Build Your Own NA Subframe Brace for $5

Posted on 13. Jul, 2009 by jeffz in Tech

The following tech project found its way into Editor Jeff Zurschmeide’s book “Automotive Welding: A Practical Guide” just out from CarTech Books. But the project was so much fun, we thought we’d give you all a sneak peek.

Miatas need some lower suspension and chassis bracing. Mazda realized this by 1991 and included a rear subframe brace starting with the 1992 model year. They added a front brace under the transmission starting with the 1994 model year. The factory braces are stamped tubes, and numerous aftermarket solutions have also been developed. (See Forever MX-5 Issue 2 for installation details on our fabulous set of Cusco braces on the Survivor.)

This summer, I was working on a manuscript for a book on automotive welding with Russell Nyberg of Acme Customs in Portland, Oregon. Russell also created the roll bar in the Forever MX-5 Time Attack Miata. We needed a project that would showcase the uses of tube aluminum and the TIG welding process, and our Lifestyle Editor Scott Fisher was in need of some cool stuff for his 1996 Brilliant Black car, so we decided to get crazy and see what we could do.

Russell had never created a subframe brace for a Miata before, so we asked him to look at the problem and design a custom brace for us. This brace uses about $5 worth of scrapyard aluminum tubing and 16-gauge sheet aluminum. It took us about 4 hours to complete the project. That makes an exciting story, but it also doesn’t account for Russ’s design savvy, fabrication skills, and TIG-welding setup. But if you’ve got the skills and access to a TIG welder, our work is your gain!

Here’s what Russ came up with, and how you can repeat this project:

1. The NA Miata has two box-section parts in its undercarriage that support the lower A-arms of the front suspension and two transmission support mount points. These will form the rectangle of the frame brace. NB and NC models have slightly different undercarriage configurations but there are always four (or more) points to conveniently bolt on some kind of brace without drilling extra holes in your floorpan.

2. Measure the distances between each of the four holes and draw your base figure on a piece of scrap cardboard or plywood as a template. Check and recheck your measurements to make sure they’re accurate. Note that the convenient mount holes do not define a perfect rectangle! That’s important because your finished piece has to line up with the mounting holes.

3. Based on your mount points, you can design a basic brace that relies on arches or triangles for support. We decided to create a lightweight brace with one arched and one straight beam, tied together only with a lightened and stressed skin.

4. Use your template to create a jig before you begin welding. Depending on the materials and method you choose, You may warp or shrink your parts out of true when you weld. One way to create a jig from your template is to copy your base figure on a sheet of thick plywood and drill holes at the locations of the mounting points. Then attach your first pieces to the plywood as you tack your work together. As you weld each part into place, the jig holds the dimensions for you. If you plan to produce several similar pieces, create your jig out of steel and you can weld right on it without fear of lighting it on fire.

5. Start with large pieces for the mount points. We used 2-inch lengths of 1.75-inch .120 wall aluminum tube. Then we cut some discs with a 1.875-inch hole saw to make cups out of our short sections of tube. Weld the discs to the ends of the tubes and then bolt the resulting cups to your jig. We used the TIG welder at 110 amps with 1/16-inch aluminum rod for this part, using the TIG’s foot pedal control to modulate welding power.

6. Now measure your distances and cut your aluminum tubes. We used 7/8-inch .120 wall aluminum tube that we picked up from the scrap bin at the metal yard. Make one brace straight and bend the other one on your bender to make a pointed arch. The two pieces may touch, and you may choose to weld them together, but we didn’t. Weld the tubes to the cups all the way around. Aluminum warps very easily, so keep the cups bolted to the template as much as possible to hold everything in place. We left the TIG welder at 110 amps and continued to use 1/16-inch aluminum rod.

7. When the tubes are welded to their cups all the way around, use some cardboard to make a template for the skin. You can make this skin from one piece of sheet metal and fold it around the straight tube, but it works just as well to cut two sides. We used 16 gauge sheet aluminum for this material.

8. If you choose, you can use a hole saw and lighten the skin by taking a few holes out of it. This is already a very lightweight piece, but the holes give it a very sporting look. We also recessed the holes with a special clamp made just for the purpose. These are available through metalworking supply houses.

9. Weld the skins to each side of the brace using 1-inch stitch welds with 1-inch gaps between stitches. We switched to 1/8-inch rod for this part of the process, as it requires more filler material. The stitch pattern helps avoid warping the part and is almost as strong under tension as an uninterrupted bead. Keep the piece bolted to the template as you install the first skin, then unbolt the piece and clamp it to the table as you weld on the second skin. When you’re done, try to flex it – you’ll find that it’s strong!

10. Bolting your brace to the Miata’s A-arm support boxes requires backing plates and the easiest way to make those is to weld a nut to a backing plate. For the Miata, the plates must be 2-inches square. We cut squares of 1/8-inch mild steel, and we welded a 5/16 nut to the back of the plates with a MIG welder.

11. To install the brace, we took along our best power drill with a reaming bit. No matter how carefully you measure, template, and build, you’re likely to be off a little bit when you go to install. So we found the best mounting alignment, and “adjusted” our mounting holes on the brace to get everything installed snugly.

So there you have it - $5 worth of materials, 4 hours of labor. What could be easier?

OK, before you head to the computer to send us e-mail, here’s the point: We know that not everyone happens to be friends with a skilled fabricator who will donate an afternoon to make a brace, and we know that if you paid a shop for its time and tools and materials, the cost would far exceed the retail price of a proven subframe brace.

The point is twofold – first to show you how, with a little thought and a little welding practice, you can design and make your own custom parts. Second, we thought Russ’s design and execution was just too cool to let it stay hidden underneath Scott’s car.

If you would like custom work done for your car, you can contact Russell Nyberg through www.acmecustoms.com.

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