What is the performance of a baseball bat or softball bat and how is it measured? Bat performance standards are set and regulated by the National Collegiate Athletic Association (NCAA), National Federation of State High School Associations (NFHS), Little League, USSSA, and Amateur Softball Association (ASA) in the United States and many countries around the world. Bat performance measurements for non-wooden bats are used to bring the game back into balance between offense and defense and to maintain the sport's safety due to high batted-ball speed as new metal bat technology evolves. The study of bat performance gets interesting when you realize that their is not a common test or testing methodology between all these organizations, such that batted-ball speed (BBS), bat performance factor (BPF), bat exit speed ratio (BESR), and bat-ball coefficient-of-restitution (BBCOR) are different measurements trying to accomplish the same goal.
NCAA and NFHS Bat Performance Measurements
In 1999, the NCAA developed a bat performance regulation based on the BESR of the reference 34" long wood bat with an assumed pitched ball speed of 70 mph and a bat swing speed of 66 mph. It was quickly realized that BESR, which measures bat-bat collision efficiency, also needed the accompany bat swing speed measurement, called the moment-of-inertia (or MOI), to estimate the ball speed coming off a bat. The targeted maximum batted-ball speed using the reference bat is 97 mph. Please note that this is not the maximum batted-ball speed possible, but a target, since both wood and aluminum bats can produce higher speeds if the pitcher is throwing faster or the swing is faster than the reference measurement. This target is set as a reference point so that wood and aluminum bats will produce the same ball speed under the same conditions within a few mph. A fact that has be validated in several field studies.
In 2000, the NCAA and NFHS decided to establish the maximum collision efficiency (BESR), maximum barrel size, and bat drop (for the same batter, a heavier bat has a slower swing speed) to keep the estimated batted-ball speed of an aluminum bat around the same speed of the reference wood bat. To be NCAA and NFHS certified, the bat must have a BESR of 0.728 or less, maximum barrel diameter of 2 5/8" and cannot have a drop greater than -3.
In the fall of 2008, the NCAA announced that they will change their bat performance measurement to use BBCOR, which is the ratio of bat-ball speed before and after the collision and measures the elasticity of the bat-ball collision. The date for mandatory BBCOR certification for bats is the 2011 season and there will be no grandfather clause for bats certified under BESR. This change will not affect the current testing methodology but will eliminate the small discrepancy between bat lengths and more directly align the measurement of bat performance with the ball's exit speed. In order to achieve certification, the BBCOR will need to be 0.50 or less with a maximum length-to-weight difference (drop) of -3 and a bat diameter less than 2 5/8 inches.
Note: I have not seen it yet, but I would expect the NFHS to follow this mandate for high school baseball.
ASA Bat Performance Measurements
Also in 2000, the ASA established a bat performance regulation by directly measuring the batted-ball speed (BBS) of a slow pitch metal bat called the "ball-in, ball-out" measurement. The ASA reference measurement is based on a ball being pitched at 10 mph and a bat swing speed of 60 mph hitting the bat at the maximum batted-ball speed performance point, called center-of-percussion (COP). From this, the ASA determined that the maximum ball speed of a hit ball should be 85 mph or lower for the bat to become certified.
In 2004, after a field study conducted at the National Tournament in Montgomery, AL, the ASA adjusted their reference metric to more closely resemble the pitched ball and bat swinging speed of the game. While maintaining their current measurement method, the ball pitched speed was increased to 25 mph and the bat swinging speed was increased to 85 mph. Additionally, the field study showed that the maximum batted-ball speed performance point (formerly labeled COP) actually varied from bat model to bat model and bat maker to bat maker. This required that the bat being certified initially undergo testing to determine the maximum performance location first and then the pitched ball is to be targeted at that location. Based on these updated assumptions, the ASA determine that for a bat to be certified the maximum batted-ball speed should be 98 mph or less. Even though the numerical value of BBS with the 2004 standard is higher than the 2000 standard, it is actually a much stricter standard.
In 2008, the ASA updated their testing procedure to account for the fact that composite bats reach their peak performance after they are broken in by hundreds of bat-ball collisions and the composite fibers and resins loosen up to provide a larger trampoline effect. Starting with the 2009 composite bat models, the ASA will simulate up to 1000 swings before a composite bat will be tested against the ASA 2004 certification tests. Composite bat models certified under the old testing procedure will be grandfathered and can be used until they are no longer safe for use. This testing procedure change will not affect 2009 metal bats since their performance decreases with use.
USSSA and Little League Bat Performance Measurements
During the early 1990s, Little League reached an agreement with bat manufactures to limit the performance on non-wood bats to the expected performance level of the best wood bats at that time. They created a metric called the bat performance factor (BPF) which must be less than 1.15 in order for a bat to be certified for Little League play. The BPF metric is equal to the ratio of the "bounciness" of a baseball off a bat (called bat-ball coefficient of restitution or BBCOR) to a baseball off a hard wall (called bat-wall coefficient of restitution or BWCOR). The BPF metric does not include any assumptions pertaining to bat speed so it is more effective at testing shorter youth bats since the effective bat speed can vary by bat length and the location of the bats center of mass (ie., end loaded or handle loaded). The USSSA uses the same bat performance metric but allows for a slightly higher BPF = 1.20 or less.
For the 2009 season, Little League has made two changes: 1) all bat must have a BPF label printed on them, and 2) all "big barrel" used in junior league (age 13 years and older) can have a maximum barrel diameter of 2 3/4". Most bat manufactures have been complying with these rules for years so little change is expected.
Over the past 10 years, the governing bodies of baseball and softball have been regulating the performance of non-wood bats (usually made of aluminum) to perform statistically the same, in terms of how fast the ball exits the bat, as the best wood bats. The organizations have chosen the best wood bats to encourage and allow the use of wood bats within their organization's teams. Although their testing methodology and batted-ball performance metric may differ, they all include some type of measurement of how "springy" a bat-ball collision is based on a preset bat-ball speed collision. Their two-fold object is to maintain the defense and offense balance and the safety of the game players.
Here is a recap of the current testing methodology.
Regulate Batted-Ball Speed Directly: ASA is based on slow-pitch reference point of a pitch traveling at 25 mph and the bat swing speed of 85 mph after determining the maximum performance location on the bat barrel. For a ball to be certified, the BBS must be 98 mph or less. Due to the testing methodology of measuring the "ball-in, ball-out" speed with a stationary bat, the "whip action" from the flexible bat handle bats, the testing of shorter youth style bats, and the higher pitching velocity normal in the fastpitch variant of softball can produce results that are either higher or lower by a few mph.
Regulate Batted-Ball Speed Indirectly: NCAA and NFHS BESR, maximum barrel size of 2 5/8" and drop standard of minus 3 limit the collision efficiency and bat swing speed. They will certify a -3 bat that has a BESR equal to or less than 0.728. For the 2011 season, the BESR certification will no longer be allowed and the new BBCOR certification will be required.
Regulate the Collision Efficiency Indirectly: USSSA softball and Little League regulate the BPF, which is a metric of the efficiency that a bat can make the ball bounce versus a ball bouncing off a wall. Due to the testing methodology of "ball-in, bat-out" with a stationary bat used by Little League, the measured results have greater variability than the "ball-in, ball-out" testing methodology used by ASA. This is due to the low bat elasticity and the ball-bat collision producing up and down vibrations that dissipate energy.
Over the past ten years, organizations have changed their bat performance metric and assumed bat-ball collision speeds to more realistically represent game situations. It can be anticipated that during the next ten years more changes will occur as organization continue to work to maintain the offense-defense balance and the safety of the game.
Photo Credit: Billie / PartsnPieces
 Bat drop is the difference between the bat length and the bat weight. For example, a bat with a minus 3 drop (-3) could be 33" long and weigh 31 oz.
Professor Alan M. Nathan - Department of Physics, University of Illinois at Urbana-Champaign - Physics of Baseball
Professor Daniel A. Russell - Department of Applied Physics, Kettering University - Physics and Acoustics of Baseball and Softball Bats
Baseball Research Center - the bat and ball testing facility at University of Massachusetts/Lowell that certifies NCAA baseball bats for the NCAA
Sports Sciences Laboratory - the bat and ball testing facility at Washington State University that certifies softball bats for the Amateur Softball Association (ASA)
ASTM International - organization that standardizes bat performance testing
The Evolution of the Baseball Bat by former MLB pitcher Bernie Mussill
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