Titanium golf clubs adapt to rule changes

American golfers looking to add some easy extra distance to their drives got a bit of bad news this summer. The United States Golf Association (USGA) kept in place its limit on the allowable coefficient of restitution (COR), for metal drivers. A measure of an object's energy transfer capabilities, COR in the golf world describes the spring-like effect of a metal clubface as it strikes the ball. According to Dick Rugge, the USGA's senior technical director and a mechanical engineer, each one percent increase in COR adds 1.5 to 2 yards of distance given a swing speed of about 100 to 120 mph. "But that's only if-and it's a big 'if'-the golfer hits the ball just right," he says.

The USGA and its European counterpart, the Royal & Andrews Society of St. Andrews, Scotland (R&A) in May had announced that they would try to come up with a joint rule on COR. Since 1998 the USGA has measured COR in a test that involves firing a ball at the clubface, measuring the rebound velocity, and expressing COR as a ratio of the velocities before and after the ball strike. After creating that test, the USGA set a COR limit of 0.830 for everyone in its jurisdiction-from the pros down to any duffer with a sanctioned handicap. The R&A, meanwhile, had no test of its own-and no limit. To level the playing field, golf's two governing bodies in May floated a proposal to adopt a temporary limit of 0.860 for the next five years. With this month's ruling, the two governing bodies did find a way to harmonize their rules over the next five years, but they also declined to adopt the higher 0.860 COR. In the U.S., the limit will stay at 0.830 for all levels of play, while the R&A will phase in a 0.830 limit over the next six years-starting with competition level play in 2003.

The prospect of even a temporary increase in COR has received plenty of media attention since May. "People get excited about COR because it is one mechanism for increasing ball velocity-and distance-overnight," acknowledges Benoit Vincent, vice president for product development at TaylorMade Golf (Carlsbad, CA). But from an engineering standpoint, Vincent argues that designing for high COR alone really represents a swing in the wrong direction.

Getting in the zone. Focusing on a high COR neglects the fact that the "hot" zones in which the spring-like effect reaches its full potential can be tiny in traditional faceplate designs. "They're about the diameter of a tee head," he says. "You would need a very high skill level to consistently hit that spot." Move even a fraction of an inch out of this zone, and ball velocity falls off, offsetting some of the distance gains promised by a high COR. Vincent cites velocity drops, for instance, of 7 or 8 mph for ball strikes that deviate even .25-inch from the sweet spot. So in his view, it's far more important that clubs have a large COR zone rather than just a high COR.

TaylorMade's most recent drivers, the R500 Series, have been designed with this larger COR zone in mind. Made from a cast titanium alloy (Ti-6Al-4V) body with a cold rolled titanium face insert, these clubs feature a new machined faceplate technology that Vincent says expands the COR zone from tee-size to quarter-size-a more realistic target for the average golfer. Instead of the flat or mostly flat face inserts of the past, the new driver has an "inverted cone" feature CNC machined into the reverse side of the faceplate with the thickness varying from about 2.5 mm at the center of the cone to about 4.0 mm at the top. The faceplate, which was carefully contoured with the help of computer models, tapers off to about 1.7 mm thick on its edges. According to Vincent, this selective thinning of the faceplate increases its flexibility, which boosts COR. At the same time, the cone shape works to distribute the energy from the ball strike across a larger area. Vincent describes the cone's role as "flattening out" the response of the thinnest, hottest regions of the faceplate. The end result, he says, is the large, consistent COR zone.

Flexible design. Aside from what the R500 can do on the links, another important aspect of its design ties back to the USGA's abortive COR rule change. The initial version of the R500 had an even thinner faceplate and a COR close to 0.860. Yet just two weeks after the USGA decision to keep the lower limit in place, TaylorMade managed to come out with a conforming version of the R500. This club comes very close to 0.830-actually 0.822 in order to accommodate a 0.008 tolerance.

Rather than revamping the whole club, the company's engineers changed the height of the cone from about 3 mm to 4 mm in the USGA conforming version. And because this change added about 12g of weight to the head, they also readjusted the mass balance of the club using a weighted cartridge system that was already part of the design. "We didn't know what the USGA would do two years ago when we started designing this club," says Vincent. "But the flexibility of our technology means that we don't have to sit around and whine about the rules."