The Aims And Compromises Of A Robust Rollcage

What goes into a racing cage? What considerations must be made when assembling that intricate collection of tubes which keep an unlucky driver from taking an ambulance ride from the track instead of their road car? As safety standards march forward and cage builders need to cope with the demands of stickier tires, more powerful engines, and increasing vehicle weight, they need to make a series of careful calculations to determine what level of complexity, what shape, and what materials best suit the job.

Among other things, a proper racing cage includes door bars—something absolutely necessary in door-to-door racing—specially welded sections to increase rigidity, and a slew of compromises made to appease the surprisingly stringent rules present in most American road racing categories. With your typical club racing car, there are concessions made to address strength, rigidity, weight, complexity, and cost. Keeping drivers safe and keeping costs minimal take precedence, but there are always a series of complicated considerations to be made depending on the situation.

The cage you start building isn’t always the one you finish building. — Greg Hoffman, Performance Fabrication

That also means we’re not discussing the ever-popular, inexpensive, and corner-cutting approach that is the bolt-in cage. It’s popular among my drifting friends, but the reality is that it’s not usually smiled upon by racing sanctioning bodies, because they bolt to weakened parts of the floor, typically. Full-cage builders generally look for the ideal amount of thickness, mounted to the strongest sections of the body or frame.

Connecting the rear shock towers in a NASA ST3 BMW M3. Photo credit: TC Design

Generally speaking, tube diameter and thickness is largely determined by the vehicle weight — meaning a heavier car will require a heavier cage. The basic racing rollcage incorporates six or eight points — a “point” being a place where a bar intersects the floor. There is an endless list of variations possible within the basic framework of a rollcage, and there are additional strengthening sections to make the car exceptionally resilient. For instance, some like to add bracing from the strut towers to the firewall, and between the door bars and the chassis, to create an even safer safety cell, but that comes towards the end of the line.

Preliminary Considerations to Be Made 

The first business I consulted for insight into its process was Performance Fabrication in San Carlos, California. “The first consideration we have to make is the driver’s height, then start measuring the seat height, which dictates the position of the main hoop,” says Greg Hoffman, welder/fabricator at Performance Fabrication. Then, simply put, they assess what the car will be used for, and try to devise the best possible compromise between weight and strength that fits within the rules.

Being able to fit a large seat through the opening is a concern in cage design. Photo credit: TC Design

“Sometimes the rules are a little light, so we add a little more tubing than required, so we tend to err on the side of caution. We tend to slightly overbuild our cages,” Hoffman concedes. “And we also ask our client how fast this car will go, and how it will be used.” However, these reinforcements are usually made in areas which need the most attention and are not always done completely symmetrically. Therefore, “driver’s side door bars usually get thickened, but not the passenger’s side,” adds Hoffman.

With lax rules, a rollcage can quickly become a jungle of tubing. Photo credit: TC Design

Steve Schmalz, owner of Performance Fabrication, feels that the tubing diameter should be thickened slightly in the vital areas, and is willing to add a few pounds to the minimum weight in the process. Schmalz tries to focus on supporting the cockpit primarily. “While we might save weight in some of the less critical areas, we might thicken the windshield bar and rollover hoop for more safety—even if it costs us something in weight.”

They’re not only concerned with thickness, but orientation. They also want the supporting rear bars/stays “to cross these two bars (in an ‘X’ shape) to keep the car from twisting,” notes Schmalz. In a similar fashion, they “add a diagonal bar from the passenger’s side of the main hoop to the driver’s side of the windshield bar, and reinforce those junctions with triangular gusset bars,” to protect the roof from torsion. “This last addition isn’t actually in the rules, but contributes to rollover protection while preventing the car from twisting,” he adds.

Safety is paramount, but many builders are also occupied with increasing rigidity. Improving rigidity with the cage happens when “tying into upper connections with the main hoop and A-pillar, or when adding a triangle,” notes TC Design Owner Tony Colicchio. “What’s interesting is how there really can’t be too much rigidity when adding to the cage. The less sticky the tire, the less rigid the chassis needs to be, and at that point, you think about weight savings,” adds Colicchio. Clearly, there are numerous considerations that need to be made.

That diagonal roof bar keeps this M3 from twisting. Photo credit: TC Design

Selecting Steel and Right Amount of Rigidity

Someone looking to build a strong cage must consider how much they’re willing to spend on the cage, possible cage repairs, the cage’s lifespan, the rules of their sanctioning body, vehicle weight, possible weight savings in the cage department, and of course, the speeds they’re likely to reach. Clearly, the ideal cage is — like most considerations in racing — the best compromise between all relevant factors. Because of the complexity involved, “The cage you start building isn’t always the one you finish building,” adds Hoffman.

These factors will determine the type of material used, of which there are three main choices today. The first is Electronic Resistance Welded tubing (ERW), which is created from a flat plate of steel formed into a tube and electrically welded. It is the least-expensive approach and falling out of fashion these days, largely because it hasn’t been allowed by the SCCA and NASA for the last five years. “The high-end chassis makers are moving away from ERW steel and onto higher-grade materials,” adds Schmalz.

That’s because there are cost-effective alternatives that are stronger. The second option is Drawn Over Mandrel tubing (DOM), which is a mild-steel tubing — usually 1018 or 1020 — shaped by a mandrel. The last and most expensive option is chromoly. Made from seamless-steel, chromoly is somewhat stronger than DOM and can allow for a lighter construction, depending on the rules.

DOM holds one considerable advantage. It provides more elongation: the ability to flex and bend slightly in the event of a crash. There’s a certain degree of flexibility that’s desirable in the event of an impact; brittleness comes from too hard of a cage, and a slight amount of give is helpful in most situations. “Since DOM bends and is easier to fix, it can be segmented, unlike chromoly,” notes Schmalz. This means that DOM typically enjoys a longer lifespan than chromoly, and is much more affordable for the average racer. Fortunately, most of the North American governing bodies design the rules for the racer without a fortune in his wallet.

The Spirit and Progression of SCCA, NASA, and FIA-Spec Cages

SCCA- and NASA-spec cages conform to a very similar set of standards. Depending on the class, they can become unregulated, but at the lower end of the totem pole, categories like Spec E30 and Spec Miata are very stringently controlled.

These classes are generally structured around a six-point cage. The sanctioning bodies are quite strict about keeping these connection points to a minimum for two reasons. The main reason is to keep costs low, but the secondary reason is to limit chassis rigidity. As these classes are low-tech and designed to put a premium on driving ability, they’re not interested in building these chassis to handle more grip. “There is a point you reach where the additional cost and weight don’t make the car any faster given that most of the spec classes run on a harder tire compound than most open-tire classes,” says Colicchio.

A simpler cage in a Spec Boxster. Photo credit: TC Design

In recent decades, the general standards have remained the same and there have only been a couple of significant changes. There are the aforementioned changes to preference in materials, and “NASA and SCCA have mandated a 360-degree plate welded at every ending point,” notes Colicchio. The main connection points have to terminate on a plate, otherwise they’re likely to shear. Therefore, there are some specific rules to the size, placement, and style of these plates and their welds.

Welds, Plates, and Points of Termination

As far as welding is concerned, there are a few hard-edged rules that Colicchio tries to follow, but the process is somewhat flexible — no pun intended. The basics are the same as with any weld. “De-greasing, cleaning scale off, deburring, and possibly chamfering edges depending on the specific joint” are all standard means of approaching any weld, but when working in such tight confinement, a little foresight is necessary. “Yes, as you build the roll cage, you are always thinking whether or not you can get to a joint to weld.” To access the joints later in the build, removal of the roof is sometimes necessary, and removal of the windshield is usually necessary.

“In some cases, you might weld a few bars together outside the car and then install as an assembly,” he notes. “You can leave the main mounting points unwelded until the end, so that you can move the cage around to get access. You can also cut access holes in the interior sheet metal panels or push sections away for more clearance,” Colicchio says. With years of experience and a few custom-tailored MIG guns to access those hard-to-reach areas, he’s seen it all, and is comfortable with cramped conditions.

The thickness of the plates differ depending on the sanctioning body, but generally it’s the same as the thickness of the tubing, and is often made from cold-processed rolled steel, which makes it somewhat denser than the tubing material. The size of the plate is often limited to prevent increasing chassis stiffness, especially in categories like Spec Miata, Spec E30, SCCA IT, and the lower NASA ST classes.

Plate location depends on several factors, but Colicchio looks for “big box sections to mount,” and when possible, tries to “tie into a multi-plane connection.” In the case of a Spec E30, Colicchio adds plates on the floor (horizontally), on the back gas area (vertically), and on the rocker (vertically), then joins those into a three-sided box, where he mounts the tube.

An example of a multi-plane box in a Spec Boxster. Photo credit: TC Design

Other than that, the rules have remained roughly the same, which speaks to the high standards they adhere to. There are a few areas where additional pieces of support, though not mandated, have become widely popular and subject to interpretation.

Understanding Door Bars

A vital part of a road-racing car’s cage is, without a doubt, the door bars. Some road-racing cars have three or more horizontal door bars which connect the front down-tube to the roll hoop, they typically curve out towards the door, and are linked by vertical bars also known as “NASCAR” bars. These curved sections “are bent outwards for more driver area,” says Colicchio. It’s a strong and effective way to strengthen the cabin’s walls, but there’s a lighter approach that offers similar levels of safety.

Instead of using horizontal bars, Colicchio likes to arrange the door bars in the shape of an X which curves inwards slightly. This design “offers more structural strength and stiffness,” he notes, and are constructed in two styles. “The door bars for an X-system can be done either with two bars: a top bar and a bottom bar, or three bars: a main bar and two additional bars to complete the X,” adds Colicchio. “Most of mine are the three bar method.”

For added strength, the X is often reinforced by sheetmetal gussets, also known as “tacos,” which keep the welded sections from coming apart during an impact. “The gussets in the center of the door bars are formed using a press-brake tool I made, that has a radius to match the rollcage tube radius,” Colicchio describes.

The aforementioned tacos strengthening some of TC’s door bars. Photo credit: TC Design

Appealing A-Pillar Support from Across the Atlantic

One item which has filtered down from the FIA rulebook into SCCA/NASA prominence is the popular vertical support bar stretching from the floor, through the door bars, all the way to where the upper windshield bar meets the A-pillar bar. “This is used widely in the faster NASA Touring classes,” notes Colicchio. This adds to rollover protection, and is most useful in cars with flatter windshields like Ferrari Challenge cars.

The vertical A-pillar support bar (left) running all the way from the rocker panel. Photo credit: TC Design

However, the vertical A-pillar shrinks the width of the window aperture, and this limits ingress and egress — the technical terms for entering and exiting. For a car with a small window area to begin with — like a Spec Miata — nobody but a malnourished string-bean can slip through, and even they might catch their suit on a tab in the process. Extrication in the event of a fire needs to be done in several seconds, thereby making this exiting post-haste a dangerous (if not impossible) task for anyone but an octopus or a contortion artist. Thankfully, this addition isn’t always necessary with a lighter car. Fortunately, for cars which do require additional support, but cannot provide a large enough aperture to enter or exit through, there is an alternative.

The popular sheetmetal gusset opens the aperture while providing all the strength of a vertical A-pillar support bar. This long, flared, perforated piece of metal protruding from the A-pillar “takes the A-pillar support bar idea, but offers more strength and better ingress/egress,” clarifies Colicchio. The problem is the time it requires to fabricate. Between the stitch welding and cutting those weight-saving holes, “these items require upwards of 25 hours,” says Colicchio. Interestingly, these build times prevent even certain well-heeled drivers, like those in Ferrari Challenge, from taking this route. Instead, they go with the basic A-pillar support. Perhaps it’s for the better — some of the Challenge drivers could stand to lose a couple of pounds.

To trim weight from the top of the car, cutting perforations out of the sheetmetal gusset is a simple and effective approach. Photo credit: TC Design

In any event, it shows just how many considerations need to be made. Between cage strength, build budget, and ergonomics, there’s at least an hour of potential discussion between builder and owner — and that’s only one of the many compromises which need to be examined. With careful analysis of the conditions, a builder can check all the boxes and keep a driver safe and secure.

About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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