How Does COR Affect Your Golf Game?
Industries, companies, and organizations love to use acronyms because the abbreviations of longer terms slash syllables and easily roll off the tongue. One such acronym that has been a buzzword since 1998 in the golf equipment industry is COR – Coefficient of Restitution.
Experienced golfers know the COR is a number which represents how “hot” the face of their clubhead(s) is made – or rather how much ball speed and resulting distance they can achieve for their swing speed. COR made its way into the golf industry’s vernacular back in 1998 when the United States Golf Association was concerned about the distance the pros were hitting the driver. Acting before ever doing any testing, the USGA blamed the pros’ distance increase on the use of the relatively new (at that time) titanium drivers and enacted a rule that placed a limit on the COR of all driver faces. The limit of 0.830 COR was based on the highest measured COR among all drivers the USGA had approved as conforming prior to the policing of the COR.
COR is a measurement of the energy transfer in a collision of two objects which is expressed as a number between 0 and 1. For example, when the USGA put a COR limit of 0.830 on driver faces, that meant no driver would be deemed to be conforming to the rules if more than 83% of the energy in the collision of the driver head with a golf ball were transferred from the head to the ball. Thus a collision which yields a COR of 0 means no energy is transferred between the two objects while a COR of 1 indicates a perfectly elastic collision in which all energy is transferred from one object to the other.
The COR rule also became known as the “spring face rule.” This term is misleading because in fact, a higher COR clubface does not really act like a spring. When you think of a spring face, it is logical to think the ball causes the clubface to flex inward, and upon flexing back out the ball is propelled as in the manner of a trampoline sling-shotting a gymnast upward.
Higher COR clubfaces do not work in the manner of a spring. In the collision between the clubface and the ball, there is always some energy lost. This is because of several reasons but mainly due to the face flexing inward while the ball is compressed against the face. Both actions result in a loss of energy. Of the two, the ball loses by far the most energy during impact because it can compress as much as 30% of its diameter against the face of the driver. To contrast the face loses less energy because the face only flexes inward at most a little over 1/16” as a result of impact. In a normal shot hit with an low COR thick face stainless steel metal wood, scientists calculate that 80% of the energy loss in such an impact came from the ball being compressed while the other 20% came from the very slight flexing of the more rigid clubface.
The idea of a higher COR face design, whether created for a driver or any other clubhead, is to allow the face to flex inward a little more so that the ball is not compressed as much against the face. When that happens, the face loses a tiny bit more energy because of the small increase in face flexing but the ball is able to lose a lot less energy because it is compressed so much less due to the slight increase in face flexing.
The net result? The ball takes off at a higher velocity and flies farther for the same clubhead speed and same loft angle on the clubface. Hence a high COR means more distance regardless of your clubhead speed.
If you like math and you’re interested to know more about the relationship of the factors that control ball speed as a result of impact or how much more distance you can get with a higher COR clubhead, here is the tried and true mathematical relationship between all of the elements that contribute to the speed the ball leaves the clubface:
Vball = speed of the ball in mph
Vclubhead = clubhead speed in mph
Wball = weight of the ball in grams (a constant at 45.93 grams)
Wclubhead = weight of the clubhead in grams
A 1mph increase in ball speed = 1.8 yds increase in carry distance
A 1mph increase in clubhead speed = 2.8 yds increase in carry distance
Tom
Hi Tom, It seems to me there are two COR effects here; the COR of the club and the COR of the ball. The club COR is limited by manufacture regulations, to 1.5 but, can the effective, overall COR (ball and club) be still significantly increased by an increased ball COR?
Schaun In a different way, the COR of the ball is also legislated by the USGA in the form of the ODS, the overall distance standard. This is the edict in which the USGA has a rule that for a specific set of driver launch characteristics of loft and clubhead speed, etc., the ball speed cannot exceed a certain figure. I’m out of town as I answer this so I don’t have the rule book close at hand to reference all the exact parameters of the ODS, but most certainly there is a limit for the ball that translates in… Read more »
Hi tom I was doing some research and can across this article. Great by the way. I recently had a lengthy conversation with a club manufacturing rep that was insistent that COR changes based on swing speed . As a result the are marketing club faces to achieve .830 , saying a player that swings 80 mph is only hitting with a core of.700 or so . Can you comment on this . Thanks
TOM COR cannot change with club head speed because COR is a product of the club head design. It has nothing to do with the golfer or any of the golfer’s swing characteristics. What changes with slower clubhead speed is the amount off ball speed that any clubhead and COR can generate. People with slower clu bhead speeds cannot generate as much ball speed from any clubhead as can a golfer with a higher speed. However, the SMASH FACTOR, which is the performance indicator of COR does not change with different club head speeds. Let’s say you have two golfers,… Read more »
Hi Tom- fascinating stuff, love to play with the math to see how far the ball will go!
can you indicate what is the approximate COR for one of your hybrids, such as 21-24 degrees. Would love to see how much more ball speed a hybrid with a hot face would get compared to the same iron head, which as you said below would be approx .78
MARK
The 775 and EQ1 hybrids are both in the area of 0.820-822 for their COR. It is much more difficult to try to get closer to 0.830 with a hybrid or fwy wood because the face area is so much smaller than a driver, which really limits how much the face can flex. But I can tell you they do achieve a very high smash factor and ball speed over conventional hybrids out there.
TOM
Hello Tom,
Shouldn’t the upper part of the velocity equation be:
Vclubhead * (1 + COR) (multiply rather than add )
Hi Steve,
Thanks for your noticing this error – now corrected.
ASH
Hello Mr Wishon, been a big fan of your articles and work for a while now.. A big thank you for all the time you spend sharing your knowledge with the rest of us! I had a question regarding the COR of clubs other than the driver; are there any standardized numbers based on club type(i.e a COR for a stock muscleback wedge) or is the variance between manufacturers of the same club type large? Furthermore, does COR drop continuously from Driver through the bag, or are fairway woods and hybrids maxed out at 0.83 as well? Am I right… Read more »
SHIV Many thanks for your kind words and your interest in the tech side of golf clubs ! No one’s more interested in it than me which is why I have spent a lifetime trying to figure out all this stuff ! You ask good questions, fun to answer for sure. Within the heads that are not designed to be a higher COR such as thick face forged or cast irons or normal cast stainless woods and hybrids, the COR will run within a pretty narrow range from 0.77 to 0.79. This does not vary from company to company for… Read more »
Tom: I realize this question is not quite in the field of this article but I was wondering as I read this about Smash Factor. More specifically if you look at the data on pgatour.com, the top 40 players on the list all have 1.5 or better SF and you need to look all the way down to Justin Rose at #186 for a Smash Factor of 1.47 which I have read from Dave Tutelman is about the highest number you could expect. So my question is where are these numbers coming from. Is it just that the radar can’t… Read more »
ADAM IF and this is a big IF, IF the clubhead speed and ball speed for a driver are absolutely accurate, the highest possible smash factor for a 0.830 COR conforming driver head is 1.495. Most launch monitors cannot read clubhead speed and ball speed perfectly. Even TrackMan which is used on the PGA Tour, has a +/-1mph error tolerance for these speeds. So that means if the ball speed is read +1 too high and the clubhead speed -1 too low, the TrackMan can output a smash factor of 1.52. Or if the ball speed is -1 and the… Read more »
So if I bought a driver that was tested CT of 239 and another at 249, there would be no difference in distance all things being considered equal on a 110 Driver SS?
Wilton There would be a carry distance difference between the same impacts for a head with a 239CT and one with a 249CT but it would be very small. On the order of around 1 to 1.5 yards of carry distance at a swing speed of 110mph with the driver so with all the other variables affecting any shot, it would in essence be imperceptible to a golfer. Now on a more controlled test situation such as a hitting robot, it could be seen but barely. 239CT = a COR of 0.822. 249CT would be a COR of 0.8264 so… Read more »
Tom,
I seem to recall a very simplified test for COR that involved
dropping a golf ball at either waist or chest height to impact your driver which was secured at floor level . Then from the rebound distance you could get a guesstimate as to the core of your existing driver or when deciding in the golf shop between manufacturers.
Could this be a very simplified measurement ? I think this came about when Veja Sing was crushing his driver years ago and his driver was subject inspection for conformity.
ROGER Not really, although the concept is sound. You’d need EXTREMELY precise controls and equipment to do a COR test in the manner you describe. First of all you would have to be able to duplicate the speed of the ball as it contacts the center of the face within 0.01 mph. Then you would need to have sophisticated electronic equipment to both measure the speed of the ball as it drops on the face AND the speed of the ball as it rebounds off the face. These would have to be accurate within 0.01mph. And you would have to… Read more »
Hey tom, please tell me…for a guy who swings around 116mph. what would a difference in distance difference be on a “standard drive”per CT difference?
HANK:
For a 116mph clubhead speed, for each 0.010 that the COR is increased, the carry distance will increase by about 1.75 yards. In converting that to CT, for each 22 usecs that the CT is increased, the carry distance will increase by about 1.75 yards. The relationship between CT and COR is, 18 CT = 0.008 COR.
Hope this helps,
TOM
thanks so much for your reply Tom. that doesnt sound like a lot at all…so 22 CT increase is only 1.75yards? surely then there is no point in getting a non conforming driver that springs more?
HANK
Yes, a CT 22 increase is virtually undetectable to 99.8% of all golfers. If you are shopping for a driver above the COR limit, you need to be digging around to find one that is far higher in COR, such as being as close to 0.900 as possible. I do not research this area of equipment so I am sorry I cannot tell you where you might find such a higher COR driver, if one does exist.
TOM
Tom You say that the COR value is the rebound speed of a ball fired at the club head at 109 MPH.this wood mean the ball speed would then be 90.47 MPH. This should be the same with as swinging a driver with a club head speed of a 109 MPH and hitting a stationary ball.The ball should then be moving at 90.47 MPH but this is not true. When a driver club hits a ball at 109 MPH the ball leaves the club head at approximately 158 MPH. So if this is the case as this the way it… Read more »
BRUNO No I mean that if you do a proper COR test, for whatever speed you shoot the ball into the face, the rebound speed off the face would be 83% of that inbound ball speed for the face to be of a 0.830 COR. When you swing a club and hit a stationary ball, when the ball speed is 1.495 times the clubhead speed you then have a COR of 0.830 on the face of the head used to hit that shot. COR testing and actually hitting a ball with a club are two different things in golf club… Read more »
Hi Tom,
I’m a trainee PGA Professional from England, and just wanted to know what your input was with single length irons, and the effect of COR.
Would i be right in saying, COR would be affected if a golfer was to alter all his clubs to the same length.
I know there are a lot more issues than COR when changing all then lengths, ie lie angles, grinds, lofts etc.
Would be great to hear from you.
Kind Regards
J Richardson
Jonathon Thanks very much for your interest in the technical stuff of clubs and for taking the time to visit to ask. COR is a part of the head design and has nothing to do with length whatsoever. What makes you think that though is the fact you know that as you go shorter you lose a little clubhead speed which brings a loss in ball speed. The face still has the same COR as always but when you hit a high COR face with a slower speed, you do compress the face a little less so the COR that… Read more »
Can you tell me the method to establish COR? Is it a tool I can buy? I am trying to figure out the cor of a drivers I own.
Thanks
Mitch Wolf
MITCH There is no economical way to accurately measure the COR of a clubhead. The equipment the USGA began with back in 1998 to measure COR was about a $10,000 piece of equipment. There is a way to reasonably ball park the COR of a clubhead if you have access to a TrackMan or FlightScope launch monitor. These are the two most accurate launch monitors made. While not perfect, both run an error tolerance for the ball speed and clubhead speed of +/-1mph, which is as close as you can get. I just tried to post a math formula that… Read more »
Hi Tom,
With regard to high-COR irons, I’ve just watched an interview with Taylormade exec Sean Toulon in which he claims that his company’s pairing of high-COR faced iron with slot technology is their most important invention since the metal wood. “If you’re not playing an iron with a slot in the bottom then you’re not playing the best,” says Toulon. Would be interested in your thoughts.
Best, Duncan
DUNCAN There are two different ways to go about engineering an iron to create a High COR face. You can do it the way we have done it since I did my first high COR iron in 2000 (the old Snake Eyes Fire Forged irons, then our 770CFE, then our 870Ti and this year the 771CSI) which is to create a separate thin, high strength metal face that you attach to an open face area in the head body by welding or press fitting. Or you can do it as TM is doing with the slot that allows the face… Read more »
That fount of knowledge Wikipedia says, “Golf balls themselves typically have a COR of about 0.78, but this varies based on the “compression rating” (a measure of the hardness of the ball). I assume this must be measured based on a collision with a solid surface like a steel plate or a concrete floor. So, I guess that the additional .05 – .78 to .83 – for a driver face must represent the amount of flexing due to face thickness. Any idea what the range of ball CORs might be? Given the marketing hype about softness/hardness in golf balls one… Read more »
DALE I have never done any sort of investigation or research into ball COR, so I am sorry I can’t tell you that range. I can tell you that I had several discussions with my main engineering mentor/teacher about matching the ball to the face through what is called impedance matching characteristics. It is possible, theoretically so, to match what really would be the COR of the ball to the COR of the clubface in a way that there could be a little more efficient energy transfer. You might remember a few years back when I think it was Top… Read more »
Tom: would you comment on the correlation between COR and CT
ED: I know you know, but for others who may read this and wonder, let me explain for their understanding what these acronyms are. COR is the Coefficient of Resitution, which is a parameter that dictates the energy loss in a collision of two objects. COR is expressed as a number between 0 and 1.0. 0 meaning in a collision of two objects all energy is LOST, while 1.0 means in a collision, all energy is transferred and retained. In golf it is the parameter that determines how fast the ball speed will be for any given clubhead speed on… Read more »