detail
Horseshoe Bearing Periodic Error:
Any bumps or depressions on the horseshoe bearing surface will result in movement of the optical axis: This error is solely a function of the size of the irregularity and the distance to the south bearing of the yoke. The shorter the distance from the horseshoe bearing to the south bearing the larger the angular error. The formula is:
error = arctan (size of irregularity / yoke length)
What counts of course is the movement of the primary mirror, not the horseshoe bearing. But it turns out that for all angles of declination and right ascension the angular movement of the mirror is the same as that of the horseshoe.
Quite small deflections result in dramatically large movements of the optical axis. But this is just the facts of life as applied to the horseshoe style mount with split ring territory being the worst case.
The drive train:
3/8" motor shaft to 1/2"x10" dia wheel, 3/4" shaft to 40.75" wheel - 1449:1 mechanical gain