1. The bar must not interfere with my comfort in the vehicle. I spend about 1 1/2 hours per day in it, and being 6'4", comfort can be a challenge today.
2. The bar must not interfere with the operation of the vehicle. The top, including the glass rear window, must function correctly.
3. The bar must be strong. I can compromise optimum strength for some of the above considerations, but it must add to the strength and safety of the car.

On the up side, I wasn't constrained by what is in the market. With an ample set of fabrication tools at hand, I can bend, notch, and weld tubing to my heart's content.

So, I began searching the net to see how others have built roll cages. By and far, the most popular seems to be Hard Dog bars, and their many variations. These seem well built, strong, and some even work with glass back windows. Unfortunately, they almost all put the hoop in front of the seat belt shoulder attachment. This makes sense for most, but I fear that my wide shoulders would rub on the bar, and my head nearly hits the top now. I decided I wanted the hoop to be behind me in a normal driving position. I noticed that other roll bars would put a cross brace in between the sides of the hoop, but this prevented the seat from being reclined all the way. A hoop by itself would allow full seat use, and minimal intrusion, but it didn't have the strength I wanted. The problem is that hoops have little strength against bending on any side or forward/rear force. A fully triangulated hoop would have strength, but that requires lots of space. Those are some of the considerations in design.

I decided to build a hoop that sits between the two shoulder harness reels. This allows it to be a little rear of where my head sits, and behind my shoulder; the head rest of the seats will be right next to the bar, not behind. To strengthen the hoop against forward/rear loads, I braced it from the rear with supports in the trunk. The angle is less than ideal, but it is worth it as it does add significant strength. To brace it against side loads, I "partially triangulated" the hoop with a bar running from the passenger bulkhead to the top of the hoop. It would have been stronger to run it corner to corner, but that would have interfered with the full use of the driver's seat. I also tacked the hoop to the shoulder harness mounts. This design, I hoped, would provide a good mix of strength, suitability for daily usage, and comfort.

With my basic design in my head, I proceeded to install. This started with the appropriate disassembly of the car. I removed the carpet, passenger seat, seat belt plastic trim, seat belt shoulder harness guide, the dozen or so 10mm bolts holding the galvanized bulkhead, and finally the bulkhead. Also, it looked like Mazda did a great job ensuring that all the wiring is well connected to the chassis, and I removed many of those connectors from the chassis to allow me to get at the sheet metal. I left the driver's seat in so I could sit in the car & ensure the design met my needs. With that, my work area was taken care of.

The first thing I did was measure and fit up the hoop. I took measurements between the shoulder belt boxes (40.5 inches), as well as height from the bulkhead to the bottom of the soft top (24 inches) at the frontmost part of the bulkhead. Per my basic design as outlined above, this is a good starting place to make the hoop. I figured that I had enough information to start the hoop, after which I could tweak the design.

Note that there is a little bit of sheet metal in the way of where I want to put the bar. Using a hacksaw, drill, and chisel, I removed the offending pieces. The main offenders were the mounting tabs for the above mentioned galvanized bulkhead. I cut the forward areas off, trying to keep the mounting bolts if possible. While not noted later in the article, I also had to remove sections for the rear struts, as well as cut & pound down the spot welded seam where the trunk & bulkhead sections attach. All-in-all, it isn't much modification, but it's possibly not for the squeamish.

Back to the bar itself. I cut a little over 90 inches of tubing with my power hacksaw, figuring it is better to have a little extra tubing than have too little. I bent up the hoop per the article I've previously written on tubing bending. I was very happy with the results. With one cut to the excess, I placed it in the car, lifted the top, and found the bar to come within about 1/4 inch from the top, and about 1/4 inch forward of the top support. I was very happy with this result, as it provides the maximum occupant room without rubbing against my top. Hoop ready, I moved on.

Due to having a glass window in the soft top, I had very little room to fit rear braces. Ideally, I would have liked them to extend at a 30 degree angle downward to ward the rear, preferably connecting at the rear shock tower. However, as the glass lays down when the top is put down, I could only attach a few inches rear of the hoop attachment. Even though less than ideal, this should add significant strength. I cut tubing at the required length, then deeply cut/notched the tubing where it attaches to the hoop. See pictures for how this worked out.

Finally, I cut a diagonal brace that sits between the two seats. I brought it as close to the driver's hoop corner as I could without interfering with the seat. The idea here is that side loads turn into tension loads somewhere in the chassis. It has to do with the concept that triangles are a lot stronger than boxes. This doesn't quite meet that design consideration, but it's close enough that it adds value. I notched the tube where it meets the hoop, and it was ready. With that, all the requisite pieces of the roll bar itself were done.

Let's discuss attaching a roll bar to the chassis for a moment. You cannot expect a roll bar welded to the chassis to survive an incident. The problem is that the roll bar will transfer huge loads into it's mounts, and the thin sheetmetal in a car will quickly punch through or tear away. The car's sheetmetal is strong overall, but you cannot distribute loads into very small concentrated areas without failure. For example, 1.5 inch tubing covers approximately 1.75 square inches, while a 3x6 mounting plate is 18 square inches. Further a mounting plate can be but against seam welds or extended to cover the edge of the bulkhead, where significant benefits in strength can be realized. For my car, I cut 3x6 11 gauge steel (.1225 inches) for each of the hoop ends & diagonal, and 3x3 plates for the rear struts. The 3x3 plates are smaller than I would prefer, but due to how the wiring is routed in the car, it wasn't feasible to make them larger. That's a total of 72 square inches of load distribution, versus the 8.75 inches that would result from welding directly to the sheetmetal. You can see how this is arranged in the pictures, and how I welded these plates directly to the strongest corners of the car. I also tried to run these toward the edge of the bulkhead as well, which is another strong area. Anyway, the summary here is that you really need to pay attention to how the loads are distributed in the car, or you may find that your efforts don't survive an incident.

It was then time to weld. My first task was to weld the mounting plates to the car. I ran small beads around the edge of each, going corner to corner to reduce warpage and blow through. My hope is that this is strong enough. I covered probably 40-50% of the edges.

Before welding the hoop into the car, it's important to note that the car needs to be level such that there is no bending in the chassis that gets "set" by welding in the roll bar. It's best to do it on a level floor, or even better, using jackstands on a level floor.

With the plates in, I refit the hoop to ensure it didn't hit with the added height. I made sure it was level in both planes. It fit just fine. I put the top down, and fit the rear struts, making sure to keep the hoop immobile. With the rear struts fit, I spot welded each them to the hoop in a number of places. Note that I didn't weld either to the car yet. This is so that I could pull the assembly out, and carefully weld the struts to the hoop on the bench. This allowed me to move the workpiece around to get these long seam welds correct, rather than contorting to weld in the car. Once welded, I put it back in the car, seam welded the hoop & struts to the plates, and welded the hoop to the corner of the shoulder harness boxes. I then welded the cross brace in, keeping it in the same plane as the hoop. With that, the bar was fully in.

For cleanup, I ground off any slag and ground the welds down flat using a 2" grinding/sanding disk. For a really finished look, I filled the joints with plastic filler (aka bondo) and sanded them down. The result was that the bar just looked great. I believe it will be very usable, and strong enough to both take an impact, as well as add to the chassis rigidity.

With the bar "done", I was able to concentrate on painting the car, which will be detailed in a separate article. Once the car was painted, I reassembled. The key piece that needed to be modified was the package tray bulkhead & carpeting. I decided that this piece of stamped galvanized steel could only be reinstalled from the front, so I cut at each edge and in the center to allow it to fit around the roll bar. I used a sheetmetal nibbler and a set of metal shears to make the modifications. Once cut to size, it reinstalled rather easily. I just had to cut slightly around the carpet to make it lay properly.

From there, I reassembled the car. It came together as I expected, fitting my needs as well as being out of the way. I did add some SFI 45.1 roll bar padding. This padding had the advantage of only being on one side of the bars, allowing it to fit in the confied space. A pad that went all the way around the bar would have possibly bound on the top going up & down. With this work done, the roll bar is complete, and has been a pleasure to live with. I hope to never have to use it, but if I do, I expect it could save me from additional injuries.