What makes a golf driver illegal? Explaining coefficient of restitution and characteristic time
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What makes a golf driver illegal? Explaining coefficient of restitution and characteristic time

Many golf fans were shocked to learn Xander Schauffele's driver was ruled illegal by the R&A ahead of the 2019 British Open Championship.

Schauffele talked about the testing, which he supports, and the resulting fallout after his second round at Royal Portrush. However, he didn't explain precisely what the R&A was testing for, nor did he discuss what made his driver illegal.

The driver Schauffle was planning to use was ruled illegal because it exceeded the R&A and USGA limit for what's called characteristic time. Characteristic time, or CT for short, is a measure of long the golf ball stays on the club face at impact. It is a measure that tells us the elasticity of the materials in the club face (and golf ball for that matter). The longer the characteristic time, the more spring-like the face is. There is a maximum time regulated by the USGA and R&A, and it's CT limit is currently 239 µs (microseconds) plus 18µs for measurement tolerance, so a total of 257µs.

According to Tony Covey of MyGolfSpy, Schauffele's driver was ruled illegal because it exceeded the tolerance by 1µs. So what does that mean in terms of performance? Really, not much. As Covey told me on Twitter, 5µs of characteristic time equals approximately 0.25 mph in additional ball speed for a flushed strike on the center of the face.

This means the manufacturing tolerance allowed by the USGA and R&A covers approximately 1 mph in ball speed. For a driver that's illegal by 1µs, there's not much of a tangible difference in performance. Nonetheless, it's illegal.

Some golf fans may not be familiar with characteristic time, but they may have heard of coefficient of restitution.

Coefficient of restitution, or COR for short, is a measure that tells you how much energy is transferred from the golf club to the golf ball at impact. It's a measure of club and material efficiency. If a club were totally efficient, COR would be 1. However, the USGA and R&A limit maximum coefficient of restitution to 0.83, meaning 83 percent of energy from a club can be transferred to a ball at impact.

However, testing COR is difficult to do in the field, so the USGA developed characteristic time as an equivalence for COR that could be more efficiently tested.


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