How Moment of Inertia (MOI) Affects Driver Playability - should you buy a square driver?

Introduction

Two of the USGA limitations on driver heads are size and MOI. The maximum driver head size is 460 c.c. plus a 10 c.c. tolerance making the absolute maximum legal size 470 c.c. The term c.c. stands for cubic centimeters. The maximum allowable driver head M.O.I. is 5900 g-cm² plus a tolerance of 100 g-cm² making the absolute maximum driver head MOI 6000 g-cm². The term g-cm² stands for grams per centimeter squared.

Regarding MOI, some confusion can exist because the numbers used to express it in our industry can be in different units. In other words, the Maltby Playability Factor for irons uses oz-in² (ounce/inches squared). Bobby Grace putters and some others use kg-m² (kilograms per meter squared). A number of golf club manufacturers use g-cm² (grams per centimeter squared) as already discussed. The MOI machine that is used in the Golf Club Design Studio can provide MOI readings in 13 different unit formats. The point is that a simple conversion factor can be applied to each format to convert directly and accurately to any other format so they all basically mean the same thing.

History

The first modern metal headed drivers (stainless steel) were introduced by TaylorMade toward the end of the 1979 model year. The very successful Tour Burner was introduced in 1983 and was a whopping 140 c.c. driver head (persimmon at the time was over 200 c.c.). Callaway for example introduced their first stainless steel metal driver head in 1988 in a Bobby Jones model. The first Big Bertha was first introduced in 1991.

Head sizes in the early 1980’s started off smaller than their persimmon counterparts and then gradually became similar in size and then bigger in size. Stainless steel as a material had limitations in making larger driver head sizes because it simply was not strong enough to keep the faces from caving in when size was increased beyond its limits. The exotic driver head materials and unique head manufacturing methods revolution started in earnest during the mid 1990’s to make heads bigger and with higher CORs (coefficient of restitution or spring face designs). It is interesting to note here that as the heads became larger in c.c. size the MOI’s also became greater as a result. However, the advertising did not tout higher MOIs but concentrated mostly on COR which was the hot trend at the time.

Alter shape, materials and/or center of gravity location to improve driver playability.

The 2007 Model Year for Drivers

The MOI measurement for modern day drivers is a function of five basic mass and dimensional properties; the head size, material, shape, weight and center of gravity location. Since the head size is limited by the USGA and the head weight is dictated by the swingweight we desire, this leaves shape, materials, and center of gravity location as the easiest variables to significantly modify to achieve higher driver MOI.

We have measured the horizontal MOI of many 450 to 460 c.c. drivers in the Golf Club Design Studio and we have found that they basically range from 4300 to 4700 g-cm². The USGA initially proposed a limit on MOI in 2005 of 4750 g-cm² but the manufacturers voiced strong opinions about this limitation and the U.S.G.A relaxed it to 5900 g-cm². According to Dick Rugge, senior technical director for the USGA, the limit was raised because the difference would mean little for golfers in increased performance. Rugge also said that if you were going from 2000 to 3000 g-cm² it would be huge but at the higher level going from 4750 to 5900 g-cm² it would have a much lesser effect. I agree with his assessment.

My research and study shows that with current technology it would be very difficult to achieve MOIs higher than 5400 g-cm². Achieving the USGA limit of 5900 g-cm² would require new head manufacturing technology and/or new head materials to be developed. This will probably be accomplished in the near future.

Shape is one of the clubhead design variables left for the manufacturers to alter the center of gravity and/or the MOI. The U.S.G.A has taken away the easiest way to effectively increase MOI by increasing the clubhead size. The manufacturers shape changes coupled with driver head size limitations can only add marginally to the current range of driver MOI. In my opinion, this puts the new non traditional driver shapes more into the current hot trend category vs. significant increased technology to benefit golfers. This is also saying that there is nothing wrong with the new non traditional shapes (square is currently the most popular) regarding comparable playability with traditional shapes. It remains to be seen if golfers will accept the square shape (or any other non-traditional shape) because of the look in the playing position or possibly sly comments from their playing partners. I do not feel that many players will actually get significant increased playability, but again this is my initial opinion and only time and further testing will tell. Be sure to actually hit these new shaped drivers before buying and take along your current driver for an accurate comparison.

Be aware that the USGA limit on MOI is only based on the MOI in the horizontal plane (heel to toe) of the clubhead. Some of the manufacturers are also modifying the MOI in other clubhead planes and claiming advantages. One of these is referred to as vertical plane MOI (face to rear). Some even make claims by measuring MOI around the hosel bore centerline. I cannot substantiate or deny their claims but I can tell you that I personally use the drivers’ horizontal plane MOI coupled with the clubheads rearward center of gravity location as two of the more important variables (mass and dimensional properties) in evaluating driver performance.

The Bottom Line for The Golfer

This is a great time for any golfer to move up to the current 460 c.c. or so drivers and get rid of their older smaller headed driver. The reason is to take advantage of the highest technology available today which translates into the easiest to hit and straightest hitting drivers ever made. Any golfer moving up to this technology should immediately notice the benefits in an easier to hit driver. This obviously makes our job easier because we can almost guarantee game improvement and a better golfing experience.

If you hit the ball off-center, higher MOI drivers, such as modern drivers with unique shapes, will help you.

The high MOI on modern drivers doesn’t mean much if you have the ability to hit the ball every time on the heads exact horizontal center of gravity location. However, any mis-hits off of the face center of gravity (so-called sweet spot) will feel much more solid, lose less distance and go straighter. The reason for this is that the higher MOI on large headed drivers better resists clubhead rotation on off center hits vs. lower MOI driver heads. This clubhead rotation occurs around the horizontal center of gravity whenever the ball is not struck perfectly and results in less force (distance) applied to the golf ball. Modern large headed drivers with higher MOIs simply have less rotation on off center hits resulting in less distance loss. It can be said that they also have much larger sweet spots than lower MOI driver heads.

Couple all this with much higher CORs, increased vertical gear effect, more rearward centers of gravity and the improved flight characteristics of modern golf ball designs and you have the reason why modern large headed drivers hit the ball so much longer and straighter than smaller headed drivers. As long as the USGA imposes the current limits on driver head size and most major golf club manufacturers keep incrementally increasing MOI, with existing head materials technology most all the current top grade driver heads should all perform exceptionally well.

It is very important to get correctly fit to the best driver shaft, loft, length, grip size and swingweight. Don’t forget to get a ball fitting on the launch monitor and also to remember that the ball must be teed higher for best trajectory because the vertical center of gravity location on the large headed drivers is actually above the face center vertically.

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