CT, short for characteristic time, is the test the USGA and R&A use to measure "spring-like effect," how much extra energy a flexible clubface returns to the ball on impact, in driver heads. The published limit, per the USGA's pendulum test procedure, is 239 microseconds, with a permitted test tolerance of 18 microseconds, meaning a driver measuring up to 257 microseconds still conforms. A driver that tests above that combined figure is non-conforming, and that's what's actually happening when a tour driver gets randomly tested and pulled: its face has flexed, or "crept," past the legal limit through repeated use, not necessarily through anything the player did deliberately.
What the CT test physically measures
The test uses a small steel ball on a pendulum, swung into the clubface, with sensors in the ball measuring how long, in microseconds, the ball stays in contact with the face before separating. A stiffer, less springy face returns the ball faster (a lower CT number); a more flexible, trampoline-like face holds contact longer (a higher CT number). The USGA and R&A test characteristic time specifically for drivers; fairway woods, hybrids and irons are tested under a related but separate coefficient of restitution (COR) method.
The published limit, and why it's stated as a range
According to the USGA's own testing procedure, the base limit is 239 microseconds. Because no measurement device is perfectly precise, a tolerance of 18 microseconds is built into the standard, which is why the practical cutoff cited by the governing bodies and repeated across golf equipment press is 257 microseconds, the base limit plus the full tolerance. A driver measuring at or below 257 microseconds on the official test conforms; above it, it doesn't.
Why a driver's CT number can rise after it leaves the factory
This is the part that actually explains tour testing headlines. A driver's face is thinnest, and therefore most flexible, right where repeated high-speed impact happens most. Over enough strikes, especially at tour-level swing speeds and volume, that face material can fatigue and flex slightly more than it did new, commonly called "creeping." A driver that conformed when it left the factory, and conformed when the player put it in the bag, can drift closer to or past the limit purely from use. This is why the PGA Tour periodically pulls random drivers for CT testing during events: it isn't accusing a player of cheating, it's checking for exactly this kind of gradual, use-driven drift.
Carrying vs using a non-conforming driver
The distinction that matters under the Rules of Golf is the same one that governs every equipment-conformance question: Rule 4.1a penalizes making a stroke with a club that doesn't conform to the Equipment Rules, and the penalty is disqualification. Simply owning or carrying a driver that happens to have crept past the CT limit, without knowing it and without making a stroke with it in a round played under the Rules of Golf, isn't itself a violation, though once a player becomes aware a club doesn't conform, continuing to play it is. This mirrors the same carrying-versus-using distinction covered in full at /non-conforming-golf-driver/.
Does this actually matter for a casual or club-level golfer?
For almost everyone, no, in the direct testing sense. CT/COR testing at the level described above happens on tour and at the highest levels of competitive golf, where officials have the equipment and the standing to test drivers on-site. A weekend golfer's driver is not going to be pulled at the turn and tested with a pendulum device. What does matter, indirectly: driver face "creep" is a real, physical phenomenon, not just a tour storyline, so a driver that's seen years of heavy use theoretically could have drifted from where it started. In practice, this becomes relevant mainly if you're buying a used or older driver and want to confirm the specific model is on the current conforming list, covered at /usga-conforming-golf-club-list/, or if you're stepping into a competition serious enough to have its own equipment-checking process.
How CT and COR differ from the size and length limits
Spring-like effect is only one of several ways a driver can fail to conform, and it's worth keeping the categories separate. Clubhead volume and overall club length are independent limits, checked by measurement rather than by the pendulum test, and covered in full at /non-conforming-golf-driver/. A driver can conform on CT and still fail on volume or length, and the reverse is also true. When a driver fails the conforming list check or gets flagged in a competition, the specific reason matters. A club rejected on CT is a face-flex issue, potentially from wear; a club rejected on volume is a manufacturing or aftermarket-modification issue from the start. Knowing which category a concern falls into is what actually determines whether it's worth checking at all for a given driver.
The COR method, briefly
Before CT testing was introduced, spring-like effect was measured with a coefficient of restitution (COR) test, a related but different methodology. COR is still the method used for fairway woods, hybrids and irons, where CT testing doesn't apply. The underlying question, how much extra ball speed a flexible face adds beyond what's permitted, is the same across both methods; the equipment and calculation differ by club type.
What "aftermarket hot drivers" are actually doing
Some aftermarket and specialty drivers are deliberately built or modified to exceed the CT limit for extra distance, sold openly for recreational, non-competitive use. This is a separate situation from natural creep in an otherwise conforming driver: it's an intentional design choice, not a fatigue effect, and a driver built or altered this way is non-conforming from the start, not just after wear. Whether hitting one is appropriate is a group-agreement question for casual play, and a disqualifying breach the moment it's used in a round played under the Rules of Golf.
FAQ
What's the actual CT limit for a conforming driver?
239 microseconds, with a permitted test tolerance of 18 microseconds, for a practical conforming ceiling of 257 microseconds, per the USGA's published testing procedure.
Can a driver that was legal when purchased become non-conforming over time?
Yes, in principle. A driver's face can "creep," flexing slightly more after extended use, particularly at high swing speeds and volume, which is why tour equipment gets periodically re-tested rather than assumed conforming forever based on its original certification.
Will my driver ever actually get CT tested at my local club?
Almost certainly not. On-site CT testing happens at the professional and elite competitive level, where officials have the equipment. Ordinary club and casual rounds don't involve this kind of physical equipment check.
Is it illegal to own a driver that fails the CT test?
No. Rule 4.1a's disqualification penalty applies to making a stroke with a non-conforming club during a round played under the Rules of Golf, not to owning or carrying one.
How is CT different from COR?
Both measure spring-like effect, but CT (measured in microseconds of ball-face contact time) is the method used for drivers specifically, while COR is used for fairway woods, hybrids and irons.
For the broader question of what makes a driver non-conforming beyond face flex, see /non-conforming-golf-driver/. For the equipment rules that actually affect an amateur golfer, see /golf-equipment-rules-for-amateurs/.