Why do golf balls have dimples?
The counterintuitive idea that revolutionized the sport
In almost every other ball-sport, the balls used are perfectly spherical. However, golf balls have weird circular dents in them. There is very simple physics behind the reason golf balls are made this way.
Back in the 1850s, golf balls were called gutties. Invented by Dr. Robert Adams Paterson, these balls were made by heating the sap from the Malaysian Sapodilla tree. This heated sap could be molded into spheres. These spheres would harden when dried. Golfers realized over time that used golf balls that had nicks and dents had a much more consistent flight. Without understanding the science behind it, much before aerodynamics was understood, golf ball manufacturers started etching and carving the balls.
Over time people started experimenting with different etching designs. As the aerodynamics was understood, people could better design the etchings. Today, the rubber golf balls have around 300–500 dimples with an average depth of about 0.010 inches. A dimpled golf ball travels, on average, about 200m which is almost twice as much as a smooth ball. The physics behind this is simple.
The golfer can control the ball’s velocity, direction, and spin only at the time of impact, which is about 0.0005 seconds. After that, the trajectory of the ball is controlled by gravity and air resistance. There are two main components of air resistance — drag and lift.
Drag is the force that always opposed the direction of motion whereas lift acts perpendicular to the motion (usually upwards in the case of a golf ball). A moving ball has a high-pressure area on its front side. The air flows smoothly along the boundary of a smooth ball, causing drag. It also leaves behind a wake region where the airflow is turbulent. The size of the wake also increases the amount of drag.
When a ball is rough (has etchings or dimples) the airflow along the boundary of the ball is turbulent, causing a buffer of turbulent air between the ball and the smooth air causing the drag. This turbulent layer clings to the surface of the dimpled ball and allows the smooth flowing air to wrap around the ball better, hence significantly decreasing the size of the wake. Hence, dimples manage to reduce drag by half as compared to a smooth ball.
Dimples also affect the lift. A smooth ball with backspin creates lift by wrapping airflow around it, like an airplane. The lower region is of higher pressure compared to the upper, which leads to an upward force. The dimples, increase this imbalance, increasing the lift force.
Dimples optimize both the drag and the lift and have been perfected with years of research. Even a 0.001 inch of change in depth or size can bring about a significant change in the ball’s trajectory. Throughout history, there has been a tradeoff between distance and accuracy for golfers. Higher the speed, lower the accuracy, and vice-versa. But with recent improvements like rubber balls and dimples, manufacturers have managed to maximize both distance and accuracy together, leaving the golf courses the only option to increase their size to maintain the relative difficulty level.
