Bench note 04 · Tessa Ruhl
Arc width and how fast the path turns
Watch a putting stroke from directly overhead and the head traces a shallow curve. It starts on the target line, moves inside it, comes back to the line at impact, and goes inside again. Everyone knows this. The interesting part is that two quite different properties of that curve get collapsed into one word.
Arc width is how far inside the line the head gets at the top of the backstroke. Path rate is how quickly it moves from the line to that point. They are not the same thing, and a stroke can be wide and slow to turn, or narrow and abrupt.
Why the distinction matters
Consider two players who both measure 1.9 inches of arc width on a ten foot putt. Identical readings, and a fitting sheet that only records width would treat them as the same case.
The first player moves off the line gradually, reaching maximum displacement only near the end of the backstroke. Their path rate is low. The head is doing something smooth and the transition into the forward stroke is unhurried.
The second player pulls the head inside sharply in the first few inches and then travels almost parallel to the line for the rest of the backstroke. Same width, very different motion. There is a direction change early in the stroke, and the head has to be redirected in order to arrive square.
Those two players want different putters. The first is well served by a heavier head, which will support the smooth motion. The second usually putts better with less head mass, because a heavy head makes the early redirection harder to time and the errors get bigger.
How we measure it
The overhead camera is on a fixed mount, and there are two reference marks on the surface at known spacing. From the footage we take the head's lateral displacement at three points: two inches into the backstroke, halfway back, and at the top.
Width is the reading at the top. Rate is the ratio of the first reading to the last — how much of the total displacement happened in the first couple of inches. A player who has already used forty percent of their arc width two inches into the stroke has a high path rate. Somebody at twelve percent is turning slowly.
The whole calculation is arithmetic on three numbers. It is not sophisticated. It just requires somebody to have decided that the second number is worth writing down.
Arc width is where the head ended up. Path rate is how it got there. Only one of them is usually recorded, and it is the less informative one.
What the combinations tend to want
Four rough cases, with the caveat that every one of them gets tested rather than assumed.
Wide arc, low rate
The classic shoulder-driven stroke. Suits heavier heads, moderate to full toe hang, and standard or slightly longer length. These players are the least sensitive to equipment changes generally, which is a pleasant thing to discover about yourself.
Wide arc, high rate
Usually a stroke where the hands take over early. Lighter heads, more forgiving face technology, and often a shorter putter than the player expects. The short club shortens the lever and reduces how far the hands can pull the head off line before they notice.
Narrow arc, low rate
A tight, controlled motion. These players often putt beautifully with face balanced mallets and benefit from high head mass, which stabilises a stroke that has very little natural momentum.
Narrow arc, high rate
The awkward one. A small stroke with an abrupt direction change in it, frequently associated with tension in the hands or a grip that is too small. Before recommending any head, we look at grip size and length, because equipment is often not the cause here.
The green speed complication
Both numbers move with surface speed, and not by the same amount. On a fast surface the stroke gets shorter, which naturally reduces absolute arc width. Path rate, being a ratio, holds up much better. That makes it the more portable measurement, and it is one reason we run every session at two different speeds.
If a player's rate is stable across both blocks and only their width changes, we are looking at one stroke adapting sensibly to conditions. If the rate changes as well, something less stable is going on, and the equipment recommendation gets more conservative.
What this does not tell you
Neither number says anything about whether the face is square at impact, which is what actually determines where the ball starts. A player can have a lovely path and still miss everything, because face angle at impact dominates start direction by a factor of roughly five to one.
Path measurement is a tool for choosing equipment that supports the motion. It is not a diagnosis and it is certainly not a lesson. If a stroke needs changing, that is a conversation for a coach.
The three-point method came out of an afternoon of arguing about whether a player's stroke was "wide" while both of us watched the same footage and disagreed. Numbers ended the argument in about four minutes.