What is CoR in Golf?

Introduction

Even for people who have been playing golf a long time, there can be many terms and phrases bandied about that you’re not too sure of. You may be wondering what the term CoR means, let’s take a look.

What Does CoR Stand For?

CoR stands for coefficient of restitution. CoR is a measure of the transfer of energy between two objects when they collide. 

What is CoR?

CoR is expressed as a number between 0 and 1. If all the energy was transferred during the collision then the CoR would be 1. However in reality you could never get such a perfect transfer of energy. If no energy was transferred between the two bodies then the CoR would be 0.

How do we Calculate CoR?

In golf, we want to know the CoR between the clubhead and the ball so we need to know the ball speed after impact and the clubhead speed before and after impact. We then use the following formula to calculate the sea OR for that club.

CoR = ball speed after impact – club speed after impact / club speed before impact

Here are some examples:

Club Speed Before ImpactClub Speed After ImpactBall Speed After ImpactCoR
80471110.8
100691470.78
120801770.808
CoR examples

The USGA and R&A imposed a limit on the CoR for clubs of 0.83. This means that the maximum energy transfer between club and ball is 83%. In actual fact, the limit is 0.822 but there is a tolerance of 0.08 making 0.83 the cut-off. This was to address the problem of the trampoline effect in drivers.

If a manufacturer produces a new driver which tests higher than 0.83 then it would be ruled non-conforming and would not be allowed to be used in competitive play at any level. 

Prior to this rule change, several drivers had been released with a CoR figure in excess of 0.83. For example the TaylorMade XR-03. Players using drivers like this would be able to hit the ball further.

To check how far you should be hitting the ball take a look at my golf club distance calculator.

How do you Measure CoR?

The governing bodies measured CoR by firing golf balls at the clubface using an air cannon. They then measured the velocities of each shot before and after striking the face.

The problem with using this method was it was very time-consuming taking around 45 minutes to perform the test for each clubhead submitted. Since 2004 a new test has been used, called the CT test. This uses a small portable pendulum that strikes the clubface with a steel ball. Sensors can then detect how long the ball remains in contact with the face. The limit is set at 239 ms with a tolerance of 18 ms. This means any club with a reading above 257 ms would be non-conforming.

Because this is a much simpler test to perform it is now carried out much more regularly at tour events. Xander Schaufele fell foul at the 2019 Open Championship and five players reportedly had drivers that failed the test at the Safeway Open that same year.

Why is CoR Important?

CoR is important because manufacturers are continually striving to make drivers that hit the ball further. They found that by making the face of your driver thinner it would act almost like a trampoline in effect giving extra yardage on your drives. This is because the face flexes more as you make it thinner and therefore there is more energy transferred to the ball giving additional distance. By imposing a limit on the CoR for a driver’s face it meant they were limiting the ability of manufacturers to tweak this feature for extra distance.

What is CoR in Golf: Conclusion

So if you’re in the market for a used driver make sure you don’t buy one that is so old it predates the CoR/CT rules as you may be unable to use it in competition. Of course, as long as your buddies agree then you can still play such a driver in your own friendly games.

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