Cages are something that are kind of hard to analyze, so it comes down to what makes you feel comfortable/best judgement. There are some best practices that should followed to get the best results and I'll list a few:
- Keep your tubes as straight as possible and avoid bends
- Tubes are likely to buckle at bends and you are pre-yielding the material, so the tubing will undergo less deformation before it tears and breaks. If you must have a bend in the tube, make it at a node.
- Minimize unsupported spans
- The longer the section of tube is without other tubes connecting to it, the more likely it is to buckle.
- Make sure you merge your nodes
- You want the theoretical centerlines of of all the tubes that are coming together at a joint to intersect at a single imaginary point (node)
- BAD
- GOOD
- Triangulation!
- Triangulation creates stiffness and resists bending by keeping the tubing in pure tension and compression. Think about your design; if the welded joints were hypothetically replaced with spherical joints, would your cage still be rigid and just as strong? If not, you need more triangulation!
- BAD
- GOOD
- Use the best material you can afford
- 4130 > 1020 DOM (A513 Type 5) > 1020 CREW (A513 Type 2) > 1020 HREW (A513 Type 1)
- Attach your seats and belts to the cage, and attach the cage to the frame
- You want to stay with the cage if it separates from the vehicle, and you want the cage attached solidly and not just by some flimsy sheetmetal.
Now, there is a big difference between how a cage needs to be built to survive a high-speed rollover, end-over-end rolls, or multiple rolls versus your typical flop or half roll that occurs on the trail. For the more violent types of rolls, you definitely don't want to skimp on your cage and are going to want to follow all the rules I outlined above; however, I have seen poorly/under-built cages (including factory Jeep cages) survive impressively well during serious rollovers. In the end, anything is better than nothing, but your cage could very well be the thing that saves your life. So, how good is good enough? Well, only you can really answer that question, but I think you should attempt to build something that could survive a pretty serious roll over, even if you "don't do anything too crazy" with your flat fender.
Here are some examples of factory Jeep cages (crappy) holding up surprisingly well:
This guy rolled over two complete times on Black Bear Pass (Here's the in-cab video:
https://www.youtube.com/watch?v=UxcY...&frags=pl%2Cwn)
This guy rolled on Tomboy Road (Imogene Pass)
All that said, I would add an "X" to the main hoop behind the seats on the Rango cage or some other form of lateral triangulation at a minimum. I would also seriously consider putting an "X" in the roof panel as well to add more triangulation and help prevent rocks or other objects from protruding into the cabin when you are upside down. Some door bars would also be a really good idea to keep you in, objects out, and add yet more triangulation into the design. As for material, I think 1020 DOM is a happy medium between strength and and cost. If you can't afford DOM, at least get CREW over HREW since it only costs slightly more and is marginally stronger. For size, you could probably get away with 1.5" dia. .120" wall as light as a flat fender is, but I would probably go 1.75" dia. .120" wall myself if you aren't going to be making it out of 4130. Tube diameter is the main factor in the stiffness of the tube, and wall thickness is the main factor in dent/crush resistance. Sure, you can trade diameter for thickness to an extent to maintain bending strength with thicker wall, smaller diameter tubing (easy mathematical formula to determine equivalent bending resistance), but it takes a lot of additional wall thickness to make up for even slight reductions in tubing diameter. You can also use stronger materials to compensate for thinner walls and/or smaller diameter (again, easy to determine equivalencies using math), but unless you are building a race car where every last pound counts, it is much more economical to use larger diameter/thicker wall lower grade material. Also, like a good painter who knows when to put down the brush, you need to know when to put down the welder. I see a lot of people who use a whole lot of unnecessary material, or don't use their material effectively (excessive bends, missed nodes, lacking triangulation) and end up with a lot of extra weight up high in their rig (ironically making a roll over more likely
) and excessive material costs.
