Answer: The standards for balls are as follows:

The size shall not be less than 1.68 inches in diameter. The weight of the ball shall not be more than 1.62 ounces. The Initial Velocity of the ball shall not be greater than 250 (+ 2% tolerance) fps (about 170mph.) The Overall distance of the ball shall not exceed 280 (+ 6% tolerance) yards. This tolerance is made up of 4% innovative tolerance and 2% test tolerance. So the real limit is 291.2 plus a 1.92% test tolerance (or 5.6 yards). The ball must be designed and manufactured to be spherically symmetrical.

The Measurements

The weight and the size are relatively easy to measure.

The Initial Velocity is measured using a large heavy wheel spinning with a striking surface traveling at nearly 98mph which strikes the ball. This does relate to the speed at which a ball will leave the clubhead.

The Symmetrical rule requires that the ball be launched spinning about two different axes and that it perform the same way within certain bounds.

The Overall distance used to require that the ball be launched outdoors using a mechanical golfer. This is no longer the case. Today a balls launch conditions can be measured using the mechanical golfer indoors. These with the aerodynamic properties of the ball can be used in a computer simulation to predict exactly how far the ball will travel.

The Indoor Test Range (ITR)

The USGA Technical Department has been developing a new test method for determining overall distance of golf balls. To do this, the USGA Research & Test Center will use the Indoor Test Range.

What is the Indoor Test Range? Simply put, it is a method for getting aerodynamic information about golf balls. This aerodynamic information, which consists of the lift and drag of the ball under different conditions, allows us to simulate the entire flight of a ball.

The first thing the Indoor Test Range (ITR) needs is a way to launch the ball down an indoor range (70 feet). For this, we employ a ball launcher, which uses computer-controlled flywheels and belts to give a precise amount of spin and speed to each golf ball. Digital photographs of the ball taken as it leaves the launcher help make sure the speed and spin are right.

Next, the flight of the ball is tracked down a length of seventy feet. It is tracked using a series of "ballistic screens", which are essentially timing gates that use light sheets to detect when and where the ball goes through. Knowing the flight track, or trajectory, of the ball, and knowing the ball’s size and weight, a computer can figure out how much lift and drag the ball had at that speed and spin rate.

This has to be done several times, at different speeds and spin rates. Why? During a trajectory, the ball slows down because of drag. Below, the red line represents the speed of the ball during the trajectory.

The ball starts at 160 mph, but goes down to speeds as low as 62 mph near the end of its trajectory. Since the lift and drag change with relative spin and speed, balls must be tested over every part of this trajectory. Therefore, each ball is tested at sixteen different speeds and spin rates to put together the whole picture of the ball flight. This information allows us to precisely calculate the distance that a golf ball will travel.

How does this information improve our ability to test golf balls? It has been recognized by professional golfers that they can use technology to learn how to drive balls farther. However, even with similar technology, it would be impractical for Iron Byron to change his swing the way a professional can.

Now, with the Indoor Test Range, there is no need to "tune" Iron Byron to drive every type of ball as far as possible. Instead, using the aerodynamic information about each brand of ball, and having found the speed of the ball using an impact test, a computer can figure out the best angle and spin to get each ball to its greatest potential distance. The Indoor Test Range can test balls year-round, and with much greater precision than was ever possible using Iron Byron.

 

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