Tom Veilleux, a senior scientist, and Vince Simonds, director of aerodynamic research at the Top-Flite Golf Company, explain.
Engineers and scientists in the golf industry study the impact between a golf club and a golf ball to determine the ball's so-called launch conditions. The impact typically lasts only 1/2000 of a second, but it establishes the ball's velocity, launch angle and spin rate. After this brief impact the ball's trajectory is controlled entirely by gravity and aerodynamics--no matter how much the golfer hopes or curses. As a result, aerodynamic optimization--achieved through dimple pattern design--is a critical part of overall golf ball development.
A smooth golf ball hit by a professional golfer would travel only about half as far as a golf ball with dimples does. Most golf balls have between 300 and 500 dimples, which have an average depth of about 0.010 inch. The lift and drag forces on a golf ball are very sensitive to dimple depth: a depth change of 0.001 inch can produce a radical change to the ball's trajectory and the overall distance it can fly. Dimples have traditionally been spherical in shape, but it is possible to optimize the aerodynamic performance of other shapes. The HX golf ball by Callaway, for example, uses hexagons (see image).
Air exerts a force on any object moving through it. Holding your arm out of the window of a moving car easily illustrates this phenomenon. Aerodynamicists break down the force into two components: lift and drag. Drag acts to directly oppose motion, whereas lift acts in a direction perpendicular to motion (it is usually directed upward in the case of a golf ball). As you rotate your hand in the air stream, you vary the amount and direction of the lift and drag forces acting on your hand.
A moving object has a high-pressure area on its front side. Air flows smoothly over the contours of the front side and eventually separates from the object toward the back side. A moving object also leaves behind a turbulent wake region where the air flow is fluctuating or agitated, resulting in lower pressure behind it. The size of the wake affects the amount of drag on the object. Dimples on a golf ball create a thin turbulent boundary layer of air that clings to the ball's surface. This allows the smoothly flowing air to follow the ball's surface a little farther around the back side of the ball, thereby decreasing the size of the wake. A dimpled ball thus has about half the drag of a smooth ball.
Dimples also affect lift. A smooth ball with backspin creates lift by warping the airflow such that the ball acts like an airplane's wing. The spinning action makes the air pressure on the bottom of the ball higher than the air pressure on the top; this imbalance creates an upward force on the ball. Ball spin contributes about one half of a golf ball's lift. The other half is provided by the dimples, which allow for optimization of the lift force.
Answer originally posted January 5, 2004.