Well, I think this is a very good post. Torsional rigidity of a ladder frame is not the best and why monocoque construction is much better for torsional rigidity. I have read where some of this ends up splitting the outside fender too.Thanks for the heads up Kurt. This is fascinating. I just went down and inspected mine too. What I found on mine is that there isn't any crack there, but if I look down from the top, I can find a subtle buckling in the sheetmetal, bowing inward toward the engine compartment on both sides. Just as everyone else here is showing, it's at the crumple zone closest to the firewall.
I'm running the ARB bumper with winch.
I have 3" of suspension lift and 35" tires.
I've carved crap out of the way to make the wheels clear all the way around, but in deep compression, I can still just get the top of the tire to kiss inside the well. It's a rare event, and I don't think that it imparts any force of significance.
Irrespective of bumper choice, the 4500 pound vehicle has all the ingredients necessary to flex the frame a bit when it gets into a twist. Although the body mounts are rubber, they still impart a force. If the bumper is brought up against the body, I'm sure it can impart even more force, but I don't think it's a deal breaker. The beauty of physics is that there is no "immovable object", there are just limits to the sensitivity of measuring tools.
This is flex fatigue. Note that the crack is happening ON TOP. Ask yourself how to apply a one-time force necessary to get the metal to TEAR at that location. It would have to be bent OVER something with down pressure in front and behind, while up pressure is applied beneath ... or some equivalent mechanism to get this to be a point of bending. That was the thinking in guessing about wheels hitting inside the well. If the body is supported in front and behind, and the wheel pushes up in the middle, that would explain it. Unfortunately there are crumple points that are much closer to the place over which the wheel touches, and if you push up inside the well at the point of contact, you can flex the actual well, without moving the stiffer support box above. The wheel wouldn't be able to do it.
That's NOT what is happening here. The twisting motion of the frame in offroad driving conditions is only partially being damped as it is transmitted to the body. The rest of the body has a lot more stiffness because of the height from floor to roof. The engine compartment is much flatter and therefore has less vertical stiffness. The body is getting "flexed" up and down (and probably in TWIST) just like the toe of a well broken in running shoe. This bending effect is amplified at the level where the low profile engine zone meets the high profile body zone. High flex meets low flex.
When metal is bent back and forth, it ANNEALS. This is a process of work-hardening that uses heat and compression to pack the crystal structures closer together. Annealing trades ductility for hardness. This is why medieval blacksmiths heated the steel bar red hot and hammered it thin before sharpening it into a sword. This is why bending coat hanger wire back and forth eventually warms it and then snaps it.
What you're seeing is an annealed flex point which eventually got too brittle to flex, and subsequently parted.
I think that the bumper may contribute to this to some degree, but my guess is that you've uncovered a design flaw that could become evident in any FJ after the right number of twisty trail runs. I'll guess that it happens sooner if the bumper design transmits more bending moment, but like the coat hanger... big bends or little bends... it eventually parts.
Ok... if that's the diagnosis, then what's the management?
My guess is that if you try to weld some sheet metal over that area, you'll just move the flex point to one of the welds and repeat the cracking process. You'd also de-engineer the crumple mechanism to some extent. I doubt it'll mean a thing if you actually wreck, but when your passenger is killed and their lawyer gets ahold of the investigator's report, you could be doing a lot of explaining. Realistically, for me, though... I doubt this would be a problem in the face of all the other massive offroad modifications I've got going on. When they get in underneath and see where I've actually cut and welded on structural points, I think the body issue would be pretty trivial.
My first thought was actually to go the other way. If this is where it's going to flex, let's not fight it, let's actually free it up to do what it has to do. I was thinking about actually completing the fracture by grinding out a quarter inch gap and then affixing a sandwich in the gap made of two pieces of sheet steel and a layer of urethane. However, I don't see that as being any better than the welded repair in the scheme of liability (which bothers me a little bit) and urethane has a habit of squeeking (which actually bothers me more). In the end, this seems like more work than it's worth!
Therefore, my plan will be to let it happen and then just leave it. If there has to be flexion at that point, then thats what it will be allowed to do. I looked at all the parts anchored onto the body forward of that location on both sides, and absolutely none of it would suffer. There's the radiator (which is attached to everthing else by flexible hoses) the battery (attached with wires) and some various other tubes and wires that I can't see having a problem with additional flex. The air system is rubber. My ARB compressor is on top of the wheel well, but its connected with wires and hoses.
If there's annoying flex and rattle after that, I'll weld on some supports and destroy the flexion point... and I'll just see what happens.
It's only body metal.
Adding more weight to the front with heavier bumpers and equipment only adds to the problem.
The best fix is to fix the frame and that could be done but it would take a separation of the frame from the body to achieve. Evidently the engineers understood this and changed the frame. How do you achieve more torsional rigidity with your ladder frame?
If you build a simple four sided frame out of wood, you can flex it easily. Add an X between two of the sides and it adds some torsional resistance. The bigger the X the better. You will notice that the FJ has no X between the frames.
Back to the wooden frame. If you add a panel across the whole frame and attach it to all of the sides, you add the most torsional resistance you can. I don't know if it could be done but welding sheet metal across much of the frame would add more torsional rigidity. Another issue would be the front body mounts to the frame. Like you said, it is obviously cracking at the crumple zones which have already been work hardened by the stamping process. Any push on the corners at the front will exacerbate the issue whether it is from body mounts or bumpers. It is the continual flexing that weakens the area, not one push.
You could strengthen the inner fenders which would reduce the crash worthiness to something like a Jeep.
BTW, this is not a serious structural issue but a design flaw. Ladder frames flex in torsion. The crumple zones are doing what they are supposed to be doing, flexing under pressure. They are just not designed to flex all of the time. When they do, like BellyDoc said, the work harden. Work hardened steel is much more brittle and can crack under stress. Just driving on washboard roads would be a problem. The inner fender is just a weak point in the design.
Just my two cents.
FYI: My 08 has a build date of 11/07. 97K and no bulge or crack. I bought it used and don't believe it has been off road.