Strut Tower Brace Design Revisited

Sean coonhound at
Mon Feb 28 16:45:03 EST 2005

Amidst all the engineering talk about forces, rebounds, and moments, I must admit to being a bit lost.  However, I'm trying to follow- and thought some interjected points might be worth considering:
1.  Why does the stress fracture occur only at the OEM hole in the sheet metal?  Does that hole lessen the strength of the gussett?  It would seem so.  Also consider that the left side gussett has no hole- and no fractures, AFAIK.
2.  Why does the fracture always seem to pass dead-center through the hole (at least, mine does)?  Does this lend credence to the "hole lessens strength" argument?
3.  Is it possible that the fracture is not a direct result of suspension movements per se, but of chassis flex?  For example, torsional (twisting) rigidity of chassis?
4.  It would seem important to note that the close-up pic of my car's stress fracture shows that after the fracture, the two edges of fractured metal do not remain aligned.  Rather, the left or outbound side (as one looks under the hood at the fracture) is about 1/8 inch higher than the right side.  In other words, it looks as if someone grabbed the sheet metal on left side of fracture and pulled it up (though nobody did so).
I'm no engineer, but it would seem to indicate that the force causing the fracture (and causing the metal to move post-fracture) is similar to the force caused by trying to pull the strut tower away from the car (as opposed to pushing it in toward the engine bay).  A similar force could be exerted if King Kong lifted the car by the bumpers and tried to twist it (torsional force).  Torsional chassis forces occur during normal daily driving- even on well-maintained roads.
5.  If fracture is due to torsional rigidity (or lack thereof) in chassis, it would seem that a strut TOWER brace (as opposed to the strut brace) might fix the problem by increasing torsional rigidity.
Comments welcome.
Anyone who missed my pics can see them at:
It is difficult to tell that the edges of the fracture do not line-up in the close-up photo due to angle, but if one looks carefully, it is apparent.
Sean Coriaty
'91 200 20V Avant

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