Mandrin - question about cut angle....

[Michael Finney]

you mentioned a few months ago that the math behind why the ball doesn't take off on the club's "delivered loft" is way over most everyone's head....i believe you.....

could you put why a cut angle exists in words as opposed to formulas? in a lay sorta way?

would cut angle exist in a vacuum?

for those who don't know, if a player "delivers" 10 degrees loft to a golf ball on a centered hit, the ball will not take off at 10 degrees but closer to 8 degrees according to some trackman data.....

thanks in advance

 

[mjstrong]

for those who don't know, if a player "delivers" 10 degrees loft to a golf ball on a centered hit, the ball will not take off at 10 degrees but closer to 8 degrees according to some trackman data.....

I thought ballflight came off lower than delivered loft with irons, but higher than delivered loft with drivers (I assumed due to COG location). Is that "old" information?

 

[Jared Willerson]

Wouldn't the driver be higher because of the ascending angle of attack?

 

[Jon Hardesty]

Guys. Mike Finney and I have been looking at a lot of trackman information for the last few days and have a lot of questions that we don't have answers to yet. In all the data we are looking at, the one thing that is missing is delivered loft at impact. This piece of the pie is critical in determining the general cut angle throughout a set of clubs from LW to driver.
What we all know from the information in Theodore Jorgensen's book is that a golf ball will leave the clubface closer to a right angle with a lower lofted club (driver) and less to a right angle as the loft increases.

 

[Gibby]

"if a player "delivers" 10 degrees loft to a golf ball on a centered hit, the ball will not take off at 10 degrees but closer to 8 degrees"

Interesting. I wonder what would happen if you hit a soft rubber cube rather than a spherical ball (with the force being delivered perfectly square to one of the flat sides [and not an edge or corner]) with a modern driver.

Crude text illustration below:

Driver Head ----> Cubic Ball, hypothetical launch angle

(_] ----> [] ----->

 

[lasse]

I am sure the math is complex but it must be due to friction?, just think of hitting a slice with a ping-pong or tennis racket, the ball certainly does not come of the face of the racket at a right angle.

I can imaging it must be very difficult to give a straight answer to your question as it depends on the friction between the face of the club and the ball...a ball of a new spin milled face would come of at a lower angle than a worn out wedge (with the same delivered loft).

 

[niblick1]

I've been reading the old Trackman newsletters lately.

Is what you are saying, using Trackman terminology, is "why does dynamic loft minus attack angle give you spin loft (the loft ball launches on)"? Or asking something else?

 

[Jon Hardesty]

Niblick1. Where have you seen any dynamic loft numbers at impact? On the Trackman newsletter?

 

[niblick1]

Niblick1. Where have you seen any dynamic loft numbers at impact? On the Trackman newsletter?

Google 4.0 Trackman newsletter, and you see a discussion of the option of being able to show dynamic loft or spin rate.

Various diagrams in the pdf newsletters describe the dynamic loft variable and show spin loft. But I am not looking at trackman output data of my own.

I am just trying to make sure I understand the question! Michael F has used "cut angle" in the past, and I have never been sure I understood what he meant by the term.

 

[Jon Hardesty]

I have some Trackman numbers from the PGA Tour (2004-2008). Like I said, one of the major missing pieces to the info. that I have is the actual "delivered loft" of a given shot with a given club. Without this info. it is virtually impossible to determine the actual cut angle of a shot, even if you have the "attack angle" you still need to know the delivered loft at impact. This stuff can get complicated huh?

 

[Kevin Shields]

Is the cut angle measured in relation to the ground or the continuing downward attack angle?

 

[Jon Hardesty]

Kevin, the cut angle is the angle at which the ball leaves the clubface.

 

[mandrin]

if a player "delivers" 10 degrees loft to a golf ball on a centered hit, the ball will not take off at 10 degrees but closer to 8 degrees according to some trackman data.....

Michael,

Exact calculations regarding what happens during impact are difficult because of the large physical deformation of the ball, the varying friction force due to sliding, sliding/rotation and rotation of ball on clubface surface, all happening during the tiny 0.0004 sec interval. Best one can do is a first approximation as done by Prof Jorgensen in ‘The Physics of Golf’.

However you asked for getting some guts feeling for the reason why the ball’s departure velocity vector is slightly inside the face normal vector. Since club and ball are both moving during impact let’s take instead the very simple case of a ball hitting obliquely a wall. The underlying physics is the same but the associated imagery is easier to understand.



We know that if we let a ball drop from a certain height it bounces back a little less high, caused by some energy loss due to deformation. Let’s assume no friction than a ball bouncing obliquely on a solid surface will have a similar reduction in the vertical component of the velocity but none in the horizontal direction.

The resulting trajectory of the ball after impact with the wall is now slightly inside the dashed line, which represents the ideal case for an ideal COR of 1. Rather simple but as soon as we throw additionally friction and deformation into the equation it becomes a totally different matter.

 

[jmessner]

Face Roll....

Also keep in mind that most drivers have roll (curvature) on the face so that high on the face the loft can up to be 2-3 degrees higher than in the middle. The reverse is true on lower part of the face. So, you almost have to know where on the face the ball was hit also.

 

[Kevin Shields]

Kevin, the cut angle is the angle at which the ball leaves the clubface.

Yes, i know. But if you toook the cut angle and measured it against the downward angle under the ground and not the horizontal ground you might end up with the delivered loft.

 

[Michael Finney]

Mandrin, thanks for your reply - it helps my understanding.

 

[dominicscaife]

Launch Angle Physics

I think of it like this: the higher the loft, the more of a glancing blow we are applying, so the clubface loft has less effect on the ball.

This article explains it in pretty understandable terms (quoted below):

Design Notes - Golf Physics p2

I'm curious how well your trackman data compare with Mr. Tutelman's I'm sure he'd appreciate in any good data you have to refine his software.

There are some graphics if you follow the link above.

Direction of the ball

Let's take another look at the release of the ball from the clubface. The picture shows the direction and the spin of the ball (the red arrows), along with two other important directions: the direction the clubface is pointing (blue arrow) and the direction the club is moving (green arrow).

The direction of the ball -- called the launch angle -- is always between the arrows, and is almost always closer to the clubface direction. If there were no friction involved, then the ball would slide up the face and release in exactly the direction the clubface is pointing. But friction causes the ball to roll on the clubface instead of slide. The upwards motion of the ball is used to get the ball spinning. Since the ball has a moment of inertia, it takes some torque (force at the edge of the ball) to make it spin. That force comes out of the upwards acceleration of the ball, so it takes off a little lower than the clubface is pointing.

How much lower? The equation is a complicated, but not nearly as ambiguous as ball speed was. The three references (C&S, Wishon, and Dupilka) agree on the numbers for launch angle. For very small lofts, the direction of the ball is nearly the same as the direction of the clubface. Put another way, the launch angle is the same as the loft.

As the figure shows, the launch angle becomes a smaller fraction of the loft as the loft increases.
For typical driver lofts, the launch angle is about 88% of the loft. For a 10° driver, the launch angle is just under 9°.
At about 20° of loft (the beginning of the irons) the launch angle is down to about 80% of the loft (the yellow dotted line). For lofts in this range, you can think about an 80:20 rule; the ball is 80% to the clubface direction and 20% to the clubhead path.
In the area of the wedges, the launch angle is still more than 60% of the loft. So, for all reasonable golf clubs, the ball's initial direction is closer to the clubface direction than to the clubhead path.
While the equations to come up with launch angle are complex, there is a simple formula that fits the curve very well. ("Fits the curve" means that the equation has nothing to do with the physics of the situation, but it happens to give the same result for all practical purposes.) This formula is:
LaunchAngle = Loft * (0.96 - 0.0071*Loft) It gives results within a tenth of a degree up to 40° of loft, and stays within about a degree up to 60° of loft.
The same physics works in the horizontal direction. If the clubface isn't square to the path, the ball takes off between the two directions, and much closer to the clubface direction.

Since the loft is probably a bigger angular difference than the horizontal lack of square, you can use the loft to set the percentage difference of direction. For instance, the graph above shows that a 12° driver has about 85% conversion of loft to launch angle. So the sidspin due to a few degrees of non-squareness is also probably 85% in the clubface direction and only 15% in the clubhead path direction.

 

[tongzilla]

why is there no reduction in the horizontal direction?

 

[mandrin]

why is there no reduction in the horizontal direction?

Entities such as forces or velocities are vectors. The advantage of vectors is that one can analyze separately for the usual 3D x,y,z coordinates. For our 2D oblique collision problem conveniently one takes the coordinates to be parallel and perpendicular to the surface. When analyzing for the parallel coordinate, and taking into consideration the absence of friction forces during impact, it follows that there is no change along this coordinate due to impact and the parallel velocity component remains hence unchanged.