[Tom Kirkham]

Typical (for the 1960's) plunging half shafts are made up of a male and female splined coupling. One of the problem with splined couped half shafts, is as the coupling cycles between static (not sliding) friction to kinetic (sliding) friction, the abrupt change in the friction forces causes inconsistencies in the suspension. The coefficient of static friction is higher than kinetic friction. This causes a jerking in the suspension as suspension moves up and down under load. Ferrari solution was to to use ball splines (linear ball bearings) to couple the shafts together. Fords solution with the GT40 was to use Rzeppa joints (now known as CV joints) and Giubo joints. Before the days of mass production on CV joints, if I recall correctly, Ford was paying about the price of a new Fairlane for each joint. Ford only used a CV joint on the outer end of the shaft and a Giubo joint on the transaxle side. Interestingly GKN now makes a "Fixed Ball Joint – SIO plus Ballspline" which is a combination of a CV and a ball spline on one shaft.