It seems like the floodgates are opening and we are going to continue to see more and more affordable indoor launch monitor options every year. It’s a great time to be an indoor golfer. However, it’s important to know that these units aren’t all the same and some of their differences could mean big variations in how well they are able to determine what the golf ball is actually doing. It seems to be a point of contention with some people, but the truth is that radar units just can’t provide the same level of accuracy as camera-based (photometric) units do in an indoor setting. They function best when they can actually track a ball throughout a substantial portion of its flight. Even Trackman, the prize-winning Thoroughbred of radar launch monitors, seems to be conceding that fact with their new, camera-based indoor launch monitor systems. So, what is it that a camera (the high speed camera in the Rapsodo MLM2PRO, for example) does so well when it comes to predicting ball flight? Much of that answer is boiled down to the ability to measure spin rates and spin axis.
What is Spin Axis?
This is a topic that could easily span many more pages than we are able to devote to it, but we think we can explain it quickly in a way that makes sense.
First, we need to understand that a golf ball spins on single axis. Imagine a horizontal line parallel with the ground going through the center of the golf ball. Since we are striking the ball with a lofted club, it spins backwards on that axis. We, of course, call this backspin, measured in revolutions per minute (RPM). While some players may see upwards of 11,000 RPMs of backspin with a wedge, they may also see less than 2,000 with a driver. In the picture below, we see the ball spinning backwards along a horizontal axis (if you can kindly ignore the imperfect model I created).
Of course, we know that rarely does the ball spin perfectly backwards. Instead, golfers usually see curvature of some sort to their golf shots. While the full explanation of why that occurs is fascinating (read up on the D-Plane if you’re interested), we will keep it simple and say that a combination of your swing path and the club face’s angle at impact can cause the ball to stray from spinning perfectly backwards. Over the years, we’ve all grown familiar with the term “side spin”. It’s a common metric on almost any launch monitor that gives us an idea of whether our ball is moving left or right. However, there really isn’t such a thing as side spin. Rather, the axis the ball spins backwards on tilts, and we call this spin axis.
A perfectly horizontal axis is measured at 0 degrees, while the ball below is spinning on an axis tilted to the right, typically labeled as a positive number. In this case, the tilt is extreme for the sake of demonstration; likely close to 45 degrees. It would represent maybe the most impressive slice off the planet that anybody has ever seen.
Conversely, the ball below would illustrate a ball that moved in draw or hook shape, and we’d show the angle of the spin axis with a negative number. For the sake a conformity, we’ll just say this one is -45 degrees. Again, keep in mind that real world golf is going to (hopefully) see much smaller tilt angles than shown here.
Why Should You Care?
In truth, a large portion of the golfing public probably doesn’t need to care about spin axis. However, anybody considering an indoor launch monitor or simulator should consider its value before making a purchase. The truth is simple: If a launch monitor can’t measure spin axis and spin rates, its ability to display a realistic and accurate shot on a screen is compromised. We can add in all the necessary disclaimers there, because there are units that can calculate their way into acceptable windows of accuracy, but the truth remains. If you want to know what the ball is doing, it needs to be measured and radar really struggles to do that indoors. A photometric launch monitor that measures things like ball speed, starting direction, launch angle, back spin, and yes, spin axis, is going to be the most accurate option to track things like distance, shot shape, angle of descent, and dispersion.
Rapsodo and Callaway RPT Balls
Prior to 2023, options in the photometric arena weren’t really included in the low-cost portion of the market. Ranging from around $3,000 for a SkyTrak+ to close to $15,000 for Foresight’s GCQuad, they weren’t quite accessible enough to be considered “affordable”. Rapsodo managed to turn that reality on its head with its introduction of the MLM2PRO, which comes in around $700. While it’s not the lowest priced unit on the market, it’s the lowest priced unit that can boast its accuracy coming from photometric measurements.
It does this with the help of Callaway’s RPT (Rapsodo Precision Technology) golf balls. These are essentially normal Callaway Chrome Soft X balls marked with a series of dots that the MLM2PRO can measure with astounding accuracy. Callaway was able to leverage their existing expertise in golf ball marking they developed with the Truvis line. This partnership has enabled Rapsodo to punch far above their weight class in terms of accuracy, rivaling units close to 10 times the price.
According to Rapsodo, “Through our partnership with Callaway, we specially engineered balls using Callaway’s TruvisTechnology. These new golf balls are called, Callaway® RPT™ Chrome Soft X® Golf Balls and are designed with RapsodoPrecision Technology (RPT™) so the MLM2PRO™ can track the in-flight spin rate and spin axis of the ball. This allows you to receive spin measurements within 1% of the accuracy of high-priced launch monitors.”
The MLM2PRO isn’t perfect, and there are aspects of the device that remind us it’s a $700 launch monitor, but its reputation for accuracy really can’t be disputed. One complaint we’ve shared is that the RPT balls, while amazingly functional, are limited in scope. For example, those customers that would rather use a regular Chrome Soft don’t have that option at the moment. We aren’t alone in wishing Rapsodo and Callaway would expand the balls available with RPT markings. Maybe if we ask nicely again, we’ll get our wish.
What are your thoughts on spin axis and launch monitors? Come discuss the topic with thousands of other golfers on THP’s forum.
