I have owned the car for over a year now and have had some fun doing work on it here and there and getting to drive it in some beautiful areas with the top down. I have enjoyed using it as a real car, but I don't want to forget the whole reason why I bought this car...FOR RACING! At this point in time, the roll cage wasn't really necessary since I am only doing the occasional track day along with some autocrosses. But I we have a tubing bender and making a cage is difficult, so I figured it would be an excellent winter project.
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| Initial Roll Cage Design
This was the initial design I made for the cage. Using the SCCA rulebook and going off the rules for improved touring/spec miata this is what I came up with. Now like any good project, this is not how it really came out and I will cover each point that is different and why when they come up in later posts. The main requirement of this cage was that I had to be able to fit in it, which as a 6' guy can be a tall order. Hehe, get it! When you look at most cages you can buy on the market, such as the Miata Cage, the main hoops all come down and mount in the corner of the floor pan. The issue with that for me is that I need the seat as far back as I can get it, and mounting there takes up a couple inches of space. So I went with a mount similar to the hard dog roll bar that I had and mounted it to the package tray. So here are some pictures of the "done" cage:
  
I have gotten to take the car to the final of the winter series autocrosses with the cage done. Here is my quick review of what I like and don't like about what I have done:
With how I have built the cage, the car doesn't seem any stiffer than it did with just the roll bar. The torsional stiffness of the chassis is likely a little higher with the cage, but there are many other suspension reasons on why it doesn't feel any stiffer currently.
With the door bars as they are, getting in and out of the car is actually not too bad. It takes a little longer, but I don't have any problems plopping in and out. Originally I was going to gut the doors and take out the windows, but I am not at that point in my life. So the door bars became a compromise to keep the windows for the time being.
With a helmet on, I clear everything just barely using the PCI seat bracket. The bracket is a tightfit and took some practice on figuring out how to get the seat in and out efficiently, but I think I have the process down. My helmet does touch the gusset on the side unfortunately, but the only way I can get past that is cutting out the factory seat brackets which I am also not ready to do yet.
Here is my list of cons on things that I did that, if I were to build another cage, I would not do again. I mounted the front feet to the floor pan and had one side welded to the door sill. Many people, like the Miata cage, actually notch the tube and mount it on the door sill. Next time I would definitely do this. I think it would add much more rigidity than I got and you also have much more meat there in the case of a roll over. Same goes with the rear supports. If you take the rear supports all the way into the trunk, you can actually get them mounted on top of the body frame rails. The last big thing that I would do differently is the main hoop. The cross brace on the main hoop still prevents my seat from going as far back as I would like it. You would likely have to cut out the seat belt towers, but I would try and lean the main hoop back ~10° which would give way more clearance.
As far as a first attempt at designing and building a cage, I am pretty pleased with it though. Solidworks FEA was very good and is up to snuff for the SAE roll over standard that all production vehicles must go through. Hopefully I will never have to test it out though.
I will work on breaking the build into some smaller chunks so I can go over all the stuff I learned and changed without make a brick of text.
BONUS:
I drove the Miata down with my very tolerant girlfriend to the final winter series autocross at PPIR. I have not put the soft top back on yet as I am going to have to figure out some method of latching it. So we drove it down at 7 in the morning, with no top, and no heater controls in 20° weather. It should be noted, she is a very very nice lady.
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