SPZ510

Started my cage, here are some pics. Tell me what you think. It's just tack welded for now so recomendations will be taken seriously. I'm wondering on the placement of the tubes from the strut towers up to the hoop. Do you think I should move them out to meet up with the upper tube that will go forward. I kind of like the slanted in look and have seen cages where those two tubes meet in the middle/top of the hoop, from the strut towers.

christaylor

Is this a full cage or just a rear bar? If it's a full cage, those braces that go back to the strut towers need to line up with the tubes that go along the top of the door. If you get in a bad wreck, with it like it is currently, it will just pretzel your main hoop.

Looks kinda small, what size tubing are you using?

SPZ510

It will eventually be a full cage. It is 1.5" tubing, one of two options from Autopower. The other is 1.75", I got the main hoop from I/O port racing and that guy is a tech inspector for NASA. He said that size would pass tech. I can move those two tubes coming from the strut towers no prob.
Anyone else?

Blake

First, put down the MIG and pick up the SCCA GCR (General Competition Regulations). This will tell you what you need. For example, 1.5 inch tubing may be fine if it's .120 wall, but not .095, for a car over a given weight. Also, all the tubing will need to be the same size and thickness; no mixing and matching. Lots of other critical stuff, too (max number of bends, pickup point locations, etc).

Specific to you question, I would agree that it is not correct to run those back braces anywhere other than to the outside main hoop bends. If you want additional diagonal bracing, run it from one of those points back to the strut-end of the opposite bar.

It might be a good idea for you to have a fabrication shop inspect your welds, too. Tech inspectors get too caught up in the letter of the rules and often fail to see defective workmanship that could be disasterous. One such example came into RaceTech Fabrication recently and it was so bad Russ called up the tech inspector and chewed his *** about letting it pass (the customer built his own cage and really did a hack job of it). It came in for some sheet metal work and left with a whole new (safe) cage. Besides the crappy welds, bad bends, and other stuff, the fire bottle was mounted outside the (passenger) door bars! So, don't put too much stock in getting something to pass tech; they are only picky when they want to be.

Good luck, and don't be afraid to take it to a pro for inspection or even completion. Honestly, if you just tacked it all in place, it would only cost you a couple hours of labor to have it welded up properly.

SPZ510

Thanks Blake. I'll definitly consider have a profesional weld it together, and I think I'll go ahead and get an SCCA rule book.

Travis R

I agree that at the very least the top of the rear bars need to move out the the bends. You could add some torsional rigidity if you made them into an X. Top left bend connected to the right side strut tower and vice-versa.
Where do you plan to run the car? Road race, autocross...?

Speed Raycer

You've got to think about your load paths when doing a cage. Idealy, every tube should be able to transfer any of its energy in a wreck (or turn) to another tube.

An X will give you great transfer and structural rigidity, the only real flaw is that they are weak if its a direct load from the front or rear. Good in side impacts, worse in straight on rear impacts.

The current cage that I'm doing has "straight" main hoop braces from the intersection of the front downbars back to the rear pads (around the spring mount/coil mount brace) and then an X from the upper door bar mount location to the rear pads. This ads quite a bit of chassis rigidity, keeps the weight low and doesn't clutter up the rear view.

In IT, all bars must be at least the minimum size required for the spec line weight of the vehicle minus 180 lbs. This means that a 2300 lb car can use 1.5" .095. All tubes must be at least that size. They can be larger, so some mix and match is allowed (can you say weight distribution )

Carl Byck

here are some pics of my cage, they illustrate some of the above posts.
https://www.rx7club.com/attachment.p...postid=2402915

Carl Byck

2 https://www.rx7club.com/attachment.p...postid=2402973

Carl Byck

https://www.rx7club.com/attachment.p...postid=2403057

Carl Byck

https://www.rx7club.com/attachment.p...postid=2403075

CypherNinja

When building a cage, you (ideally) want all of the tubes to carry a load (including crashes) along its length in either compression or tension, never in bending. This means that all your tubes will end up coming together in common joints, forming triangles. You want as much of the cage to be made up of triangles as possible. Often referred to as Triangulation. Tubes are MUCH stronger in compression and tension than bending.

Just my .02, whatever its worth.

Bonus material: http://e30m3performance.com/tech_art..._symposium.htm

in2twins

Those are awesome, thanks. Carl

Blake

Sorry for the late reply; I'm not really a regular here. Anyway, this statement is not exactly true. GCR, 18.1.6 para c. last sentence:

"If any of the above bend requirements cannot be met, all components of the
roll cage shall be fabricated from the tubing size(s) listed for the next
heavier category of automobiles"

Granted, if you are not violating any of the bend requirements (e.g. four max on the main hoop), then I suppose the issue is open to interpretation, but it would still need to be "sold" to the tech inspector. I also have trouble with the idea that anyone would voluntarily mix and match tubing. I would use the minimum specified or upgrade the whole thing as a system to the next size/wall up. Where is the advantage of mixing and matching, when the weakest link is the most critical part? Oversizing some parts doesn't necessarily benefit you in any way, but it most certainly gives you a weight and CG penalty. If you need to make a minimum weight, the best solution is balast down low; not oversize tubing up high.

Just my $0.02.

Speed Raycer

The statement was exactly true. All tubes must meet the min. required size... so nothing smaller than 1.5" or thinner than .095. The earlier statement of " all the tubing will need to be the same size and thickness; no mixing and matching" wasn't true.

Nobody suggested putting the heavier bars up high. That would be plain stupid. You can utilize the heavier wall tubing to help offset the weight of the heavier driver side (driver, seat, NASCAR bars, battery, etc) by using it low on the pass. side door bars and bracing. It's definitely not any higher than the big mass of weight steering the car! The point is, you can put weight where you need it as opposed to where the ballast has to be.

What is there to "sell" to the tech inspector as long as all tubing meets the min. required?

Blake

I will admit it is a matter of argument, but it is not so clear cut as you seem to think. First, if the GCR doesn't say you can do it, then you can't, technically. And, those times when you must upgrade the tubing (e.g. too many bends in the main hoop), then you are required to upgrade ALL tubing to a higher spec. This is a very clear rule, as I pointed out. Roll cages are a system and they should be treated as such. Honestly, I had never imagined anyone would intentionally mix and match for any reason other than ignorance of the standard and practices of building a proper roll cage system.

Please tell me why they would not allow you to just upgrade the main hoop when you put an extra bend in it (like a peak in the middle, for proper clearance), but they somehow would let you mix and match for other reasons? It doesn't follow logic. When they say a size is "minimum", it does not prevent you from upgrading to a larger size/wall, but it does not grant you permission to do it *****-nilly; the presumption is you will do it as a system.

As I said, you have an argument that might get pass a tech if you "sell" it to him, but it is not specifically sanctioned by the rules and I fail to see any advantage in it. Few techs would ever check all the bars or bother to ask, so it is probably a moot argument anyway. Ultimately, safety is in your hands, so do as you please if you can sneak it by the tech.

As for your weight distribution argument, I don't buy it. All but a few inches of the cage is above where you would normally place ballast.

Speed Raycer

I had a big long reply typed out and just deleted it. You've obviously got your methods of cage building and I've got mine. I've seen my cages impact tested at speed and stand behind how I build them. We could argue GCR interpretations all day long but I've got a cage to build and another one coming in two weeks.

I don't have to sneak or sell a cage through tech. In fact, I've had tech guys request my cars to show people how its done. Enough said.

Blake

Fair enough. It is a minor issue, at best, provided the cage is done at least to minimum specs.

SPZ510

Better?

SPZ510

another.

Speed Raycer

Much Better!!!!

Now add a bar runnng from the top of the hoop down to the opposite rear pad and get some triangulation going!

Silkworm

That cross bar is pretty low to mount harnesses to, are you going to do that or add another bar?

I forget the exact angle, I want to say 40*, from horizontal is the maximum allowed for harnesses to mount, and based on where your harness slots are in the seat, and how close your bar is to your seat, I think you'd be more like 45-50*.

You can solve that with an additional bar, or by moving that bar up.

PaulC

SPZ510

Thanks Racer. Any further out and it gets pretty close to the trim panel and it would get hard to weld without breaking loose the main hoop spot welds and sliding the whole thing forward and down. There's a couple more bars I will add, I just did'nt want to comit myself without asking for advise.
I'll fix the belt angle with different seat mounts. Right now they are Sparco/OMP seat mount, both made by the same L.A. shop, witch are garbage if you ask me. They make the seats sit extremely high, so much that my head is almost hits the headliner and I'm only 5'8"! And only 3 out of the 4 bolt holes line up. You can add this one to the product review section. I feel like a trucker when I drive this thing, I just need one of those ***** on the steering wheel and fifth wheel welded in the trunk and I can start haulin.

Speed Raycer

Good catch on the harness angle. IIRC, most belt manufacturers recommend from 5 deg. below the shoulder to 30 deg. above. The GCR mandates how far the crossbar is allowed to be under the shoulder line, but that escapes me right now.

There is also an illustration in the GCR showing that you also have to have 2" of clearance between your helmet and the top of the hoop.

Sire Riptoid

When you have someone weld it have them stick weld it because stick welding is much more stronger