All about Gear Effect - 2a

Dave Tutelman  --  March 28, 2013

Vertical gear effect revisited:

Clubhead designer's view


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.




Last modified - Mar 28, 2013