Identifying altered softball bats and their effect on performance

Transcription

1 DOI: /jst.58 Research Article Identifying altered softball bats and their effect on performance Lloyd V. Smith and Curtis M. Cruz Washington State University, USA Identifying altered softball bats Progress in bat design has increased the disparity between the potential and allowed performance of baseball and softball bats. This disparity has motivated some players, particularly at the amateur softball level, to have their bats altered to increase performance. This study reviews the mechanisms contributing to bat performance and how alteration techniques take advantage of these mechanisms. The study involves 28 bats that were altered by adding weight, reducing the wall thickness, artificially breaking in the barrel, or painting a non-certified bat to appear as an approved bat. The average hitting performance of the altered bats increased by nearly 5 per cent. Bat performance was observed to improve with decreasing natural frequency and radial stiffness of the barrel. However, variations between bat performance and these measures of barrel stiffness appear to make them unreliable indicators of alteration. Three comparable bats were hit in a batting cage to measure performance change from normal use. The average performance change from the batting cage increased only 1.6 per cent. & 2008 John Wiley and Sons Asia Pte Ltd Keywords:. softball bat. performance. altering. test method 1. INTRODUCTION Improvements in the technology used in the design and fabrication of non-wood baseball and softball bats have significantly improved their batted ball speeds. One study showed that softball bat performance increased 17 per cent over a 30-year period [1]. In an effort to maintain the traditional balance between offense and defense, most governing associations have placed limits on bat performance. Improvements in bat design and regulation methods have increased the difference between the potential and allowed bat performance. The increasing disparity in bat performance has motivated some players to have their bats altered to increase their performance and gain a competitive advantage. The problem is most acute in adult amateur softball, where equipment is purchased and controlled by the individual players. *Washington State University, 201 Sloan, Spokane Street, Pullman, WA USA lvsmith@wsu.edu In the following we seek to determine the prevalence of bat alterations; to understand the common bat altering methods; to quantify the effect of normal use and alterations on bat performance; and to evaluate test methods to identify altered bats. 2. BAT PERFORMANCE A number of studies of bat performance can be found in the literature. Nobel and Eck [2] considered the effect of weight on performance. They found that weight added to the knob or distal end of the bat increased performance. This observation was supported by House [3] and Adair [4] who argued that the advantage of added mass was greater than the disadvantage of a reduced swing speed. Work by Bahill [5,6] also support increased weight, although he found a limit and suggested a methodology for optimizing the bat weight for the individual player. The effect of bat weight on swing speed has also been studied to quantify its effect on performance. Nichols et al. [7] compared wood and aluminum bats and observed that swing 196 & 2008 John Wiley and Sons Pte Ltd Sports Technol. 2008, 1, No. 4 5,

2 Identifying altered softball bats speed depends on the bat s mass moment of inertia (MOI). Fleisig et al. [8] compared baseball and softball players and found the swing speed increased linearly with decreasing bat MOI. Smith et al. [9,10] considered the effect of mass and MOI separately with slow and fast pitch softball players. Swing speed was observed to increase non linearly with decreasing MOI. Bats with constant MOI and varying mass had a nearly constant swing speed. A significant contribution to bat performance involves the trampoline effect associated with bats of hollow construction. Narou and Sato [11] measured the coefficient of restitution (COR) between a ball and tubes of differing flexural and radial stiffness. They found that the COR increased with increasing flexural stiffness and decreasing radial stiffness. This observation was supported by Brooks et al. [12] who studied ball impacts against flat plates. They found that plate compliance can be tailored to achieve an optimal COR. Nishikawa [13] extended the idea of an optimal COR to bat ball collisions. He found the contribution of the bat s flexural stiffness was less than that of the radial stiffness of the barrel. The trampoline effect was studied by Nathan et al. [14] who observed that most of the energy loss in a bat ball collision was from the ball deformation. They showed that compliant barrels not only reduce energy loss from ball deformation, but store the otherwise lost energy in the elastic barrel, returning it to the rebound speed of the ball. 3. BAT ALTERATIONS The options available for producing an altered bat are seemingly numerous. Limited resources of the average amateur player have reduced the methods to painting, shaving, and accelerated break-in processes [9]. Painting involves altering a commercially-available bat that is not certified for a desired league of play. The bat is painted so that it resembles a bat that is allowed under league rules. While this method is termed painting, it typically involves adhesive decals and a clear coat. The decals are usually computer generated. The refinished bats differ in quality, but in general, require skill and careful examination to distinguish them from factory-decorated bats. The non-factory decals are often slightly thicker than factory decals. A clear coat is required to provide an acceptable surface finish and can result in a bat diameter exceeding the allowable limit. However, most painted bats still pass the common ring test used to identify dented metal bats. Some painted bats are sanded before the decal is applied to disguise the diameter increase from the decal and clear coat. Most manufacturers track their bats with unique serial numbers, which allows painted bats to be identified at the factory, but provides little assistance for the umpire in the field. Painting bats violate the copyright of both the bat manufacturer and the associations whose marks are reproduced. A number of bat painters have been successfully challenged using copyright law [15]. Adding or removing weight from the bat changes its swing weight or MOI. Even though bat performance has been shown to increase with MOI, some players prefer a lighter bat [16]. Weight alterations are usually done in the handle at the knob or at the distal end in the end-cap. Although many bat models are marketed in different weights, after-market changes in weight are not allowed by governing associations [17]. As will be discussed later, the effect of MOI on performance is relatively small. A high-performance bat could nevertheless exceed the allowed performance limit with appropriately placed weights. Since bats are labeled at the factory by their weight, bats altered in this way would also be mislabeled in the field. Shaving involves reducing the wall thickness in the barrel region of the bat. Shaving is usually accomplished by removing the end-cap and placing the bat in a lathe. A boring bar is used to remove material from the inside of the bat. The end-cap is then glued back in place. In some cases, the concentric sleeves of multiwall bats are separated and given a similar thickness reducing treatment. Reducing the barrel wall thickness increases the trampoline effect but also reduces bat durability. The latter characteristic places the batter at risk of being caught. In the event the barrel breaks open in play, the machining marks will be as apparent as a wood bat that has been corked. The material used to form composite bats involves a fiberreinforced polymer. The fibers are selected, placed, and arranged according to the bat s design. The stiffness and strength of the polymer and fiber differ significantly. When placed under load, this difference in strength can cause cracks to form. This type of damage often occurs on the interior of the bat and is not readily visible or detected. Crack growth and coalescence softens a composite bat increasing the trampoline effect. Bat manufacturers use toughened polymers with varied levels of success to minimize crack growth. Many players have found that the performance of composite bats increases with use. Accelerated break-in (ABI) processes, which are unique to composite bats, are intended to accelerate and extend the region of crack growth beyond what normally occurs in play. ABI processes involve deforming the barrel to levels exceeding that which occurs in play. The simplest ABI method replaces the ball impact with a harder object, such as a stiff post or a mallet. Damage growth is accelerated because the impact forces are generally larger and act over a smaller area than would occur with a ball. Another ABI method deforms the barrel in a quasistatic fashion using a vice or hydraulic press with machined platens that conform to the shape of the barrel. Rolling is an ABI technique where the platens are replaced by rollers aligned normal to the length of the bat. The barrel portion of the bat is passed through the rollers while it is compressed. The process typically involves multiple passes where the bat is rotated circumferentially to distribute the damage. ABI treatments usually damage the barrel uniformly along its length, while composite bats used only in play generally have damage concentrated near the sweet spot. The performance gain of many altered bats is less than 2 per cent. A competitive advantage can still be gained; however. Even a small increase in bat performance can turn a catchable fly ball into a home run. It is for this reason that governing associations are interested in ensuring that bat performance does not exceed specified limits, even by relatively small amounts. Sports Technol. 2008, 1, No. 4 5, & 2008 John Wiley and Sons Pte Ltd 197

3 Research Article 4. TEST METHODS There is interest in developing tests to identify altered bats during play. Two methods will be described that are portable and have been considered for field comparisons. Both methods consider the barrel stiffness. A laboratory device that is currently used to measure bat performance will also be described and used to evaluate the portable methods. A Modal analysis is routinely used to describe the vibrational response of complex structures. A frequency response function (FRF) is obtained from the ratio of the fast Fourier transform of a response signal and a stimulus input [18]. The input was provided at various locations along the length of the bat through a hand-held instrumented impact hammer (PCB model 086C02; PCB Piezotronics Division, Depew, New York, USA). The response was obtained from a small accelerometer placed at a fixed location on the bat (PCB model 352C22; PCB Piezotronics Division, USA). Assembling the FRF as a function of impact location provided a measure of the frequency and shape of the numerous vibrational modes. Flexural frequencies were found by placing the accelerometer in the taper region of the bat. Hoop frequencies were found by placing the accelerometer in the barrel region, 901 from the hammer impacts response. The FRF of a representative flexural and hoop-dominated responses are presented in Figure 1. While remnants of all modes are apparent in both curves, the hoop frequencies are clearly more dominant in the hoop curve. The frequency of the hoop vibration will increase with increasing barrel stiffness and decrease with increasing mass. The dependence of the hoop frequency on barrel stiffness gives it potential as a measure of relative bat performance. It should be noted, however, that a modal analysis is a measure of the free vibrational response, while the response during impact is from a force input. Thus understanding how the different free vibrational modes and frequencies contribute to bat performance is not trivial. The lowest vibrational mode will have the largest magnitude and is often considered the most significant. In the following, the frequency of this mode will be used for comparison. An apparatus that can be used to measure the barrel stiffness quasistatically is depicted in Figure 2. The test procedure involved a preload of 67 N, after which the cylinder was displaced 0.51 mm and the load was recorded. The cylinder was then displaced at an additional 1.3 mm where the load was again recorded. The barrel stiffness was found from the ratio of the difference in the load and displacement before and after the 1.3 mm displacement. The method is commonly used in industry and has been proposed as an American Society for Testing and Materials (ASTM) standard. It has also been used informally by a number of associations to compare bats, and in some cases, identify altered bats in local tournaments. While both the quasistatic test and hoop frequency are sensitive to barrel stiffness, the former is insensitive to inertial effects. A laboratory test used to measure bat performance directly is depicted in Figure 3 and described in ASTM F2219 [19]. Here balls were fired at an initially stationary bat at 49.2 m/s. The ball speed was found using light screens as depicted in Figure 3. The ball speed was recorded before and after impact, while the bat was allowed to recoil after impact about a fixed point, 152 mm from the knob. The recoiling bat speed was found from a momentum balance of the bat ball impact. The incident ball speed was unique to slow-pitch softball, obtained from the sum of the average game pitch and bat speed, 11 and 38 m/s, respectively. The collision efficiency, e a, was obtained from the ratio of the rebound and incident ball speeds. Bat performance was compared on an ideal hit-ball speed scale [20], found using the following equation: v h ¼ e a v p þð1 þ e a Þv b in which v h is the ideal hit-ball speed, v p is the reference pitch speed (11 m/s), and v b is the bat speed. By using bat and ball speeds representative of play in Equation 1, v h describes the Load cell Cylindrical impact surface L. V. Smith and C. M. Cruz ð1þ Figure 2. Schematic of the test apparatus used to measure the barrel stiffness. Figure 1. FRF of representative flexural and hoop response of a metal softball bat. solid line, flexural; dashed line, hoop. Figure 3. Schematic of the test apparatus used to measure bat performance & 2008 John Wiley and Sons Pte Ltd Sports Technol. 2008, 1, No. 4 5,

4 Identifying altered softball bats Table 1. Quantity and types of bats used for each method of alteration v r v p v b speed a ball would be hit in play by the bat under consideration. Bat and ball speeds were measured in a field study involving 14 adult male, slow-pitch softball players and bats of varying mass and MOI [16]. Markers placed on the bats were tracked using high-speed video from which rotational speed and the instantaneous center of rotation of the bat just prior to impact were found. Because the bat was rotating, its speed was found to depend on the impact location with the ball and its MOI according to the following equation: v b ¼ v r p c 1 2 ð2þ in which p is the distance from the impact location to the average instantaneous center of rotation found from the field study (216 mm). The reference bat speed, v r, was 38 m/s at 559 mm from the pivot point taken from the average bat speed from all players and all bats in the field study. The distance from the instantaneous center of rotation and reference-bat speed location, c, was 775 mm. The coordinates and bat speeds are depicted in Figure 4. The reference bat MOI, I r, was 165 g m 2, while I b represented the test bat MOI (relative to the pivot point). The reference bat MOI and exponent of one half are empirical parameters used to fit the field study results. 5. RESULTS Laboratory pivot point p c The 28 softball bats that were compared in the study are summarized in Table 1. While the primary aim of the study was on multiwall composite bats, wood, single, and multiwall aluminum bats were also examined. Using Equation 1, the performances of the bats in their initial and altered configurations is compared in Figure 5. The average performance of the weighted bats increased 2.9 per cent. The weight of these bats was increased by approximately 10 per cent and placed at the distal end to increase the MOI by 20 per cent. Most players use bats within 5 per cent of their preferred bat weight. Using bats outside their preferred weight range is viewed to negatively impact the player s swing Ir I b Field center of rotation Figure 4. Reference bat speeds and locations for determining bat performance. c, reference bat speed location; p, distance from the impact location to the average instantaneous center of rotation found from the field study; v b, bat speed; v p, reference pitch; v r, reference bat speed. Single-wall Multi-wall Multi-wall Alteration Wood Aluminum Aluminum Composite Weighted Shaved 6 4 Batting cage 3 Accelerated break-in 8 Painted 3 Altered (m/s) New (m/s) Figure 5. Comparison of bat performance of new and altered softball bats. solid circle, weighted; open circle, shaved; solid triangle, ABI; open triangle, painted. mechanics. Thus the 10 per cent increase in weight represents an extreme limit of performance change achievable through added weight. The average performance of the bats given an accelerated break-in process increased on average by 4.3 per cent. As expected, the variation in the performance of these bats was larger than the weighted bats, ranging from 0.7 to 8.5 per cent. The variation could be due to the severity of the accelerated break-in process. The bats achieving the largest performance increase, for instance, typically exhibited surface cracks in the barrel after the accelerated break-in treatment. The average performance increase of the shaved bats was 4.2 per cent. Shaving had a larger effect for the composite bats, increasing their average performance by 6.6 per cent, compared to the 2.6 per cent average increase for aluminum. The largest change of 7.9 per cent was with a composite bat, while the smallest change of 1.7 per cent occurred with an aluminum bat. Shaving should lower the durability of both metal and composite bats. Composite bats can be shaved more aggressively than metal. However, damage in composite bats is more difficult to detect than dents in metal bats. The performance of the painted bats was on average 5 per cent higher than the bats they were painted to represent. The effect of painting is obviously a function of the performance of non-certified bats and the performance of target certified bats. Three multiwall composite bats were hit in a slow-pitch batting cage by recreational-league batters. The barrel stiffness of each bat was measured every 400 impacts, while the laboratory performance was measured every 2000 hits. The ` Sports Technol. 2008, 1, No. 4 5, & 2008 John Wiley and Sons Pte Ltd 199

5 Research Article L. V. Smith and C. M. Cruz relative change in bat performance and barrel stiffness is presented in Figure 6 for a representative bat. The average performance of the three bats increased 1.6 per cent, while their average barrel stiffness decreased 16 per cent. The batting-cage bats experienced a decrease in barrel stiffness of approximately 10 per cent during the first 500 impacts with no appreciable change in performance. This result could be a characteristic of the multiwall construction. Small changes in friction and clearance between the bats walls during the first few impacts may have a larger effect on the barrel-stiffness test than on hit-ball performance. The bat ball impact, for instance, involves larger barrel displacements and engages the interior walls of the bat a larger portion of the impact than occurs in the barrel stiffness test. The batting-cage results show that the performance of composite bats can increase with normal use without alteration. Bat performance is shown as a function of hoop frequency in Figure 7. If a bat is modified to lower the barrel stiffness (and increase V h ) the hoop natural frequency will also be lower. This result was observed on an average sense, where the hoop frequency of the altered bats decreased 5.7 per cent compared to the average performance increase of 4 per cent. The variation in hoop frequency with performance was sufficiently large; however, it did not appear suitable in identifying altered bats. Bat performance is shown as a function of barrel stiffness in Figure 8. As with hoop frequency, one would expect performance to increase with decreasing barrel stiffness. The average barrel stiffness of the altered bats was 10.4 per cent lower than when the bats were new. A stronger trend of performance with barrel stiffness was apparent than was observed with hoop frequency. The bats hit in the batting cage showed a similar change in barrel stiffness as the altered bats. Thus bats identified as having low barrel stiffness require further scrutiny to distinguish altering treatments from normal use. Bat policing can be considered on a relative (rather than absolute) basis. Measured properties of the bat (such as weight, MOI, barrel stiffness, and frequency) may be viewed as Figure 6. Comparison of the bat performance (v h ) and barrel stiffness (BS) of a representative bat hit in a batting cage. Open square, v h or hit-ball speed; open diamond, barrel stiffness. Figure 7. Comparison of new and altered softball bat performance as a function of the first mode hoop frequency. v h, hit-ball speed; open circle, new; solid circle, altered. Figure 8. Comparison of new and altered bat performance as a function of the barrel stiffness. v h, hit-ball speed; open circle, new; solid circle, altered. a type of bat fingerprint. Changes from these nominal properties could be an indication of alteration. This approach would require the creation of a database that would need to evolve with new bat models. It would also have to account for processing variation and normal bat wear. There is some resistance to this approach, as manufactures regard processing variation as proprietary, and normal bat wear is difficult to quantify. Changes from normal wear can also be large, as evidenced by the barrel stiffness in Figure 6. The change in bat performance is plotted as a function of the change in barrel stiffness and hoop frequency for each of the altered bats in Figure 9. The correlation of barrel stiffness and hoop frequency with bat performance was found to improve. However, variation still posed challenges. The performance of some bats increased 4 per cent with little change in barrel stiffness or hoop frequency. In some cases, the alteration produced an increase in hoop frequency (where a decrease was expected). This is in contrast with the laboratory-measured bat performance, which increased for all the altered bats & 2008 John Wiley and Sons Pte Ltd Sports Technol. 2008, 1, No. 4 5,

6 Identifying altered softball bats 3. House G. Chapter 10: Baseball and softball bats. In: Kreighbaum E, Smith M, eds. Sports and Fitness Equipment Design. Human Kinetics Publishers: Champaign, IL, 1995; Adair R. The Physics of Baseball, 3rd edn. Perennial: New York, Bahill T, Karnavas W. The ideal baseball bat. New Scientist 1991; 130 (1763): Bahill T. The Ideal Moment of Inertia for a Baseball or Softball Bat. IEEE Transactions on Systems, Man, and Cybernetics-Part A: Systems and Humans 2004; 34(2): Nicholls R, Elliott B, Miller K, Koh M. Bat kinematics in baseball: implications for ball exit velocity and player safety. Journal of Applied Biomechanics 2003; 19: Fleisig G, Zheng N, Stodden D, Andrews J. Relationship between bat mass properties and bat velocity. Sports Engineering 2002; 5: Smith L, Cruz CM, Nathan AM, Russell DA. How bat modifications can affect their response. In: Fuss F, Subic A, Ujihashi S, eds. The Impact of Technology on Sport, Australasian Sports Technology Alliance Pty Ltd: Melbourne, 2005; Figure 9. Change in bat performance as a function of change in barrel stiffness and hoop frequency. v h, hit-ball speed; open circle, BS or barrel stiffness; open triangle, frequency. 6. CONCLUSION The study has considered the effect of bat alterations on bat performance and methods of identifying altered bats. It was observed that altering methods involving added weight, thinning the barrel wall, painting, and accelerated break-in processes increased performance on average 3 to 5 per cent. The average performance of three composite bats from normal use in a batting cage was also shown to increase 1.6 per cent. For the bats considered here, the average performance of the accelerated break-in process was more than 2.5 times that of the bats used in a batting cage. Barrel stiffness and a modal analysis were compared with laboratory bat performance for their potential to identify altered bats in the field. The natural frequency and stiffness of the barrel were not able to reliably identify the altered bats considered here. Preventing the use of altered bats in play has been shown to be a challenging task and may require increased cooperation between manufacturers and regulating associations. REFERENCES 10. Smith L. Oklahoma fast-pitch softball field study. Presentation to the NCAA Softball Rules Committee. November 18, Naruo T, Sato F. Performance of baseball bats. Proceedings of 5th Japan International SAMPE Symposium. SAMPE, Tokyo, Japan, 1997; Brooks R, Knowles S, Mather J. Design and Construction of a High Performance Composite Cricket Bat. In Scott M, ed. Proceedings of ICCM-11. Gold Coast, Queensland, Australia. Cambridge: Woodhead Publishing, Nishiwaki T. Designing of CFRP baseball bats. SAMPE Journal 2002; 38(2): Nathan A, Russell D, Smith L. The physics of the trampoline effect in baseball and softball bats. In Hubbard M, Mehta R, Pallis J, eds. The Engineering of Sport 5, Vol. 2, International Sports Engineering Association: Sheffield, UK, Amateur Softball Association. ASA Court Judgment. asasoftball.com/communications/getstory.asp?nid [14 October 2008]. 16. Smith L, Broker J, Nathan A. A study of softball player swing speed. In: Subic A, Trivailo P, Alam F, eds. Sports Dynamics Discovery and Application. RMIT: Melbourne, 1993; Amateur Softball Association Official Rules of Softball Team Edition. Amateur Softball Association, USA: Oklahoma City, OK, Avitabile P. Experimental modal analysis a simple non-mathematical presentation. Sound and Vibration, 2001; 35(1): ASTM International. Standard Test Methods for Measuring High-Speed Bat Performance. In Annual Book of ASTM Standards, F2219, ASTM: West Conshohocken, PA, 2075, 15(07). 20. Nathan AM. Characterizing the performance of baseball bats. American Journal of Physics 2003; 71(2): Smith LV. Why ASTM F2219? SGMA Annual Meeting, Dallas, TX. October 2, [14 October 2008] 2. Noble L, Eck J. Effects of selected softball bat loading strategies on impact reaction impulse. Medicine and Science in Sports and Exercise 1986; 18(1): Received 20 March 2008 Accepted 5 August 2008 Published online 9 December 2008 Sports Technol. 2008, 1, No. 4 5, & 2008 John Wiley and Sons Pte Ltd 201