Centre of Gravity (COG)
|Centre of Gravity heights, photo by @kirkoguri on Twitter|
Centre of Gravity (CG) is a point in the head that represents all the mass of head. The above photo shows the position of the centre of gravity in 4 different heads by height. Only minimal differences but enough to make the heads perform quite differently.
The CG is sometimes referred to as the sweet spot of the club. The sweet spot is the projected position of the CG from inside the club on to the club face. It is quite a small spot and is not bigger in some clubs then other.
When the ball is hit on the 'sweet spot' the club face doesn't twist or rotate and the maximum amount of energy is transferred to the ball.
The CG can be moved around the club head by shifting weight, move weight towards the toe the CG goes towards the toe (a small bit, not a huge amount). Moving the weight around influences the flight of a ball by utilising gear effect to help the golfer. More weight in the heel will promote a draw from centred strikes and straighter shots from heel strikes. The spin rate can also be influenced by moving the CG higher or lower.
Moment of Inertia (MOI)
Moment of inertia (MOI) is the measure of an object's resistance to twisting. The higher the MOI the more the object will resist twisting. For a golf club this means for shots that are mishit a head with higher MOI will twist less offline then a club with lower MOI.
In golf heads weight is moved to the perimeter to increase MOI and make them more forgiving. So a cavity back will be more forgiving then a blade. This is also the process that makes a 'sweet spot' bigger, it's not making the 'sweet spot' bigger but the club head twists less giving more forgiveness and the illusion of a bigger sweet spot.
It also helps with putters a big mallet style putter with weight moved to the extremes will not twist as much as a blade style putter meaning the ball will start more online with mishits.
Roll & Bulge
Roll is the curvature on the face of a wood from crown to sole.
Bulge is the curvature on the face of a wood from heel to toe
Bulge and roll are there to counteract what is known as gear effect. When a driver is struck off-centre, the head wants to twist, roughly about it's centre of gravity. In the period of time between impact and separation, the friction between the club face and ball adds spin to the ball in the opposite direction to the rotation of the club head. this is much the same way that in a 2 gear system, if one gear is rotated, the other rotates in the opposite direction (thus the name gear effect.)
The purpose of bulge is to modify the initial launch direction to compensate for the shape in ball flight caused by an off centre hit. For example on a heel strike the ball will move more left to right and as a result we would like the ball to start further left to compensate for this.
Roll directly effects loft in the vertical plane on the face. A ball hit higher on the face will spin less due to gear effect but will launch higher due to the roll, a ball hit lower on the face will spin more due to gear effect but will launch lower due to roll. These compensations lead to better results from mishits.
Any questions or comments are appreciated.