Martin Hopley
By Martin Hopley

A key phrase you will encounter when reading about golf clubs is Coefficient of Restitution (COR).

What is the Coefficient of Restitution?

The technical definition for COR is the ratio of velocity out to velocity in, so it is a measure of how much energy comes out of an impact versus how much energy was put it in. It gives you a measurement of the efficiency of an impact so in a perfect impact where no energy was lost the COR would be 1.

Why is COR important?

COR has a direct influence on ball speed and therefore distance. In the 1990's golf club manufacturers used new technologies to improve average COR’s from 0.78 to the realm of 0.86 which resulted in a significant increase in distance. In 2003 the R&A and USGA brought in a unified standard to limit the maximum amount of COR a clubface could have.

What are the limits?

The increases in ball speed, and therefore distance, is something that is closely monitored by the games governing bodies who are concerned that further advances may impact upon the game. The R&A and the USGA have set the COR limit at 0.83 meaning that a maximum of 83% of energy can be transferred to the ball at impact.

How is COR measured?

COR is measured by launching a ball at the face of the golf club and measuring the speed velocity of the ball before and after it has made contact with the clubface. This is a relatively complex thing to do and as a result the R&A and USGA introduced a measurement called Characteristic Time (CT) to measure the elasticity of the clubface.

Are COR and CT the same thing?

COR and CT are not the same thing. CT was introduced by the governing bodies to allow measurement of the spring like effect of the face in a more portable environment. CT is a pendulum test and it measures the amount of microseconds a metal ball is in contact with the face when it is swung against it.

A higher CT indicates the club has a higher spring like potential and can impart more energy to the ball and make ball speed higher. The CT limit is currently 239 µs (units) plus 18µs for manufacturing tolerance, so most manufacturers aim for 257µs. As manufacturing processes comtinue to improve they are able to get closer to this limit without going over it.

Are all golf clubs at the regulated limits?

Most leading drivers and fairway woods are at the limits and even hybrids are very close. Even if the centre of the clubface may be at the limit, designers are focussing on approaching the maximum across the whole area of the clubface to maximise ball speed across the entire face on off centre hits.


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