To most golfers the short game is a mystery. Whenever we take a significant break from playing, it's these little shots that usually acquire the most rust. Figuring them out again is tough, because there are so many ways to experiment with technique. Unlike with the full swing, our notion of textbook form around the greens is less specific.
That said, at the moment of truth—impact—well-struck short-game shots are not mysterious, at least when viewed through the realm of physics. Using a TrackMan launch monitor and my best students, I recorded data on three essential shots: a greenside bunker shot, a low-flying "skid-spinner" chip and a flop shot. I then shared this data with John McPhee, Ph.D., a member of the Golf Digest Technical Panel and professor of systems design engineering at the University of Waterloo in Canada. We examined the physics of successful shots and discovered some common elements, which we'll share with you here.
As complicated as the science might seem, a basic understanding can really help. The next time you're grinding at the practice area, you'll at least know what you're trying to do.
1. THE BUNKER SHOT
Match your speed to the moisture
On a greenside bunker shot that spins and stops quickly, there's less sand between the clubface and the ball. Think of the sand on the face as layers that slide over one another. The fewer the layers, the greater the transfer of shearing force to the ball (see diagram, F), which creates more backspin (S).
Moist sand has higher shear strength than dry sand (this is why wet sand castles can be built taller), so moist sand layers can withstand higher shear forces before they slide. When sand is dry and powdery, more of the club's energy is spent on sliding the sand layers than on imparting backspin to the ball.
The takeaway here is how to plan. With moist sand (not wet, heavy sand), you can try for backspin. If the sand is dry, expect more rollout. We also see how critical clubhead speed or velocity (V) is. In our tests, I had to swing the club 64 miles per hour to fly the ball 20 yards (above). In dry sand, I had to swing 68 mph, or near my physical limit with a wedge.
To get faster through the sand, swing to a fuller finish. But if you're a slow swinger, you're better off adjusting your club's entry point. The closer to the ball you enter the sand, the less you'll be robbed of speed—and spin.
2. THE SKID SPINNER
When low on the face is actually good
For this low chip that halts quickly after its second or third bounce, the ball must be hit low on the clubface. This rotates the clubhead clockwise during impact (see diagram), which has two effects: First, the club is delofted by the impact force, so the ball's trajectory is lowered. Second, because the ball compresses and rolls up the clubface, the clockwise club rotation creates counterclockwise ball rotation, known as vertical gear effect.
This dynamic adds to the spin (S) that results from the club's loft. The glancing blow allows more of the club's velocity to go toward spin than speed.
This is such a fun shot. It comes off like a skull and bewilders your playing partners when it sizzles to a stop. What we know is, you have to hit the ball low on the face—and a bit toward the toe helps. The key is to make a shallow pass through the ball.
Widen your stance and feel that the club swings close to the ground, especially on the follow-through (above). This allows the club to skid through impact right where the bottom of the ball meets the ground to create a tangential blow. You shouldn't take a divot, so it's easier to achieve this shot off of firm turf or when the ball is perched high on a bed of grass.
3. THE FLOP SHOT
The club's bounce goes under the ball
On this shot, the ball is hit higher on the clubface with more direct impact than the skid spinner. The ball stops quickly on the green because of its steep landing angle, not its backspin. For Cameron's 30-yard flop shot, the backspin (S) is only 4,100 revolutions per minute, but the launch angle is up to 51 degrees. The ball speed is similar to that for the 30-yard skid spinner, even though the club's velocity (V) is much faster. The reason is simple: More energy is going into hitting the ground with the bounce of the club to create a slightly upward angle of attack on the ball.
At address, lay the clubface so it points up to the sky. This open face, where you can see the leading edge, creates a high probability that you'll deliver maximum loft. How, you ask, can you strike the ball high on the clubface when it's open this much? At impact, the club's sole is going to push down into the ground to create room for it to slide under the ball. The key is fast wrists—and no tension.
The wrists should hinge so that the clubhead rises higher than the shoulders on both sides of the swing (above). Feel that the clubface passes under the ball very quickly, like a spatula sliding under a pancake.