About this site

What is the Bowlidex?

The Bowlidex is a free resource for bowlers and pro shop operators to quickly compare bowling balls across major brands. The advanced filtering and sorting controls make it easy to determine whether a particular ball fits into a bowler's current arsenal or to find a ball that has characteristics similar to an older model.

Where does the data come from?

All data is collected directly from the manufacturers' web sites.

I have an idea for a feature or a ball I'd like to see included. What can I do?

Please enter any comments, suggestions, or corrections below and tap Send:

Blank feedback isn't terribly useful and is a waste of electrons.
Unable to send feedback because of a server issue. Please try again later.

Glossary of terms


In spite of its appearance, a lot more goes into a modern bowling ball than just an eye-catching color scheme. Manufacturers pour countless hours into designing new chemical formulations for coverstocks and new geometries for cores so that their bowling balls will perform as expected on the numerous lane conditions found in today's game.

The coverstock is the material on the outside of the ball. As the only part of the ball that touches the lane, the coverstock is responsible for most of the ball's total reaction. On most balls, the coverstock is less than an inch thick, but in some designs the coverstock extends all the way to the core.

The coverstock's role is to regulate friction between the ball and the lane. Different coverstock formulations produce different levels of friction by either changing the surface roughness or coverstock hardness. Generally, a rougher surface increases friction by having more defined microscopic peaks and valleys that can reach to the lane surface through thicker layers of lane conditioner, while a harder surface reduces friction by reducing the contact area between the ball and the lane.

Modern coverstocks can be divided into three categories: polyester, urethane, and reactive resin. Polyester (or plastic) coverstocks generate the least amount of friction and are generally only found on spare balls and entry-level balls. Urethane coverstocks generate a low amount of friction and have a reputation for smooth, controllable reaction shapes. Reactive resin coverstocks are made from urethane with additional plasticizers and other chemicals and produce the greatest amount of friction possible. Most performance balls released today have reactive resin coverstocks.

Within reactive resin coverstocks there are three types. Solid reactive coverstocks tend to have smoother reaction shapes by generating friction earlier, and thus have a less pronounced reaction downlane. Pearl reactive coverstocks have additional chemicals mixed in that tend to reduce friction, causing the ball to generate friction later and store more energy for a stronger downlane reaction. Hybrid reactive coverstocks are formed by pouring roughly equal parts solid and pearl into the coverstock mold at the same time, resulting in a ball that has characteristics of each type.


After the coverstock, the core, or weight block, is the next most important factor in determining how a ball reacts on the lane. In a modern 15-pound bowling ball, most of the weight is concentrated in the core. The United States Bowling Congress places limits on certain aspects of the core shape to which every manufacturer seeking USBC approval must adhere.

Every core shape has at least two stable axes. If the core is rotating around one of these axes it is said to be in a stable rotation. If the core is not rotating around a stable axis, the laws of physics force its orientation to change until it reaches a stable axis. This affects ball motion in the following way.

When the bowler releases the ball, the ball (and therefore the core) rotates around a particular axis. (Where this axis is and how fast the ball rotates depends on the bowler's unique characteristics.) The angular distance between this rotation axis and the core's nearest stable axis determines how unstable the core is at the moment of release. As the ball travels down the lane, the core causes the rotation axis to migrate around the ball, getting closer and closer to the stable axis with the lowest radius of gyration. The effects of this migration can be seen in an additional change in direction in the ball's path beyond that which is generated by friction between the coverstock and the lane.

In order to help bowlers compare the relative strength of core shapes, manufacturers provide the three core measurements described below.

Radius of Gyration (R.G.)

Every three-dimensional object has a particular radius of gyration, or R.G., around a given axis of rotation. This measurement identifies the distance from the axis at which all the object's mass can be concentrated without changing the objects moment of inertia (or, in other words, the amount of force needed to get the object spinning at a certain rate).

Every possible axis drawn through the center of the object has its own radius of gyration. Depending on the precise shape, the object may spin more easily in one configuration than another, much in the same way a figure skater spinning in place spins faster with her arms tucked in than with her arms extended.

In terms of bowling ball cores, the radius of gyration given by manufacturers is for the axis with the lowest possible R.G. for that particular core geometry. This is called the "low R.G." axis and is marked by a circular pin or other mark embedded into the coverstock.

On the lane, a ball with a lower R.G. will generally reach the roll phase of its motion sooner. However, this can vary greatly between balls depending on the coverstock formula, the lane condition, the layout (positioning of grip holes), and the bowler's unique release.


A core's differential measures the difference in the radius of gyration between the low R.G. and high R.G. axes. The differential regulates how quickly the ball's rotation axis migrates as the ball travels downlane. A higher differential can result in faster axis migration, more overall ball reaction, and wider separation between the rings of oil picked up by the ball as it rolls.

Geometrically, a core with a smaller differential will be more spherical and have little difference between its width and height, while a core with a larger differential will tend to be elongated.

Intermediate Differential

While most cores are symmetric (meaning they have rotational symmetry around the low R.G. axis), some cores are asymmetric. Asymmetric cores have more mass on one side of the core than the other. This produces a third stable axis around which the core "prefers" to spin. The intermediate differential (also called "mass bias" or "PSA differential") measures the difference in the radius of gyration between this axis and the high R.G. axis.

Generally, a ball with a higher intermediate differential will respond to friction quicker. On the lane, this results in the ball spinning up to its maximum rotation rate very soon after reaching the dry part of the lane, producing a more angular reaction shape.