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In general, the SCTA rulebook can be described as straight forward in some areas, yet ambiguously complex in others. When it comes down to it, the rulebook is up for interpretation just like any race series. However, it's important to realize up front that it's only the chief tech inspector's interpretation of the rules that counts at the end of the day. To avoid any disputes when the car was finally built and ready to race, we made sure to keep Steve Davies, the SCTA chief tech inspector, in the loop during the construction of our roll cage.
The SCTA rules require that all cars weighing over 2,500 pounds and competing in classes where the record exceeds 175 MPH, use a minimum of 1-5/8″, .120″ wall mild steel tubing or 1-1/2″, .120″ wall E4130 Chromoly tubing. The rulebook goes on to state that these tubes must terminate into ¼” thick floor plates that have a minimum perimeter of 22″. To give you a relative comparison, NASA requires that cars weighing between 2,500 to 3,000 pounds use 1-1/2″, .120 wall or 1-3/4″, .090″ wall tubing and use a minimum of an .080″ thick floor plate. Remember the video? This is the reason for SCTA's larger diameter and wall thickness requirements in comparison to road racing.
To make sure we satisfied the SCTA rules, Jim Pierce constructed the main structure of the roll cage from 1.75″ x .120″ wall, DOM mild steel tubing. We chose DOM mild steel alloy over Chromoly for its weight and cost benefits. Chromoly has chromium and molybendum, which gives it improved strength properties over 1018 alloy steel. Inch per inch, and pound for pound, there is little difference in weight between chromoly and mild steel tubing. The weight savings in a chromoly cage comes from the fact that the added strength of chromoly allows the cage to be built with a smaller diameter, thinner walled tube. In land speed racing though, a heavier car is actually desired as it is more stable at high speeds. Many classes in land speed racing have restrictions on the use of aerodynamic aids, so land speed racers often address stability by adding weight, sometimes getting very creative in doing so.
This crew of this Dodge SRT4 filled the factory intercooler with buckshot (bee bees from shot gun shells) and then used the factory fuel tank to hold 12.5 gallons of water. After all the weight additions, the car was reported to weigh in the neighborhood of 4,000 pounds!
Chromoly needs to be TIG welded and then have each weld normalized or heat treated for its strength to return in the weld area. If the normalizing is not done correctly, the weld area will be weak and brittle, which is obviously not a good thing. All this highly skilled TIG welding and heat-treating adds to the labor cost of a Chromoly cage making it much more expensive than a mild steel cage. Although I'm not exactly in my college years anymore and barely getting by on the frugal nutrients of top ramen, I would actually like to get done building this thing and go race so the reduced cost of the mild steel cage was a plus.
Roll cages in land speed applications usually mimic that of drag racing. Actually, if you trace drag racing back to its roots you'll find that it's the other way around. Many of the NHRA rules actually spawn from land speed racing. Hence, the similarities in roll cage construction. Nonetheless, knowing that I might have somewhat of a road race bug that may need to be itched again later on in life, Project 240 LSR's cage design is closer to that of an overbuilt road race cage instead of the typical drag race or land speed cage.
