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View Full Version : Research-a-thon Case Study: Trackman and off-center impact



Jeff Martin
February 12th, 2013, 03:01 PM
Over at Richie's, gmbtempe, a Research-a-thon participant, posted the following a couple weeks ago:


I saw this post on Twitter from teacher Andrew Rice:

"Did you know that a strike a half an inch off center with a driver can be enough to tilt the Spin Axis up to 20 degrees!"

He is a big Trackman user.

My question though is 20 degrees a lot? What does 20 degrees do to the ball?

When you get the path and face angle, how does Trackman know where you struck the ball? How do you know, without measuring impact on every shot, where you impacted it and how do you factor that into learning with Trackman?



Here is my response:


Excellent questions.

I assume Andrew is referring to the influence of bulge on the actual clubface angle at impact. If you know the radius of the bulge on your driver, simple trigonometry will tell you how much a toe-side hit "opens" the clubface compared to center-impact, or how much a heel-side strike "closes" the face angle compared to center-impact. Obviously, a more "open" or "closed" face will add spin axis tilt. But, rather than speak in terms of spin axis tilt, which no one is really familiar with, why not just look at the number of degrees of face angle that are involved?

You'll recall that at the Reserach-a-thon we sprayed the clubface with foot powder before each shot, then took a picture of the clubface post-impact. Here is one of your driver (BTW, this is your drive that landed smack on the target line, at least according to Trackman, the best we recorded!):


http://i223.photobucket.com/albums/dd102/jeffy10028/GB-6-01R_zps3e68c751.png


Using ProAnalyst imaging software, it is easy to precisely estimate how far from the center of the clubface contact was made. ProAnalyst uses the ruler to calibrate pixels into centimeters, then, by setting the "origin" in the clubface's geometric center, can tell us how much towards the toe impact was made as well as how high on the face. Then, using the measured bulge and roll, we can calculate in Excel how much more "open" impact was and how much more loft was "added" compared to center impact. Here are your results:


http://i223.photobucket.com/albums/dd102/jeffy10028/gregbrownloftadjustments-marked_zps31dc8ba7.png


For that particular drive, which was coded GB - 6 - .1R, contact was 0.84 inches towards the toe and 0.55 inches high on the face. As a result, that impact location was almost 4 degrees more open than center impact, and added 2.64 degrees of loft.

Now, how does Trackman "know" the impact location? Well, it doesn't. It uses collision physics to "back into" the likely face angle AT POINT OF IMPACT. Based on the observed clubhead path and the initial ball flight, the Trackman software estimates what face angle relative to the path would produce the observed initial ball flight.

Importantly, the actual spin axis tilt is measured, so off-setting gear effect is reflected there. As a result of gear effect, an open face-to-path relationship that produces a slice with center impact might, on a toe-hit, produce hook spin axis tilt, as happened with this drive. Here are the Trackman numbers:

Path: 1.6 degrees (in-to-out)

Face angle: 3.4 degrees (open)

Face-to-path: 1.8 degrees (open)

Spin axis tilt: -3.4 degrees (hook spin)

As I mentioned, that ball landed dead on target.

By looking at the displayed path, face angle and spin axis numbers, experienced Trackman users can make pretty good guesses about impact location.

What this means for you is that, whenever you use Trackman, apply to the clubface spray powder (DON"T use Dr. Scholl's: it comes out too thick; buy the cheap stuff) or dry erase markers like are used on white boards, and, after enough experience, eventually you'll be able to deduce impact location simply from the displayed numbers.

One other thing worth knowing is that Andrew's tweet is wildly misleading. Andrew is ignoring the simultaneous, offsetting influence of gear effect in his tweet. Gear effect is precisely why bulge is built into drivers in the first place. If a driver existed where a half-inch mishit actually created 20 degrees of axis tilt, no one would ever buy it. That kind of "information" from Andrew can do more harm than good.




Jeff

gmbtempe
February 12th, 2013, 03:15 PM
I tweeted him back, his response was its too complicated to explain and go to Trackman.

Also Tman got the "net" face somewhat correct at 3.4 open, but if I was trying to learn from the numbers I might assume I came into the ball with an open face, when in reality the face was not open but the strike "opened" the face, if I am understanding your post correctly.

I wonder how close I was to delivering a square face, maybe a picture right before impact?

Jeff Martin
February 12th, 2013, 03:33 PM
I tweeted him back, his response was its too complicated to explain and go to Trackman.

Well, it isn't too complicated: I explained it!!


Also Tman got the "net" face somewhat correct at 3.4 open, but if I was trying to learn from the numbers I might assume I came into the ball with an open face, when in reality the face was not open but the strike "opened" the face, if I am understanding your post correctly.

I wonder how close I was to delivering a square face, maybe a picture right before impact?


If the Trackman face angle is correct, the center of the clubface was about 0.6 degrees closed at max compression (3.4 degree face angle at the impact location less 3.99 degrees of bulge "opening"). But it was the impact location and bulge that opened the face angle nearly four degrees, not the face twisting open because of the off-center strike.

Since that strike was on the toe-side, the clubface might have opened a tiny bit between initial contact and maximum compression, but in your case I doubt it. That particular swing wasn't recorded by the Phantom (not my fault! blame Kelvin!), but two other drives with toe-side impact did not open between initial contact and max compression, although they both opened after max compression.



Jeff

gmbtempe
February 12th, 2013, 03:35 PM
Well, it isn't too complicated, I explained it!!




If the Trackman face-angle is correct, the center of the clubface was 0.6 degrees closed at max compression (3.4 degree face angle at the impact location less 3.99 degrees of bulge "opening").

Since that strike was on the toe-side, the clubface might have opened a bit between initial contact and maximum compression, but in your case I doubt it. That particular swing wasn't recorded by the Phantom (not my fault! blame Kelvin!), but two other drives with toe-side impact did not open between initial contact and max compression, although they both opened after max compression.



Jeff

What happens if you are using a Trackman, to dial in your swing to the nearest millimeter, and you dont use impact marking, and dont know the buldge of the driver being used?

Jeff Martin
February 12th, 2013, 03:47 PM
What happens if you are using a Trackman, to dial in your swing to the nearest millimeter, and you dont use impact marking, and dont know the buldge of the driver being used?


You're wasting your time???

Spin axis tilt and face-to-path will give you an idea of impact location: negative spin axis tilt with an open face-to-path indicates a toe strike, for example. Apparently, Tuxen is so good at this it is claimed he can guess clubface location within a quarter-inch. However, we are also seeing significant deviation between the Phantom and Trackman on face-to-path, and the Trackman spin axis tilt numbers have yet to be independently verified, so maybe my first answer is the correct one!


Jeff

gmbtempe
February 12th, 2013, 03:49 PM
You're wasting your time???

Spin axis tilt and face-to-path will give you an idea of impact location: negative spin axis tilt with an open face-to-path indicates a toe strike, for example. Apparently, Tuxen is so good at this it is claimed he can guess clubface location within a quarter-inch. However, we are also seeing significant deviation between the Phantom and Trackman on face-to-path, and the Trackman spin axis tilt numbers have yet to be independently verified, so maybe my first answer is the correct one!


Jeff

Just trying to understand things correctly. I suppose the teachers, like BM, that has received training from the likes of Tuxen could use the device and help out a student from the numbers alone.

Jeff Martin
February 12th, 2013, 04:05 PM
Just trying to understand things correctly. I suppose the teachers, like BM, that has received training from the likes of Tuxen could use the device and help out a student from the numbers alone.


I learned today that Quintic was hired by the UK PGA to evaluate Trackman because of a concern that UK instructors were "teaching" from Trackman "straight off the computer screen" without adequate knowledge of what the numbers meant or the machine's limitations. This was Quintic's conclusion:

1621



Jeff

gmbtempe
February 14th, 2013, 11:47 AM
Update:

Had an email from Andrew Rice explaining the spin axis changed 20 degrees with an off center hit. I had incorrectly interpreted that to mean a change in direction of 20 degrees when in affect it was 20% of the side spin tilt changed, which has a much smaller affect it appears. The ball could still easily end up close to the target when other factors are considered, where to me that souned wildly offline as I was relating degrees in terms of change of direction.

Anyway, I am glad he clarified the information he provided on Twitter with the following backup. No ill will on my end and hope to follow Mr. Rice and expand my knowledge of the golf swing.

From email,

Dave Tutelman (who has no affiliation with TM of FS and is pretty much just a mathematician) has a pretty easy to understand description of the gear effect here:




http://www.tutelman.com/golf/ballflight/gearEffect.php



For drivers he estimates the sidespin (the "tilt" component to backspin) with the following formula:




SideSpin(rpm) = 16.4*BallSpeed(mph)*HorzMiss(inches)



The magic 16.4 comes from looking at actual drivers and seeing how the location of CoG varies with MoI. In general the closer the CoG is to the face, the lower the MoI of the club.



The formula for spin rate if in line with the CoG (from Fredrik Tuxen from TM):




SpinRate(rpm) = 2.3*ClubSpeed(mph)*SpinLoft(deg)



So in the following example:




ClubSpeed = 100 mph/SpinLoft = 13 deg (Driver)/HorzMiss = 0.5 in



SpinRate(Dpl) = 2990 rpm



Assuming center strike (max Smash Factor)




BallSpeed = 1.5*ClubSpeed = 150 mph



SideSpin(HGE) = 16.4*150*0.5 = 1220 rpm



1220 rpm sidespin compared to 2990 rpm backspin gives 22 deg tilt of spin axis.

Jeff Martin
February 14th, 2013, 01:43 PM
Update:

Had an email from Andrew Rice explaining the spin axis changed 20 degrees with an off center hit. I had incorrectly interpreted that to mean a change in direction of 20 degrees when in affect it was 20% of the side spin tilt changed, which has a much smaller affect it appears. The ball could still easily end up close to the target when other factors are considered, where to me that souned wildly offline as I was relating degrees in terms of change of direction.

Anyway, I am glad he clarified the information he provided on Twitter with the following backup. No ill will on my end and hope to follow Mr. Rice and expand my knowledge of the golf swing.

From email,

Dave Tutelman (who has no affiliation with TM of FS and is pretty much just a mathematician) has a pretty easy to understand description of the gear effect here:

http://www.tutelman.com/golf/ballflight/gearEffect.php

For drivers he estimates the sidespin (the "tilt" component to backspin) with the following formula:

SideSpin(rpm) = 16.4*BallSpeed(mph)*HorzMiss(inches)

The magic 16.4 comes from looking at actual drivers and seeing how the location of CoG varies with MoI. In general the closer the CoG is to the face, the lower the MoI of the club.

The formula for spin rate if in line with the CoG (from Fredrik Tuxen from TM):

SpinRate(rpm) = 2.3*ClubSpeed(mph)*SpinLoft(deg)

So in the following example:

ClubSpeed = 100 mph/SpinLoft = 13 deg (Driver)/HorzMiss = 0.5 in

SpinRate(Dpl) = 2990 rpm

Assuming center strike (max Smash Factor)

BallSpeed = 1.5*ClubSpeed = 150 mph

SideSpin(HGE) = 16.4*150*0.5 = 1220 rpm

1220 rpm sidespin compared to 2990 rpm backspin gives 22 deg tilt of spin axis.




Andrew neglects the offsetting influence of bulge built into the driver clubface. Apparently, he only read the first half of Tutelman's gear effect explanation! The clubface curvature built into a driver with a 12-inch bulge radius (like yours) would "open" the clubface by 2.4 degrees on a half-inch toe-side mishit, introducing offsetting "slice" sidespin.

Below is a table produced by Tutelman using similar numbers to Andrew's. As you can see, the "net" hook sidespin would be around 437 rpm, not 1,220, producing just a 6 yard miss. Without offsetting bulge (last column), Andrew's 22 degrees of spin axis tilt would send the ball into the next fairway (or out-of-bounds)! This stuff is complicated enough without "experts" going around explaining it all wrong.



1635



http://www.tutelman.com/golf/ballflight/gearEffect1.php







Jeff

gmbtempe
February 14th, 2013, 02:02 PM
Thats why I said other factors.

Gotta love modern engineering.

Jeff Martin
February 14th, 2013, 02:17 PM
Thats why I said other factors.

Gotta love modern engineering.


BTW, 437 rpm of net sidespin is about 8 degrees of spin axis tilt if the ball has 3,000 rpm of backspin.




Jeff