MOI, or moment of inertia, describes a golf clubhead’s ability to resist twisting on off-center hits. If you hit a ball off the toe or heel of any golf clubface, the clubhead will twist to some degree, costing you distance and direction control. Equipment manufacturers seek to engineer the highest possible MOI into clubs from putters to drivers to make clubs more forgiving. Golfers should consider a club's forgiveness when purchasing equipment.
Forgiving Mis-hit Golf Shots
Frank Thomas, former technical director of the United States Golf Association, calls MOI “the moment of forgiveness.” He says that manufacturers generally use two methods to increase forgiveness at the moment of inertia. First, they try to move as much weight as possible away from the club’s sweet spot, the point on the face where the club doesn’t twist on impact. You can see examples of this type of engineering in heel-toe weighted putters, perimeter weighted irons and hollow-headed metal drivers. Second, they increase the size of clubheads, which allows designers to move weight further from the sweet spot. Modern drivers, for example, have 460-cubic-centimeter clubheads, with most of the weight located in the shell away from the sweet spot for greater MOI.
Perimeter Weighted Irons
You usually can spot an iron with high MOI by looking for weight distributed between the toe and heel and the top and bottom of the clubhead, creating a hollow space, or cavity, behind the sweet spot in the back of the club. Club makers measure moment of inertia in grams per centimeter squared. A perimeter-weighted, cavity-back iron probably has an MOI between 2,000 and 3,000 grams per centimeter squared. Club makers consider that an appropriate degree of MOI for a game improvement club suitable for most amateurs.
In 1986, Jack Nicklaus won his sixth Masters Championship title using a putter with an unconventionally large, heel-toe weighted head. Nicklaus’s putter was an extreme example of a high-MOI club at the time. Like other clubs, putters with high MOI are designed to resist twisting if you make accidental heel or toe contact. Just as with other clubs, to increase the moment of inertia, manufacturers move weight to the ends of the club and increase the size of the clubhead. Putter designs can incorporate very high MIOs, over 10,000 grams per centimeter squared, particularly in large-mallet clubheads. Club makers argue that you will find it easy to return the face of a high-MOI putter to square at impact because the club face will resist opening and closing during the stroke.
The Driver Controversy
High-MOI drivers are a source of controversy among golf’s rule makers. The large clubheads common to most drivers are not only more forgiving, they also allow golfers to hit the ball much farther. Moving weight way from the center of the clubface creates a spring-like effect that propels the ball off the face with greater speed. The United States Golf Association and other governing agencies have limited clubhead size to 460 cc in an attempt to control the distance tour pros and better amateurs hit the ball. However, Thomas points out that most casual players need to hit the ball farther and straighter. They could benefit from all the MOI forgiveness possible in a club, Thomas believes.
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