 |  | | | Turning a sports car on its lid is not something any enthusiast expects to happen, but you should be prepared for the worst. Damaging your ego or your pride-and-joy is one thing, but more serious is the potential for personal injury. Adding to the danger is the fact that many sports cars are stuck with 1960s technology when it comes to safety considerations. Modern production cars have crumple zones, airbags, rollover protection and built-in side beams to prevent intrusion.
Traditional European sports cars often have no rollover protection, or roll bars that are more cosmetic than structural. Not only that, many aftermarket roll bars are not made to racing standards. Although they are better than nothing, the effectiveness of any bar is dependent on many factors including design, wall thickness, tubing type, diameter and the method of attaching it to the frame. Many bars would fail if "tested" in an actual accident, while others not only make the cars safer, but handle better as well!
Roll Play
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The first consideration when installing or upgrading a roll bar is usage. If track events are planned, the design part may be done for you. Simply follow the regulations provided by racing clubs. Autocrossing involves relatively low speeds and events seldom require rollover protection. However, high-speed solo events that take place at racetracks usually require rollover protection. Wheel-to-wheel track or drag racing, no matter how well regulated, is dangerous business and roll bars or cages will usually be required. Even if they aren't, you don't want to risk your neck in an unsafe car.
To get started, ask any clubs or tracks that you plan to compete with for a copy of their regulations. Follow them as gospel, but if you decide to improve on them check to make sure you are within the regulations. Extending the roll cage through firewalls may stiffen the chassis, but is illegal in some clubs or classes. If you are not going to race, it still pays to get copies of regulations as they are written by organizers more knowledgeable about high-speed accidents than you are.
Cage or Bar?
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First decide whether you need a roll bar, cage or a partial cage. If you do not intend to run track events other than autocrossing, then a simple bar may suffice. With a street-driven car you may have to design a bar that will fit under a top or satisfy some other practical consideration. A partial cage may include a roll bar combined with side-impact bars and/or a cowl bar that fits under the dash. This maintains the original lines, but beefs up both protection and chassis rigidity (albeit at the expense of ease of entry).
Your next considerations will be the bar material and size, which will vary depending on vehicle weight. The Sports Car Club of America (SCCA) specifies roll cage sizes for their racing series, and at one time they had specs for roll bars as well (the SCCA no longer allows roll bars alone). Since the SCCA puts on many events that sports cars could compete in, let's use them as an example of building rollover protection to a set of rules.
SCCA Specs
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The SCCA allows only four materials for roll bar construction: seamless, DOM and ERW mild steel and chrome moly. Seamless tubing is produced using a process that is similar to forging. It has a refined grain structure and has a slight spiral twist for added strength. Drawn Over Mandrel (D.O.M.) tubing is made from steel strips and welded together. It is then cold drawn down to a smaller diameter and thinner wall to give it more strength. Electric Resistance Welded (ERW) tubing is made by rolling flat steel into a tube and welding it together. It is easily identifiable by the short flashing on the inside of the tube.
Any of these materials are usable, but ERW tubing needs to be thicker than the others for the same strength. Mild steel is available in many types depending on the metals it is made from. Some are stronger, while others are easier to form. The SCCA requires roll bar or cage material to be SAE 1010, 1020 or 1025. Due to its greater strength, chrome moly (SAE 4125 or 4130) can be thinner than mild steel but is much harder to weld properly. It must be "normalized" after welding, which involves heating and slowly cooling the entire structure in a specialized oven. If this is not done weld embrittlement can weaken the structure.
Some racecars have been built with stainless steel or aluminum roll bars, but these are highly specialized (and expensive). They are often not allowed in amateur racing due to the difficulty of welding and normalizing them, which can lead to failure even if the welds look good to the naked eye. Mild steel is actually the best material as it is easy to weld properly (by a professional) and does not require treating before use.
The diameter and wall thickness of your bar will be determined by the weight of your car. A good starting point is 1.5 inches in diameter x .120 wall for cars under 1,500 pounds; alloy (chrome moly) could be 1.375 x .090 wall. For 1,500 to 2,500 pound cars 1.75 x .120 (mild) or 1.625 x .102 (alloy). For cars over 2,500 pounds consider 2.25 x .120 (mild) or 2.00 x .102 (alloy).
Roll cages can use smaller tubes since a cage is a stronger structure. For under 1,500 pounds, tubing could be 1.375 inches x .095 (mild) or 1.375 x .080 (alloy). For 1,500 to 2,500 pounds the SCCA requires 1.5 x .095 (mild) or 1.375 x .095 (alloy). Over 2,500 pounds required 1.50 x .120 (mild) or 1.50 x .095 (alloy). These dimensions are for properly designed and triangulated cages. If you are planning for a more basic cage, then stronger tubing would be appropriate. Some other clubs require a 3/16-inch inspection hole be drilled in a non-stressed portion of the bar, so tech inspectors can verify the wall thickness.
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