Nissan Spec-V: Part 3: Building a Roll Cage
7/20/2006
Last time, we removed the sound deadener and seam sealer from the inside of our Spec-V. This week we are starting to work on the roll cage. Richie Watanabe of Technosqaure will be constructing the cage and doing most of the heavy fabrication. Most people know the roll cage protects the occupants, but the cage is also the car’s backbone. With a stiffer chassis, the car is more responsive to changes in the suspension setup, making it easier to dial the car to conditions. In addition to being protective, energy absorbent and stiff, the cage also has to be lightweight. An overbuilt cage slows down the car.
Like all racecar projects, before you weld or cut anything you must first check the rules. We consulted the NASA CCR as well as Super Streets time attack rulebooks for requirements. The rules require a minimum of 1.5-inch diameter main tubes with a wall thickness of 0.120 inches or a 1.75-inch tube with a wall thickness of 0.095 inches for our weight. We went to the larger-diameter thinner-walled tube as it is stiffer for the same amount of material compared to a smaller-diameter thick-walled tube.
For materials we decided on 1020 drawn over mandrel (DOM) mild steel. When DOM tubing is made, a flat sheet of steel is drawn into a tubular shape while cooled over a mandrel and electrically seam-welded to make a complete tube. This cold drawing gives it better mechanical properties.
When we fabricated the roll cage we took extra care to ensure the mitering was very precise, and that the tubes were as close to the shell as possible. We had to cut holes in the floor so the cage could be lowered to complete a difficult series of 360-degree welds before raising the cage. We then covered the holes with the tube’s foot mounting plate.
The main tubes of the cage were backed up to the floor by plates of steel .125 inches thick. Sometimes boxed plates were built to integrate the tube foot fully into the frame rails for added strength. The forward legs were attached to the side frame rails as far forward as possible. The main hoop was placed as far rearward as possible. The rear suspension trailing arms were tied into the structure with the rear downward legs mounting to the top of the shock towers. More tubes will be added later but this is the basic backbone of the car.
Ritchie Watanabe carefully welds our cage together with a MIG welder. Watanabe uses both MIG and TIG welding on his cages.
To lighten the chassis and to make room for the cage tubes, Watanabe cuts out much of the interior sheet metal between the trunk and the cabin. He also removed the myriad of sheet metal brackets, studs and other now-unneeded bits of metal that can add weight and injure the driver or people working on the car.
The cage’s main hoop of attaches to the main frame rails. The floor plate is a complexly curved piece of steel to get maximum contact between the frame rail, suspension mounting point and the main hoop. This adds strength without adding weight
Side impact is probably the greatest danger for occupants. The door bars feature two solid, uninterrupted beams bent to form and X at the center and reinforced with gussets. This forms a near-impenetrable barrier between the driver and the outside world.
The front leg is attached to this box pillar welded to the main frame rail as far forward as possible. The box allows the cage to be dropped in order to create 360-degree welds around some of the tubes located in tricky spots. Then we raised up the cage and boxed it in for a tight fit to the roof. While time consuming, this creates a superior cage. 
Our cage starts to come together and the Spec-V starts to look more like a real racecar. Cages do more than protect in crashes. No mere bolt-in roll bar, our cage is more like a semi-tube frame designed to stiffen the body structure and improve handling.