When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I could be missing something, but I don't see how the doors are going to add much stiffness. They have two front hinges with at least a little slop, and basically no solid attachment at the other end. If your door's edge is contacting anything other than the pin that the mechanism latches onto, then it will rub and mess up the paint and not close properly. If it has no solid attachment at one end, it can't provide significant resistance to distortion in any plane. Note that the targa top, in contrast, is bolted in solidly.
The doors have a big metal pin don't they? Even if the doors got loaded the pin would take the force as the pin fits pretty snug in the "slot". This is a separate pin than the latch pin. But I agree with the hinges "slop" I doubt they do anything. I really doubt the frame deflects enough to make the doors a temporary structural member. Some people claim they can't open the hood or doors, etc. when their C4 is jacked up but I have never had this issue. I have had my 84' jacked up on both extreme ends of the car (Under the engine cradle and under the aluminum frame in the rear) and the targo still bolts on easily and the hood/doors open fine with me and my buddy sitting in the car. But like many have stated that's not the problem deflection anyways.
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by MatthewMiller
When the frame of the C4 flexes in torsion, the targa roof panel is loaded in shear. The metal frame by itself wouldn't do much to add stiffness - it would distort like a parallelogram. It requires the clear panel of plastic to act as a shear web in order to provide much stiffness to resist the shear loads. Think of a shear web as the same idea behind the middle part of an "I" beam: it doesn't have to be strong in any other direction except shear loading.
I guess this goes to the question of strength of materials including fiberglass roofs -- as well as the clear ones. And, my point about the body panels too. I wasn't as "deliberate" in concluding the roof's metal frame isn't "stiff" like a frame component, but I did hastily assume they aren't in the same ballpark...and that the acrylic/fiberglass "insert" of the roof has to help it's rigidity...just like the body would help the frame. Maybe not LOADS -- but some.
Originally Posted by MattewMiller
I could be missing something, but I don't see how the doors are going to add much stiffness.
Yeah...I'm with you on this one too. The doors essentially "float" in their rubber (weatherstrip) isolation from the body. I view them as being deliberately designed to act independently of chassis flex -- as not to be damaged, bent, or binding.
Originally Posted by MatthewMiller
.... reminiscing about F-bodies. I had a 2nd-gen and a 3rd-gen, and the 2nd-gens were the worst at torsional stiffness of any car I've ever worked on. The 3rd-gens were an improvement, but still pretty susceptible to permanent deformation. I think the last couple years, where they added some form of adhesive bonding to a lot of panels, made a real improvement. I wonder if the same is true of C4s. My 96 is not nearly as flexible as your friend's 90. I jack it from either rear/side jacking point all the time, and never have issues. I've never tried it with the roof off, though. That said, we have to remember that the C4 was basically a 1980 design in terms of analysis technology. All cars of that era were vastly inferior to modern designs in terms of frame stiffness. The 911 Targa of that era was jokingly called the "flexible flyer" for the same reason. It wasn't just GM that hadn't grasped the importance of torsional rigidity. Of course, back then spring and damping rates were generally a lot lower, tire sidewalls were more compliant, and tires had a lot less grip. So the need was somewhat less than it is now, too.
True except that the C4 specifically became THE first car to break the 1G cornering threshold and you would THINK that conversation about structural rigidity would become part of that conversation. Still....Maybe GM was partially thinking sportscar on a smooth "tarmack". It obviously doesn't have the type of suspension a 4x4 does -- making it poorly designed for rough/off- roading.
When you look at newer cars, I have to wonder where the line was breached between a "grocery-getter" and a track-ready car that you can buy from the factory? IOW...what is the real target of the Corvette-buyer and when did it transition from a street car to a race car? Cause I think it's there now....with 600hp and higher iterations.
I'm always a bit nervous about jacking my vette and have been since day one -- because of a few stories here/there. The more time goes on, the less I worry because I've yet to see it really "bind" in a jacking situation. That doesn't mean I think it's stiffer than the 90 in question. That's too hard to quantify w/o back-to-back comparison.
My real question (from your paragraph above, Matt) is what years did they see improvement in F-bodies? (was it LATER than 90?) Since Camaro's 3rd generation ENDED in 1992, maybe they were learning something specifically from [harder-hitting?] C4s in their 4th gen....especially the early/stiffer C4s...and Z51s? I asked the question about the use of frame adhesive but I haven't seen it answered.
The metal frame by itself wouldn't do much to add stiffness - it would distort like a parallelogram. It requires the clear panel of plastic to act as a shear web in order to provide much stiffness to resist the shear loads.
Right. My clear top was totally ruined with the webbing in the clear top. I threw it away. It was well beyond just around the edges. I have seen other cars that have what looks like an X brace installed in the roof line, still had head room for the tall drive to fit inside.
Originally Posted by MatthewMiller
I could be missing something, but I don't see how the doors are going to add much stiffness. They have two front hinges with at least a little slop, and basically no solid attachment at the other end. If your door's edge is contacting anything other than the pin that the mechanism latches onto, then it will rub and mess up the paint and not close properly. If it has no solid attachment at one end, it can't provide significant resistance to distortion in any plane. Note that the targa top, in contrast, is bolted in solidly.
I made the point in response to the SMC panels adding any frame support. Sure maybe you are right, and when it twists it will just pivot on the latch pins.
Out front, I'm not on a particular "side". But, I have to ask IF SMC doesn't add SOME rigidity, then why does installation/removal of an SMC roof factor into anecdotal feedback?
See below for more detail but the roof's metal (aluminum or magnesium) frame, combined with the glued on panel makes the roof strong in the one direction that is important; shear. Windshield does the same thing to the w/s frame. Anyway, the w/s frame, roof and halo effectively "box" the frame of the car; they provide a 3D cage which makes it stiff. Remove the roof, and the halo and w/s frame are out of the picture...the rocker rails are left to do the entire job on their own. That is why they were made with such a huge cross section. That large cross section give the thin steel great strength. The way it was tacked together, IMO, took some of the designed in stiffness away.
Originally Posted by GREGGPENN
I also think like WVZR-1 in concluding outright structural "insufficiency" would result in reports of frame/body failure....and on a significant scale.
Keep in mind that "strength" and "rigidity" are two different things. A piece of glass is pretty rigid...but not strong. A leaf spring is tough and strong, but not rigid. The C4's frame is strong enough to meet the criteria; last a long time and provide occupant crash safety. Therefore, we can have an unacceptable amount of flex (with the roof off) and not experience any frame failures....we just experience unpleasant user perception (shakes, jitters, and rattles).
Originally Posted by GREGGPENN
Of course, I also have to consider the significant number of people wanting a more rigid chassis AND that manufacturers strive exactly FOR that! We know bridges and other major structures MUST flex to avoid breakage but they aren't suspended on the same level of articulating suspension. So...I'm forced to look at what other manufacturers tout...that a stiffer frame IS better....GM probably decided the same thing here. Both can be considered "functional" as-is. Maybe not "ideal", but functional.
Definitely not ideal, and McLellan expressed his disappointment in the stiffness of the final product in both his book and it was even further detailed in Michael Lamm's book, "The Newest CORVETTE From A through Z52"
Remember, the Frame ended up strong enough...but not stiff enough -when leads to operator perception issues that we're all familiar with.
Originally Posted by GREGGPENN
I guess I'd want to see someone push down on the RR of their full-bodied Corvette -- using the same technique as Tom. We all KNOW it wouldn't flex as much. Right?
That is an excellent question that I had asked myself. I think the answer is "yes"...and "no". I intend to try this experiment when I get feeling better...hopefully Sunday, with my '92. Here is what I THINK; all the extra weight of everything on the car will make my force input proportionally smaller; I'll be trying to twist the frame AND lift 100's of lbs of FRP, stereo, fuel tank, bumper, wiring, sound deadening, carpet, trim, glass, hardware, glue...etc. The KART breaks the frame down to the simple essentials and therefore, I think makes things easier to scrutinize...to SEE. Also, there isn't much to GRAB ON TO, on the *** of the car to really try bending it. So IDK that the test will yield the same results. But I want to see for myself, so I intend on standing the '92 the same way, as I have the '89 Kart and try the same test. Stay tuned.
Originally Posted by bill mcdonald
How are the rocker panels (body)attached?
The FRP rocker panels attache to the rocker frame rail with what is essentially sheet metal screws. However, these screw thread into plastic inserts that "snap" into slots in the rocker rail stamping. The FRP Rocker panel is thin, flimsy and is a finishing piece...not a structural one. The door sill FPR is attached with two rivets -maybe three, and a whole lot of GLUE. It too, is thin, flimsy and is only meant to put a finish on the "mechanicals" -not to add strength.
Guys, this is true for virtually all of the FRP; the entire rear tub has some strength; enough to carry cargo in the back, but it doesn't add any strength to the frame of the car. The whole rear tub, fenders, and rear bumper cover are only attached to the frame, along the rear of the "rear wall" (behind the seats) with rivets and glue, and it was glued just below where the rear cargo light is. That's it. The whole rear end of the tub is supported by a pair of rubber pads near the rear of the frame rails, that the whole rear FRP assembly (tub, fenders bumper cover, hatch latch and glass) rests on. So, there just ain't no way, that all that fiberglass is adding to the stiffness. It's not. The assembly ain't stiff itself...and it's barely attached to the car! I'd say that it's just barely strong enough to hold all of it's own weight, together/up.
Originally Posted by MatthewMiller
When the frame of the C4 flexes in torsion, the targa roof panel is loaded in shear. The metal frame by itself wouldn't do much to add stiffness - it would distort like a parallelogram. It requires the clear panel of plastic to act as a shear web in order to provide much stiffness to resist the shear loads. Think of a shear web as the same idea behind the middle part of an "I" beam: it doesn't have to be strong in any other direction except shear loading.
This is exactly right. The (aluminum or magnesium -not steel) roof frame is strong...but the way it's acted on the panel glued to it makes it much stronger (in that direction). Another part of the car that behaves a similar way? The windshield. The w/s frame is stamped tin square tube, also poorly spot welded. It's kind of strong, but remove your roof one day, and push down on the upper corner of the w/s frame; you can flex it too. But you can't fled it side to side...b/c that big slab of glass is glued into it, making it strong in shear, as Matt says, too. Like the roof panel.
Originally Posted by MatthewMiller
I could be missing something, but I don't see how the doors are going to add much stiffness. They have two front hinges with at least a little slop, and basically no solid attachment at the other end. If your door's edge is contacting anything other than the pin that the mechanism latches onto, then it will rub and mess up the paint and not close properly. If it has no solid attachment at one end, it can't provide significant resistance to distortion in any plane. Note that the targa top, in contrast, is bolted in solidly.
I agree with this too. The doors aren't designed to and don't have the proper shape or mounting system to strengthen the car in torsion. If they actually tried to, the long lever arm that is the door, would pretty easily bend the hinges, which are spaced relatively close together. No, the frame of the car should provide a rigid enough structure to support the doors, and not the other way around.
Originally Posted by MatthewMiller
That said, we have to remember that the C4 was basically a 1980 design in terms of analysis technology. All cars of that era were vastly inferior to modern designs in terms of frame stiffness. Of course, back then spring and damping rates were generally a lot lower, tire sidewalls were more compliant, and tires had a lot less grip. So the need was somewhat less than it is now, too.
Anyway, the w/s frame, roof and halo effectively "box" the frame of the car; they provide a 3D cage which makes it stiff.
This is why I said that the harness bar that I put in helps triangulate the box and stops some of the trapezoidal effect. The same way that the Foxbody Mustang coupes were stiffer than the hatchback coupes. The metal bulkhead behind the back seat triangulated the body.
I had one (a 90 hatchback), and the struts would move if you hit a bump while turning. That was even with Saleen sub-frame braces. It needed all the rest of the braces that have anchored the top of the struts.
The domestic car companies learned a lot about making cars stiffer and better performing since our cars were built.
If the HSLA (High strength Light Alloy) rails are as poorly done as you hint the SMC panels 'on all cars' would be just a mass of splinters and crazing through the finish. That is very seldom ever seen. You would actually have to drill and remove all of the spot welds to actually diagnose the efficiency of the assembly process. The SMC actually adds substantially to the build. Removing the SMC as in your 'kart' results of course in a lesser product. A typical cage would certainly improve 'the kart'. I didn't read the entire thread (didn't see the need).
Someone commented about 'unibody' - this is very close to nearly identical. There's very little that's different.
You can maybe sort the HSLA structure from this 'typical' coupe structure. Note the gussets etc.
I have thought about stiffening those areas up using an extra layer of sheet metal that bridges over the abutments and then riveting it all together, maybe even glueing the panel first.
The problem is inducing extra holes that would allow moisture to penetrate and breaking the coating itself by drilling holes. Airplanes achieve stiffness with riveted panels, I bet would could really stiffen it up, without adding a lot of weight if we did it correctly. The key is getting to the areas to do it at.
Check this out, some is pretty simple, but there is some really interesting information here.
True except that the C4 specifically became THE first car to break the 1G cornering threshold and you would THINK that conversation about structural rigidity would become part of that conversation. Still....Maybe GM was partially thinking sportscar on a smooth "tarmack". It obviously doesn't have the type of suspension a 4x4 does -- making it poorly designed for rough/off- roading.
Even though I included tire grip on my list of things that have improved over the years, it's probably the least important reason to desire torsional rigidity in the frame. The fact is that lateral acceleration (i.e. cornering grip) doesn't necessarily load the frame in torsion very much. It all depends on the relative loads of the contact patches front and rear combined with the relative roll rates (the wheel rate in roll). If the car has 50/50 weight distribution, equal-sized tires front and rear, and wheel rates tuned for neutral cornering, then there isn't much force trying to twist the frame. OTOH, if the car is severely front-heavy and is running low wheel rates in front and high wheel rates in back (maybe a giant rear sway bar) to try to get rid of understeer, then the frame will have to react all that difference in front and rear rates or else the handling won't change. In that case, yes the tire grip does make it worse.
This reminds me of a pic from an autocross last year of me using Hoosier A7 tires (black-hole-gravity levels of grip):
You can see several inches of air under the left front tire as I exit a corner under hard acceleration. This is because 100% of the front loads had been transferred to the right tire, and my rear tires were relatively equally loaded (arguably not enough rear wheel rate, or too much front rate). If my chassis were a completely wet noodle, that tire couldn't be up in the air - it couldn't react the torsional loads.
When you look at newer cars, I have to wonder where the line was breached between a "grocery-getter" and a track-ready car that you can buy from the factory? IOW...what is the real target of the Corvette-buyer and when did it transition from a street car to a race car? Cause I think it's there now....with 600hp and higher iterations.
The thing about newer cars is that engineers have figured out that higher torsional stiffness is not only good for handling, but it is also better for ride quality, NVH, squeaks/rattles, and maybe even safety. So all cars are going with stiffer frames, not just performance cars. As an aside, the beauty of modern performance cars is that they are more comfy and practical than ever, even though they have race-car levels of performance.
I'm always a bit nervous about jacking my vette and have been since day one -- because of a few stories here/there. The more time goes on, the less I worry because I've yet to see it really "bind" in a jacking situation.
FWIW, when I jack one side of my C4 up from one of the two rocker-panel jacking points, the hood gets some shear loading and doesn't want to open cleanly. If the hood was already open and I try to close it, it doesn't line up with the receiving holes (or whatever you call them) properly. But the doors still work fine. I think Tom was calling attention to the joints between the frame rails and the A-pillar, and this would lend credence to that theory. On my 2nd-gen F-body (1975 Trans Am), the doors wouldn't even open if I jacked it up from any point on the side.
My real question (from your paragraph above, Matt) is what years did they see improvement in F-bodies? (was it LATER than 90?) Since Camaro's 3rd generation ENDED in 1992, maybe they were learning something specifically from [harder-hitting?] C4s in their 4th gen....especially the early/stiffer C4s...and Z51s? I asked the question about the use of frame adhesive but I haven't seen it answered.
Going from hazy memory, I think the use of adhesive in F-bodies started in 91. I know my 92 had it. It was clearly better than early 3rd-gens, but it was still not all that stiff.
PS - I just ordered The Newest Corvette. Thanks JoBy for posting that image of some of the content. That pushed me over the edge.
Last edited by MatthewMiller; 02-24-2018 at 12:04 AM.
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by Tom400CFI
That book is awesome. I love the depth of technical detail that it goes into. Much more than even McLellan's book.
Yeah...It gives me comfort KNOWING they actually DID put a lot of thought (and execution) into the suspension!
Until I read it (in print), it hadn't occurred to me the early stages of computer development and how they would add to later generation. Duh! Look at the ODB1 computers! Even between 80 and 96, light-years of computer development would come into the everyday engineers wheelhouse.
I'd hope the transition to adhesive-added frame would be IN my 89 but who knows? The might have waited (or had more motivation) with the change to late body-style? OTOH, the frame itself seems basically the same sans crash bumpers.
I'm going to hope MY car was GLUED together! LOL
When I originally started thinking about rigidity (earlier in this week), I was looking at the "uniframe" as 3 sections: Front, Rear, and side rails. It SEEMED if the front/rear sections resisted trapezoidal deflection, the side rails wouldn't "bend" as Tom's video seems to imply. But the F/R sections SEEM to have "square" bracing to prevent trapezoidal shifts. (Wish I could find the frame pics on my PC...They made it easier to see the lateral "plates" I'm talking about. In WVRZ-1's diagram, one of them is labeled 61 (or behind it).) Before this line of thinking, I wondered if an "X-brace" placed laterally at the front of the cargo area would help. BUT...there is a welded square just below it! That's when I came to the conclusion what Tom says...the frame rails bend and/or bend where connected to the F/R sections. Finally, that McLellan's own solution to frame rigidity POINTS to what Tom sees: The weakest point being the rail-to-front assembly joint.
I found it interesting to read how they arrived at the need for the "K-bar" behind the engine AND the bracing (I assume) placed on each side of the dash.
The lack of "body loading" in the article implies (to me) that the body does NOT add rigidity. It's VERY interesting to read they needed to abandon quick-release Targa latches in lieu of bolts -- to handle the 1000lb force the roof sees. Obvious it IS more of a frame stiffener than I would have conceded considering [my perception of] it's construction.
I hate to bear bad news, but I don't believe that they ever used adhesive in the frame assy. Obviously, the DID use a ton of glue to adhere the FRP parts onto the steel frame...and to one another as well. But the connecting points for all of the steel frame parts are either spot welds, or threaded fasteners.
The only changes made to the frame from '84 to '96 occurred in '88 and included:
*The "Wonder bar" increased in size (although look at the way it's spot welded to the frame rails!)
*The "K" elements that tie the K member to the frame rails increased in size
*They added some bulky gussets to the inner front frame rails.
These changes are illustrated by the white pieces in this picture...
Now that you went and posted that picture showing the braces added to the Z51/Z52 packages, I am going to have to go out and look to see if they were carried all the way up to 1996
Now that you went and posted that picture showing the braces added to the Z51/Z52 packages, I am going to have to go out and look to see if they were carried all the way up to 1996
They were. I believe those parts were incorporated into early(er) C4's only in the performance package cars and I believe I've read that it was a result of the World Challenging racing cars. That may not be right, but I think i've read that.
Anyway, all those upgrades became standard on all '88^ cars.
They were. I believe those parts were incorporated into early(er) C4's only in the performance package cars and I believe I've read that it was a result of the World Challenging racing cars. That may not be right, but I think i've read that.
Anyway, all those upgrades became standard on all '88^ cars.
This is becoming one of those threads that is so highly educational, that I leave the "thread subscription" on it so I can go back and reread it. Knowing that the braces were only added to specific packages on earlier cars is worth a lot. A whole lot.
I love what this thread has evolved into. There are tons of great pics (even with the Photosuckit damper), tons of fantastic ideas, input, thoughts...My hope is that I can contribute something worthwhile to it, at some point.
It's funny when I go back and read my first post, how little I understood the car....I just hadn't really thought about it at all to that point.
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by Tom400CFI
Great observations, Greg.
Meh.....Maybe not so much. I'm coming to the realization that the side rails extend ALL THE WAY front-to-rear. And, that the A and B pillars are "straddle" welded over the side rails. (Before, it LOOKED like the side rails were a separate piece. Of course, that wouldn't make sense.) After all who'd build a car w/o one-piece side rails? Duh...
So...with complex bends to add rigidity, how would the side rails even flex/bend -- as you observe? Seems like any bowing would be uniform along the entire length of the side beam AND that it would take more than you leaning on it to create the amount of deflection seen in your video? Hmmm...
Do you get the same amount of torsional deflection if you push down on the LR? Are there any broken welds in the back end (or front cross member) of your "Kart"? Loose K-brace?
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by drcook
This is becoming one of those threads that is so highly educational, that I leave the "thread subscription" on it so I can go back and reread it. Knowing that the braces were only added to specific packages on earlier cars is worth a lot. A whole lot.
I agree....a LOT. It's not clear if the wonder bar and K-brace were increased in size OR if either was actually missing in earlier cars. I interpret the feedback to mean they were increased in size/complexity to add rigidity.
One has to wonder if this should be (or is) listed on some of those websites that detail differences from year-to-year. It's been almost 20 years since I bought mine but I remember thinking 89 was the earliest year I should buy....Until I read about wiring upgrades a year/two later! LOL
And, of course, one/two people SEEM to think LT1's are a whole lot better!
02-23-2018, 09:01 AM
Even between 80 and 96, light-years of computer development would come into the everyday engineers wheelhouse. 
