I have been looking at tubular chassis all over the internet and I keep coming back to the idea of raising and reinforcing the center tunel...

what are the advantages of having our tall longs as well as a backbone structure tied in?

my example of my thoughts are probably the c6 vette.

see the c6 chassis here

a video I took in 04 for the c6 dealer training at Pheonix International Raceway (got to drive lots of vettes, base911, boxterS, z4) the c6 Vette is amazing.

DIVX Video

Hi...

Im planning a tube frame 914 that will incorporate something similar I think. The plan is to use 2X4 box tube for the perimiter frame and then make substructure reinforcement working toward the center of the chassis, similar to what the Corvette has. These reinforcments may be 2X3 or round tubing or a combination of both.

As for mounting, I plan to notch the longs and slide the 2X4 frame up into the area that is occuppied by the Heat Tubes. Since Im running a V8 anyway this should work out fine. Inside the 2X4 box tube frame will be the cooling lines for the front mounted Radiator.

Not sure if I will have to raise the center tunnel though. Im hoping I dont have to. IF I do, then it will call for a Cable Shifter.

The only problem that I have run into so far is the mounting for the Rear Control Arms and Coilovers...but that should not be that difficult.

As I come up with more planning I will post my progress...

Thanks...

Steve

Are you sure that Vette tunnel is structural?


EDIT: Nevermind I just googled it and I guess it is...

It would take a good deal of structural analysis and strength of materials investigations to gain any real value I think. I believe the C-6 vette has that beefy center tunnel to allow narrow door sills, as you allude, but I think that the loads may be different with beefier sills, so that the center isn't really stressed much. Sorta an "either/ or" situation. I'm just speculating, mind you.

it couldn't hurt...when I had my full caged 914, I was going to cut out the center tunnel but was advised not to touch it until the cage was installed....

I think the C6 is a different creature than a 914. That big boxy tunnel houses the torque tube, keeping the engine in line with the transmission. But since the engine is much closer to the transmission in a 914, there won't be so much twisting. Plus the difference in power (400 hp versus 200?)

The backbone chassis has some limitations. Torsional stiffness isn't its strong suit.
http://www.autozine.org/technical_school/c...ech_chassis.htm

If you were to put the car in torsion, i.e. jack up the car on opposite corners, where would the least amount of deflection be located? Down the center tunnel, most likely. So why bother stiffening something that isn't going to see much work, and, in turn, can't work against the other elements in the car?

Another (oversimplified) way to look at it is this. A tubes torsional rigidity can either be increased with wall thickness, or diameter. You could put a solid 2" rod right down the middle of the car. Or you could put in a 4" diameter .090" wall tube instead. Now slowly increase the diameter of the center tube until it is as wide as the car (and correspondingly thinner). The suspension points now act directly on the tube, and forces don't have to be carried (at right angles) to the center of the car, fore-aft, then back to the outside of the car.

So, you'd be better off putting the stiffness at the theoretical largest diameter -- the longitudinals -- which locate the suspension, and then the firewalls, which locate the longs with each other.

I've run out of word pictures. More to come...(?)

so what you are saying is a roll bar or tie-ins over the suspsension consoles in the rear from long to long would be more worth while?

I've been thinking about putting a hoop in line with the rear tin from console to console angled back towards the rear engine tin opening which on my car is removed...

if i dont go subaru right away I'm going to modify the tin to fit the entire whole vs adding back in all the permanent shelf that i removed. eventually it will be subaru or sbc

how about the front? does angling from the front suspension points to the gas tank firewall? whatever you call it help significantly with handling?

Mainly I want to stiffen the car a lot, but not go across the windsheild frame to the roll bar. I want to keep the top completly open but i want the car to be prepared for a lot of power and hard autocross.

When ya gonna get some meats on that Dodge?

Very interesting read

when the 914 is done... (so it'll be a while) - actually I upgraded to the 245's from the 225's

IMO it is best to just hoop the car and tie the longs together for the best torsional rigidity. To increase rigidity between the wheels, tie tubes to the suspension points through the firewalls. Increasing the center tunnel to get the same effect would be far more difficult, as you would have to reinforce the firewall tie-in points as well. Probably a lot heavier in the end v. a simple cage. the new Z06 tho' !!!!!!!

I started out to use 2x3 but couldn't get chromoly tubing in that size - if you use 2x4 mild steel you car will get heavy fast, and 2x4 is a huge section for such a small car. I went with 2" diameter as the "square" that surrounds my interior (which is too big as well frankly, but I wanted side impact stability) and 1-3/4" extends front and rear from there and most of the floor support in my car is a combo of 1-3/4" and 1-1/2", all .090" chromoly to keep weight down. All said and done my car is still over 2600lbs, tho' I will be getting fiberglass lids, putting my gutted, early doors on it for the track, and so forth. Probably still be 2500#.

Longs? I don't need no stinking longs...
Click to view attachment
Click to view attachment

What is hard to see is that I have the longs sitting on 1-1/2" tubes during construction to lift the body up relative to the new floor - helmet clearance was tight before with the race seats and remember, you sit on the lowest part of the car without tubes, so adding them to the floor puts you up 1-3/4" inches, so I set the whole body up from my new "bottom".