drcook

Throwing this out for discussion.

Ever since Tom posted up his thread about about frame deflexion, and what I learned from the before and after sounds and handling characteristics of my low mile 1996 (added a camber brace and a harness brace that is attached at the seat belt bolts for the upper), I have wondered how these cars would have handled had they been offered in a true fixed roof coupe.

Even with the top on, there is still looseness that allows the body to move a slight amount. A little bit here and a little bit there adds up.

We all know it was supposed to be a T-top. So I even speculate that some stiffness was lost due to that misbegotten decision. The only way to know was to have been one of the ones that actually got to drive a prototype that was a T-top.

HOWEVER, the T-top Z28 Camaro I had (1980 bought new) was the loosest POS I ever drove. That body twisted more than a C4 does. It popped the windshield loose, if you jacked up the front end at the jacking points, both doors would swing open, The neighbor kid had a 1979 with T-tops. He hooked it up one night, the body twisted and stripped the paint off the roof in 1 big flake. The paint just separated from the metal. I dumped that car. Subsequently I learned to feel for "V" bends in the pillar behind the door and could feel where the body had bent.

I also wonder how much the show Magnum P.I. influenced the decision. It debuted on ‎December 11, 1980. From everything I can tell, each of the Ferraris that were used had a targa top.

Kevova

The Porsche 911 Targa influenced GM management concerning the "T"bar. The 1 piece roof was planned hiding the bar from view when in place. With the roof off it was in full view and had to go. The convertible was an afterthought and required additional brackets and braces to reduce flexing. With optional hard top installed the convertible is more ridged than the coupe.
Mr Konner of Konner Chevrolet influenced the equipment installed on the Corvette Challenge cars. So there would not be a stripped down fixed roof racer like the LE1 F bodies.

dizwiz24

drcook

The Corvette Challenge didn't come into well into the run. I am talking about from the beginning or just as a version of the car. Ford marketed t-top Mustangs, sun roof mustangs, convertible Mustangs, fixed roof Mustangs and notch back Mustangs. The notch back Mustangs are stiffer than the roofed coupe. I had a 90 that I had the Saleen sub frame braces welded in. However even it had its issues. If you hit a dip in the road while turning a corner, it would move the top of the struts. I had that car aligned so many times. I even drilled down through the plates to put bolts in to hold it in place.

The convertible with the roof bolted on also has the X-brace underneath to help with the rigidity.

There was a very big difference in the way that T-top Z28 I had drove and felt over the fixed roof versions. The windshield was popped loose in the first 30 days of its existence. 1979 was the first year GM went to the bigger style T-top, leaving less of the roof metal.

Without taking a coupe and fabricating up the parts to weld in, we will never know, but I can speculate the car would have handled a lot better had it been built as a FRC. The fixed roof rails along the top on the side would have formed a box with the frame, such as the braces designed by G.K. (sold by Vettetovette) do. Even though they are tightened up for compression, they still form a box and another parallel structure with the frame rails.

Side profile to side profile, a C4 Corvette looks more like a Ferrari than it does a Porsche.

FAUEE

If there had been no bolts to remove the roof and it was just fixed in place, it would have been no different.

And let's not take this out of context, the c4 game is still VERY stiff, it's just not stiff BY TODAYS STANDARDS,. They didnt have hydroformed frame rails, ultra high strength steel, and things like that to work with. The frame structure of a c4 ia fundamentally similar to that of a c7, just with far older tech, and far older materials.

The c4 pulled over a g in the skidpad on tires that would be laughable and likely considered too poor performing to be legal.

Tom400CFI

I think you have to be careful here on what you mean. "Handling" means two things to most people;
1. what the car can do (objective numbers)
2. how the car feels; how easy it is to attain that/how good it feels/ride.

The C4 already handles (what the car can do) incredibly well. Well enough that it's objective handling data is not far from or even matches the objective numbers of today's cars. In the '80's it was paradigm changing. But how does the car feel? How attainable are it's limits? How does it inspire confidence? Well...that is where a more rigid structure really helps, and where 30 year newer cars stand well above the C4.

I don't think that stiffening the structure would improve the objective handling data that the C4 can produce, but I do feel that it would make the car feel better/safer and limits more accessible.

drcook

I completely understand the difference between the late 1970's technology (welded together stampings) and what came later. I am just talking about the car as it stood. I think it would have had less squeaks, felt better as you say. I was not and am not comparing it to any later models, only itself.

Bolting the roof on versus a structure that was welded into the frame ? Not different ? I would argue that point. If there had been a structure that connected the hoop to the windshield frame structure and was welded together, it would have been quite a bit stiffer than bolting it in place. Bolts are always going to have slop as they are not precision machined and matched together. In order to build it on the line, there had to be tolerances built in. If you had bolts and their mating threaded components that were just a grade below an interference fit, you would have to assemble the structure together and fix in place otherwise there wouldn't be enough give to put them together. There would always be a mismatch. I have built enough components/assemblies/dies/molds and such during my career in precision metal working to understand what you have to do when things are that tight.

I would bet that if a person fabbed up a bunch of rigid brackets to attach 1" travel dial indicators to (various spots in the interior/body/etc) and drove the car around with cameras filming the indicators, you would see how much it does move.

Roll cages that are welded in and tie the front to back and such do this in a much more solid way, but for everyday driving, my belief is the car would have been different.

Tom400CFI

that a welded in metal roof structure would be noticeable compared to the bolted in roof. Many points of contact vs. 4 points, an optimized structure rather than a light weight/see through roof, etc.



.

MatthewMiller

My guess is that the C4 with its roof bolted in is at least as stiff as it would have been with a T-top, and probably more so. It's probably a little less stiff with the roof off, although they did seriously increase the frame rail size/stiffness to accommodate the targa plan. With the roof bolted on (and if everything fits well), the roof is in shear and there shouldn't be much weakness or movement to it.

Comparing this to any 2nd-gen F-body (like a 79 Z28) is problematic, because those cars were terribly limp even with hardtops. They were based on 1960s unibody technology, so there's just no comparison to a C4 that was at least a decade newer and designed with a higher priority on performance and less on economy. In comparing the C4 to the FRC/C5Z/C6Z, its important to remember that those aren't "fixed roof" designs in the way you're thinking, either (welded metal frames, etc.). Their roofs bolt on, too. They just use more bolts (I think), and aren't set up to make removal and installation an end-user task. The C4 and C5 bare frames, don't look a lot different from each from a distance, and both have windshield and b-pillar hoops that are not connected by any structure. As I understand it, the increased torsional stiffness of the C5/6 comes mainly from the one-piece hydroformed frame rails and the improved union between the floor/tunnel and the firewall and frame rails. All in all, I doubt that welding a roof panel the the C4 frame would make much real difference compared to a C4 with a good-fitting roof bolted in.

drcook

I agree 100 %. It is a vast improvement on the way frames are made. The technology to do it, was in existence a bit of time before implemented by GM. As we all know, GM had lots of issues that were ingrained into their culture, hence the "GM Nod" and having to be shown how to build a precision engine by Mercury Marine, ie: the LT5.

I was throwing out the Camaro and Firebirds, not to compare to the C4 Corvette, but to compare against themselves. The fixed roof Camaro was a lot stiffer than the T-top models.

Now I am not suggesting that a welded in structure for the roof would have increased the limits of the basic design, only that it would have made the basic design better up to the limits. A while ago there was a post on here of some folks building a C4 into a race car and welding up a lot of the structure, improving on the spot welds. I just think, that as Tom says, the structure would have felt better and drove better up to its limits.

On a good hot day, when I pull into my drive, I can feel the car moving around, even with the braces I have. We will see how it feels next summer when I get the convertible x-brace I have installed. It will help the twist on a diagonal. The GK braces help on a front to back.

I also wonder how the car would feel using a combination of x-brace and GK brace. Of course they add weight, that I understand. But if the "experience" is better, it is worth it, because that is what I am building for, "the experience".

Tom400CFI

I agree with the first sentence, but according to Dave's book the removal of the T-bar killed their design/strategy. "...all our work and effort went out the window" -I think were his words. I don't "Get it" as to how the T-bar could have done so much for the structure compared to the wider, bolt-in roof, but the way Dave wrote the book, it sounds like it did a lot.
When I was messing around w/the frame of the 'Kart, my mind's perception of what was happening to allow the C4 frame to flex was blown out of the water. I've always totally believed that the "problem" was between the rear wall (behind the seats) and the fire wall. "proof" of that is when you drive with the roof off the front of the car (dash forward) feels and appears to move torsionally, independent of the rear of the car (seats back). With the kart, I thought I proved that further when I made and posted some vids on here of my flexing the frame by hand, torsionally. I filmed it looking at the top of the halo, relative to the top or windshield. Here are those vids:




Later however, I tested it differently and the torsional flex revealed itself in a totally different way. The rocker rails weren't flexing at all....it was the whole firewall region, the rear x-member, and the "rear wall" that was flexing (twisting laterally through the car) allowing the two frame rails to rock opposed to each other, side to side. There is your twisting. I posted another vid of my '92 confirming that it exhibited the same behavior. I'll see if I can find the vids.





Anyway, Dave said in his book that there was a "hinge point" in the frame that they couldn't fix. I thought he was talking about the "s" bend in the rail ahead of the firewall, but now I think he was talking about the firewall itself -what I've observed. Now, I don't see how a T-bar would mitigate that in a meaningful way. It would help....but all it could do is limit fore and aft movement of the halo to windshield. Could it do that more than the roof can? I don't see how, but Dave's words make it seem like it can.

As for roof on, roof off, Dave posted actual Hz figures for the car roof on vs. off and it's pretty substantial. I'll see if I can find those....
EDIT: here we go...









A lot more reading/detail from a different perspective (different book) right HERE.

drcook

If you read what Tom posted about additional reading, you get to where they are discussing the targa roof. Initially the prototype had quick release latches (according to the article) which actually would pop open. They say they found over 1/2 ton of force going through the latches. Notice in the text it talks about carrying the "required loads" and "buildability requirements". That is what I was talking about above regarding manufacturing tolerances.

The most telling quote:

"We still felt we were structurally below where we wanted to be, but we did want to keep the open targa roof."

So they go on talking about bolting the roof in and having to add the "wonder bar" to control front end torsion.

Based upon what Tom has graciously provided, I can read into it that if the C4 had been built as a FRC, there would have been structural benefits.

When I worked with metal manufacturing machines (especially cutting machines) I could feel the vibrations (I also hear low level frequencies). I learned to tune in the cut by getting rid of the vibrations through varying speeds,feeds and the cutting tool itself, along with where and how to cool the chip. I hear and feel those same vibrations from motor vehicles. The neighbor guy (a couple years older than me) was deaf. He tuned his motors the same way, by feeling the vibrations and tuning them out.

(Knowing about vibrations and how it breaks metal, I have seen a lot metal break, is one reason I won't fly on an airplane, but that is another story).

You might think I am a mad man, but I lay awake at night thinking about this stuff. I have been thinking about the FRC ever since Tom did the post about his cart and his findings.

MatthewMiller

I get what you all are saying, and that passage in the book does seem to imply that they had a stiffer structure with the T-top than with the targa roof bolted on. My take is that I think that's probably not the case, and that McLellan didn't really mean to imply that. What I think happened is that the frame was first designed with the T-top to an acceptable stiffness, and probably with earlier tires that caused less grip/torsion and had a lower structure frequency.

I can see how the T-bar structure helped with chassis stiffness in torsion. In order for the frame to twist, the windshield frame and rear hoop have to move laterally in opposite directions, and therefore the length between any two points (one on the windshield frame and one on the rear hoop) has to get longer. A T-bar would resist that with tension, thereby resisting that torsion. However, it wouldn't fully prevent the frame and hoop from twisting, because they could just get closer to one another, either pulling the roofline lower and/or pulling the top of the windshield toward the rear hoop. That shouldn't be as good as a full shear web, which the targa roof forms, especially with the frame improvements they had to make to get the C4 to have tolerable stiffness without the roof. OTOH, you can take the two T-tops off and the car doesn't lose any rigidity. Ergo, my guess that the T-top wasn't going to be any stiffer - probably not even as stiff - as the targa with the roof bolted on, but that it was way better than the targa with the roof off. But I could certainly be wrong about that. I suppose the best test would be to take torsional load tests on a C4 with its roof bolted on, and then with one of those aftermarket tubular roof braces on in place of the targa top. Those braces appear to me to be very similar to a T-top brace.

drcook, my main point is that the bolted in targa top really isn't much different than an FRC's or Z06's roof. They aren't welded in - they are bolted in. Again I could be wrong, but I doubt they are getting much more stiffness from their roofs than a C4 does from its bolted-in targa.

PS - The Ferrari 308 started life as a FRC. Actually, it first was made with a fiberglass body if memory serves. The bodies on these cars were never structural - they were tube frame cars with bodies hung on that. The GTB (fixed roof) version had tubes running along the edges of the roof to tie the structure together. The targa came later, and along with the removing the roof panel (which again was non-structural) they also had to leave out the tubes running from windshield to rear hoop. I think they used a beefed up roll hoop structure to retain its stiffness. I believe I've read that Tom Selleck - who is about 6'4" - couldn't sit in the car with the roof on, and it's also obviously desirable to see the star of the show rather than cover him a roof. So those are the reasons you never saw him drive the car with the roof on.

Tom400CFI

That is a thorough analsys of the text and car. You could be right. IDK. I SURE wish that tool JVP or whatever would entertain C4 questions to Tadge. Only an engineer who was on the project at the time, could speak to this with more clarity and knowledge than whatever we can try to decipher from Dave's book and "The Newest Corvette".

If DrCook moves forward with some kind of experiment here....I'm ALL

drcook

I admit I could be completely off the reservation here myself. It is just something I have been wondering about, and you folks are the only ones that I can discuss this with.

Probably FRC is a bad term, because it pulls you to think about those cars, which, with their tubular, hydroformed frame are very stiff. Maybe a better way of saying would simply be a full welded structure ?

Also M.M. you and Tom bringing up the subject of the bolt in brace. It is still bolted in, but it adds reinforcement laterally along two places of the roof/windshield/hoop structure and then ties them together in a much stronger way than than the targa top can.

At the time when Tom (Gaspar) was finalizing his last runs of that brace, I just didn't have the money to buy one. Probably should have scraped it together somehow.

The flexing of the chassis was really brought home when I had the stripes painted on my car. The guy that did the painting, took the top off for some reason (maybe to buff it easier, or paint it after the stripes were layed out). He forgot to tighten the front bolts, just did the back. After I pulled out of his place and hit a spot in the road that wasn't smooth and even, the front of the top was bucking and banging, I tried holding it down until I got into a place to stop.

Down the road if I ever get a chance to get another car, maybe that would be a modification to look into (ie: welding in a permanent roof).

But I do thank you for taking part in the discussion. I learned some information out of it regarding the placement and stiffness of the shocks and how they stiffened up the bushings throughout the suspension to help.

I bought the "pin top" shock mounts from Banski.

https://www.banskimotorsports.com/pi...ck-mounts.html

They will be installed when I redo the suspension with the rest of the components from him and the Del-Alum bushings up front I bought from Global West. I had built a similar arrangement back in the early 80's for the 65 I had. I saved scraps of teflon (bar ends) from different runs of teflon parts we cut for the military (I don't remember the actual part now, suffering from CRS {can't remember sh*t}). I cut bushings for the shocks, as well as stainless "washers" and gave them the ability to pivot. Along with the same thing I made for the sway bar, the handling of the car was "quick" to say the least. I followed the guy home I sold it to, and while he was a very experienced Corvette builder, he danged near wrecked it getting off the X-way as it steered so quick. He was used to the sloppy factory steering.

I also realize everything is a compromise in a production, for the consumer market. What I like and enjoy driving is too stiff for the average Joe. There also has to those pesky "manufacturing considerations" that would send costs soaring if you had to slow the process down to build a true precision assembly.

MatthewMiller

I'd be interested in a thread with some pics of that shock mount installation. Those are on my list of things to eventually upgrade on my car.

On tangent now: if someone around here were able to machine some Delrin and urethane pieces, there's a couple pieces that used be in VBP's inventory and are no longer available since they closed their doors. The front lower control arm bushings are available in poly as standard, concentric items; but VBP used to sell offset bushings that really helped in getting more negative camber up front. I have a set, but people can't find those anymore. Also, the VBP rear leaf springs used poly blocks in place of the stock sandwich of rubber, metal, and paper for the center clamp isolators. They were just rectangular blocks with two nubs on one end to mate with the metal clamping plate (they remind me of a Lego block), and they are glued onto the spring with 3M windshield glue. Those seem prone to disintegrate over time, possibly with help from the exhaust system that runs close under them. If you or someone you know is able to make a run of these, it would be useful to the C4 community.

drcook

Actually I do know someone. However, we would need some pics and possibly a sample to get some dimensions from and unfortunately, some prepaid orders. My buddy (he is a 1990 owner himself) got sucked into a deal and did work up front and had folks bail on him. He was stuck holding the bag so to speak and out all the money for materials, electric and time.

He has a Bridgeport and a lathe, I do some of my work there myself. Originally he was a gunsmith, but it is too hard to make any money at that trade anymore, unless you are doing high end work.

I really wish I had a shop full of the equipment that I ran as a kid. I could do wonderful work. After working in shops, I switched over to writing commercial software for a living.

And by the way, drcook = david r. most folks just call me Dave.

I know another person that has a very well equipped shop. Same deal as above, prepaid orders (since it is such a low run). He could make both the offset bushings and the blocks but would need a sample. He was a tool and die maker/general machinist, but his family's place got put out of business due to companies sending all their work to Eastern Europe.

Because of the short sightedness of this country, if we ever get into another dustup where we really need people to be machining stuff in a hurry, we are out of luck. Machinists have been replaced by operators of CNC machines and even the small job shops that had those have disappeared, at least all the small places I drifted around and worked at.

MatthewMiller

Thanks, Dave. I will keep this option in mind. I don't need these part right at the moment, but I expect that I will someday. There might be a useful group-buy that could be set for current owners of VBP rear springs who would want to keep an extra set or two of the isolator blocks on their shelves. I'm not sure how likely it is that we would get a large group of buyers for the LCA offset bushings. Maybe I'll work on this one of these days...

drcook

Do you want to see them installed or just some good info or an evaluation ?

They are all well made components. Based on my experience with the 65, the design really helps the suspension components do what they were intended to do, instead of squishing rubber and then working.

I also am thinking about getting some spherical bearing sway bar end links.

If you access the order page:

https://www.banskimotorsports.com/sh...er-center.html

there are pictures and installation instructions. When I tried to post the links here, they didn't open correctly, but downloaded to the computer and opened locally

MatthewMiller

Banski has good pics and some info on them on site, as you say. So I don't need that or a how-to-install writeup. I'm just interested in your assessment of them.

That's another thing I'm considering. Is there a particular brand you're considering, or maybe homemade? I'm still not sure if there's enough bind in poly bushings for the end links to make the heim-joint style worth the expense.