Bat Rolling and the Science behind it.

Composite Bats are made by pressing layers of resin and fiber together (see video). The resin can be broken up in between each layer of composite by Bat Rolling. The bat is placed in between 2 rollers and pressure is added to flex the bat in order to break up the resin. As the resin breaks up the fiber is free to flex which gives the bat more of a trampoline effect. This trampoline effect equates to greater batted ball speeds. This effect can be achieved with aluminum or alloy bats also. The metal becomes more malleable which also causes a trampoline effect and more distance to the batted ball.

Perpendicular bat rolling was the first type of bat rolling machine to be manufactured. Perpendicular bat rolling was the industry standard in the infancy of rolling. The Perpendicular method breaks the resin up in 1/4 of an inch stripes along the barrel at maximum pressure (see fig 1-2). As bat rolling evolved so did the machines and the parallel roller was created. The parallel roller is able to put consistent pressure along the entire circumference of the barrel to break up resin completely (see fig 3). For this fact perpendicular rolling followed by parallel rolling has become the bat rolling standard.

Parallel rolling is optimized with a 6 inch roller. Most slowpitch softball bats have a 9 inch sweet spot (see fig 4). Parallel rolling will cause the barrel to flex about 1 1/2 inches past the roller at maximum pressure (see fig 5). The flex should not extend into the end cap area or the taper area for risk of damaging the integrity of the bat. The 6 inch roller will not damage the taper area or end cap area but a larger roller has the potential to flex the composite material in these areas and cause structural problems with the composite.

There have been a number of studies showing that (assuming a player can maintain the same bat-swing speed) using a bat with a larger moment-of-inertia (MOI) will enable the player to hit balls faster and farther. So it is not surprising to find that another popularly used technique is to alter the weight distribution; either by adding weight to the barrel end or by moving weight from the knob to the barrel without altering the total weight so that the balance point moves further out from the handle and the moment-of-inertia increases. According to some Senior Softball discussion groups, one of the more popular high performance bats (Miken Ultra 2) is often end-loaded by simply removing the iron rod in the handle.
The bar chart above shows the batted-ball speeds measured for four bats (single-wall aluminum, wood, multi-wall composite, and multi-wall aluminum) before and after the moment-of-inertia had been increased by 20%. The increase in moment-of-inertia was obtained by adding weight to the barrel end of the bats in a fashion similar to that used by some of the more popular bat doctors. In all four cases, the increase in moment-of-inertia resulted in an increase in batted-ball speed, usually by about 3 mph.