Coefficient of Restitution
Coefficient of restitution (COR) is a number between 0 and 1 that measures how efficiently energy passes from one object to another when they collide. In golf, it describes how much of a swing’s energy the clubface transfers to the ball at impact.
What is coefficient of restitution?
Coefficient of restitution comes from physics, where it describes any collision between two objects. A value of 0 means every bit of energy is absorbed and nothing bounces back, like a lump of putty hitting a wall. A value of 1 would mean a perfect collision with zero energy lost, which never happens in the real world.
Golf borrows the term to describe what happens in the fraction of a second the clubface and ball are touching. A higher COR means less wasted energy. More of it ends up pushing the ball forward, which shows up as ball speed, and ball speed is what carries the ball down the fairway.
You will often hear COR called the “spring-like effect” or “trampoline effect.” Thin metal clubfaces flex inward slightly at impact and snap back, returning some of that stored energy to the ball. The hotter the face, the higher its COR, and the more help the golfer gets without swinging any faster.
How COR works at impact
The clubface and ball stay in contact for an astonishingly short window. By one comparison, the average human blink takes about 350,000 microseconds, and the ball sits on the face for more than a thousand times less time than that, according to Morton Golf Sales.
In that instant, the face compresses the ball and then releases it. A perfectly rigid face would behave like a wall, sending the ball off at a fixed speed. A face engineered to flex and rebound gives the ball an extra nudge as it springs back to shape. That difference is what COR captures.
A flawless rebound is impossible, though. The club and ball are built from different materials and have unequal weights, so some energy always disappears as heat and deformation. A plain stainless steel driver face with no spring effect measures roughly 0.78 COR, according to club designer data published by golfclub-technology.com. Reaching the legal ceiling takes deliberate engineering.
The USGA and R&A COR limit
In 1998, the United States Golf Association capped the COR of metal woods at 0.822, with a measurement tolerance of 0.008, which puts the working limit at 0.830. Any driver face above that mark is non-conforming and cannot be used in sanctioned competition, as documented in USGA equipment records and reporting by Golf.com.
The reasoning was distance. As ultra-thin titanium faces appeared in the early 2000s, governing bodies worried that springier drivers would let players overpower classic golf courses. Holding every manufacturer to the same 0.830 figure keeps equipment on a level field and protects the challenge of older layouts.
A COR of 0.830 means up to 83 percent of the collision’s energy reaches the ball. That sounds generous, but it represents the practical edge of what current materials allow, which is why so much club design effort goes into getting as close to the line as possible across the whole face rather than just the center.
COR vs. characteristic time (CT)
Here is where a lot of golfers get confused. The USGA stopped using the COR test to police drivers back in 2004 and replaced it with a measurement called characteristic time, or CT. The two are not the same thing, but they track each other closely, so a club with a high CT will also have a high COR.
The reason for the switch was practical. Measuring COR meant firing a ball from an air cannon and timing about 45 minutes per clubhead, according to Golf.com’s reporting on USGA testing. CT uses a small portable pendulum that taps the face with a steel weight and records how long the two stay in contact. It is quick enough to run inside a tour van on site.
| Coefficient of restitution (COR) | Characteristic time (CT) | |
| What it measures | Energy returned to the ball in a collision | Contact time between face and a steel pendulum |
| Units | Ratio from 0 to 1 | Microseconds |
| Legal limit | 0.830 | 257 microseconds (239 + 18 tolerance) |
| Test method | Ball fired from an air cannon | Portable pendulum tap |
| Used today for | Clubs other than drivers | Driver conformance |
A reading of 239 CT corresponds to roughly 0.822 COR, the same value the old limit was built around. Drivers are now judged by CT, while COR still governs the rest of the clubs in the bag.
How COR affects distance
More COR means more ball speed, and more ball speed means more carry. The gains are smaller than people expect, though. Golf club designer Tom Wishon has calculated that the difference between a face measuring 0.820 and one at the 0.830 limit is about 4.2 yards of carry for a 100 mph swing, as cited by LiveAbout.
Swing speed changes the picture. Faster swingers gain more yards from each increment of COR, while slower swingers see a smaller return from the same face. Either way, the 0.830 cap applies to every driver on the market.
COR never works alone. Loft, where the ball strikes the face, clubhead speed, and the ball itself all shape the final distance. A hot face buys a golfer free speed, but a poor strike or the wrong loft can erase that advantage in a hurry.
COR across different clubs
Not every club carries the same COR, and that is by design. Drivers chase the highest legal figure, since their only job off the tee is raw distance. Irons and wedges sit lower. A golfer wants control and predictable spin from those clubs, not the maximum spring a driver chases.
Fairway woods and hybrids land in between, but they struggle to reach the driver’s number. Their faces are simply smaller, which limits how much they can flex. Wishon Golf notes that well-built hybrids tend to land around 0.820 to 0.822 rather than brushing the 0.830 ceiling.
Related Golf Terms
- Shaft torque — A shaft‘s resistance to twisting during the swing.
- Kick point — The point along a shaft that flexes most, influencing ball flight height.
- Center of gravity — The balance point within a clubhead that affects launch and spin.
- Swing weight — A measure of how heavy a club feels when swung, based on weight distribution.
- Moment of inertia — A clubhead’s resistance to twisting on off-center strikes, a key forgiveness measure.
Frequently Asked Questions
Is a high-COR driver illegal?
Only if it exceeds the limit. Drivers are measured by CT, and anything over 257 microseconds (about 0.830 COR) is non-conforming. Almost every retail driver from a major brand passes, since manufacturers design to the rule.
Do golf balls have a COR?
Yes. The ball’s own rebound is part of the collision, and the USGA regulates ball performance through an overall distance standard rather than a single published COR figure. Clubface COR and ball behavior work together at impact.
Is COR the same as smash factor?
No, but they are related. Smash factor is ball speed divided by clubhead speed, a number a golfer can see on a launch monitor. COR is one of the physical properties that influences smash factor.
Can a clubface lose COR over time?
A thin driver face does change with heavy use. In some cases, it speeds up and creeps over the legal limit before it eventually fatigues. For the average golfer playing a normal amount, any change is too small to notice.
Sources
- United States Golf Association. “Equipment Standards.” Accessed June 2026.
https://www.usga.org/content/usga/home-page/equipment-standards.html - Golf.com. “Gear 101: What is a CT test, and how does it differ from a COR test?” Accessed June 2026.
https://golf.com/gear/gear-101-ct-test-cor-test-golf-clubs/ - Titleist Learning Lab. “What is the coefficient of restitution in golf?” Accessed June 2026.
https://www.titleist.ca/learning-lab/technology/golf-coefficient-of-restitution - GolfNewsNet. “What makes a golf driver illegal? Explaining coefficient of restitution and characteristic time.” Accessed June 2026.
https://thegolfnewsnet.com/ryan_ballengee/2019/07/20/what-makes-a-golf-driver-illegal-explaining-coefficient-of-restitution-and-characteristic-time-115474/ - LiveAbout. “What Is COR (Coefficient of Restitution) in Golf Clubs?” (quoting Tom Wishon). Accessed June 2026.
https://www.liveabout.com/what-is-cor-1563310 - Wishon Golf. “How Does COR Affect Your Golf Game?” Accessed June 2026.
https://wishongolf.com/2012/07/03/how-does-cor-affect-your-golf-game/ - Morton Golf Sales. “What Does COR Mean and Why Does it Matter?” Accessed June 2026.
https://blog.mortongolfsales.com/what-does-cor-mean-and-why-does-it-matter/
