“Moment of inertia,” or MOI, is a property of physics that indicates the relative difference in how easy or difficult it will be to set any object in motion about a defined axis of rotation. The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion. Conversely, the lower the MOI, the less force needed to make the object rotate about an axis.
To understand MOI, think of a spinning ice skater. At the beginning of the spin, the skater extends her arms and the rotation speed is slow. As the skater pulls her arms in closer to her body, the speed of the spin greatly increases. Thus when the arms are extended, the skater’s Moment of Inertia is very high, and the result is a slower spin because the high MOI of the skater is resisting the speed of rotation. Conversely, the reason the spin speed increases when the skater pulls in her arms is that as the arms get closer to her body, the skater’s MOI falls lower and lower, creating less resistance to the rotation.
There are several different moments of inertia that are factors in the performance of a golf club. Remember, MOI has to first be defined by identifying what axis the object is rotating around. There is an MOI for the whole golf club which, when swung, is “rotated” around the golfer during the swing.
There are also three different MOIs which can be measured for the clubhead itself. Two of these MOIs are important in the design of any clubhead.
First, when you hit a shot off the center of the face, even though the head is secured to a shaft, the head will try to rotate around the vertical axis going through the clubhead’s center of gravity. Second, and at the same time, when the golfer swings the club on the downswing, the clubhead is rotating around the axis through the center of the shaft.
The first example refers to the MOI of the clubhead about its center of gravity. In marketing terms, this is the head design property that has a bearing on the amount of “forgiveness” a clubhead offers for off-center strikes. The larger the clubhead, and/or the more the designer incorporates perimeter weighting, the higher the MOI of the clubhead about its center of gravity vertical axis will be. The higher the MOI of the head about its vertical CG axis, the less the head will twist in response to an off-center hit, and the less distance will be lost from that off-center hit.
The smaller the head and the more head weight is positioned close to the center of the head, the lower the MOI of the head will be around its vertical CG axis, and the more distance will be lost when the ball is hit off center.
Again, higher MOI equals more resistance to the object being rotated around an axis; lower MOI equals less resistance to the object rotating around an axis.
The second example refers to the MOI of the clubhead about the shaft axis. Little is spoken about this MOI in equipment marketing, but it is an important head design factor that can affect the accuracy of the shot, not the distance. The bigger the head or the more weight that is placed far out on the toe of the clubhead, the higher the MOI of the head will be about the shaft’s axis. The smaller the head or the more weight that is positioned in the heel area of the head, the lower the MOI of the head will be about the shaft’s axis. The higher the clubhead MOI around the shaft, the more tendency there is for a golfer to leave the face open at impact. The lower the clubhead MOI around the shaft, the more tendency there is for a golfer to rotate the face more closed at impact.
As stated earlier, the whole golf club also has an MOI. The longer the club, the heavier the head, the heavier the total weight of the head, shaft and grip added together, the higher the MOI will be for the whole club. Conversely, the shorter the club, the lighter the head, the lower the weight of the head, shaft and grip, then the lower the MOI will be for the club.
The MOI of the club is important to matching the swing feel of all the clubs in the bag. Clubfitting theory states that if all clubs in a set are made to have the same, identical MOI, the golfer will be more consistent because each club will require the same effort to swing.
The current method for matching clubs in swing feel is called swingweight matching. Swingweight is an expression of the ratio of the weight in the grip end of the club to the weight in the rest of the club on down to the clubhead. Swingweight-matched golf clubs are not matched for MOI, but come relatively close to MOI matching. MOI matching of clubs is a swing matching system currently offered only by more advanced custom clubmakers. You can learn more at centerofgravitygolf.com