Bowling balls have been made a few different ways over the years and no one way has been truely dominant.
Two of the most common ways bowling balls have been made for many decades now is with either 2-piece or 3-piece construction.
Three-Piece Bowling Balls
These are balls that are made by filling the vast majority of a ball from the middle all the way to the shell with a basic filler material.
At the “top” of this filler material is a small pancake shaped dense weight block that provides some extra weight to compensate for the removal of material when finger holes are drilled around it.
These 3-piece bowling balls are commonly refereed to as having pancake cores that are not-dynamic in any way. They are basically symmetrical and provide little to no differential – meaning they don’t flare which causes hook potential to be very low.
In modern bowling only the most basic balls are made with three piece construction, plastic spare balls come to mind first.
Two-Piece bowling balls
Two-Piece balls have been the construction choice for many decades for those balls meant to perform, hook, and strike a lot.
A Two piece ball refers to a ball with a central core otherwise called a weight block.
These cores contain the dense material that provides weight to the ball and allows for differential and track flare.
Many older two-piece balls used a central symmetrical core and then a thick shell to surround it which gave the bowling ball it’s final shape however most modern 2-piece balls use the central core and then a filler material between the core and the outer shell.
The reason for this has mostly to do with the costs of the high tech material that goes into shell construction and the weight and density of the central core. To keep weights in acceptable ranges the “outer core” that sits between the central core and the overstock is often lightweight as it’s not intended to affect ball motion or come in contact with the lane surface.
Many bowling balls use the outer core in their ball designs but despite the three distinct layers these balls are still considered to be two-piece bowling balls.
Two Piece Balls Hook More… Usually
Another basic rule-of-thumb that is easy to remember is that three piece balls are for going straighter and two-piece balls are for more hook.
Balls with central weight blocks have the ability to be drilled in such a way as to increase the flare potential and hook potential of a ball. When the weight block is engineered to increase flare then each time a ball rolls over on a lane surface a fresh (dry) track line is formed which greatly increases friction and hook potential.