# 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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• herrsonic
• 12:49 PM - May 14, 2008

Thank you for that information. There are hardly any manufacturer's out there making driver heads smaller than 460cc. There are quite many variables, but I'm not always sure whether it is marketing or actual physics. We tend to think that bigger is better, but I'm not exactly sure that 460cc is the ideal size compared to say a 400cc head. In any case, if we are going with what is being made, our only option is 460cc. Sure there are heads out there smaller than 460cc, but they're usually old stock. I do agree that club fitting is important. I've probably easily spent more than \$1000 on fitting in the past as I've been fitted many times, gone through multiple clubs, and the one club that is hardest to fit well is the driver. That's also because my swing has evolved.

• Ralph Maltby
• 11:43 AM - May 14, 2008

herrsonic, the importance of the larger 460cc driver head sizes along with maximum COR (CT), higher MOI and the clubheads center of gravity location is to hit the ball more solid on off center hits and have it fly longer on a better trajectory. There are also a number of benefits on perfect center hits such as the ball spin reducing vertical gear effect, which is very minor on the smaller driver head sizes.The small heads are old technology and cannot do that simply because of their size. However, there are still golfers who prefer the smaller head size because of the gigantic look of the newer 460cc heads. The aerodynamics of both head sizes do come into play, but are relatively insignificant compared to the higher playability attributes of all the other more significant factors I listed here for the larger driver heads. There is quite a bit of information on this website regarding driver sizes, MOI and COR (CT). Keep reading and we will really get you hooked on this stuff and hopefully as a result you can better fit yourself or simply understand proper fitting and ultimately play better golf.

• herrsonic
• 12:57 AM - May 13, 2008

I'm just beginning to get into the physics of golf equipment so I stumbled onto this site from a search. How important is the characteristics (size, COR, MOI, shape, etc.) of the driver head in providing additional distance? We all have been told that the two key factors to hitting a long shot is (1) clubhead speed and (2) a well struck ball. If that was the only case, then why is the club head characteristics of the driver so focused upon. Would a 330cc head size increase distance because one could generate more clubhead speed with a smaller head size? That is, theoretically, if one gets the same loft angle with a 330cc head size as a 460cc head size, hit in the sweet spot for both, with the same clubhead speed, would they both go the same distance?

• Ralph Maltby
• 08:42 AM - May 05, 2008

Antony, on each clubhead design and the specific material used, along with the heat treating process, the thickness of the face will vary to obtain the maximum COR. Driver face thickness is different on most every driver. You cannot tell the performance of any driver by comparing face thickneses because there are too many variables to consider.

• Antony
• 08:53 AM - April 26, 2008

Ralph. In terms of the thick of a Driver face (considering for example one made of a Beta Ti), how this characteristic interact with the C. O. R., Should a thicker face perform better than a slim one. Also, What is thick of the face of the CT250 Carbon Titanium driver and the MXO HM450.

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