Today, we hear a lot of acronyms being thrown around by equipment manufacturers when talking about their equipments and technology. We, as golfers, sometimes misunderstand what they actually mean and how these properties actually affect our game of golf. Hopefully, at the end of this article, you will become a little more informed about what these manufacturers and their marketing people are talking about.
COR
COR is an acronym for “coefficient of restitution.” Coefficient of restitution is a measurement of energy loss or retention when two objects collide. The COR measurement is always expressed as a number between 0.000 (meaning all energy is lost in the collision) and 1.000 (which means a perfect, elastic collision in which all energy is transferred from one object to the other).
It is impossible for the collision of the golf club and golf ball to produce a perfectly elastic collision (COR of 1.000) in which all energy is transferred for two reasons:
The clubface and the ball are made from completely different materials;
The clubhead and the ball are of two totally different weights, or masses.
The current USGA rule limiting the coefficient of restitution of a clubhead states that the COR cannot be higher than a measurement of 0.830. This means that when the clubhead impacts the ball, there cannot be more than an 83 percent transfer of energy to the ball.
To give a frame of reference for performance, with a driver, the difference in carry distance between a head with a COR of 0.820 and another with a COR of 0.830 would be 4.2 yards for a swing speed of 100 mph. It is true that as swing speed increases, the distance difference increases. This is one of the reasons why the USGA rule which limits the COR of a clubhead has the effect of penalizing the slower swing speed golfer much more than the higher swing speed player.
MOI
Moment of inertial, or MOI, is a property of physics that indicates the relative difference in how easy or difficult it will be to set any object in motion about a defined axis of rotation. The higher the MOI of an object, the more force will have to be applied to set that object in a rotational motion.
There are several different moments of inertia that are factors in the performance of a golf club. Remember, MOI has to first be defined by identifying what axis the object is rotating around. There is MOI for the whole golf club which, when swung, is “rotated” around the golfer during the swing.
There are also three different MOIs which can be measured for the clubhead itself. Two of these MOIs are important in the design of any clubhead.
First, when you hit a shot off the center of the face, even though the head is secured to a shaft, the head will try to rotate around the vertical axis going through the clubhead’s center of gravity. Second, and at the same time, when the golfer swings the club on the downswing, the clubhead is rotating around the axis through the center of the shaft.
The first example refers to the MOI of the clubhead about its center of gravity. In marketing terms, this is the head design property that has a bearing on the amount of “forgiveness” a clubhead offers for off-center strikes. The larger the clubhead, and/or the more the designer incorporates perimeter weighting, the higher the MOI of the clubhead about its center of gravityvertical axis will be. The higher the MOI of the head about its vertical CG axis, the less the head will twist in response to an off-center hit, and the less distance will be lost from that off-center hit.
The second example refers to the MOI of the clubhead about the shaft axis. Little is spoken about this MOI in equipment marketing, but it is an important head design factor that can affect the accuracy of the shot, not the distance. The bigger the head or the more weight that is placed far out on the toe of the clubhead, the higher the MOI of the head will be about the shaft’s axis. The higher the clubhead MOI around the shaft, the more tendency there is for a golfer to leave the face open at impact. The lower the clubhead MOI around the shaft, the more tendency there is for a golfer to rotate the face more closely at impact.
The whole golf club also has an MOI. The longer the club, the heavier the head, the heavier the total weight of the head, shaft, and grip added together, the higher the MOI will be for the whole club. The MOI of the club is important to matching the swing feel of all the clubs in the bag.
CG
The center of gravity (CG) of any object is the one small point which represents the intersection of all the possible balance points of that object. In a golf clubhead, the CG can be determined by balance the headof its face, sole, or any place on the head – the intersection inside the head of all these different balance points is the center of gravity of the clubhead.
Because the center of gravity is a single point inside the clubhead, its location has to be defined in 3-dimensions. This means a clubhead has a vertical CG location (how high up in the head the CG from the sole). It also has a horizontal CG location (how far over it is from the center of the shaft in the hosel of the head). Finally, the center of gravity is also defined by how far back from the face it is located.
The lower the center of gravity and the farther back the center of gravity is from the face of the club, the higher the trajectory of the shot will be for any give loft angle on the clubhead. Of the two CG locations that affect the height of the shot, the CG back from the face has greater effect on the height of the shot than does the vertical CG.
