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goose175 previously said:
BellyDoc,
Thanks for the thorough analysis. I agree in most parts with your statements.
The flexing causes the final cracking and fatigue of the metal sheet. This is for sure not caused by a single impact. But I still do believe that the tire causes the upward bucking of the rail. A flex would cause a rather constant torque load along the rail, rather than a point load at this crumble zone. Let me know what u think about this theory...
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I disagree with one aspect of this. Imagine tack welding a thin steel rod onto a heavy steel block. Then imagine flexing it up and down. The deflection will manifest as a curvature in the rod, but the fixed point at the weld will receive the most stress because one end is not moving at all. if it tears, it will tear at edge of the weld metal lump.
When you buy a garden hose, many of them are equipped with a rubber sleeve that slides over the screw-on connector for the hose bib. This is so that there will be a transition zone in stiffness, and one will be less likely to damage the hose-to-connector junction by accidently yanking it to the side. This is again a place of concentrated stress.
In the running shoe example I used earlier, imagine an old pair of running shoes that gets used till the sole physically rips apart. It tears at the point just under the ball of your foot. Again, the portion where the more flexy stuff meets the less flexy stuff... where the force is concentrated.
In your FJ, the body's passenger compartment has a great deal of floor to ceiling height compared to the hood and fenders. It's not going to flex. This is the same physical principle involved in constructing a bridge truss. The sides of the vehicle add stiffness. When the fender panels get pushed up because the frame beneath is flexing, they have less height and flex more. The force will concentrate at the base of this lever arm... where it connects to the stiffer portion of the body.
I have the ARB bumper which is being discussed here as having more gap and therefore less likelihood of direct force application to the body. I think Kurt is exactly right about this. Bumper contact will only exacerbate the problem.
However, we ALL have a body mount point just behind the bumper and in front of each of the front wheels. This is where the up and down force is being applied as the frame flexes. I can grab the edge of the fender panel when the hood is up and flex it with my own muscle. I'm sure the 4500 pound vehicle has much less problem than I do at this.
The next mounting point back is the one that sits on the bracket that people are chopping. Everything in between floats. The only POSSIBLE candidate for a source of force application between these is the tire, and if the tire was the cause, then it would flex the metal DIRECTLY over the contact point, or at the closest place to that location that had a weakness... it would NOT flex at a comparatively THICK point that was back against the RIGID FIREWALL.
Todd's picture of the metal buckling is exactly what I saw in mine. The reason that it's buckling is that it's been stretched by TENSION and has gotten longer. The only way it has to go as it lengthens is out of line to the side. It's getting PULLED on the bottom by up-flex, and its getting PULLED along the top with down-flex. As it flexes back and forth, it's getting annealed and it's growing less ductile.
Note that if you look at your FJ from the side, you'll see that this point is at the thinnest of the entire car. It's at the very back corner of the front wheel well. There is less than a foot of vertical height between the top of the plastic fender flare and the top of the fender panel where it meets the hood. The hood adds basically zero strength to the system.
Bernd
, man do I feel dumb. Do you have a wider angle so I can place it?
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