Why the Bay Reads Long, and by How Much

A golfer walked in last spring with a printout from another studio showing a seven iron carrying 171 yards. He plays a course we know well and he had been coming up short of that number by a full club for two months. He was not lying, the other studio was not lying, and the printout was not faked. The reading was correct for the room it was taken in and wrong for the golf course he plays, and nothing on the sheet warned him about the difference.

The gap between a bay reading and a turf result is not one error. It is four, and because they all lean the same way they accumulate instead of cancelling out.

One: The Model Is Running on Standard Air

Everything past the screen is calculation. The ball travels perhaps eight feet before it is stopped, and every yard of carry reported after that point is a flight model taking the launch conditions it observed and predicting where the ball would have come down.

That model needs air to fly through. Unless somebody has fed it live weather, it uses a default, and the defaults in commercial software are close to sea level at a mild temperature. Oklahoma City sits around twelve hundred feet, which on its own is worth a couple of yards on a mid iron. The bigger effect is day-to-day: a cold, high-pressure December morning and a humid August afternoon differ enough in air density to move a seven iron by four or five yards, and a bay running on defaults reports the same number on both days.

Two rooms in the same city, running the same software, can therefore disagree by several yards purely because one of them has a weather station wired in and the other does not. Neither is broken.

Two: The Mat Is Kinder Than the Ground

A synthetic mat has a hard base under a compressible surface. When a club arrives a little steep, the sole skids off that base and into the back of the ball. The strike that would have been a fat shot on grass, with a chunk of turf taken before the ball and several yards lost, becomes a slightly-thin-but-acceptable shot with almost full speed.

This is not a small correction and it is very unevenly distributed. For a golfer with clean turf contact it is worth almost nothing. For a golfer who catches one in five heavy on a fairway, the mat quietly removes the penalty on all of them, which raises the average carry without changing a single thing about the swing.

Averaged across the people we measure, the mat effect on a seven iron sits somewhere between two and six yards. For fairway woods it is larger and considerably more variable, which is why the fairway wood block insists on a lane confirmation rather than treating it as optional.

Three: The Ball Is Often Not Your Ball

Many bays run on a limited-flight ball, a range ball, or a premium ball that is not the one in your bag. Spin is the column that moves most between models, and spin drives carry through launch and through drag. A soft urethane cover and a firm ionomer cover can differ by several hundred revolutions on a mid iron, and by well over a thousand on a partial wedge.

Where the ball has been marked for optical tracking, there is a second, smaller effect: the ball has to be found and read, and any reading that fails gets discarded, which subtly biases the sample toward cleanly struck shots.

Four: People Swing Differently When Nothing Lands

This is the one nobody can measure cleanly and everybody who has spent time on a fitting floor has seen. There is no consequence in a bay. The ball hits a screen either way, nobody watches it drift into the trees, and the tempo that produces the best number gets rehearsed for twenty minutes.

It is not universal. Perhaps a third of golfers swing meaningfully harder indoors, a small number swing more carefully, and the rest are indistinguishable. But it is real, it is directional, and any studio that tells you the room has no behavioural effect at all is describing a population that does not exist.

Four effects, all pushing the same way. That is why the answer is never "the bay is roughly right" and always "the bay is long by a specific, seasonal, club-dependent amount".

What the Four Add Up To

The table below is our own offset table, condensed. It comes from the quarterly lane mornings, where the same two staff hit the same reference clubs indoors and then on grass within a few hours of each other. The numbers describe our bay, our mat, our reference ball and our instruments. Do not apply them to somebody else's room.

Two things are worth noticing. The offset shrinks as loft increases, because a higher, shorter flight spends less time being modelled and less time in the air for air density to act on. And the winter column is the widest, because the bay is heated and the lane is not, so the model is working from indoor air while the ball is flying through outdoor air.

What to Do With This

If you have a fitting sheet from anywhere, the useful question is not whether the numbers are real. They are. The useful question is which of the four effects above were present and whether anybody accounted for them.

• Was live air data fed to the model, or was it running on defaults?
• Was the surface a mat, and was any correction applied for it?
• Which ball was used, and was it yours?
• Was any part of the session confirmed outdoors?

Four questions, and any studio working carefully will have all four answers written down already. A sheet that can answer them is usable on a golf course. A sheet that cannot is still a fine record of which club beat which other club in that room on that afternoon, which is genuinely useful and is a different thing from a yardage.

Corrections: none since publication. The seasonal figures above are re-derived every quarter and this page is updated when they move by more than a yard.