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.

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.

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.

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.

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.

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.

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

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.

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

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

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!

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.

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.

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.

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

- Carl

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.

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.

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.

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.

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.

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.

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.

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.

All welded up and ready to go into the car.

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.

Dave is getting some of the welding done.

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

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.

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.

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.

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.

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

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