There are a few different Moments of Inertia (MOI) that influence on the performance of a golf club. In swing feeling the most important is MOI of the rotation of the whole club.
Moment of Inertia of a golf club is the physical measure of heft, the quantity that resists turning the club from cocked to released. It is a basic physical quantity that governs a golf club’s behavior. Any component of the club has its Moment of Inertia. The bigger MOI causes the more torque it will take to produce a certain amount of rotational acceleration.
Moment of Inertia is defined by the mass and the axis the mass is rotating around. Quantitatively, the moment of inertia of each grain of mass is its mass times the square of the distance to the axis.
At impact the whole club rotates around the hands somewhere near the wrist hinge. The method of matching individual clubs in one club set is about aligning the MOI of all clubs. The MOI matching is based on the assumption that the club in swing rotates around its butt. Therefore, the MOI value in relation to the axis of rotation located at the butt of the club is taken into account.
MOIG – BioMatch matching
Clubfitters also began to use another method called BioMatch. Important elements of this method are the values of MOIG and BMI. MOIG is an acronym for a Moment Of Inertia around the center of the Grip. Accordingly BMI is an acronym for a BioMatch Index. In general the MOIG matching is based on the assumption that the club in swing rotates around the center of its grip.
Obviously the geometric center of the grip (component of a club) is approximately 5 inches from the butt. Whereas the center of the grip (place where the player is holding the club) is typically located about 100 mm down the shaft from the end edge of a golf grip. So in MOIG matching the axis of rotation is located 100 mm (a little less than 4 inches) from the butt.
Obviously, the longer the club and the heavier the components (head, shaft, etc.), the higher the whole club MOI and MOIG will be. Conversely, the shorter the club, the lighter the components, then the lower the MOI will be.
Moment of Inertia is expressed in units of mass-times-length-squared. Clubmakers use pound*inch², kg*inch² or kg*cm². See conversion to other units on MOI units conversion subpage.
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update: November 2021