Understanding and Building an NHRA 8.50 and 10-Point Roll Cages
When building a race car, one of the most frustrating and misunderstood parts of the build can be the roll cage. Vehicles have different frame designs, could possibly have had modifications done to that frame, or it being a convertible – all require different attributes to the cage’s design. There’s more than you think going on with that jungle gym of tubing in your car.
I could easily write an article five times this size when it comes to explaining how roll cages work in a variety of ETs and car configurations. I am going to explain the key rules and requirements on building a 10 point, or 8.50 and slower, roll cage for unibody vehicles per the 2010 NHRA Rule Book. We will be using our 60 horsepower, soon to be 1000 horsepower 1965 Mustang Project “Biting the Bullitt” as our example. The cage we will be using is a pre-bent piece from Chris Alston’s Chassisworks.
The Goal of our ’65 Mustang
As with any project, it’s a good idea to figure out what you’re going to do with it in the long haul. Meaning if your vehicle is going to run low 10s, but then might see the 9s sometime, building the faster cage in the car will lead to less headaches down the road. Biting the Bullitt will run 9s off the bat, and there aren’t any immediate plans to push it under an 8.50 ET. Our power will come from a Dart block-and head-equipped 427ci small block Ford that features an AED blow thru carburetor and a Paxton NOVI-2500 supercharger to produce in the neighborhood of 1,000 horsepower. This is going to be achieved through a Crower hydraulic roller valvetrain and a moderate amount of boost. This will allow us to drive the Mustang around town when we want, then throw the slicks in the trunk and head to the dragstrip.
Our Roll Cage – a Chris Alston’s Chassisworks pre-bent 10-point
There are two ways to build a roll cage; starting with a pre-bent cage or bending the tubing up yourself. If you have a popular vehicle, there is a good chance Chassisworks already makes a pre-bent cage for it. “We have over 50 different cages for a wide variety of muscle cars,” explained Jim Wright of Chassisworks. “When we make a production roll cage kit, we actually get the vehicles in here at the shop and test fit the parts accordingly, and not just rely on given measurements.” A pre-bent cage reduces the cost of installation and the need for a tubing bender. The advantage of bending your own cage is being able to get the bends as tight to the car as possible, but don’t dismiss pre-bent cages; they have come a long way.
The Alston kit we selected is made from 1 5/8-inch mild steel with 1 1/4-inch main hoop support bars. “If a customer wants a custom cage that we do not already offer, they can use our custom roll cage worksheet and we can bent it to their specifications,” said Wright. The tubing size requirement on 8.50 cages are 1 5/8-inch x .118-inch mild steel or .083-inch chromoly. For 10-second and slower cars that only need roll bars, the size minimum is 1 3/4 O.D. x .118 mild steel or .083 4130 chromoly tubing, so this is another aspect to take into play when building or expanding a roll cage as your vehicle gets faster. Also, while a chromoly cage is typically lighter due to its lower minimum wall thickness, it can only be TIG welded, while mild steel can be TIG or MIG welded, though the welds need to be clean, complete without holes, and show no signs of grinding.
The cage we selected comes with:
• Main Hoop
• Main Hoop Support Bars
• Door Bars
• Front Down Bars
• Windshield Bar
• Cross Brace/Harness Bar Rear Down Bars
• Extra Straight Tubing For Optional Extra Bars
Chassisworks can custom-design a cage order specific to your liking, using a wide variety of bends and straight configurations to get you there.
In addition to the roll cage, we also opted for a pair of swing out door bars to make getting in and out of the car easier, as well as subframe connectors with X-brace from sister company Total Control Products.
Caging the Sloth – Six Cylinders Sees 10-Points
Nothing screams speed like a 60 horsepower Mustang with a 10-point roll cage in it. Though it might be safer driving it now, it sure hasn’t aided in the acceleration factor with the added weight. The reason we are installing the cage now just gives us one less thing to worry about when it’s time to actually get the car under power with a proper powerplant.
When it comes to preparing to install a roll cage, you will want to remove everything from inside the car that you can. This includes carpet, seats, and door panels. As you know, welding is extremely hot and it isn’t shy when it comes to burning a hole in anything. If you have a headliner that needs replaced and want to keep one in the car with the cage, replace it beforehand to save yourself a big headache. We will go through the steps later when it comes to protecting the headliner during the final welding.
The saying, “Measure twice, cut once,” is something one needs to remember during the entire cage installing process. The pre-bent cages are purposefully made long to accommodate for variances in vehicle builds. Also, tack weld the entire cage in place before completely welding it to ensure a better fit. Additionally, whatever seat and seat mounts you plan to use for the vehicle should be installed. The driver, or someone of similar build, should be present for the variety of clearance checks. We took the Mustang to local racing shop Mckinney Motorsports to have the cage installed.
Floor Support Plates
The Chassisworks roll cage kit comes complete with pre-cut 6-inch x 6-inch plates for your floor mounting. Per the NHRA Rule Book, “On unibody cars with stock floor and firewall (wheel tubs permitted), the roll cage may be bolted or welded to the floor/rocker box.” If you are going for the bolted option, the .125-inch steel plates must be on top and bottom of the floor and bolted together with at least four 3/8-inch bolts and nuts. Welding the plates to the car does not require the use of bottom plates but must be completely welded to the floor.
The Main Hoop
Generally the main hoop is installed as close to the roof or headliner as you can without it rubbing. The NHRA rules state, “All roll bars must be within six inches of the rear, or side, of the driver’s head, extend in height at least three inches above the driver’s helmet with driver in normal driving position or be within one inch of the roof/headliner in the area above the driver’s helmet, and be at least as wide as the driver’s shoulders or within one inch of the driver’s door.” You can lean the main hoop back or forwards to get the proper measurements as well. Additionally the bottom of the main hoop must be welded to a pair of the aforementioned 6-inch x 6-inch plates.
The roll bar from the Chassisworks kit fit amazingly well. “From a safety standpoint, we would always recommend a roll cage instead of a roll bar because it’s safer in a roll over condition, even if your ET doesn’t call for it yet,” explained Wright. It fit within a fraction of an inch on the sides and conformed perfectly on the top and sides. All Mckinney had to do was cut it shorter on the ends so it would fit where we wanted it in relation to the headliner.
The Cross Bar/Harness Brace – The First Tube to Notch
The cross bar or harness brace serves multiple functions. It increases the structural rigidity to prevent forward or lateral collapse. Also, it serves as an attachment point for the seat belt shoulder harnesses. The harnesses typically attach via a welded bracket and a bolt to a bracket on the belt’s harness or a wrap around-style belt. The bar must be installed within four inches of the driver’s shoulders vertically, but not above them.
A critical step in building a roll cage is tube notching. You cannot weld a flat-cut tube end to an intersection of another piece of the cage and expect it to be safe. Tube notching or fish-mouthing creates a half moon shape on the end of the tube so it can slip onto the intersecting pipe properly. Usually a specifically designed tube notcher or drill press that allows for variable angles are used with a hole saw on the end. If the cost is too much, a hand grinder can also work, though it can be less accurate.
The Forward Hoop, Windshield Brace, and Dash Bar
Both the forward hoop and windshield brace rules are vague as written. The front bar must be one continuous piece of tubing with the same diameter and thickness specs as the main hoop. It welds to the top corner of the main hoop and follows the roof and A-pillar line to another 6-inch x 6-inch plate in the floor. The forward hoop can go through or before the dash. The straight bar coming off the main hoop should be parallel to the roof before it makes its first bend. Also there can be bars that connect to the front down bars, go through the firewall and connect down into the front frame rails. Wright explained, “When you get into the 8 second zone the down bars to the front frame rail really help stiffen the chassis. They are typically harder to put in though with the stock dashes, heater, and wiring installed.”
Not wanting to hack up the dash in our Mustang, McKinney installed the bar before the dash, set back about an inch from it. This gave them the space they needed to fit a stock dash pad back in and also for the kick panels that include speakers. The Chassisworks forward hoop bars come pre bent and fit awesome. All McKinney needed to do was shorten them to fit accordingly.
The upper windshield bar is another important piece of the cage. It acts as a supporting brace for the forward hoop and also to keep the roof from collapsing. Again, with the same spec tubing as the forward bars and main hoop, it must be welded at the top of the windshield support to the two forward bars. The bar has a slight bend on both ends so that it can conform to the windshield’s shape and to tuck it up out of the driver’s view. “The windshield bar is one of the most important bars in a roll cage during a roll over situation,” explained Wright. “It keeps the front of the roof from crushing in on the driver.”
A dash bar is not required in vehicles that have their firewalls altered by one square foot or less, though a dash bar can be useful to tie the lower part of the front cage together to reduce chassis flex. Before-dash forward down bars can be tricky when it comes to installing a dash bar as it can be in the way of your legs, though there isn’t anything that states you couldn’t bend the bar back towards the firewall and run it across there. Though we decided against the dash bar for now, the NHRA rule states, “If the OEM firewall has been modified (in excess of one square foot for transmission removal, not including bolted in components) a lower windshield or dash bar of 1 1/4 x .058-inch 4130 chromoly or 1 1/4 x .118-inch mild steel is mandatory connecting the forward cage supports.”
Door Sidebars, Swing Outs, and Rocker Bars
Both door sidebars and rocker bars must be constructed of the same 1 5/8-inch .118-inch thick mild steel as the main hoop and and forward down bars. The door bars are there to protect the driver in a side impact crash and must be welded to the main hoop. From the main hoop, it must pass in between the driver’s elbow and shoulder. There is no specific angle or point in which the forward side of the door bar must intersect, though it should weld to the forward bar near the bottom 6-inches.
Swing out door bars can help a driver and passenger get into a vehicle without needed to go to Yoga classes, and as long as your car is going to be slower than an 8.50 ET, are completely NHRA legal. The Chassisworks kit features a clevis-type connector that is over the minimum thickness required by the NHRA. The swing-out rules are pretty broad and are as follows:
1 5/8-inch O.D. x .083-inch CM or .118-inch MS minimum. Bolts/pins must be 3/8-inch-diameter steel, minimum and in double shear at both ends.
b. Male or female clevis(es) permitted. Male clevis must use two minimum 1/8-inch-thick brackets (CM or MS) welded to each roll- cage upright; female must use minimum 1/4-inch-thick bracket (CM or MS) welded to each roll-cage upright. Pins must be within 8 inches of the vertical portion of both the forward and main hoops. A half-cup backing device must be welded to the vertical portion of the main hoop (inward side) or the upper end of the swing-out bar (outward side), minimum .118-inch wall (CM or MS) extending at least 1 5/8 inches past the center of the pins. A clevis assembly using a minimum .350-inch-thick male component and two minimum .175-inch-thick female components may use a 1/2-inch-diameter Grade 5 bolt and does not require a half-cup backing device.
c.Sliding sleeves of 1 3/8-inch x .083 CM or .118 MS, with minimum 2-inch engagement, are permitted in lieu of the upper pin/cup.
d. All bolt/pin holes in the swing-out bar must have at least one-hole diameter of material around the outside of the hole.
As mentioned above, the swing out door bar pins must be within eight inches of the vertical sections of the front and rear main hoop. Typically, the clevis receiver is welded to the roll cage at the shoulder. The lower portion can use a small piece of pipe welded to the bottom of the front main hoop bar and then the clevis is attached at this point. This allows the needed height for the swing out to function properly. What we did was install the door bar higher up on the front main hoop and in the same fashion as the rear hoop – welded directly to the hoop.
The larger clevis joint is designed to slip into the end of the door bar. Since there are variances in the minimum wall thickness on the tubes, some grinding on the inside of the tubes might be required to fit the clevis in snugly. After that, it is completely welded to the pipe.
Rocker bars are only required when the OEM frame rails don’t run below and outside of the driver’s legs. The frame rail acts as a side impact support and a rocker bar must be added in place if it. This also goes for any car with a modified floor or rocker box, though modifying the transmission tunnel up to six square feet is permitted. The NHRA rule book states:
“A rocker or sill bar, minimum 1 5/8-inch x .083 CM or .118 MS or 2-inch x 2-inch x .058-inch CM or MS rectangular, is mandatory in any car with a modified floor or rocker box within the roll-cage uprights. Rocker bar must be installed below and outside of driver’s legs and must tie into the main hoop, the forward hoop, frame, frame extension, or side diagonal. Rocker bar may not tie into swing-out side bar support. If rocker bar ties into side diagonal more than 5 inches (edge to edge) from forward roll-cage support or main hoop, a 1 5/8-inch x .083 CM or .118 MS brace/gusset is mandatory between the diagonal and forward roll cage support or main hoop.”
Almost Done! Main Hoop Support Braces, Rear Down Bars, and Roll Cage Gussets
Believe it or not, this section is the easiest part of it all. The “D” bars or main hoop support bars are designed for unibody vehicles in which the roll bar is connected to the floor instead of a separate frame. The D bars are designed to support the main hoop from movement under impact as a triangulated point of reference. Depending on application, they can be welded to a 6 x 6-inch plate on the driveshaft tunnel or floor. On the top side, the D bars are welded in the corner of where the harness bar and roll bar meet. Since we are installing a set of TCP subframe connectors, we placed the D bars over the top of the connectors. This way we can weld the connectors to the plates from underneath the car to provide even better protection. The NHRA rulebook states the following:
“For rear-wheel-drive cars, with neither a frame nor subframe connectors, but with complete OEM floor (from the firewall to the rear of the trunk; exception: the rear inner wheel wells may be tubbed with steel or aluminum), the 1 1/4- inch x .058-inch CM (or .118-inch MS) “D” bars may be welded to conventional 6-inch x 6-inch x 1/8-inch form fitted/contoured plates attached to the driveshaft tunnel. Otherwise, the “D” bars must be attached to frame, subframe, or subframe connectors.”
The rear down bars or rear support bars must attach to the floor of the trunk… somewhere. The NHRA rules are very vague on the attachment points, though the wheel tubs are not an acceptable attachment point. Also, there is a variety of rules on the total diameter of the tubing depending on the length and amount of cross braces utilized. If using 1 5/8-inch tubing, the rear bars can be any length or angle. This means you could write your name in cursive with the rear bars, and as long as they connect to the rear floor, they will technically be legal. With this tubing, the top side of the rear down bars can be welded anywhere on the horizontal section of the main hoop or five inches or less vertically down the sides.
We opted for the angled rear bars on our kit for two reasons; first to allow for more head room for any rear passengers, and second was so we could re-install the rear seat. We wanted the rear bars to weld right behind the rear seat to allow for more trunk space since most of the rear trunk is actually occupied by the fuel cell. Since we will be adding in mini tubs later, we allowed for the additional four inches of space we needed from the stock tubs. This spot is also very close to the frame rail and we will later box the plate to the frame rail when we install the tubs. Having some extra material left over, a support bar was welded between the two down bars for additional strength.
While there are measures you can take to weld the majority of the top welds, in some instances (especially with headliners) it can be hard to weld the top-most part of a bar. I called up NHRA to ask their tech department about that and they mentioned that if at least 75% of the joint is welded, a gusset on both sides of the tube can be added to take place of the missing welding. While the tech did not have specs on the gusset’s requirements, some online sources have said they must be 1 1/2-inch long on each side and be at least a quarter inch thick.
Installing the Total Control Products Subframe Connectors
Subframe connectors should be anyone’s first purchase on a unibody vehicle. Not only are they cost effective, but they tie the front and rear frame rails together and drastically improve the vehicle’s resistance to flexing. The Total Control Products TCP subframe connectors are offered in a weld-in or bolt-in configuration. For our project, we selected the weld-in versions. The connectors are specifically designed for first generation Mustangs and fit perfectly in place.
The subframe connectors can be optioned with Chassisworks’ bolt-in cross brace that can also be optioned with a driveshaft safety loop. Since our stock exhaust is in the way for now, we did not install the brace. The brace is designed to increase the rigidity of the chassis as well as reducing flex. It attaches with two bolts on all four corners, though the front bolt holes have to be drilled through the frame rail as well. The driveshaft safety loop attaches to the brace with four bolts.
Bottom Line – Get a Rule Book!
While we offered a lot of tips and tricks when it comes to installing a 10-point roll cage, it does not take place of a rule book. They are cheap and indisposable when it comes to having your cage certified. Also, there are other rules that must be met to make your vehicle legal for a certain time and speed outside of the roll cage. It’s a good idea to consult with the tracks you race with locally to inquire about any additional rules they require to race. Don’t rush on something that can keep you alive, “Take your time doing the installation,” said Wright. “All of our cages are made oversized in length so you can fit it as tight as possible and the tighter you can fit it, the better results you are going to have”. Remember, a roll cage is designed to keep you safe in case there is a crash, so don’t skimp on your safety!