Effects of RG and Differential

Did you know that RG and differential have a huge effect on ball reaction? Knowing just some of the basics can really help you choose the right equipment for your game.


RG stands for ‘Radius of Gyration’ and is defined as the distance from the axis of rotation at which the total mass of a body might be concentrated without changing its moment of inertia. Simply put, RG dictates when the core begins to rev up. Weight blocks can be designed in many different shapes, such as light bulbs, diamonds, ovals, footballs, etc.


A football is an ideal concept to explain how a weight block functions. The football can spin easily either end-over-end or as a spiral. It would take longer to make one rotation at the same speed end-over-end than it will rotate as a spiral because the radius of the former axis is longer than the spiral axis. This means the end-over-end axis is rotating in the high RG plane and the spiral axis is the low RG plane.


Within that shape, RG is determined by the location of the majority of the mass. The lower the RG number, the earlier the hook. High RG means the exact opposite: the higher the number, the more length the ball will create. When we see the RG number advertised on a ball box, they provide the low RG plane number. This is industry standard across the board for all balls.


RG ranges from 2.460” – 2.800”

  • 2.46” – 2.51” (Lower RG)
  • 2.52” – 2.56” (Medium RG)
  • 2.57” – 2.80” (Higher RG)


With a better understanding of RG, it’s now time to discuss differential and its effects on ball reaction. We define differential as the difference between the maximum and minimum radius of gyration, better known as the X and Y-axes of the weight block. RG-differential indicates the bowling ball’s track flare potential. Therefore, the higher the differential, the more the ball will flare. The more flare on a bowling ball, the more the bowling ball cover touches the lane surface. With more of the surface touching the lane, the earlier the ball wants to hook. With low flare, less of the ball surface touches the lane causing the ball to create more length due to the surface rolling over the same part of the bowling ball which is now covered in oil. Less surface equals less friction promoting more length.


Differential ranges from 0.000” – 0.060”

  • 000 – 0.025 (Lower Flare Potential)
  • 026 – 0.046 (Medium Flare Potential)
  • 047 – 0.060 (Higher Flare Potential)


How will these factors affect my decision in choosing the correct ball for my game? For this example, let’s use the Storm Proton PhysiX™ which has an RG of 2.48 and a differential of .053. Based on the value chart we used earlier, this is a lower RG/higher differential ball that will want to get into an earlier roll while providing plenty of flare. In addition to the sanded-solid coverstock, this will likely be the strongest ball in your arsenal. This ball performs best in fresher, high-volume conditions.


Now compare the Proton PhysiX to the Hy-Road™ Pearl with an RG of 2.57 and differential of .046. The Hy-Road Pearl features a much higher RG and slightly lower differential, which suggests this ball will provide much more length and less flare overall. The Hy-Road Pearl will maintain a straighter line longer than the Proton PhysiX. This type of ball will be most suitable for higher friction conditions.


The !Q™ Tour Emerald is a unique example. The RG is 2.49 with a differential of .029. Just based on the numbers, this ball wants to roll early, but it will create natural length with low-medium flare potential. Coverstock plays the most crucial part in ball reaction because of that combination. Since this ball features a 1500-grit polished finish, this ball will have length and a strong, yet controllable down-lane motion. However, if you were to take away the polished finish, it would yield a smoother backend reaction.


This information just scratches the surface of core dynamics, but hopefully, this helps simplify the selection process of your next Storm ball.


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