Ever since Tiger Woods claimed that some tour pros were using “hot” clubs — drivers deemed nonconforming by the U. S. Golf Assn. — there has been growing interest in a quick way to determine if a club is illegal. The point is not merely academic. The association's website currently lists over 250 nonconforming drivers, including versions of well-known models such as Great Big Bertha, King Cobra, TaylorMade, and Titleist.

The USGA's coefficient of restitution (COR) test measures the springlike effect of the club face. A cannon fires a golf ball at the face of a driver and rebound velocity cannot exceed 0.83 of impact speed. That is, a ball hitting at 100 mph cannot rebound faster than 83 mph. The air-cannon test requires the heads be removed from the shaft and can take several hours to perform.

To measure COR faster and more accurately, Marc's Golf Service, Palm Desert, Calif., built a custom device that impacts the club face with a pivoting striker. It lifts a 10-in. arm with a 130-gm contact head to a 90° position. A solenoid releases the arm and the contact head impacts the driver face. The critical part is precisely measuring contact time and correlating that to COR.

The company turned to ATMD (acam time-measuring device) software and hardware from acammesselectronic GmbH, Stutensee, Germany, and Transducers Direct, Cincinnati, said to be a powerful, economical system for high-precision, time-difference measurements in the picosecond range.

Control and programming are via a PC, which connects to the ATMD by a 16-bit AT-bus or PCI interface card. The unit comes with Windows-based software, and users can write control programs in any standard language. Various time-to-digital converters are available with resolution as small as 25 psec.

The system has two operating modes. The polling mode reads results directly via the AT-bus or PCI interface. Maximum speed of the interfaces limits data rate to about 250,000 measurements/sec. In burst mode, results are temporarily cached in a FIFO on the motherboard, making rates up to 2.5 million/sec possible. Results show 350,000 ns coincides with the USGA's 83-mph limit. Marc's uses the system to customize the COR to fit a customer's individual swing speed. For instance, selectively machining the face of a driver with a conforming COR might raise it to 400,000 nsec, and increase distance. When the face of the driver is made thinner, the ball remains in contact with the head longer and launches off the face at a greater speed.

Precisely measuring contact time between the striking head and driver lets technicians determine if a golf club conforms to USGA regulations.