I'd like more details on the pic that Skycrawler posted. That had to be a pretty damn hard roll or he used conduit for the cage instead of mild steel tubing. My 92 XJ looked about the same (opposite side being crushed though) with no doors or cage and that was a pretty hard roll. If that cage was made out of HREW or DOM of a decent size then it must have been a rather violent roll.
When it comes to cages for your typical recreational wheeler I am a firm believer in using DOM on the main hoops and HREW on triangulating pieces. I also believe strongly that 1.75x.120 tubing should be used on a free standing cage while 1.5x.120 tubing is acceptable on a cage that is properly integrated into the structure of the vehicle. To break is down into our terms 1.75x.120 for a Wrangler and 1.5x.120 for an XJ assuming that the cage on the XJ is tied into the structure of the XJ. The pillars of the cage should be stitch welded to the pillars of the jeep using connecting gussets and the tubing should be as tight as possible to the body, either internally or externally. Another very important thing is the mounting points themselves. Floor plates should be at least 1/8" thick and of sufficient size to distribute the load being transferred to the floor. If you are welding the plates to the floor then only a top plate is needed. If its going to bolted to the floor then you need a bottom plate too. The plates should be of different size so as to not cause a sheering effect on the floor.
The advantage of the XJ being unit body is that you can basically make the cage and the body one piece and they're reinforce each other.
I also like to look at the rule books from sanctioning race bodies. SCCA specifies that 1.5x.120 wall DOM is acceptable for vehicles weighing 2700lbs or more, however they do not allow ERW of any type. Ultra4 specifies that for vehicles of our weight 2x.120 is needed, they don't allow HREW but do allow CREW.
I do believe in a bit of common sense however too. I'm not running my Jeep down the open desert or around a track at the speeds the SCCA and Ultra4 vehicles are.
The WERock stock modified class rules are probably the most applicable to us. They limit your tire size to 35" which may or may not be smaller than what we run, but the type of wheeling is similar and/or beyond what we do on our trails. They go slow over big rocks and obstacles, we go slow over little ones.
WERock Stock Modified Class Roll Cage rules state:
4.16.1:
Six (6) point mounting cages covering the driver are required.
4.16.2:
OEM bars are approved for a portion of the roll cage.
4.16.3:
Handles are required on the interior portion of the roll-cage or vehicle.
4.16.4:
Round steel tubing (D.O.M Preferred) 1.5” O.D with 0.120” wall is compulsory for the
basic roll cage. Aluminum and/or soft metals are not permitted. Roll bar construction must be
welded. A W.E.ROCK official must approve roll cages made of other material or in other wall
thickness/diameters.
4.16.5:
Connection positions of the roll cage must tie in to the frame of the vehicle; Body mounts
are considered a tie in point.
4.16.6:
The front-most position must be no farther toward the rear of the vehicle than fifteen (15)
inches behind the throttle and brake pedals.
4.16.7:
The Cage must have a space no wider than 24” above the driver’s head, and at least 1
spreader bar between the front main bar and rear main bar are required unless the cage top is
24” wide or less.
4.16.8:
Gussets must be welded in the four corners of the “halo”. Gussets may be tubing or plate
steel.
4.16.9:
A minimum of.040 magnetic expanded or flat sheet metal, or 1/8” aluminum, must cover
the area immediately over the driver seat and be welded or bolted to the roll cage. Steel tubing
must surround the roof panel.
4.16.10:
W.E.ROCK recommends a spreader bar to be mounted under the dash area to connect
the right and left “A” pillars.
4.16.11:
If doors are not ran, a bar running from the “B” pillar, at approximately shoulder height, to
the “A” pillar, at approximately shin height, must be ran. This can be a bolt in piece.
4.16.12: A “periscope bar” (a bar sticking straight up from the roll cage) is not allowed
Take it for what its worth, and everybody has a different opinion, evening different sanctioning bodies. In the end unless you are competing you can do what ever you wish and they type of wheeling you are doing will dictate how much protection you really need.