Locating Shaft Spine

By Carter Penley

There are varying opinions as to where the spine should be located in a finished club. Based on tests I had conducted years ago, I defined two planes in a golf shaft:  the droop plane and the swing plane. The droop plane is parallel with the clubface and the swing plane is perpendicular to the clubface. For our purposes, the spine of a golf shaft is the stiffest plane of the shaft.

Finding the stiffest plane can be performed by several different methods. One method is by using the Lockie board. When a shaft is placed on the board and bent, a state of equilibrium exists when the weaker plane is the bend plane. The stiffest plane (spine) is on the sides of the shaft and perpendicular to the bend plane. Equilibrium exists because the stronger plane is not in a state of compression and tension simultaneously (as it is when it is bent). Performing the same test on a long flat ruler is the easiest way to visualize this. A shaft with a very dominant spine resembles the ruler. The weaker plane bends easily and the “spine” of the ruler is on the sides since it is more difficult to bend.

Another method is using the Autoflex Machine. The Autoflex machine performs the same basic test as the Lockie board, but bends the shaft up instead of down. The Autoflex then marks the inside of the curved shaft (the weak plane).

The H.O.A.M. (Harmonic Oscillation Analysis Modeling) machine finds the spine in a much different and more precise manner than either the Lockie board or the Autoflex machine. It has a strain gauge that measures the force required to bend the shaft. The H.O.A.M. machine analyzes the force readings, finds the stiffest part of the shaft and prompts the user to mark that as the dominant spine. If the long flat ruler were placed in the H.O.A.M. machine, it would treat the unbendable side as the spine.

There are varying opinions as to where the spine should be located in a finished club. During the days of  Penley Sports, we conducted a series of field tests with the spine located in either the droop plane or the swing plane. And this is what we discovered in our extensive testing:

Someone driving the ball about 350 yards with the spine located in the droop plane gained only a yard or two when the spine was located in the swing plane. Based on drive distance alone, there is no significant reason to position the spine in the swing plane. Additionally, the players could feel and react to the club more easily when the spine was located in the droop plane, placing the weakest plane in the swing plane. It was much harder to control the club if the droop plane was the weaker plane.
Our second position is that by having the spine in the droop plane, the shaft has a more positive and consistent reaction at impact than when the weak plane is in the droop plane.

Based on the testing (with more coming shortly) the club performs and feels better with the spine located in the droop plane. Since Penley shafts are checked on the Autoflex machine which marks the weaker plane, graphics are placed on the shaft with Penley located 90° from the Autoflex mark (on the spine). Once again, visualizing the ruler, Penley shafts would be printed on the unbendable side, and the flexible plane of the ruler would be in the swing plane.

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