Impact Laws

Every golfer will have a ball flight of some description.  Every ball flight has a characteristic also known as the D-Plane or descriptive plane. The D-Plane is made up of the golf balls direction, trajectory, curvature & distance traveled.

The D-Plane is determined by the clubface conditions at impact.  During impact the ball compresses, slides and rolls up the clubface creating backspin (lift).  The lift force varies linearly with the backspin angular speed, and the velocity of the ball.

The backspin imparted to the ball by impact makes it rotate around an axis.  Just like the wings of a plane, when the axis is tilted, the ball will curve the same way a plane banks when the wings are tilted until gravity pulls it down.

 

spin axis

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The clubface conditions are also known as the impact laws.  The Impact laws refer to the physical forces that are absolutes in influencing the flight of the ball.  (Hume at al, 2005, PGA, 2003)  The five impact laws include:

 

  • The horizontal & vertical alignment of the clubface at impact
  • The swing path
  • The angle of approach
  • Clubhead speed
  • Centeredness of strike

 

Club face Angle

 The clubface angle can be viewed in both the horizontal & vertical plane.

The club face angle in the horizontal is the degree at which the leading edge is pointing to both the target and also the swing path (PGA)

The clubface angle of an iron at impact has a 75% influence on the golf balls launch direction with the swing path accounting for the remaining 25%. (Rose, 2008).

 

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The loft on the club will affect the golf balls spin axis, trajectory & curvature.  A more lofted club will impart more backspin; more backspin helps the ball rise higher but travel shorter distances (Kempton, 2013).  This will also reduce the sidespin which causes the golf ball to curve through the air (PGA, 2011).

The formula for calculating spin rate is:

clubhead speed x

dynamic loft x

club factor (2.6 for an iron)

= Spin rate

(Rose G. Titleist Performance Institute)

 

Swing Path

The swing path is the direction that the clubhead is travelling in before, during and immediately after impact.  Along with clubface alignment the swing path has a major influence on the overall shape of the shot.

 

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Angle of Approach

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The angle of approach or attack angle is the angle formed by the ascending or descending arc of the clubhead in the forward swing in relation to the slope on the ground.

The angle of attack also plays a significant role in determining the club path (Rice 2018).

 

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Clubhead Speed

 Clubhead speed is the velocity the clubhead is travelling at impact.  Clubhead speed influences distance, trajectory & curvature.  As the clubhead speed increases, the amount of backspin imparted on the golf ball will increase, the total distance will increase, the maximum height achieved will increase and the amount of curvature of a non-square face position will increase.

 

Centeredness of Strike

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A centred strike will ensure that no distance is lost (Kempton, 2013).   An off centre strike will not only slow down the clubhead but will cause it to twist about its centre of gravity (Cochran & Stobbs, 2005).

Cochran and Stobbs state that at impact the ball very strongly resists the clubheads attempt to send it into instantaneous flight.  It resists strongly enough to flatten itself out against the face of the club before reacting into flight.  As the reader can imagine, the force involved is very great.

If this force is applied to the middle of the clubface, in line with its centre of gravity it will slow the whole clubhead up quite appreciably but without twisting it or throwing it off balance in any way.  This is precisely what happens during a perfectly struck golf shot.

 

Gear Effect

With an iron, because the centre of mass is close to the clubface (due to the thinner head design), the point of contact with the face will just move straight backwards on a toe or heel strike.

A strike located at the toe will open the blade & a strike located at the heel will close the blade.

 

However!

Due to the design & size of the driver head (it’s deeper), this puts the COM of the clubhead farther back in the head.

Because of this, on an off-center hit the contact point will not only move backwards (as in the iron), but it will also move more to the right on a toe shot. This is reversed for a heel shot.  The result is that the golf ball will spin in the opposite direction like a gear.

 

Toe shot – clubface twists right, imparts more hook spin/less slice spin to the ball

Heel shot – clubface twists left, imparting more slice spin/less hook spin to the ball

High on the clubface – Less back spin

Low on the clubface – More back spin

 

Please note this is the effect of a non-centred strike which is not the same as clubface angle!

 

 

 

 

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