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 Part  I             Part III

 THE STRIKE ZONE / Scott Bailey

Part II: The basic science of bowling ball physics

Hello everyone, and welcome to a new season of fun and excitement! I hope you all had a wonderful summer and were able to sneak in a few games of practice to prepare for the upcoming season.

Earlier this summer, I began a three-part series dealing the basics of bowling ball physics. The first segment dealt with what most industry experts consider the most important factor in determining positive ball reaction, the bowling ball's surface. As I stated in the article, no other aspect of the ball's construction will impact as significantly on its reaction as its surface structure and texture.

The second most influential aspect is the makeup of the ball's interior, what we call the core design. How the core mass is distributed throughout the interior of the ball will determine the relative dynamic characteristics of the ball. Factors such as Radius of Gyration, Differential, Core Torque, and Track Flare are mainly determined by the internal core structure. Understanding the definitions of these variables will greatly enhance your ability to purchase the right ball for a given ball reaction.

Radius of Gyration is a measure of the mass distribution inside the ball. Core designs with the mass located further from the geometric center of the ball have a higher radius of gyration, or RG. Conversely, core designs with the mass located closer to the center of the ball have a lower radius of gyration. Because RG is a product of moment of inertia, this core factor influences the relative spin rate of the ball. It also gives the bowler an idea of the relative break point the ball will have.

Balls with low RG cores will tend to spin faster and hook earlier than balls with higher RG readings. Bowlers with faster ball speeds and above average axis rotation will tend to benefit more from low RG core designs, especially when bowling on more heavily oiled lane conditions. Conversely, bowlers with slower ball speed or less axis rotation usually see better reaction out of high RG core designs. High RG designs also work well when the lane oil begins to dissipate.

Differential Radius of Gyration is defined as the measured difference in RG readings between a bowling ball's high, low, and intermediate RG axes. This concept sounds a lot more difficult than it is; please let me explain.

As stated above, RG is the measure of the distribution of mass inside your ball. Every bowling ball has three major spin axes: the high RG or "X" axis, the low RG or "Z" axis, and the intermediate or "Y" axis. The large locator pin that most high performance balls have marks the "Z" axis. If you were to measure the RG of your ball, the lowest reading you would record would be on this axis.

Now, if you were to draw a line from the pin through the label (CG) to a specified distance, this would place you very close to the "X" axis. If you were to measure the RG on this point, you would receive the highest RG number. The difference between the highest and lowest RG measurement is the Differential Radius of Gyration.

The DRG greatly influences the amount of track flare potential your bowling ball will have. Track Flare is the movement of your ball track due to the core's precession as it travels down the lane. If you notice several thin but separate oil rings in your ball track, that is track flare. Track flare maximizes the friction between your ball and the lane surface, so the more track flare you have, the earlier and stronger your ball will hook. If you have trouble achieving strong hooking action, you may need a ball with higher RG differentials. However, if there never seems to be enough oil on the lane for you, I would suggest trying a lower RG differential core design.

Internal Core Torque also deals with the mass distribution inside the ball, as well as the internal lever arms created by the core. Basically, it determines a ball's relative ability to delay "roll out." A ball with high core torque such as the Black Thunderstorm will have less roll-out tendency than the Danger Zone, which is a low torque ball. This does not mean that high torque is better than low torque. To achieve positive ball reaction, you must have the right amount of core torque for your style of bowling on your lane conditions.

If you notice that your ball stops hooking on the back end, causing it to hit the pins with little power, you may need a core design with higher torque. Likewise, if your ball never seems to achieve a strong roll, lower torque cores may work better for you.

With that said, you are probably wondering how this all affects your buying decisions. Unlike the surface texture, the core design parameters cannot be altered once the ball is drilled. Therefore, it is very important to choose the proper design before you purchase the ball. Your physical game and the lane conditions on which you participate determine which types of core designs will work best for you.

To help facilitate your decision, I have included a chart of bowler types, lane conditions, and possible core design choices that will work best together. This chart is a very basic guideline, so it is best to consult your local IBPSIA pro shop technician. He or she will be of invaluable help in this area.

Lane Condition


Straight Player


Power Player

Oily Heads; Dry Back-ends


Low RG; Medium DRG


Low RG; Medium DRG

Medium Heads; Dry Back-ends


Medium RG; Medium DRG


High RG; Low DRG

Dry Heads; Dry Back-ends


High RG; Medium DRG


High RG; Low DRG

Oily Heads; Oily Back-ends


Low RG; High DRG


Low RG; High DRG

Medium Heads; Oily Back-ends


Medium RG; High DRG


Medium RG; Medium DRG

Dry Heads; Oily Back-ends


High RG; High DRG


High RG; High DRG

I also will make available a list of bowling balls with their core design parameters available to anyone who wishes a copy. If you would like a copy of this list, please contact me at 703/ 560-BOWL or via E-mail at

Next month, we will take a look at drill pattern layouts, which I believe are the most misunderstood aspect of bowling ball dynamics. Too often, bowlers become so engrossed in choosing the proper drill layout that they forget that surface and core design will make up 85 percent of the ball's reaction potential. We will discuss this in great detail, so until then, play well!

Scott Bailey operates The Strike Zone Professional Bowling Store in Vienna, Virginia.


 Part  I       Part II       Part III