Garland
This is how they now test for the Spring Like Effect. Measure the impulse time and then they must compute this number and convert to a COR for an impact with a USGA standard ball at a fixed club head speed (120mph)
U.S.G.A. Characteristic Time-New Test
USTA C.T. Machine
The method of measuring the SPRING LIKE EFFECT in a clubface, recently developed by the USGA is to use a pendulum on the end of which is a metal hemisphere which bounces against the clubface and the contact time relates directly to the SLE. The results of the conformance tests are used in determining conformity of the club head to the Rules of Golf,
Using the pendulum testing apparatus, a golf clubhead is impacted several times by a small steel pendulum. A characteristic time between the clubhead and pendulum is recorded for each impact. The characteristic time is directly related to the flexibility of the golf clubhead
The characteristic time of the clubhead shall not be greater than 239 µs. A maximum test tolerance of 18 µs is associated with this test. Therefore a maximum characteristic time of 257 us. or units, is allowed.
This test measures the response of a shafted golf club. If there is no shaft in the clubhead, one must be temporarily installed prior to testing.
The normal procedure calls for the measurement to be made at the face center, and is meant to replace the old coefficient of restitution method.
Here's the older "cannon" test to measure the COR where they used a Pinnacle as their standard ball:
In 1998, the United States Golf Association ("USGA") decided to regulate technological improvements through a liberal interpretation of Rule 4.1 of the Rules of Golf, as set forth by the USGA and the Royals and Ancient Club of Saint Andrews. The USGA determined that a golf club head having a coefficient of restitution ("COR") greater than 0.83, on a scale of 0.00 to 1.00, would be non-conforming under the Rules of Golf as a club head having a spring-like effect.
In order to determine the COR of a golf club head, the USGA devised a laboratory test that necessitates the removal of the shaft of a golf club. The test is conducted at the USGA testing laboratory requiring that a golf club be submitted to the USGA for conformance. The un-shafted golf club head is placed on a pedestal without securing the club head to the pedestal. A PINNACLE.RTM. Gold two-piece golf ball is fired at the face of the club head at 160 feet per second. The club head is knocked-off the pedestal, and the COR is measured by the rebound of the golf ball. A grid is established on the club face using the scorelines and etched vertical lines, further destroying the club and creating further uncertainties. The procedure is repeated at random sites on the grid on the face of the golf club until the point with the highest COR is determined from the test. The outbound velocity of the golf ball after impact with the face is determined using a light gate systems such as described in U.S. Pat. No. 5,682,230. A more detailed explanation of the test is provided at the USGA web site.
It is obvious to anyone skilled in the art that such a test is inapplicable to on-course testing, and requires a specific laboratory with skilled technicians to perform the test. Further, the "cannon test" results in destruction of the club. Yet further, the test is conducted on an unshafted club head, completely ignoring the shaft and grip. What is needed is a test that can be performed on course, with consistent repeatability, and minimal operator error.