The repulsion coefficient and forged cup faces

The compression and decompression of clubfaces was capped by the rules in 2008. The rules are referred to as the SLE (Spring-Like Effect) rules because of the “springing back” function of the face.

Currently, the rule for repulsive performance is measured by CT value (characteristic time). The upper limit set by the rules is 239μs (microseconds), but in consideration of manufacturing tolerance, a leeway of 18μs is allowed. Therefore, the actual upper limit is 257μs. Many golfers may know this.

Until a few years ago, club manufacturers manufactured drivers with a margin to ensure that the repulsive performance conforms with the rules. It was rare for a head to exceed 230μs, and many heads marked less. This is because targeting beyond that will inevitably exceed the limit due to manufacturing errors.

Later, as manufacturing accuracy improved, increasingly more manufacturers began to actively go for the upper limit of 257μs, the actual upper limit, let alone the prescribed 239μs limit. As a result, drivers these days have higher repulsion coefficients and more high-rebound areas than before.

However, 18μs means 18 millionths of a second. Controlling that small amount of contact time is not an easy task. Many manufacturers are incorporating various technologies to aim for rebound closer to the limit than before, but margins are set regardless. This is because it would be catastrophic for the manufacturer if they exceed the rules and a recall was to occur.

Most drivers nowadays are manufactured by welding a rolled face material from the back to a body that looks as if only the face section has been hollowed out. Amid targeting a difference as small as a few millionths of a second, the welding and polishing processes of their manufacturers generate impact in one way or another. The reason PROTOCONCEPT uses a cup face is that the face itself requires minimal work, and the performance of the face created by ultra-high-precision forging can be utilized as it is, facilitating uniform product precision.