Roll cage. Where do I start? Big brain storming fiasco help. partial plan mess

[JonRB]

:hammer:I'm getting close to starting on the cage for my 2nd gen Firebird. The hard parts;

I'm probably within a week of finishing my tube bender, which should bend 1&3/4 up to 120degree with the potential to bend more degree with new dye$. And I have a 200amp tig machine. I've welded roll bar kits in cars before, even with subframe connectors seam welded with out riggers to the cage. I had to build my own tube bender cause I want this cage tight to the roof, and A-pillars, through the dash, and I couldn't afford to buy a bender. And my cage has to be tight to the roof - like perfect.

Now here's what I want; I want the whole thing tube framed, pretty much. I want bars from the front, (somewhere between the A arms and the radiator) to the back (in the trunk); my front end is all glass with no inner fenders. I want working wipers and the option for an engine set back alittle. The car is a Doug Nash 5-speed Pontiac V8 and I want it to remain street legal.

Here's my problem(s). I plan on going with 1&3/4. And I may swap the rear suspension in the future. As you know, the factory has half a frame in the front, nothing in the middle(theres CE bolt in connectors in there now), and rear frame rails made of basically stamped sheet metal. It just doesn't seem right to me to weld a good quality cage to stamped sheet metal in the back. Even the CE weld in rear rails are only .083 mild or something, but they wouldn't work for a future rear suspension swap anyway. I know the back of the car is light, and doesn't need to support the weight that the front does. But really, a 120 wall hoop to a 120 plate welded to a stamped sheetmetal floor doesn't seem right to me. I know it's legal for alot of racing classes, but I just don't really want it done that way.

So for the middle section of the floor, should I weld in connectors made of 120 with provisions for cage mounts and start there? Or should I go about it another way? I don't want to chop out the whole floor and weld the body on the new frame(tube drag style), cause I want it to keep some of it's original charicter. Ideas? Where should I run the rails?; in the stock places or should I lop the back of the subframe and move the rails out board to the rocker? Maybe four rails from the front to the back?

And now the really hard part. The rear. Right now it's on landrum race springs with konis in the original salisbury/leaf suspension. I'd kind of like to just leave that for the time being untill I sort out my future rear suspension, Which I've been collecting ford SVT 8.8 termminator parts for, but I havent made the subframe, Dual A arms, nor sorted out any of the geometry for that yet, and that may take years. I just have the aluminum center, halfs, and dual A knuckles as of yet (I plan on going full spherical and no subframe isolation for the irs). So should I just run the bars that come down from the cage and harness bar down to the original floor in the back, and expect to dang near back half all of that in the future? Or should I do somthing now, preparing for enough room for the irs whil reataining mounts for the leafs so the car isn't sitting with out an axle for possibly years and I could move it around, even drive it?

I started going with the irs cause I want rear tow and camber. And the irs seemed to make more sense than a floater axle.

Thoughts, words of caution, suggestions, ideas. I need as every thought, idea, and knowledge you guys could offer.

What about fixtures? Super serious, or I have a brain and I could get this straight with out one? I think I could get it straight, I think fixtures would make it easier, but consume more time and materials in making them for a one-off car.

[Evldoer]

I used .083 wall 2x3 tubing for my cars frame rails. My car is also unibody construction, but I did not put a cage in it. I removed the factory rear rails, then I cut a 2 inch gap in the floor, from the rear bumper to the front of the car where the front subframe bolts to the car on both sides. I then cut and welded the 2x3 to follow the contour of the floor, and welded it in that 2 inch gap, front to rear. You have no idea how much that will stiffen up the car, even though it is only.083" wall.
I just feel that the .120 wall is just to heavy. You could still tie the cage into .083 tubing and the car would be even stiffer then mine.
I think there is such a thing as building a car to strong. I believe there needs to be a crush factor somewhere in the mix when building a chassis. If your car is so strong that it does not absord energy in a crash, all that force is transferred to the driver.
Anyone else have any thoughts or ideas on this?

[JonRB]

Thanks for the reply.

I don't know that I'm going to run .120 out to the rear bumper. Actually I don't know that I'm going to run .120 at all. I know I'm not going to run it all the way out to the front, as I plan on running a glass enduro nose, but hiding a crash bar in there to protect the radiator incase of a small bump, and distribute the load more evenly to the frame rails in the case of a big bump against and uneven surface. I am going to loop the cage/tube around the motor and use it to help stiffen the suspension.

My thoughts are, that if I hit something in the front, It's most likely my fault. And as hitting something up front, being the end result of to much speed, and a higher probablility of me hitting somthing with little or no "give", ie; tree, concrete barrier, etc, I should build more of an impact absorbtion zone up there.

As for the back though, maybe I should treat any offender as hostile. There is a possibility that I could spin the car around, and send it into a mess rear first (which does happen). But if I drive like I'm supposed to, any hit in the rear would most likely be the result of another vehical hitting me. Perhaps I should risk a strain to my neck, and force their vehical to do most of the energy absorbtion. I have a second gen, and my tank is back there.

Also there are some other factors. Weight and speed of the vehical resulting in the energy of the impact. Perhaps, though it would be better to have a soft car run into a hard object at a moderate speed and and walk away without internal injuries and strains. That same soft vehical, may not be up to the task of hitting something hard at high speed - where I may suffer a great deal of internal bruising and possibly fatal neck injury, I wouldn't be crushed in a pile of mangled steel, which is often the case with un-reinforced production vehicals in high speed mishaps.

The weight of the vehical is of great concern. As I'm no where near complete with my engineering studies, yet have aquired skills in fabrication, it is really to easy for me to build a car that is entirely to strong, and to heavy.

I was following most of the rules provided for race classes when I was contemplating .120. It could be a factor that .085 seam welded in to a floor could be just as strong, if not stronger than a .120 tube structure. But I do intend to gusset the cage to the body and roof as much as I can.

Also. Being that I'm over most displacement limits, will take free-reighn in my suspension designs, and intend to run wipers and lights, resulting my not really fitting in any real competition class. pehaps I don't need .120. Possibly I could build the car out of .095, or .085CM, still have a safer car on the streets, better ridgity, and save a great deal of weight. The car is basically a compilation of thin gage sheet stampings from the factory right now. That's not ridged or safe at all for the power I'm putting in it. Seems almost any tube I put in the thing is a step in the right direction. Hmmm...

Dang it. I need the engineering math of my courses, but I'm tied up in a computer course right now, as well as a full time job, and can't even do the last parts of my tube bender build untill maybe this sunday. I'm on it though, and the build is happening as fast as I can do it. Shoulda just skipped the courses and sprung for one of those new auto-cads with the stress dynamics.

[mikedc]

Hi. This thread is a few weeks old now but I was interested by it.


IMHO the safest car shape is probably gonna be a stock F-body in the front & rear clips, but with a seriously thick & beefy rollcage in the passenger area from firewall to back seat. IMHO the additional rollcage tubing that extends out into the engine bay and the trunk should only be used for stiffening the suspension mounting points. At least in terms of safety alone. For racing around mostly under 100 mph I would rather err on the side of a soft front & rear clip and a center cabin that's strong as a tank. Just my opinion.

A big thing swaying my opinion is the fact that the OEMs make the rear clips so soft these days. The stock Camaro rear subframes + the rear rollcage tubing structure around the fuel cell = you've already probably got a stronger rear end than they tend to build for street use today. (And don't forget how heavy modern cars are. Your race car is probably gonna come out a good bit lighter than a modern sedan, which calls out for leaner frame structures just to maintain the same kind of crumple resistance.)


I can think of another example. This one may seem irrelevant but I think we can really learn something from it - Hollywood stunt cars. They start with stock production cars, beef them up for tremendous purposely-done wrecks, and the ONLY thing they care about with regards to car preparation is safety. Nobody cares if the car can't run another 100 laps after getting hit. Nobody cares if the car can't be repaired within 7 days to do another race. Nobody cares if the shock towers aren't stiff enough for a racecar suspension to be tuned right. All they wanna do is whack something at a very high speed one time and walk away from it. So what do they do? They build a brutally-strong rollcage in the center cabin of the car and they don't beef up anything else whatsoever. Their cars are beefier inside the cabin, and weaker outside the firewalls, than a common tube-chassis racecar.