Moment of Inertia (MOI)
The moment of inertia (MOI) is the measure of the resistance of a body to change its speed of rotation about an axis, then the higher the MOI greater will be the force required to put a body in rotation with respect to an axis, for against lower is the moment of inertia less force will be required to put it into rotation.
As an example that we all know just think of the skaters when they make spinning tops: when they spin with open arms they have a high Moment of Inertia and then rotate more slowly, when then they put their arms close to the body, the MOI collapses creating less resistance to rotation, and then allowing the skater to accelerate its rotation.
But as you insert the MOI speech in golf? The golf club may have various MOI, it depends on which axis of rotation is taken into consideration.
There is, for example, a moment of inertia for the entire club if it is considered as the axis of rotation the golfer that performs the swing, but there are also 3 different MOI relatively to the club head and two of them are very important in design of any golf club.
The first is when you hit the ball out of the center of the face: even if the head is attached to the shaft, it will tend to rotate around the vertical axis passing through the Center of Gravity (CG) of the club. In marketing terms, this is the value that determines the "forgiveness" of a golf club in case of shots taken outside the center of the face. The larger the head and more designers shift their weight on the perimeter of the clubhead, the higher the MOI of the head so the head will rotate consequently less and you will have less loss of distance in shots taken out of the Sweet Spot. Of course, just the opposite in the case of small heads with less weight on he perimeter.
The second one refers to the MOI of the club head in relation to the axis of the shaft. In this case it’s not the distance of the shot to be affected but the accuracy of the shots. Greater will be the size of the head or the greater the weight placed away from the shaft, to the toe of the head, the higher the MOI relative to the axis of the shaft, then the greater the tendency for a golfer to leave open the face at impact. In the case of smaller heads or more weight toward the heel of the club head it will be the exact opposite: the greater the tendency for a golfer to turn the face closed at impact. We can see it by analyzing many modern drivers where there is the possibility of change the weight position on the head: In fact we note that if we move it toward the heel we go in direction of the word DRAW while if we move it towards the toe we go towards the word FADE.
As I said before there is also a MOI for the entire club. The greater the length of the stick, the weight of the head, the weight of the complete club, the higher will be the MOI, and vice versa.
The MOI of the club is important to be able to have the same feeling of swing with all the clubs. In theory, if all the clubs have the same identical MOI player is more consistent as each club takes the same effort for the swing.