Composite bats have evolved quickly to become hot-hitting bats. Although many vendors advertise that their bats are hot with your first hit, these bats produce greater ball exit speed after the bats have been broken in, according to Kettering University applied physicist Daniel Russell. Although it's a fairly common belief that breaking a bat in makes the sweet spot bigger or increases the size of the hitting surface, in reality, breaking a bat in involves an entirely different mechanism.
Composite Bats
Composite bats are made from Kevlar fibers, carbon and glass embedded in a plastic resin. They've been produced since 1985, but the early composite bats did not perform well. By 2001, design and manufacturing changes yielded the Louisville Slugger Genesis, a composite slow-pitch bat that ranked number one in performance at the 2001 Bat Wars. Early Genesis models struggled with durability issues. They tended to break after 50 or so good hits. By 2002, the durability of composite bats had improved and though they are still vulnerable to cracking and breaking, especially when used in especially hot or cold conditions, their performance continues to make them the favorite of many players.
Trampoline Effect
When a ball impacts a bat, the bat and the ball both compress. As the ball changes directions, it springs back into its original shape, but it loses some of its kinetic energy during the compression and decompression process. If the barrel of the bat is softer, the bat compresses more and the ball compresses a bit less, losing less kinetic energy. When the bat decompresses, it pushes against the ball as the ball exits the bat, creating what is called a trampoline effect. The ball loses less of its kinetic energy, and the re-expansion of the bat adds to the exit speed of the ball.
Breaking In
Just as you break in shoes, a glove or a favorite chair by using them, you break in a composite or multilayer aluminum bat by using it. Each time a ball strikes a bat, it causes some compression that doesn't decompress. The ball breaks bonds between layers of the bat and crushes composite fibers, creating gentle dents or what are called "waves" in the barrel of the bat. Waves reduce the stiffness of the walls of the bat, increasing the trampoline effect.
How Many Hits
Kettering University applied physicist Daniel Russell notes that the speed of a ball leaving a bat improves by 2.5 to 3.5 mph after 500 hits. Conventional wisdom says, according to TheCoachReviews.com, that a bat can be fully broken in with between 100 to 200 hits. Because of the increase in ball exit speed after a composite bat is broken in, the Amateur Softball Association altered its regulations in 2009, specifying that a bat must conform to BPF and BESR maximum exit speed standards throughout its lifetime, and not just after the bat has been freshly broken out of its packaging. After about 1,000 hits, the trampoline effect starts to dampen, and exit speed decreases.
References
- Kettering University; Are Composite Bats better than Aluminum Bats?; Daniel Russel; March 2005
- Kettering University; Do Accelerated Break-In Techniques Really Work?; Daniel Russell; March 2011
- Kettering University; The Sweet Spot of a Hollow Baseball or Softball Bat; Daniel Russell; November, 2004
- Kettering University: What Happens When Ball Meets Bat?
- TheCoachReviews.com: Taking Care of Your New Fast Pitch Softball Bat
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