When you're talking safety, the first thing that comes to mind is a roll cage. It's fundamental to the safety of the driver of any race car, and greatly improves the chassis rigidity. If you're looking to get a cage into your car, you have a few options. You can get a bolt-in cage, have a fabricator weld in a cage for you, or you can fabricate your own. I'm partial to the latter, but any way you go, there are a few things you should know about cage building. In this article, I'll tackle some of the fundamentals. Look for articles in the future to address each area in itself.
Design considerations: When considering building a roll bar or cage, you need to consider your requirements first. Are you building strictly to a set of competition rules? If so, which rules? Is chassis rigidity more important than additional side impact protection? Is this a daily driver, a full-out competition vehicle, or a rock crawler? General design considerations such as those (and others) should be the key guidance to how you build your cage.
Competition rules: SCCA has a very clear set of competition rules, and many sanctioning bodies for road racing in the US use SCCA rules for their competitions as well. If you're planning on road racing, or just plain want to design a good cage, pick up a copy of the SCCA "General Competition Rules" or GCRs. An order form is at http://www.scca.org/interactive/merchandise.pdf. If you're planning on drag racing, you probably want to check out their rules, at http://www.nhra.com/contacts/tech_faq.html. The point here is that your sanctioning body may very well require a certain design, tubing type & thickness, and have other requirements for you to compete. Even if you don't compete today, you may want to compete at a later date, and changing an already-built design is a lot of work.
The tubing: When people think "roll cage," they usually are thinking of mild steel tubing welded into a car. The builder has a variety of choices of base metals including "mild steel", chrome moly 4130. Tubing comes both in different diameters, as well as a variety of thicknesses. These properties dictate the tubing's strength as well as it's weight. Your choice will be directed by the welding equipment you have available, the weight you're willing to live with, as well as the strength you want out of the finished product. You'll want to understand the trade-offs in each.
The bender: If you're like me, you'll want to do it all yourself, and that includes the bending of the tubing. However, it's not as simple as buying a "pipe bender" at Harbor Freight (I have lots of their tools -- not a bad store). You have to ensure you have mandrel bends in all your tubing. You see, the pipe benders will collapse the wall of the tubing, resulting in lost strength. Mandrel benders such as those from jd2 and Williams Lowbuck are the most economical way to get the correct tubing bends. I just purchased a used jd2 bender, and will discuss benders in detail in a later article.
The cage design: Cages provide two main purposes: protect the occupants and stiffen the chassis. The cage design needs to take those into consideration along with your other requirements. For instance, a tube with no bends is much stronger than a tube with one or more. Tringulation of the cage is very important to ensure stiffness. You want to tie the cage into areas which receive the most loads, such as shock towers and frame rails. This too would be a good topic to discuss in detail.
The builder: If you're building it yourself, you should be a pretty good fabricator and welder. If you're not building it yourself, you need to find a fabrication shop you can trust. Look for someone who understands the rules you're competing with. Not to say that a circle track guy can't build a safe cage, but is he familiar with your rulebook to ensure you pass tech? I would also recommend talking with the builder, reviewing his work, and talking with a couple customers. A good fabricator can take your design considerations and turn it into a cage that meets or exceeds your expectations. Cage fabrication isn't cheap; you want to ensure you're getting your money worth.
The welds: This is what holds all that tubing together, and the welds had better be good enough to withstand a large impact without cracking. Penetration should be thorough, all the way around the weld. I recall an episode of "Monster Garage" where a guy broke all the welds with his bare hands. Imagine what all that flying tubing might do to you if you are to get into an accident. Make sure the welds are strong and safe.
Conclusion: This was only a brief survey of some of the major considerations in building a roll cage for your car. Hopefully, this article, and the follow-on articles, will help you design and build a better roll cage.
