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Measure twice, cut five or six

Posted by Adax , 06-08-2013 · 1,746 views

Step one is always the main hoop; everything else is based off this piece. I used to make an ironic statement about how this first step in building a cage was also the most difficult but I've since learned that it's all difficult. I learned a bit while building the roll bar for the green car so it wasn't my first rodeo, but as mentioned previously, I got bucked off pretty bad in the first go-round.

The main hoop can either be based off the floor or the package tray. Valid arguments can be made for both but I decided to go off the package tray. It provides a little more room in the main cabin but the biggest factor was that it allows the door bars to be based off the chassis rather than off the main hoop in the rear. This adds strength to the door bars and rigidity to the chassis by distributing the load over a much greater area. Rules state that the mounting plates can be up to 144 sqin but not more than 15 inches on one side. I planned to take advantage of this allowance on order to optimize frame rigidity.

The main hoop needs to be as close to the B-pillars as possible. Most also put bend in the center to get it closer to the roof, although you don't really need the clearance here and is weakens it a bit in side impacts. I planned to have enough bracing inside the hoop that I wasn't too concerned about this last factor and went ahead with the bend. It does make the fabrication of the hoop much harder though as you no longer have two simple 90 degree bends. I approached the process by measuring out the perimeter of the cabin the hoop would occupy pretty closely then sketching this out on the floor with a dry-erase marker. Use a sharpie if you are single.

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The math is easier if you do the center bend first so you can then determine the angle of the corner bends. It looks like the verticals are not parallel in the image below but it's just due to the perspective of the image.

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As with any fab project, use tacking welds until the last possible stage as they can be easily ground away leaving no record of your mistakes.

Side-bars come next. These can be pretty simple two-bend pieces but you will have easier entry and exit (easy exit can be pretty important) if they fit the door openings more closely. Mine were quite complex, having 4 bends, all in different planes. This means you need to rotate the tube in the bender between bends. There is not a terribly accurate way to do this with a manual bender. I used a digital level (the one off my SmartCamber tool which worked out pretty well. Once you get one side done, you need to create an exact mirror image for the other side so take good notes of what you did on the first one, both what you planned to do and what you actually did. One of the more challenging aspect of these pieces, as if the bends were not challenging enough, is that they connect to the main hoop at the bend. It's not possible to accurately notch the end to fit the bend precisely so a bit of finish work is needed.

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Actually the first step should probably be the mounting plates. Note that I don't have the mounting plates in yet. This is not intentional, it's because the 3/16" plate had not arrived yet so I tacked on some 1/8" scrap I had.

No need to tack the mounting plates so once materialt arrived I started working on them. I went with a box-type mount for the frame rail landing perches. You want it large enough that the down-tube can be securely welded to it but no so large that it interferes with your feet. Also best to keep them rounded to decrease the potential for a lateral maleolar fracture during a side impact. Sean at MiataCage has a nice solution http://www.miatacage.com/ but they are only available with the complete kit.

I planned them using a contour gauge but it is really overkill. Here's an image of the inside weld of one before attaching it to the rail.

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Welding note: I can weld 3/16" angle joints all day; it's probably the easiest weld. Welding 3/16" plate to sheet metal of varying thicknesses (probably as thin as 22g) while achieving good penetration of the 3/16" without blowing through the sheet metal is difficult beyond anything I've ever tried. If you have not welded this before, practice before you ruin your tub. Then figure out how to do it vertically. The key is to spend 90% of the time withe puddle on the thick stuff then sweep the puddle down to the thin stuff.

Here's one of the package tray perches.

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I made the rear down-tube perches pretty long in order to incorporate most of the length of the rear subframe tub mount. This area connects the car to the road so the more rigid it is the better. Note those welds, they didn't come easily.

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With both side tubes in place the top windshield tube and the knee bar or dash tube come next. These had 4 bends each as well but all in the same plane. I contoured the dash bar very close to the firewall. This is one of those bars that is difficult to fit until it is notched so it's a bit of a leap of faith fabrication. The top bar is easier. There is quite a bit of play in the fore-to-aft mounting direction, just make sure you can get to the top mounting points once it is in place. Mounting the front of the top to the cage is also legal if you find that works better.

Top bar:

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I don't have a good image of the dash bar alone, here's one with more of the cage completed.

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It's worth noting that the cage can be built in two pieces: the main hoop with diagonals and such; and everything forward of that. The side tubes. dash and windshield bars can be removed from the car for final welding, same with the main hoop. This allows you to easily do all the back-side welding which would not be possible in the car. Welding the to two together should be the last step. Note the black racer tape I have holding the two together. Probably not compliant.

Harness bar and main hoop bracing was next. Take a look at the rules but there's quite a bit of freedom here. I probably have more than is needed.

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I seem to be reaching some sort of link or image limit. Part three of cage to follow once I get this figured out.

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