Is your driver breaking the rules without you knowing? 

What makes a driver non-conforming? How do testers catch them? And could your gamer be breaking the rules without you knowing? 

Non-conforming drivers have been one of golf’s biggest talking points in 2025. 

Rory McIlroy had to swap drivers during the week of the PGA Championship after his TaylorMade Qi10 failed a Characteristic Time (CT) test (more on what that means in a moment). Scottie Scheffler suffered the same fate, albeit more quietly, and Rory later admitted he was “pissed off” his name was the one that got leaked. 

But what actually makes a driver non-conforming – and how do officials catch them?

What makes a driver non-conforming? 

The USGA and R&A set out rules for all golf equipment – from maximum head sizes to groove depth to how ‘springy’ the face of a driver can be. 

It’s that last one – the ‘springy face’ – that’s been the issue with non-conforming drivers. 

There’s a specific test – known as a CT test – which determines whether a driver face is legal or illegal.  

What is CT testing in golf? 

In golf, a CT (Characteristic Time) test measures how long a golf ball stays in contact with the clubface at impact. The longer the ball lingers, the more spring the face has – and that can mean faster ball speeds and more distance. 

The legal limit for CT is 239 microseconds, but there’s an allowed tolerance of 18 microseconds. So, anything up to 257 microseconds is technically within the rules. Go beyond that, though, and you’re into illegal – aka non-conforming – territory. 

How are drivers tested? 

The driver head is secured in a vice, and a small metal ball on a pendulum is swung into the face. Sensors measure how long the ball stays in contact with the face. The longer the contact, the hotter the face. 

It’s not destructive, but it’s precise – and it’s become a key tool for checking conformity on tour. 

How does a driver become non-conforming? 

Manufacturers have to get their clubs tested and approved before release. If they pass, they’re added to the USGA and R&A’s official List of Conforming Driver Heads list. 

But just because a driver starts out conforming doesn’t mean it’ll stay that way. 

Over time – especially under the strain of high-speed impacts – the face can wear down. That wear can make it thinner and more flexible, causing something known as ‘CT creep’. 

Basically, the driver becomes springier the more it’s used. 

That’s why golf’s governing bodies do random CT spot checks at tour events – to see whether players are using drivers that have worn down beyond the allowed limit. 

How often are drivers tested on tour? 

Roughly 30 drivers are tested at each tournament, chosen at random.  

Some players – including Scheffler – have suggested that more widespread testing would make the process fairer. 

The USGA have defended their approach, saying that the current level of testing is sufficient, but it’s a topic that’s likely to keep coming up. 

What happens if a driver fails on tour? 

If a player’s driver fails the CT test, it’s immediately deemed non-conforming and must not be used. The player will switch to another, identical head. Most players have multiple heads ready for action in case of a failed test or any damage that may occur during practice.  

There’s no penalty or disqualification for a failed test, unless the player had knowingly used a non-conforming club during competition. In most cases, it’s treated as a technical issue, not a breach of the rules. 

That’s why some players – including McIlroy – have been frustrated when failed tests are leaked, as they’re not evidence of wrongdoing, just equipment that’s aged beyond the limit. 

Could your driver be non-conforming? 

Let’s be honest: there’s probably not going to be a CT test machine waiting on the 1st tee of your next midweek stableford. 

But if you’re a high-speed player who practises a lot, or you’ve been gaming the same driver head for years, it’s not out of the question that your club could creep beyond the legal limit without you knowing. 

That said, most amateur golfers don’t swing fast enough, or hit enough balls, to wear down their driver faces the way pros can.  

Aside from the little matter of being illegal, is there a downside to a ‘hot’ driver? 

Tempted to scour eBay for a well-used driver head that might sneak you a few extra yards? 

Hold your horses. 

While a hotter face might give you more ball speed, it often comes at the cost of forgiveness. The wear won’t be evenly spread across the face, so you may see inconsistencies in launch, shot shape, flight, and spin. And a driver that’s past its best is more likely to misbehave on off-center strikes. 

If you think your driver isn’t performing how it should, or the performance has changed, it’s worth booking a custom fitting session to get the numbers and compare it to some other models. You might walk away with more distance, better consistency – and peace of mind that your driver is legal. 

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