Over 100mph, my right mirror would fold back.
The problem is the stem protrudes past the bottom of the mount hole,
so tightening the nut does nothing, just spins the mirror.
I tried using a shim in the mount hole to keep the stem higher,
but that required a better fit than I was willing to spend time on.
I added a washer at the bottom of the stem that just fits over the stem.
Now the bottom nut tightens up and the mirrors don't move.