# Moment of Inertia

TECH NOTE 24: I've been using an Integra IV driver with an SK Fiber Lite Revolution shaft. I've been reasonably happy with it and hit it at least 20 yards further than my previous GBB style driver. Unfortunately I don't always hit it on the center of the clubface. It still flies further than I'd expect but it just doesn't have that pleasing feel of a center hit. I recently built an Intergra IV 3 wood. It's a very oversized head and is very difficult to hit off the fairway (don't pay any attention to what the salesman says). This 44" club is fantastic off the tee however. I hit it a mile and 90% feel dead solid perfect. I recently played in the Wisconsin State Match Play Championship. I hit driver off of the first hole and ended up scrambling to save par. I hit the three wood on the next 13 driving holes, never missed a fairway and shot 72. I fooled around with the driver for some time and with all my tweaking was never able to get it to feel like the three wood. I wish I could figure it out.

I've never really looked into the subject but I've read a bit about matching clubs with moment of inertia rather than swingweight. Swingweight is basically a mass times length measurement. This is a static measurement and many say it really bears no relationship to what goes on in the golfswing. We match swingweights basically because it's easy. The real measure of the resistance of your hands to the acceleration of the club during the swing is its moment of inertia. Swingweight is basically mass times length. Moment of inertia is mass times length squared. It can't be measured with a simple balance system.

MOI is really not that hard to measure. It basically requires a stopwatch. If a club is clamped in a bearing arrangement about 5"down from the butt end and allowed to swing like a pendulum it's period of oscillation is proportional to the square root of its moment of inertia. I built a simple bearing system to mount the club and using a stopwatch I measured the period of oscillation of about 20 cycles of the pendulum. It was quite easy to get repeatability to about 1%.

Since MOI is a function of length squared whereas swingweight is a linear function of length it would appear that my driver should have a lower swingweight than my three wood to get matching MOI's. I haven't gotten around to doing all these measurements but I'm not convinced MOI is the answer either. I've fooled around with the swingweight of my driver and it feels worse as the swingweight goes down.

It's the middle of summer and I'm too busy golfing to really get into these experiments. Hopefully this winter I'll get the time and publish another Tech Note on the subject.