In physics, center of gravity (CG) is defined as a point at which the entire mass of an object may be considered to be located. In club making, CG is a critical concept to control a number of effects that are important for determining the performance of golf clubs.

CG and Gravity Angle - In designing drivers or fairway woods, club manufacturers have found many ideas, including club-head design concept with tungsten inserts found in such models as Callaway Great Big Bertha Hawk Eye Titanium Drivers or Taylor Made FireSole Drivers to make the CG lower and/or deeper. TaylorMade driver "r7" is a more sophisticated version as it allows golfers to change the CG position and the gravity angle by repositioning the cartridges. The lower CG is beneficial for getting the ball in the air, and the deeper CG is the source of gear effect. Also, the position of CG relative to the shaft determines the angle of club face as shown in the above right diagram. This angle is called "gravity angle", which influences the ball trajectory. It biases the ball trajectory either to draw or fade depending on the angle. This is different from a "hook face".


Draw Bias - Callaway (Hawk Eye Model) positioned the tungsten screw in such a manner that the gravity angle of the driver was biased toward the draw side, and the company calls it "Draw Bias" -- a mechanism to decrease the possibility of hitting a slice, and increase the chances of drawing the shot from right to left. The company also claims that " The position of the heavy Tungsten Gravity Screw on the driver -- back toward the heel of the club-head helps the average golfer bring the club-head back to square at impact much more easily. This action on the ball not only counteracts the spin that creates a slice, but also produces the spin necessary to effect a draw."

An offset club with minimum face progression is another design concept to help reduce the degree of slice. Club manufacturers like Cobra and Taylor Made, for example, claim that the off-set hosel helps keep the club-head behind the hands, making it easier to keep the face closed at impact, and that helps promote a right to left ball flight pattern.

Ping's approach to move the hosel position to control ball trajectories is also noteworthy (click here for more information). The other approach is to control the ball trajectory by varying the shape of the clubface to fit each individual swing type and control ball spin and the amount of gear effect. Adams Asymmetrical Face Curvature (AFC) system employs this approach.

Gear Effect - The gear effect was found accidentally around 1850 in an effort to protect the club face from the impact of striking hard Gutta balls (see "Golf Balls" for more details). The club face was rounded, and it also resulted in the better control ability as shown in the right diagram.

The deeper CG position and the rounded club-head design allowed a larger amount of moment of inertia (MOI) of the side spin of golf ball generated from off sweet spot shots maximizes the amount of gear effect. Persimmon drivers meet such design criteria for the maximum gear effect, but earlier metal drivers did not. That is why persimmon drivers had been used until late 90's by many pros at the Masters. However, a certain type of titanium drivers with deeper CG now can be used like persimmon divers, enabling ball control from right to left or left to right at will.

Nowadays, some drivers are designed to use a new concept of the vertical gear effect (VGE) by making the deep club face with the low CG - which is believed to produce a higher launch angle with lower backspin - the launch conditions that deliver longer drives for most golfers.

Toe-down and Shut-face Phenomena - A large club-head offers many advantages, but it also brings disadvantageous effects in the total performance of the club – namely the toe-down and shut-face effects. With a large clubhead, CG of a clubhead is apart from the shaft line, and the centrifugal force acting on a clubhead bent the shat down (toe down) at the impact.

The amount of tortional deformation appearing on the shaft (shut-face phenomena) is a function of the centrifugal force and the CG/shaft line length, which determines the amount of face to become closed at impact. In other words, by locating the CG closer to the shaft line, the shut-face phenomena can be minimized.

These effects are propotional with the centrifugal force and the position of CG relative to the shaft line as well as the shaft properties (flex and torque).