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C4 FRAME TECH. Talk about frame specs and flex solutions...

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Old 07-05-2010, 03:49 PM
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Tom400CFI
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Default C4 FRAME TECH. Talk about frame specs and flex solutions...

There have been a few threads recently that have stimulated my thinking about the frame of the C4...and what can be done to improve it.

There are SOME/FEW bolt on items that are claimed to improve the strength of the structure, but absolutely no objective data to support these parts' effectiveness. Some people say some items help....some say "not so much".

Here is a quote I found...most of it's content are "common knowledge", but to keep it all together...

The old perimeter-type ladder frame was replaced by a steel backbone design not unlike that pioneered by England's Lotus. In the Corvette, the "spine" took the form of a C-section beam rigidly connected to the differential and carrying the driveshaft. This arrangement reduced weight and opened up more cockpit room by eliminating the transmission and differential crossmembers, and by permitting the exhaust system to be run beneath the driveshaft instead of alongside it.

Welded to the backbone* was what Chevy called an "integral perimeter-birdcage unitized structure" or "uniframe," making the new model the first Corvette to employ modern unit construction instead of the old body-on-frame configuration. The "birdcage" formed the windshield and door frames, lower A-pillar extensions, rocker panels, rear cockpit wall, and front subframe. It also included a "hoop" above and behind the cockpit, both for additional rigidity and as a hinge point for the lift-up rear window. Galvanized inside and out for corrosion resistance, this structure effectively comprised a skeleton onto which the fiberglass outer panels would attach. Completing the basic assembly were an aluminized bolt-on front suspension carrier and a bolt-on extension for the back bumper.

From looking at my donor car...If the cowl or rear roof sections are removed from the frame it would be weakened and require some crossmember reinforcement to be added to keep the frame from twisting.


*EDIT; 2/24/28: The "integral perimeter-birdcage unitized structure" or "uniframe," is in NO WAY, "welded to the "back bone". The "Back bone" isn't really a back bone; it's nothing more than a way to support the rear diff and the azz end of the transmission.

Items I know of so far are
*X-Brace
*Targa Truss (only helps when roof is off)
*Camber brace
*Longitudinal rods(?) http://forums.corvetteforum.com/c4-g...me-braces.html

I am starting this thread because I would like to gather as much info in one place as possible about the C4 frame. It seems that pics/info is hard to come by on this. I'll start by posting all that I have, below.

I think the biggest problem w/the C4 frame is lack of torsional strength. I think there is torsional weakness ahead of the firewall and between the firewall and the roll hoop. Aft of that, I'm not sure what to think.

Some questions that would be nice to know the answer too:
*What is "high strength steel"? What makes it "high strength" and how much stronger is it than "low(?) strength steel?
*Chassis hard numbers; frequency, torsional strength in ft-lbst/degree and bending strength
*How or why to the Gordon Killebrew/vette2vette rods work? What are they adding to?

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Old 07-05-2010, 04:31 PM
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The killebrew rods seem to be a bit of a play off the concept of subframe connectors used on Camaro's. The structure of those cars are different, and subframe connectors for those cars have a much more valid approach in design. Unless some data exists revealing a lot of bound or bounce in the center frame section I'm wary of how effective these are. They are merely adding bounce stiffness to those outer frame rails which would theoretically remove twist and flex under load. I'd personally need to see some raw data to believe in it.
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Old 07-05-2010, 04:49 PM
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Frame Perimeter


Roll Hoop

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Old 07-05-2010, 04:51 PM
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These pics are to show that the fundamental structure has not changed, C4>C6...






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Old 07-05-2010, 05:06 PM
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Originally Posted by arctic 92 View Post
Unless some data exists revealing a lot of bound or bounce in the center frame section I'm wary of how effective these are. They are merely adding bounce stiffness to those outer frame rails which would theoretically remove twist and flex under load. I'd personally need to see some raw data to believe in it.
I don't get it either. It seems like they'd be more effective if they didn't have heim (sp?) joints at the ends. And even more if they were tied into the stock rail, along their path...like F-bod subframes. As they are now, I don't understand what they do.
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Old 07-05-2010, 05:11 PM
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I think the C4 "Harness Bar" is one of the most effective pieces available to strengthen the "B" pillar area.

http://www.ecklers.com/corvette-harn...1991-1996.html

I'm going to fabricate my own version though.

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Old 07-05-2010, 05:18 PM
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Originally Posted by Tom400CFI View Post
These pics are to show that the fundamental structure has not changed, C4>C6...
I'm not sure that's true? I thought the center section was part of the structure on a C5. I suspect this is why the frame rails are smaller and less intrusive. The reason they could do this was because of the transaxle. On a C4, the transmission is in the center, making that kind of structure impossible. McLellan talks about this in Corvette from the Inside. If you haven't read it, it's worth a read.
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Old 07-05-2010, 05:18 PM
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I can see that. In the C6, the "rear wall" comes up much higher and provides torsional stiffness there, I think. A cross car brace in a C5 or 6 wouldn't help much, IMO. But in the C4, the rear wall terminates much lower and a brace may actually help in that area.
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Old 07-05-2010, 05:24 PM
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Originally Posted by Aurora40 View Post
I'm not sure that's true? I thought the center section was part of the structure on a C5. I suspect this is why the frame rails are smaller and less intrusive. The reason they could do this was because of the transaxle. On a C4, the transmission is in the center, making that kind of structure impossible. McLellan talks about this in Corvette from the Inside. If you haven't read it, it's worth a read.
Copy that. I agree, the C4's tunnel shape is all out of whack for providing "structure". The C5/6's tunnel is much more "rectangular" shaped, which would be better. The more I think about it, the more I don't know WHAT to think about that. I thought that the newer frame rails were smaller and less intrusinve because they met the criteria by being one piece hydroformed, rather than stamped tin, spot welded together.

Here is a pic that shows how the C6 tunnel interfaces w/the frame rails:




I don't know...

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Old 07-05-2010, 05:25 PM
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That's some great info you posted up there...."saved as". I gotta study over some of this.
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Old 07-05-2010, 06:07 PM
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0Paul Ruggeri
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The C5/6 tunnel also has a plate that bolts to the entire lenth of the bottom of the tunnel. Making it a tube, no doubt this is a weak point of the C4 by comparison.
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Old 07-05-2010, 06:10 PM
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Originally Posted by Tom400CFI View Post
Copy that. I agree, the C4's tunnel shape is all out of whack for providing "structure". The C5/6's tunnel is much more "rectangular" shaped, which would be better. The more I think about it, the more I don't know WHAT to think about that. I thought that the newer frame rails were smaller and less intrusinve because they met the criteria by being one piece hydroformed, rather than stamped tin, spot welded together.
Dave talks about Lotus, and how they prefer an "X" frame. Basically a strong center backbone that goes out to the suspension mounting points. Of course they also do mid-engined RWD, or in the case of the Elan, front-engined FWD, cars. So they don't have the packaging problems to deal with.

The C5 was an attempt to get this benefit but in a more practical drivetrain layout.
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Old 07-05-2010, 06:33 PM
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Tom,

This is some great stuff. What are you thinking about doing, cause if your going to do it, ME TO.... I know when I take the top off and make a hard corner it feels like it is comming apart and has some squeaking and groaning.
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Old 07-05-2010, 06:36 PM
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Originally Posted by Tom400CFI View Post
*Chassis hard numbers; frequency
I've saved some data on this:

C6:
-ZO6 25 Hz

C5:
-Coupe 23 Hz (top on) 21 Hz (top off) torsional str 720 kn/rad
-Z06 (&FRC) 24 Hz torsional str 800kn/rad
-Convertible 20 Hz

C4:
-Coupe (top on) 13 Hz and the torsional strength is 214 KN/rad
-convertible: (??) and the torsional strength is 140 KN/rad

Data are from the SAE Automotive Engineering Mag, Aug 1997



1997 Corvette Production and Performance

Model Year Highlights

* Door sills are lower due to a new perimeter frame built with
hydroformed steel tubing. These single-piece side rails are much
stiffer than the 14-piece rails of the previous generation.
* Floorpan consists of a lightweight composite sandwich with a core
of balsa wood that aids in chassis rigidity and noise isolation.
* Entire chassis structure is 4.5 times stiffer than the chassis
structure of the C4 generation.
* Chassis is rated at 23 hertz. Previous C4 generation chassis was
rated at 17 hertz.
* 34 percent fewer parts are used in the C5 over the C4 generation.
This equates to approximately 1500 fewer parts used.
* New C5 weighs approximately 69 pounds less than the 1996 Corvette.
This is due to the use of new SMC (sheet molded composite) body
panels instead of the previous generation's FRP (fiberglass
reinforced plastic) body panels.

*References:* AutoWeek, January 6, 1997, pg. 14-17
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Old 07-05-2010, 06:46 PM
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I think the X brace for the C4's beneath is probably the most feasible solution to adding some level of torsional rigidity without sacrificing creature comfort. If your willing to sacrifice some rear storage or top storage ability there are definitely options to stiffen the rear that are minimally intrusive. There are certainly more aggressive means to stiffen it, but you'd have to get pretty involved as far as tearing into the car.

It sounds like due to the multipiece side rails of the C4 that stitch welding them could be an option. If these are merely created similar to a unibody, then stitch welding or using adhesives can make them significantly stronger. Obviously this would involve exposing them though...haha. Not exactly a small job.

High strength low alloy steels are defined by the SAE. High-strength low-alloy (HSLA) steel is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. Their yield strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi). Due to their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels. HSLA steels are usually 20 to 30% lighter than a carbon steel with the same strength. HSLA steels are also more resistant to rust than most carbon steels, due to their lack of pearlite. Formability and impact strength can vary significantly when tested longitudinally and transversely to the grain. Bends that are parallel to the longitudinal grain are more likely to crack around the outer edge because it experiences tensile loads.

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Old 07-05-2010, 06:52 PM
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Originally Posted by Tom400CFI View Post
I don't get it either. It seems like they'd be more effective if they didn't have heim (sp?) joints at the ends. And even more if they were tied into the stock rail, along their path...like F-bod subframes. As they are now, I don't understand what they do.
Well, it appears they're attaching to structurally stronger sections of frame rail at the bends. If the frame is going to bounce it'll have to elongate towards the center. If you could pre-load the lower portion of the frame with a rod like that then you'll resist elongation of the lower portion of the rail.

Like I said though...I'm wary of that designs effectiveness.
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Old 07-05-2010, 07:20 PM
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This is incredibly good info. Thanks for participating!

Arctic 92, thank you for talking about the high strength steel. That was informative! A question about the HSLA steel; can it be welded to like regular steel? Or are there specific techiques that are required?

Ardwolf, your post is fantastic! Thank you. KN/Rad; that is Kilo Newton/radius? I don't know what a rad is. I'm confused on that one.

Wiki: radian (SI unit) rad: The angle subtended at the center of a circle by an arc whose length is equal to the circle's radius. One full revolution encompasses 2π radians. = 1 rad EDIT: It seems that "1 rad" is ~57 degrees. I can't imagine that they are twisting the frames 57* from the front bumper to the rear, for these tests....are they??

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Old 07-05-2010, 07:20 PM
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Originally Posted by Paul Ruggeri View Post
The C5/6 tunnel also has a plate that bolts to the entire lenth of the bottom of the tunnel. Making it a tube, no doubt this is a weak point of the C4 by comparison.
I completely agree.
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Old 07-05-2010, 07:42 PM
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Old 07-05-2010, 08:23 PM
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Originally Posted by Tom400CFI;

Ardwolf, your post is fantastic! Thank you. KN/Rad; that is Kilo Newton/radius? I don't know what a rad is. I'm confused on that one.

Wiki: radian (SI unit) rad: The angle subtended at the center of a circle by an arc whose length is equal to the circle's radius. One full revolution encompasses 2π radians. = 1 rad EDIT: It seems that "1 rad" is ~57 degrees. I can't imagine that they are twisting the frames 57* from the front bumper to the rear, for these tests....are they??

You don't have to twist it a full radian to measure the torque per radian. It is simply a different unit than a degree. You can convert something like lbs-ft/degree to it.
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