All about Gear Effect - 6
Dave
Tutelman --
March 19, 2009
Rolling right along - is GRT great?
Figure 6-1
Figure 6-1 shows the reasoning that leads to GRT. If you put constant
loft on a driver instead of roll, you will get constant launch
conditions, because the launch conditions are a function of loft,
clubhead speed, and angle of attack. So low-face hits will not be low
ropes, and high-face hits will not be sky balls.
This concept, unfortunately, is doomed from the start. One of the
launch conditions is spin. Now we know that, due to
vertical gear effect, the spin will be very different from a low hit to
a center hit to a high hit, if the loft is constant.
The function of face roll is to change the loft across the face to get
a better set of launch conditions based on the height of impact. For
instance, a high hit will have a lot less backspin than a center hit.
So you want somewhat more loft higher on the face, to (a) raise the
launch angle to better match the reduced spin and (b) increase the
backspin a little. By balancing (a) and (b), you get better launch
conditions and more distance. Similarly, a low hit will have more
backspin than a center hit, so a lower loft is called for.[1]
Tom has explicitly taken the position that vertical gear effect
contributes
relatively little spin. If that were the case, then GRT would be a
great idea. But a look at the Hotstix
data shows this not to be the case. The data was taken from a
driver with some face roll. If vertical gear effect were negligible,
then backspin would be lowest for a low-face hit, and increase as
impact moved up the face. Instead, exactly the opposite was observed;
there is a significant decrease
in backspin as impact moves up the face. The only reasonable
explanation is enough vertical gear effect to overwhelm the
change in backspin due to loft. This casts serious doubt on the idea of
a constant loft driver. |
A closer look at GRT
| Let
me start this discussion with a disclaimer. Most of the
conclusions
below are based on a physical model.
The prototype testing that I have done is limited and anecdotal; those
tests support the conclusion of the model, but I need to be as cautious
about my own anecdotal evidence as I remind others to be. |
Height
on
clubface |
Loft:
ideal |
Loft:
GRT |
| +0.8" |
15.7º |
14.7º |
| +0.4" |
13.2º |
13.2º |
| center |
10.7º |
11.8º |
| -0.4" |
8.1º |
10.7º |
| -0.8" |
5.6º |
9.6º |
While it is clear from physical arguments that
a true zero-roll driver clubface must have performance problems, GRT is
not
zero-roll.The GRT clubface in practice is
as close as Wishon could reasonably go to a zero-roll clubface. It has
a 15" roll for the top third of the face, and a 20" roll for the bottom
two thirds. The table at the right shows the loft progression up the
face, for the optimized roll and GRT. The GRT does not have a flat face
by any means, but it has a lot less loft change than the model says is
ideal. (Note that the two faces were chosen to have the same loft at
+0.4", the height that gives maximum distance for a 100mph clubhead
speed.)
Tom's rationale, as I understand it, is:
- Large-head
drivers have tall faces. With the usual face roll of 9"-12", this
causes in a big loft difference between low-face hits and high-face
hits. The result is too high and too low trajectories to be very good.
- The initial goal was the flattest face possible for the
bottom 2/3, to keep
the loft at optimum everywhere on the face. Because of the difficulty
of fabricating a perfectly flat face, a curvature of 20" radius was
adopted.
- The
top third of the face has a 15" roll, mostly because many
golfers fit
themselves -- through ego -- into too low a loft on their driver. So a
bit of roll might help them. There was also a feeling that high-face
roll might be advantageous in and of itself, but that was a secondary
consideration.
- This may produce ballooning for clubhead speeds over
115mph, but is beneficial below that.
I'm not sure Tom will agree with my synopsis. You can read a very
detailed account in his own words in his
forum.
He also talks about the testing they did, and which golfers benefit
most from GRT. Wishon has always been very ready to share technical
information with the clubmaking community.
If the gear effect model is valid (and it has been validated),
then it looks like GRT is not an
optimal design. The model says that roll should be about 8"-10", not
15"-20".
That
conclusion immediately precipitated a discussion on the TWGT
forum. The discussion pushed me to evaluate GRT more
carefully. Here's what I found...
First, some personal experience.
This is not a careful, controlled test, but rather a single anecdotal
data point. Still, it told me what to examine, just in case the model
had not.
In the summer of 2008, I got into a swing pattern that had me
hitting my driver shots
thin more often than not. Entirely too many drives at "quail height". So
I got a Wishon
915CFE driver head with a GRT face to apply a band
aid while I tried to
fix my swing. I built it to the same specs as my usual driver, which
has
10" of roll.
The GRT face definitely gave me better-looking
drives. The worm-burners were a thing of the past. When I took both
clubs to the driving range, it was clear that the trajectory difference
was the
GRT face and not my swing. The 915's loft did not fall off low on the
face,
and you could see the difference in the launch angle.
There was a downside, however. At the range and on the
course, the carry distances were not very different between GRT and a
conventional-roll driver. The GRT drives
looked a lot better in the air, but they landed no farther up the
range. And the total distance -- where
the ball
stopped -- was significantly in favor of the roll face driver.
Eventually the GRT driver left the bag, and I lived with low line
drives until my swing came around in late Fall.
It
is worth adding that I should be an ideal candidate for GRT. Tom
reports that his testing showed that it might produce ballooning for
clubhead speeds over 115mph (higher than the average on the PGA Tour),
but was advantageous below that. He also said that the 20" roll radius
solved that problem. My aging swing is 80-85mph on a good day. Suffice
it to say that I was disappointed that, while GRT solved my trajectory
problem, its effect on my overall driving distance was negative.
Can the model supply an explanation?
Turns out it can. I used the model to plot carry distance vs face
height, for both the GRT roll and the ideal roll. But, because my GRT
driver got a lot less total distance -- even when the carry distance
was similar -- I also looked at angle of descent. (Angle of descent is
the primary factor in runout, how far the ball goes after it lands.)
Here are the results.
Figure 6-2
The
carry distances are pretty similar -- within a couple of yards --
for strikes above the center of the clubface. They depart on the lower
half. By 0.4" below center, GRT is giving up ten yards of carry to the
ideal loft.
Height
on
clubface |
Ideal
roll |
GRT |
| Loft |
Spin |
Loft |
Spin |
| +0.8" |
15.7º |
2600 |
14.7º |
2250 |
| +0.4" |
13.2º |
3050 |
13.2º |
3050 |
| center |
10.7º |
3500 |
11.8º |
3900 |
| -0.4" |
8.1º |
4000 |
10.7º |
4800 |
| -0.8" |
5.6º |
4400 |
9.6º |
5650 |
But carry distance is only part of the story. The angle of descent is
even more telling. Except for strikes quite high on the clubface (above
+0.4"), the GRT roll produces higher angles of descent (AoD)
-- considerably higher for the lower half of the clubface. An informal
industry rule of thumb[2]
says that you
are losing substantial overall distance if you let your AoD
get above 40º. GRT produces an AoD above 40º for all impacts
from the center of the clubface down.
Why
should this be the case? The table at the right is the same as the
table we saw above, but with columns for spin as well as loft. From the
center of the face to the bottom, where GRT is helping the trajectory,
it is also adding lots more backspin than the roll clubface. In
addition, the lower launch angle of the roll clubface produces a
trajectory that prevents ballooning and keeps down the angle of
descent. (Launch angle can be estimated by eye from the table; it
is 85%-90% of the loft.)
This would explain the results I saw from my GRT driver. Yes, I got a
pretty trajectory even from a low-face hit, but it was associated with
a lot of backspin. For a GRT face, the backspin was 5000rpm
for impact a half inch below center. For the ideal roll driver, it was
only 4000rpm. That, together with the lower launch angle, result in a
much lower AoD and a bigger runout.
What about Wishon's caveat that there might be ballooning for low hits,
if the clubhead speed is 115mph or more?
- 115mph is around -- actually, slightly above -- the
Tour average clubhead speed.
- The calculations above are based on 100mph clubhead
speed, so they should see the benefit of GRT if Tom is right. But they
do not.
- My
own clubhead speed is dropping with age; nowadays it is 80-85mph on a
good day. I should certainly have seen the benefits of GRT. I saw quite
the contrary.
- Elsewhere
in this study, there are calculations of the sensitivity of
optimal
roll to clubhead speed. Speed does not seem to matter much. From 80mph
to 120mph, the difference in
optimal roll radius is less than 2".
Finally, let me mention -- in case you haven't noticed already
-- that I have said nothing about fairway woods and hybrids. This
discussion is only about drivers. Fairway woods have much shorter faces
and shallower centers of gravity, so gear effect is not nearly as much
of an issue.
|
Re-evaluating the
model
So far, the model says that GRT is not
as good as the commonly-found face roll in commercial drivers. Wishon
has criticized this work as being just math without benefit of
prototype testing. He claims that his prototype testing validates the
superiority of GRT, but has not put forth any actual data.
Be aware that not all tests agree with Tom's. My own experience
and at least one other forum poster have observed problems from too
flat a face, especially on below-center hits.)
Added in July 2012:
I exchanged email with a longtime Internet golf friend with whom I have
actually played several rounds, even though we live at far corners of
the country. Dale has a Wishon driver with a GRT face, and I had
suggested he do a test run using impact tape to distinguish a high face
hit from a low face hit. Here is an excerpt from his response. OK,
Dave, you were right! You wrote in an earlier email, “A GRT clubface
will launch the ball at nearly the same launch angle as the high-face
hit. But, due to gear effect, there is a lot more backspin on the ball.
This results in some ballooning that will hurt carry distance (even though it still looks like a good ball flight).
It will also result in a substantially higher descent angle, seriously
curtailing rollout after landing.” [Emphasis added is Dale's.]
So
I paid much more attention to impact this past weekend, and I am
missing below center. Ball flight, as you point out, still looks fine,
so I didn’t realize I was missing low until I started verifying at your
urging. That explains a lot, and tells me what I need to work on. The
ball flight and lack of roll you describe above exactly matches the
drives I’m disappointed in.
Still, Tom's excellent
track record of innovation invites further investigation of why his
tests of the GRT clubs seem to be at odds with what the
validated model predicts. Given Tom's record,
there's enough
chance
that I am applying the model badly. So let's check in depth what might
plausibly be changed about the model, that might cause it to show an
advantage for GRT.
I re-evaluated a bunch of aspects of the computation I used to find
optimal face roll. The details are in the appendix,
but here is a summary. The aspects evaluated were:
- Change the formula for vertical gear
effect spin, 25yVb.
This would be the most effective way to prove in GRT, because GRT seems
predicated on an estimate of gear effect spin much lower than the model
predicts. Vb and
y are clearly
proportional factors in spin, so the only thing left to change is the coef=25.
Changing the coefficient enough to make GRT look good produces results
completely inconsistent with the measured Hotstix data. This
is a non-starter, because it requires a model at
odds with real, measured spin data. We'll have to look at less
all-encompassing changes.
- Assume that shaft tip flex will limit
gear effect spin. This was reported by
Upshaw, but we could verify only relatively small effects, up to maybe
15% of gear effect spin. Looking at the practical issues (e.g.- keeping
the shaft in a reasonable range for shaft fit), I tried a very generous
5% reduction of gear effect spin.
- Try a lower estimate of the backspin
produced by loft. We used a backspin formula that
produced a best fit to several sources of backspin data. If we just fit
to the source reporting the lowest backspin, we get 7% less spin. This
also allows a smaller coefficient for the gear effect spin, in order to
stay consistent with the Hotstix data. This is a fairly plausible
change, since it corresponds to one of the sources of backspin data.
- Increase the corrections for clubhead
rotation. Our model used the midpoint of
the clubhead rotation to estimate the correction. While this
is sound, I tried a [physically impossible] estimate based on the full
rotation, to see what it would do to the optimal roll.
- Factor in the rolloff of COR away from
the center of the face. As COR decreases, so does
ball velocity Vb,
a direct factor of gear effect spin. Making Vb fall
off away from the center will reduce gear effect.
Figure 6-3Figure 6-3 is a graph of all the factors and
what each
contributes to
roll radius. Evaluating whether each could justify GRT:
- Tip stiffness is slightly significant, but not very
plausible. It requires ignoring proper shaft fitting to choose a shaft
that minimizes gear effect.
- Reduced backspin from loft is significant and
reasonably plausible. As the only factor with both characteristics, I
have temporarily adopted it for the GRT analysis above (Figure 6-2). So
it is already factored in when you look at those
results; there is no more to be had from the 7% backspin reduction.
- Rotation correction of loft is very significant, but
not that likely. It is easy to justify a half-rotation correction (as
all the roll calculations in this article have done). It is impossible
to justify the full correction in the graph. It is a stretch, but might
be worthwhile, to take half the advantage (corresponding to three
quarters of the rotation).
- Rotation correction of speed is negligible, so we
won't even worry about its plausibility.
- Falloff of COR is also negligible. Although it is
very plausible, its contribution is barely visible.
If we add up the plausible factors -- reduced backspin and half the
advantage of rotation correction -- we can drive the optimum roll to
about 10", but no further. Dividing things up low and high, as GRT
does, the optimum roll is 9" high on the face, and 11½" low on the
face. That is not at all close to GRT.
Even if we
were to add up all the factors, no matter how implausible, we are left
with 12" high and 15" low, still well short of GRT.
So I don't see anything wrong enough with the model to
justify GRT. Tom and I will have to remain in disagreement.
|
Conclusions
All this leads me to the conclusions:
- The concept of a flat face -- a single loft at all face
heights -- is decidedly inferior to a proper roll.
- TWGT Graduated Roll Technology does have some small
curvature; it is not flat for practical reasons. Still, the proper roll
is considerably more curved than real-world GRT.
- There
is a significant and noticeable distance penalty, both carry distance
and runout, for a low-face strike on a GRT driver. The model says so. I
have experienced it.
- One
thing the GRT implementation got right: it looks like the lower 2/3 of
the clubface wants to have about 20% less roll than the top third. But
GRT uses 15" and 20"; the model works hard to justify even 10" and 12".
Notes:
- For a
raw-data measured example, the Hotstix data shows that moving impact up
one inch reduces the backspin by 1300rpm, while the launch angle
actually increases by more than 5º because of face roll. A quick
calculation shows that, if the face had been flat, the change in
backspin would have been well over 2000rpm.
- For instance, see
the
Golf Digest article on the 40º AoD rule of thumb. In fact,
the article quotes Tom Wishon. "What's amazing is how visible
it is," Wishon says. "You can easily see
the difference between an angle of descent of, say, 45 degrees and one
of 38. Anything under 40 degrees is a quality angle of descent for
enhancing the roll. The secret to total distance is to use the driver
that allows you to hit the lowest trajectory that carries close to the
farthest."
Last modified - April 7, 2009
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