"THE GEAR EFFECT" How And Why It Happens
by Eric Cook. P.Eng. GSEM. PCS.

As published in Flagstick Magazine

The gear effect is more easily understood if a few basic principles related to the Physics of Impact are explained first. I have met very few golfers that really understand how a golf ball is hit. That may sound like a strange statement in view of all of the books, video tapes, Golf Channel instruction sessions and other sources of information that are available to you. Let me explain.

Most golfers, experienced or not, believe that the clubface is SQUARE to the target line at IMPACT and that you contact the back of the ball during a perfect stroke. As I proceed, you will see that this is really a false assumption because the truth is that THE CLUBFACE SHOULD BE SQUARE TO THE TARGET LINE AT SEPARATION NOT AT IMPACT. The diagrams shown in figures 1 and 2 illustrate proper ball contact and separation positions.



Figure 1 shows the club path coming from the inside to the outside, as it should be for a properly executed stroke. The clubface is approximately two degrees open to the target line when the ball is struck by the clubhead. The ball stays on the clubface for about 5/10,000ths of a second, during which time the clubhead travels 3/4 of an inch to the position shown in Figure 2. This is the point of SEPARATION at which time the ball leaves the clubface going towards the target. Under these conditions, if you hit the back of the golf ball with the face square to the target line, the clubface will be closed by the time the ball separates from the clubface and the ball will hook or at least go to the left of your target.

If you have set up to the ball correctly, with the clubface square to the target line at address and the ball positioned for a Wood Head so that the inside edge of the ball is lined up with the centre of the clubface (see Figure 3a), execution of the swing as described above will result in a dead centre hit on the face of the club (Figure 3b). This dead centre hit should send the ball down the target line as planned with no side spin imparted to the ball. The ball is picked up by the clubhead and carried on the face of the club for about three quarters of an inch to the position shown in Figure 3c at which time the ball separates from the clubface and proceeds down the target line.

If everything has gone according to plan, you will have hit a straight shot towards the target, but what would have happened if you had hit the ball on the heel or toe of the club?

I am sure that you have, at some time, hit a ball on the toe of the club, saw it start out to the right of your target and mysteriously draw back to your target. If so, you have witnessed the GEAR EFFECT that I mentioned earlier.The GEAR EFFECT is the result of an off-centre hit because a centre hit should result in a straight shot as indicated by Figure 3c. A dead centre hit should not create side spin and the ball should go straight.



The Laws of Force and Motion dictate what will happen to both the clubhead and the ball if the ball does not contact the clubface in line with the centre of gravity of the head (see Figure 4). A common misconception has golfers believing that the ball hitting the toe of the club, as shown in Figure 4, will force the clubface to open causing the ball to go to the right of the target line. Fortunately that’s not true. The rotational motion of the clubhead is due to the reaction of the ball leaving the clubface.



Newton’s 3rd Law tells us that "for every action there is an equal and opposite reaction"; therefore, as the energy transferred to the ball reacts with the clubface, the ball begins to separate from the clubface. The reaction to this separation from the clubface forces the clubhead to rotate around the centre of gravity in a clockwise direction for the toe hit illustrated in Figure 4. Since the golf ball is still in contact with the club, the clockwise rotation of the clubhead imparts a counterclockwise rotation to the golf ball (see Figure 5). This is the GEAR EFFECT.

A basic rule resulting from the Laws of Force and Motion tells us that THE BALL ALWAYS GOES WHERE THE CLUBFACE IS POINTING WHEN THE BALL SEPARATES FROM THE CLUBFACE. That means that a slice is the result of an open clubface at separation and a hook means that the clubface was closed when the ball left the clubface.

Depending on the location of the centre of gravity of the head and the amount of horizontal face bulge built into the clubface, the golf ball may leave the clubface to the right of the target line on a toe hit, but the GEAR EFFECT will normally cause the ball to move left, back towards the target line. Of course the reaction of the ball also depends on the club path so for the purpose of this article we must assume that the clubhead was swung on the ideal inside to outside club path.

The GEAR EFFECT happens to some degree on all off-centre hits and is a direct result of the Laws of Force and Motion and the effect is usually more dramatic with Woods than it is with Irons. The one thing that you can be sure of is that THE GEAR EFFECT works and is the cause of those mysterious draws and fades that so often bring your errant shots back to the target line.