cage design...

I've recently been entertaining the idea of caging the Miata, and have the notion to try something new and interesting. most bolt-in or weld-in cages run one tube from the rear hoop along the top of the door and down the A-pillar to either the floor or the firewall and into the engine bay, but after looking at the cage on a Marlin 5EXi (british kitcar built on a spaceframe chassis), that one appears to run the "roof" area of the cage much like an old T-top, with a separate front hoop connected to the rear hoop by two bars running down the middle of the roof. to me, this seems like a better idea, especially if the car will be run with the hardtop on most of the time and serve dual purpose as a track car and occasional street car, as it doesn't make the already-small door opening of a Miata seem even smaller, but still gives me a full cage and the added rigidity that provides. would a setup like this be SCCA/NASA legal for trackdays and time trial/wheel-to-wheel racing, or is a traditional cage design likely to be better? here's a few pictures of the Marlin's cage so you can visualize what I'm talking about, or at least try to:

I'm no cage expert, but what the hey... Hopefully somebody with actual knowledge will pipe up soon. In the meantime:

I don't like that it doesn't reinforce the perimeter of the area around your head. I don't know how often something tries to intrude there, but it makes me nervous.

I was about to say that I didn't think it would be as rigid as the traditional perimeter, but I may be wrong about that. I'd want to build the classic little balsawood models of both versions. Obviously, with balsa you'd just do an X-shape instead of the back-to-back C-shapes, but frankly I think an X there would look less goofy, not involve any bent tubes, and provide most of the benefit in terms of not restricting ingress/egress...

Ah, here's a structural downside in terms of a basic rollover: With a traditional cage, if you try to bash in the top corner of the windshield from the front, that force is passed along the tube along the top of the door to the point which is triangulated by the rear hoop and rear brace. In the case of the curved tubes above, that force essentially tries to tighten up the radius of the C-shape by bending that tube. That worries me. The X version I mentioned earlier should be somewhat better, but at the angles involved the force on the tube would be higher, increasing the likelihood of buckling.

For a street roadster, that would be fine as it would be safer than many of the style bars around.

For the track? No. While it could be legal (check out Formula and Sports Racer) cages it wouldn't be as safe as a more normal cage.

One issue I would have with this cage desing from a NASA Racing perspective is there would need to be a way to attach a window net, I don't readily see a way to attach one to this cage, I suppose you could attach the the net to the door frame on some cars but not a Miata.

Looking at the NASA CCR's there is little chance that this cage design would be allowed on a race car.

Nice for head room but that could be a problem in other circumstances

It would do little to prevent intrusion into the cabin. I'm not a fan.

Also - If headroom/entry/exit is a concern, consider putting a kink int he halo bar. Here's a cage I built -

That little 5* kink made a huge difference in headroom.

I don't like that at all. While i always wanted to have them built on the big side to keep the bars away from the driver, You should never be the outer most part of the car, in a rollover if you stike an object upside down you will hit before the cage which pretty much defeats it's purpose. In a closed car I like Dave's idea, and in an open car make it taller then the windshield if that makes you comfrotable.

To make it somewhat SCCA compliant, you could make a front hoop and a main hoop. Tie them together with a double center bar that's not intrusive. If at some time in the future, you need full compliance, you can add outer bars along the roofline that are either permanent or removable. If SCCA racing is your end game, I suggest you read the GCR. Once you read thru all of the safety requirements, you come away with a feeling of "that makes sense what they are asking for".

Number one: there are no rearward 'stays' from the main hoop to the car to keep the bar from folding over. I'm going to say that right there will make it non compliant.

Number two: there is no 'diagonal' across the rear hoop to keep it from folding sideways. Again this would make it non compliant.

Number three: that double center bar', while attractive and certainly better than nothing, does not add any real strength. Envision the car getting on its head while moving forward (a very likely scenario, if you think about it), that would have the forces on the front bar pushing rearward. Follow the 'load path': there's nothing to properly transmit that force to the main hoop and then to the body of the car (the reason for the rear stays) so the front hoop would very likely fold right over.

My verdict: it's a 'show bar' which is better than nothing but is not truly good protection and I'm going to say it probably wouldn't pass tech.

On an open car, SCCA rules for a 'race ready' cage say that if there is not a front cage hoop, then the rear bar must be tall enough so that if you drew a straight line between the top of the rear hoop to the front of the car (think of it as laying a straight edge from the hoop mto the part of the car that would be on the ground if it were upside down) that line must be at least 2" higher (IIRC) than the driver's helmeted head. That's why the Abomination's 4 point roll bar is so high. About adding headroom: when we built our last LeMons car there's two of us who are rather lengthy and the previous car's cage did interfere with the melons. So we built this halo with extra height in mind. The bends at the rear of the halo allowed us to tuck it up inside the car's roof and greatly increased the headroom, notice in the side picture it's nearly invisible from the outside even though it's 1 3/4 .095 wall DOM tubing. The halo also ties in directly at the point where the rear stays come in (remember the load path thing). The cage passed LeMons tech (and should pass SCCA), but the more Toyman and I have looked at it the more we keep saying we should triangulate it at the rear, from the halo down to the main hoop. We will probably triangulate or gusset it at the front as well.