Jacking (again)

[Paul Workman]

I'm new to Zs, but not new to physics and mechanics. So, a couple flags went up when I read the jacking seciton and comments, such as...

From researching the tech section, there are essentially two thoughts w/ regard to jacking the C4s (ZR-1); one using a single jack and the other is to use two jacks. But, there are some conflicting statements w regard to stress and procedures to minimize damage (appearantly to the vulnerable(?) windshield):

Single Jack Method:

The single jack method calls for lifting a front corner just far enough to put a jack under that corner before moving the jack to the rear to lift just far enough to put a stand under that corner, and repeating a little at a time in order to minimize stress to the frame.

But, here's the "rub": I think we all agree there is some stressing of the frame/body when one corner of the car is lifted. However, when lifting a front corner of the car, the frame is so stiff that the rear corner also lifts. At that moment the stress to the frame and body does not increase with height - continuing to lift will not increase stress to the frame.

Multiple "back and forth" stressing: Stress is temporarily relieved as the jack then lifts the rear corner up to the point where it is equal to the front corner. Then new stress is introduced from the rear corner should that corner be lifted above the level of the front corner (NOT a good idea). Lifting the car a bit at a time results in the frame/body being stressed back and forth from one corner to the other several times as the jacking shifts from end to end - sort of like lifting the car several times, in terms of stressing, no?

So, barring other practical considerations, it makes more sense (to me) to do all the lifting from one (front) corner whenever possible. For example, jack the front corner to the desired level before placing the jack stand beside the floor jack, and then move to the rear and jack it to the desire height and place the jack stand. Not only might this be quicker, but it prevents that back and forth stressing by moving the jack back and forth.Note: It goes w/o saying that due to the engine/trans weight, to minimize stress and possible damage to the body, etc, if one is going to lift only one corner at a time be sure to lift from the front. Furthermore, when shifting to the rear jacking point, simply bring the frame rail level with the front corner will minimize stress, i.e. especially do NOT attempt to raise the front corner off the jack stand in the same way the rear lifts off the ground when jacking the front corner. Some serious stresses would be brought to bear as the frame attempts to lift the engine/trans from a rear corner!!
Clearly, if a single floor jack is all that is available, lifting the entire front or rear from a point that applies uniform lift to the entire end of the car avoids any undue stress concerns and would be the preferrable method for a single jack situation.

Two jack system:

If lifting one side of the car at a time, clearly the two jack method avoids undue concern about stressing the frame/body and short of a method that lifts equally from all four wheels, the two jack method would be my common sense choice.

Which brings us to the body parts: To open or not to open, that is a question!

I don't have any stress analyses to go on, but using a SWAG, I'd think the doors and hood and hatch (to say nothing of the roof) could add structural rigidity to the body/frame, assuming their ability to withstand stress is not exceeded. I don't have an answer, but I've seen several examples of both procedures being used. However, in no case has a windshield broken or any other mishap occur that I've seen. I'd like to know...Is it better to button up or open up??? (For what it's worth, I accidently drove over the end of my ramps with my '95LT1..on two occations...resulting in the frame crashing down - coming to rest onto the steel ramps. One time the fender pannel behind the right front wheel shifted and had to be re-adjusted, but nothing broke either time. I can't even imagine the amount of stress that introduced to the frame/body! :rolleyes:Did I say how much I hate ramps now?)

For now, I only have one jack. What I really would like is drive-on lift, but in the mean time, a jig that would allow a single jack to do the lifting w/o introducing excessive stress would be nice. Until then, I'm going to lift a front corner to the necessary height and put a jack stand under it, and then lift the rear until the frame is parallel again and jack stand the rear.

What do y'all think (besides the fact that I might have too much time on my hands??:rolleyes:)

P.

[bobbyhi]

For now, I only have one jack. What I really would like is drive-on lift, but in the mean time, a jig that would allow a single jack to do the lifting w/o introducing excessive stress would be nice. Until then, I'm going to lift a front corner to the necessary height and put a jack stand under it, and then lift the rear until the frame is parallel again and jack stand the rear.


This is the method that I use. Doesn't seem to have any problems (yet):mrgreen:

[GOLDCYLON]

. What I really would like is drive-on lift:mrgreen:

Dont we all? :mrgreen:

[tccrab]

I can understand keeping the targa top in when jacking the car.
Drive the car once on a bumpy road with the top out and you will surely understand why leaving the top in is a really good idea.
I can't imagine that the doors or the rear glass add to the rigidity significantly. The weakest link is the latch assembies, as that is the part which would take the most stress and the most likely place to give in a stress situation.

[petefias]

I lifted mine many times with single jack starting on the front, then on the back. Always had the roof in. Had the hood open, but closed doors. When letting the car down I went slow to release the jack. Once on jack stands the doors would open/close easily meaning the frame is not stressed.

Never had any issues due to this procedure of raising the car.

This procedure puts an extra stress on the stabilizer bushings, but so what. I'll put poly-bushings on if they fail.

[Norsky4360]

Last spring when I took "The Beast" in to Discount Tire for new GYs all of their lifts were in use at the time so they used the two jack method to lift the front first from the jack points, set jack stands, then lifted the rear from the jack points and set jack stands. They also opened doors and popped the hood and rear hatch before lifting or lowering.

At home I only have one floor jack and have had the car up a couple of times by first driving up on pieces of 2X10 laid flat to get a little clearance for the jack. Then I lift the front by the front cross member, set jack stands, lift the rear by the differential (with a 4x4x4 block on the jack), and set jack stands. I leave everything closed while lifting or lowering. No problems so far using that method.

[QB93Z]

I have successfully raised my ZR-1 by jacking one SIDE at a time. A single jack placed on the under the frame below the A-pillar raises one side of the car. If you find the balance point you can raise the front and back wheels together. Then I place jack stands front and rear, and move the jack to the other side and repeat.

To me, this method results in very little twisting of the frame so the stresses added should be small.

I jack with the doors shut and the hatch installed.

I had a bad experience trying to jack both side of the rear of the car together with one jack. I have an adapter that is supposed to slip between the tailpipes and lift the differential. My exhaust system is not stock, so apparently I didn't have the jack properly placed as I was raising, the jack slid out from under the diff and dropped the car. Luckly there was no damage. But, it made me rethink the procedure.

Jim

[tomtom72]

One more vote for "wish I had a lift" & air compressor!:mrgreen:

I also use the one jack, on one side at a time. I just pick the factory jack point to get to and start there. All doors & hatch closed, top securely in place & windown up(doubt if that means anything) and usually hood closed till I get to my desired altitude. I try to make sure the car is level f to r & side to side when done going up.Seems it makes no difference starting @ front or rear jack point....go aways & the whole side comes up after a bit.

Takes longer to get it in the air than most of the things I want to do...I go up in stages & down the same way.....since it's such a "process" I try to plan a bunch of things to do so it seems worth the effort!:o

:cheers:
Tom

[rkreigh]

I finally got a 4 post lift (mostly so I could stack the zr1 and the z06, they fight for the bunk bed) but before that I would drive the car up on some poor mans ramps I made (2 X12s nailed together to "stair step") and used the rhino ramps in back. that gets the car up.

I then made a home made "jack a vete" with marine ply in a T shap cut to fit the curve of the diff

that way I can jack up the back by the diff and the front using boards on the cross member.

my car has coil overs and is lowered so jacking it up was a PITA.

I have also just used the facory jack and it works just fine. poping the hood is probably a good idea but I don't the car isn't THAT flexible.

the frame does twist a bit, but not too bad. I do think popping the hood is a good insurange policy against cracking. I did notice that the tire shops pop the hood and open the doors as there is a bit of flexing going on, but not horrible. :mrgreen: