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When fabricating your own roll bar, first consider that bars must be shaped on a special bender that does not wrinkle, kink or "neck down" the tubing in the bend area. A muffler shop bender will produce a bend with a curved area of a smaller diameter than the rest of the tube—and this is not legal with any racing group. It is also not legal to heat the tube and bend it. All bending must be done "cold." The radius of each bend is important, as too tight a radius will weaken the bend. The radius should be at least 3 1/2-times the diameter of the tubing.
Mock Bar
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We recommend that you start your roll bar project by buying PVC pipe and bends in the size you have selected for your actual bar to create a mock-up. First mount your seats in the position you will be driving in. Driver comfort is of primary concern. Set up your PVC tubing and check for clearance around the driver's head. No tube member should be located where the driver's head can make contact in the event of an accident, since serious injury could result even if a helmet is worn. With the driver properly belted in, you will need at least two inches of clearance under the top of the bar to the top of the helmet.
Also check for locations to mount your harness, and make sure nothing is interfering with the shifter or parking brake. All tubing that could be contacted in an accident must be covered with foam cell padding available from racecar part suppliers. A headrest or tall seatback should be used to prevent the driver's head from snapping backwards.
Reinforcement
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Your bar or cage must be firmly anchored to the main frame structure of your car. If your car has a separate frame then the bars should be welded or bolted to the main frame members, not small auxiliary tubing or sheet metal panels. TIG, MIG or arc welding is best for roll cages and bars, as they provide superior penetration and strength.
When a car is inverted in a crash, the bar or cage must be able to hold up the weight of the car as it is dropped onto the bar, and then continue to hold as the car slides on the bar, often at high speeds. There will be a lot of force trying to flatten the bar, so it should be reinforced diagonally and longitudinally.
With a driver's side hoop, some sort of fore and/or aft braces will be needed to keep the bar from folding over. Some bars have one forward bar that starts on one side of the hoop and extends down to the passenger-side foot-well area where it ties into the mainframe members. Others have a rearward-facing brace that also ties into the mainframe tubing. For this configuration to work there must be a convenient frame member strong enough to withstand crash loading.
For a full-width roll bar, a diagonal bar will be needed to triangulate the structure. Usually two fore or aft braces (or both) are used to keep the bar from bending over. The braces should always be the same size and wall thickness as the main hoop. Professional racecar builders will add steel gussets to the tube junctions for even more strength.
Side Beams
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If you intend to do wheel-to-wheel racing you may want to add side beams or bars to your structure. These are located behind the doors and are intended to protect the driver in case of a "T-bone," (side impact). NASCAR cars have six or more huge side bars to keep 3,000+ pound stockers from getting inside, but a lightweight sports car can make do with less. Side bars may also tie into a tube that runs behind, or just below, the dashboard for even more stiffness.
Side bars stiffen the frame and can make the car handle better. A stiffer frame also makes chassis tuning more precise by providing a stable platform for the suspension to react against. If the frame is not rigid enough, stiffening the suspension with springs or a sway bar will only cause the frame to deflect more, partially negating the adjustment. By stiffening the frame, small changes in suspension adjustment can easily be felt by the driver.
Removable Bars
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Since many sports cars are dual-purpose street and racecars, a removable roll bar or cage may be a good option. Roll bars may also need to be removed to facilitate body panel installation. There are several ways to accomplish this. Roll-bar tubing can have 3/16-inch thick mounting plates welded to the bottoms, and this plate can be bolted to the frame with Grade 8 SAE or NAS (aircraft) hardware. If one tube slips inside another to facilitate removal, the inner tube must bottom in the outer tube and be secured with two bolts and nuts (welding the nuts to the back of the tube can facilitate removal).
Removable braces can attach to the main hoop with 3/16-inch thick double-lug mounts and either NAS bolts or quick-release pins available from race car and aircraft part suppliers. Bolts should have a grip (the smooth section between the head and the start of the threads) that extends clear through the lugs so that no threads are loaded in a shear application. For added strength, these braces must have bolt-diameter tube sleeves welded into them so tightening cannot crush the tube, and then the main tube ends capped. Don't bolt braces directly to the main bars as the bolt holes weaken the structure and provide a place for stress to start in an accident.
The body on many sports cars will have to be pierced for the roll bar to fit through. There are many ways to do this and maintain a neat appearance. Sometimes a brace can be made removable, or a slot or notch can be cut in the body or trunk lid.
Many sports cars have ready-made bars that can be purchased off the shelf and installed. Others will require fabrication from scratch. Get an experienced fabricator to bend and weld your bar, and make sure it's a proven design—your life may depend on it.
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