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If everyone struck a putt on the horizontal center of gravity location of the putter face there would be no need for putter head playability factor. We all know this is not the case. The actual ball impact point on the putter face is both a function of the golfers’ ability and how long the putt is. It is a fact that better players usually hit the ball more consistently close to the center of gravity and for most all players, the longer the putt, the harder it is to hit the actual putter face center of gravity.
A term that has been commonly used over the years is “sweet spot.” I think by now all golfers realize that the size of this so-called sweet spot varies from club to club, whether we’re talking about putters, woods or irons. There are a number of factors that help to determine a putter head’s sweet spot. However, there is one very significant factor that determines this. That factor is the putter head’s Moment of Inertia (MOI). MOI is simply defined as a measurement of the putter heads resistance to twist or turn when acted on by an outside force. An example would be a putt struck off-center or more correctly a putt not struck on the putter head’s horizontal center of gravity location. This will make the putter head twist or turn at impact. The farther off-center the hit, the more twisting that occurs. If the hit occurs on the center of the face, then no twisting occurs and MOI has little meaning. The more twist that occurs the more the putt will usually be off the intended line.
Moment of Inertia (MOI) is mainly responsible for the size of a putter’s sweet spot.
This brings us back to sweet spot. The higher the MOI, the larger the sweet spot and conversely the lower the MOI the smaller the sweet spot. Obviously, it would be very important for a player who has trouble hitting the putt consistently near the center of the putter face to use a very high MOI putter head. Common features of high MOI putter heads are head shapes with more material in the heel and toe areas, longer putter head lengths and heavy weights such as brass, lead or tungsten added in the heel and toe areas.
The Maltby Playability Factor (MPF) places putter heads in different categories. These are the similar categories that are used in iron head MPF but putter heads are calculated differently. Since the MOI is mainly responsible for the size of a putter’s sweet spot, it is the predominate mass dimension used for determining the putter heads MPF. The MOI is taken accurately and measured in inch-ounces squared or grams per centimeter squared. This MOI number is used, along with other weight distribution data to calculate the MPF category the putter head fits into.
Putter head playability based on Moment of Inertia and other mass and dimension properties was thoroughly tested and proven using actual robotic putting machines and ultra high speed photography. Hundreds of GolfWorks School students visited the Golf Club Design studio throughout the year and were actually shown these capabilities with live demonstrations. Every school student came away with a much better understanding of how putters work, how to fit putters better and to say the very least, they were all amazed at the differences in putter playability from the various MPF categories.
The complete method and all explanations for determining putter head MPF will be published soon in a new book titled The Maltby Playability Factor, Understanding Golf Club Dynamics – Book Three, Putters. Until the book is published I have included on this website a listing of putters and their MOI’s that can be compared by the reader. This listing will be updated periodically as new putter heads are measured.
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See charts that illustrate the relationship between MOI and putter sweetspots.