Patent Publication Number: US-11660512-B2

Title: Double-barrel ball bats

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/268,413, filed Feb. 5, 2019, which is a continuation of U.S. application Ser. No. 15/894,365, filed Feb. 12, 2018 and issued as U.S. Pat. No. 10,220,277, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Ball bats, particularly composite ball bats, have been designed with various stiffness properties to meet the preferences of various players. Many players, for example, prefer the feel and performance of ball bats having barrels that exhibit high compliance (for example, high radial deflection) and low stiffness. There are challenges, however, in making an effective, durable ball bat having these properties. In addition, there are challenges in making a ball bat with high compliance that can meet league or association rules, such as rules associated with the Bat-Ball Coefficient of Restitution (“BBCOR”), the Batted-Ball Speed (“BBS”) value, or other rules associated with collision efficiency of a bat and a ball. 
     SUMMARY 
     Representative embodiments of the present technology include a method for making a ball bat. The method may include forming a bat frame with a handle and an inner barrel structure. The method may include providing two or more spacer elements extending radially outwardly from the inner barrel structure. The method may further include forming a barrel shell with one or more layers of composite laminate material. Forming the barrel shell may include forming a main barrel and a tapered section. An inner diameter in the tapered section may be equal to an outer diameter of a first one of the spacer elements. The method may further include mechanically locking the barrel shell to the bat frame by passing the handle through the barrel shell and moving the barrel shell toward the inner barrel structure until the barrel shell contacts the first one of the spacer elements such that a gap is maintained between an outer diameter of the inner barrel structure and the barrel shell. 
     Another method for making a ball bat may include providing a bat frame, the bat frame having a handle and an inner barrel structure, and positioning a release material on the inner barrel structure. The method may further include forming a barrel shell around the release material with one or more layers of composite laminate material, wherein forming the barrel shell includes forming the barrel shell to coextend with the inner barrel structure, and curing the one or more layers of composite laminate material of the barrel shell. The method may further include removing the barrel shell from the bat frame, removing the release material from the bat frame, providing a first spacer element to the bat frame, the first spacer element being positioned in a tapered region of the inner barrel structure, providing a second spacer element to the bat frame, the second spacer element being positioned adjacent to a distal end of the inner barrel structure, and positioning the barrel shell onto the inner barrel structure by first sliding the barrel shell over the handle and then onto the inner barrel structure. The first spacer element and the second spacer element maintain a gap between the barrel shell and the inner barrel structure. Positioning the barrel shell onto the inner barrel structure may include engaging the first spacer element with a tapered section of the barrel shell. In some embodiments, the gap may vary along a length of the inner barrel structure, for example, by varying an outer diameter of the inner barrel structure between the spacer elements. 
     Another representative embodiment of the present technology may include a ball bat having a frame with a handle and an inner barrel structure, the inner barrel structure including a tapered region joining the handle and the inner barrel structure. The ball bat may include a barrel shell with a proximal end and a distal end positioned opposite the proximal end, and a tapered section positioned adjacent to the proximal end. The barrel shell may include one or more layers of composite laminate material. The barrel shell may be positioned around the inner barrel structure and spaced apart from the inner barrel structure along at least a portion of a length of the barrel shell to form a gap between the barrel shell and the inner barrel structure. A mechanical locking feature may be provided and configured to retain or secure the barrel shell to the frame. The gap may generally have a uniform width along its length between spacer elements, or it may have a varying width. For example, the gap width may be narrower at a center of percussion of the ball bat. 
     Other features and advantages will appear hereinafter. The features described above can be used separately or together, or in various combinations of one or more of them. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, wherein the same reference number indicates the same element throughout the views: 
         FIG.  1    illustrates a perspective view of a ball bat according to an embodiment of the present technology. 
         FIG.  2    illustrates a perspective exploded view of the ball bat shown in  FIG.  1   . 
         FIG.  3 A  illustrates a cross-sectional view of the ball bat shown in  FIGS.  1  and  2    in an assembled configuration. 
         FIGS.  3 B,  3 C, and  3 D  each illustrate a portion of the ball bat shown in  FIG.  3 A . 
         FIG.  4 A  illustrates a cross-sectional view of a ball bat according to another embodiment of the present technology. 
         FIG.  4 B  illustrates a portion of the ball bat shown in  FIG.  4 A . 
         FIG.  5    is a flow chart illustrating a method of making ball bats according to an embodiment of the present technology. 
         FIGS.  6 A- 6 E  illustrate stages of assembly of a ball bat according to an embodiment of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     The present technology is directed to double-barrel ball bats, and associated systems and methods. Various embodiments of the technology will now be described. The following description provides specific details for a thorough understanding and enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions, such as those common to ball bats and composite materials, may not be shown or described in detail to avoid unnecessarily obscuring the relevant description of the various embodiments. Accordingly, embodiments of the present technology may include additional elements or exclude some of the elements described below with reference to  FIGS.  1 - 6 E , which illustrate examples of the technology. 
     The terminology used in this description is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this detailed description section. 
     Where the context permits, singular or plural terms may also include the plural or singular term, respectively. Moreover, unless the word “or” is expressly limited to mean only a single item exclusive from the other items in a list of two or more items, then the use of “or” in such a list is to be interpreted as including (a) any single item in the list, (b) all of the items in the list, or (c) any combination of items in the list. Further, unless otherwise specified, terms such as “attached” or “connected” are intended to include integral connections, as well as connections between physically separate components. 
     Specific details of several embodiments of the present technology are described herein with reference to ball bats. Embodiments of the present technology can be used in baseball, softball, cricket, or similar sports. 
     As shown in  FIG.  1   , a baseball or softball bat  100 , hereinafter collectively referred to as a “ball bat” or “bat,” includes a handle  110 , a main barrel  120  (constituting at least part of a hitting surface), and a tapered section  130  joining the handle  110  to the barrel  120 . The free end of the handle  110  optionally includes a knob  140  or similar structure. The main barrel  120  is optionally closed off by a suitable plug or end cap  150 . The interior of the bat  100  is optionally hollow, allowing the bat  100  to be relatively lightweight so that ball players may generate substantial bat speed when swinging the bat  100 . 
     The ball striking area of the bat  100  typically extends throughout the length of the main barrel  120 , and may extend partially into the tapered section  130  of the bat  100 . For ease of description, this striking area will generally be referred to as the “barrel” or “barrel region” throughout the remainder of the description. The barrel region generally includes a “sweet spot,” which is the impact location where the transfer of energy from the bat  100  to a ball is generally maximal, while the transfer of energy to a player&#39;s hands is generally minimal. The sweet spot is typically located near the bat&#39;s center of percussion (COP), which may be determined by the ASTM F2398-11 Standard. Another way to define the location of the sweet spot is between the first node of the first bending mode and the second node of the second bending mode. This location, which is typically about four to eight inches from the free end of the bat  10 , generally does not move when the bat is vibrating. For ease of measurement and description, the “sweet spot” described herein coincides with the bat&#39;s COP. 
     The proportions of the bat  100 , such as the relative sizes of the main barrel  120 , the handle  110 , and the tapered section  130 , are not drawn to scale and may have any relative proportions suitable for use in a ball bat. Accordingly, the bat  100  may have any suitable dimensions. For example, the bat  100  may have an overall length of 20 to 40 inches, or 26 to 34 inches. The overall main barrel diameter may be 2.0 to 3.0 inches, or 2.25 to 2.75 inches. Typical ball bats have diameters of 2.25, 2.625, or 2.75 inches. Bats having various combinations of these overall lengths and barrel diameters, or any other suitable dimensions, are contemplated herein. The specific preferred combination of bat dimensions is generally dictated by the user of the ball bat  100 , and may vary greatly among users. 
     The ball bat  100  may include two or more separate attached pieces (for example, a portion of the bat  100  that includes the handle  110  may be separate from, but attached to, a portion of the bat  100  that includes the main barrel  120 . In some embodiments, a portion of the bat  100  that includes the handle  110  may include a portion of the tapered section  130 , and a portion of the bat  100  that includes the main barrel  120  may also include a portion of the tapered section  130 . In some embodiments, the portion of the bat  100  that includes the main barrel  120  may overlap with the portion of the bat  100  that includes the handle  110 . In some embodiments, the tapered section  130  may be mostly or entirely included in the portion of the bat that includes the main barrel  120 . As used herein, the “handle” and “barrel” may include portions of the tapered section  130 . 
     In particular representative embodiments of the present technology, the ball bat  100  may be constructed from one or more composite or metallic materials. Some examples of suitable composite materials include laminate layers or plies reinforced with fibers of carbon, glass, graphite, boron, aramid (such as Kevlar®), ceramic, or silica (such as Astroquartz®). In some embodiments, aluminum, titanium, or another suitable metallic material may be used to construct some portions or all of the ball bat  100 . For example, in some embodiments, the main barrel  120  may be formed with one or more composite or metal materials. The handle  110  may be formed from the same materials as the main barrel  120 , or the handle  110  may be formed with different materials. In some embodiments, the handle  110  may be formed with a metal material and the main barrel  120  may be formed with a composite material. 
       FIG.  2    illustrates a perspective exploded view of the ball bat  100  shown in  FIG.  1   . In some embodiments, the ball bat  100  includes a frame  210  and a barrel shell  220 . The barrel shell  220  may be a generally hollow, tapered, cylindrical structure, and it may be positioned over and onto the frame  210 , where it is mechanically locked with the frame  210  (as further described below). The barrel shell  220  may form an outer barrel in a double-barrel structure. The frame  210  may include the handle  110  and an inner cylindrical backstop or inner barrel structure  230 , and it may generally resemble the shape of a ball bat. The handle  110  and the inner barrel structure  230  may be formed with separate components or they may be integral (for example, the frame  210  may be made a unitary, integral component using composite materials or a metal material, such as one or more of the materials described herein). One or both of the handle  110  and the inner barrel structure  230  may be hollow (for example, they may be formed in a cylindrical shape with one or more layers of composite materials, or with a metal material). The inner barrel structure  230  optionally includes a tapered region  240 , which may have a shape that generally corresponds with the shape of the tapered section  130  of the barrel shell  220 . For example, the tapered region  240  may gradually transition from the outer diameter of the inner barrel structure  230  to the smaller outer diameter of the handle  110 . 
     The barrel shell  220  includes the main barrel  120  and it may include at least part of the tapered section  130 . In some embodiments, the barrel shell  220  may be configured to coextend with the inner barrel structure  230 . The barrel shell  220  may be made with composite materials described herein, and it may be made with the same or different materials as the inner barrel structure  230 . For example, the barrel shell  220  may be made with plastic (with or without fiber reinforcement), thermoplastic composite reinforced with fibers (such as chopped fiber, very long fibers, or continuous fibers), or other composite materials described herein, such as laminate composite materials. 
     When assembled, as further described below, the barrel shell  220  is positioned over and onto the inner barrel structure  230 . The end cap  150  is attached to the distal end of the barrel shell  220  or the frame  210 . The optional end knob  140  may be attached to the proximal end  250  of the handle  110 . An optional collar  260  (also visible in  FIG.  1   ) may be positioned at an interface between the handle  110  of the frame  210  and the barrel shell  220 . The collar  260  may serve an aesthetic purpose (for example, providing a smooth appearance for the bat  100 ), or one or more functional purposes (for example, assisting in locking the barrel shell  220  to the frame  210 , or closing a gap between components to resist debris penetrating the assembly). 
     The barrel shell  220  forms an outer barrel that is substantially separated or spaced apart from the inner barrel structure  230  by a gap, which is illustrated and described below with regard to  FIGS.  3 A- 3 D , for example. As described in additional detail throughout this disclosure, the barrel shell  220  provides some compliance during a hit to create a trampoline effect, while the inner barrel structure  230  provides a backstop to limit the radial deflection of the barrel shell  220 . Ball bats according to various embodiments of the present technology provide improved hitting feel and sound without substantially increasing swing weight. In addition, ball bats according to various embodiments of the present technology may provide reduced shock or vibration for improved player comfort. 
     Referring to  FIGS.  3 A- 3 D , a space or gap  310  is provided between the barrel shell  220  and the inner barrel structure  230 . The gap  310  may result from the barrel shell  220  having a larger inner diameter  320  than an outer diameter  330  of the inner barrel structure  230  along at least portions of the length of the ball bat  100 . In some embodiments, the gap  310  may extend along the bat  100  between the end cap  150  and the collar  260 , with optional breaks or interruptions in the gap  310  formed by spacers or fillers, as described below. 
     In some embodiments, the gap  310  may have a gap width W that is generally uniform along all or part of its length (for example, at least 50%, or 100%, of the striking area). For example, in some embodiments, the gap width W may be between approximately 0.1 inches and 1.0 inch. In specific embodiments, the gap width W may be 0.10 inches, 0.125 inches, 0.140 inches, 0.50 inches, or another suitable dimension. Bat designers may select the gap width W based on several factors, such as the thickness or composition of the barrel shell  220 . In one exemplary embodiment, a one-inch gap width W may be used in a ball bat  100  having an outer barrel diameter of 2.75 inches. In some embodiments, the gap width W may be greater than 150% of a thickness of the barrel shell  220 . In yet further embodiments, the gap  310  may have a varying gap width W along its length. 
     The gap  310  between the barrel shell  220  and the inner barrel structure  230  may be maintained by one or more spacer elements positioned in the gap  310 . For example, when the bat  100  is assembled, a first spacer element  340  may be positioned adjacent to a proximal end  350  of the barrel shell  220  (optionally, within the tapered section  130 ), and a second spacer element  360  may optionally be positioned adjacent to a distal end  370  of the barrel shell  220 . The spacer elements  340 ,  360  may contribute to maintaining concentricity between the barrel shell  220  and the frame  210  or the inner barrel structure  230 . 
     A representative example of a spacer element is illustrated in  FIGS.  3 A- 3 D . In some embodiments, each spacer element  340 ,  360  may be in the form of a partial or complete ring positioned between the barrel shell  220  and the inner barrel structure  230 . In some embodiments, one or more of the rings forming the spacer elements  340 ,  360  may be discrete elements attached to the frame  210  or the inner barrel structure  230 , or they may be integral with the frame  210  or inner barrel structure  230 . For example, in some embodiments, the material forming the inner barrel structure  230  may be molded to include one or more contours or projections along the length of the inner barrel structure  230  to form the shape of the spacer elements  340 ,  360 . In some embodiments, one or more of the rings forming the spacer elements  340 ,  360  may be attached to or integral with the barrel shell  220 . In general, the spacer elements  340 ,  360  include projections extending radially outward from the inner barrel structure  230 , or radially inward from the barrel shell  220 . 
     The spacer elements  340 ,  360  may be made of any suitable material, and various materials may affect the bat&#39;s performance. For example, the spacer elements  340 ,  360  may be made of the same material as the barrel shell  220  or the inner barrel structure  230 . In some embodiments, the spacer elements may be rigid, such that they may be formed with one or more plastic (with or without fiber reinforcement), metal (such as aluminum, steel, magnesium, titanium, or other suitable metals), or composite materials. In some embodiments, the spacer elements may be formed with one or more resilient elastomeric materials, such as foam, foaming adhesive, rubber, thermoplastic polyurethane (TPU), or other suitable resilient elastomeric materials. In a particular representative embodiment, elastomeric materials used in the present technology may include polyurethane foam having a density of approximately four pounds per cubic foot (the inventors determined that the damping characteristics of such a foam helps a bat designer comply with BBCOR or BBS regulations, in various exemplary configurations). 
     Additionally or alternatively, in some embodiments, one or more resilient elastomeric materials may be positioned in the gap  310  between the spacer elements  340 ,  360 . Such elastomeric materials may include elastomeric materials described throughout this disclosure, or other suitable elastomeric materials. For example, an elastomeric material may partially or completely fill the gap  310  between the spacer elements  340 ,  360 . 
     In a representative embodiment, a layer or band  395  of elastomeric material (including any elastomeric material described herein, or any other suitable elastomeric material) may be positioned to be centered directly in the middle of the spacer elements ( 340 ,  360 ), or near the center of percussion, or at any other suitable position along the striking area of the bat. In some embodiments, the band  395  of elastomeric material may be positioned on and around the inner barrel structure  230 , or it may be positioned on and around the inner diameter  320  of the barrel shell  220 . Such a band  395  of elastomeric material (whether positioned on the inner barrel structure  230 , the barrel shell  220 , or both) may have a thickness between approximately 0.003 inches and 0.250 inches, depending on designer preferences and the gap width W. In a particularly representative embodiment, the band  395  may be between approximately 0.010 inches and 0.10 inches thick. In some embodiments, the location and thickness of the elastomeric material may affect the net gap width and the performance of the bat, for example, by providing a different rebound speed in one part of the bat than another. The band  395  may have a length L between 0.75 inches and 3.0 inches along the length of the bat, or in some embodiments, 0.125 inches to 6.0 inches along the length of the bat, depending on placement and desired performance or feel. 
     When an elastomeric material is positioned in the gap  310 , it may be positioned to completely fill the gap  310  along a radial direction between the barrel shell  220  and the inner barrel structure  230 , or it may only partially fill the gap  310  between the barrel shell  220  and the inner barrel structure  230  along the radial direction. In some embodiments, the gap  310  is otherwise filled with air. In other embodiments, the gap  310  may be a sealed vacuum space. 
     In some embodiments, some or all of the inner barrel structure  230  itself may have elastomeric properties. For example, the inner barrel structure  230  within the interior of the barrel shell  220  may be formed from an elastomeric material, or it may be at least partially covered or coated with an elastomeric material, such as a urethane material, rubber, polyurethane, thermoplastic polyurethane, thermo-plasticized rubber, thermo-plasticized elastomer, or another suitable material. In some embodiments, elastomeric materials may have a hardness value of Shore 70A or less, for example, between shore 20A and shore 40D. In some embodiments, the barrel shell  220  may include elastomeric materials in a similar manner. For example, it may be coated with an inner lining formed with an elastomeric material. In some embodiments, a gap may still be located between the inner barrel structure  230  and the barrel shell  220 , such that the elastomeric material is engaged only when the ball impact is of sufficient energy to cause the barrel shell  220  to bottom out against the inner barrel structure  230  or the elastomeric material. 
     In some embodiments in which the spacer elements  340 ,  360  are formed with soft, resilient, or elastomeric materials, or in which elastomeric materials are positioned in the gap  310  (such as the band  395  or any coatings or other elastomeric structures described above), such elastomeric materials can soften or dampen the impulse of the barrel shell  220  when it contacts the inner barrel structure  230  during the bat&#39;s  100  impact with a ball. Accordingly, ball bats  100  according to the present technology may comply with BBCOR or BBS regulations at least partially because the elastomeric materials tend to dampen and absorb energy during bat-ball impact. Increased damping characteristics of the materials selected for the spacer elements  340 ,  360 , or elastomeric materials positioned in the gap  310 , are associated with decreased BBCOR or BBS. Increased damping characteristics may also reduce shock felt by the player during a hit, or sound heard by the player during a hit, and may enhance bat durability. 
     The spacer elements  340 ,  360  may be positioned at any suitable locations along the length of the bat, and more or fewer than two spacer elements may be used. In a particular representative embodiment, a distance D 1  between the spacer elements  340 ,  360  may be at least 25% of the overall length of the barrel shell  220  to correspond with all or part of the striking area. For example, the distance D 1  may be 80% or more (such as 100%) of the overall length of the barrel shell  220  to allow the gap  310  between the spacer elements  340 ,  360  to correspond with most or all of the striking area. The spacer elements  340 ,  360  may have any suitable length or thickness to support the barrel shell  220 . 
     In various embodiments of the present technology, materials and dimensions may be selected to create a desired level of flex and compression of the barrel shell  220  relative to the inner barrel structure  230  (for example, the amount of trampoline effect of the barrel shell  220 ). For example, the position, spacing, and composition of the spacer elements  340 ,  360 , elastomeric materials in the gap  310 , any elastomeric materials in or on the inner barrel structure  230  or barrel shell  220 , the thickness and composition of material(s) forming the inner barrel structure  230 , the thickness and composition of material(s) forming the barrel shell  220 , or the width of the gap W may be selected individually or in various combinations to create the desired level of flex and compression of the barrel shell  220  relative to the inner barrel structure  230 . 
     In the art of ball bat design, designers may measure compression values by determining the amount of force required to compress a cylinder or ball bat in a radial direction. For example, designers may rely on compression values based on testing under the ASTM F2844-11 Standard Test Method for Displacement Compression of Softball and Baseball Bat Barrels. 
     Compression values of the inner barrel structure  230  and the barrel shell  220  may be selected to tune the feel or trampoline effect of the assembled ball bat  100 . In some embodiments, the barrel shell  220  may have a lower (such as significantly lower) compression value than the compression value of the inner barrel structure  230 . In some embodiments, the barrel shell  220  may have a higher compression value than that of the inner barrel structure  230 . The discussion of specific compression values below is only representative of the technology for illustration, and is based on measuring compression under the ASTM F2844-11 standard, at a location approximately 6 inches from the distal end of the inner barrel structure  230  or the barrel shell  220 , which may correspond to within approximately 3 inches of the center of percussion of an assembled ball bat. Compression is generally measured in a location away from the spacer elements ( 340 ,  360 ). 
     In a particular representative embodiment of a fast-pitch softball bat, the barrel shell  220  may have a compression value between approximately 130 to 150 pounds, while the inner barrel structure  230  may have a compression value of approximately 190 pounds or more (such as 270 pounds). Some representative compression values or ratios that the inventors have discovered to provide improved or optimal performance and feel include, for example: (a) a barrel shell compression value of 130 pounds and an inner barrel structure compression value of 190 pounds, or a ratio of inner barrel structure compression to barrel shell compression between 140 percent and 150 percent; (b) a barrel shell compression value of 154 pounds and an inner barrel structure compression value of 195 pounds, or a ratio of inner barrel structure compression to barrel shell compression between 120 and 130 percent; (c) a barrel shell compression value of 220 pounds and an inner barrel structure compression value of 400 pounds, or a ratio of inner barrel structure compression to barrel shell compression between 180 and 190 percent; and (d) a barrel shell compression value of 240 pounds and an inner barrel structure compression value of 76 pounds, or a ratio of inner barrel structure compression to barrel shell compression between 25 and 35 percent. 
     In a particular representative slow pitch softball bat according to an embodiment of the present technology, the barrel shell  220  may have a compression value of approximately 50 pounds, while the inner barrel structure  230  may have a compression value of approximately 270 pounds, or there may be a ratio of inner barrel structure compression to barrel shell compression between 200 percent and 600 percent. 
     In some embodiments, in which a designer must comply with BBCOR or BBS requirements, higher compression values may be used. For example, compression values may be approximately 500 to 600 pounds or more, to approximate the BCCOR value of a solid wood baseball bat. In some embodiments, to maintain compliance with BBCOR or BBS limitations, the spacer elements  340 ,  360  may be soft (a softer connection between the barrel shell  220  and the inner barrel structure  230  correlates with lower performance). In general, compression values may be selected such that the final assembled ball bat  100  complies with league or association rules. 
     Embodiments of the present technology allow bat designers to create an overall bat assembly with a compression value less than 300 pounds while meeting performance limits set by various leagues and associations. A combination of performance and adherence to standards and rules, while maintaining durability, has been a challenge for bat designers in the past. 
     The barrel shell  220  may be mechanically locked to the frame  210  or the inner barrel structure  230  to prevent it from sliding off the frame  210  or the inner barrel structure  230  during use. A suitable mechanical locking feature may include a snap-ring configuration, a tongue-and-groove configuration, a projection on either the barrel shell  220  or the frame  210  and a corresponding notch in the other of the barrel shell  220  or the frame  210 , or any other locking arrangement between the barrel shell  220  and the frame  210  or the inner barrel structure  230 . In some embodiments, elastomeric materials or other materials positioned in the gap  310  may resist separation of the barrel shell  220  from the frame  210 . 
     In some embodiments, the proximal end  350  of the barrel shell  220  may be tapered and configured to be in an overlapping, interference fit with a corresponding tapered region  240  of the frame  210 . Such an overlapping interference fit may form a mechanical locking feature to secure the barrel shell  220  to the frame  210 . More specifically, a proximally positioned inner diameter of the barrel shell  220  in the tapered section  130  of the ball bat  100  may be smaller than a more distally positioned outer diameter of the frame  210 . In some embodiments, the spacer elements  340 ,  360  create the mechanical locking feature by providing an interference fit with the barrel shell  220 . For example, an outer diameter of the first spacer element  340  may be equal to an inner diameter of the barrel shell  220  near the proximal end  350  of the barrel shell  220 . The tapering of the barrel shell  220  in that part of the bat prevents the barrel shell  220  from sliding off the frame  210  in a distal direction. The coextensive tapers of the inner barrel structure  230  and the barrel shell  220  may also prevent the barrel shell  220  from sliding off the inner barrel structure  230  in a distal direction. 
     In some embodiments, the end cap  150  may be positioned to engage an inner diameter of the inner barrel structure  230  of the frame  210 . The end cap  150  may close or cover a distal end of the gap  310 . In some embodiments, the spacer element  360  adjacent to the distal end  370  may be omitted and the end cap  150  may include a projection or spacer extending into the gap  310  to maintain the spaced and concentric relationship between the barrel shell  220  and the inner barrel structure  230 . Concentricity between the barrel shell  220  and the inner barrel structure  230 , along with spacer elements such as the spacer elements  340 ,  360 , may facilitate radial deflection of the barrel shell  220  without pivoting relative to the frame  210  during a hit. 
     As shown in  FIGS.  3 C and  3 D , in some embodiments, a ring  373  of elastomeric material may be positioned adjacent to one or more of the spacer elements  340 ,  360 . The ring  373  may be positioned a space  380  between the first spacer element  340  and the proximal end  350  of the barrel shell  220  (outside the space between the spacer elements  340 ,  360 ) to support an overhanging part of the barrel shell  220  at its proximal end  350 . The ring  373  may partially or completely fill the space  380 . Likewise, another ring  373  of elastomeric material may be positioned in a space  390  between the second spacer element  360  and the distal end  370  of the barrel shell  220  (outside the space between the spacer elements  340 ,  360 ), to also support an overhanging part of the barrel shell  220  at its distal end  370 . Although the ring  373  is described as being formed with an elastomeric material, it may be rigid in some embodiments. The ring  373  may prevent cracking or other damage at the proximal  350  and distal  370  ends of the barrel shell  220 . 
     Referring to  FIGS.  4 A and  4 B , a ball bat  400  is similar to the ball bat  100  described above with regard to  FIGS.  1 - 3 D  in most aspects, except that the inner barrel structure  410  of the frame  420  has a shape or contour that creates a gap  430  of varying width W between the inner barrel structure  410  and the barrel shell  220 . In some embodiments, the gap width W may be smaller in or near a chosen reference region  440  along the length of the barrel than in other locations along the length of the barrel. The gap width W may be varied by varying the outer diameter of the inner barrel structure  410  along its length. For example, the outer diameter of the inner barrel structure  410  may be larger in the reference region  440  than the outer diameter of other parts of the inner barrel structure  410 . 
     In particular representative embodiments, the reference region  440  may include one or more of the striking area of the bat  400 , the center of percussion, or other regions of the bat  400 . In a more particular representative embodiment, the reference region  440  may span a two-inch distance from either side of the center of percussion. 
     The narrower gap width W may provide an area of reduced performance or BBCOR (or BBS) due to the outer barrel structure  220  being limited in the amount it can radially deflect or compress before being stopped by the inner barrel structure  410  during impact with a ball. For example, a ball bat  400  according to an embodiment of the present technology may be designed such that the gap  430  in the reference region  440  is relatively small, so that the bat  400  exhibits a BBCOR (or BBS, or other performance measurement) value that complies with regulations. 
     The gap  430  outside of the reference region  440  may facilitate increased trampoline effect and BBCOR (or BBS) relative to the gap  430  in the reference region  440  to enhance the overall bat performance along the length of the barrel, or to broaden the areas of the bat where peak performance can be achieved. Optionally, the gap width W may be selected to maintain compliance with performance limitations along the full length of the barrel. In some embodiments, the gap width W may be reduced to zero, or omitted, in the reference region  440 . 
     Embodiments of the present technology also include methods of making double-barrel ball bats, including but not limited to the ball bats disclosed herein.  FIG.  5    illustrates a method  500  of making ball bats according to the present technology. In block  510 , composite laminate material may be laid up or otherwise positioned around a mandrel to form a frame (with or without the spacer elements described above). In block  520 , a release material may be wrapped or otherwise positioned or applied around the inner barrel structure of the frame (which may be cured or uncured at this point in the method). The release material may have a thickness corresponding to the desired gap width between the frame or inner barrel structure and the barrel shell. The release material maintains the gap width during the manufacturing and curing process. The release material may include one or more of silicone sheet, elastomeric sheet, polyamide, cellophane, vinyl, polymer materials (such as PTFE), or other materials suitable to prevent bonding between the barrel shell and the frame during the molding and curing process. In some embodiments, the release material may be in the form of a tube or a sheet wrapped around or positioned on the frame. 
     In block  530 , the method may include laying up further composite laminate material around the inner barrel structure of the frame to form the barrel shell (with or without spacer elements, as described above). In block  540 , the frame and barrel shell may be cured. In block  550 , the barrel shell may be removed by sliding it off the frame, for example, in a direction toward the handle. The release material prevents the barrel shell from becoming integral with the frame during the curing process. In block  550 , the release material may also be removed from the frame. 
     In block  560 , one or more spacer elements described above may be attached to the inner barrel structure of the frame as described above. In some embodiments, spacer elements may be formed in block  510  as part of the layup of the frame. In some embodiments, optional elastomeric materials described above may be attached or bonded to, or positioned around, the inner barrel structure of the frame or inside the barrel shell. 
     In block  570 , the barrel shell may be slid back onto the frame and locked in place using one or more embodiments of mechanical locking arrangements described above (such as the corresponding coaxial tapers of the barrel shell and the inner barrel structure or the interference fit between the barrel shell and one or more spacer elements). Assembly of the barrel shell onto the frame according to embodiments of the present technology is described below with regard to  FIGS.  6 A- 6 C . 
       FIGS.  6 A- 6 C  illustrate assembly of the barrel shell  220  onto a frame (such as the frame  210  or  420  described above). As shown in  FIGS.  6 A and  6 B , the barrel shell  220  is moved toward the frame ( 210 ,  420 ) such that the distal end  370  goes over and around the handle  110  first, followed by the proximal end  350 . In some embodiments, before the barrel shell  220  is slid onto the frame ( 210 ,  420 ), spacer elements (such as the spacer elements  340 ,  360  described above) may be installed on the inner barrel structure ( 230 ,  410 ) of the frame ( 210 ,  420 ) or the barrel shell  220 . In some embodiments, elastomeric materials may be applied on the inner barrel structure or the barrel shell, as described above. In other embodiments, one or more spacer elements or elastomeric materials may have previously been installed or integrally molded or formed with the inner barrel structure. 
     As shown in  FIG.  6 C , the barrel shell  220  is mechanically locked into position around the inner barrel structure of the frame (such as the inner barrel structures  230 ,  410 , which are visible in  FIGS.  6 A and  6 B  but covered by the shell in  FIG.  6 C ). As described above, a gap (such as the gaps  310  or  430 ) may be maintained between the frame or inner barrel structure and the barrel shell. 
     In some embodiments, an exposed area  610  may remain between the barrel shell  220  and the handle portion  110  of the frame ( 210 ,  420 ). The exposed area  610  may be left as-is, or it may be filled or otherwise covered for aesthetic purposes or for further improving the mechanical lock between the barrel shell  220  and the frame ( 210 ,  420 ). For example, as illustrated in  FIG.  6 D , a collar  260  may be positioned around the exposed area  610 .  FIG.  6 E  illustrates an embodiment of a complete bat ( 100 ,  400 ), which may include an optional knob  140  and cap  150  that may be installed at any suitable point during assembly of the bat. 
     In some embodiments, the barrel shell and frame may be molded separately from each other and then connected. In such embodiments, the frame may have spacer elements or elastomeric materials applied or installed prior to attaching the barrel shell to the inner barrel structure of the frame, or the frame may have spacer elements or elastomeric materials integrated therein. 
     With reference again to  FIG.  5   , in another embodiment, the inner barrel structure of the frame may be laid up in a manner similar to that described above with regard to block  510  of  FIG.  5   , but with one spacer element positioned near the tapered region of the inner barrel structure ( 240 ), such as the first spacer element ( 340 ) described above and show in various figures. After laying up the inner barrel structure according to such an embodiment, the inner barrel structure may be wrapped in a release material, or a release material may be otherwise applied in a manner similar to that described above with regard to block  520 , such that the release material may have a thickness and length corresponding to the desired gap between the barrel shell and the inner barrel structure. Then, similar to the steps described above with regard to  530  and  540 , the barrel shell may be laid up around the inner barrel structure and release material, sandwiching the release material between the inner barrel structure and the barrel shell, similar to the process described above. The assembly may then be cured. 
     After curing, the release material may be pulled out from between the barrel shell and the inner barrel structure, leaving the gap between the barrel shell and the inner barrel structure. The remainder of the ball bat may then be assembled in a manner similar to that described above with regard to  FIGS.  6 D and  6 E . In some embodiments, the cap (such as the cap  150 ) may have a lip or spacer positioned between the inner barrel structure and the barrel shell to form a spacer element at the distal end of the ball bat. 
     In some embodiments, the frame may be made of metal. In such embodiments, the frame may be cast, machined, drawn, swaged, or otherwise made from metal, and then the barrel shell and other components may be added in a manner similar to that described with regard to  FIGS.  6 A- 6 E . In some embodiments, the frame may be made of wood and assembled in a manner similar to that described with regard to  FIGS.  6 A- 6 E . 
     Bats according to embodiments of the present technology provide improved feel and performance advantages for players. The gap between the frame ( 210 ,  420 ) and the barrel shell  220  facilitates a limited amount of “trampoline effect” that can be tailored with variation of the dimensions of the gap, materials used in the structures, and the spacer elements or materials in the gap. The barrel shell  220  exhibits compliance until it bottoms out against the inner barrel structure or materials in the gap. In some embodiments, the inner barrel structure exhibits some compliance. Accordingly, bats according to the present technology can have high or limited performance, improved feel, and improved durability as described herein. 
     Bats according to the present technology may be tamper-resistant in that a) the barrel shell is sufficiently flexible that typical “rolling” procedures (or other artificial break-in processes) may not affect the shell; b) deflecting the barrel shell so deeply in rolling to affect a change in the bat performance may damage the bat beyond use; or c) shaving or thinning of the frame or inner barrel structure may weaken or degrade the frame to a point where it may no longer be useful. 
     From the foregoing, it will be appreciated that specific embodiments of the disclosed technology have been described for purposes of illustration, but that various modifications may be made without deviating from the technology, and elements of certain embodiments may be interchanged with those of other embodiments, and that some embodiments may omit some elements. For example, in bats intended for use in softball, the barrel shell may be formed with a very flexible composite material, which may provide high performance. In bats intended for use in baseball, where performance limitations may be lower or more regulated (such as in the NCAA or in USA Baseball, which regulate a lower performance value), the barrel shell may optionally be made of a metal material so that the barrel shell is more stiff (for example, as stiff as a solid wood bat). 
     Further, while advantages associated with certain embodiments of the disclosed technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology may encompass other embodiments not expressly shown or described herein, and the invention is not limited except as by the appended claims.