Spin axis and side spin

What is spin axis?

It's the tilt of the line the ball spins around, measured from the horizon just after the ball leaves the face.¹

Think of an airplane's wings. Wings level means no tilt and a straight flight. Tilt the wings and the plane banks. Trackman uses this same comparison.¹

Bushnell calls the same idea spin tilt axis. It's the axis the ball rotates around to create curve and lift.²

How do I read the sign?

A negative spin axis means the ball curves to the left. A positive one means it curves right. Zero means no curve.¹

Trackman treats a spin axis between minus 2 and 2 as a straight shot. Under normal conditions it'd be hard to see any bend in that range. The bigger the number, the more bend you should see.¹

How do the names fit the numbers?

Trackman matches the names to the numbers like this.¹

Table source:¹. Wikipedia agrees on the basics. A draw moves right to left for a right-hander, and a fade moves left to right.³

So what is side spin?

Side spin is one part of total spin. Bushnell says it defines ball curvature, or shot shape, and that it relates to the spin tilt axis.²

So the two numbers describe the same bend from different sides. One is an angle and the other is a spin amount.²

Where does side spin come from? Wikipedia says it comes from the club, or the path of the club head, not being perpendicular at impact. A draw can come from an in-to-out path or a closed face.³

How far will the ball bend?

Trackman gives examples. On an optimized 150 yard shot, a spin axis of 10 gives about 11 yards of curve. On a 200 yard shot, the same 10 gives about 15 yards.¹

Negative numbers do the same to the left.¹

Does wind change it?

Trackman says the spin axis is set at impact and stays the same through the flight. Wind can push the ball around, but the spin axis number won't change.¹

How does my unit get it?

It depends on the mode. In Trackman's outdoor mode, the first 30 yards or so of flight give the spin axis. In indoor mode, it's calculated from spin loft and face to path.¹

That's a good reason to read your own unit's manual. Different units and modes may arrive at the number differently.

What links spin axis to my swing?

Face to path. Trackman calls it a key factor in the curve you can expect.⁴ The club path and face angle page walks through how those two numbers combine.