Tom400CFI

I can. Anything in particular that you're looking for?

ddahlgren

At the end of the day the least intrusive thing I can see from your testing is a much stiffer trans tunnel securely welded to vertical planes at the A and B pillar. If I had the money and time and really wanted to make my C4 better. I would borrow some thoughts from a car like a 308 Ferrari or of all things a Fiero (wish i never sold the thing to be honest). The front plane welded to the tunnel needs to go from floor pan to windshield base and rear one needs to do the same meaning the rear hatch is gone and replaced with a window that goes from top of rear plane to bottom of halo. Fab a rear deck lid that turns into a trunk and put sail panels on the side for style for the most part. Make the rook have removable sections like a C3 and many of other cars with a backbone between between halo and windshield. A ton of work GM should have done given they spend over 2 years to start with. Even in '91 you don't build a car the see if it is ok. They had enough engineers and computing power to know before the frame jig was made for the first one. Appreciate they landed men on the moon more than 15 years earlier than a C4. I am betting the drone leaves with the frame flex. Cheap and easy no but would stake my money on that bet.

Tom400CFI

I think someone else asked me to run the same test, as my first test back on page 8, but with the 'full bodied '92. I finally had that up on jack stands tonight, so I purposefully standed it the same way, and ran the same test...the results? Not good. The same...maybe even worse. NOTE: Unlike the '89, this car has never been in an accident, AND this car has a RD Camber brace installed. Hope you all like Poison/Bret Michaels.




This vid, the cam is on the halo, you can see the relative movement of the w/s as I lift on the right rear 1/4 panel.




In this video, look beyond the seats at the dash...and w/s frame. While focusing on the dash, you can see the "rear of car" movement with your peripheral vision. This is me lifting on the rear 1/4, by hand. The part where the rear bumper looks like it's flexing/bouncing...that's the frame bouncing...not so much just the plastic.




This side view shows the pivot point of the right frame rail, around the firewall area...just like I saw on the Kart. You can see the front of the right frame rail dipping as I lift on the rear.

bill mcdonald

I am not sure if what I am seeing is real.
I think we need to rebuild these cars.
Looks like you are totally deforming the body panels.



Is there all this drama, with the targa top in?

grandspt

Wow, to me the kart didn't look as bad as your '92!

Tom400CFI

It's funny that ^you say that, b/c I thought the exact same thing! I was thinking the '92 might be stiffer or appear stiffer because:
*it hasn't been in an accident like the kart has
*I thought the weight of everything on it might make it harder to manipulate.
*It feels fairly good compared to other C4's that I've driven or ridden in.

But when I started shaking it, but first thought was, "Are you kidding me!?"


It's less with the top on...not surprisingly. But you can still SEE the same movements...and that was a surprise to me.

v8vette84

This blows me away... You can actually see the engine bay lifting and dropping where the hood bolts. Its moving in relation to the opposite side that is lifting and dropping. Crazy... LOTS of twisting

I would LOVE to see Dave M. comment on your last videos...

GREGGPENN

Uhhhh....me too. With reference to being "blown away", I'm still trying to decide what to say....if/when I can get my jaw off the floor. When I saw the second vid, I thought to myself "There's no way you can be lifting on the actual frame -- to get that much flex in the rear body panels".

It's so unbelievable, I wish I was there (to have seen it in person)...or watched as an expert declared nothing is broken!





Any progress on welding the 89?

v8vette84

Dave's book says in regards to chassis stiffness " In the end, we were successful in raising all the roof-on structural frequencies above wheel hop--but not as much as we would have liked."

I'm curious there reasoning behind the fact that they are signed off on this chassis. Maybe they determined it didn't substantially hurt the cars performance... Or they literally had no choice because GM brass was breathing down their neck.

A following sentence reads "With the roof off, the first torsion mode of the structure was still below wheel hop, and there was nothing we could do about it."

Sounds like the GM brass just wanted the car done.
Too bad I'll bet it would be harder than anything to get a hold of the research those engineers did and what they determined would fix the issue. You know they must have presented the GM brass with a solution but it was probably "too costly" in the eyes of the bean counters and was turned down.

Here are the pages...







Also Tom, sorry if you already posted these pages. I think you posted some earlier in the thread but I didn't want to search for them.

Tom400CFI

No worries at all. More data is good.

Going into this thing w/the Kart, I really thought I could recognize "the problem" and create a solution. The more I look at the car, and the more that I think about it, IDK. Dave makes it very clear in the book how badly the targa roof decision hurt the structure. But I really wonder about that; the original car had taped rockers that were no more intrusive than those of a 3rd gen f-body toward the front. I've seen pics of that chassis, though I can not find them now. Anyway, I have two thoughts about this:
1. I can't believe that a spot welded in, stamped tin T-bar would provide the torsional stiffness that the bolt-in roof can. It's just way too narrow to effectively resist those forces, IMO. The roof is a much better shape to handle those forces and loads, I believe.
2.The original rocker design isn't as rigid as the final design.

So I wonder if the original car may have been worse, roof on, then what we ended up with? IDK.



In the vid where you can see the rear flexing so much as I lift on it, some of that is simply the SMC flexing; as I have said before, not much attaches the whole rear of the car, to the frame. What IS shocking (to me) is the "bouncing" when I release it...that is the frame.

In the last vid, I was using a floor jack, jacking directly on the rear frame rail.

v8vette84

Yea I have seen that chassis too. I also have no idea where I saw it.

Figured I would throw a few excerpts from Corvette A - Z51. There is data about chassis flex in picture 2/3 but I'm not sure how to read it.

v8vette84

My only guess is that the dotted lines either represent the movement or the baseline for the chassis tests they speak about in the book.

Tom400CFI

IDK...I've tried to decipher that same data....I don't "get it".

Someone with a bigger brain than mine needs to explain it to me.

v8vette84

Any engineers on CF?

dclafleur

The short of it is they went through and found just about every place they could gusset or brace without cutting up the prototype and added to it. The frequency of the suspension components is close to frequency of the body which means that you feel and hear more of the road noise as those two values approach each other.

The interesting part is they talk about how they were getting what they thought was an odd "beaming" frequency (which would be along the axle line) or a torsional frequency (which is caused by rotating masses, most likely the motor). But a combination of the two creating a galloping sensation. Their solutions were the k-frame braces we've mentioned that later got beefed up further, and the wonderbar which is the secondary bracing in front of the engine mount.

I'll have to pick up this book it's fascinating.

Tom400CFI

No, not road noise. What you get is an "excitation" of the frame when you hit bumps. In my second vid, I lift the rear 1/4, the sort of release it quickly...the frame quickly "bounces" a few times, before I lift it again. The "quickly bouncing" is the frame flexing back and forth like an undampened spring. The frequency at which it's doing that is close to the same frequency that the suspension wants to move at....so you hit a bump, and it can or will potentially cause the structure to "bounce" similar to, and at the same rate as you see in my video. The sensation that you get when the frame is doing that is crappy, and makes you think/feel, "this car rides poorly".

I agree with you that it sounds like they did what they could...sort of. But what's funny is that reading about how the car was "before", then they "fixed it all"....I feel like the "before" car is the car we ended up getting.



No, torsional frequency is cause by hitting bumps in the road. The tq imparted on the frame by the engine (and diff) aren't causing this to happen....unless you're in 1st gear, and whacking the throttle on/off at about 14 Hz.



Definitely. Both "Corvette from the Inside" and also "The Newest Corvette From A to Z51". Both books are awesome and should come in the passenger's seat of every C4!


.

dclafleur

I'm just curious how much of what we experience now is the result of the rubber changing over time vs new. He mentions how they went rounds with a supplier over providing the wrong durometer of rubber between the front and rear.

Everything keeps going back to from a pure handling perspective there isn't a ton of low lying fruit beyond alignment and tire choice. I've never really had much issue with the feel of the car other than it gets worse as it gets older and it gets better when I replace something worn out. Thank you for taking the time to video everything!

drcook

You have me thinking about using 3 cheap 1" travel dials from Harbor Freight to do some actual measuring to see the differences side to side.

Meaning, if you lift the right rear frame amount X-amount, you get this much lift amount at this point -vs- this much on the other side.

Also what would be interesting to see is the difference (IF ANY, I REPEAT IF ANY) that my car shows, having a camber brace and the rear harness bar tied into the structure, against one that doesn't have the same.

Also another variable we would have to consider is age and usage. I have read here (one of you, Tom ?) posted that they saw popped spot welds on a car(s) that had been driven hard. Structural/metal fatigue comes into play.

With all that movement of the chassis, how long will hard driven (I don't mean race prepped and driven hard from competition) just hard driven cars last before structural fatigue really starts to be a serious concern. Years, decades, miles, weather exposure, quality of roads, etc.

Tom400CFI

All good questions. I don't believe that the metal will fatigue if it isn't bent beyond it's elastic limit.

I agree with dclafleur about the rubber bushings and I've thought about those in my car too. IDK. You get people who install banski (no bushings) and they say it rides better.

The car with the pulled spot welds was the 'Vette Kart, and it had been in an accident (slid the rear into a curb). The accident had pulled some spot welds where the mounting ears for the bat wing are spot welded to the rear frame rails.

drcook

I have a couple dial indicators, one NIB, "put up for safe keeping :-) ", as soon as I find it, I will try and get some measurements. A good straight edge ruler with a line scribed, or pointer attached (I have some real small c-clamps) to the jack will be able to measure how much lift on the rear.

So for speaking purposes only !

Say 2 inches of lift on the right rear, results in x amount of lift on the left side measuring point -vs- the same measuring point on the right. However, I can visualize that the result would only be good for that specific amount of lift. We would have to go through multiple amounts of lift to see if the amount of deflection is linear or what and at what point (if not linear) does it start to get better or worse.

It is unfortunate that one of us does not have free access to a chassis deflection measuring device, that would go through all the permutations and capture them electronically to be able to be presented in a graph. There are many planes/axises of movement that the chassis can move in.

Lloyd Reuss, mentioned above as the cause of this situation, ultimately was fired.