Thanks for the good foam link. I'm going to Lake Powell right nwo for the weekend...check it thoroughly on Monday. Thanks!

 

Converting KN-M/rad to lb-ft/deg:

85 Targa (top in): 2756 lb-ft/deg
86 Convert (top down): 1803 lb-ft/deg
97 Targa (top out): 9275 lb-ft/deg
99 FRC: 10,305 lb-ft/deg

for fun comparison:

62 frame: 1587 lb-ft/deg
63 frame: 2374 lb-ft/deg

1 lb = 4.45 N
1 M = 3.28 ft.
1 rad = 57.3 deg

 

Cool, you woudn't happen to have the frame data from the after 86 frame "redo"? I seem to remember that GM did some beefing up of the c4 frame after 86 but I could be wrong.

 

Another thing I think about its with all that constant flexing & twisting our c4s do while driving,does that fatigue the metal & can possibly cause something in the chassis to break after years of constant flexing & twisting?Metal breaks or gets weakened from flexing & twisting.Anyone ones elses thoughts on this,especially engineers?Would be nice to have a corvette engineer that helped developed the c4 here on the forum to answer these as well as other questions

 

Where were you able to find the info for the C2/(C3) frames?

 

Info taken from Cor-vette specs C4 1984-1996 models by Mike Antonick
"..... here are the not so visible changes: (for 1986 verts)
Increased section on number two bar--or Wonderbar--between frame rails forward of the engine.
Substantially larger section K-braces, connecting frame rails to under-engine crossmember.
Added front torque box reinforcement.
Added X-brace criss-crossing frame from door hinge pillar to oppostie rear torque box.
Stiffened steering column and steering column mounting system, including reinforced IP beam, cowl bracket, and newly designed steering column tube and capsule bracket.
Added spring-loaded conical wedge pins in place of current wedge pins for more solid joint across door.
Added number three bar across top of rear torque box from rail to rail behind seats (visible above load floor).
Double panel seat back riser for additional cross-car stiffness.
Aluminum frame for the Roadster top mechanism helps reduce mass.
Electrically operated rear deck lid latches. The Corvette Roadster's suspension also receives substantial tuning refinements, including:
10mm ride height increase to maintain ground clearance with X-brace.
9 1/2 inch wheels on all Roadsters places cars' handling between base and Z51 equipped Corvettes for subtle performance improvement with no additional ride harshness. Tire pressure dropped from 35 to 30 psi for supple ride.
First application of Delco Products deflected disc shock absorbers improves isolation and wheel control.
Restrictors in power steering lines to dampen torsional steering shake, characteristic in roadsters.

 

Star Spangled Sports Car, p.141

edit: One other interesting tidbit about the '63: They had a torsionally stiffer frame designed, but it rode so rough that they softened the frame!

 

Nope. Sorry.

 

"Originally Posted by mustclime
Cool, you woudn't happen to have the frame data from the after 86 frame "redo"? I seem to remember that GM did some beefing up of the c4 frame after 86 but I could be wrong."
that was a long time ago and hard to remember,but what i recall is making the wonder bar larger,putting larger bolt-on braces from the wonder bar to the side frame,and changing the front shock towers.i dont remember any changes to the basic frame members.

 

Thanks! I've got that book boxed up somewhere in storage. I need to find it.

I'd sure like to know more about the stiffer frame (ie: what was different about it).

 

Here's a quote from the book: " 'In the course of frame development,' recalled Kai Hansen of Chevrolet, 'the point arose that if stiff was good, stiffer was better. Consequently, a frame was built with torsional stiffness considerably greater than the production design.' This had been of similar layout but with tubular members, compromising less with the needs of production and interior room. 'Evaluation of a vehicle equipped with the stiffer frame proved the ride to be unacceptable' Hansen reported. 'This approach was quickly abandoned.' "

Another result from GM that surprised them:

An experienced autocrosser at GM did an instrumented study of a Camaro (I think an LT1-powered generation). They ran the car through a course and recorded data, then cut the top off and ran it again. The car ran exactly the same time. It felt worse, but ran no slower. The conclusion from the study was that if the car is stiff enough to allow the spring, bars, and shocks to balance the car, that is stiff enough.

 

This is something I've been adding to discussions for a long time. 99% of these products are sold to street drivers. They have very little actual driving skill. They want to spend money making the car better but have very little knowledge. They bolt on hundreds of dollars of products that weigh quite a bit, and the car feels better (or has less cowl shake). They are then happy and make voluminous posts about how good the car corners now. But there is no data to back that up. Sometimes I wonder if changing how the car feels, even if it makes it slower, allows these people to drive the car closer to the adhesion limits. I'm not bashing people that do this, we all love car parts, just an insight!

Grassroots Motor Sports has an article about this several months ago.

 

You could do all the Math and Calculating you want but, in the real world lap times are the proof.

 

stiff frames may not be necessary for most of us. but I would imagine that if you're racing, or you're taking your car to trackdays regularly, or maybe just endulging in spirited driving on the back roads, the stiffer frame would allow you to determine what's going on with the suspension and tires. it's simply easier to stay close to the limit. Sort of like the difference of well-tuned sportscar power steering vs SUV power steering.

regarding stress fatigue, I would think that if the cars still stay together after 26 years on the road, fatigue probably isn't going to be much of a problem. but IMHO, beefing the frame up wouldn't be a bad idea either way.

 

I'm no chassis engineer, but doesn't the above seem like an opportunity to improve ride without hurting to handling? Would not a softer suspension tied to a stiffer frame provide more forgiveness as opposed to the C4 pull .92 on a smooth surface but only .52 over broken pavement?

This was one of the huge leaps forward in the C5 was much better ride and better handling. Not due entirely to the improved chassis stiffness but certainly helped make it possible.

It kind of pisses me off that they took that approach...then spend the next 16 years trying to improve the ride without hurting handling.

 

^This guy "gets it". It's been provent that the C4 is every bit a capable as the 5/6. New, the lat accel, slalom braking...all but acceleration numbers are right on par w/a brand new C6. Mod for mod, handling remains comparable. But the C5/6 RIDE much better...while maintaining that level of handlling. IMO the stiffer the frame is, the softer (more compliant) the suspension can be, to achieve the same results. The softer the frame, the stiffer the springs and shocks need to be. -I think.

Whether that is true or not, having the steering wheel moving in your hands, and the dash moving in front of your eyes, it doesn't "add to the experience".

OTOH, I agree that there are chassis mods that do nothing. -Or close to nothing.

 

I don't really understand what those chassis engineers were thinking. A soft frame is simply an undamped spring. Make the frame softer, and it has a similar effect as putting softer springs on the car, BUT there are no frame shock absorbers!

And as far as the useless frame mods go, that might not be entirely true. With regards to pushing a car to its limits, such as racing, the predictability of the car is just as important as its absolute handling limits. It's difficult/impossible to consistently drive a car to its limits if they are hard to find, or change greatly over different surface irregularities.

 

I completely agree. An undampened spring. WELL SAID.

 

The "cross-frame" brace and "camber" brace did a lot to improve my comfort level in my '91 coupe.
I always felt the rigidity of the cross-frame could be improved if the "floating," open center,one size fits all design was "fixed" while still allowing the easier two piece removal/install.
I ended up welding the bottom plate "cleat" to the other frame member and bolting the intersecting overlap together with a 7/16X2" grade 8 bolt.
Scientifically, I can't prove anything but without a lot of rhetoric, top off driving is a lot more relaxing and the car is very solid with the roof panel installed.

 

Tom I have read this hole thread now, and owning both a coupe and a convertible, and being that I have cut apart and backhalved the coupe, I can tell you that the frame has a ton of twist, and having played with the CAD on this, and then going out to the car I have learned a couple of different things that would help to improve the stiffness. Mind you that coupe is going to ride like a rock, but I bet by the time I have tightened up a couple things on the chassis, and then have them both on the road, there will be a ton of difference in the two. But I can say that my Convertible is much stiffer than the coupe was. Once I get the Convertible on the road, take your coupe, come down to Sandy and drive my convertible and your vette on the same path, and then tell me if just the simple X brace that came under the convertible helps. Mind you to really do a true comparison you should take the top off of yours, and drop my top. The Convertible is much stiffer. I am also working on a Targa Bar to put in when you take your top off that would help the top. I am going to fill both the Competition Engineering sub frame and the main Chassis frame that is under the passenger compartment with Structural Foam that we used in the Precast Industry. I am sure that with the extra stuff not required by IHRA or NHRA to pass the 10 point cage, that will go in, will make some difference. Look into what some of the 10.5 and 8.5 cars are doing to their cars and you will see where I am coming from.

http://www.dragzine.com/tech-stories...vy-horsepower/

Look at some of the pictures of what they did under the cars, and in the back of some to meet the new SFI 25.3 Spec's

Might give you a couple of ideas. I am sure you have access to a tube bender, and also have a Mig welder and Plasma Cutter, but if not, I have them down here and you are more than welcome to come down to my shop and have at it.