Golf Club Moment of Inertia – Is There a Point of Diminishing Return?

Have you noticed that people in general are preoccupied with numbers? Golfers are no exception, especially when speaking about golf club specifications. Case in point is Moment of Inertia (MOI) which has become THE buzzword in golf today. Moment of inertia is a measurement of the clubheads ability to resist twisting about a known axis. One year ago at this time there was only mumblings about the April 2006 change in the Rules of Golf by the USGA as they approved the implementation of a clubhead moment of inertia test and a limit of 5900 gm-cm2 (about the face plane). An explanation of MOI and the meaning of these measurements can be found on the article “MOI – Not Too Technical Version”. Now at least there was a number to associate what “high” MOI meant, plus it gave the manufacturers something to try to achieve. In 2007, all of the sudden, the shape of clubs changed dramatically to push the envelope on MOI. When the first metal woods were introduced around 1980, those 150cc stainless steel drivers were said to be more forgiving than the wooden heads they were replacing. As a point of reference, those metal wood drivers had a MOI of approximately 2000 gm-cm2 which was about 25% greater than the wooden drivers. In comparison traditional “pear” shaped 460cc drivers are in the 4200 gm-cm2 range. Now clubs are becoming broader from face to back, square (like the Mantara XL Driver) and even triangular shaped to increase this further. But the question needs to be asked, how much more forgiving is a traditional pear shaped driver with a 4200 gm-cm2 and another 460cc driver stretched out in a more geometric shape which has a MOI of 5000 gm-cm2 or higher? Remember, the higher the number, the greater resistance there is to twisting. Let’s say we stuck a gripped golf shaft in the left rear fender of a 2008 Ford Mustang and this was to be your new driver. Well, the MOI on the left or driver’s side of the car would be in the neighborhood of 3 billion g-cm2. If a golf ball collided anywhere along the side of that car, I can guarantee you that car will not twist whatsoever. Now this would be high a MOI!!! However, a human could never get the car in motion simply by applying a torque to the shaft with their arms. The shear weight of the vehicle makes it prohibitive to propel a golf ball off of a tee. So having a high MOI number by itself becomes meaningless. Luckily golf club drivers do have some physical limits imposed by the USGA. The key limits are 460cc for volume and 5” (127mm) long, 5” (127mm) wide (breadth) and 2.8” (71mm) tall. In addition, even though there is no USGA limit most drivers are 200g +/- 5g. The other factor to consider is the MOI is only tested in one plane and that is about the face plane or how the clubs resists impacts made on the heel or toe side of the center of gravity. Moment of inertia can be measured in an infinite number of planes. For instance, how does the club react when impact is hit low or high on the club face or how the does the head rotate around the hosel? To the left is an example of a 460cc pear shaped driver with a relatively deep face (60mm), but the breadth of the club is only 102mm. In order to maintain 460cc, a head with a higher face MOI measurement one would have to stretch out the length and the breadth. Current drivers are already near the 5” limit on the length from heel to toe, so rearward is the only direction to go. However, to increase the breadth of the club, then the depth of the face and crown have to be shortened or the face area reduced. Remember the original 150cc metal woods eventually became larger in all directions without compromises as there is when working with a fixed volume. The club on the right would be the same 460cc volume, but where the breadth is closer to 120mm. Notice how much less area there is to strike the ball above and below the equator of the face which may not be as desirable for those that make contact both high and low on the face. Another method in saving volume is to taper the face, effectively reducing the face area. In all driver heads, the length of the face from heel to toe is shorter than the maximum dimension as the toe and heel toward the center of the club extends beyond the face. However, on some models the faces are proportionally smaller than others. Even though the MOI on a driver may increase over another, by limiting the face area, it may make it more intimidating and have less room for error for higher handicapped golfers who may hit all over the face. Another attempt to reduce the effective volume of a club is to change the shape of the crown. Rather than the traditional convex shape, create a convex or inverted shape. However, the tradeoff for this is the higher pitched sound that a good number of golfers are not fond of. Even rumors of the 2008 Nike Sumo Squared driver will be without the carbon crown or the dished out shape present in the ’07 model in return going to a traditional smooth top to eliminate the very high pitched sound. However, what will the trade-off be in the face height are effective face area? One last consideration is the MOI about the hosel. By creating a larger footprint or approaching the 5” by 5” dimensional limit, the center of gravity can become a greater distance away from the hosel or centerline axis of the shaft. In this case, the ability for the club to square up at impact is reduced. The head may have to be designed with a more closed face, internal weighting positioned inside the head to create a draw bias condition or the selection of the shaft itself might be paramount in producing a playable club for the average golfer. Realize there was a quantum leap in the forgiveness of drivers going from the 150cc stainless steel to the modern 460cc titanium models. Years ago in the persimmon days, it was found that the average distance loss on a golf ball struck 1” from either side of the center resulted in a 14% distance loss. I remember seeing a test by Golf Digest in 1998, that the titanium driver of the day resulted in a loss of only 9% on the same 1” mis-hit. Today drivers are closer to losing only 5% of the distance a center impact makes. The fact that clubs produced today or shortly in the future can be made to a higher MOI than what is presently does not automatically make them significantly more forgiving as there are certain limits that have to be maintained. When changing one dimension, such as the breadth of the head, then there can be a compromise in another feature such as face height, face area or MOI in another plane. Regardless, manufacturers are attempting to eliminate as much weight possible and redistribute that weight were it can be utilized best in order to make your mis-hits even
more playable. But don’t be disappointed that your driver may only have a MOI of 4500 g-cm2 compared to what the maximum 5900g-cm2 as you are still possessing a very forgiving head.

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  1. […] Forget for a second that you have a driver with the maximum allowable volume, spring-like face and high MOI paired with a premium shaft and grip.  When you make that tape-measured drive, do you remember […]

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