Coefficient Of Restitution (C. O. R.)
A DISCUSSION ON BETTER UNDERSTANDING C.O.R.
C.O.R. is easier to understand with a few additional examples and a little deeper explanation. First, recognize that when two objects collide there are 3 basic conditions of impact that exist. There are perfectly elastic collisions, perfectly inelastic collisions and everything else in-between.
An almost perfectly elastic collision is best demonstrated by two ivory que balls. When they collide, no or very little mechanical energy is dissipated so the speed-in of one ball is equal to the speed-out of the other ball after the collision.
So, the C.O.R. is 1.00 or 100%.
A perfectly inelastic collision is best demonstrated by using two balls of putty, let’s say the size of golf balls. When one ball of putty impacts the other ball of putty, they both stick together, so the speed-out of the second ball of putty is 0. So, the C.O.R. is 0 or 0%.
The third type, which is all the other collisions, applies to golf club heads impacting golf balls. It is very important here to put a few things into perspective, because we tend to apply way too much credence to C.O.R. as the major factor in hitting golf balls longer with today’s drivers.
There is no doubt that increasing C.O.R. relates to increased hitting distances, but it is not the only reason and it also requires a perfect impact location on the face of the driver to take full advantage of it. Here’s my explanation: The C.O.R. on average persimmon driver is about .78 or 78% efficiency. Today, we can make legal titanium drivers around .83 or 83% efficiency. So, the clubface material and construction type has given us approximately a 9% increase in C.O.R. (.78/.83.) If we use persimmon as a base impact material, it tells us that the ball accounts for most of its initial velocity and the newer spring faces of drivers accounts for a much smaller increase of the ball’s total initial velocity. On a 230 yard carry drive, this could be as much as 10 to 12 years but only if a very centered impact location occurred to maximize the spring effect. The ball itself is very important to overall distance and we all want the extra 9% C.O.R., but it is harder to achieve the full 9% than most golfers realize.
The biggest gains in distance with today’s drivers most relates to club lengths which are 1 1/2” to 2” longer, head weights that are heavier because of graphite shafts which are much lighter than steel and a total club weight which is also lighter. This means that longer, lighter drivers can be swung faster. Also, the newer larger driver heads have higher moments of inertia and as a result have bigger sweet spots, which allows greater distance to be achieved even if the impact is not in the middle of the face.
Keep in mind that larger sweetspots do not necessarily correlate to larger C.O.R. (spring face) impact areas. Once again, it is a very small impact area that achieves maximum spring face effect. The larger sweetspot on its own merits means that you will achieve greater distance and a much better solidness of feel on off-center hits vs. a smaller sweetspot.
Here’s How To Get Maximum Driver Distance For Most Golfers:
1. Pick one of the new 450cc to 460cc heads made with a C.O.R. of around .82 to .83. Most driver heads produced from 2006 and on have these qualities.
2. Pick a lighter, high quality graphite shaft (usually 65 grams and under). I like 55 to 65 grams.
3. Make the length as long as you can handle and still keep the ball in the fairway. I like 44” – 45”, but I don’t like 46” and longer if accuracy suffers.
4. Have someone knowledgeable fit this club to you with the correct head weight (swingweight), loft, shaft flex, bend point and grip size. The end result that you are looking for is to launch the ball around 13 degrees for maximum carry and mostly be hitting the driver consistently solid and in the fairway. A properly calibrated launch monitor is a useful tool in determining this, but must be used by a trained and knowledgeable individual.
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