If you go to this post in the hyperlink you will see the shallow center drop (step) on the older style (Top photo) vs the larger center drop (step) on the newer design (Bottom Photo). The larger center drop increases the cross sectional moment of Inertia and decreases the Hoop stress at the outer edge where the rubber meets the rim and the cracks develope. This is especially important at the inboard side of the rim, as there are no spokes as in the outboard side. Less support = higher bending stresses. Since I won't get my old cracked wheels back I won't be doing any destructive evaluation of rim. I still theorize that minor manufacturing flaws in the castings combined with repetative bending stresses eventually leads to a fatigue crack. This along with the impact loads (hitting a bump or pot hole) imparted by the stiff side wall of the RFT tranfered to the rim combine to propagate the crack growth.
The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to critical size. Therefore, it can be said that fatigue failure occurs in three stages – crack initiation; slow, stable crack growth; and rapid fracture.
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