Understanding Ball Flight - Why Did the Ball Curve That Way?

In order to understand ball flight there are three questions to ask.

1. Why did the ball start there?
2. Why did the ball curve that way?
3. Why did the ball go that far?

In the last blog post, the first question, why did the ball start there?, was answered, in this post the second question, why did the ball curve that way?, will be answered.

Of the five impact factors, clubhead speed, club path, club face angle, angle of attack and centeredness of strike the main ones that affect ball curvature are club path, face angle and centeredness of strike.

Spin Axis

What causes a ball to curve is a tilt in it's Axis of Rotation. The axis of rotation is the axis in which the ball is spinning around. The ball spins around one axis, sometimes people believe there is a combination of backspin and sidespin, but this doesn't happen.

 

The line going from left to right is the axis the ball is back spinning around. The axis is also tilted to the right in this photo meaning the ball will move from left to right in the air.

Imagining an airplane turning is a good way to see how a tilt in the axis of rotation will affect the flight. With the wings tilted to the right in this photo the plane is turning right. A ball with a tilt to the right will move from left to right and a tilt to the left will have the ball moving right to left.

Trackman figures show that for every 5 degrees of tilt in the spin axis the ball will curve approximately 3.5 yards to the side for every 100 yards of carry.

Spin Axis tilt is caused by a difference between club face angle and club path, Trackman names this difference as Face to Path. If the face is open to the path, the spin axis will tilt to the right and the ball will move left to right. If the face is closed to the path, the spin axis will tilt to the left and the ball will move right to left. 

Gear effect on clubs, mainly woods, also has a major influence on spin axis. Heel strikes tilt the spin axis to the right, resulting in left to right shots. Toe strikes tilt the spin axis the the left, resulting in right to left shots.

Influence of the D-Plane on Spin Axis Tilt

The D-Plane or Descriptive Plane was coined by Theo Jorgensen in his book 'The Physics of Golf'. The easiest way to describe D-Plane is it is the plane that contains the initial flight of a golf ball after impact with the club face. The D-Plane is created by the club path and the club face orientation at impact.

This picture from www.brianmanzella.com is a good illustration of the D-Plane. The face is closed to the path, causing the spin axis to tilt to the left, the ball will move from right to left and initial starting point is closer to the clubface than the club path.

Gear Effect

Earlier on it was mentioned that the strike point on the face would affect the spin axis. This is due to Gear Effect. A golf club will rotate around it's center of gravity. A golf ball that impacts on the face in line with the center of gravity will have not have any face rotation due to the contact collision. Is the ball strikes more towards the toe the club face rotates open and vice versa for a heel strike.

Gear effect is the counter rotation inflicted on the golf ball due to the interaction of the rotating face on the golf ball. A toe strike causes the spin axis to tilt to the left and a right to left ball flight. A heel strike causes the spin axis to tilt to the right and a left to right ball flight.

For irons the center of gravity is close the the club face and irons exhibit minimal gear effect. With woods the center of gravity is quite far from the face and woods exhibit significant gear effect.

For a driver Trackman indicates that an impact that is 0.5 inch from the center will impart 20 degrees of spin axis tilt. This is quite a large number and is a major factor in ball flight. It can override the influence of the D-Plane at times.

Here is a photo from www.tutleman.com illustrating gear effect.

Face Closure Rate

One other factor that effects curvature is the rate of closure of the clubface through impact. If the face is closing around a point behind the face it will impact the spin axis like heel contact gear effect.

Summary

The factors that determine curvature of the ball are the difference between the club path and club face orientation at impact. The D-Plane illustrates this. Gear effect also has a big impact on spin axis and how the ball curves. The least known factor is the face closure rate, a the face is closing around a point behind the face it will impact the spin axis like heel contact gear effect.

As a result the main way to hit a straight golf shot is to have a centered strike with little face closure combined with a club path and face angle that are pointing in the same direction at impact. A straight shot can also be obtained when a non centered strike counteracts a face to path difference.




Any questions and comments are always welcome.

Understanding Ball Flight: Why did the ball start there?

If we are looking to become better golfers, a better ball flight helps. If we understand what is causing a ball flight we can make improvements.

The three questions we can ask about ball flight are:

1. Why did the ball start there?
2. Why did the ball curve that way?
3. Why did the ball go that far?

If we know the answers to these questions, we can start to make changes and improvements.

In this blog post we will look at the first question. Why did the ball start there?

Where the ball starts is called Launch Direction. Launch Direction is defined by Trackman as the horizontal direction of the movement of the golf ball's center of gravity immediately after leaving the club face.

There are five impact factors, club face angle, club path, angle of attack, club head speed and centeredness of strike. The main ones that affect launch direction are club path and club face angle.

Club Path versus Club Face Angle?

Club Face Angle has a much greater effect on launch direction than Club Path. An easy was to illustrate this is to hit a 20 yard chip with a 5 iron. First hit the chip to a target and watch where it starts and goes, then with a second shot aim the 5 iron face at a 45 degree angle to the left. Hit the chip shot with the same swing (keeping a similar path) as the first one, the ball will start a long way left, close to the 45 degree angle the club face was pointing, showing cub face angle has a much greater effect than club path.

Club face angle accounts for approximately 85% of the launch direction for drivers and 75% for irons. Let's use an example to see what launch direction a club face angle and club path might produce for a 5 iron.

For a club path of -6 degrees (minus values indicate to the left of the target, positive to the right) and a club face angle of 4 degrees the ball will start ((-6x0.25= -1.5) + (4x0.75 = 3) = 1.5), so with this path and face angle the ball will start 1.5 degrees to the right. Even though the path has a greater value, the face will have a greater influence on where the ball starts.

The above photo from http://www.golfdigest.com/golf-instruction/2012-12/sean-foley-law-of-the-draw is a good illustration of club face influence on the launch direction.

There is an exception - at high dynamic lofts >70 degrees, club face angle has little impact and club path dictates launch direction, a good example of this would be a super high lob shot, the ball starts along the swing path rather then the club face, which can be pointing to the right (for a right handed golfer)


Club Face Angle

What influences Club Face Angle?

Club Face Angle is the horizontal club face orientation at the center of contact between club face and golf ball at maximum compression of the golf ball, i.e. where the club face is pointing at the point of impact between ball and club face.

This is slightly different to club face direction. Club Face Direction is the horizontal club face orientation of the center of the club face at impact. There is a roll and bulge on driver faces, i.e. the face is curved. This affects Club Face Angle on off center hits.

Note the curvature of the face in the above photo.

On off center hits the club face angle will differ from the club face direction. Toe hits have a club face angle more to the right and heel hits more to the left.

Club Face Angle is a combination of Club Face Direction and impact point on the face. A heel shot may start on target if the club face direction offsets the club face angle at impact i.e. the club face direction is to the right. As there is no roll or bulge on an iron, this happens with woods and hybrids mainly.

Club Path

What factors influence club path? There are three major factors:

1. Swing Direction
2. Attack Angle
3. Vertical Swing Plane

Swing Direction is the horizontal direction of the plane defined by the club head's center of gravity movement prior to impacting the ball. Looking at a swing through a camera this looks like the only factor that determined club path but attack angle and vertical swing plane also determine it.

Attack Angle is the vertical direction of the club head's center of gravity movement at the maximum compression of the golf ball.

How does this affect club path? As the club is travelling down it is moving from in to out and as it is moving upward it is travelling from out to in. So if you are hitting up on a driver a swing direction will need to be positive to balance out the the negative club path value of the attack angle and create a straight launch direction.

This photo (from http://www.andrewricegolf.com/tag/trackman-article/) is an upward attack angle (pencil pointing up) with a square swing direction (hula hoop pointing straight), which gives an in to out club path (pencil is pointing left)

The hula hoop needs to be moved to the right to get the pencil pointing straight.

The amount that the swing direction needs to be compensated for the attack angle is dependent on the vertical swing plane. For a driver the vertical swing plane is close to 45 degrees, for irons it averages around 60 degrees. For a vertical swing plane of 45 degrees (driver) we get a one to one relationship between attack angle and club path i.e. if the attack is -3 a swing direction of +3 will give a straight launch direction.

For a vertical swing plane of 60 degrees (irons) there isn't a one to one relationship between attack angle and club path, it is approximately 57%, so for 1 degree of attack angle we get .57 degree of club path. With an attack angle of 4 degrees down we need a swing direction of 2.28 left to make a straight launch direction.

Summary

Club Face Angle has the biggest influence on where the ball starts. 85% influence for drivers and 75% for irons. Club Face Angle is composed of club face direction and impact point.

Club Path has some influence on where the ball starts. 15% for drivers and 25% for irons. Club Path is a combination of Swing Direction, Angle of Attack and Vertical Swing Plane.

What this means is, if your ball is not starting where you want it to, the club face angle is the main reason for this. Change the club face angle and the ball will begin to start closer to where you want it to.

Comments and questions are always welcome.