FLO Testing Explained: Finding the Flat Line
FLO stands for flat line oscillation — a second, independent way of finding a shaft's spine that doesn't rely on a deflection board at all.
A deflection board measures spine by applying a fixed load at points along a shaft and reading how much it bends. FLO testing takes a completely different approach: it clamps the butt end of the shaft, hangs a weight from the tip, and sets the whole assembly oscillating like a pendulum. Where the tip naturally settles as the oscillation dies down reveals the same spine information, measured a different way.
Why Test It Twice
Any single measurement method can produce a slightly noisy reading, especially near the transition zones on a shaft where stiffness changes gradually rather than sharply. Running both a deflection board test and a FLO test on the same shaft and confirming they agree gives a build far more confidence than either method alone — if the two disagree, we re-test rather than guess.
What "Flat Line" Actually Refers To
During oscillation, a shaft settles into a stable back-and-forth motion along a single plane once the wobble dies down — that stable plane is the "flat line." It corresponds to the shaft's spine axis, because the shaft naturally wants to oscillate along its stiffest and most flexible directions, not at some arbitrary angle in between.
A shaft doesn't need to be told where its spine is. Given the chance to settle into its own natural motion, it shows you.
Reading the Result
Once the shaft settles, we mark the plane of oscillation directly on the shaft with the tip pointed toward the observer, which gives a repeatable reference line for orientation. That mark is what the build bench uses when gluing the shaft into a head or ferrule, so the orientation decided during fitting carries through exactly into the finished club.
Where This Fits in a Session
FLO testing happens right after the initial deflection board pass, before we ever move to the launch bay. Confirming spine this early means every subsequent decision — head pairing, loft, even ball position — is being made with accurate information about how the shaft actually behaves, not an assumption.
Watch a FLO Test on Your Own Shaft
Every fitting session includes both a deflection board and FLO confirmation before any build decision is made.