The preferred orientation, or planar oscillation plane, of a golf club shaft
is located by measuring the oscillation of the shaft when an impulse is applied.
Preferably, the out-of-plane oscillation is measured at a large number of angular
positions about the shaft axis, and the principal planar oscillation plane is identified
by that pair of opposed angular positions in which the out-of-plane oscillation
is smallest. The location of the preferred orientation may be marked on the shaft
and used to assemble a golf club with the planar oscillation plane in a predetermined
orientation. The straightness of the shaft can also be determined by deriving its
spring constant from its oscillation frequency and then measuring the restoring
force when the shaft is deflected by the same nominal amount at different angular
positions; differences in restoring force can be attributed to differences in actual
deflection distance resulting from lack of straightness.
