Symmetric vs Asymmetric Bowling Ball Cores Explained

Symmetric vs asymmetric bowling ball cores in brief

The core – also called the weight block – sits inside the ball and determines how its mass is distributed. A symmetric core is described by two values, RG and differential. An asymmetric core has a third value, the intermediate differential, because its mass is distributed differently along all three axes. Manufacturers generally describe symmetric balls as smoother and more predictable when they meet friction, and asymmetric balls as sharper and more angular.

The core is only one part of the ball, though. How it moves on the lane also depends on the coverstock, the surface, the drilling layout, your release and the oil pattern. The sections below explain the core values and what they tell you.

RG: radius of gyration

RG describes how the mass is distributed relative to the ball's axis and is given in inches. A low RG means the mass is concentrated towards the centre ("centre-heavy"): the ball revs up more easily and tends to enter its roll phase earlier. A high RG means more mass towards the cover ("cover-heavy"): the ball resists rotating up and tends to go longer before it reacts.

Differential: potential for track flare

The differential is the difference between the ball's highest and lowest RG. It indicates the ball's flare potential – how far the oil track can migrate across the ball as its axis shifts during the roll, so that fresh, dry cover keeps meeting the lane. A higher differential generally means more flare potential and a ball that can hook more sharply; a lower differential means a more controlled reaction. Under USBC equipment specifications, the differential may be no more than 0.060 inches.

The differential describes potential, not a guaranteed amount of hook. How much of it you use depends on the drilling layout and your release; how much friction is available depends on the cover and surface.

Intermediate differential and PSA

In an asymmetric core, the three principal axes each have a different RG. The intermediate differential – also called mass bias – describes how strongly asymmetric the core is: the higher the value, the more pronounced the asymmetry. Asymmetric balls carry a marking for the preferred spin axis (PSA), or mass bias, which the driller uses as an additional reference when choosing the layout. That gives more layout options.

In a symmetric core, two of these axes are equal, so there is no separate PSA. Manufacturers classify their balls and publish the values on the specification sheet for a stated weight. Values can differ at lighter weights, so compare specifications at the weight you intend to use.

Which core type suits you?

Symmetric cores

Manufacturers describe symmetric balls as producing a smoother, more predictable arc. They are often recommended for bowlers with a high rev rate relative to their speed ("rev-dominant"), who already create plenty of rotation and benefit from a controlled reaction.

Asymmetric cores

Asymmetric balls tend to respond more sharply to friction and change direction more decisively. They are often recommended for bowlers with more speed than revs ("speed-dominant"), who need help getting the ball to turn.

These are starting points, not fixed rules. A strong cover or a rough surface can make a symmetric ball read the lane early, and a smooth surface can calm down an asymmetric ball. Which ball suits you also depends on the oil pattern – see our guide to reading lane conditions and oil patterns.

What drilling changes

The published values apply to the undrilled ball. Drilling removes mass and changes how the finished ball behaves. The layout – where the holes sit relative to the pin and, on asymmetric balls, the PSA – decides how much of the core's potential comes into play. Since 1 August 2020, USBC rules no longer permit balance holes, so the reaction is determined by the core, the cover and the layout.

Comparing balls: a short checklist

1. Compare at the same weight. RG, differential and intermediate differential can change with the ball weight.
2. Look at cover and surface first. They create the contact with the lane and often shape the reaction more visibly than the core.
3. Read the core values together. RG, differential and intermediate differential describe different aspects; none of them is a rating of strength on its own.
4. Add your game. Speed, rev rate and your axis determine which layout a pro shop will recommend.

Further reading: Bowling ball coverstock types · Reactive or plastic bowling ball · Bowling ball drilling and finger fitting