I read through Dave Tutelman's comments on the shaft behavior model that
FitChip is relying on and I agree with him that their model is wrong. You
can't treat a golf club swung by the hands the same as a club clamped in a
vise. The vibration mode is not the same. First, the hands are far too
soft to 'clamp' the end of the club (as Dave pointed out). Once the
initial bending of the club is established at the top of the downswing and
the club starts recovering from this bending it is more reasonable to look
at the club as a shaft vibrating between to massive weights on the end (the
hands on one end and the clubhead on the other, similar to the way a guitar
string oscillates when plucked in the middle (fundamental oscillation, I
don't what to hear about harmonics)). The mass of the shaft is
significantly lower (specially for drivers) than the mass of either the
hands or the clubhead. The clubhead appears to lead or lag at impact, but
in reality the clubhead is very nearly at the same place it would be with
an infinitely stiff shaft, but the shaft is bowing so the clubhead appears
to be leading or lagging. This isn't absolutely correct, but it's much
closer than most people think.
I did some tests with 200-g tip weights (no offset c.g. effects) on driver
shafts and monitored the shaft behavior with an InPractice video system
(60-frames per second). The data is not good enough to make truly
quantitative statements, but good enough for some qualitative
statements. Shaft bow at the impact point is a function of shaft
stiffness, sometimes the 'head' leads, sometimes lags, depending on the
vibration frequency of the shaft and the timing of the swing. It does
exactly what you would expect it to do. I used an 'A' flex shaft and an
'S' flex shaft and, with my swing, the 'S' flex shaft had oscillated
through straight, bowed forward, and recovered back to straight at the
impact point. Remember this is with a 200-g tip weight so there are no
offset c.g. effects. With the lower frequency of the 'A' flex shaft it had
recovered to lead slightly from the initial bend at the impact point. With
a higher frequency shaft, or a lower frequency shaft there could easily be
a 'lag' at the impact point.
With a real clubhead installed the offset c.g. effects add to the shaft
oscillatory motion. With the 'S' flex shaft the shaft was bowed slightly
forward at the impact point. With the 'A' flex shaft the lead was greater
than with the tip weight. The offset c.g. effects are sufficient, I
believe, that it is unlikely that you would ever see a 'lag' at impact with
an actual golf club.
Regards,
Alan Brooks
At 07:28 AM 10/26/2006 -0400, you wrote:
Tom. Please take a look at the following link:
http://www.fitchip.com/fitchip4club.htm
I always believed that the shaft reacted just the opposite from the pics
above the graph. Your take please.
André.
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