A good cage provides more than just safety. It’s the foundation upon which a racing car is built. A good cage strengthens the weaker parts of a chassis. Any adjustable element is more clearly felt when the cage is properly designed and installed.
For those reasons, it’s worthwhile to invest in a builder who understands the myriad compromises involved to make a solid cage. To better educate NASA members, we’ve reached out to two top cage builders so they could share their wisdom: Tony Colicchio of TC Design in Campbell, Calif., and Mark McMahan of McMahan Autosports in New Franklin, Ohio.
Popular Materials
There are three main considerations when picking the material a cage will be made from: material cost, overall weight, and the rules.
Electric Resistance Welded tubing (ERW) is created from a flat plate of steel formed into a tube and electrically welded. ERW was, for many years, one of the heavy hitters in the material market, largely because of its low price. Nowadays, it’s virtually non-existent because it hasn’t been allowed by most of the sanctioning bodies for the last 10 years. In 2024, the major players are DOM, 4130 and DOCOL.
Drawn-over-mandrel tubing is mild-steel tubing — usually 1018 or 1020 — shaped by a mandrel. DOM is still the preferred material chosen for the majority of road racing cages built today because it costs less, it can be MIG welded fairly easily, and it’s malleable.
Being able to elongate and bend slightly in the event of a crash means it will absorb the second, third, and fourth impact better than others might. This means that DOM typically enjoys a longer lifespan than chromoly, and is much more affordable for the average racer.
4130 makes the odd appearance in cage design — particularly in higher-end builds. Also known as chromoly, 4130 is roughly twice as expensive as DOM, has about 30 percent more tensile strength — the resistance of a material to breaking under tension — and about 30 percent more yield strength, but roughly the same rigidity. Though its added tensile strength makes it harder on cutting tools, the more concerning limitation of the material is that, by design, 4130 requires a highly skilled welder.
It can be MIG welded if the welder is talented and the fitment is tight, but it really should be TIG welded. A less-skilled welder runs the risk of making the material even more brittle with excessively large gaps that they fill in with their welds.
“Often, the inexperienced welder fails to get the notches close enough, and in an attempt to compensate for their poor notch placement, they add more material and end up overheating the joints,” says Colicchio.
“When welding chromoly, we’re asked to use mild steel filler rod that has better elongation in the event of a crash. You want the weld area to be milder than the rest, because that’s where the stresses are concentrated,” says McMahan.
In reality, the added tensile strength of 4130 requires a specific sort of vehicle to fully appreciate. To take advantage of the additional tensile strength, an extremely rigid construction is needed, ie. a tube frame composition. “Even if we tie the cage into multiple planes on a traditional chassis, we cannot realistically see the benefit of the chromoly,” Colicchio notes.
DOCOL R8 is probably the most appealing of the three when cost is no object. A relatively new material, DOCOL combines the malleability of DOM with the 4130’s greater tensile strength. However, DOCOL is produced almost exclusively on the East Coast. Once the material is shipped to the West Coast, the price is roughly twice that of DOM. This material is rapidly becoming the preferred material for top club/pro level builds.
Spatial Constraints
Once the material is decided, the cage design/complexity must be addressed. To do this, it is essential to establish the process by which the various tubes are added.
Typically, the first piece of the cage is the main hoop: the curved bar mounted at the floor behind the driver’s seat rising vertically to the roof, then spanning laterally across the car and back down to the floor on the opposite side.
A driver’s height rarely determines the height of the main hoop. “We want to have the main hoop fit tight in the chassis, period. The only time the height of a main hoop has a driver influence is in a production-type race car with no windshield or top,” Colicchio adds.
However, the builder decides just how snugly they’d like the hoop to stand relative to the roof and the sides of the car. There’s a big decision to make: a low-slung cage that provides ample protection to the driver is often easier to install. Form-fitting, low-clearance cages are prettier and can increase structural rigidity, but installing them correctly requires foresight.
That entails a little more than just painting the bars before installing the cage. As the builder is assembling the cage, they need to think about whether they will be able to access a joint to weld. How the cage is shaped and assembled outside the car determines the level of access.
With years of experience and a few custom-tailored MIG guns to access those hard-to-reach areas, Colicchio has learned how to work in cramped conditions. “In some cases, you might weld a few bars together outside the car and then install it as an assembly,” he begins. “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.”
Forty years of experience have given McMahan the sort of knowledge needed to work around these spatial limitations without wasting time. “One feature of my cage design is that I take it as close to the roof whenever I can,” he says. “The fact that the edge of the cage is going to touch the roof is a plus, not a minus. I find I can get a little added stiffness from additional contact points, even if they’re not welded. I drop the cage through the floor before I weld the plates so I have access to the tops of the weld near the roof.”
Obviously, following the correct order of steps is essential to the fitting process, but these tips and tricks aren’t necessarily intuited by the inexperienced builder. “For example, I’ll make the door bars before I weld the cage to the floor. I ratchet strap the main hoops and door hoops to give myself enough room, and if I’ve done my planning, it should line up with the door bars — then I weld. This approach helps me relax, since I know I won’t get into a jam that would force me to cut the cage out,” McMahan explains.
There are spatial constraints and temporal constraints. Because many drivers are reluctant to completely convert their once-cushy road car into a dedicated racecar, there is an order they can follow to transition more gradually. People who aren’t ready to take the plunge can add steadily to the scaffolding surrounding them.
However, some things can be put off until later while others cannot — not without removing the roof or making another costly alteration. McMahan prefers to avoid scalping his cars whenever possible, so he decides on what can and cannot be done at the earliest stages. “Some things can be added later: door bars, foot protection, and the X-bar in the plane of the main hoop. Other things, like a diagonal bar across the roof (something nice to have when there’s a composite sunroof panel in place) can’t be added later—not without chopping off the roof, anyway!” he explains.
Mass Matters
Compromises can be seen throughout a cage in obvious ways. Since weight has such a strong influence on performance, keeping the cage design as light as possible is preferred. Some rigidity can be gained from a more involved cage, for instance, but if the tire is restricted by the rules, or if the car is particularly light already, for instance, it might not make sense to make a cage any more complicated than it needs to be.
In short, a Spec Miata requires a little less from its cage than a TA2 car requires from its own cage, and finding the right compromise between weight and strength requires weighing each car’s respective constraints.
However, most cage builders will err on the side of caution by reinforcing various vital areas or using slightly thicker tubing, “usually because the customer requests it — they think they’ll gain a performance advantage from a more elaborate cage, though they probably won’t,” McMahan says. There are diminishing performance gains past a certain point of complexity.
For instance, the bar connecting one C-pillar to the other C-pillar is subject to a complicated weighing of variables. Choosing between a single diagonal crossbar and an X-bar is decided by the weight of the vehicle, the innate rigidity of the vehicle, the grip of the tire, and the cost of the build. Clearly, the added triangulation of the X-shaped crossbar increases the rigidity of the car, but just because it has twice the mass does not mean it doubles the rigidity. “In reality, you’ll gain an additional 25 to 30 percent from this,” Colicchio states.
Multi-Plane Mounting
In something like a Spec Miata, the number of attachment points are usually limited by the rules to cut costs, and keep the rigidity comparable across the entire class. This is a series that prides itself on parity, even if there are ways to find an advantage.
The number of overall attachment points is just as important as where the mounting points are placed. When deciding on where those should be, Colicchio aims for areas that incorporate at least two planes, a 3D mounting point. This means the mounting plate incorporates the floor and a rocker panel, for instance.
“NASA has 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 rules that determine the size, placement, and style of these plates and their welds.
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 and the slower NASA Super Touring classes.
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.
McMahan’s typical floor plates consist of two plates mounted on different planes inside the car and joined before the tube is connected. “When the main hoop is welded to the car for the final time, it’s welded down at the bottom near the base of the B-pillar. The second plate is already welded along the side by the rocker. The two end up touching, the first flat on the floor, and the second standing vertically,” McMahan elaborates. These two plates are welded together. The duo of eighth-inch steel ends up having roughly 8 inches of surface area.
Details: Door Bars
With the broader strokes brushed, the builder can consider several different details to improve ergonomics, stiffness, and safety.
The most popular door bar design today is the FIA-inspired pyramid X-bar design. The latter will typically bow outward in a pyramidal shape for more space. The apex of the pyramid, the point nearest to the door, being the center of the X.
For added strength, the center of the X is often reinforced by pieces known as “tacos,” which weld adjacent bars together and keep them from coming apart during an impact. FIA cars often use carbon inserts to further fortify this area.
Then there’s the NASCAR-style horizontal design, which weighs more than a comparable X-bar does. The NASCAR bars are thought to limit ingress and egress, but they can be more accommodating than the X-bar if the NASCAR-style top tube is angled down from the B-pillar toward the driver’s feet. This means the driver can swing one leg over to get in and out, whereas with most X-bars, they usually have to put two feet in simultaneously.
The main strength of this design is that it generally allows for more room, which comes in handy when designing a cage for a cramped cabin, like that of a Spec Miata.
Details: A-Pillar Reinforcement
Another interesting cage detail is the A-pillar reinforcement. Since this joint where the A-pillar meets the roofline is in shear by design and is put under enormous strain in a rollover, it should be reinforced beyond what’s provided by the typical hoop that runs along the A-pillar, door frame, and back behind the driver to the main hoop.
Any vehicle with a steeply angled windshield, like a Ferrari 458 or any Corvette, needs to have this area fortified.
The most popular and cost-effective option is the FIA bar. As it has been proven to be hugely helpful in withstanding a multiple rollover event, this basic design has been mandated in FIA rally. It does cramp the OEM seating position slightly with a vertical bar that runs at a tangent to the A-pillar and joins the main hoop with the door bars.
The other option is the visually appealing sheet-metal gusset. This flared, often die-perforated piece of metal protrudes from the A-pillar and tapers slightly as it runs toward the B-pillar. It has the Star Trek look, requires less real estate, and is one of the stiffer options, but it’s roughly four times the cost of the X-bar.
Some will go further and mount a support tube connecting the main hoop to the side hoop, sometimes with a triangular gusset. Mounted at roughly 45 degrees, this serves as a double-failsafe for the welds along that vital joint, and it rarely impedes ingress or egress.
Tying it All Together
After understanding all that goes into a good cage, it’s natural to want to avoid such complicated considerations. Therefore, a car with the scaffolding in place will look pretty appealing to the racer looking to get started soon. As much as it might seem like a shortcut to buy a car with a cage already in place, be careful — it’s worth having it inspected. If the cage is low-quality, removing it to start over, as all prudent people should, is costly.
“The real horror stories start after you begin taking stuff out – the stuff that didn’t pass tech. Then you see all the welds that weren’t done properly. If the builder doesn’t plan ahead on how they’re going to get to every weld on the car, they’ll sometimes cut corners and hide their mistakes,” McMahan recounts.
Therefore, it’s worth taking your time and finding the right person who can build your cage well — and build it once. “The cage is where the build really begins,” says Colicchio.
