Thanks Burner, but I'm only thinking out loud and hoping someone will expand upon an idea. I was thinking about this today and I believe a smaller tube will work as well and be a lot easier to work with. I had also thought about using driveshaft loops to bridge the spine.

You do know this would be a good reason to use side pipes, don't you? Sorry, but I already have a Grand tied up in the SS exhaust.

I know I can get 3" pipes down the middle, but I wondered about getting a second tube under there. I honestly believe that a 2x4 or even a 2x2 would work just as well when used with a driveshaft loop and even 1x1" tube trussing.

I was also wondering how the C-5 got past vibration with their shaft tunnel bolted to the motor and the rear transaxle? I'm not at all familiar with its design.

I am not an engineer, wish I was, somehow there is a way to figure this out.

I wish I had a way to calculate the strength of the spine and it structural traits to ascertain how small it could be and still be effective.

I was laughing today at the thought of using a 3" pipe as the spine and the exhaust pipe combined. I could easily build a structure under there with only one tube on each side. Anyone have a flexible 3" Kevlar tube? I could use a couple as a transitional isolator between the engine exhaust header and the new structurally integrated frame exhaust ducting. :lol: :lol: :yesnod:

Maybe I’ll just put in a swing-out door bar with a street-friendly 8 point. Reckon’ I could conceal a sliding swing-out bar in a custom door panel?
JIM :lol:

 

jim, if you can figure out how to get that spine in there, let me know. as i and others have stated previously, the crossbrace that i made has substantial vibration however i just got back from willow springs for a track day and i can tell you i believe it has made a substantial difference in the torsional rigidity of the frame. i'm not sure the same benifits would be apparent if rubber or poly trans bushings were adapted but then again the availability of solid mounts for such events would just make the change over an additional step in preparation.
i think if you were going to make a truely custom car, it wouldn't really be that difficult to fabricate the 'spine' for our cars. the biggest disadvantage as i see it is the transmission tunnel aft of the shifter location. it tapers down to such a point that you would loose a great deal of the cross section that would be so benificial. now if you didn't mind making some rather radical changes to the interior, what i envision is a steel or aluminum plate for and aft of the cabin that start at the top of the trans bell housing in front and the top of the rear bulkhead in back, both tapering down to the framerails. then two vertical panels to wall the trans tunnel from the inside and a horizontal panel on top with a removable panel on the bottom to box everthing in. the biggest problems i see with this design would be the requirement for a custom interior and the clearance for the rear bulkhead. if you could make things fit though, you would have essentially created that elusive c-5 style backbone.
yes, sidepipes would give you so much more room to work with.

 

just thought i'd post a pic of what a c-3 should look like going through the turns ;) :flag

 

:D Car behind looks like it is ready to roll over :D

 

I had a thought whilst sitting on the pot. I was wondering. If you went to sidepipes and got your exhaust out of the way, you could run a bit of square channel around your driveshaft. and create a couple of I beam triangles at either end of the square channel butted against the rear Ladder support. and starting after the where the sidepipes go out. That would make it almost a facimility of the C5 frame posted earlier. It just would make drivetrain maintenance rather difficult. Just thought I would throw that out there.

 

I went out to the shop and found a HPbooks-1055 Chasis Engineering by Herb Adams. I think this may have some of the answers that I have been looking for.

I wish you guys had one to review also, maybe we could find what we are looking for. It has a lot of information on the Corvette Chassis.

There is a section on a tunnel backbone style of center section. The article states that it is more inclined to work with stronger front and rear bulkheads than a system with an X center section.

I will see if I can enter some of this info if nothing else. JIM

 

Clutch' My car and yours are almost identical. I have a White '80 yours an '81.

From what I can see the answer is to create a 2x2 tube bulkhead at the cowl section with supports running toward the front bumper, and another bulkhead, only a lighter version, across at the radiator cradle. I think the front one could be much lighter in weight.This covering the front section pretty well.

The rear section would constitute building a loop at the rear of the seat where the present wall between the seat and the battery is. This would be supported with a cross truss as low to the floor as possible and reaching the level of the rear floor on the second cross bar. This section from the attachment at the frame up to the height of the rear deck floor should be 2x2 square tube again. The two cross bars need to be tied together verticlly with either a sheet of lightweight sheetmetal forming a sheer panel or using small tubes forming a conventional truss. The upper part of the Loop can be round if desired. I would then place my rear kicker bars about a foot or so higher and running toward and through the rear wall and attching to the rear bumper mounting area. If needed a small lightweight tube truss about 12" high could be placed across the rear frame rail ends. This could also protect the fuel cell. This would pretty well take care of the rear section.

Now for the magic section. The middle! This is going to hurt .

Looking inside we see our interior with seats and console. Here is what it looks like. We see our console as a flat top with flat sides forming a right angle. this is merely an illusion as the floor is still in the old round tunnel configuration. Following the design of th esquared console I sugget we take a saw and cut out the tunnel to the lines of the console that is presently installed. Then using a 1x1 tube create a truss on each side just smaller than approximate height and width of the original console.These trus will connect the front cowl bulkhead and the rear section at the loop.

This backbone will be re-covered with another transmission tunnel that adheres to its configuration.

Whew! how about it? JIM :smash:

 

I've been thinking about this same thing too. One for a performance benefit and two to get rid of my annoying rattles. I think the back bone is definitely the way to go to stiffen the frame significantly without cutting up the interior with a full roll cage like I used in my pro-street cutlass.

I'm a dynamics engineer for an aircraft engine company and I have access to a
finite element program at work. I intend to put a model of the C3's frame into it and see if I can answer the question about how much improvement of a backbone in a C3 can make. I'll have to assume 0% stiffening from the body since it won't be in the model, but it should tell me if the backbone brace will make a difference in both torsional and lateral bending of the frame. I'll try to the the model done sometime this week, and then I'll try to figure out how to fit a brace in the tranny hump after X-mas while I'm vacation. I probably wont have any info until mid Jan, but I'll keep you guys posted when I figure something out. :cheers:

Oh, and one more thing that I'll try to get a sketch of is my rear crossmember mount. I took Greenwoods advise about raising it up to get better rear suspension geometry, and while I was in there I converted to a 5 point mounting system using polyurethane bushings to reduce rearend movement.

http://temp.corvetteforum.net/c3/greatwhite/

 

jason, please do keep us posted. i feel pretty confident in saying most of us don't have access to that type of resource so if you do, that might be able to keep our cost down experimenting on things that don't work.

jim, i think you and i are pretty much on the same page with the backbone idea. our major point to reconcile is what to do with all the crap the factory tucked between that console and tunnel. i agree that our backbone needs to have roughly the same dimentions as the center console but then we have to do something with all the stuff in there.
i hadn't really considered that much about fore and aft bracing but if we have the room, we might as well.

i think that the idea of having sidepipes and freeing up room underneath the car to triangulate the frame would be a substantial increase in torsional stiffness, however it doesn't address the frame buck (sorry, i can't think of the correct terminology for that). there is a draw back in the amount of weight added versus benefit received in that.
if we can figure out the way to do a backbone, i intend to do it in aluminum to reduce the weight penelty. additionally, i don't want to loose focus of the original intent and that is to stiffen the frame enough that a more compliant suspension can yield better comfort and improved performance.

 

I should really read this entire thread before posting but i'm gonna cheat this time. I have a simple four point roll bar in my 74. I can lift one side of my car up entirely equal with a jack placed right near the transmission crossmember. If I tried this in my 79 the door would not open. I think I have a picture of this somewhere.

 

Here a couple of pictures...excuse the mess! These cars are alor stiffer than some think. Having a clean, dry, unmolested chassis with a simple 4 point can work out pretty good.

 

Here's how you cure chassis flex: ;) :)

 

Jason, a FEA model is a great idea. It's the best way to try a few things in quick sucsession. I'm interested in what you come up with. What program are you using?

 

marck, now THAT is stiff! now let's try to get that kind of stiffness into a car with air conditioning and doors that open. :smash:

stingy, my concern is the chassis flex in dynamic motion. the c-3 frame is substantially stiffer than most ladder frames in a static state but the dynamic forces encountered while on a twisty mountain road, race track or new england type freeways give us the twitchy, nervous feeling in addtion to the creaking and popping sounds we all enjoy so much. :rolleyes:

 

who says the doors don't open? :)

Something similar, I see this as perfectly streetable ;) :D

 

Yeah, sure that's streetable. Hehe. I think the first one you posted was pretty good. It didn't intrude very much into the passenger space.

 

who needs passengers ;) they're excess weight :D

 

I was talking with a buddy who's into the Twin Turbo Z's and he was telling me that a buddy of his with a Z was injecting foam into the frame to stiffen it up. Would that benefit us at all? Very little weight, kind of a pain to inject, but is supposed to be VERY solid once its in there.
-terry

 

:nonod: I hate this subject.....not sure about the '76 models --- but I'm willing to bet my '68 is worse ! :eek: :jester

 

The FEM program I have is called Nastran. I use a graphical interface call Patran. I typically use it to analyze tubing for resonant frequencies, but it can also analyze static deflections.

I figure I'll probably model the frame using "beam" elements . Basically, the model looks like stick art. You tell the computer the cross sectional properties of each area and it does the rest for me. It's pretty slick. If I can't use this technique, I'll resort to the more painful full 3D method of modeling that uses "shell" elements. Either way will work, but the beam model would be alot simpler and faster to do.

I think the first idea I'm going to try is using two small thickwall tubes that will go done the tranny hump. At the front they will have to angle out to the frame and at the rear I think I might be able to connect them to the frame after it kicks up in the back for the rear suspension. This way it will create a box when viewed from the side. I have to get the car up in the air to see exactly what kind of brace I can get up in there since space is at such a premium. I'm also intending to try and work 2 drive shaft loops into the brace for safety sake. It bothers me having the drive shaft that close to me with no safety features, so I want to take care of that concern as well.