Play Better Golf.
The best way to understand and explain moment of inertia (MOI) is to demonstrate it. I have taken a series of photos which explain how to do this. First, keep in mind that MOI is a measurement that determines the resistance of clubhead rotation on off center (off horizontal center of gravity) hits. MOI has very little effect on exact horizontal center of gravity hits.
Demonstrating MOI is easy. Give it a try!
This is a very sophisticated MOI machine made by Inertia Dynamics which is a division of Rockwell International. This is the same unit that Nike and the USGA use plus a number of other major golf club manufacturers. It is interesting to note that The GolfWorks was one of the very first companies to buy and use this machine. We have had it since 1995.
It works by mounting any object on the spindle making sure that the centerline of the spindle projects through the objects center of gravity. In this photo we are measuring moment of inertia through the horizontal center of gravity plane, thus the clubhead is mounted on the horizontal center of gravity. If we wanted to know the vertical center of gravity we would mount the clubhead on its toe with the mounting point projecting through the vertical center of gravity. Actually we can make MOI measurements on any clubhead axis we desire as long as the mounting point on the MOI machine projects through the center of gravity plane.
The horizontal MOI is the only axis that the USGA has a limitation on (that’s a USGA rule) and is by far the most important in explaining playability differences from one club to another. The manufacturers use other MOI axes (mostly vertical) in their advertising when making claims but these are more minor and difficult to put into actual playability differences that a golfer would detect.
This photo shows two objects that look like dumbbells. Each one is the same exact total weight and also the individual components are the exact same weight. The only difference is that one has the weights positioned toward the middle of the rod and the other has the weights positioned out onto the ends of the rod.
The dumbbell with the weights toward the inside is a low MOI weight distribution and measures 15.025 oz–in² (ounce inches squared).
The dumbbell with the weights toward the outside is a high MOI weight distribution and measures 95.696 oz-in². Note that this dumbbell’s weight distribution produces an MOI that is over 6 times greater than the dumbbell in photo 3.
Let’s assume that these two dumbbells are actually putters. You could basically impact the ball almost anywhere on the face and hardly feel any twisting of the clubhead. The putt would also feel solid and would roll almost the same distance as a center face hit. A high MOI clubhead (any club in the bag) will produce much more energy to the ball on any off center of gravity hit vs. a low MOI clubhead. If you struck a putt off center of gravity with the low MOI putter in our example, the putt would feel unsolid, the putter head would rotate at impact and the ball would come up far short of the hole.
You can easily demonstrate high and low MOI by using any two wedges or putters. Actually, any other golf club will work, but their lengths are quite long and make the demonstration more cumbersome.
Using one hand, grasp the two clubs with the heads toward the middle. This is a low MOI demonstration. Simply rotate the two clubs like an airplane propeller, but only rotate them approximately 1/3 revolution clockwise and then 1/3 revolution counterclockwise and keep repeating a few times. Notice that the clubs rotate rather effortlessly and you can rotate them quite fast.
Next, reverse the clubs and grasp them with the heads toward the outside. You now have the grips overlapped in your hand. Next, rotate the clubs as before. Note that this is now very difficult to do as the MOI is very high because the clubhead weight is on the very outside vs. the inside. This configuration greatly resists rotation in the opposite direction. Every time you go from clockwise rotation to counterclockwise rotation it feels as if you will never get both clubs stopped. Also remember to point out that you have not changed the total weight of the two clubs, only the weight distribution of the two clubs, which also changed the MOI.
Click here to get Ralph’s Insider Emails
See charts that illustrate the relationship between MOI and putter sweetspots.