As the Royal Scamp project moves along, we move past the initial bolt-ons and get into the fabrication. This section of the build is about two things- safety and performance. With expected time-slips in the mid-10 second range, a roll bar is required. Besides the obvious roll-over benefits, the roll bar also adds some stiffness to the chassis, especially for a unibody vehicle like the Mopar A-body. Most sanctioning bodies require roll bars in cars running 11.49 (13.49 for convertibles) or faster, and a 10-point cage once you get into the 9s (10.99 with a modified floor or firewall). The Royal Scamp should be in the 10.5 range (hopefully), so we decided a 10-point cage was best. For roll bars, you must use 1 3\4” OD x .118” mild steel or .083” chrome moly tubing, for cages you can go down to 1 5\8” x .118” mild steel or .083 chrome moly tubing.
Roll bars and cages are described by their attachment points. A typical roll bar has 6-8 points, and cages start at 10 and go up from there. A point is where the bars connect to the chassis or frame. The main hoop is 2 points, the rear down bars are 2, the door bars are 2 and so on. What seperates a cage from a roll bar is the B-pillar. A roll bar mounts behind the B-pillars, with only the door bars extending beyond the B-pillar. A rollcage adds a roof hoop with A-pillar down bars and can extend into the engine bay.
Where a roll bar adds stability to the chassis, a cage goes even further. The roof hoop and windshield bars connect the front subframe to the rear subframe, reducing twist and flex. Another benefit of an 8- and up point system is the frame bars that intersect the lower half of the main hoop. These bars tie into the seat crossmember, main hoop and the door bars, and cross the center of the car to the floor. This increases tuneability for the rear suspension, and decrease chassis reaction time.
Adding a roll bar or cage seriously limits back seat access. In the Scamp, we were planning on this, so the rear seat was gutted. The main hoop is not the problem, it is the seat crossmember and the frame bars (is used) that make it really tough to get in the back seat. As part of our installation, we left the floor bars out until we complete the rear firewall and tubs.
Installation of a rollcage is not easy. This is a safety device and proper installation is key to its success. You hope you never need it, but if you do, it needs to work. You need a few tools to install a cage-
Quality welder- DOM mild steel can be welded using a MIG, but if you want chrome-moly, you have to TIG it. Good penetration is a must and your welds must be free of slag and porosity. You also can’t grind the welds, they must be exposed for certification.
Tubing notcher- While this is not absolutely necessary (we didn’t use ours), it helps. JD Squared offers an excellent tubing notcher that clamps in a vice or bolts to the work bench and runs off a regular drill. Using good quality bi-metal hole saws, the notches will be perfect everytime.
Chop saw- You don’t always need a fish-mouth cut, most of the floor\frame mounts require basic angle or flat cuts. The chop saw is the perfect tool.
Plasma cutter- The alternative to a notcher is the plasma cutter. In some ways, it is actually better. Putting together a cage requires a lot of fit and re-fit. Unless you have built several cages, getting those notches right on the first try is probably not going to happen. With a plasma (or oxy-acetylene, which you also need for bending the floor plates) torch, trimming the tubes to fit is a quick process. Once you have the tub trimmed to the basic shape, you use a right-angle grinder to clean up the notch.
Tubing bender- Most beginners should buy pre-bent bars. S&W Race Cars offers pre-bent bars and cages for most cars, which is where we sourced our cage. That said, custom applications require bending tubes. While we didn’t need it for this portion of the build, we picked up a manual tubing bender from JD Squared. Beware of the cheap Chinese pipe benders you find at the budget tool stores. These are not tubing benders, they are designed for heavy-wall pipe. They will crush the tubes, not bend.
An assistant- It may seem silly to list this as a tool, but you really need an assistant to help you hold, mark and cut the tubes for the cage, especially when you are working on the overhead hoop. There are ways to do it by yourself, but it is incredibly more time consuming and difficult.
With the interior stripped and the parts ready for installation, we set out to stiffen up the Scamp. We built our cage to be within NHRA specs. If you are planning on racing, you need to check the class rules for regulations. Cars running faster than 9.99 seconds, or 135 MPH, need to have the cage certified every three years by NHRA officials. The entire installation of our cage took about three days.
1. The roll cage shipped as three groups of bars wrapped with metal banding. It is important to lay all the bars out on a flat surface and check them for flatness. The roof and main hoop are very susceptible to twisting during shipping.
2. There are a few specific measurements required for proper installation. The main hoop must be located no more than 6 inches from the back of the driver’s helmet and within 5 inches from the top. Any bar that can come into contact with the helmet must also be padded.
3. We marked the position of the main hoop on the edge of the inner quarter panel. We plan on welding the main bar through the floor to the suspension crossmember, so we just tacked it to the floor for now. Otherwise, the main hoop would have been welded to a 6×6-inch plate to the floor. For frame cars, all of the bars must be welded through the floor, directly to the frame.
4. For the rear down bars, you must locate the subframe. In the trunk, this can be done by locating these tell-tale spot welds. Before welding anything, the floor must cleaned up with a grinder, removing any rust, scale and paint.
5. The rear down bars have to run through the rear deck. Unless you custom bend these bars, you will either lose the back seat or have to notch it. We used the plasma torch to trim it out, trying to cut only what was required, keeping the structural integrity of the rear deck intact.
6. We used magnetic welding triangles to hold the bars in place so we could lay down some tack welds. The down bars should run parallel to each other.
7. Three tack welds on each bar will keep it from moving during the rest of the installation process. Placing these in a triangle pattern proved to be the best solution.
8. The next step is the seat crossmember. This bar must not be positioned above the driver’s shoulders and no more than 4-inches below. We tacked this bar in as well.
9. This is the best solution to installing the roof hoop. A long 2×4 and a pair of clamps. This part of the build is a lot easier with a helper, positioning and trimming the bar took several tries, so a helper saves time.
10. We used this really cool tool from PiperMaster to help us get the right angle for the upper hoop. The tool slides over the tube, and uses a bunch of needles to get the right shape. This is a complicated joint, and it needs to be a tight fit, you don’t want to fill gaps with weld.
11. The corresponding shape was marked on the pipes. It is really easy to get the pipemaster out of place, so careful handling is important. The pipemaster was slid back to the contact edges of the pipe so that the notch will fit. Keep in mind that the front edge of the tube will sit further rearward than it did before the cut.
12. Using the ESAB plasma torch, we made quick work of the 1 5\8”x0.083” mild steel tubing.
13. Once the big cut was made, we used a grinder to clean up the notch. We removed any slag from the plasma cutting and added a beveled edge to get a clean, flat weld.
14. The final notch has clean radii and the bevel. This should butt up to the hoop nice and smooth. Note the flat edge top and bottom- this is an important feature, as the inner radius will interfere with the overall fitment.
15. Then the upper hoop was repositioned in the car and held in place with the 2×4 and clamps. We put the hoop under tension to pull it tight to the roof and tacked it in place. Don’t remove the upper supports just yet.
16. We took a pair of the supplied 6×6 floor plates and cut them in half for the windshield down bars. We could have heated and bent them, but we just needed a simple angle, so it was easier to cut and weld. We used the Pipemaster to get the angle on the lower windshield down bars.
17. With the angles cut for the upper joint on the windshield bars, they were tacked in place. We covered the windshield with craft paper and masking tape. This protects the windshield from welding spatter. Nothing looks worse than a rusted windshield.
18. The next step was dropping the cage through the car. To do that, we cut out holes surrounding each of the four hoop bars (the main hoop and the windshield bars) with the plasma torch. You can do this before you set the main hoop, but then you have to support the main hoop from the floor.
19. The entire assembly dropped through the floor 6-8 inches. This makes it much easier to do the finish welding on the upper hoop. This is all you want to finish weld at this point.
20. Working inside the car, we used out MillerMatic 211 welder to lay down some really nice welds. This is a tough part of the build, there is not much room to get the gun between the roof and the main hoop, so take your time. A flashlight is nice to have to look for missed areas.
21. The upper hoop welds are critical for any roll bar\cage. You can’t have porosity or slag and most tech look for raw welds, not dressed and smoothed welds. You should keep a logbook of your installation for the inspection tech.
22. We raised the cage back into the car and supported it with the 2×5 and clamps. Then we slid the floor plates under the main hoop and welded them to the floor. The upper section of the cage was completed at this point, and the 2×4 support was removed.
23. The frame support bars were next. Since these will be positioned through the floor to the suspension crossmember, we held off on installing these until the rear suspension was completed.
24. The last step for the installation is the door bars. For a roll bar, only the driver side is required. This bar must cross through a point between the driver’s shoulder and elbow, and is usually positioned at the same point as the seat crossmember on the main hoop. The same 6×6 steel plates were used to secure the door bars to the floor.
S&W Race Cars
Miller Electric Manufacturing Co.
Esab Cutting and Welding