Core Characteristics and Ball Strength

Looking at the 2008 USBC Ball Motion Study, we can conclude that the cover accounts for ~70% of the reaction whereas the core accounts for ~25%.

If you study the graphs in the study carefully, I interpret that when the surface finish is at 2000 grit or above, the Ra, or microscopic height of the peaks, are less of a differentiating factor between balls. There is always new cover technology coming out which could change this (e.g., “Chemical Friction”), but my interpretation is based on the latest data available. On-lane friction is part of the cover to lane interaction but this would also not be a differentiating factor between 2 balls thrown on the same board on the same lane, since it is being dictated by the lane itself. Think of it as a constant factor if you are comparing 2 pieces of equipment.

So the next biggest factor is going to be the core itself. We have become familiar with core specifications known as Rg or Radius of gyration and differential which is the difference between the largest radius and smallest radius of gyration. Some liken differential to the torque of an engine.

We’ve talked a little bit about this in various articles, but here are some simple keys to start:

  • Low RG means the ball wants to get into a roll quickly. (Picture a figure skater with arms tucked spinning very fast)
  • High RG means the ball will take some time to get into a roll. (Picture a figure skater with arms out, spinning slower than the arms tucked)
  • Low differential means that the ball will have little flare as there is little difference between the largest and smallest radius.
  • High differential means that the ball will have high flare as there is a large difference between the largest and smallest radius.

Don’t be scared, keep reading. I will simply this as we get to the end!

What exactly does that mean?
Remember the ball goes through 3 phases as it goes down lane, skid, hook, roll. The ball will skid for as long as it is not encountering enough friction to grab the lane. This is why the predominant affect on a ball’s motions is it’s own friction characteristics (Ra, Rs), the lane’s friction characteristics (on-lane friction) and oil absorption. Then we can talk about the core affect. The study showed that “performance” decreased with higher RG and lower differential.

As of July 1st, 2010, the USBC has set the lower RG limit for a core at 2.46. The upper limit remains 2.80. There are balls already on the market with a lower RG than this, but they will remain legal for competition. So the closer the ball is to the 2.46 limit, the earlier the core will want to get into a roll. The closer the ball is to the 2.80 upper limit, the later the core will want to get into a roll. The differential has no lower limit, but the upper limit is set to .60.

The RG had a bigger impact than the differential. The way I look at it is the RG will determine the core’s strength, or it’s want to impart motion on the ball’s overall motion. The higher differential will tend to create a smoother hook, but in theory a bigger hook. That’s because each time the ball flares, a fresh part of the ball is touching the lane creating a new oil ring. That allows the oil to be better absorbed into the ball. So the differential is helping to increase surface friction to some extent.

Comparing this to our Strong vs Weak ball article, here is a brief tip.

General core specs:
Strong cores will tend to be between 2.46 and 2.53
Medium cores will tend to be between 2.53 and 2.60
Weak cores will tend to be between 2.60 and 2.80

Differentials specs:
High will tend to be .050 and .060 (stronger hook, smoother reaction)
Medium will tend to be between .040 and .050 (medium hook, potentially more angular reaction)
Lower will tend to be under .040 (depends on RG so could range from smooth reaction to more angular reaction)

It takes a little bit of time and experience to understand the impact of the core on overall ball motion. The easiest way is to compare two balls with the same cover and finish but different core. The Storm Furious and Reign of Fire could be set up for such a test. I will try to publish this informal study as soon as possible.


  1. Good article coach. Can you dig a little more into the RG thing. I have always believed that a high RG ball will “lope” down the lane and it’s hook will be delayed. Low RG will have a tendency to get into a roll sooner.

    But you were using the term STRONGER to describe a low RG ball.

    ****The RG had a bigger impact than the differential. The way I look at it is the RG will determine the core’s strength, or it’s want to impart motion on the ball’s overall motion. The higher differential will tend to create a smoother hook, but in theory a bigger hook. *****

    Can you explain this a different way. I would not be able to explain this to somebody else if I had to.
    Keep up the good work!

    • HI David, Thanks for the comments and good question. You are correct about the idea of lope. Let me explain with a visual. Imagine you have a core that looks like a bowling pin and one shaped like a sphere. Without looking too much into how the ball is drilled which is critical, this is more a matter of comparing the 2 cores relatively speaking. The bowling pin may have 2 preferred spin axes, one end over end and one rolling on it’s side. Regardless, this shape will have the opportunity to “lope” until it reaches one of those axes. That allows it to get further down lane before it gets into a roll. This is a high RG example. The sphere on the other hand, basically gets into a roll immediately because it in a shape that can only roll. This would be an extreme case of low RG just for visualization.
      Hope that helps. Good Luck!

      • I’m good w/ that explanation. But I still don’t get how low RG equates to stronger. Seems to me the high RG ball would be stronger since it gets down the lane farther, store more energy and then have a stronger move on the back end. Ebonite Gamebreaker is a low RG ball. It has a nice predictable move to the pocket. I would equate that to a weaker ball.

        Does that make sense?

        • Hi David. Again, you make a good point. I’m not keen on the terminology of “strong vs weak” myself but let me explain. A lower RG ball has a stronger capacity for steering the ball from within. Since it naturally wants to get into a roll sooner, it will be less sensitive to drilling. You should watch my ball review of the Motiv Cruel to see how a low RG core creates early roll. So generally speaking, ball manufacturers will put in lower RG weight blocks in aggressive covers and create a heavy oil ball. In that case, the cover and core both create traction, causing the ball to want to move in heavier oil. That’s the same reason why “heavy oil” balls tend to cover fewer boards in lighter oil. The core and cover create so much traction that the energy transfer is complete well before the breakpoint. That means the ball is rolling early and the breakpoint has no real effect on the ball motion anymore. So even if the breakpoint is 40 ft, you might find the balls transition from hook to roll is less than 40 ft.
          Hope that makes sense.

  2. Very good explanation. Less sensitive to drilling is good terminology.

    Basically if you are looking for an oily lane ball, you would tend steer away from higher RG. And higher RG balls for when you want to push the ball thru the heads?

    • Yes, I think that would be fair to say. It’s not an absolute. It really depends on what ball reaction you are looking for. I think it’s fair to say that many times lower RG balls are “control” balls since the core wants to steer right away. They will have a predictable motion because they want to dominate the ball reaction. High RG balls will tend to make bigger moves because the core will not turn over as quickly and it’s roll will be closer to the length of the pattern. So you will have the core turning over as well as increased friction combining for a big move. You just may not want that when lane conditions are tough.
      With that being said, “controlling” the lanes does not always equate to carry. Let me pull this together on a typical house shot. A ball like the Gamebreaker has a very low RG. Personally, I love the roll of the V2 core. In fact, I think it’s the best core ever made, IMHO. So it “controls” the lanes so well. However, THS is not a tough condition. Hence, sometimes, the more “explosive” ball reaction may carry better in a given game. Say on a fresh shot, a medium or higher RG ball may actually garner better carry because of higher energy transfer to the pins. It’s really up to the bowler to know when a “control” reaction is needed. A transition game, like Game 2 of a set, many times requires a control ball reaction. We know the carry will not be great necessarily anyway, but better to throw a 215 than a 185.

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