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What is COR in Golf?

In this article we explain COR, how it relates to golf, your performance and what the future is of driver technology within the game of golf.

What is COR in Golf?

COR stands for Coefficient of Restitution which explains energy transfer when two bodies come into contact. In golf, COR limits refer to the energy transfer between your driver head and the golf ball upon impact. COR ranges from 0 to 1, with 1 referring to a perfect transfer of energy from club to ball and 0 meaning there was no energy transfer.

In 1998, the United States Golf Association (USGA) set the Coefficient of Restitution (COR) limit for woods at 0.822 with a test tolerance of .008, creating a legal limit up to 0.830.

How is COR measured?

COR was historically measured by firing a golf ball into the club face at a set speed and measuring the speed afterwards. For example, if a golf ball was fired towards a driver sweet spot at 100mph, and after impact, it rebounded with a speed of 78mph the COR would be 0.78. If it rebounded with a speed of 99mph the COR would be 0.99.

How can drivers alter COR?

The golf ball deforms upon impact (up to 30%), which causes a loss in energy. Small amounts of energy are also lost through sound and heat. However, engineers discovered that by creating golf club faces that flex more upon impact, the golf ball will deform less and more energy is conserved. More face flex resulted in higher ball speeds in drivers and longer drives!

Modern testing of COR – Characteristic Time

In 2004 a more modern, portable test was created for testing this effect, known as Characteristic Time (CT). This test involves swinging a metal pendulum against a driver face and measuring the contact time between the two. The greater the contact time, the more face flex (and COR) is present. At the time of writing CT is limited to 239 microseconds (+ 18 microseconds for manufacturing tolerances).

The future of drivers in golf

Drives have always aimed to be as long as possible as golfers search for the best drivers for distance. This legal limit has prevented further gains from perfectly struck drivers, but now the race is on to optimise the performance of drivers from all areas of the club face, pushing them towards the COR and CT limits.

This is generally achieved through variable face thickness technology, the materials used on the face and crown and shifting mass around within the club head.

I hope this has been a useful guide.

Happy golfing – Will @ Golf Insider.

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Will Shaw, PhD, MSc, PGA Pro

Will is a PGA golf professional, with a PhD in Biomedical Science and MSc in Sports Biomechanics & Psychology. He spent 10 years lecturing part-time at Leeds Beckett University and the University of Leeds in Biomechanics and Motor Control before becoming the Head of Golf for the University of Exeter. He currently runs Golf Insider UK, Sport Science Insider around wider consulting and academic roles in sport performance and motor control.

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