Size, Depth, and Vertical Location of Balance Hole
By Brian Pursel
For maximum effect of balance holes, a depth of 2 1/2 to 3 1/2 inches deep is recommended. This depth ensures that the balance hole will remove core mass, effecting its overall shape and flare potential greatest. Larger holes, drilled shallower, will remove the less dense coverstock and/or outer core material. This will result in a less dramatic difference in ball reaction. If you are trying to slightly tweak your reaction, I'd suggest using a shallow hole. You can always drill deeper for a greater reaction change. Obviously, the larger the bit you can use and the deeper you can go (and still remain within the ABC limits of side weight), the greater the difference of the ball's reaction you can expect. Being able to use a larger, deeper hole requires that you have the CG in a position to create excess positive weight. I would recommend placing the CG in a position anywhere up to 2 ounces of positive weight before drilling a balance hole. Slower ball speeds will be best served with smaller balance holes, hence smaller CG shifts. Faster ball speeds will find greater satisfaction with larger, deeper holes created by larger CG shifts.
The position of the CG dictates the possibilities of a balance hole's size and depth. The relation of the CG to the pin is a useful tool to manipulate this. Placing the pin in a 1:30 direction to the CG (10:30 for lefthanders) will keep the CG closer to the center of span. This will allow only a smaller hole. This is best for slower ball speeds, lighter oil conditions, and less axis rotation (more end-over-end roll). Placing the pin at 12:00 will position the CG further away from the center of span, allowing a larger volume balance hole to be used. This is better for the mediums (of oil, speed, and axis rotation). Placing the pin in a 10:30 direction from the CG (1:30 for lefthanders) will place it furthest from the center of span, giving us the opportunity to use the largest volume hole. This is best for heavier oil, faster ball speeds, and larger amounts of axis rotation (side roll) and axis tilt (spin).
The vertical position of the balance hole also determines the rate in which the ball loses axis rotation (side roll). As the ball travels down the lane, friction causes it to lose side rotation (skid) until it rolls end-over-end. This loss of axis rotation creates hook. Once the ball has entered into a complete end-over-end roll, it will cease hooking. We have a drilling pattern that we named "rev-leverage". This drilling pattern utilizes a balance hole 6 inches from the center of span in the thumb/positive quadrant. This is the area of the ball located in a 4:30 direction (southeast) from the center of the span (lefthanders would be in a 7:30 direction, or southwest). Placing a balance hole here allows the balance hole to join forces with the thumbhole to create a huge flat spot on the core. This flat spot maximizes flare potential (it is located roughly 6-7 inches from the pin) and also creates a strong mass bias. This mass bias causes the ball to lose energy at a faster rate, creating a quicker transition from side roll to end-over-end roll. Heavier oil patterns, faster ball speeds, and higher amounts of axis rotation and tilt need a ball reaction that loses side rotation faster. Conversely, lighter oil patterns and slower ball speeds will find that this position of the balance hole will cause the ball to lose energy prematurely, causing the ball to "hook out" before it hits the pins. This hook-out results in maximum deflection and insufficient hitting power.
Balance holes can be your friend if used correctly, your worst nightmare if used incorrectly. Every bowler's style and lane condition combination must be evaluated before deciding to use a balance hole. Please consult your local IBPSIA or Striking Effects Pro Shop for expert advice.