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MOI matching is a snap when you use eight-gram increments.
By Jeff Sheets, Vice President of R & D at Golfsmith

Every year we see golf technology changing at an increasing rate that general consumers are unable to keep up with. The evolution from persimmon-shaped woods to the new variety of square, bullet-shaped and triangular heads is one of the most obvious examples. I have often touted that golf equipment design is no different from fashion design. A keen eye can easily tell the model year of a club because of its design cues, simple elements like sole widths, the lines on a toe or the shape of a topline. However, alongside changes in the physical appearance of equipment, often there are invisible factors changing, too. For example, one non-visual element we’re changing in most of our 2008 iron models is the weight of our iron heads.

For more than 75 years, iron heads have been separated by a 7-gram weight increment between each loft and the next. This progression allows identical swingweights among clubs with half-inch length increments. The swingweighting system has worked well for many decades, so why fool with it? You must understand that there is a method to the madness: Because we have recognized the advantages of moment of inertia (MOI) matching, a process that head weights have a direct influence on, we are changing the weight increments on most of our 2008 iron models accordingly.

MOI matching actually predates swingweighting, but it involves some tricky mathematics to arrive at the appropriate head weights, shaft lengths and overall club specifications. With the development of the swingweight scale in the 1920s, swingweighting became the club fitter’s shortcut to MOI matching. Clubmakers seeking a rapid assembly procedure have stuck with it ever since.

In the past few years, MOI matching has become popular again because of high-tech electronics that enable a club fitter to perfectly fine tune a set without dealing with the arithmetic. In fact, for the past year the Auditor MOI Scale (No. 245246) has been one of the hottest-selling tools we offer. In using the MOI scale, one piece of information has become very apparent — MOI matching a set is more easily accomplished by using 8-gram increments between the heads than by using the traditional 7-gram increments.

Using half-inch length increments and 8-gram weight increments between irons leads to an MOI matched set. So how does this compare to a swingweighted set? None of the elements of the MOI matched set will differ except for the swingweights. Instead of the swingweight being constant, there will be a progression: Each club will swingweight a half-point heavier than the next-longer club. For example, if you assemble an MOI matched set with the 3-iron as your longest club at D0, the swingweights will increase successively to D0.5, D1, D1.5, etc., until you reach the pitching wedge, which will be D3.5.

As you can see from the chart, the longer irons in an MOI matched set are lighter, and therefore easier for most golfers to swing, than those in a traditionally swingweighted set. At the same time, the shorter irons are heavier in the MOI matched set. As the shaft becomes shorter and lighter, the heavier mass in the head provides better feel while placing more mass behind the ball at impact.

Change is good when it helps achieve the objective at hand — in this case, controlling distance and consistency. In a recent focus group, test shots were compared between a swingweighted set and an MOI matched set of otherwise identical irons. Using a TrackMan™ radar system to measure shots, we found that short iron distances were identical between the two sets, while the long iron shots showed a much greater distance disparity between the two. All participants hit the MOI matched long irons at least a half-club farther than their swingweighted counterparts. More importantly, the yardage dispersion between the MOI matched clubs was much more consistent than the yardages among the swingweighted set. Overall shot-making quality was measurably better with the MOI matched set than with the swingweighted set.

The 5-irons in our sets will continue to weigh 254 grams, just as they have in years past. Longer irons will be lighter than previous versions by a gram or two, and shorter irons will be heavier than the old models. A change like this may appear radical to some club fitters. However, we view it as another technology improvement that all golfers will benefit from. A number of the major OEMs have also recognized the merits of the 8-gram increment, but they are not talking about MOI matching. Instead, they focus on the merits of progressive swingweighting. Regardless of what you name it, this set configuration leads to a better performing product for all levels of golfers.

Previously existing models will continue to be offered in 7-gram weight increments. Using these older designs makes traditional swingweighting available to those club fitters who prefer that technique. However, we anticipate that 8-gram increments will be the iron spec of the future.