One point made that I disagree with as a hard and fast rule. The main hoop does not need to be vertical. The horizontal portion of the main hoop should be in a place where it best protects the driver and co-driver, but if the legs of the hoop need to be at a 30° angle from vertical to better tie into the vehicle's structure then that is how it should be built.
Case in point, if the main hoop in a Baja Bug cage is leaned forwards at the top then the bottom of it's legs can be tied into the rear torsion housing. Which is the most rigid part of the entire stock vehicle. If you insist on the vertical rule then all you have to tie the legs into is a single thickness piece of sheet metal or the middle of a horizontal tube - which violates the "No Mid-Span Junctions" rule. Clearly to lean the main hoop forward will require good triangulation for it to be strong, I contend that done right it will be a stronger structure than a vertical main hoop regardless of what the legs are tied to.
Regardless of orientation it does need to be triangulated. You have to quit thinking right angles (orthogonal, rectilinear, however you want to phrase it) and start thinking in triangles.
I see a LOT of what I'll call "Sport Cages" built with massive triangulation in the over-head portion of the cage and little none in the rest of it. Not only useless, but it makes no sense. I'll agree with 68Ford's statement, most cages that I've seen in EB's are better than nothing, but not by a lot.
IMSA is road-racing, not circle track racing.
I totally agree. I think we need to delineate a few things for the sake of the group, especially the new guys.
I see a lot of people referencing race sanctioning body rules to drive cage design. This is not a bad place to start since there is no guidelines for street driven classic vehicles. However, we need to keep in mind that these rules were written with the hobbyist or fabricator in mind. The rules masked the engineering or empirical data involved and provide guidelines. Good for most folks, but that doesn't mean you can't "violate the rules" if you know what you're doing.
Where I take issue, is the data that is masked by the "rules" keeps people focused on the B-pillar hoop. While the B-pillar helps form the backbone of the cage in most cases, that is not where the action is at.
Most of our vehicles spend their life on the street. Some spend the majority of their time on the trail, but rarely on a race course. Therefore we should be looking at the street-rollover data and building from that. Thankfully the .gov helped us out by doing all the research and developing Federal Motor Vehicle Safety Standard 216 (FMVSS 216) which governs Roof Crush Resistance of new vehicles. If you read the data behind this standard, you would see that the vast majority of high-speed rollovers impact the A-pillar at a prescribed angle, with 3-4 Gs of impact force. Look at the example photos below:
My cage was designed around this reality. I also could not put X-bracing in the B-pillar hoop, which violates many race-cage rules, but those guys don't have passenger access issues. I was able to get around this by angling the C-pillar forward and creating what amounts to 2 "hoops" that form an "X" when viewed from the top. The cage is tied to the floor and the belt-line at 6x locations each. The A-pillars now get crush resistance from the opposite C-pillar connections. Overall the cage is solid yet doesn't conform to what most consider good practice.
So take all of our advice with a grain of salt and do your research, because ultimately you are responsible for your own safety.
;D
