Gear & EquipmentJune 20, 2011

One-thousandth of an inch might not be as much as Johnny Miller thinks it is

When Johnny Miller uttered on Saturday's broadcast that "one-thousandth of an inch on a driver is 20 yards of hook," I was perplexed. Now that I have some data in front of me, I want to revise my opinion. That calculation is monstrously wrong.

I'm looking right now at some of the robot testing numbers for the 14 drivers that made the 2011 Hot List, many of which are being used by players on tour and competing in this year's U.S. Open. As a group, for a toe hit these drivers produced 98 percent of the ballspeed of an on-center hit. In other words, on 95 mile-per-hour, average-golfer swing speed, you might lose three miles per hour of ballspeed. Let's call that 6-8 yards.

(Related: You can get the ballspeed percentages for all drivers on the iPad version of the Hot List on the Golf Digest App here.)

As a group, the average launch angle and spin rate differed compared to an on-center hit by 17 rpm (virtually the same) and .02 degrees (again, virtually the same). What about dispersion? On off-center toe hits, as a group, the average was about 5.5 yards off the center line.

But here's the thing, Johnny. Our off-center toe hits are measured at THREE-QUARTERS OF AN INCH from the center of the face, or approximately 750 times one-thousandth of an inch.

Maybe he's referring to face angle. If so, and doing a little noodling in my head, let's say one degree might mean .03 inches of face rotation (a driver face is about 4 1/2 inches wide so a full, 360 degrees of driver rotation would equal about 14 inches). So "one-thousandth of an inch" would be equal to about one-thirtieth of a degree of face closure.  The effect of rotating the face one-thirtieth of a degree closed at impact? Not sure, but you can bet it's a whole lot less than 20 yards. I can tell you that TaylorMade once documented that its R9 drivers accounted for 40 yards of flight correction with a range of four degrees of face angle change. That's 10 yards for each degree, so one-thirtieth might mean .33 yards.

So let's add it up: Maybe 0.1 yards shorter, maybe 0.33 yards off line. I'm no math expert but even if you put those two together it doesn't add up to 20. TaylorMade's chief technical officer Benoit Vincent, who is an expert, actually did the math, if you're interested. "One-thousandth of an inch miss is not making a 20-yard hook for a driver. This is only 0.0254 millimeters off the center, which is in the noise for off center hits. With the bulge and roll of the face, there is no deviation there. We don't know how to calculate what angle this could be. Here is another way to estimate the effect: if the face is lagging by 0.0254 millimeters at an impact of one-inch from the center, then this angle would be 0.06 degrees, which would have minimal effect because one degree of face angle equals approximately 10 yards of deviation, so 0.06 degrees would equal approximately 0.06 yards or two inches deviation.

"Sorry for the complex math, but the answer is: one-thousandth of an inch creates a deviation of two inches, not 20 yards, because it would correspond to a 0.06 degrees open or closed face."

As John McPhee, a professor of systems design engineering at the University of Waterloo and a member of the Golf Digest Technical Panel put it, "I doubt that one-thousandth of an inch off-center would cause a significant hook. Otherwise, by extrapolation, my own drives would be several hundred yards in the woods!"

Miller's the most entertaining analyst in golf. He is quick, controversial and speaks with a rare and refreshing insight. But in terms of modern equipment physics, he might be off here. Way off.

The good news? Today's drivers are a whole lot more forgiving than Johnny Miller thinks they are. Of course, the converse is true, too. Since we occasionally hit it out of bounds, what that really means is that we've mis-hit it by a mile.

--Mike Stachura