Understanding Golf Club MOI

[golfdigest]

A reader named George joins me in confusion about MOI. Given that it's Hot List season, it seems appropriate to clear up the confusion. Or try, anyway:

Writer Max Adler in Golf Digest mag writes about Moment of Inertia of clubs. I need help in understanding this--my question is--MOI is measured about a particular axis--is the axis the shaft axis or is it the axis thru the face of the club? Where can I find articles that address the technical issues of MOI and center of mass and center of percussion, etc??

George, Crudely put, moment of inertia (MOI) is an object's resistance to twisting and it can be measured along any axis. An inner tube floating down a river has a very low MOI about its horizontal axis (it will spin in a circle easily) but a much higher MOI about its vertical axis (it would require much more force to flip end over end). When we talk about golf, the important MOI measurement, the measurement of which the United States Golf Association has placed a limit of 6,000 grams-per-centimeters-squared, is the horizontal MOIabout the clubhead's center of gravity. On a toed shot, the clubhead would have a tendency to twist clockwise about this axis. A larger clubhead with a higher MOI will resist this twisting more. MOI about the vertical axis of a clubhead (which resists twisting on high and low impact shots) matters, but is probably a less important a factor in off-center hit performance. MOI about the shaft axis is the third measurement possible with golf clubs. Golfers often refer to this MOI when they talk about how difficult it is to square a larger clubhead immediately before impact. The common complaint is that the larger clubheads tend to want to lag open. An egghead might insist this is more a matter of conservation of angular momentum, a closely related physical reality.

That said, all three of the MOI's share a direct relationship. The higher the horizontal MOI, the higher the vertical MOI, and the higher shaft axis MOI. The first two want to help while the third hurts your likelihood o fhitting a good shot. If you really want to delve deep, check out "How Golf Clubs Really Work and

How to Optimize Their Designs" by Frank D. Werner and Richard C. Greig. Max Adler

I am tempted to say, "You had me at inner tube," but that would only reveal my abject ignorance of these things. If, like me, you want to look at those new clubs flaunting their MOIs, check out the Hot List, and especially the gallery of

products that lead in Technology

.

--Bob Carney

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