Expert: - 1/30/2009

Question
What do the "RG#" & "Dif.#" on a bowling ball mean?

Answer
R,
RG is Radius of Gyration, the technical definition involves the relation of the area or mass of a figure to its moment of inertia.
In bowling, a low RG ball will change from it's forward path more readily. If you throw hard, or bowl on more oil, a ball with low RG numbers is more likely to change direction earlier. Simply, low RG is a more center heavy mass (core) inside a ball.

A high RG ball is less willing to change direction. So, if you have slower ball speed or bowl on a dry(er) lane, the initial path (forward down lane) will be maintained longer because of the dynamic of the mass inside. Simply, the core of a high RG ball is typically taller with mass to both top and bottom of the ball.

Diff, or RG Diff is actually Differential Radius of Gyration. In bowling, the difference between the high RG and low RG axis of a bowling ball. Simply, less Diff. (.010-.030) results in less flare potential, a medium diff ball (.031-.040) is greater flare potential and high diff (.041-.060 or max diff. undrilled) the greatest potential for a ball to flare.

Bowling ball core strength is a measure of how low the RG of the core and how high the RG Differential is.

From the USBC "The radius of gyration of a 13.00 lb. or more bowling ball, about any axis, shall not be less than 2.430 inches nor more than 2.800 inches (MAX). In addition, the maximum differential radius of gyration between any two axes of the same ball shall not exceed 0.060 inches. These shall be tested in accordance with an USBC approved test procedure."

Thanks for the question.