An explanation of why the arm supporting the rotor drops (i.e. a increase in the angle
) during
precession
will be made with reference to the following set up (although a similar
explanation is still valid when dealing with other set ups such as that
in video 7).
. It can be seen in the following video that as precession occurs the angles
and 
increase until
the arm is resting against the stand and so
is effectively clamped, thus the gyroscope no longer precesses.Video showing how the angles changes during precession
Explanation of motion
A similar dropping motion is also observed in other videos, and so an analysis of this motion carried out.Go here for mathematical analysis of motion.
From this analysis it is found that the dropping motion of the arm is caused mainly by friction in the bearings of the upright. If the arm is released from horizontal the analysis of the motion gives the following expression for the initial change in theta.
Where QF represents the friction in the bearings of the upright, and Q3 friction in the rotor.