Should Golfers Keep the Flagstick in the Cup When They Putt?

A recent rule change has given golfers a conundrum and a new analysis reveals how complicated the decision is.
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Golfer putting
Peter Gwynne, Contributor

(Inside Science) -- As if golf weren’t already difficult enough, the game’s international authorities added a level of complexity two years ago. They overturned a rule that penalized golfers for hitting the flagstick when they putted their ball on the green.

That raised an obvious question: Should golfers keep the flagstick in the hole or take it out when they putt?

Physicists, statisticians, golf gurus and PGA professionals immediately weighed in on the decision. Perhaps not surprisingly, they’ve come to different conclusions. 

Now, Steve Kuchnicki, a mechanical engineer at York College of Pennsylvania, has created a mathematical model of the interactions among the ball, the hole and the flagstick, which is also called the pin.

He found that factors such as the distance and speed of the putt and the golfer’s confidence in his or her ability to make it should inform a golfer's decision.

If you’re really sure about the curving path the putt will take toward the hole or not sure about the ball’s speed on the way, "you might want to leave the flagstick in," Kuchnicki said.

"In the first case, it will act more like a backboard," he explained. "In the second, if the ball is going fast, the stick will keep you safer for the next shot."

On the other hand, he added, "the flag can keep you out of the hole" in certain cases.

The United States Golf Association and the U.K.'s Royal and Ancient Golf Club changed the rule mainly to speed up play. Since most golfers don’t use caddies, the need for individual golfers to remove the flag when they reach the green can increase their time on it. The new rule addresses that issue.

Almost immediately, analysts offered advice to the golf world on how to react. Dave Pelz, a physics graduate long regarded as a putting guru for professional and amateur players, recommended keeping the pin in the hole all the time, based on a loss of energy when the ball hits it. PGA professional Bryson DeChambeau, who also has a physics background, said that keeping the flagstick in is an easy decision for putts longer than 10 feet. In a paper in the journal Chance, University of Nebraska-Lincoln statistician Christopher Bilder quoted DeChambeau as saying that the ball has better potential to stay in the hole with the pin in than with it out.

Tom Mase, a mechanical engineer at California Polytechnic State University in San Luis Obispo, disagreed, based on studies with a device called the Perfect Putter. He concluded that keeping the pin in prevented some breaking putts from entering the hole, specifically those putts that began to drift away from the hole’s center as they approached it. The same putts succeeded every time with the pin out. Mase speculated that the most likely explanation for the failures is that the flagstick is not always perfectly vertical.

Kuchnicki’s analysis, Mase continued, “has the essence of what I saw.” 

An avid golfer whose handicap of 5 strokes puts him in the top category of amateurs, Kuchnicki wanted to avoid confusion about the in-or-out decision. "My putter’s my best club," he said. "I wanted to make sure that it remains my best club."

He decided to use physics and geometry to determine how the ball rebounds off the flagstick. He based his analysis on a 1991 study of the interaction between the ball and the hole by Brian Holmes, a physicist at San Jose State University. 

Holmes had focused in part on the impact of the ball with the rim of the hole opposite its entry point. Kuchnicki adapted that approach to include the flagstick. "It’s all about how the ball rebounds, off the flagstick and off the hole," he said.

Details of the rebound can be complex. The ball can sometimes roll slightly up the flagstick when it hits. "That changes the ball’s spin, which might help diminish its downward velocity. So the ball will have a greater chance of staying out of the hole," he said.

In other words, the ball’s tendency to bounce away from the flagstick will override its tendency to drop into the hole.

Holmes, who used various hole rims made of grass, cork or wood in his original studies, pointed out other factors related to the flagstick. "What happens with the stick at different angles?" he asked. He also noted the importance of the "bounciness" of the ball that determines how far it will rebound from the flagstick -- a factor known as the coefficient of restitution. DeChambeau, in fact, has studied the effect of flagstick materials with different coefficients of restitution.

Kuchnicki’s analysis revealed two conditions under which the flagstick should be kept in the hole: when the golfer is certain that the ball will contact the hole near the center of the flagstick, and when the golfer is uncertain how hard he or she should hit the putt. In the second case, the stick may keep the ball closer to the hole after it rebounds, thus making the next putt easier. 

"In all other cases," Kuchnicki said, "the flagstick is shown to be nonbeneficial."

Kuchnicki himself generally takes the stick out. But for weekend amateurs, he advised, "it’s better to keep it in if you’re going to miss anyway."
 

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Peter Gwynne is a freelance writer and editor based in Hyannis, Massachusetts, who covers science, technology and medicine.