Theo,
It's Easter, so I guess I'll be the list's Rust Bunny again. I was
going to refer you to Tom's excellent tech tip, but he did that himself.
Of all the areas insulted by the brownbonic plague on my first car, by far
the most interesting was the forward rear spring hanger / back-of-Xframe
interface. There is a tremendous difference in degradation potential here
for cars from 'salt' areas. The gentleman who dipped my car introduced me
to the term 'impacted rust'. This occurs where layers of sheet metal are
(usually spot) welded together like two (or three) pieces of bread in a
sandwich. Unfortunately, the 'filler' in these sandwiches is eventually a
saline solution which capillaries in there from road splash that you've paid
the DMV to doctor to prevent your wife from doing 360's in the Suburban.
The area in question is jammed with just these kinds of matings: the
triple plate that the spring bolt goes through (twice), the top of rear
frame to (vertical) rear floor pan area that Tom mentions, the winglike
piece that slabs underneath the back of the Xframe and then connects to both
the rear frame and the inner rocker, and finally (though probably least
important structurally), the little tab on the driveline side that you
found. It was my impression on dissecting (quite literally) my car, that
the cracking of that little plate may have alot more to do with weakening
and associated movement of the other components (especially radially in the
spring hanger itself) than with it's own inadequacy.
Chris Hill
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