## What is Radius of Gyration (RG)?

What is RG (Radius of Gyration)? When looking at ball specs, we always see the term RG. But what is it and what does it mean? To go into detail of what it actually is, we would need more than just a quick post. What it means is what we will discuss.

The RG number you see on every ball is derived from an equation. The equation is (Y axis RG + Low Axis RG)/2. The Y axis is the height of the weight block & the X axis is the width. In our last post about differential, we used a weight block with a Y axis RG of 2.512 & an X axis of 2.462. Plug that into the equation and we have (2.512 + 2.462)/2=2.487. On the spec sheet we will then see the RG of the ball as 2.49. Ok, great, so what does that mean?

In layman’s terms the RG defines how quickly the weight block likes to “rev up”. The lower the RG of a ball, the quicker the weight block wants to read the lanes. In technical terms, the lower the RG, the quicker a core wants to reach its preferred spin axis (PSA). Our weight block with an RG of 2.49 would be considered a lower RG ball and therefore will want to “rev up” quicker. A high RG core will want to “rev up” later down the lanes.

Generally speaking, low RG cores are used for when the lanes are wet and high RG cores are used for when the lanes are dry. There is no clear cut definition of what an high RG or low RG ball is. Everyone has their own opinion. Generally speaking, a ball with an RG below 2.50 is considered “low” and an RG above 2.54 is high.

The reason we see different RG numbers in different weight balls is due to the density changes of the weight block and filler material we use inside bowling balls AND the fact that the density of the coverstock does not change regardless of ball weight. So if you imagine that 1 pound of reactive resin is used in every ball, regardless of weight, that means that the resin is a different percentage of the total weight of the ball. So in a 16# ball, 1 pound of resin is approximately 6% of the total weight of the ball but in a 14# ball, resin is around 7% of the total weight. This is why the RG’s change.

But here is the catch. Whenever we drill a ball, the RG numbers change. Depending on how the ball is drilled and what type of weight block is in the ball depends on how much the change is. The type of cover that is on the ball also is a big factor. An aggressive, “grippy” cover will translate the core characteristics to the lane surface better than a weak cover. If we had a very low RG core in a plastic ball, the ball still won’t hook much because of the weak cover. In a case like that, the ball will actually hydroplane over the oil….but that is another post.

So what do we do with all this information? Most pro shops will use RG’s as a reference point only. If a ball driller wants to choose a ball for heavier oil and needs a ball that starts to hook early, a low RG ball helps narrow down the selection process. Conversely, needing a ball for drier lanes, the pro shop will begin by looking at high RG balls. If you throw the ball 20 mph, your pro shop will probably select lower RG balls. If your ball speed is 12 mph, your arsenal most likely has more higher RG balls.

This may be a bit of information to process but there is no need to over complicate it. Try not to read too much into the RG rating. As mentioned earlier, If you are shopping for ball, use the RG as a reference point only to help narrow down the selection process.