heinrich

Duuuude ... sheesh that looks like a suicide machine

deliriousga

Ron, if you take Tony's 4x4s or even 6x6s and use 2-3/8" bolts (bolted through both pieces with 3/8" washers on both ends), do you know approx. what the shear or pullout values would be? If it is still too low, how many or what size would be good to use if you bolted wood or Hacker's steel pieces together?

In Tony's first pic it looks like the car is resting on the jack stands with the wood used for additional stability or "just in case". I like that set up. If you made the bottom layer of beams under the tires twice as wide as it is in the picture and placed them the other way (across the tire tread), would you be able to almost eliminate tipping. If those are 6x6 beams, that would make it 22" wide and it seems like it would be very difficult to tip.

Finally, after dr bob's and others' suggestions, I'm looked at making a pad that's about 2'x2' wide and 7" high and bolting the stands to it with flooring joist angles (3/16" steel angles). If it's strong enough to withstand shearing (4 3/8" bolts through the wooden base holding the stand on) then wouldn't it be pretty stable and very difficult to reach a tipping point? With using Hacker's beam to make it easy to lift and tight in the lift points and a wider base attached to the stands it should be pretty pretty difficult to bring down.

hacker-pschorr

944's are built with a center jacking point as well. I wonder why the 928 was left without one? Too heavy in Porsche's eyes to be jacked up in the center with the supplied jack?

Ron_H

Garth: I had envisioned the center section of this device with the 4" dimension vertical. The bending moment should be taken by the upper and lower horizontal faces (analogous to flanges in a WF section) and horizontal shear taken by the vertical faces. I assumed a 0.25" wall thickness of structural steel tubing (Fy=46psi). I would turn the "C" section channel so that the long dimension is vertical, if I used one.

Here is a suggestion re: weight. Try an "M" section I beam, M4X13, which has a section modulus of 10.5 in4 in the X axis and 3.36 in4 in the Y axis, (required section modulus for this problem is 1.23 in4), and weighs just 13 lbs per foot.
Compare that to the 4x4 tube at 12.21 lbs per foot and a section modulus of 4.11 in4. Or the 2X4 tube at 8.81 lbs per foot and a section modulus of 2.35 in the X axis or 1.54 in the Y axis. I would use the tube with the 4" face vertical as the web (vertical dimension) takes the horizontal shear and gives a higher allowable moment.

The smallest channel shape I find that is worth considering is a C10X20, which has a weight of 20 lbs per foot, a section modulus of 1.32 with the Y axis horizontal (I assume you need to jack under the wide dimension, and I certainly wouldn't jack with the section turned the other way with this shape), which is the weakest dimension. The moment of inertia for this shape is 2.81 which is half that of a 2x4 tube which is understandable since there is only one flange. Turning this member vertically gives a moment of inertia of 78.9 and a section modulus of 15.8 (lift one corner of the garage!!) So why not bolt two channels together, say C3X4.1 and get a stable jacking shape? One of these has a section modulus of 1.10 in3 in the X direction vertical, but weighs 4.1 lbs per foot X 2 = 8.2 lbs per foot. Why not stick with the 2x4 tube or the M shape? Certainly bolting to the C section or the M shape is easier than bolting to the tube shape, if that is the preferred method of attaching the jack stand saddles to the beam. I would weld them and be done with it all. The result would be about 45 lbs. +/-. Place the member on the jack stands and run the floor jack under it and then raise the stands when the car was high enough.

Another consideration is impact loading. If you begin to lower the car but stop midway down abruptly, which I often do, loads will be increased. Sometimes I grab the handle of the jack and twist in delight at the job being "finished" rather than gently lowering the car. Then I twist it back quickly to stop the descent so abruptly. I would go with a stronger section for this reason. I would pick the M section or 4X4 tube.

As for the wood blocks bolted together, my gut reaction tells me that 3/8" diameter bolts with large washers (2") both sides should handle it. There is 900 lbs on each wheel, say 1 kip vertical on each wheel. I would use the wood as firewood rather than stack it. Then I would go to Heinrich's house and polish all his sharks before I would get under that arrangement. Will it hold? Probably. But it is wood; look at the check/split in one of the pieces in the stack. No thank you. Probably doesn't put my mind at ease. Nothing is so final as death.....alone in the garage late at night. What is the coefficient of friction of masonry versus wood on concrete? Try a monolithic concrete block before wood. I have placed blocks of 4 x 12 wood flat on a jack saddle and tried to jack an engine/transmission in my 914/6 only to have the wood split and falll away before my eyes. And the engine/trans. fall down on the saddle and wobble as I got religion right then and there !!!

dr bob

JUST BUY THE TALL STANDS..

SteveG

Thread res-erection: and adding what seems to me to fit here - floor jack recommendation

I survived the Harbor Freight jack stand fiasco of 2020 (I believe mine are not the faulty ones). My back says I deserve to make these jobs as easy as possible if I am to continue the DIY life.

So I'm looking at the long handle, "quick lift" models at HF. I have not seen lift bars for the GTS or Cayman so the front has no short cuts. But for the rear, I have been using a small jack with a short handle (16"?). I place it in the center on the xmember under the transmission or thereabouts and then put stands under each side. It is difficult making the initial placement b/c of how far under the bumper that xmember is. It is about as far as I can reach prone. And the short handle is a problem.

So I'm looking for recommendations on the longhandled jacks. So do I need a 3-ton when the GTS is 3600 and the Cayman is a lot less?

dr bob

Hi Steve --

There are lift-bars available for the GTS. Same ones as fit under the S4 jack pads.

If you don't mind the larger investment, the long-frame high-lift jacks are a better option for the rear under the crossmember. They also lift a few inches higher than the regular-frame versions. Only available in steel though, so it's a bit more work moving them around vs. the popular aluminum jacks.

Most floor jacks are very easily resealed with a cheap piston seal from a local hydraulics place. I resealed my MaxJax lift cylinders for under $30 worth of seals installed on my pistons by experts. Just had to clean the cylinders and reassemble. The floor jacks are smaller pistons and maybe a little more hunting, but there's usually just one and the parts are easier to get to. On my jack, the piston and cylinder is all in one with the two pump pistons, so dropped out easily for seal replacement. I spent more time on the ten minute rides to the hydraulic place than they did with placing the seal on piston for me. You might try flushing the old fluid out and replacing with new before you tear into anything serious. Sometimes that's all it will need. Waste No Time or $$ on leak stop oils or additives; the piston ring seals are neoprene and don't expand with the solvent additives like soft seals might.

SteveG

Dr. B: re: lift bars, I thought the contact point with frame was a mismatch. I'll look into it. Thanks.

linderpat

liftbars work for all 928's (altho for pre-86.5 cars, get the right style) as well as the Boxster/Cayman, 996 and 997 (and maybe the 991?). I use them for my 997 and previously my 03 Boxster S, as well as for all 928's. I have QuickJack 5000 too, but liftbars are faster and less hassle to use.

Wisconsin Joe

I thought the Liftbars site showed them for a wide variety of cars.

Ken is pretty responsive to e-mails. I'd suggest asking him.