Springlike effect or COR in Driver


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About the 50th Anniversary of Frank Thomas' Invention of the Graphite Shaft

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	WHAT IS COR? COR is a measure of the efficiency of the transfer of momentum between two colliding bodies. To better understand the concept of COR imagine that a ball is flung at a rigid brick wall at 100 mph and it bounces back at 75mph the COR is 75/100 = 0.75 . Exit Velocity divided by Approach velocity = COR It is not possible to get a COR of 1.00 (100/100) as this would mean that there was no energy loss during impact. There is a limit to COR which is governed by the Laws of Physics. We all intuitively know that a ball will not bounce higher than from where it was dropped, no matter how resilient the ball or how "springy" the surface. No matter how technology advances or new materials develop, the resilience of impact is limited. From a practical point of view knowing that even sound is a form of energy loss the estimated high on COR is about 0.930 COR OF A CLUB AND BALL COLLISION Taking into account the weight of the ball and the club head and the speeds of these two colliding bodies, before and after impact one can calculate the COR. It has been found that the COR of the collision between the club head, which does not exhibit any Spring Like Effect (SLE) and a standard ball is about 0.768 at a head speed of about 110 mph (see above Wooden Clubs). If the face of the club deforms and recovers during impact, like a trampoline (see above Titanium Clubs), this takes up some of the deformation that would have otherwise been in the ball, the COR would increase as there are fewer losses in the deformation and recovery of the club face than in the ball. COR RULE In 1998 The USGA set the COR limit for woods (metal woods) at 0.822 with a test tolerance of .008 effectively taking the limit up to 0.830. This is about 0.06 above no SLE and equivalent to about 10-15 yards in distance based on the resulting increased ball velocity and change in launch conditions. Looking at the History of this rule may help: • 1909 the rule was first introduced and required that the "club shall not contain any contrivances such springs." • In the mid 1950s the rule was changed to read that the club face shall not contain any finish for the purpose of unduly influencing the movement of the ball. • In 1984 the rule was specifically changed to read "The clubface must not be designed and manufactured to have the effect at impact of a spring… or have any other effect which would unduly influence the movement of the ball." This rule is essentially unchanged today; however there seems to be some divergence from the literal translation of the rule and the way it is being interpreted and enforced. The rule allows for “some” spring-like effect. A decision of convenience and compromise.