Originally Posted by
FBinNY
You may have that backward. The preload pressure is by it's very nature equal in both stacks. (Newton's Laws), while the forces generated by riding are mostly in the lower stack. (headtube flex acts like preload and loads both bearings equally, but steel head tubes don't flex much, if at all).
Very possible I do. I'm visualizing the forces due to leverage from the stem and ground, with the fork flexing because of the head tube angle, which appears to me to lessen the force against the top bearings relative to the bottom bearings. I'd need to draw a vector diagram to be sure, but visualizing it is more often right than not.
Assuming that the difference in forces is similar to the forces of the pre-load, that would account for the wear patterns being different from what you'd expect with equal preload.