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Core Technology 101: An Analysis of Spherical, Symmetrical and Asymmetrical Core Design

Updated at February 26, 2010 13:36

By Randy Teitloff
Ebonite International, Inc.

Editor’s note: This article is a good illustration of the technical knowledge that pro shop professionals must command in order to provide the level of service to serious bowlers. This is only a very small part of the basic information which goes into properly selecting and drilling a bowling ball. Your Smartbowler Pro Shop is your resource.

In this article we are going to talk about cores in order to build an understanding of what it all means, breakthrough the mysticism that surrounds this aspect of a bowling ball, and get to the facts and simple truths. When talking about the subject, as with anything, there is always some degree of skepticism or uncertainty regarding the validity of claims one might hear that a concept in technology brings. Some of the claims that have been made are nested in truth while others stem from misconceptions. This article will present the proven results of years of work into the insight and understanding of the mechanics of core technology.

Cores come in a variety of shapes and sizes and, depending on the type of core used in a ball, will impart a particular property or characteristic to the ball. This article will discuss the attributes and features that make up the core and talk in terms that describe the nature of the core itself. In order to do this it is necessary to first establish a common frame of reference when discussing this matter and also to reflect on the technology of the past in an effort to appreciate the current trend in technology.

At this point it is probably prudent to define some terms that are used to describe certain qualities of a core including Mass Bias or intermediate differential, Radius of Gyration (RG) and Differential (Diffs). To begin with, Mass Bias or intermediate differential is not really as it states, the purposeful vectoring of mass from the geometric center of the ball to induce an imbalance. It actually is changing the mass properties, generally core shape, in a way that creates a difference in the RG values about the three axes of a bowling ball. To describe what a core is, we in the industry use measurable terms like RG and Diffs that quantify how tall or wide a core is, whether a ball is mass centered or cover heavy and also how uniform or non-uniform the shape of the core happens to be.

There are three basic, fundamental or distinctive types of cores: spherical (Figure 1), symmetrical (Figure 2), and asymmetrical (Figure 3). Obviously the spherical core is merely a standard, full size core that has been used for years. It imparts very little dynamics to affect ball performance. Other than coverstock variations, there just aren’t enough inertial differences in the mass properties of the ball to create strong processional forces to steer the ball toward the pocket to the degree of other core technology types. This stems from the fact that there is very little difference in RG values anywhere on the ball, particularly the principle axes of the ball that has a spherical core in it.

In 2-piece cores, like the standard "light bulb" symmetrical or more complex asymmetrical type, they can and do possess the forces necessary to affect a change in the path of the ball down the lane to a greater degree than that of the spherical core design.

In the past, some have referred to the "light bulb" design as an asymmetrical core. This is a misconception. While it is more asymmetrical than a spherical core, it is still not a good idea to refer to it as a truly asymmetrical core shape since there is symmetry in the core. A "light bulb" type core has a low RG inertial axis and a high RG inertial axis that is perpendicular to the lowest. Moreover, the RG inertial reading that is taking 90 degrees to the pin all around the ball has relatively the same inertial value. What you wind up with is a ball that has the concentric or circular distribution of differing RG values that are "symmetric" about the pin and hence the term symmetrical core.

An asymmetrical core is one where there are three distinctive RG values along the cores principle axes: the low RG inertial axis, which is usually in the area of the pin, an intermediate RG inertial axis that is 90 degrees to the pin and a high RG inertial axis that is 90 degrees from both the pin and median axes. Does this matter? The answer is yes. In order to distinguish the differences between core types it is better to use terms that best fit the description of the mass properties the core happens to exhibit.

Why does Ebonite International make so many different types of cores? The main reason is to affect the kinematics of the ball. The technology of core designs is making use of the inertial dynamics that are possible in bowling balls. It is exploring the limits of mass distribution to create new potential tools that affect ball motion.

What are the advantages of the different types of cores? Before this is answered directly, let’s take a look at the progression of ball technology over the years. First is the development of balls that utilized large, full-sized cores. If you were to measure the inertial dynamics of these types of balls, you would find the inertial measures were the same no matter where the measurement was taken. This can be seen in Figure 1 where a value might be taken as an RG of 2.510 at the pin and an RG of 2.513 that is on the perpendicular axes. At most, the values may be different by only a small factor. The only real mechanism, other than the veneer, to manipulate ball motion was the use of static weight imbalance. It is widely known the use of finger/thumb, positive/negative, and top/bottom weights have a direct impact on ball motion with these types of balls.

As technology progressed through the years, core designs changed. They became less symmetric. The familiar "light bulb" shape became the staple core and since then a myriad of complex and bizarre shapes emerged. However, they all possess the same fundamental premise as the original Hammer type core that started this phenomenon.

As mentioned previously, these balls have inertial properties that are different than those of the full size spherical cores. In Figure 2, the values for the pin RG might be 2.510 and perpendicular to the pin in any orientation about the pin can measure the same RG of as little as 2.52 to 2.57 as an example. This is an incredible leap over spherical cores.

Later it was found that static weights played less of a part in manipulating ball motion and use of pin position dictated a predominant influence. This was mainly due to the fact that a new property became manifested by virtue of the mass properties from the core. It seemed that the differential in inertial parameters induces a secondary axis of spin brought about by the rotational axis. This secondary spin axis is what we call the bowler’s PAP migration or precession movement. This precession movement gave rise to what is known as track flare.

Now we come to the totally asymmetric cores or the ones with the mass bias. This type of core creates differences in all three principal axes of the ball. In Figure 3, we see that a measurement about these axes will yield three different RG values. The values between the pin and 90 degrees to the pin is the same as that of non-mass bias cores; however, in this case, there is a difference in the RG values of the two 90 degree to the pin. This value can be as low as 0.005 and as high as 0.030 differential RG.

These core types function in much the same way as symmetric cores do; yet, they have added features to exploit that are not possible with symmetrical cores. With this core type you have a ball that has the progressively non-concentric or an elliptical distribution of differing RG values that are "asymmetric" about the pin and hence the term asymmetrical core.

The first benefit is derived from the inherent strength contained in the mass bias in that it can influence ball motion to the same degree as static imbalances did for full size spherical cores. In a sense, it could be like having a dynamically imbalanced ball. That is, orienting the mass bias in different areas of the ball will have the same relative effect on the ball motion like finger/thumb, positive/negative and top/bottom positions. These changes were used in spherical type cores everyone has come to know and use through the years to change ball motion. In the case of this type of core, the effect is magnified and much more pronounced in changing the motion of the ball.

A core is considered an asymmetrical core when the RG values of the three main axes are different. Some cores may look asymmetric but are not because it is not merely the shape that dictates what a core is, it is the numbers that do. How different one might ask? Anything above a 0.005 begins to exhibit non-symmetrical behavior; the higher the number the stronger the influence. But for the sake of simplicity, if the Mass Bias or intermediate differential is at least 0.010 then it is generally considered a true asymmetrical ball.

One might think when looking at Figure 3 that an asymmetrical core is merely a symmetric core with a static imbalance that is positioned perpendicular to the primary imbalance of the bowling ball for the purpose of making the ball pin out rather than tilting or offsetting the core. Due to the limitations of the amount of top weight that a ball can have, the offset of the core is minimal and the highest value of an intermediate differential that can be generated is less than 0.005. A symmetric core is not going to be an asymmetric core just by tilting or positioning the core slightly offset to produce a pin position that is away from the pin.

Another way we can look at the three basic types of cores is in Figure 4 that shows all three core types together. The colored lines represent the principal axes of the core and the boxes represent the core. The diagram on the left represents the spherical core. As you can see, all sides are equal and will yield three axes of the same RG values. The diagram in the middle represents the symmetrical core and two of the sides are equal and one is different. In this case the core is taller than it is wider but the converse could be true and hold the same configuration in that two sides out of three would be equal. In the case of the diagram on the right, all three sides are not equal and would yield three different RG values, making it asymmetrical. It looks like a cigar box where the one side is taller; one side is narrow and the other is wide.

-Randy Teitloff is the Vice President of Research and Development for Ebonite International, Inc. This article, which is protected under copy right, is courtesy of Ebonite International, Inc. and should not be used without the company’s consent.

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