Well, I'm still feeling like crap, but I couldn't stay away so I spend some time in the shop fiddle-farting w/the Kart today.

First, I the supported side of the rear of the car, and flexed it again...same results. So Greggpenn, that should answer that question for you.

Isolating the frame flex is going to be more difficult than I originally thought. My method, was to affix a rod to parts of the car that I "know" are solid, running parallel with parts that I believe are "hinged" where they attach. If I tweak the structure and the car part moves relative to the rod, then I've proven that the welds between the anchor of my rod, and the reference point are the "hinge".

Unfortunately, the plan didn't work out the way that I expected. My first move was to take wood clamps (bar clamp), and clamp one to the rocker rail vertically, with the bar rising up so that the end of the bar was near the corner of the w/s. If I flex the car, and the proximity of the end of the bar moves, relative to the top/corner of the w/s...then I know my "hinge" is between those points (likely at the front end of the rocker rail).

I put another bar clamp on the halo, with the end of the bar about 1" above the w/s frame. This one makes it easy for me to see that I'm getting the movement that I originally recorded in the videos.

When I Tweaked the frame, everything happened as I expected; the proximity of the vertical/rocker rail clamp and the top/corner of the w/s changed with the twisting...and those movements "jibed" with the halo bar clamp's end, just above the w/s. "The car is flexing at the front of rocker rail", I thought.

To try to narrow in on it, I moved the rocker rail clamp, and clamped it to the door post, so the bar ran towards the rear of the car, parallel with the rocker rail. Like this:






I figured that the rocker rail would rise and fall, and the bar would not...and this would prove where the flex is. Unfortunately, when I flexed the frame again (this time, using a floor jack so I could watch the action carefully), the end of the bar clamp bar moved "lockstep" with the rocker rail. Huh.

How can the rail move relative the the w/s....but not to the door post? The w/s is mounted to the door post.

In running the test, I also noticed, through using the floor jack and having a different perspective, that when I jack the RR of the frame, the RF of the frame, by the bumper drops. The fulcrum point is where my RF jack stand is, about under the fuel filter. I'm not sure what to make of this yet. Right now, I'm thinking the action of the flex is a lot more complex than I had originally thought. But I also think that I need to run more of the same kinds of tests, moving the bar around more, and just start to become more familiar with the behavior of the frames reaction to the twisting.

 

If I followed everything you said, it SOUNDS like the side rail is stiffer than we expected. It sounds like the entire rail is "pivoting" on the jack point...and not really bending?


Oh yeah...Please refresh our memories regarding the year of your Kart? (to clarify whether you are dealing with pre/post K/Wonder bracing.

 

Does anyone close to Tom own one of the targa top frame braces, such as the ones made here in Ohio by Tom Gaspar ? It would be interesting to see what happens and most importantly be able to see what happens because it wouldn't block line of sight out with one put in place.

Harbor Freight has cheap 1" dial indicators. A couple of those would help see where the structural components are moving and how much they are moving (if any). Depending on the day and which sale is running, they sometimes can be bought for $5.00 a piece, but are usually a bit more than $10.00.

Dial indicators and some magnetic bases could be a very useful diagnostic tool for you as you work out the structural deflection and if it can be remedied as regards your cart there.

 

That is what it looks like, but I need to do more experimenting. The observation that the top of w/s to rocker rail moves says it's more than just a rigid rail pivoting, but I'll have to try more, to see.



'89, so it's a "late frame". Frame has all the same features as my '92.



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Features.

 

^Right?



Whoo....that's a lot of questions. Only 3 stands when I'm pushing/pulling down/up on the rear. IDK if the opposite corner is rising. When I was using the jack, it LOOKED like the far side was pretty stationary on it's two jack stands, but I didn't measure...just "looking". That could use further investigating.

Fire wall still intact, all frame braces are still installed. IDK what the hingepin-to-frame is. front "bumper" crossmember is still there. K-braces are tight.

It would seem that if the opposite corner isn't rising/moving, the the rear halo would HAVE to be "trapazoiding".


I'll say this....it's WAY harder to tell precisely what is happening than I thought. There is so much happening all at once.

 

Maybe what I should do is support the rear of the car in the middle, rather than one side. I might be able to see what's happening more uniformly that way....maybe.

 

I looked at your vids (post 143, pg 8) again. In the second one look how the front w/s is dropping (compared to shelving behind it). It looks like the ENTIRE frame has torsional twisting....which SEEMS to point back to the A and/or B pillar having trapezoidal movement. (Edit: And explains the "unity" of movement you saw with clamp position #2)

When you jack, consider you might not find the same flex bending a rail in the opposite direction. It's not a symmetrical piece.

EDIT: Oooops...I see you quoted my second-to-last post I decided to delete. Most of the questions weren't making sense to me -- the more I thought about it. That's why I looked back at the videos...then considered my questions flawed. The hood pin receiver brace might be more clear....the one(s) that brace down/forward/inward from the A-pillar to the engine cradle area.

Oh yeah...I'm surprised no one has mentioned bushings spec'd in different durometers. Makes me wonder if/how my swap to poly changed handling? Or, really, how heim-jointed rears changes things? IOW...what was GM after using different stiffness? Was that only in early years? Is it a big enough deal to mention -- along with the bracing issue for potential buyers?

 

Man....I have had my azz whooped in the past couple weeks. I was sick for about two weeks (!), and as soon as I started to feel better....I screwed my back, snowblowing. That's been a week and it's just starting to get better, finally. I feel like a LAME-O.

Anyway, i worked on the kart today, a little. I put the exhaust back on (had it off for some welding), I re-routed the brake lines, and I farted around w/the frame some more.

Greggpenn, when I jack the rear corner of the frame, that rail does appear to "pivot" around the firewall. The whole rear of the car doesn't appear to have much strength to resist "parallelograming". Anyway, if I jack the right rear frame, the front right, ahead of the front jack stand, drops. The front left drops a little too, but not as much as the side I'm jacking. It almost seems like a vertical "X brace" in the rear would help a lot, by tying the two rails together in the rear, in vertical planes.

Anyway, I moved the front stands forward, to underneath the sway bar mounts, as that is where a good deal of torsional forces are introduced into the frame when driving, then re-ran the same tests. The results were similar with the two rails seeming to pivot about the firewall again....but with more weight on the rear (with the front stands moved forward), I noticed a new thing. Check this out....




I've always assumed that the "rear of car" was probably the strongest/stiffest part of the car; the rail cross sections are huge, the length of the rails is short, and they're made of high strength steel. I guess the rails may BE strong and stiff...but not what they're attached too! Yikes.

The rear halo does flex forward some during this test...but not as much as the rear wall seems to deform and let the rear rails move.

I still nee to try the original test (vid) on my '92 with the roof off. Maybe tomorrow....


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I think some of the already in production fixes are a good idea, but with time and structural component advancement, the current fixes redesigned with new HSS or UHSS or many of the other stronger and lighter material may be the fix but there are likely so few C4 owners interested enough (at this some 2018) in this and able or willing to afford this would likely make a new run of braces etc in the new material non profitable.

 

Watching the video it seems like it is begging for a diagonal brace between the rear door pillar (B pillar?) to the rail in front of it. A steel rear fire wall and front firewall with a trans tunnel I would be fixes the whole thing. An X brace between th rear halo to the opposite frame rail on both sides and betting you have rigid then. I am thinking both could be mocked up with 2X4s either screwed or clamped on.

 

The biggest problem as I see it, is the effectiveness of anything "bolt on". I think most 'Vette owners are reluctant to weld on their frames (or have the frame welded on). The more I look at this car, and play with the frame, the more I think real fixes are going to have to involve welding, 'glass cutting and basically modifications that you can't "undo".



I agree with all of that. I thought that rear part of the car -the part below the B pillar- was a lot stronger than I realized it is. You're right; a steel tunnel would help, and angled gussets would help a lot too.

 

You might also try the Kart with the targa bolted in. I'll be very curious how the other car compares.

After your first vid, I was thinking that rear "wall" must be flexing quite a bit. Not surprised you are seeing that. Guess I'd wonder what a roll cage (vertical x-brace), vert x-brace or V2V brace, and targa brace would accomplish (in lieu of a roof)?

One thing I can tell for sure....The less I watch this last vid, the less it bothers me! The MORE I watch it, the harder it is to discern f/r flex vs side-to-side torsional twist. I'd probably try putting your board under both rear frame extensions and lift -- and watch the A/B piller gap....to confirm side rail flex.

Obviously, lifting up on the VERY rear imparts a bit more leverage than at the axles.

 

I'm probably not going to put the targa back in; I've already removed the mounts from the w/s and rear halo. I'd have to install them, align them then bolt the roof on from my '92...all to prove that which we already know; that the car is way stiffer with the roof on.

I can do the same test w/the wood in the middle of the rear. It won't bend as much b/c it'll lift the rear of the car off the rear jack stands with less force per rail...but it'll still bend I bet.

The majority of the bending in this latest video is where the rear rail meets the "rear wall". I put a clamp on the halo and ran the bar forward over the w/s. When doing this test, it moved a little...but not as much as you can see the relative movement of the rear rail to halo but noting the clamp that I've got on the halo, running to the rear rail. The amount of flex in the rocker rail for this test seems minimal.

It didn't show great in the vid, but in person you can actually see all the metal of the "rear wall" bending, as the rear rail rises.

Also, in this last test, the front stands were under the sway bar mounts, the rears were under the rear jack points.

 

The more I look at this and the more I think about it, I'm thinking I agree with you. It appears bolt on stiffening might have some ride quality improvements but I'm not sure it would have a distinct traction improvement since they appear to just preload an existing structure acting as a spring to its maximum compression. Cool stuff!

 

It's cool to scrutinize and the opportunity that this "naked 'Vette" is providing us to SEE what is going on is interesting.


I've had this idea for a long time that there was ONE place in the frame that was the worst....and that if that was solvable with a bolt on, that part would be a silver bullet. The more I look at this car though, the more I think that the problem is just everywhere. Everything in the frame seems to have compliance.

I just went out and ran the test GREGGPENN requested with the jack jacking the middle of the rear. I behaved how I expected; the rear rails bent up slightly before coming up off the rear jack stands. I re-examined the bar clamps between the halo and w/s and halo and rear rail...it kind of looks like the flex is about 50/50 between the rear rail and rocker rail...though the rocker rail is a lot longer and all of the rear rail flex appears to be centralized where the rail meets rear wall. See vids. The first one does an OK job of showing the flex in the Rocker rails by showing the displacement of the rear halo to the w/s. I couldn't hold the camera still enough to show the same/similar displacement at the rear-to-halo bar clamp...I got frustrated. Sorry for the f-bomb.




The second vid does a better job of showing the displacement of the rear or rear rail, to the halo by showing the relative movement of the rail to the bar which is attached to the halo.




I was looking forward to working on a solution for C4's, that was bolt on or easily weldable. I'm not smart enough or educated enough to understand what's happening throughout the frame, however. It just seems like everything bends. Therefore, I'm going to abandon my "silver bullet" idea and proceed with just welding up all the seams in the frame...then get on with the cage.


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I appreciate you going through all of this. It'll be cool to see how much welding it up changes things.

 

You could just tie off some strings between the 2 points you think are moving. If the rear goes up bending the rear wall forward the string will droop. It would be closer to reality if front stands are mounted under the center of the A-arms as that is where most of the load is concentrated.

 

With as much torsional movement you're seeing, I'm surprised people running WITHOUT roofs don't see and hear door wearing/creaking/binding.