RG Ratings Explained

A Quick Explanation of a Bowling Ball's Radius of Gyration

Roto Grip Grenade urethane bowling ball
Roto Grip Grenade urethane bowling ball. Photo courtesy of Storm Products, Inc.

When looking to purchase a bowling ball, you see all kinds of specs, numbers, and phrases that make no sense to beginners and even a lot of experienced bowlers. One of these—and one of the most important to selecting the best ball for your game—is RG (Radius of Gyration).

This number explains how the mass is distributed in the ball, which will give you an idea of how the ball performs. That is, when will the ball start​ to rotate?

Even in spherical objects, weight is not distributed evenly. The most noticeable proof of this in a bowling ball is the core, which has a shape that clearly weighs more in some spots than others. Still, how can mass be distributed throughout a bowling ball to your advantage? Scientifically, of course.

RG Scales

Every ball will rate somewhere between 2.460 and 2.800, although many ball manufacturers have converted to a 1-10 scale to give consumers an easier frame of reference. Still, how easy can it be when a term like "radius of gyration" has such a strange range of scale? Spheres are generally hard to comprehend, anyway. So, as best we can deduce, what do these numbers mean to a regular human being?

The Meaning of the Ratings

A ball with a high RG rating (close to 2.800 or 10, depending on which scale the manufacturer uses) will have a mass distributed toward the cover, which is often referred to as “cover-heavy.” This type of mass distribution will give your shots more length.

That is, the ball will travel through the front part of the lane while saving energy so it can start rotating as it nears the pins. These balls are well suited for dry or medium lane conditions when you don’t want the ball to hook too early.

Conversely, a ball with a low RG rating’s (close to 2.460 or 1) mass will be distributed toward the center, otherwise known as “center-heavy.” These balls are valuable on oily lane conditions, as they’ll begin rotating earlier, giving you more time to grab the lane and get the ball to the pocket.

If you were to use a ball with a low RG rating on a dry lane, you may have trouble with overhooking your shots. If you were to use a ball with a high RG rating on a wet lane, you may have trouble getting the ball to hook enough. This is one reason so many bowlers, especially those who bowl in a number of different bowling centers, carry an arsenal of bowling balls, giving them options when needing to adapt to a given lane condition.

There is no definitive RG that is better than any other. Like everything else in bowling, the ideal RG is dependent on all the other factors in play. To retain energy in the ball longer down ​the lane, go with a high RG rating. To get the ball rolling as soon as possible, go with a low RG rating. While there are general guidelines that can help you guess, the only trustworthy method is to actually throw a shot on the lane and figure things out from there.

When combined with your drilling layout, style of bowling and everything else that goes into throwing a shot, the RG of your bowling ball will have a huge impact on how your ball actually rolls.