makes sense. while i'm doing bushings, springs and shocks i'll do it. i'll be sure to have jackstands right where the weight would be if it were on the tires so all will be straight. i know i should have thought of that since i did it on both fox bodied mustangs i had.

 

Wow I guess I got pretty lucky. I can jack my '76 anywhere at any number of points, and absolutely nothing flexes. The doors open and close smooth as glass no matter what, and I'd be very worried if they didn't! I find it hard to believe that any flex is acceptable regardless of the car's configuration be it BB/SB/Coupe/Vert/AC/PS/PB etc.

 

Keep spreading this word around and someday you convertible owners won't be able to claim how much more valuable they are than coupes. May need to revise the old saying about convertible collector cars, at least when referring to C3s, to say "when the top goes down, so does the nose." Up until reading this, I just thought they squeaked and rattled more.

 

My C2, 67 Convertible rattled, squeaked, and you could see the gap at the top of the door moving as I drove the car, it flexed that much. Turned me off to convertibles. I sold it after buying my 70 coupe... Could have been it needed some major chassis work. The body loses rigidity without the T bar. I recall the convertibles have extra bracing behind the dash,

But.... if the majority of buyers prefer the ragtop, price is going to be higher. Much like matching numbers or not...

 

Theres' a good reason Gm gave the convertibles cowl bracing and the extra lock pins on the doors. Mine has no rattles even with a pretty firm suspension, but it's not nearly as rigid as a coupe.

If you put a convertible up on stands, fronts at the stabilizer mount and rears at the kickup, there will be no flex bb or sb.

Check your body mount bushings if the flex is too great.

 

Check your body mount bushings if the flex is too great.
I'm new to all this and I don't understand how bad rubber body mount bushings can add to the body flex while it's sitting on jackstands. It seems like the frame would flex, and then the body would flex a little less because the rubber would have some give. Please elaborate....

Thanks,
John

 

Simply put, the body bolted to the frame even with rubber bushings becomes an "assembly" sort of like a floor or roof truss, with the "whole" being much more rigid than the individual pieces.
There really isn't that much give with new or good bushings. Even some of the original bonds letting loose will cause for flex. that's fairly common in the cowl/fender area.
A weak car is usually a combination of several things deteriorated a bit over the years, that's why some people report no noticeable flex.
If yours is fairly tight while driving, don't worry about it, just jack it in the appropriate spots when necessary.

 

It could be a simple bushing or a tight door adjustment. My 74 BB427 would flex and when I shut the door with the window up it would catch the molding on the jack stands. If you jack your car up one corner at a time can twist it up a little too. It happens to most stock rubber body mouted cars!

 

My summary from my '73 SB non air coupe:

1) with original frame, neither door would open or close with the car on jack stands.

2) with new-to-me replacement frame, both doors will open or close easily with car jacked in any position.

 

I think it depends on what you mean by "flex". If you jack up one wheel, you should be able to see the frame flex , like one rail taking on a different angle relative to the ground than the other. A frame that had no flex in that way would ride incredibly stiffly, and you would probably have problems with a wheel being off the ground more than you would like on uneven surfaces. When I say "I am always surprised at how much the frame flexes", it is that type of flex I am talking about.

If what you mean by flex is the frame rail taking an inverted "U" shape to it when the car is lifted off the ground, then I think flex should be minimal. The frame being metal, it has to flex some or it would crack, but if the car looks like it is bending in the middle, I would think that would be a problem.

 

There is no visible flex in my car when I jack up one corner. Granted I haven't attacked it with a level and an angle finder to verify my perception, but the doors still open and close exactly as they would if all 4 wheels were on the ground.

That's what your suspension is for.

I honestly think what a lot of people have to come to accept as "normal" is actually very "worn out".

 

If this were true, by the time you got the right front wheel off the ground, your left front would also have to be off the ground.

Yes, of course. Maybe I worded that one badly. I don't care to get in an argument over this. Frames are made of metal, and metal flexes or it cracks. If you've got the magical frame that neither flexes nor cracks, then I'm happy for you. Every frame on every car I have ever worked on in the last 30 years, new cars included, has flexed under certain conditions. Maybe all the new cars I bought came from the factory with worn out frames, so maybe that experience is inapplicable.

 

I can't jack just one wheel front or back without the other wheel on that side coming off the ground too, I guess my frame is pretty stiff. The jack points i use are in the manual. You would think they would tell you that this was going to happen, especially when new ?

So now I use two jacks or if I am lazy just one in the middle and put the whole side on jack stands. No point in working on the back wheel with the front just dangling there with no support. Anyone else have this happen to them ?

 

It's going to depend on exactly where you jack as well as the weight distribution of the vehicle, but with most cars the wheel on the same side front or back will also lift. My experience is the same as Motorhead's...

I used to have an Integra that would lift 3 wheels in the air with 1 jack.

Metal only cracks if it's loaded past its yield point. And guess what, keep taking material out of the frame via rust, and what is left will eventually crack. Of course all frames will have some degree of flex, but if it's enough to see with the unaided eye or enough to bind up and/or damage things attached to it (e.g. doors) then that is too much. And the fact that there are some cars which do this and some which don't sends a very clear and obvious message to me.

 

how about these new tubular aftermarket frames.do they have that much flex or are they real strong?

 

Who makes aftermarket C3 frames? Currie? Art Morrison?

 

http://www.colliertechnologies.com/main.html

 

Both of mine do exactly the same thing. You can tell the 80 Coupe is stiffer than the 75 Convertible, but on both of them the doors open and close normal on jack stands or on the ground.

 

Mine does that.

 

So then, the car was balancing on two points? The one remaining tire on the ground and the jack? I'd sure like to see that one.

If I jack my car at the front jack point in the owner's manual, I could lift both wheels of one side from there. If I am just changing a tire, the front wheel will come off the ground before the rear wheel, so I'll still be on three wheels, but the rear spring on that side will be mostly decompressed. I'm sure I'm not communicating well here, which is why my first post in this thread started with "a picture is worth a thousand words".

Let's try it this way: I pulled my starter and changed the solenoid. In order to do this, I jacked the frame right at the right front wheel and removed that wheel to get my fat gut under there to reach the starter. Jacked in this manner, the right rear spring was somewhat decompressed, but nowhere near coming off the ground, and the frame was visibly flexed in what I would describe as an "axial" manner. IMO, this is normal even for new cars. But, I think the OP was talking about jacking the car in the middle, and having the frame rail look like an inverted "U". The frame is metal and will flex somewhat in that direction, and BB AC cars being heavier in the front, I would expect more frame flex than a SB non AC car, but if flex in that direction is dramatic, it would be a good idea to check the condition of the frame. If we can't agree on that, I am willing to agree to disagree.