Tip Trim Driver Shafts?

Our general advice on whether or not you should have the shafts in your driver trimmed when building your club is straightforward and uncomplicated: don't do it. OK, if that's too extreme and you absolutely positively believe you must cut something, then maybe a half inch when you are in between flexes but take note that just about every premium shaft maker in the industry recommends no tip trimming on their driver shafts.

We know tip trimming is a common practice by a wide range of clubmakers both professional and amateur, but just to be clear our advice is: don't do it. Don't request it. Don't have it done on your driver. If someone wants to hack an inch off the tip of your costly new shaft just ask why not jump up a flex in stiffness or 10 g in weight and retain the performance design the shaft engineer intended. Or try a different shaft.

We were motivated to write this blog because of how frequently we encounter this practice with shafts that were not designed to be tip trimmed for use in drivers. We have seen some truly outrageous trimming done on driver shafts that effectively ruined the performance designed into them. OK, we know we'll be challenged on this recommendation. So here's why we take this view.

First, let's consider how shafts are designed by shaft engineers. Driver shafts are built to have a short parallel tip section to allow insertion into the clubhead with enough extra length to allow for the stiffness and torque of the tip region to respond fully to the swing motion. 

The parallel tip section is 2.5 inches to 3.0 inches on most premium shafts. With a common hosel depth of 1.5 inches to 1.75 inches, there's not much allowance for trimming before affecting performance of the design. Trimming 1.5 inches to 2.0 inches doesn't make any sense to us with these shafts. Even an inch is highly questionable -- we are talking driver shafts here, not fairway woods or expensive alignment sticks.

If you like the performance and feel of a particular shaft in your clubfitting session but want a stiffer or more controlled feel, then try a stronger flex from the same manufacturer or try a similar EI stiffness profile from a different source. In that way you will keep the responsiveness and control that works for you. There are so many options for using stiffer or heavier shafts to achieve a desired feel, it takes away just about all the arguments for trimming. So why do it?

We think the current questionable practice of trimming driver shafts comes from earlier days of clubmaking when options for shaft design were limited, production techniques were less sophisticated, steel shafts in drivers were common, parallel tip regions were longer, graphite shaft technology was new. 

Is there ever a circumstance where tip trimming a driver shaft is reasonable? Well, we don't rule out a half-inch trim but we still ask why is it being done, what alternatives have been investigated, and have the effects on shaft feel been considered? In our experience, there is almost always an alternative to shaft trimming that should be explored before over-riding a particular performance design.

I recognize that the pot of common practice is being shaken not stirred, but it's a point that bears discussion. Let me know what you think. Send me an email and I'll follow up on a subsequent blog post. 

Gear Effect in Golf

Let's begin by considering what happens to ball flight when the ball is not hit in the center of the clubface. If the impact of the ball is not in line with the center of gravity of the clubhead and the target, the club will twist around its center of gravity.

The clubhead is deflected back away from the ball by the off-center hit, and spin in the opposite direction is imparted to the ball. The ball and clubhead rotate in the opposite direction like connected gears.

Ball spin force in the opposite direction of clubface movement adds curvature to the flight of the ball. A toe hit sends the ball to the left, and a heel hit sends the ball to the right. A driver with a flat face would yield unacceptable shot dispersion. These effects are not intuitive unless you think about the ball and the clubhead as connected gears that rotate in opposite directions as a result of the impact. 

To minimize the hook or slice effect, modern drivers are typically designed with a radius of 12 - 13 degrees of horizontal heel-to-toe curvature (bulge) and vertical sole-to-crown curvature (roll) to counteract the undesired ball flight effects of an off-center impact of the ball on the clubface. An example of bulge is illustrated below. 

Bulge functions as a compensation for excess slice or hook trajectories that come from off-center hits. Curvature of the clubface starts the ball further to the right or left allowing gear effect to bring the ball closer to the desired target direction than would otherwise be possible with a flat club face. 

Vertical roll has become significant in modern drivers with tall clubface dimensions by increasing or decreasing the effective loft at impact. Ball contact on the upper surface of the clubface opposes backspin, and contact on the lower section of the clubface enhances backspin.

Gear effect can change the tilt of the ball spin axis. The magnitude of the change reflects the difference in loft and face angle that is generated by the bulge and roll design. The gear effect generated by vertical roll can change backspin by more than 1000 rpm (about 20% - 30% of total backspin for most driver impacts) and have a substantial effect on ball trajectory and distance. In combination with horizontal bulge action as in low heel or high toe hits, gear effect can have a major impact on the shape of the ball flight.

Vertical gear effect is affected by the center of gravity of the driver. When the center of gravity is close to the clubface, gear effect is less than when the center of gravity is further back or lower in the clubhead. 

Monitoring the ball contact location on the clubface via impact tape or impact powder spray is an essential technique in clubfitting, especially with the driver. Measuring club parameters that maximize center contact and minimize heel and toe hits is an important key for achieving distance and accuracy. 

Ball Spin Fitting

The spin of a golf ball is initiated at impact by friction caused from the ball sliding up the clubface. Increased loft angles and greater club head speed generate more spin although in higher-lofted wedges roll of the ball may become significant. The upper edges of grooves on irons and wedges can enhance spin, but channeling water and grass away from the clubface might be grooves' most significant function.

The golf ball spins backwards around a single axis. The axis is horizontal when the club face and club head path are oriented in the same direction to yield a straight ball flight. Our blog posts on the D-Plane describe the importance of face angle in tilting the spin axis and affecting ball flight.

Students of physics learn that when the flow speed of a gas or liquid is increased, local pressure is decreased (the Bernoulli effect). Spin effectively increases air flow speed in the direction of the spin near the surface of the ball. The resultant decreased pressure over the top of the ball together with a corresponding increased pressure at the bottom surface provide lift forces to keep the ball in the air.

As the ball moves through the air, increased resistive pressure at the front surface deflects air in streamlined flow patterns around the ball. Air pressure gradients develop between a slow-moving boundary layer of air at the ball surface and the surrounding layers of faster streamlined flow. In the trailing wake of the ball, air pressure is decreased, causing trailing air flow to become turbulent and create a resistive "drag" force.

Dimples on the surface of the ball disturb the boundary air layer near and create micro layers of turbulent air that act to delay (extend) the formation of turbulent air in the wake of the ball. The effect of the modified turbulence is to reduce resistive drag forces and allow the ball to remain in the air longer. We can see why considerable research is directed towards finding optimal dimple patterns for given ball speeds and desired trajectories. There's big money in those dimples.

The aerodynamics of the golf ball are further complicated by the compressive properties of the ball, impact position on the clubface, ball speed, and the effects of wind force and wind direction. Understanding what contributes to lift and drag forces that extend the flight of the ball, and how impact forces are transmitted to the ball to affect its trajectory (the D-Plane) are sufficiently complex that fitting for ball spin remains a mix of art and science. 

There are cost effective choices to be made in finding the right ball for each player. It's both illuminating and fun to test golf balls in a controlled environment (hitting into a net) with a launch monitor and a professional clubfitter who understands how your swing mechanics and golf equipment translate into ball performance.

The bottom line is that clubfitting and ball performance testing with launch monitor technology is required to identify optimal ball spin and ball design for each player. Significant effects on both distance and accuracy are at stake for players over a wide range of skill levels. Spin matters. Test, don't guess.

D-Plane and Ball Spin

The concept of the D-Plane introduced by T.P. Jorgensen in The Physics of Golf, second edition, 1999, describes what happens to the flight of the ball immediately after it impacts the clubface -- we reviewed it in our previous blog, D-Plane and Ball Flight. Here we continue our discussion on how knowledge of D-Plane helps you understand your ball flight.

Upon impact, the ball slides up the clubface and ball spin is generated by frictional resistance between ball and club. The rotational direction of the spin is under and back -- we think of it as underspin or back spin. The spin occurs around an axis that is perpendicular to the direction of the spin.

In a straight shot where the clubface orientation and the club path are in the same vertical plane, the axis of ball spin is horizontal. When the face angle is open relative to the club path, the ball spin axis is tilted to the right as illustrated below. 

When the spin axis is tilted to the right, the ball flight trajectory will fade or slice for a right-handed golfer. If the club path is straight towards the target and the clubface is open the rightward tilt of the spin axis sends the ball slicing to the right.

If the club path is towards the left with an open clubface the spin axis is still tilted right. The ball trajectory will start out to the left then fade or slice to the right. The amount of curvature in the direction of the ball flight is a direct function of how much tilt there is in the spin axis. There is no separate side spin affecting direction.

When the ball contacts the center of the clubface and the face angle is not the same as the club path, the spin axis tilt is perpendicular to the orientation of the D-Plane. However, when ball contact is elsewhere on the clubface in the horizontal (heel to toe) direction, the force of the impact will cause the clubhead to twist slightly. Clubhead twist changes the face angle and causes the spin axis to tilt. 

When the ball impacts the heel of the clubface, the clubhead twists in a counter clockwise direction causing the ball to rotate in the opposite direction -- clockwise to the right. The spin axis tilts right and the ball flight goes right.

The opposite rotations of the clubhead and ball at the moment of impact can be described in terms of a gear effect. The ball and clubhead always rotate in opposite directions. The side spin from the gear effect adds to the spin of the D-Plane and tilts the spin axis. Gear effect is helpful for understanding ball flight and we'll consider it in a future blog post. 

D-Plane and Ball Flight

The concept of D-Plane (descriptive plane) comes from T.P. Jorgensen in The Physics of Golf, second edition, 1999, to describe what happens to the flight of the ball immediately after it impacts the clubface. The author's conclusion about ball flight differed somewhat from conventional wisdom and was largely ignored until radar launch monitor data verified the concept less than 10 years later.

What Jorgensen pointed out was that the initial ball flight path lies within the geometric plane determined by a line perpendicular (termed "normal" in geometry) to the clubface and a line along the path of the clubhead at impact. The direction of ball flight in this plane depends on the orientation of the club face relative to the swing path of the club through the impact zone. How far the ball travels and the direction of the ball later in its flight depends mainly on clubhead speed, launch angle (and angle of attack), and ball spin.

Here's what the concept means for a straight shot where the path of the club and the clubface orientation at impact are identical. The D-Plane is vertical and the ball moves straight to the target. A vertical D-Plane can also be generated by a straight pull to the left or a straight push to the right. Here is a side view of a vertical D-plane as illustrated by Jorgensen. 

Arrows illustrate a line perpendicular (normal) to the clubface and the vectors (a vector is a quantity with magnitude and direction) for clubhead motion, initial ball flight direction, and lift force on the ball. We can also think of the normal line as representing the loft of the clubface at impact.

The D-Plane helps us understand the precision that's required in aligning the clubface and club path vectors. But hitting the ball straight can be difficult for most golfers. Let's consider a more common shot: e.g., when the club swing is on an outside to inside path (left direction) and the clubface is aimed straight at the target.

The figure above (coutesy of David Nel from FlightScope) illustrates an iron shot in which the clubface is aimed at the target but the club is moving towards the left on an outside-to-inside path. Measured ball flight trajectory is closer to the aim of the clubface than to the direction of the club path. Consistent with this illustration, launch monitor data for different clubs and all types of shots shows that face angle orientation accounts for about 80 % of ball flight direction through the impact zone.

We will consider what happens to the subsequent flight of the ball in our next blog on the D-Plane and Ball Spin.