The next installment of my series on clubhead characteristics for beginners looks at the delights of COR…
You can think of the coefficient of restitution (or COR) as a measure of the spring-like properties of the front of a golf club.
The higher the COR, the “bouncier” the clubface, and the further it propels a golf ball that it hits.
Now let’s look at that in detail.
Technically, the coefficient of restitution is a term used in physics as a measure of the ratio of the speed of an object before and after a collision. Let’s explain that in simpler terms!
If you roll a ball along the ground and it hits a wall, the ball bounces back.
COR measures this bounce effect. For example, if the ball hits the wall at 50mph and bounces back at 30mph, then the COR of that collision is…
30 divided by 50 = 0.6
If it came back at 40mph, the COR would be…
40 divided by 50 = 0.8
The higher the COR, the faster the ball comes back.
A COR of 1 would mean that the ball bounced back at 50mph - exactly the same speed as it had when it hit the wall. We know intuitively that can never happen - the ball always comes back at a slower speed. So COR is always less than 1 (in practice, it’s always a good deal less).
The nature of the ball and the wall obviously have a big influence on COR. Imagine rolling a ball into a wall made of wet clay. It probably wouldn’t bounce back at all. So the COR of that collision would be…
0 divided by 50 = a big fat zero.
A golf club hitting a ball is also a collision. So COR is relevant here, too.
The higher we can get the COR, the faster the ball would shoot off from the “collision” - it would go further.
This is a big issue in drivers - the golf clubs designed to hit the ball as far as possible.
Manufacturers raise a driver’s COR by ensuring the clubface isn’t completely rigid, but has some elasticity (think of a trampoline).
Of course this isn’t the kind of elasticity you’d associate with a balloon or rubber band - it just means that the surface of the driver gives a little when it hits the ball. As it rapidly returns to its original shape it acts like a spring and gives the ball an additional push.
This adds distance to the drive; the COR is higher.
The rules of golf governing clubs state that such springlike effects are not allowed. But rather than such a rigid interpretation, the USGA (the governing body for golf in the USA and Mexico) instead effectively defines a limit on how much this springlike effect can be.
And they do this by placing an upper limit on the COR: currently 0.83. In other words, a golf club is considered valid only if the COR it produces when hitting a standard ball is equal to or less than 0.83.
Needless to say, driver manufacturers strive to get as close to this upper limit as they can (remember: the higher the COR, the further the ball goes) without exceeding it. And there have been heated debates between manufacturers and the golf authorities about the conformity (or not) of new club designs.
Indeed, some clubs knowingly cross this COR limit - their COR is higher than 0.83. That’s usually the issue when you hear of “non-conforming drivers” - drivers whose COR does not conform to the limits defined in the rules.
Such clubs are technically “illegal” according to the formal rules of golf.
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