Todd’s Spec Miata Roll Cage – Part 2

Part 2 and the finishing of Todd’s Spec Miata Cage.

Overall it took around 8 days to finish this cage, with Todd, Dave, and myself, including the braces for the back of the Ultrashield race seats he is going to be running on both sides of the car.

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Here’s a good shot of the finished cage. You can see on the 2 forward bars that go down to the footwells, we’ve bent them in a way that gives maximum clearance for the drivers head under the Miata hard top.

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With the mount secured to the truck, you can see how much leverage is needed to get the tubes to bend. It’s Todd’s turn here to bend one of the forward tubes.

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Basically the bender works like a large ratchet, where you need to bend about 7 degrees at a time. Each time you reach the limit of the bend, you pull back to one more notch in the ratchet and bend again.

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The trickiest part about bending these steel tubes is the elasticity of the material itself. When the bar is being bent, and is remains in tension, it gives a false reading on the dial gauge, you have to un-bind the bar from the bender, and re-set it in the mandrels to get a proper reading. Once you get the feeling for how far you need to bend beyond what you’ve determined for the bend, you can easily stop where you need to. I find the steel needs to be bent about 4 degrees beyond where you need it, for it to bounce back to the desired dimension.

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The order of building the cage can be important as well. With the 2 forward bars in place, and the main frontal roof bar in place, the diagonal for the roof was hard to get in, since it could not be rotated in or simply slid in. This required a bit of grinding of welds, plus some muscle and a BFH to get it to snap in.

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Here you can see 2 symmetrical bars that were bent for the start of both of the Nascar style door bars.

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A great image to show just how much a difference quality tools and consumables make. These 2 hole saws were used in a proper tube notching tool, and lasted around 10 cuts each. Just look at all the missing teeth, not to mention they seemed very out of round and walked all over the place.

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Dave gets a turn at doing some notching for the vertical supports in the Nascar door bars.

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Got chilly in the shop when we didn’t have the door shut and the heater on!

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After going through 3 of the cheap Chinese Samona hole saws, we popped down to Home Depot and bought a Ridgid bi-metal hole saw. Was about 40% more expensive, but the difference it made was unreal. It seemed to cut through the steel tubing like butter and made some really nice copes/fishmouths.

This hole saw did about 35 odd cuts at the end of the project and still looks like a champ, no missing teeth! :D

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Nascar door bars are becoming increasingly popular as safety becomes a main priority of these racers. It’s pretty rare that someone would get T-boned in a racing incident, but you always need to be prepared for the worst.

The other option that some still opt for is the X-brace in the door bars. Which can be achieved in a couple of ways. Either having 2 contigous bars, one that bends over the other to form an X, or one contigous bar with another intersecting it in 2 pieces.

The 2 piece bar isn’t as desireable as there has been photos on the web of failed welds where the short intersection bars have detached from the center weld and basically become a small spear in which the driver could get seriously injured.

The former option is much safer as the plasticity of steel would resist fracturing in almost all cases and the X would stay in-tact becoming less of a hazard.

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The finished cage! You can see we mounted the dash bar just below the steering column, this is where it made most sense to fit with the stock dash (as mandated by SCCA rules), and to allow hard mounting of the steering column to the dash bar, increasing Todd’s safety.

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If you look at the horizontal bars that run between the main hoop where the harnesses will be mounted, you can see the rectangular tubes that will be the back braces for the Ultrashield seats. I designed these to be as efficient and light as possible, as well as fully adjustable if the seat needed to move or change angle at any time in the future.

Todd did a great job of cutting these up and making them work, and I welded them to the harness bars. They consist of some 13/4″ square steel tubing for the hard mount on the cage, and some square aluminum tubing for the sliding mount that will attach to the rear of the seat back via some aluminum U-channel that will act as a pivot and flat mounting surface. Will post pictures up later.

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Was a pretty fun project and Dave and I are really happy with how the cage turned out. Now who’s next?

- Carl

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