What is horizontal bulge in a wood
Have you ever noticed that the club face in woods is curved across the face while the clubface in irons flat and straight. This feature in the woods is called the ‘Horizontal Bulge’ and is more pronounced in the driver as compared to the fairway woods and is responsible for creating gear effect during a shot. This is because the further back the centre of gravity of a club, the more horizontal bulge is required to create a gear effect. Irons are flat and do not have horizontal bulge as their centre of gravity is quite close to the club face as not behind as in the case of woods
A shot hit from the toe of the club will tend to go to the right and one hit from the heel of the club will tend to go to the left. Gear effect helps in reducing the movement to the right and left when a shot is hit from the toe or the heel respectively.
When a shot is hit off the toe of the club face of a wood, the gear effect gives a right to left spin to the ball enabling the ball to either hold its line that it takes off on or sometimes even come back towards the left, thereby reducing the effect of the mishit. Similarly, when a shot is hit off the heel of the club face of a wood, the horizontal bulge gives a left to right spin to the ball enabling the ball to either hold its line that it takes off on or come back towards the right.
This is how horizontal bulge on the face of a wood helps in hitting straighter shots.