.
Then lift with a crane and see if you can pull it right off.
You know, for Chas.

[patrickt]

Here are two teaser pics to give you a rough idea of what it will look like after it's drilled and mounted. Obviously it will be cleaner and crisper when it's bolted through the face of the QJ. Yes, it will look cool.

[xb-60]

Patrick,
I know you can't in the current environment, but really...you need to get out more

Stay sane
Glen

[tomshep]

What about drilling a hole in the actual quick jack just above where the lift rod would go and put the red ring directly in the quick jack? You already have a vertical plane for the ring. From a front view you wouldn't see the hole. Sideways there would be a small hole visible. Most wouldn't even notice the small hole.

Tom

[KDubU]

Looks nice and beefy. Only on one side? Perhaps I missed the whole engineering piece on pulling up on a flat bed but prefer two straps even length to the one tow cable on the flatbed but that is me. I still really like this tow hook and agree go beefy or go home. You do NOT want a failure.

[1795]

The engineers covered this in earlier posts. Puling from both quickjacks creates bending force that pulls the quickjacks towards each other.

[Argess]

Doesn't a flatbed have the winch in the center, so the pull won't be straight if that's the case. Gets worse the farther up the flatbed you are. Honestly, I never really noticed if a flatbed winch can be moved laterally.

And any side load would be doubled with only one strap....

Also, there will be a twisting torque on the whole car when you pull it from one side, so there will be side-loads even if it is a straight pull. If the whole business is beefy enough, everything will be OK.

But is it? Patrick's ERA looks different than my older one, if I remember correctly. I wouldn't trust the threaded rods in mine for this purpose as I think they would bend quite easily.

Mind you, Patrick is committed now as he's drilled the hole, so he might as well try it and see what happens.