MOI - Swing Weight and Dead Weight - What and Why?



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BY ERIC H. COOK P. Eng.

Swing weight, dead weight or total weight, static balance and dynamic balance are words that most golfers have heard but few understand, simply because a complete explanation of these terms is difficult to find. These characteristics of any golf club are directly related to how the club feels to the golfer and how well each club is matched to the other clubs in the set.

Most golfers think they know what they are getting when they buy a "matched" set of clubs, but they are usually quite wrong because, while they will have a set of clubs that all look alike, that is normally where the similarity ends. But then what do we really mean when we say "matched" clubs? Advertising would have you believe that "matched" sets of clubs all feel the same and all react the same way to your swing.

"During the backswing, and the start of the downswing, the weight and motion of the clubhead cause the shaft to bend; storing energy in the shaft. For the golfer to hit a good shot two conditions must be met during the downswing; the clubhead must return square to the intended line of flight at separation and the stored energy must be transferred to the ball for the golfer to attain maximum distance. Ideally, the shaft should be perfectly straight (unflexed) when the ball separates from the clubface."

Swing weight has been widely accepted by the golf industry and golfers for many years only because there was no better yard stick available. It has also been widely misunderstood, particularly by golfers who are led to believe that swing weight is a measure of sameness within a set of clubs.

Actually, swing weight is a static measurement taken with the club at rest on a balance scale, which bears little resemblance to the dynamic state of the golf club during the swing and impact with the ball. If we are to attain consistent feel throughout a set of clubs any measurement of sameness should logically incorporate a dynamic measurement rather than a static one.

Newton's second law tells us that Force = Mass X Acceleration which is the force created by a body in motion in a straight line. The Force created by a golf club travelling in a circle is represented by Angular Momentum which is equal to the Moment of Inertia X Angular Acceleration.

Since Angular Acceleration is directly related to how fast the golf club is swung the only constant in this equation is the Moment of Inertia. If a set of clubs is made so that the Moment of Inertia is constant, the resistance to the swing force will be the same with every club contributing to constant feel. Whereas swing weight is Mass x Length, moment of inertia is Mass x Length Squared. Therefore, if swing weight is matched, moment of inertia is mismatched, which actually detracts from a constant feel while the clubs are being swung.

iso-vibe inc, creators of the Swing-Sync System, developed a test procedure to determine the ideal moment of inertia for any particular golfer. These tests have shown conclusively that the standard approach to headweight selection, using the swing weight system, is inconsistent with matching golf clubs by feel. Contrary to popular belief, a person may not swing a lighter club faster than a heavier one. It depends more on the weight distribution of the club than on the overall weight itself, since moment of inertia relates directly to the clubhead speed that can be generated by the individual. In most cases golfers are swinging clubheads that are too heavy.

You may have concluded by now that this is a very complex subject which certainly warrants the continued research and development which we are doing in our never-ending search for better golf clubs. However, research indicates, very conclusively, that the feel of a golf club is related to shaft flex and moment of inertia, both of which must match the golfer for effective results.

Swing weight is being questioned by more and more clubmakers and will, we are sure, be replaced someday by a dynamic system that relates to a golf club in motion. iso-vibe inc. is a pioneer in this endeavour.

All of this information is condensed into a simple test procedure that will allow golf professionals to accurately select the frequency and moment of inertia for their customers. Combining these test results with other appropriate measurements, Swing-Sync USA is able to build custom fitted golf clubs for the customer that all feel the same.

But alas! What excuse will you have left?