Though it's been a while since we've seen Project Devin in KIT CAR, it's coming along nicely, and the latest addition is a full-width roll bar. Since I intend to run my '59 SS in vintage racing events, I want a bar that would satisfy the regulations and keep my skull off the pavement, should the worst occur. Anyone with a kit or replica vehicle who intends to drive quickly on the road or track should strongly consider adding a sturdy roll bar (if your car doesn't already have one).
If your car has a roll bar, don't assume it's sufficient for track use. Different racing groups require various roll-bar configurations, so be sure to study the regulations before heading out for the track. Being sent home on the trailer is a rotten way to start a weekend.
Some roll bars aren't strong enough to protect you in case of a rollover. For instance, many bars sold for pickup trucks come with a warning that they are for cosmetic purposes only. Many are made of poor-grade steel that has too little wall thickness or is too brittle to hold up in an accident. I have seen muffler pipe used in low-buck circle-burners, but the money saved won't touch the bill for a pine box.
Some bars are too small a diameter for the weight of the vehicle, or they may be improperly plated (chroming may weaken the metal). The most common cause of roll-bar failure is unsafe mounting, where the base of the bar is bolted or welded to a sheetmetal floor (or worse, fiberglass!) panel that will tear loose when the weight of the car is suddenly applied to it. I once saw a Firebird roll over at a track event, and the roll bar posts were sticking out of the bottom of the floorboards by a foot! The driver survived, but good luck is no substitute for proper engineering.
If you have a Cobra replica, you have it made since most of them came with roll bars, or they can be purchased from a variety of sources, such as Cobra Restorers (770/427-8658). Other kits may require custom installations like the one we came up with for the Devin. For serious road racing (like SCCA or NASA events) you will probably be required to have a roll cage that completely surrounds the driver and is reinforced with side bars to protect you in case you are T-boned (hit from the side, usually following a spin). These cages are triangulated with diagonal bars to prevent the structure from bending in a violent accident. For low-speed events, like autocrossing, you will seldom be required to have a roll bar at all, or the regulations may allow any form of bar. Vintage racing falls in the middle of these two extremes, with most clubs requiring some sort of roll bar in open cars. Once again, be sure to check with the clubs you intend to race with before building your roll bar. My favorite clubs require the same roll-over protection that was required when the car was built (in the Devin's case, 1959). Since the rules weren't very stringent back then, most clubs heavily recommend that the 1972 SCCA roll-bar regulations be followed instead. The SCCA no longer allows simple roll bars (now requiring cages), but their old regulations from 1972 are endorsed by most vintage racing clubs.
These regulations stipulate that the material must be ERW or DOM mild steel tubing or chrome moly SAE 4125 or 4130. Although stronger, chrome moly must be welded by an expert and "normalized" to relieve stress after welding. Good race car shops know how to do this (involving slow cooling of the tubing to prevent cracking). Mild steel tubing is both cheaper and easier to weld.
For a car that weighs less than 1,500 pounds (like a Lotus 7), the 1972 SCCA rules require 1.5-inch-diameter mild steel tubing with .120-wall thickness. Chrome moly can be thinner (1.375-inch x .090-wall). For 1,500-2,500 pound cars (like VW-based kits and some Cobra replicas), tubing sizes go up to 1.75-inch x .120-wall (mild steel) or 1.625-inch x .102-wall (chrome moly). Over 2,500 pounds requires 2.25-inch x .120-wall (mild steel) or 2-inch x .102-wall (chrome moly). Roll cages are generally made from 1.5-inch or 1.75-inch tubing. The Devin is expected to weigh in at 2,100 pounds, so I went with 1.75-inch mild steel with a .120-wall thickness.
Mike Minette at Minette Enterprises (Dallas, Texas, (972)241-7184) has built roll bars for everything from dragsters to road racers, so I felt comfortable having him do the job. I decided on a full-width bar that would slip into sleeves so that it could be easily slid out for body removal. The following photos show how he did it. KC
Minette used an adjustable angle finder to make sure the bend was exactly 90 degrees. | 
The bar was then laid on the floor while Minette found the exact center. This is critical to make sure the bar fits correctly. | 
Once the center had been marked, Minette marked the tubing for the location of the second bend. |