Symmetric vs Asymmetric Cores

Knowing whether you need a symmetrical ball or an asymmetrical ball for the next piece of your arsenal is more important than you may think. Understanding the difference between the two can be a daunting task even for the seasoned professional, but once you have familiarized yourself with the main factors engineered into the ball construction process the sport becomes much clearer and adjustments become easier. Please keep in mind, however, that the information which follows may lead you to your nearest bottle of aspirin! It can be quite technical in nature, so don’t be alarmed if you need to re-read this article a few times before it starts to make sense.

The term differential is the common nomenclature for the difference between the maximum and minimum RG values. The larger the number, the greater the flare potential becomes for the bowling ball.

The radius of gyration, or RG as commonly known, is a measurement in inches from the axis of rotation at which the total mass of a body might be concentrated without changing its moment of inertia. Low RG balls rev up faster and more easily, creating more ball motion, or change of direction.

Total differential (flare potential) can be described as the difference between the X (low RG) and Y (high RG) axes of any bowling ball, symmetrical or asymmetrical.

Intermediate differential is typically only expressed on asymmetrical balls and is the difference in the RG between Y (high RG) and Z (intermediate RG). Intermediate differentials exist on most symmetrical balls, but is not large enough to make a significant impact on the ball’s overall motion.

Differential ratios mandate how asymmetrical a ball is and can be found by dividing the intermediate differential by the total differential. Balls with a larger ratio have a higher degree of asymmetry. Symmetrical balls have the lowest differential ratios in the industry.

There’s a Time and Place

A symmetrical core has an RG (radius of gyration) values of the Y (high RG) and Z (intermediate RG) axes of the ball do not differ by more than 5% of the total differential of the ball. An asymmetrical core is a ball where the RG values of the Y and Z axes of the ball differ by more than 5%. It’s generally accepted that symmetrical drilled balls have a smooth, controllable motion. Asymmetrical balls have a defined, angular shape downlane that respond to friction quicker than symmetrical balls, given the same coverstock composition and preparation. All balls, symmetrical or asymmetrical, become asymmetrical after drilling. Simply put, asymmetrical cores are not in equal proportion top to bottom like a symmetrical core is.

Asymmetrical balls can exhibit large amounts of track flare even with long pin-to-PAP (positive axis point) distances. A 6″ pin-to-PAP distance layout on a symmetrical ball will typically result in a very low-flaring ball. In a strong asymmetrical, however, a 6″ pin-to-PAP distance layout might result in a very high-flaring ball. This is the critical difference between symmetrical balls and asymmetrical balls. This leads to another interesting conclusion: asymmetrical balls can, in general, provide a ball driller with more reaction options than symmetrical balls. Symmetrical balls have only two ball motion “tuning parameters”: pin-to-PAP distance and pin buffer. Asymmetrical balls add a third variable to the equation in the placement of the PSA (preferred spin axis) in relationship to the bowler’s PAP. The higher the undrilled intermediate differential is, the more significant the PSA position becomes.

Bowlers who favor the use of an asymmetric core need a little extra help curving the ball. These balls rev up fast and finish strong with a more aggressive movement downlane. Asymmetrical balls are great for heavy amounts of oil or longer patterns which don’t provide a lot of friction while symmetrical balls are typically smoother and yield a benchmark type of reaction that are more controllable. Symmetricals have two principal moments of inertia (X and Y axes) and asymmetricals have three (X, Y, and Z.) This greater degree of asymmetry is responsible for the highly dynamic moves asymmetrical balls can create.

And finally, don’t forget that there has to be a proper marriage between cover, core, and layout for the ball to react optimally, but we will save that for a later discussion.


15 thoughts on “Symmetric vs Asymmetric Cores

    1. Gary,

      Primarily so that it is the same method to lay out any bowling ball whether its symmetrical or asymmetrical. The distance itself is insignificant, but it is still a tool in fine-tuning ball motion. As you can see, the 5 x 2 x 2 Torrent needs an extra hole that can further manipulate the ball reaction. Choosing shorter numbers for the PSA-to-PAP distance on symmetrical balls, will kick the CG further out making extra holes necessary. It’s just another way to further manipulate the ball reaction to more closely match up to the bowler. If you have any further questions, feel free to reach out to me at Thanks for reading the article.

    1. With the correct layout yes, their intermediate differentials can help immensely with carry-down issues.

  1. Didn’t touch balance holes. They certainly influence ball motion. Why doesn’t Storm endorse the Dual Angle Drilling Method? It is much simpler to understand and adjust from.

  2. I find that symmetrical balls respond to fiction ie check up way too fast compared to asymmetrical balls. That’s why I don’t use them. But I love my alpha crux. Y’all need to put that ball back out 😀

  3. Didn’t touch balance holes. They certainly influence ball motion. Why doesn’t Storm endorse the Dual Angle Drilling Method? It is much simpler to understand and adjust from. Don’t you think ball surface will dictate where the ball reads the lane vs drilling method? (per you example, same ball cover, regardless of core)

    1. Hello Tim.

      There is another excellent article written by Alex Hoskins that goes into great detail about balance holes. And I understand your position on the Dual Angle technique. I used it exclusively for 10 years when I operated my own pro shop. Its fast, easy to lay out a ball, and take a lot of the guesswork out of the job. However, now that I work closely with the VLS system I can clearly see the major differences. The Dual Angle method, while very generalized, does not take the shape of the weight block into account and solely relies on the relationship of the VAL and drilling angles summed together. The VLS system places an importance on the physical location of things like the PSA in relation to a bowler’s PAP. Say you have a 60° x 5 x 30° layout on a ball for a bowler with a 5″ straight over PAP. A 60° drilling angle on an asymmetrical ball would imply a later, sharper breakpoint. I agree with that. A 30° VAL angle (pin up) suggests the ball will rev up early and transition the quickest. I also agree with that. However, if we change the VAL angle to 60° to make it pin down and not change the drilling angle, the physical location of that PSA in now in a completely different location than it was before in relation to the bowler’s PAP. And depending on where and how big their finger/balance holes are, the RG and differential numbers post drilling can change drastically because of the SHAPE inside the ball. Again, physical distances and locations of these motion tuning points on the ball matter more than the relationship of their angles. If you would like to email me at I can send you additional information proving these points.

  4. Why don’t they mark the MassBias on symmetrical balls when the first thing they say in instructions is to mark it on the ball?

  5. Alex,

    Excellent article on the difference between symmetrical and asymmetrical. This information will great when I purchase another Storm bowling ball. Just one questioned, so symmetrical ball would probably be better on shorter oil patterns as they don’t react as fast?

    1. Hello David.

      Yes, you tend to see the higher caliber player using symmetrical shapes on shorter patterns for their control and predictability. Then, they take it one step further and make it urethane/symmetrical!

  6. 65 year old, still in good shape, I’ve always been straight up 10 type bowler.I wear a brace to increase my revs..I’m going to switch to an out to in…throwing towards gutter from around middle to 3rd arrow….asep ball is what I’m thinking…can you give me names of some good storm balls that well help me..thanks and have a Blessed Day!!, RICK

  7. Why are the lower weight balls not available with asymmetrical cores. I have RA and was just curious. I try like hell to throw a 12, but just can’t most days

    1. The lower weights do in fact have asymmetric cores, just different shaped designs. This has to do with the density of the core itself. The 14-16 pound balls need the heavier weight block to create the ball. Those cores are very dense and cannot be put into the lighter balls. The lighter balls need less dense cores but the shape doesn’t match up closely to the RG and differential values so we have to find a shape along with the correct density to be able to make that ball in the lighter weights. Hope this helps explain!

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