Originally Posted by
Paul_P
It seems to me that the axis of rotation can't be anything other than the steerer tube itself. Handlebar grips move on a single circle centered on and perpendicular to the axis of this tube and can be in front, behind or beside the axis. The diameter of the circle is the shortest distance between a grip and the steerer tube axis.
That's basically it. The important thing is the relationship between the axis of rotation (the steerer tube) and the the grips are. The stem/handlebar configuration doesn't directly affect the handling, only the location of the grip position relative to the steerer. You would get the same handling with a backwards stem and flat handlebar as you would with a forwards stem and a swept back handlebar, as long as the sweep put the grips in the same spot.
You can't move the axis, because it is welded/brazed/bonded firmly onto your frame. If you need to find the axis of rotation of a part on a bike, it is usually where the bearings are.
The idea that the shape of the connection changes the behavior, even though the relationship between the axis of rotation and the point you're applying force to is the same, is the one that brought us L-shaped crank arms like
these. It's been pretty thoroughly disproven.