Todd’s Spec Miata Roll Cage – Part 1

Last week I started work on a Spec Miata roll cage for a friend of mine from the Need For Speed team. Todd handles the physics tuning for the cars in our game, and is taking the leap, getting into real road racing.

Dave Halabourda, Todd, and myself have been working on this for a few days.

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First off I needed to build a solid stand for the mandrel bender to mount on. You can anchor the bender to a concrete floor, but to make it mobile, and for ease of installation, I built a mount to slide it into the tailgate of my Tundra. Works well!

You can see the sample bend I made on the tailgate. This allows us to mark any future tubing for where bends need to begin, and where they need to line up once placed in the bender.

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Todd busy cleaning up the shop. He managed to strip the interior in a day, and sold just about all of what he didn’t need the next day. I guess there is a demand for second hand Miata parts.

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In Spec Miata, the rules allow you to run either open, or with the factory Mazda hard top. Todd made the obvious choice to go with the hard top. Once we fit the hard top, we could get a basic idea of where the cage would run based on the seat position and Todd’s height. He chose a tall Ultrashield racing seat, that combined with his height made it a bit of a challenge to get the cage high enough and close enough to the hard top.

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The main hoop is always the first peice of the puzzle. It’s critical to get this right, and get it tack welded in, as the rest of the roll cage all ties into the main hoop.

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We used some soapstone to outline the main hoop on the concrete floor. This will allow us to make sure it is symmetrical from left to right.

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Test fitting the main hoop, its nice and tight against the roof, perfect for maximum clearance. It’s also as far back as it needs to be to clear the seat and the driver.

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Dave began working working on the boxes for the main hoop floor mounts, as well as the 2 forward bar floor mounts. Here is one of the forward bar mounts tacked together.

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All welded up and ready to go into the car.

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We continued to fit the other bars to the cage. We’ve got the 2 rear bars, the diagonal brace, and the passenger side harness bar fitted here.

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Dave is getting some of the welding done.

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Pre-heating some of the thicker areas helps get proper weld penetration in those critical areas.

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Dave is grinding a bit of weld so that the other bars can be fitted with tighter tolerances. Getting multiple bars to meet at one nodal point is the best way to assure maximum stiffness for the structure. However, getting multiple bars to meet at one point makes notching them, or fish-mouthing them, one of the biggest challenges of building a roll cage.

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With the rear x-brace tacked in, we’re ready to continue welding the cage up to finish the rear section. Pretty happy with how it’s come out to this point.

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The rear bulkhead/raised fuel tank of the MX-5 makes it difficult to mount the x-brace for the rear bars right down at the bottom, so we mounted them as low as we could get them.

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Underneat that bulkhead, the rear bars are mounted directly above the frame structure in the uni-body, and right next to the rear damper mounts, making sure any forces coming from the suspension, are transferred directly into the roll cage and to the frame structure.

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Here’s Todd clearancing another weld so we could get the best fitment for the first of the forward bars.

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First of the forward bars mounted and welded in. You can see how it gets raised slightly above the main hoop, since the stock Hard Top has a slight bubble to it, we wanted to take advantage of all the space we have in the car.

You can also see the drivers side harness bar is now mounted in, bowed out to the rear to give more clearance in case Todd ever needs to move the seat rearward.

- Carl

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