Bench Journal / Frequency

Butt Frequency and What CPM Really Tells You

The most quoted number on a build bench, and the one most often quoted without the conditions that make it mean anything.

Carry Lab Bench Filed under Frequency About 8 minutes

Clamp a golf club by the grip end, pull the head down a few inches, and let go. The club oscillates. Count how many times it completes a cycle in a minute and you have its butt frequency, expressed in cycles per minute — CPM.

That is the entire measurement. It is simple enough to explain in a sentence, which is part of why it gets over-interpreted. It is also the single most repeatable way to describe how stiff an assembled club actually plays, which is why we keep using it.

What the Number Is Responding To

Butt frequency is a property of the whole assembled club, not of the shaft alone. Four things move it:

Shaft stiffness. The obvious one. Stiffer shaft, higher reading.
Club length. Shorter club, higher reading, and the effect is large. This is why a 7 iron always reads higher than a 4 iron built from the same shaft model.
Head weight. Heavier head, lower reading. Adding a few grams of tip weight will pull the number down measurably.
Grip and build-up. Less than the others, but not zero. Extra wraps of tape change the clamped mass at the butt.

That list is why CPM cannot be used to compare clubs of different lengths, and why quoting a frequency number without saying which club it came from is meaningless. "My irons are 310" tells you nothing until somebody says which iron.

Clamp Length Decides the Answer

Here is the detail that undermines most cross-shop comparisons. The frequency reading depends on how much of the butt is held in the clamp. A five-inch clamp produces a different number from a six-inch clamp on the identical club, and the difference is not small.

Our standard is a five-inch butt clamp, used on every reading, incoming and outgoing. Any bench that is internally consistent is useful. A bench that varies its clamp length is producing numbers that cannot be compared to each other, let alone to anyone else's.

The practical consequence: if you had a set measured somewhere else and the numbers do not match ours, that is usually clamp geometry rather than either bench being wrong. What matters is that the relationships between clubs in one set, read on one bench, are consistent.

What CPM Cannot See

This is the important limitation and it is routinely glossed over. Butt frequency is one number describing an entire stiffness curve. Two shafts can produce identical butt frequency readings while having very different profiles — one soft in the tip and firm through the middle, the other the reverse.

Those two clubs will feel similar in the hands and produce clearly different ball flights. CPM cannot distinguish between them. Only profiling along the length can.

Frequency is an excellent check that a build matched its plan. It is a poor tool for choosing which shaft the plan should have specified.

So we use it for two things. First, as a diagnostic on incoming clubs, where scatter across a set reveals build problems immediately. Second, as verification on outgoing builds, confirming the finished club landed where the plan said. Shaft selection itself is a profiling and monitor job.

What Scatter Looks Like

Read a set that was never checked after assembly and you will typically see the frequency climb from long irons to short ones, as expected, but with clubs sitting off the line by anywhere from a few cycles to fifteen or more. That scatter comes from ordinary manufacturing tolerance combined with nobody verifying the result.

A club sitting well above the line plays stiffer than its neighbours and tends to come up short. One sitting below plays softer and can fly further and higher than the club above it in the bag. Both produce the complaint that a particular iron "just does not work", which players almost always attribute to themselves.

Using the Number Sensibly

Some working guidance, from a bench that reads several thousand clubs a year:

Only ever compare clubs of the same length, read on the same bench, with the same clamp.
Judge a set by the shape of its line, not by any single club's absolute value.
Treat a two-cycle difference as noise. Treat eight cycles off the line as a defect worth fixing.
Never choose a shaft from a frequency target alone. It cannot see the profile.
Always record which club produced which number, or the record is useless later.

Do that and CPM becomes what it should be: a fast, cheap, repeatable check that the club in your hands is the club somebody intended to build.