Bowling Terminology (Part One): Ball Dynamics

Bowling Terminology (Part One):  Ball Dynamics

by James Goulding III


I am writing this blog post to give bowlers some basic, intermediate, and advanced information on terms used pertaining to ball dynamics.  You have probably heard terms like Differential and Radius of Gyration (RG) and never really understood what they actually mean.  I am going to list some of the terms myself, and other experienced bowlers and pro shop operators, use pertaining to ball dynamics.  This list is a combination of terms used by experienced bowlers and ball drillers (like myself), and put together by a member of the site, whose name is Sean Cross.  While I have added my own terminology and expertise to the list (which you can find on the FAQ section of ballreviews), the list was put together by Sean, so I feel he deserves the credit for combining everything together on this list.  Thanks Sean, and I hope everyone can learn from these terms, and effectively use them in their bowling language dictionaries.


Internal or core torque refers to the mass distribution within the core and the internal lever arms created by the core. Core torque is an assigned value of the ball’s ability to combat roll out, the complete loss of axis tilt and axis rotation.  High torque balls are more effective than lower torque balls at delaying roll out.  Core torque can also be one indicator of the type of reaction that a bowler can expect at the break point with high torque balls having the propensity to be more “violent” and the lower torque balls tending to display a more even, predictable transition from skid to roll.

It is the difference between the lowest and highest RG values of a bowling ball. You compute the high rg value and subtract the low rg value, and you have the differential.  There is no minimum for differential.  What differential tells you:  RG Differential is an indicator of track flare POTENTIAL in a bowling ball.  Differentials in the .01 s to .02 s would mean that a ball has a lower track flare potential, .03 s to .04 s would be the medium range for track flare potential, and the .05 s to .08 s would indicate a high track flare potential. These ranges above are not based on cardinal rules. They are BTM (Bowling This Month)  in-house rules of thumb because there are no published guidelines.  Also, differential is a guide to the internal versatility of a ball.  It can indicate just how much of a length adjustment can be made through drilling.  A low differential will allow for only a modest variance in length (from shortest drilling to longest) which may translate into as little as a foot or two on the lane.  An extremely high differential may translate into a length window in the neighborhood of eight feet on the lane.

The planned apparent imbalance in balls due to high tech cores and drilling techniques.  Many people claim that this has created balls that hook out of the box with a lessening requirement to have the skill to impart the hook and power by the bowler themselves.  This is still up for debate, but the increase in scoring pace of the game of bowling over the past 15-20 years can not be ignored.

In the old days, before the advent of modern core design in bowling balls, the center of the ball was, more or less, symmetrical.  In today’s high tech computer designed ball, in the cores and multiple cores designs, you can have cores that are not evenly balanced and distributed within the center of the ball.  This allows balls to be drilled and designed in a manner that the apparent “weight” of the ball can shift depending on the drilling pattern i.e., it is not “static” it is “dynamic”.

The migration of the ball track from the bowler’s initial axis, the axis upon release, to the final axis, the axis at the moment of impact with the pins. The more flare created by the core, the more hook potential for a given cover stock.  Higher differential bowling balls will flare, on average, significantly more than a lower differential bowling ball due to the increased track migration of the bowling ball with the higher differential.

Simply put, the mass bias in a bowling ball occurs when the mass (weight block or portion of weight block) is bias (more dominant) in one direction inside of an object (in this case a bowling ball).  If you took a bulb shaped, single density core and positioned it dead center from side to side inside the ball, there would be no mass bias.  You also would have a ball that is a pin in (< 2″).  In order to kick the c.g.  (center of gravity) away from the pin to create a pin out ball (> 2″), you have to “tilt” the core inside the ball, or place the entire core slightly off center.  This became a common practice among manufacturers as the demand for pin out balls increased.  When this is done however, you create a “dynamic imbalance” inside the ball because the mass is more dominant or “bias” in the direction of the “tilt” or “offset”.  That is the most important factor when discussing the mass bias, it is a DYNAMIC POINT ON THE BALL.  Positioning the mass bias in different positions when laying out a ball will have a great impact on the “motion” the ball will make as it is going down the lane (even arc, hook/set, skid/flip and so on).  

There are people who will argue that static imbalances (finger weight, side weight etc.) are more important than dynamic imbalances.  My reply to this is that a dynamic imbalance is a real point in the ball, it is constant and does not change unless you alter it by drilling into it with a drill bit.  A static imbalance, or the CG (center of gravity), will change as soon as you put one hole in the ball.  It will change again with each additional hole you put in the ball as well.  While static weights can be used to “fine tune” the reaction of the ball at the break point, it is the dynamic lay out that dictates the roll of the ball.  If a pro shop operator truly understands the principals of the mass bias and how to apply them, they can greatly increase your overall satisfaction with the ball you purchase.   On a ball that doesn’t have a pre-marked MB it’s theoretical position can be found by measuring from the pin through the CG 6.75″.

A Pin-in ball (when the pin is located within two inches of the CG) is excellent choice for control and less overall hook.  A Pin-out ball (> 2″ away from the c.g.) usually can be made to hook more and flip more dramatically than pin-in balls.  They (pin out balls) often give the driller more options as far as fine tuning reaction shapes of the bowling balls for varying styles of bowlers.

This is the final position of the axis after the ball has lost all axis rotation and tilt. The length of time it takes for the ball to reach it’s PSA and it’s post drilling PSA are influenced by the amount of friction, the drill layout, and bowler’s spec’s.

The measurement that tells us the core’s impact on the skid potential of the bowling ball.  It identifies how fast a ball begins to rotate once it leaves the bowler’s hand.  Three designations for the RG of bowling balls are:  low, medium, and high.  A high RG ball goes further down the lane before hooking because it takes longer to begin rotating and stores its energy on dryer conditions.  A low RG ball revs up early and is a more evenly arcing ball used on wetter conditions.  There are three axis on a bowling ball used to measure RG (radius of gyration).  The lowest RG axis (usually denoted by the letter Z) is the axis through the pin.  The highest RG axis (usually denoted by the letter X) is located 6-3/4 inches from the pin through the center of gravity (CG or heavy spot).  The intermediate RG axis (usually denoted by the letter Y) is located 6-3/4 inches from both the low and high RG axis.

Even though all bowling balls of a given weight are about the same size (minimum diameter of 8.500 inches to a maximum of 8.595 inches), these balls are constructed differently.  Some use two materials (one shell and one core), others use three, four or five or more pieces to construct the shell(s) and core(s).  Each of the materials used has a density (which roughly translates into weight per unit of volume).  Zirmonite (as used in the Columbia pin) is denser (heavier by volume) than Bismuth Graphite (used in the core of the Brunswick Zones) which is denser (heavier by volume) than the fired ceramic that is used in the Columbia and Track cores.  These, and the other dense-material cores used by other manufacturers, are all heavier by volume than the material used in the main cores.  The main core material is denser than the foam-like material used as outer cores or inner shells, the purpose of which is to keep some balls in compliance with the USBC (United States Bowling Congress) weight limitation and to help pinpoint a certain RG value.  Then there is the urethane used for the outer shell of the ball, which by density fits in between the core materials.

Even though you may have a bowling ball with as few as two parts or as many as five or more, all balls have one characteristic:  they will act as if all of their weight is located at a point some distance away from the rotational axis.  This distance is the radius of gyration (RG). 

For example, a bowling ball has a maximum allowable diameter of 8.595 inches (maximum radius = 4.2975 inches).  Theoretically, the RG could be any distance from just over zero inches by placing ultra-dense materials in the center of the ball and extremely lightweight filler beyond, to just under 4.2975 inches by placing ultra-dense materials near the outer shell and filling the inner areas of the ball with lightweight foam.

In the first example, the ball would be as center heavy as possible.  In the second, it would be as shell heavy as possible.  The problem with unlimited RG is that the two extremes would produce variations in ball performance that would be enormous.  One would roll immediately and the other would “lope” all the way through the pin deck. 

The USBC (United States Bowling Congress), in an attempt to limit the amount of variation in ball performance that could be achieved through construction, placed minimums and maximums on RG.  The rule states that the minimum RG can be no lower than 2.430 inches and no greater than 2.800 inches.  This means that every ball must act as if its entire weight (mass) is rotating at a distance of not less than 2.430 inches or more than 2.800 inches from the axis.  Since the total span of RGs ranges from 0 to 4.2975 inches, technically all bowling balls fall within the overall medium RG range.  However, when anyone in bowling talks about RG, they are not referring to the total range of possible RGs, but instead only to the RG range allowed for the sport, which currently is 2.430 to 2.800.

In the At a Glance chart, and in ball reviews and comparisons in BTM, the following scale is used for low flare potential balls:

Low RG = 2.430 to 2.540
Med RG = 2.541 to 2.690
High RG = 2.691 to 2.80

There is a slight upward adjustment for high flare potential balls. Determining the RG for BTM and fellow ball geeks, the formula for finding the radius of gyration (usually denoted by the letter k) is:  the square root of the ball’s moment of inertia divided by its mass (k squared = I / m).  What RG tells you: like with everything else in bowling, RG in and of itself tells you very little.  It is ONE indicator of length.  The characteristics of the three types of balls are as follows:

A low RG ball will be easier to “rev up” and it will rev faster, quicker because most of the mass is located relatively close to the center of the ball.  Since it revs faster, sooner, it also wants to hook earlier.  Medium RG balls are intermediate length balls.  They are a little more difficult to spin (takes more power), so most bowlers will see a slight loping characteristic through the heads and early mid lane, followed by a faster revving action and later hook than you would get with the low RG ball.  High RG balls are the hardest to rev up, since the mass is concentrated farthest from the center, and therefore bowlers will see longer lope, much later revving action, and the latest hook from these balls.

Well, there it is in a nutshell.  Hopefully by seeing what these terms mean, you can make more sense out of what goes into a bowling ball manufacturing process, as well as how these technical terms apply to you out on the lanes.  Next time I will explore terminology that relates to bowlers and how they throw the bowling ball (i.e. axis tilt and rotation).  Thanks again to, where I helped to compile the information, and to Sean Cross who crunched all the info together on ballreviews.

James Goulding III


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