Patent Publication Number: US-11647804-B2

Title: Protective helmet with integrated rotational limiter

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/990,567, filed on May 25, 2018, which is a continuation of U.S. patent application Ser. No. 15/638,121, filed on Jun. 29, 2017, the disclosures of which are incorporated by reference in its entirety herein. 
    
    
     TECHNICAL FIELD 
     Aspects of this document relate generally to helmets with an integrated rotational limiter. 
     BACKGROUND 
     Protective headgear and helmets have been used in a wide variety of applications and across a number of industries including sports, athletics, construction, mining, military defense, and others, to prevent damage to a user&#39;s head and brain. Contact injury to a user can be prevented or reduced by helmets that prevent hard objects or sharp objects from directly contacting the user&#39;s head. Non-contact injuries, such as brain injuries caused by linear or rotational accelerations of a user&#39;s head, can also be prevented or reduced by helmets that absorb, distribute, or otherwise manage energy of an impact. This may be accomplished using multiple layers of energy management material. 
     Conventional helmets having multiple energy management liners are able to reduce the rotational energy transferred to the head and brain by facilitating and controlling the rotation of the energy management liners against one another. Some conventional helmets, such as, for example, those disclosed in US Published application 20120060251 to Schimpf (hereinafter “Schimpf”) employ a continuous surface interrupted by a recess in the outer liner that a projection from the inner liner extends into. Additionally, other conventional helmets, such as those disclosed in US Published application 20010032351 to Nakayama (hereinafter “Nakayama”) employ an inner liner and an outer liner that both have interlocking recesses and projections. 
     Some conventional helmets employ structures or objects that bridge energy liners that must break, deform, and/or deform an elastic material for the liners to rotate against each other. Such a method of energy absorption has advantages and disadvantages; while the energy is absorbed by the failure or deformation of the projections, it either happens over a short period of time, thus doing little to attenuate the rotational accelerations experienced by the user&#39;s head and brain, or the liners may tend to rotate out of one another, reducing the helmet stability. 
     SUMMARY 
     According to one aspect, a helmet includes an outer liner having an interior surface comprising a shelf extending inward from the interior surface proximate a perimeter of an opening at a lower edge of the outer liner. The shelf includes an arresting surface. The helmet also includes an inner liner having an exterior surface, an interior surface and an edge connecting the exterior surface to the interior surface. The edge is facing the arresting surface of the shelf. The inner liner is slidably coupled to the interior surface of the outer liner through at least one return spring and slidably movable relative to the outer liner between a first position in which the edge of the inner liner is separated from the arresting surface of the shelf by a first gap, and an arrested position in which a portion of the edge of the inner liner is in contact with a portion of the arresting surface of the shelf in response to movement of the outer liner relative to the inner liner caused by an impact to the helmet. Furthermore, the at least one return spring biases the inner liner toward the first position. 
     Particular embodiments may comprise one or more of the following features: the interior surface proximate a majority of the perimeter of the opening may include the shelf. The at least one return spring may be composed of an elastomer material. The first gap separating the edge of the inner liner from the arresting surface of the shelf while the inner liner is in the centered position may be between 12 mm and 15 mm. The shelf may include a plurality of shelf pieces. The arresting surface of the shelf may be discontinuous. The outer liner may include a front, a rear, and/or two sides opposite each other and connecting the front and the rear, Also, a first portion of the shelf may be located proximate the rear of the outer liner, a second portion of the shelf may be located proximate one of the two sides of the outer liner, and a third portion of the shelf may be located proximate the other of the two sides of the outer liner. The first gap may be substantially uniform across the arresting surface when the inner liner is in the first position. The outer liner may include a plurality of vents passing through the outer liner. The inner liner may include a plurality of channels passing through the inner liner. The plurality of channels may at least partially overlap with the plurality of vents, and may form a plurality of apertures from outside the helmet to inside the helmet. Each of the plurality of vents may be beveled at the interior surface of the outer liner. Each of the plurality of channels may be beveled at the exterior surface of the inner liner. Additionally, at least one of the interior surface of the outer liner and the exterior surface of the inner liner may include a surface of reduced friction. Finally, an air gap may exist between a majority of the exterior surface of the inner liner and the interior surface of the outer liner. 
     According to another aspect, a helmet includes an outer liner having an interior surface including a shelf extending inward from the interior surface proximate a majority of a perimeter of an opening at a lower edge of the outer liner. The shelf includes an arresting surface. The helmet also includes an inner liner having an exterior surface, an interior surface and an edge connecting the exterior surface to the interior surface. The edge is facing the arresting surface of the shelf. The inner liner is slidably coupled to the interior surface of the outer liner through at least one return spring. Also, the inner liner is slidably movable relative to the outer liner between a first position in which the edge of the inner liner is separated from the arresting surface of the shelf by a first gap that is substantially uniform across the arresting surface, and an arrested position in which a portion of the edge of the inner liner is in contact with a portion of the arresting surface of the shelf in response to movement of the outer liner relative to the inner liner caused by an impact to the helmet. Lastly, the at least one return spring biases the inner liner toward the first position. 
     Aspects and applications of the disclosure presented here are described below in the drawings and detailed description. Unless specifically noted, it is intended that the words and phrases in the specification and the claims be given their plain, ordinary, and accustomed meaning to those of ordinary skill in the applicable arts. The inventors are fully aware that they can be their own lexicographers if desired. The inventors expressly elect, as their own lexicographers, to use only the plain and ordinary meaning of terms in the specification and claims unless they clearly state otherwise and then further, expressly set forth the “special” definition of that term and explain how it differs from the plain and ordinary meaning. Absent such clear statements of intent to apply a “special” definition, it is the inventors&#39; intent and desire that the simple, plain and ordinary meaning to the terms be applied to the interpretation of the specification and claims. 
     The inventors are also aware of the normal precepts of English grammar. Thus, if a noun, term, or phrase is intended to be further characterized, specified, or narrowed in some way, then such noun, term, or phrase will expressly include additional adjectives, descriptive terms, or other modifiers in accordance with the normal precepts of English grammar. Absent the use of such adjectives, descriptive terms, or modifiers, it is the intent that such nouns, terms, or phrases be given their plain, and ordinary English meaning to those skilled in the applicable arts as set forth above. 
     Further, the inventors are fully informed of the standards and application of the special provisions of 35 U.S.C. § 112, ¶ 6. Thus, the use of the words “function,” “means” or “step” in the Detailed Description or Description of the Drawings or claims is not intended to somehow indicate a desire to invoke the special provisions of 35 U.S.C. § 112, ¶ 6, to define the invention. To the contrary, if the provisions of 35 U.S.C. § 112, ¶ 6 are sought to be invoked to define the inventions, the claims will specifically and expressly state the exact phrases “means for” or “step for”, and will also recite the word “function” (i.e., will state “means for performing the function of [insert function]”), without also reciting in such phrases any structure, material or act in support of the function. Thus, even when the claims recite a “means for performing the function of . . . ” or “step for performing the function of . . . ,” if the claims also recite any structure, material or acts in support of that means or step, or that perform the recited function, then it is the clear intention of the inventors not to invoke the provisions of 35 U.S.C. § 112, ¶ 6. Moreover, even if the provisions of 35 U.S.C. § 112, ¶ 6 are invoked to define the claimed aspects, it is intended that these aspects not be limited only to the specific structure, material or acts that are described in the preferred embodiments, but in addition, include any and all structures, materials or acts that perform the claimed function as described in alternative embodiments or forms of the disclosure, or that are well known present or later-developed, equivalent structures, material or acts for performing the claimed function. 
     The foregoing and other aspects, features, and advantages will be apparent to those artisans of ordinary skill in the art from the DESCRIPTION and DRAWINGS, and from the CLAIMS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The inventions will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and: 
         FIGS.  1 A and  1 B  show embodiments of a helmet with multiple energy management liners as known prior art; 
         FIG.  2    is a perspective view of a helmet; 
         FIG.  3    is an exploded view of the helmet of  FIG.  2   ; 
         FIG.  4 A  is a front cross-sectional view of the helmet of  FIG.  2    in a first position taken along cross-section lines A-A; 
         FIG.  4 B  is a view of the helmet of  FIG.  4 A  in an arrested position; and 
         FIG.  5    is a side cross-sectional view of the helmet of  FIG.  2    in the first position taken along cross-section lines B-B. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure, its aspects and implementations, are not limited to the specific material types, components, methods, or other examples disclosed herein. Many additional material types, components, methods, and procedures known in the art are contemplated for use with particular implementations from this disclosure. Accordingly, for example, although particular implementations are disclosed, such implementations and implementing components may comprise any components, models, types, materials, versions, quantities, and/or the like as is known in the art for such systems and implementing components, consistent with the intended operation. 
     The word “exemplary,” “example,” or various forms thereof are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Furthermore, examples are provided solely for purposes of clarity and understanding and are not meant to limit or restrict the disclosed subject matter or relevant portions of this disclosure in any manner. It is to be appreciated that a myriad of additional or alternate examples of varying scope could have been presented, but have been omitted for purposes of brevity. 
     While this disclosure includes a number of embodiments in many different forms, there is shown in the drawings and will herein be described in detail particular embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the disclosed methods and systems, and is not intended to limit the broad aspect of the disclosed concepts to the embodiments illustrated. 
     Conventional helmets having multiple energy management liners reduce the rotational energy of an impact transferred to the head and brain by facilitating and controlling the rotation of the energy management liners against one another. Some conventional helmets employ liner interfaces interrupted by a recess in one liner that a projection from another liner extends into, limiting the ability of one liner to rotate with respect to the other. See, for example,  FIG.  1 A , which shows a helmet  100  with a continuous outer liner  102  having a recess  108  with dampening material  110  and a continuous inner liner  104  having a projection  106  extending into the recess  108 , similar to the helmet shown in  FIG.  15    of the prior art reference to Schimpf referenced previously herein. Upon impact, rotational energy is absorbed as the outer liner  102  moves with respect to the inner liner  104  and the projection  106  compresses the dampening material  110 . See also  FIG.  1 B , which shows a helmet  150  with a continuous outer liner  152  and a continuous inner liner  154 , each having a series of interlocking recesses  158  and projections  156  separated by elastic material  160 , similar to the helmet shown in  FIG.  6    of the prior art reference to Nakayama referenced previously herein. 
     Conventional helmets employing structures such as these have the disadvantage of relying on one or more small projections, and friction between liners, to absorb all of the rotational energy of an impact. The absorption is either done over a small period of time, thus doing little to attenuate the rotational accelerations/decelerations experienced by the user&#39;s head and brain, or is spread over a range of relative displacement of the liners that stability is compromised, and one liner will possibly rotate out of another, compromising the head protection for the wearer. 
     Additionally, some conventional helmets include a continuous interface surface between an inner liner and the outer liner. See, for example, the continuous outer liner  102  and a continuous inner liner  104  of the helmet  100  of  FIG.  1 A , and the continuous outer liner  152  and a continuous inner liner  154  of the helmet  150  of  FIG.  1 B . Such a design allows for the rotational energies to be absorbed by more material, whether through protrusions extending into recesses, or deformable structures bridging liners. However, conventional helmet designs configured in this way are conventionally manufactured for football or motorcycle helmets, and are not suitable for implementations where ventilation is a concern, such as conventional bicycle helmets where a large portion of the helmet is required to have air flow openings and gaps extending from the innermost area of the helmet through all energy management liners. Relying entirely upon interlocking protrusions and recesses, or deformable bridging structures, may constrain the size of the airflow openings, lest the liner not be able to withstand the forces exerted by the projections and/or deformable bridges. 
     Contemplated as part of this disclosure are helmets having multiple energy management liners that are able to effectively rotate against one another upon impact while still being limited in the range of rotation by an integrated rotational limiter. Specifically, by using a rotational limiter, such as a shelf or a series of partial shelves or shelf pieces, on an interior surface of an outer liner to interface with an edge of an inner liner, a protective helmet may effectively attenuate rotational energy of an impact while also retaining and stabilizing the inner liner inside the outer liner. 
       FIGS.  2 - 5    illustrate a non-limiting embodiment of a helmet  200  comprising an outer liner  202  and an inner liner  204 . The interior surface  300  of the outer liner  202  comprises a shelf  400  ( FIGS.  4 A- 5   ) with an arresting surface  402 , and the inner liner  204  comprises an edge  306  facing the arresting surface  402  of the shelf  400 . The inner liner  204  is slidably coupled to the interior surface  300  of the outer liner  202  through a series of return springs  500 . Upon impact, rotational energy is initially absorbed by the outer liner  202  sliding with respect to the inner liner  204 , as well as by the deformation of the return springs  500  as the outer liner  202  moves away from a resting position (see first position  414  of  FIG.  4 A ). If the rotational energy of the impact is sufficient to slide the outer liner  202  with respect to the inner liner  204  far enough that the edge  306  of the inner liner  204  is in contact with the arresting surface  402  of the shelf  400 , additional energy is absorbed by the energy management materials of the inner and outer liners. 
     This is advantageous in relation to conventional helmets, such as helmet  100  of  FIG.  1 A  and helmet  150  of  FIG.  1 B , which absorb rotational energy through small projections bridging energy management liners. In contrast to the sharp decelerations and sharply localized energy absorption associated with conventional helmets, the contact between the edge  306  and the shelf  400  absorbs the rotational energy across a wider, stronger portion of the liner over a longer time than a small projection compressing a small amount of elastic material, and prevents the inner liner  204  from rotating out of the outer liner  202 . This results in better attenuation of the rotational acceleration/deceleration of the user&#39;s head and brain while stabilizing the helmet and reducing the chance of liner separation. 
       FIG.  3    shows an exploded view of a non-limiting example of a helmet  200 . As shown, helmet  200  has an outer liner  202  and an inner liner  204 . The inner liner  204  may be slidably coupled to the interior surface  300  of the outer liner  202 , according to various embodiments. In other embodiments, additional liners may be included. 
     Reference is made herein to inner and/or outer liners comprising an energy management material. As used herein, the energy management material may comprise any energy management material known in the art of protective helmets, such as but not limited to expanded polystyrene (EPS), expanded polyurethane (EPU), expanded polyolefin (EPO), expanded polypropylene (EPP), or other suitable material. 
     An outer liner  202  is exterior to the inner layer of a helmet and is composed, at least in part, of energy management materials. In some embodiments, the exterior surface of the outer liner  202  may comprise an additional outer shell layer, such as a layer of stamped polyethylene terephthalate (PET) or a polycarbonate (PC) shell, to increase strength and rigidity. This shell layer may be bonded directly to the energy management material of the outer liner  202 . In some embodiments, the outer liner  202  may have more than one rigid shell. For example, in one embodiment, the outer liner  202  may have an upper PC shell and a lower PC shell. 
     According to various embodiments, the outer liner  202  may be the primary load-carrying component for high-energy impacts. As such, the outer liner  202  may be composed of a high-density energy management material. As a specific example, the outer liner may be composed of EPS. 
     The outer liner  202  may provide a rigid skeleton for the helmet  200 , and as such may serve as the attachment point for accessories, such as a chin bar, or other structures. Although not shown in  FIG.  2   , the helmets of this disclosure may comprise any other features of protective helmets previously known in the art, such as but not limited to straps, comfort liners, masks, visors, and the like. For example, in one embodiment, the inner liner  204  may include a fit system to provide improved comfort and fit. 
     As shown, the outer liner  202  has an opening  206  at the lower edge  308 , where a user would insert their head. The perimeter  320  of the opening  206  of the outer liner  202  is bordered by a front  310 , a rear  312 , as well as two sides  314  opposite each other and connecting the front  310  and the rear  312 . In some embodiments, the outer liner  202  may comprise one or more vents  316  passing between the outside of the liner to the inside. In other embodiments, the outer liner  202  may be continuous and unvented. As previously discussed, the outer liner  202  also has an interior surface  300  comprising a shelf  400  extending inward proximate the perimeter  320  of the opening  206 . The shelf  400  will be discussed in greater detail with respect to  FIGS.  4 A and  4 B . 
     Also shown in  FIGS.  2  and  3    is a non-limiting example of an inner liner  204 . An inner liner  204  refers to an energy management liner of a helmet that is, at least in part, inside of another liner, such as outer liner  202  or another inner liner. The inner liner  204  is composed, at least in part, of an energy-management material. 
     The inner liner  204  has an exterior surface  302  and an interior surface  304 . The perimeters of these surfaces are connected by an edge  306 . The edge  306  might also be referred to as an edge surface, or an edge face. In some embodiments, the edge  306  may interface with the exterior surface  302  and the interior surface  304  at an angle. In other embodiments, the edge  306  may smoothly blend into the exterior surface  302  and the interior surface  304 . In some embodiments, the edge  306  may be a flat surface, while in others, it may be a curved, wavy, or multi-faceted surface. Furthermore, in some embodiments, the inner liner  204  may comprise one or more channels  318  passing between the exterior surface  302  and the interior surface  304  to facilitate ventilation. In other embodiments, the inner liner  204  may be continuous and unvented. 
       FIGS.  4 A and  4 B  are cross-sectional views of the non-limiting example of the helmet  200  of  FIG.  2   , taken along the line A-A, while  FIG.  5    is a cross-sectional view of the same non-limiting example, taken along the line B-B. As shown, the interior surface  300  of the outer liner  202  comprises a shelf  400  with an arresting surface  402 , and the inner liner  204  comprises an edge  306  facing the arresting surface  402  of the shelf  400 . The shelf  400  extends inward from the interior surface  300 . In some embodiments, including the non-limiting example shown in  FIGS.  4  and  5   , the shelf  400  is proximate a perimeter  320  of the opening  206  of the outer liner  202 . In other embodiments, the shelf  400  may be located on the interior surface  300  of the outer liner  202 , away from the perimeter  320  (i.e. the inner liner  204  would be much smaller than the outer liner  202 ). 
     According to various embodiments, the shelf  400  serves to lock the inner liner  204  in place after it is placed inside the outer liner  202 , and provides a hard stop to the motion, be it rotational or linear, of the inner liner  204  with respect to the outer liner  202 . Other embodiments may include additional, or different, structures, surfaces, bumpers, and/or features to constrain the motion of the inner liner  204  relative to the outer liner  202  to desired bounds. In some embodiments, at some points the inner liner  204  may be fixed in place, while at others it may move freely. 
     Advantageous over conventional helmets, the use of a shelf  400  such as those described herein may be adapted to a variety of helmet types. For example, the non-limiting embodiment shown in  FIGS.  2  through  5    is a bike helmet. These methods may be applied to any other helmet known in the art that may be used to protect against injuries due to rotational forces. 
     In some embodiments, the interior surface  300  of the outer liner  202  proximate a majority of the perimeter  320  of the opening  206  may comprise a shelf  400 . In other words, a majority of the perimeter  320  may be proximate to a portion of the shelf  400 . For example, the non-limiting example shown in  FIGS.  4  and  5    depict a helmet  200  having a shelf  400  with a first portion  404  of the shelf  400  proximate the rear  312  of the outer liner  202 , a second portion  406  proximate a side  314  of the outer liner  202 , and a third portion  408  proximate the other side  314 , opposite the second portion  406 . In some embodiments, the helmet  200  may further comprise a portion of the shelf  400  proximate the front  310  of the outer liner  202 . As shown, these portions are also all proximate the perimeter  320  of the opening  206  of the outer liner  202 . Of course, in other embodiments, the shelf  400  may extend along less than a majority of the perimeter  320 . 
     In some embodiments, the helmet  200  may comprise a plurality of partial shelves or shelf pieces  410 . In some embodiments, a shelf piece  410  may be a portion of a shelf  400  (e.g. first portion  404  of  FIG.  4 A ) directly attached to another portion (e.g. second portion  406  of  FIG.  4 A ) such that together they form a single contiguous shelf  400 . In other embodiments, a shelf piece  410  may be a portion of a shelf  400  that is distinct from other shelf pieces  410 , each shelf piece having its own arresting surface  402 . 
     As shown, the shelf  400 , comprises an arresting surface  402  to interface with the edge  306  of the inner liner  204 . As previously discussed, the edge  306  of the inner liner  204  faces the arresting surface  402  of the shelf  400 . In the context of the present description and the claims that follow, the edge  306  of the inner liner  204  is considered to be facing the arresting surface  402  of the shelf  400  when the orientation of the edge  306  relative to the arresting surface  402  is such that when the inner liner  204  slides with respect to the outer liner  202  such that the inner liner  204  makes contact with the shelf  400 , the edge  306 , or a portion  418  of the edge  306 , is in contact with the arresting surface  402 , or a portion  420  of the arresting surface  402 , of the shelf  400 . 
     In some embodiments, the edge  306  and the arresting surface  402  may be shaped such that when they make contact, the edge  306  is mated with the arresting surface  402  where contact is made. In other embodiments, the arresting surface  402  may be shaped such that it captures, cups, wraps around, and/or retains the edge  306 , such that the inner liner  204  is prevented from rotating out of the outer liner  202 . In some embodiments, the arresting surface  402  of the shelf  400  may be a continuous surface. In other embodiments, the arresting surface  402  may be discontinuous. For example, the arresting surface  402  of a shelf  400  may be discontinuous when the shelf  400  comprises a plurality of shelf pieces  410 , each separate and distinct from the others. 
       FIG.  4 A  shows a cross-sectional view of a non-limiting example of helmet  200  with an inner liner  204  in a centered or first position  414 . In the context of the present description and the claims that follow, the centered or first position  414  refers to the ideal or neutral position of the inner liner  204  inside of the outer liner  202 . According to various embodiments, including the non-limiting example shown in  FIGS.  4  and  5   , when the inner liner  204  is in the first position  414 , the edge  306  of the inner liner  204  is separated from the arresting surface  402  which it faces by a first gap  412 . In some embodiments, the first gap  412  may be between 12 mm and 15 mm. In other embodiments, the first gap  412  may be larger, while in still others it may be smaller. 
     In some embodiments, the first gap  412  between the arresting surface  402  and the edge  306  may be substantially uniform. In the context of the present description and the claims that follow, substantially uniform refers to the size of the first gap  412  being within a particular distance range throughout the arresting surface  402 . For example, the difference between the smallest first gap  412  and the largest first gap  412  throughout the arresting surface  402  may be 1 mm, 2 mm, 3 mm, or more. In other embodiments, the first gap  412  between the arresting surface  402  and the edge  306  may be non-uniform. As a specific example, the first gap  412  between the edge  306  and the arresting surface  402  may widen to make space for a ventilation duct through the inner liner  204  and the outer liner  202 . 
     The inner liner  204  is slidably movable between the first position  414  and an arrested position  416 , in which the edge  306 , or a portion of the edge  306 , of the inner liner  204  is in contact with the arresting surface  402 , or a portion of the arresting surface  402 , of the shelf  400 .  FIG.  4 A  shows a cross-sectional view of a non-limiting example of helmet  200  with an inner liner  204  in an arrested position  416 . It is worth noting that all discussion of motion, rotational and/or linear, of one of the liners is relative with respect to the other liner. For example, any discussion of motion of the inner liner  204  with respect to the outer liner  202  could be reframed as motion of the outer liner  202  with respect to the inner liner  204 . 
     In some embodiments, forces may be needed to return the inner liner  204  to a pre-impact position (e.g. first position  414 ). See, for examples, the return spring  500  of  FIG.  5   . According to various embodiments, the inner liner  204  may be directly coupled to the interior surface  300  of the outer liner  202  through at least one return spring  500 , which returns the inner liner  204  back to a first position  414 . The return springs  500  may also serve to attenuate some of the rotational energy from an impact. 
     A return spring  500  may be composed of a variety of elastic materials, including but not limited to an elastomer such as silicone. According to various embodiments, a return spring  500  may have a variety of shapes, including but not limited to bands, cords, and coils. In some embodiments, one or more return springs  500  may directly couple an edge  306  of the inner liner  204  to the interior surface  300  of the outer liner  202 . In other embodiments, one or more return springs  500  may directly couple the outer liner  202  to locations on the exterior surface  302  of the inner liner  204  that are not proximate an edge  306  of the inner liner  204 . 
     Some embodiments may employ one or more return springs  500  to return the inner liner  204  to the first position  414 . Other embodiments may employ additional, or alternative methods. For example, in some embodiments, the first gap  412  between the edge  306  and the arresting surface  402  may be empty. In other embodiments, the first gap  412  may contain a bumper composed of an elastic material, which may serve to absorb impact energy and return the inner liner  204  to the first position  414 . In some embodiments the shelf  400  may be integral to the outer liner  202 , and may be composed of the same material as the rest of the outer liner  202 . In other embodiments, the shelf  400  may be composed of an elastic material that may absorb additional impact energy transferred through motion of the inner liner  204  and assist in returning the inner liner  204  to the first position  414 . 
     As shown in  FIG.  3   , the outer liner  202  comprises a plurality of vents  316  that pass through the outer liner  202 , and the inner liner  204  comprises a plurality of channels  318  that pass through the inner liner  204 . As shown in  FIGS.  4  and  5   , the plurality of vents  316  at least partially overlap with the plurality of channels  318  to form a plurality of apertures  422  from outside the helmet to inside the helmet. According to various embodiments, the exterior surface  302  of the inner liner  204  and the interior surface  300  of the outer liner  202  may not be continuous, and may comprise vents, channels, openings, and/or other features which introduce voids in the surfaces. In some embodiments, including the non-limiting example shown in  FIGS.  2  through  5   , such voids may provide fluid communication between outside the helmet and a user&#39;s head, improving ventilation while the helmet is in use. In other embodiments, such voids may be employed to reduce the overall weight of a helmet. In still other embodiments, such voids may be employed for other reasons. While the following discussion will be in the context of vents  316  and channels  318 , it should be recognized that the methods and structures described may be applied to any other void in a rotation surface (e.g. exterior surface  302  of the inner liner  204 , interior surface  300  of the outer liner  202 , etc.). 
     While use of vents  316 , channels  318 , and/or apertures  422  in helmets is well known in the art, an inner liner  204  slidably coupled to the inside of an outer liner  202  through return springs  500  presents an issue not faced by conventional helmets. Therefore, according to various embodiments, the edges (i.e. the boundary where the liner surface tips inward to start a void in the surface) of vents  316  are shaped at the interior surface  300  and the edges of channels  318  are shaped at the exterior surface  302  such that rotation of the outer liner  202  with respect to the inner liner  204  is not impeded (e.g. the edge of a vent getting caught on the edge of a channel, etc.). 
     In some embodiments, including the non-limiting example shown in  FIGS.  2 - 5   , the vents  316  are beveled at the interior surface  300  of the outer liner  202 , and the channels are beveled at the exterior surface  302  of the inner liner  204 . In the context of the present description and the claims that follow, beveled means having a sloping edge. Examples of a sloping edge include but are not limited to one or more angled planes, and a curved surface. Thus, a vent  304  beveled at the interior surface  300  would, at least initially, narrow as it extends through the outer liner  202 . 
     As noted above, attenuation of rotational energy occurs when the exterior surface  302  of the inner liner  204  and the interior surface  300  of the outer liner  202  rotate against each other. In various embodiments, one or more of these surfaces may be modified to facilitate that rotation. For example, in one embodiment, the exterior surface  302  of the inner liner  204  may comprise a surface of reduced friction  322 , having been treated with a material to decrease friction. Materials include, but are not limited to, in-molded polycarbonate (PC), an in-molded polypropylene (PP) sheet, and/or fabric LFL. In other embodiments, a material or a viscous substance may be sandwiched between the two liners to facilitate rotation. 
     According to one embodiment, there may be an air gap  502  between the two liners, or between a majority of the exterior surface  302  of the inner liner  204  and the interior surface  300  of the outer liner  202 , to help allow for movement. For example, the air gap  502  between the two liners may range from 0.3 mm to 0.7 mm. In other embodiments, there may be other distances of air gap  502  between the two liners. 
     Where the above examples, embodiments and implementations reference examples, it should be understood by those of ordinary skill in the art that other helmet and manufacturing devices and examples could be intermixed or substituted with those provided. In places where the description above refers to particular embodiments of helmets and customization methods, it should be readily apparent that a number of modifications may be made without departing from the spirit thereof and that these embodiments and implementations may be applied to other to helmet customization technologies as well. Accordingly, the disclosed subject matter is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the disclosure and the knowledge of one of ordinary skill in the art.