First a bit of a disclaimer. Safety is #1. I'm not a certified welder, a mechanical design engineer, or the like. I'm just a guy who does pretty decent work on cars. I figure I might as well share my designs and thoughts with the reader with the idea that if you're going to do this stuff anyway, you may as well get all the information you can. Remember though -- it is possible to make a cage or roll hoop that is more of a hazard than a help if improperly engineered. Some of you may read this and decide to have a professional fabricator build the cage, and perhaps you'll be able to talk a little more intelligently about how to build it. But there are those adventerous few that build their own cage, and this article is targeted at you.

So, with a few sticks of raw tubing, a gutted car, a welder, notcher, and bender, how does one go about building a roll cage? Well, for me the process boils down to a few critical steps:

• Prep the car (this article)
• Carefully size, bend, and cut the hoop (part III)
• Build the rear portion of the cage, tack in (part III)
• Build the hoop forward, including the door braces, tack in (part IV)
• Weld cage in, clean it up, and prep for paint (part V)

You may ask why I call the hoop out as a separate portion in the whole build, and it's not even welded in at that point. To me, the hoop is the critical foundation from which the cage is built. It must be exactly where you want it, sized correctly, level, and straight, or the cage will not fit right. This is the unit that dictates the placement of most everything else, as well as protects your noggin in a rollover. You want to get it right.

Preparing the car

I was lucky in that the Miata had been stripped before it was handed over to me. After getting it home, I took a careful look at where everything was to be placed. At those areas, I stripped the paint off using grinding discs, and prepared to weld in mounting plates. These plates are what the cage tubing is welded to, tying the car's chassis to the cage. Note that it is not advisable to ever weld the cage tubing directly to the car's sheetmetal. The body sheetmetal is quite thin in most places, and in the event of a rollover can easily be punched through. Additionally, the sheetmetal is moveable, allowing the cage to move a little, which reduces some of the chassis rigidity benefits a cage can provide. I used thick mounting plates I had ordered some time ago from a steel supply house listed on the web. They provided a stack of cut 8" square plates of 11ga steel (.1196 inches), which is at least as thick as wall of my cage tubing (required by SCCA). In most cases, I cut these in half for mounting, but the extra width did come in handy when fabricating the wedges for the a-pillar tubes.

This is an 8 point cage, meaning it will attach at 8 points to the body. Two are at the hoop & bulkhead, two are near the hinge in the doors for the a-pillar & windshield, two are forward of that to the firewall, and two go to the rear near the shock mounting points. I have to create mounting points at each with these steel plates.

I started with the bulkhead, intending to run the mounting plates from the seat belt attachment point down toward the corner of the floor, both of which are heavily reinforced from the factory. The idea is that this will provide a solid foundation for the most critical part of the cage, the hoop. After cutting my plates in half, I used a big hammer and a vice to bend them to conform to the shape of the bulkhead. After test fitting, tweaking the bends, and setting the plates into place (on both sides), I was able to move on to other areas of the car.

At the rear, I cut mounting plates to the appropriate size to fit between the gas tank and the shock mounts. I also took note of how closely I'll be welding to the gas tank -- not too appealing.

I actually built the forward mounting plates a bit later in time, but for clarity, I'll provide the information here. Rather than bringing the forward tubes down to the floor, I copied what I had seen in the other race car. I built "wedges" that would attach to the rockers of the car. These rockers are extremely strong, so this is a great mounting place. I built the wedges to weld in near the driver's and passenger's calves, following the contour of the rocker. I first bent a full size mounting plate about 105 degrees, resulting in a slightly acute angle. Once I had it right, I cut out ends for these wedges that fit into the angle, then tacked them in. Finally, I spent a bunch of time with a hacksaw & grinding wheel getting the cutout to follow the rocker contour just right. Once done, these wedges almost snapped in, but they did require a lot of work.

The final area that needed mounting pads is the reinforcement that will come from the "wedges" toward the firewall. If you look at the firewall, there are already reinforced curved areas. These are a perfect place to put the mounting plates. Again, this required cutting, bending, and fitting to get a proper mounting pad.

With all this done, I was able to weld the new mounts into place. For most pads, I seam welded around the edges. However, because of the criticality of the bulkhead, as well as the large area the bulkhead mounts cover, I also drilled holes to plug weld into.

And with that, I was able to move on to the next step… building the roll cage hoop.