Why MOI?
Dave Tutelman -- 1/21/2002
Some people have expressed confusion about what moment-of-inertia
(MOI)
matching does for golf clubs. This short article is intended to
convey
that, but you'll have to look elsewhere for other things like:
* What MOI is. I've explained elswhere about first and second
moments.
This article assumes you know what is there.
* How to compute the MOI of a club, and how to match clubs for
MOI.
Again, I've covered that in other articles.
OK, so why MOI-match clubs? When is it a good idea and when is it not?
The point of matching a set of clubs (especially a set of irons)
is so
that all the clubs will be at the same point of "release"
when they hit
the ball. If they are released the same then (1) the clubface
will be
square, and (2) the loft at impact will be as the club is designed.
Well,
that assumes the clubs are matched to each other and to THE GOLFER.
If
they are well matched to each other but not to the golfer, the
clubface
may be a consistent amount open or closed, rather than always
square.
Ultimately the point of custom fitting is to fit EACH CLUB
to the golfer.
Matching clubs to each other assumes there is something consistent
about
the subject golfer's swing; you can design one ideal club for
the golfer
and match the rest of the clubs to that one. This is a flawed
assumption,
so we need to know a little more about golfer's swings, how they
work,
and how they vary from club to club.
HITTERS VS SWINGERS:
The first distinction we need to make is between two extreme styles
of
swing, the "hitter" and the "swinger".
* A pure hitter develops clubhead speed by turning the club with
the
hands and the wrists, hitting the ball with those extremities.
* A pure swinger develops clubhead speed with body turn, allowing
centrifugal force to throw the club from "cocked"
to impact and
leaving the hands and wrists free to hinge.
Of course, there is no such thing as a pure hitter, and very
few pure
swingers. Most golfers tend in one direction or the other, though,
and
it will have some effect on what parameters need to be balanced
in order
to match clubs across the set. In particular:
* With a pure hitter, the release of the club is timed by the
club's
moment of inertia, the "second moment" of the club.
* With a pure swinger, the release of the club is timed by the
ratio of
the first moment to the second moment.
In both cases, the moment is taken around the axis of the hinge
between
the arms and the shaft, which depends on the golfer and is somewhere
between the wrists and the middle of the grip.
BALL POSITION:
Does the golfer place the ball at the same point in his/her stance
for
every club? For instance, middle of the stance, or maybe an inch
inside
the left heel. If so, then you want to match the clubs so the
release
occurs at the same point in the swing.
But a lot of golfers do not position the ball the same for
all clubs.
For instance, I like to play my sand wedge from an inch or two
behind
the middle of my stance, and the long irons well forward (2-3"
inside
the left heel). I position the ball gradually more forward as
the iron
gets longer. For my swing, a "matched set" would delay
the release as the
club gets longer.
MATCHING SETS FOR REAL GOLFERS:
How do these swing differences play out in trying to match a set
of irons
for a golfer?
As it turns out, a pure swinger is relatively immune to difference
in
heft of the clubs. The less "hit" in the swing, the
less you need to do
(indeed, the less you CAN do) for the golfer by matching swingweight,
MOI, or any other measure of club heft.
So any heft-matching we do is for the hitter. You needn't be
a pure
hitter for this to matter (in fact, you are almost assuredly not
a pure
hitter); any amount of "hit" in the swing can be matched
by the following
rules of thumb.
* If the golfer plays the ball from the same point in the
stance for
every club, then you want to match MOI. That means the the
timing of
the release will be the same for all the clubs.
* If the golfer plays the ball more forward as the clubs get
longer,
then you want the MOI to increase as the clubs get longer (to
delay
the release until the clubhead reaches the ball). It turns
out that
a swingweight match causes the MOI to increase with increasing
club
length, and so it is a good first cut to match clubs for this
golfer.
This assumes that the golfer's swing itself does not change
from club to
club. If it does, then you will have to individually match the
golfer to
each club in the bag; it will not do to match the clubs to each
other
with any of the conventional matching criteria.