It might be overstating the obvious to say that a roll cage in a racecar is the last line of defense in a big shunt or a major wreck. That’s why good cage design, as well as excellence in fabrication, are mandatory if a racer is to avoid injury during an on-track accident. With that in mind, we decided to talk to cage expert Clint Wolff, lead fabricator at Kontrolle Engineering of Rancho Cordova, Calif., about designing and building cages for Spec E46 cars – a fast-growing NASA class – to gather tips, tricks and best practices.
The Rules and Building a Compliant Cage
Per the NASA Spec E46 rulebook, Section 5.4, “the roll cage must be of the standard six-point design, or a “6 + 2” design with an additional two bars and associated chassis attachment points added for foot protection, provided that the foot protection bars do not go through the front firewall. Cars may utilize OEM type steering column and transmission tunnel attachment points to the dash bar if equipped. A driver seat mounting cradle integrated into the cage structure may attach to the chassis at the transmission tunnel at two points, each with a maximum mounting surface of 40 square inches. Factory brackets 41118239493 and 41118233493 (the left front and rear seat consoles) may be modified or removed but may not provide additional attachment points. No additional tabs or attachment points are permitted.”
What six point means is that the cage attaches the car’s unibody in six places on the floor, which is to say the main hoop, front hoop along the windshield and two backstays in the rear. Additional bracing and triangulation is helpful, says Wolff, who has built more than 40 SpecE46 cages and more than 20 complete cars in the last five years. He also adds that once past the “6 + 2” rule requirement, the number of additional tubes and attachment points is not relevant for basic safety, but more tubes and triangulation add stiffness to the overall structure. The rules further stipulate that more tubing can be added to prevent wheel intrusion into the footwells, provided those tubes and bracing don’t go past the firewall.
The factory cross bar that runs behind the dash also can be removed, and the rules state that “cars may utilize OEM type steering column and transmission tunnel attachment points to the dash bar if equipped.” Wolff notes that the factory cross bar can be tied into the cage, but Kontrolle’s preferred method is to replace that part with a piece of tubing, such that they can position their own cross bar much closer to the firewall and take approximately 8 pounds out of the car by pulling the factory piece. Not only does this help reduce weight and lower the car’s center of gravity, it enables a lower placement of the dash inside the driver’s compartment.
When it comes to seat mounting, the NASA rules state that factory brackets, BMW part numbers 41118239493 and 41118233493, “may be modified or removed but may not provide additional attachment points.” The aforementioned factory brackets run transversely along the floor of the driver’s compartment, and can be used in the building of a Spec E46 using a commercially-available seat mount/adapter, but Kontrolle removes these mounts and installs a fabricated seat mounting cradle, which is integrated into the structure of the roll cage.
The rules state that any seat cradle may attach to the chassis at the two points along the transmission with a maximum mounting surface of 40 square inches each, and the team at Kontrolle feel that tying the seat cradle to the cage is a safer option in the event of a crash. This is because any crash forces are more directly transferred to the cage, rather than having the factory sheet metal seat mounts twist or deform, raising the possibility of the seat being torn from the floorboards in an extreme circumstance.
If a builder does use a premade versus custom seat mount, Wolff considers the ones made of plate as reasonably strong, but cumbersome. He finds the Bimmerworld and VAC Motorsports offerings to be workable, but he cautions that if a driver is picky about seat fitment, these offerings aren’t as effective as something custom-fabricated.
And regarding spreader plates – the flat plates that cage tubes attach to – Wolff notes that he has seen them attached to both the parcel shelf and to the wheel wells of various BMW builds. Both of these are weak structural areas in any car, and likely will fail in an accident – so don’t build it that way, or buy a car that has such a cage installed.
Cage Materials and Weight
In terms of materials, drawn-over-mandrel tubing is the standard, for its more consistent and exact inside and outside diameter, as well as its potential for greater weld strength. The preferred material is 1.5-inch diameter DOM grade 1020 tubing with .120 inch wall thickness steel, or 1.75 inch diameter tubing with a .095 inch wall thickness.
For cage attachment points/spreader plates, Wolff suggests they be much larger than the minimum size of nine square inches, made from .120 or eighth-inch-thick flat plate, versus the minimum thickness of .080 inch as specified in Section 15.6 of the NASA CCR.
A typical Spec E46 cage build takes five 18-20 foot tubes, or approximately 90 feet of tubing for a standard cage, which, when complete, weighs 160-180 pounds. While 1.75-inch tubing weighs slightly less per foot, the benefits of 1.5-inch tubing in cage construction are notable, according to Wolff. He prefers the 1.5-inch tubing for its greater sidewall strength, and the fact that it is easier to weld because more heat can be put into the weld itself. In addition, 1.5-inch tubes can be bent more tightly because the overall diameter is smaller, given that the minimum radius of beds should be three times the tubing diameter. These factors allow for a closer fit of the cage to the contours of the vehicle, and he notes that the extra quarter inch of clearance during fabrication and installation is useful.
Technically, if one wanted to build the lightest car, mixing tubing sizes could be beneficial, but Wolff believes the tradeoffs aren’t worth it. He notes that it looks odd visually to have two different tubing sizes, and it slows down the building process because the fabricator must to use different dies for the tubing bender.
After building so many Spec E46 cages and complete cars, the team at Kontrolle have accumulated numerous “lessons learned” when it comes to building the E46 into a racing machine. They adhere to NASA CCR, of course, and employ a number of prep tricks to make their builds more elegant and functional, including cleaning up the interior of the car and making it look more professional. When building their cars, they remove the factory sound deadening and insulation. The preferred technique is using dry ice to freeze everything, thereby making it easier to break apart and remove. Otherwise, he recommends gently heating the material with a torch and then using a large, wide scraper to remove the material and adhesive. Occasionally, some gentle heat needs to be applied to the back side of the panel to get the adhesive to release. Then Wolff recommends wiping down the panel with WD-40 and acetone, and it is ready to go, or to be prepped for paint.
For other areas of the passenger compartment, cleanup consists of removing small brackets, such as those on the rear parcel shelf. Typically, removal is accomplished by drilling out spot welds or grinding the pieces off, followed by sanding or grinding them smooth. Wolff notes that removal of the sunroof in an E46 is much easier than it used to be with E30s, because most E46 models have factory sunroofs, which can be easily unbolted – he characterizes E46 sunroof removal as a “five-minute job, versus an hour” when compared to E30s.
Model Differences & Known Weak Areas
Asked about the preference of E46 coupes versus sedans, Wolff states a preference for the coupe. Conventional wisdom suggests that sedans with b-pillars are usually stiffer, and over the years, many a touring car has been based on a four-door platform. A bit of internet sleuthing also suggests that coupes are less stiff, but Wolff thinks there is very little in the way of a practical difference in torsional rigidity, particularly after a cage is installed.
In terms of other practical matters when it comes to racing, four-doors are approximately 60 to 80 pounds heavier than the coupes. While sedans offer better access for car building and serviceability, thanks to the rear doors, they are taller and carry more aerodynamic drag. The major benefits of the coupe are that the larger doors make ingress and egress easier, and as such, heavier and taller drivers will find it easier to deal with a coupe, which also enjoys the aero benefits of its lower roofline.
In terms of “known” weak areas, the E46 chassis has a few. The rear subframe and floor usually need reinforcement, and there are a number of aftermarket vendors who supply parts and guidance to addressing the issue. Bimmerworld has a comprehensive explainer, as does Turner Motorsport. The bottom line is that this area of the chassis must be tended to before hitting the track.
Front sway bar mounts can also break, and many vendors offer weldable reinforcement kits, consisting of a beefier piece of flat plate that gets welded onto the fame rail. Of course, it makes sense to start with a rust-free car, and East Coast racers may want to search for clean cars from dry-weather states when beginning their race build.
Some Final Tips
Wolff suggests that racers buy or build the best Spec E46 they can afford. This means not skimping on safety, and figuring out the desired result before embarking on a build. He recommends prioritizing trade-offs and compromises, and seeking the opinion of professional fabricators in advance. This due diligence includes checking a shop’s work and reputation, to ensure that the materials and craftsmanship are high-quality. When it comes to the costly and time-consuming effort of building a racecar, investing time up front makes real sense and will give you a safer and stronger car that might just save your life.