All about Gear Effect - 2a
Dave
Tutelman --
March 28, 2013
Vertical
gear effect revisited:
Clubhead designer's view
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Or maybe there is yet another explanation!
It has
been said that god is in the details. There is one detail we
have been ignoring.
Figure 2-3
So far, we have treated the force as being in the direction of the
clubhead's movement, as shown in the left image of Figure 2-3. Let's
remember that the force is Newton's "equal and opposite reaction" to
the departure of the golf ball. A more accurate picture would have the
force exactly opposite the departure direction of the ball -- the
launch angle -- as shown in the right image of Figure 2-3. This will
make a very small difference in C
and a much larger difference in y,
both in such a direction as to reduce the gear effect. Specifically:
C = D cos
a
y = H - D sin a
Where
- a = launch angle
- D = depth of CG
from the center of the face (what we had been calling C
until now.)
- H = height of
impact above the center of the face (what we had been calling y
until now.)
That is not going to make much difference in C
(only about 2%, for launch angles below 12°), but it will make a big
difference in y. Let's re-do the
table above, with this more accurate picture. (We will use D=1.3",
consistent with the driver data from inpakuto.com.)
Of course, we are going to have to include the launch angle and the
actual value of y. We won't
bother re-computing C, as the
difference will be minimal, way less than any precision in our estimate
of D.
Height
of
impact |
Loft
at
point of
impact |
Launch
angle |
Actual
y |
Backspin
due to
loft |
Topspin
due to
gear effect |
Net
backspin |
Distance
(yards) |
Angle
of
descent |
| Center |
11° |
9.7° |
-0.22" |
3135
rpm |
-825
rpm |
3960
rpm |
237.2 |
41° |
| 0.2" above |
12° |
10.5° |
-0.04" |
3416
rpm |
-150
rpm |
3566
rpm |
241.6 |
39° |
| 0.4" above |
12.9° |
11.2° |
0.15" |
3668
rpm |
563
rpm |
3105
rpm |
244.5 |
37° |
| 0.6" above |
13.9° |
12° |
0.33" |
3947
rpm |
1238
rpm |
2709
rpm |
245.7 |
35° |
| 0.8" above |
14.8° |
12.7° |
0.51" |
4198
rpm |
1913
rpm |
2285
rpm |
244.9 |
34° |
This is more like it! This
is what we should
have expected. We can wring quite a few extra yards of carry out of a
strike 1/2" to 3/4" above the center of the clubface -- and get a bonus
of more roll after landing (a consequence of the lower angle of
descent).
A few points to carry away from this table:
- A
center strike will create vertical gear effect to increase the
backspin. That is because the force passes below the CG and rotates the
face downward.
- In fact, you have to strike almost 1/4" above
center face just to be gear-effect neutral -- no spin due to gear
effect. You need that just to get the "nominal" distance out of the
driver.
- The topspin due to vertical gear effect looks much
more
reasonable here. It is still higher than most estimates, but not by
nearly as much as before. But...
- If we looked at the consequence of a low hit, we see
oodles of gear effect backspin.
If we go between 3/4" high and 3/4" low (a total of 1.5") we still see
the same spin difference of 5600rpm. But it is biased more
towards backspin, since the spin-neutral point is almost 1/4" above the
center of the face.
The conclusion from this is that vertical gear
effect is a very good reason to try to hit your driver high on the
clubface.
Before
we leave the subject, I'd like to point out that the real gains will
probably be smaller than those in the table. The table was based on a
ball speed of 150mph
for all the rows. But there are small losses of ball speed as impact
moves up the face, due to:
- As we can see in Figure 1-3,
the sideways velocity of the face (which creates the gear effect) is
accompanied by a backwards velocity. This is essentially a loss of
clubhead speed. For a strike 0.6" above center, this is a loss
of 1.5%. (But that is not a loss compared with a center
strike,
which also has a gear effect loss of about 1%. The lossless strike
occurs where y=0, about 0.22"
above center.)
- There
may be some falloff of COR away from the center of the face. How much?
That depends on the clubhead designer, and how well it was designed to
keep the maximum COR over as much of the face as possible.
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Last modified - Mar 28, 2013
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