Traditional sets of irons are built to a certain swingweight established by the manufacturer. By building sets that employ shafts of different flexes to an identical swingweight, the manufacturer can keep the difference between flexes exact. In order to maintain identical swingweight throughout the set, the assembler of the clubs uses an age-old formula: The head weight increases by an identical amount as each golf club in the set gets shorter.
Clubs that are moment of inertia (MOI) matched are assembled in the same manner, except for one detail. With MOI-matched sets the swingweight is progressive throughout. On average, each headweight increases by 8 grams per half-inch reduction in length, compared to 7 grams. The result is a set of irons in which the MOI of each club is constant and the swingweight is progressive, i.e., the swingweight increases as the irons get shorter.
In the long irons or short irons, MOI-matched sets may feel different compared to traditional swingweight-matched sets, or you may have difficulty detecting any difference at all. If the head weights of the longest irons in both sets are exactly the same, the short irons of the MOI-matched set will be heavier than those of the swingweight-matched set, and they’ll have higher swingweights. If the head weights of the two sets’ short irons are similar, then the long irons of the MOI-matched set will be lighter than those of the swingweight-matched set.
It is quite simple to build a set of MOI-matched irons without the expense of an MOI-matching machine. The first method involves building a set of irons using the traditional half-inch length increment between heads. Simply adjust the head weights to 8-gram intervals, instead of the typical 7-gram difference. The MOI will match very closely throughout the set. Another method is to alter the length difference to 3/8 inch and keep the head weights 7 grams apart. Since the majority of heads are manufactured with a 7-gram weight interval, this is a simple assembly method — especially when adding weight to a head is not easily accomplished.
In our continuing effort to help clubmakers move to the forefront of technology, Golfsmith has altered most of our 2008 new clubhead designs with an 8-gram weight difference between heads. This will streamline the building process for any clubmaker who wishes to try their hand at MOI-matching. Building to swingweight will still be possible if you wish. Simply adjust the head weights, starting with the heaviest head, and add weight to make the heads 7 grams apart.
|From 8 gram to 7 gram Set Adjustment
|8 gram Set
||Weight to Add
||7 gram Set
If you are mathematically inclined, the MOI of any club can be figured by using the appropriate formula. In this case MOI= Length² x (Head weight + Shaft weight/3). The length is the golf club’s final length, the shaft weight is the shaft’s cut weight before installation, and the head weight is simply the weight of the head. After doing the math, use the first four digits to the left of the answer and try to keep each club within one percent of the favorite club MOI.
Related article: Matching Made Easy