Sport Science

 Baseball/Softball bats

When aluminum were introduced over 40 years ago, there was a notable increase in bat performance.  This came to a head in 1993 when titanium bats and double-walled aluminum bats upset the balance between offense and defense in softball.  The softball associations attempted to ban these new bats, but since an objective measure of performance did not exist, there were certainly allowed bats that out-performed the disallowed bats.  This lead to widespread confusion, recriminations, and lawsuits.  In late 1993, the Sporting Goods Manufacturers Association (SGMA) therefore initiated a collaboration among the four major metal bat manufacturers (Louisville, Easton, Worth, and AMMCO) to arrive at a suitable measure of performance. Louisville, Easton, and Worth each proposed performance tests based on their existing testing procedures, whereas AMMCO requested that I develop a suitable test.  After about six months of lab and field testing, the manufacturers involved agreed on the technology that I developed, which uses the bat performance factor (BPF) as the measure of performance, since it was the only proposal that correlated well with the field test data.  BPF values are measured by impacting a bat attached to a rotatable holder with balls projected from a specially designed air cannon.  (A photo of our setup is shown in Fig. 3.1.)  Measurements of the incident ball speed and the resultant bat speed determine the coefficient of restitution (BBCOR) between the bat and ball, and the ratio of this BBCOR and the coefficient of restitution of the test ball (BLC) is the BPF of the bat.  This ratio is approximately independent of the test ball for a limited range of BLC values.  The complete testing protocol is described in the ASTM standard F1890-11.  I measured the BPFs of nearly 100 softball bats in late 1994, and determined the BPFs of the spectrum of available bats.  This information was presented to the softball associations in late 1994.  The test was eventually adopted by the major softball associations (ASA, USSSA, etc.) and by the NCAA for college baseball, and it has become an ASTM standard.  We have patented the testing procedure and have licensed the patent to SGMA, ASA, USABB, and other organizations. 

The BPF bat performance protocol has evolved and improved since SSI introduced it in 1993.  In the original protocol, bats were impacted at the center of percussion COP (relative to the rotation point) only. When high-performing composite (mainly carbon-fiber) bats were subsequently introduced, it was found that these standards had to be generalized in order to account for the fact that the new bats could have sweet spots at locations that are different from the COP. This possibility was taken into account by impacting a tested bat at various locations in order to determine the sweet spot location.  It was also soon realized, however, that the performance of composite bats improved with usage.  Each ball impact experienced by these bats tended to delaminate or otherwise damage the carbon fibers, causing the bat to become more elastic and more powerful.  These bats could therefore be found compliant with the existing standard when they were tested before being used in the field, but could become non-compliant (too powerful) if tested after sufficient usage.

To address this issue, an “automated break in” (ABI) procedure was proposed in which a bat is broken-in by rolling it between cylinders compressed into it a specified distance. In these protocols, performance measurements on a tested bat are alternated with the “breaking-in” procedure using the rolling device. This procedure is continued until either the bat’s performance exceeds a specified limit or the bat exhibits visible damage.  (A performance measurement is made after a bat’s elasticity has decreased by a specified amount.)  If the damage occurs first, the bat is considered compliant, but if the performance limit is exceeded first, the bat is not considered compliant.  The hope is that similar damage on a compliant bat caused by impacts during a ball game could then be observed, and the bat could be then be removed from play before it became non-compliant.  

 There are a number of serious problems with this rolling ABI protocol.  It is complicated and time-consuming to use.  It is not precise, accurate, controllable, or repeatable.  It adds an element of influence by the tester to the otherwise accurate performance measurements.  It requires the tester to determine the degree of compression and to determine visually if and when damage occurs anywhere on the bat barrel.  The result of a test can depend on details of the rolling procedure not precisely controllable, such as the exact compression distance, the rolling speed, and the bat’s alignment during the rolling.  The rolling cannot be accurately performed on bats with tapered barrels.  Another serious problem is that a bat could show damage when rolled to a sufficient distance, but not when impacted by balls.  Such a bat could become non-compliant (too powerful) because of impacts with balls, but would not be removed from play because it would not display visible damage arising from these impacts. Also, the rolling itself is a broad and harsh procedure that lacks adequate controls.  The procedure is inefficient because it softens the bat everywhere and not in the way that ball impacts soften it.  Also, the elasticity measurements are executed with two opposing cylindrical sections instead of with a spherical-like section at a single area, which is the way a struck ball experiences the elasticity of a bat.  

We have invented an alternative ABI procedure that overcomes all of the above difficulties.  The first step is to determine the point of maximum performance on the bat.  All subsequent performance measurements and compressions are made it this point.  The elasticity the bat at each step is determined with the bat held in a cradle so that the bat is compressed on only one side, as it is when the bat is impacted by a ball.  The compressing solid is a spherical section, with a radius equal to that of a softball or a baseball.  The compression is therefore similar to a compression arising from an impact with a ball.  This same compression mechanism is used to execute the break-in of the bat.  These break-in compressions, to specific increasing distances, are therefore also similar to compressions arising from an impact with a ball.  This procedure is thus very efficient, because the same device is used for both elasticity measurements and bat break-ins, and precise, because, unlike for rolling devices, the compression distances can be set very accurately.  And since the bat is compressed in the ABI procedure the same way that it is compressed during hits in ball games, a bat that shows damage in the ABI procedure before it becomes too powerful will also show damage when impacted by balls before it becomes too powerful.  A compression element and cradle, along with the required distance and force gauges, are illustrated in Figure 3.2.

The accelerated break-in procedure that is described above subjects composite bats to realistic compressions in order to simulate the performance increasing break-in effects arising from the use of the bat in ball games.  These compressions soften composite bats and generally improve their performance.  Compliant bats are required to show visible damage (cracks, flaking, delaminations, etc.) before they exceed the designated BPF limit (1.20 for softball, 1.15 for youth baseball).  (The BPF for softball incorporates a 0.05 subtraction, so that the real average BPF limit is 1.25.)  Bats that show visible damage after compression can be safely used in games because performance increases arising from ball impacts will show similar damage.  Officials would be instructed to remove a bat from play if it showed such damage.  A typical BPF report is shown in Fig. 3.3.