Patent Publication Number: US-2022227456-A1

Title: Bicycle propulsion system for electric bicycle conversion

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a continuation application of U.S. patent application Ser. No. 16/793,588, filed on 18 Feb. 2020, which claims the benefit of U.S. Provisional Application No. 62/806,817, filed on 17 Feb. 2019, and U.S. Provisional Application No. 62/806,898, filed on 17 Feb. 2019, each of which is incorporated in its entirety by this reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to the field of electric bicycle propulsion and more specifically to a new and useful electric bicycle conversion system in the field of electric bicycle propulsion 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic representation of a system; 
         FIG. 2  is a schematic representation of the system; 
         FIG. 3  is a schematic representation of the system; 
         FIG. 4  is a schematic representation of the system; 
         FIG. 5  is a schematic representation of the system; 
         FIG. 6  is a schematic representation of the system; 
         FIG. 7  is a schematic representation of the system; 
         FIG. 8  is a schematic representation of the system; 
         FIG. 9  is a schematic representation of the system; 
         FIG. 10  is a schematic representation of the system; 
         FIG. 11  is a schematic representation of the system; 
         FIG. 12A  is a schematic representation of the system; 
         FIG. 12B  is a schematic representation of the system; and 
         FIG. 13  is a schematic representation of the system. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The following description of embodiments of the invention is not intended to limit the invention to these embodiments but rather to enable a person skilled in the art to make and use this invention. Variations, configurations, implementations, example implementations, and examples described herein are optional and are not exclusive to the variations, configurations, implementations, example implementations, and examples they describe. The invention described herein can include any and all permutations of these variations, configurations, implementations, example implementations, and examples. 
     1. Bicycle Propulsion System 
     As shown in  FIGS. 3 and 6 , a bicycle propulsion system  100  includes: a concentric rotor assembly  102 ; and a chassis assembly  104 . The concentric rotor assembly  102 : comprises a first rotor element  134  attached to a first sprocket bracket  151  configured to engage with a first bicycle sprocket; comprises a second rotor element  136  attached to a second sprocket bracket  152  configured to engage the first bicycle sprocket; and is configured to define a circular outer drive surface  132 , to define a circular inner retention surface  133 , and to transiently engage around the first bicycle sprocket in an engaged configuration of the concentric rotor assembly  102  via the first sprocket bracket  151  and the second sprocket bracket  152 . The chassis assembly  104 : is configured to transiently secure to a bicycle frame element; includes a retention subassembly configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104  while the concentric rotor assembly  102  is engaged around the first bicycle sprocket and the chassis assembly  104  is secured to the bicycle frame element; includes a drive subassembly configured to engage the concentric rotor assembly  102  via the circular outer drive surface  132 ; and comprises a motor  162  configured to rotate the concentric rotor assembly  102  about a center axis of the circular outer drive surface  132  via the drive subassembly, the motor  162  causing rotation of the first bicycle sprocket while the concentric rotor assembly  102  is engaged around the first bicycle sprocket. 
     One variation of the bicycle propulsion system  100 , shown in  FIGS. 3, 6, and 7 , includes: a concentric rotor assembly  102  and a chassis assembly  104 . In this variation, the concentric rotor assembly  102  is configured to rigidly and transiently engage around a sprocket of a bicycle and includes a set of sprocket brackets  150  arranged about the concentric rotor assembly  102 , the set of sprocket brackets  150  configured to engage with teeth of the sprocket of the bicycle. In this variation, the chassis assembly  104 : is configured to transiently secure to a frame element of the bicycle; comprises a retention subassembly configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104 ; includes a drive subassembly configured to engage the concentric rotor assembly  102 ; includes a motor  162  configured to rotate the concentric rotor assembly  102  about a center axis of the concentric rotor assembly  102  via the drive subassembly; includes a sensor arm  171  configured to engage with a chain of the bicycle via a chain roller  176  biased against the chain of the bicycle; and includes an electronics subsystem  180 . In this variation, this electronics subsystem  180  is configured to: detect deflection of the sensor arm  171  caused by tension in the chain of the bicycle; and activate the motor  162  to rotate the concentric rotor  130  based on the deflection of the sensor arm  171 . 
     One variation of the bicycle propulsion system  100 , shown in  FIG. 3 , includes a concentric rotor assembly  102  and a chassis assembly  104 . In this variation, the concentric rotor assembly  102 : defines a circular outer drive surface  132 ; defines an inner retention surface  133 ; comprises a set of sprocket brackets  150  arranged about the inner retention surface  133  of the concentric rotor assembly  102  and configured to engage with teeth of a bicycle sprocket; and is configured to transiently engage around the bicycle sprocket, wherein a center axis of the circular outer drive surface  132  is concentric with a rotational axis of the bicycle sprocket. In this variation, the chassis assembly  104 : is configured to transiently secure to a stay of the bicycle; includes a retention subassembly configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104 ; and includes a drive subassembly configured to engage the circular outer drive surface  132  of the concentric rotor assembly  102 ; and includes a motor  162  configured to rotate the concentric rotor assembly  102  about a center axis of the circular outer drive surface  132  via the drive subassembly. 
     2. Applications 
     Generally, as shown in  FIG. 1 , a bicycle propulsion system  100  includes: a rotor configured to transiently mount to a sprocket of a bicycle (e.g., a rear sprocket of the bicycle, front chainring of the bicycle); a chassis assembly configured to transiently mount to a frame element of the bicycle (e.g., a chain stay of the bicycle, a seat stay of the bicycle); and a motor arranged in the chassis assembly and configured to drive the rotor, thereby generating additional torque about the sprocket of the bicycle and assisting a rider operating the bicycle. 
     More specifically, as shown in  FIGS. 3, 4, 5, 6 and 7 , the bicycle propulsion system  100  includes: a concentric rotor assembly  102  including a set of sprocket brackets  150  configured to transiently engage the sprocket of a bicycle in an engaged configuration and configured to release from the sprocket in a disengaged configuration; and a chassis assembly  104  that secures to the concentric rotor assembly  102  in the engaged configuration, transiently mates to a chain stay or seat stay of the bicycle, and includes a motor  162  and drive subassembly that transmits torque to the concentric rotor assembly  102 . The concentric rotor assembly  102  is configured to open (or “split”) to enable installation over and removal from a sprocket of a rear cogset of the bicycle without necessitating removal of the rear wheel from rear dropouts of the bicycle frame. The concentric rotor assembly  102  is also configured to close and latch around the sprocket in an engaged configuration in which the outer surface of the concentric rotor assembly  102  forms a continuous, circular outer drive surface  132 ; and in which sprocket brackets  150 , extending toward the radial center of the concentric rotor assembly  102 , engage teeth of the sprocket in order to transmit torque between the outer drive surface  132  and the sprocket. The chassis assembly  104 : includes a set of rollers configured to engage and retain the concentric rotor assembly  102  in a “hub-less wheel” configuration when the concentric rotor assembly  102  is installed on a sprocket; includes an electronic motor; and includes a toothed drive belt  164  that runs between these rollers and the outer drive surface  132  of the concentric rotor assembly  102  and transmits torque from the electric motor into the concentric rotor assembly  102 , which then transmits torque into the sprocket via the sprocket brackets  150 . Furthermore, the chassis assembly  104  includes a boss or rest configured to engage a seat stay or chain stay near rear drops of the bicycle frame and thus prevent rotation of the chassis assembly  104  about a pitch axis of the bicycle when the motor is actuated; and a strap or other coupler configured to wrap around the seat stay or chain stay of the bicycle and thus constrain rotation of the chassis assembly  104  about a yaw axis of the bicycle while the concentric rotor assembly  102  and the rollers cooperate to constrain translation of the chassis assembly  104  and rotation of the chassis assembly  104  about a roll axis of the bicycle. 
     Thus, the chassis assembly  104  can be installed on a bicycle frame by wrapping the strap around a right seat stay or right chain stay of the bicycle near a rear dropout of the bicycle without additional tools. The concentric rotor assembly  102  can then be opened, a first sprocket bracket  151  that is coupled to a first rotor element  134  at a proximal end of the first sprocket bracket can be located on the innermost sprocket or the rear cogset of the bicycle. The first rotor element  134  can then be fed into the chassis assembly  104  and along the retaining rollers. The second sprocket bracket  152 , attached to a second rotor element  136 , can be closed against an opposite side of the sprocket and latched to the first rotor element  134  and, by extension, the first sprocket bracket  151  coupled thereto. The concentric rotor assembly  102  and the chassis assembly  104  can thus cooperate: to fully constrain the chassis assembly  104  on the bicycle frame; and to enable the concentric rotor assembly  102  to rotate with the rear wheel of the bicycle by passing along the rollers through the chassis assembly  104 . A chain roller  176 —coupled to a sensor subassembly  170 —can then be folded to engage and ride along the bicycle&#39;s chain, a battery assembly  106  can be set in a bolder holder mounted on the bicycle frame, and a power cable can be routed from the battery assembly  106  to the chassis assembly  104  to complete assembly of the bicycle propulsion system  100  on the bicycle, such as in under one minute. Later, these elements of the bicycle propulsion system  100  can be similarly removed from the bicycle over a similar duration of time in order to return the bicycle to an unassisted configuration. 
     Therefore, the bicycle propulsion system  100  includes a chassis assembly  104  and a concentric rotor assembly  102  that cooperate: to enable rapid installation onto a bicycle when a user desires pedal assistance (e.g., in preparation for a commute); and to enable rapid removal from this bicycle, such as when the user parks the bicycle in a public space or when the user no longer desires pedal assistance (e.g., in preparation for a bike ride with friends or when cycling in a park), all without additional tools and without necessitating removal of a wheel or other native component from the bicycle. 
     Furthermore, as shown in  FIGS. 12A and 12B  the bicycle propulsion system  100  can: monitor tension in the chain of the bicycle—which is related to application of torque to pedals of the bicycle by a user—via the chain roller  176  that rides on the chain and the sensor subassembly  170  coupled to the chain roller  176 ; interpret a target output torque or output power of the motor directly from this chain tension; and modulate the torque or power output of the motor accordingly. Because the sensor subassembly  170  and the chain roller  176  are integrated into the chassis assembly  104 , the bicycle propulsion system  100  can monitor this control signal (i.e., chain tension) and implement closed-loop controls to modulate output torque or output power of the motor without necessitating installation of an additional sensor or module on the bicycle. 
     Thus, the bicycle propulsion system  100  can be quickly installed on bicycles—of a wide range of geometries and sizes—by locating three subsystems (i.e., the concentric rotor assembly  102 , the chassis assembly  104 , and the battery assembly  106 ) on the bicycle without any additional tools. When in use, the bicycle propulsion system  100  can supply additional power to the rear cogset (e.g., a bicycle cassette, a freehub) of the bicycle in order to assist a cyclist in propelling the bicycle forward as a function of power output by the cyclist. Similarly, the bicycle propulsion system  100  can be quickly removed from the bicycle, again without additional tools. Therefore, the bicycle propulsion system  100  can enable convenient and temporary conversion of a purely-manual bicycle to an electric bicycle with pedal assistance and vice versa, thereby enabling a cyclist to rapidly and seamlessly transition a single bicycle between a purely-manual configuration (e.g., for sport) and an electric bicycle configuration (e.g., for commuting). 
     2.1 System Overview 
     In one example, the bicycle propulsion system  100  transfers power to the cogset of a bicycle via a concentric rotor assembly  102  installed around a sprocket of the bicycle (i.e. a bicycle sprocket in a cassette, freehub, or chainring). The bicycle propulsion system  100  can include a concentric rotor assembly  102  configured to engage with a rear sprocket of a bicycle or, more specifically, an innermost sprocket in the cogset of the bicycle. Thus, the bicycle propulsion system  100  occupies one sprocket in a derailleur-based transmission of a bicycle while in the engaged configuration. In an alternative implementation, the bicycle propulsion system  100  includes a concentric rotor assembly  102  configured to engage with a front chainring of a bicycle. 
     Therefore, in the engaged configuration, the concentric rotor assembly  102  is configured to circumscribe a sprocket and inwardly extends a set of sprocket brackets  150  (e.g., two or more) configured to engage the teeth of the sprocket, as shown in  FIG. 11 . The concentric rotor assembly  102 , in the engaged configuration shown in  FIG. 8 , also defines a circular outer drive surface  132  (with which the drive subassembly can engage) and a circular inner retention surface  133  (with which the retention subassembly can engage), thereby enabling components within the chassis assembly  104  to rotate the concentric rotor assembly  102  about a central axis aligned with the rotational axis of the sprocket, where the concentric rotor assembly  102  acts as a wheel in a hub-less wheel system. Additionally, the concentric rotor assembly  102  can include two hinged (shown in  FIG. 9 ) or fully separable members which, when engaged with each other, cause the set of sprocket brackets  150  to align with the sprocket, thereby rotationally constraining the sprocket relative to the concentric rotor assembly  102 , as shown in  FIG. 10 . 
     As shown in  FIGS. 3, 4, 5, 6, and 7 , the bicycle propulsion system  100  includes a chassis subsystem that secures the bicycle propulsion system  100  to a frame element of a bicycle and prevents rotation of the non-rotating components of the bicycle propulsion system  100  (such as the motor, electronics subsystem  180 , drive subassembly, and retention subassembly) about the sprocket&#39;s rotational axis upon application of torque to the concentric rotor assembly  102 . Therefore, the bicycle propulsion system can efficiently power transfer to the sprocket while preventing damage to the bicycle or the bicycle propulsion system  100  due to unintended movement of the bicycle propulsion system  100  relative to the frame of the bicycle. In addition to constraining the non-rotating components of the bicycles relative to the frame of the bicycle, the chassis assembly  104  houses the motor  162 , the electronics subsystem  180  that controls the motor  162 , the drive subassembly that transmits power from the motor  162  to the concentric rotor assembly  102 , and the retention subassembly that prevents translational movement of the concentric rotor assembly  102  relative to the chassis assembly  104  while enabling rotation of the concentric rotor assembly  102  as a hub-less wheel. 
     In one implementation, the bicycle propulsion system  100  is configured to secure to a rear chain stay of a bicycle. In this implementation, the chassis assembly  104 : includes an attachment element that secures the chassis assembly  104  to the right chain stay of the bicycle; and defines a form factor that houses the abovementioned subassemblies and components between the right chain stay, the rear wheel, and the rear derailleur of the bicycle. 
     In another implementation, the bicycle propulsion system  100  includes a chassis assembly  104  that further includes a sensor arm  171  extending from the chassis assembly  104  to engage with a chain of the bicycle, as shown in  FIG. 2 . The sensor arm  171  can include a chain roller  176  that is pressed or biased against the chain (e.g., via a spring acting on the sensor arm  171 ). Therefore, as a cyclist applies torque to the pedals of the bicycle, the chain tension increases, presses on the chain roller  176 , and causes the sensor arm  171  to deflect relative to its initial position. Thus, the bicycle propulsion system  100  can estimate the power applied by a cyclist to the pedals of the bicycle during operation and can scale the power output of the motor  162  based on this estimation. 
     In one implementation, the bicycle propulsion system  100  can include a separate battery assembly  106  connected to the chassis assembly  104  via a power cable  182  and/or a throttle assembly  108  for modifying the level of assistance output by the bicycle propulsion system  100 . Additionally, the bicycle propulsion system  100  can interface with an application installed on a mobile computation device (e.g., a smartphone, tablet, bike computer) to enable the cyclist to modify settings of the bicycle propulsion device. 
     2.2 Examples 
     In one example application of the bicycle propulsion system  100 , a cyclist may install the bicycle propulsion system  100  in order to convert her standard road bicycle into an electric bicycle to facilitate commuting or to traverse more difficult terrain. The cyclist can then easily remove the bicycle propulsion system  100 : to use the bicycle for exercise; to comply with legal restriction on electric bicycles in a particular area; to prevent theft of the bicycle propulsion system  100  while parking her bicycle; or for any other reason. Likewise, the cyclist can easily reinstall the bicycle propulsion system  100  whenever she desires pedal assistance. 
     In another example application, a bikeshare operator can install an instance of the bicycle propulsion system  100  on each bicycle in a fleet of bicycles in order to electrically assist users of this fleet of bicycles and improving the utility of these bicycles to commuters in an operational region. Upon mechanical failure of any bicycle propulsion system  100 , the bikeshare operator can remove the bicycle propulsion system  100  from the affected bicycle and replace the bicycle propulsion unit with a functional bicycle propulsion system  100  while the original bicycle propulsion system  100  installed on the affected undergoes repairs. Therefore, by utilizing the bicycle propulsion system  100 , as opposed to a pedal assistance system integrated with the bicycle, the bikeshare operator can minimize downtime in the fleet of electric pedal assist bicycles. 
     3. Concentric Rotor Assembly 
     Generally, the bicycle propulsion system  100  includes a concentric rotor assembly  102  that is clamped around or that otherwise engages a sprocket of a bicycle cogset, as shown in  FIGS. 1, 8, 9, and 10 . More specifically, the bicycle propulsion system  100  includes a concentric rotor assembly  102  configured to transiently engage around a sprocket of a bicycle and including a set of sprocket brackets  150  arranged about the concentric rotor assembly  102 , the set of sprocket brackets  150  configured to engage with teeth of a sprocket of the bicycle. Additionally, the bicycle propulsion system  100  includes a concentric rotor assembly  102  that: includes a circular outer drive surface  132  and a circular inner retention surface  133 , thereby defining surfaces for the retention subassembly to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104  and for the drive subassembly to transfer power from the motor  162  to the concentric rotor assembly  102 . Furthermore, as shown in  FIG. 9 , the bicycle propulsion system  100  can include a concentric rotor assembly  102  that further includes: a first rotor element  134  attached to a first sprocket bracket  151  and a second rotor element  136  attached to a second sprocket bracket  152 , where the concentric motor  162  assembly defines the circular outer drive surface  132  and the circular inner retention surface  133  during engagement of the first rotor element  134  and the second rotor element  136  in an engaged configuration. Thus, the bicycle propulsion system  100  includes a concentric rotor assembly  102  that can easily be engaged and disengaged from a sprocket of a bicycles and can efficiently and securely transfer power to the sprocket of the bicycles from the drive subassembly and motor  162  of the bicycle propulsion device. 
     In one implementation, as shown in  FIG. 9 , the bicycle propulsion system  100  includes a concentric rotor assembly  102  that further includes two approximately semicircular rotor elements attached at one end by a hinge and defining male and female components of a latch  140  on the first rotor element  134  and the second rotor element  136  respectively. Upon engagement of the first rotor element  134  with the second rotor element  136 , the first rotor element  134  and the second rotor element  136  define the circular outer drive surface  132  and the circular inner retention surface  133 . 
     In another implementation, the bicycle propulsion system  100  includes a concentric rotor assembly  102  that further includes two approximate semicircular rotor elements that a configured to be fully separable via two latch  140   s.  Therefore, a user can couple each end of the two rotor elements to the corresponding end of the opposite rotor element, thereby defining the circular outer drive surface  132  and the circular inner retention surface  133 . 
     Additional components and implementations of these components are described in further detail below. 
     3.1 Concentric Rotor 
     Generally, the bicycle propulsion system  100  includes a concentric rotor  130  as a primary component of the concentric rotor assembly  102 . More specifically, the bicycle propulsion system  100  can include a concentric rotor  130  including a centerless disk defining a circular outer drive surface  132  and defining a circular inner retention surface  133 , where the circular outer drive surface  132  defines a toothed (i.e. geared) surface and the circular inner retention surface  133  is characterized by a diameter greater than the bicycle sprocket with which the concentric rotor  130  is configured to engage. Additionally, the bicycle propulsion system  100  can include a concentric rotor  130  that defines a thickness such that the concentric rotor  130  is laterally stable under load when being driven by the drive subassembly and while engaged with a bicycle sprocket, such as a thickness between 0.5 centimeters and 1.5 centimeters. 
     In one implementation, the concentric rotor  130  is manufactured as a single piece of rigid material before being divided into two or more separate rotor elements. For example, the concentric rotor  130  can be manufactured from a metal such as stainless steel or aluminum alloy (such as 6061 or 7075). For example, the concentric motor  162  can be milled and/or lathed from a single piece of metal. Alternatively, the concentric rotor  130  can be stamped from a single piece of metal. However, the concentric rotor  130  can be manufactured in any other way. 
     In another implementation shown in  FIG. 8 , the concentric rotor  130  can include a set of slots in order to reduce the weight of the concentric rotor  130  while leaving sufficient material to maintain structural stability of the concentric rotor  130  under load from the drive subassembly. 
     3.2 Outer Drive Surface 
     Generally, the bicycle propulsion system  100  can include a concentric rotor  130  defining a geared outer drive surface  132 , in order to interface with the drive subassembly. In one implementation, the bicycle propulsion system  100  can include a concentric rotor  130  configured to interface (i.e. mesh) with a toothed drive belt  164  (i.e. timing belt) housed by the chassis assembly  104 . In this implementation, the outer drive surface  132  can define a set of curved teeth configured to interface with a rubber timing belt. Thus, by interfacing with a timing belt, the bicycle propulsion system  100  can reliably transfer power from the motor  162  to the concentric rotor  130  without lubrication or frequency maintenance. 
     In one implementation, the bicycle propulsion system  100 , via the retention subassembly, holds the concentric rotor assembly  102  relative to the chassis assembly  104  with a pair of retaining rollers that ride along the outer drive surface  132 . However, in order to prevent damage to the retaining rollers due to impact with the toothed outer drive surface  132 , the bicycle propulsion system  100  can include a chamfered edge at the base of the teeth of the outer drive surface  132  configured to engage the retaining rollers of the retention subassembly. 
     3.3 Inner Retention Surface 
     Generally, the bicycle propulsion system  100  can include a concentric rotor  130  defining an inner retention surface  133  at along its interior circular edge in order for the retention subassembly of the chassis assembly  104  to translationally constrain the concentric rotor  130  while feeding the concentric rotor  130  through the chassis assembly  104  such that the concentric rotor  130  rotates about its center axis. More specifically, the bicycle propulsion system  100  can include a concentric rotor  130  that defines a smooth inner retention surface  133  configured to engage with inner retaining rotors. Additionally, to prevent procession of the concentric rotor  130  during rotation, the concentric rotor  130  can define a circular inner retention surface  133  that is concentric with the outer drive surface  132  and the rotational axis of the sprocket with which the concentric rotor assembly  102  is configured to engage. In one implementation, the concentric rotor  130  defines an inner retention surface  133  that includes a chamfer corresponding to an interior chamfer of the inner retaining rollers  122  in the retention subassembly. 
     3.4 Rotor Elements 
     Generally, as shown in  FIGS. 6 and 7 , the bicycle propulsion assembly includes a concentric rotor  130  that further includes two (partially or completely) separable rotor elements, each rotor element defining an arc of the complete concentric rotor  130 , in order to enable a cyclist to open the concentric rotor  130  around the sprocket of the bicycle and clamp the concentric rotor  130  around this sprocket. More specifically, the concentric rotor assembly  102  includes: a first rotor element  134  attached to a first sprocket bracket  151  configured to engage with a bicycle sprocket; and a second rotor element  136  attached to a second sprocket bracket  152  configured to engage the bicycle sprocket. In one implementation, the concentric rotor assembly  102  includes: a first rotor element  134  attached to a first sprocket bracket  151  in a set of sprocket brackets  150 ; and a second rotor element  136  attached to a second sprocket bracket  152  in the set of sprocket brackets  150  and configured to transiently couple to the first rotor element  134  to define the circular outer drive surface  132  and the inner retention surface  133 . Thus, upon engagement of the first rotor element  134  with the second rotor element  136  (e.g., via latches and/or a hinge), the first rotor element  134  and the second rotor element  136  combine to define the circular outer drive surface  132  and the circular inner retention surface  133 . 
     In one implementation, the bicycle propulsion system  100  can include a concentric rotor  130  manufactured from a single piece of material prior to being cut into a first rotor element  134  and a second rotor element  136 , thereby ensuring a precise fit between the first rotor element  134  and the second rotor element  136 . 
     In another implementation, the bicycle propulsion system  100  can include a first rotor element  134  and a second rotor element  136  that are approximately equal in size to ensure approximately equal load is applied to each side of the sprocket of the bicycle via the first sprocket bracket  151  and second sprocket bracket  152  during rotation of the concentric rotor assembly  102 . 
     In yet another implementation, the bicycle propulsion system  100  can include additional rotor elements each attached to corresponding sprocket brackets  150  in order to more fully circumscribe the sprocket of the bicycle with sprocket brackets  150 . In this implementation, the set of rotor elements can include multiple latches and/or hinges to enable a user to engage the concentric rotor  130  around the sprocket of the bicycle. 
     3.4.1. Rotor Hinge and Latch 
     Generally, as shown in  FIG. 9 , the bicycle propulsion system  100  can include a set of rotor elements coupled by a rotor hinge  138  at one side of each rotor element and transiently connected, in an engaged configuration, by a latch  140 . More specifically, the bicycle propulsion system  100  can include a concentric rotor assembly  102  that further includes: a hinge connecting a first rotor element  134  to a second rotor element  136 , the hinge defining a rotational axis parallel to the center axis of the circular outer drive surface  132 ; a latch  140  inset into the first rotor element  134 ; and a locking pin within a lot of the second rotor element  136  configured to engage the latch  140  and prevent separation of the first rotor element  134  from the second rotor element  136  in the engaged configuration of the concentric rotor assembly  102 . Thus, a user may install and remove the concentric rotor assembly  102  around a sprocket of a bicycle without tools and within a short period of time and, while in the engaged configuration, the concentric rotor assembly  102  remains rigidly engaged around the bicycle sprocket sufficient to transfer torque from the drive subassembly to the sprocket of the bicycle. 
     The concentric rotor assembly  102  can include a latch  140  inset into a first end of the first rotor element  134  and a latching pin  146  traversing a slot in a second end of the second rotor element  136 . Thus, when a user brings the first end of the first rotor element  134  and the second end of the second rotor element  136  together and slots the latch  140  of the first rotor element  134  into the slot in the second rotor element  136 , the latch  140  latches around the latching pin  146 , thereby preventing disengagement of the first rotor element  134  from the second rotor element  136 . Additionally, the concentric rotor assembly  102  can include a latch  140  configured to release a latching pin  146  upon translation of a sliding member  148  mechanically coupled to the latch  140  and configured to enclose the latch  140  inset in the rotor element. 
     In one implementation, the concentric rotor assembly  102  can include a latch  140  that further includes a spring-loaded linear cam  142  configured to engage with a hooked follower  144 , as shown in  FIG. 9 .  FIG. 9  shows the latch  140  in the locked position despite showing the first rotor element  134  and the second rotor element  136  as separated from each other for clarity. Upon engagement of the rotor elements, the hooked follower  144  catches the latching pin  146  on the opposite rotor element and rotates about a follower pin  145  until the linear cam  142  can translate into a slot left by the rotation of the hooked follower  144 , thereby preventing back-rotation of the hooked follower  144  and, as a result, prevents disengagement of the latching pin  146  from the hooked follower  144 . The latch  140  can also include sliding member  148  (not shown for clarity in  FIG. 9 ) that is mechanically coupled to the linear cam  142  to enable the hooked follower  144  to back rotate such that, upon application of a force separating the first rotor element  134  from the second motor  162 , the latching pin  146  can be removed from the hook of the hooked follower  144  as the hooked follower  144  back-rotates. 
     In another implementation, the concentric rotor assembly  102  is configured to cooperate within the chassis assembly  104  in order to hide the latch  140  within the chassis assembly  104 , thereby preventing access to the latch  140  and effectively locking the concentric rotor assembly  102  around the sprocket of the bicycle for the purpose of theft prevention. More specifically, in this implementation, the chassis assembly  104  can include a solenoid, or another electromechanical latch within the chassis assembly  104 , configured to engage with a corresponding slot, an indentation, or the outer drive surface  132  of the concentric rotor  130  such that, while the solenoid or latch is engaged, the concentric rotor is locked in place and the latch  140  is concealed by the chassis assembly  104  (i.e., the outboard frame  114 ). Additionally, the latch or solenoid can be actuated by a physical key or via wireless communication with an application executing a mobile computation device of the cyclist in order to engage and remove the latch or solenoid from the slot of the concentric rotor  130 , thereby enabling the concentric rotor  130  to freely rotate again. Furthermore, the bicycle propulsion system  100  can: store a predefined position of the concentric rotor for which the latch  140  (and sliding member  148 ) is blocked against the interior surface of the outboard frame  14 ; and, in response to receiving a command to lock bicycle propulsion system  100  to the bicycle, the bicycle propulsion system can actuate the motor  162  to move the concentric rotor assembly  102  into the predefined position and engage an electromechanical pin preventing rotation of the concentric rotor  102 . Therefore, the bicycle propulsion system  100  can be locked to the frame of the bicycle remotely without physical intervention by a user. 
     In yet another implementation, the bicycle propulsion system  100  can include other locking mechanisms such as integrated U-locks, cable locks, or folding locks configured to secure the concentric rotor assembly  102  and/or the chassis assembly  104  to the frame or wheel of the bicycle. Additionally, the bicycle propulsion system  100  can include a GPS chip and an inertial measurement unit and can, while the bicycle is not in use (or upon activation of this security feature via mobile computational device of a user): detect movement of the bicycle and/or the bicycle propulsion system  100 ; and transmit the GPS location of the bicycle propulsion system to a mobile computational device of the user. Thus, the concentric rotor assembly  102  can define security features configured to prevent removal of the concentric rotor  130  from the sprocket and/or removal of the chassis assembly  104  from the bicycle. 
     However, the concentric rotor assembly  102  can include any type of latch  140  capable of securing the first rotor element  134  to the second rotor element  136  when engaged with the sprocket of the bicycle and under load by the drive subassembly. 
     3.5 Sprocket Brackets 
     Generally, as shown in  FIG. 11 , the concentric rotor assembly  102  can include a set of sprocket brackets  150  configured to engage with teeth of a bicycle sprocket such that torque applied to the concentric rotor  130  is transferred to the sprocket of the bicycle. More specifically, the concentric rotor assembly  102  can further include a set of sprocket brackets, each sprocket bracket defining: a set of outboard retaining teeth  154  configured to engage the outer surface of the sprocket of the bicycle; a set of inboard retaining teeth  155  offset from the outboard retaining teeth  154  by greater than the thickness of the sprocket of the bicycle and configured to engage the inner surface of the sprocket of the bicycle; and a set of engagement features  156  configured to engage with pitches of the sprocket of the bicycle arranged between the set of outboard retaining teeth  154  and the set of inboard retaining teeth  155 . Thus, the concentric rotor assembly  102  can engage with a sprocket of a bicycle via the set of sprocket brackets  150 . 
     The sprocket bracket can define a set of engagement features  156  that are configured to sit within the pitches (i.e. between the teeth or spurs) of the bicycle sprocket when the sprocket bracket is engaged with the sprocket of the bicycle. Therefore, the sprocket bracket can define engagement features  156  that include a series of half-cylindrical spurs mimicking one side of the rivets of a bicycle chain. In one implementation, the sprocket bracket can define engagement features  156  that include a series of half-cylindrical spurs that are characterized by a diameter less than the diameter of bicycle chain rivets configured to engage the bicycle sprocket. By including slightly smaller diameter engagement features  156  than the rivets of a bicycle chain matched to the bicycle sprocket, the sprocket bracket can more easily be installed onto the bicycle sprocket. 
     Additionally, the sprocket bracket can define a set of inboard retaining teeth  155  and outboard retaining teeth  154  on either side of the engagement features  156  in order to prevent lateral disengagement of the sprocket bracket from the sprocket of the bicycle (e.g., due to non-axial torque applied to the concentric rotor assembly  102 ). Therefore, the sprocket bracket can include inboard retaining teeth  155  and outboard retaining teeth  154  characterized by a thickness less than the intra-sprocket spacing of the cogset of the bicycle. Furthermore, the sprocket bracket can include inboard retaining teeth  155  and outboard retaining teeth  154  that alternate on either side of the engagement features  156  in order facilitate engagement of the sprocket bracket with the sprocket of the bicycle by a user of the bicycle propulsion system  100 , as shown in  FIG. 11 . 
     The concentric rotor assembly  102  can include a set of sprocket brackets  150  with engagement features  156 , inboard retaining teeth  155 , and outboard retaining teeth  154 , configured to engage with a sprocket of a particular size (i.e. number of teeth), with a sprocket configured for a particular chain standard (e.g., half-inch pitched chain, eighth-inch chain, three-sixteenths-inch chain, 5.3-millimeter chain, 5.5-millimeter chains, six-millimeter chain, 6.5-millimeter chain, and/or seven-millimeter chain), and with a sprocket characterized by a particular sprocket spacing. Thus, the sprocket bracket can define engagement features  156 , inboard retaining teeth  155 , and outboard retaining teeth  154 , characterized by dimensions corresponding to the sprocket of the bicycle with which the sprocket bracket is configured to engage. 
     In one implementation, each sprocket bracket in the set of sprocket brackets  150  defines an engagement arc characterized by a radius equal to or greater than the pitch radius of the sprocket of the bicycle with which the sprocket bracket is configured to engage. Thus, the curvature of each sprocket bracket in the set of sprocket brackets  150  approximately matches the curvature of the bicycle sprocket with which the sprocket bracket is configured to engage. 
     In another implementation, the concentric rotor assembly  102  can also include a set of sprocket brackets  150  that define a total arc length that is greater than 25% of the pitch circumference of the bicycle sprocket. Thus, in implementations of the concentric rotor assembly  102  that include a first sprocket bracket  151  and a second sprocket bracket  152 , the first sprocket bracket  151  and the second sprocket bracket  152  can be configured to engage with greater than twenty five percent of teeth of the first bicycle sprocket in the engaged configuration of the concentric rotor assembly  102 . For example, the concentric rotor assembly  102  can include a first sprocket bracket  151  attached to a first rotor element  134  and a second sprocket bracket  152  attached to a second rotor element  136  configured to engage a sprocket defining 28 teeth. In this example, the first sprocket bracket  151  and the second sprocket bracket  152  together define an arc length and engagement features  156  configured to engage with at least seven teeth of the sprocket. 
     In yet another implementation, the concentric rotor assembly  102  can include a set of sprocket brackets  150  that define a total arc length less than sixty percent of the pitch circumference of the bicycle sprocket. In this implementation, the set of sprocket brackets  150  can engage with less than 60% of the teeth of the bicycle sprocket. For example, the concentric rotor assembly  102  can include a first sprocket bracket  151  attached to a first rotor element  134  and a second sprocket bracket  152  attached to a second rotor element  136  configured to engage a sprocket defining 28 teeth. In this example, the first sprocket bracket  151  and the second sprocket bracket  152  together define an arc length and engagement features  156  configured to engage with sixteen or fewer teeth of the sprocket. 
     Generally, each sprocket bracket in the set of sprocket brackets  150  attaches to a corresponding rotor element via a set of sprocket struts configured to secure to a face of the concentric rotor  130 , as shown in  FIG. 5 . In one implementation, the set of sprocket struts define a set of threaded bores aligned with threaded bores inset into a face of the concentric rotor  130 , as shown in  FIG. 11 . Thus, the set of sprocket brackets  150  are replaceable and/or exchangeable by a user of the bicycle propulsion system  100 . 
     In one implementation, the concentric rotor assembly  102  includes a set of sprocket brackets  150  configured to engage with an innermost bicycle sprocket in a bicycle cogset (e.g., the largest-diameter sprocket in the cogset). More specifically, in implementations of the bicycle propulsion system  100  including a first sprocket bracket  151  and a second sprocket bracket  152 : the first sprocket bracket  151  is further configured to engage with an innermost bicycle sprocket in a bicycle cogset; the second sprocket bracket  152  is further configured to engage the innermost bicycle sprocket; and the concentric rotor assembly  102  is further configured to transiently engage around the innermost bicycle sprocket of the bicycle via the first sprocket bracket  151  and the second sprocket bracket  152  in the engaged configuration of the concentric rotor assembly  102 ; the retention subassembly is further configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104  while the concentric rotor assembly  102  is engaged around the innermost bicycle sprocket and the chassis assembly  104  is secured to the bicycle frame element; and the motor  162  is further configured to rotate the concentric rotor assembly  102  about the center axis of the circular outer drive surface  132  via the drive subassembly, the motor  162  causing rotation of the innermost bicycle sprocket while the concentric rotor assembly  102  is engaged around the second bicycle sprocket. Thus, the concentric rotor assembly  102  can include a set of sprocket brackets  150  configured to attach to the inboard side of the concentric rotor  130  to avoid interference with other sprockets of the bicycle and defining a curve back outward such that the engagement features  156  are located between planes defined by the inboard and outboard faces of the concentric rotor  130 , as shown in  FIG. 10 . In this implementation, the set of sprocket brackets  150  can define filleted edges to reduce force concentration in each sprocket bracket. 
     The set of sprocket brackets  150  can be manufactured from any hard-wearing and lightweight material capable of transferring torque from the concentric rotor  130  to the sprocket of the bicycle, such as aluminum or steel. The set of sprocket brackets  150  can be manufactured via stamping milling, additive manufacturing, or any other manufacturing techniques. 
     3.5.1 Sprocket Bracket Kit 
     In one implementation, the bicycle propulsion system  100  includes multiple sets of sprocket brackets  150 , each set configured to engage with a different type of bicycle sprocket (e.g., for sprockets defining a different number of teeth or in compliance with a different standard). More specifically, in implementations of the bicycle propulsion system  100  including a first sprocket bracket  151  and a second sprocket bracket  152  in a first set of sprocket brackets  150 : the first sprocket bracket  151  is further configured to engage the first bicycle sprocket, the first bicycle sprocket characterized by a first number of teeth; the second sprocket bracket  152  is further configured to engage the first bicycle sprocket, the first bicycle sprocket characterized by the first number of teeth. The bicycle propulsion system  100  can further include: a third sprocket bracket configured to attach to the first rotor element  134  in replacement of the first sprocket bracket  151  and configured to engage with a second bicycle sprocket, the second bicycle sprocket characterized by a second number of teeth different from the first number of teeth; and a fourth sprocket bracket configured to attach to the second rotor element  136  in replacement of the second sprocket bracket  152 ; and configured to engage the second bicycle sprocket the second bicycle sprocket characterized by the second number of teeth. In this implementation of the bicycle propulsion system  100 : the concentric rotor assembly  102  is further configured to transiently engage around the second bicycle sprocket in the engaged configuration of the concentric rotor assembly  102  via the third sprocket bracket and the fourth sprocket bracket; the retention subassembly is further configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104  while the concentric rotor assembly  102  is engaged around the second bicycle sprocket and the chassis assembly  104  is secured to the bicycle frame element; and the motor  162  is further configured to rotate the concentric rotor assembly  102  about the center axis of the circular outer drive surface  132  via the drive subassembly, the motor  162  causing rotation of the second bicycle sprocket while the concentric rotor assembly  102  is engaged around the second bicycle sprocket. Thus, the bicycle propulsion system  100  can include a kit of sprocket brackets  150  including multiple sets of sprocket brackets  150 , where each set is configured to engage with a particular type of cogset. The bicycle propulsion system  100  can therefore engage with a number of different types of cogsets defining varying numbers of teeth, chain standards, or sprocket spacing by exchanging one set of sprocket brackets  150  for another set of sprocket brackets  150 . 
     4. Chassis Assembly 
     Generally, as shown in  FIGS. 3, 4, 5, 6, and 7 , the bicycle propulsion system  100  includes a chassis assembly  104  that: houses the retention subassembly that translationally constrains the concentric rotor assembly  102  relative to the chassis assembly  104 ; houses the drive subassembly that is configured to transfer power from the motor  162  to the concentric rotor assembly  102 ; houses the electronics subsystem  180  that controls the motor  162  and executes pedal assist and safety processes; and secures the bicycle propulsion system  100  to the frame of the bicycle in order to prevent rotation of the system relative to the frame of the bicycle while the concentric rotor assembly  102  is in the engaged configuration. More specifically, the bicycle propulsion system  100  includes a chassis assembly  104 : configured to transiently secure to a stay of the bicycle; comprising a retention subassembly configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104 ; comprising a drive subassembly configured to engage the circular outer drive surface  132  of the concentric rotor assembly  102 ; and a motor  162  configured to rotate the concentric rotor assembly  102  about a center axis of the circular outer drive surface  132  via the drive subassembly. Additionally, in implementations where the bicycle propulsion system  100  secures to another frame element of the bicycle, the bicycle propulsion system  100  includes a chassis assembly  104 : configured to transiently secure to a bicycle frame element; comprising a retention subassembly configured to translationally constrain the concentric rotor assembly  102  relative to the chassis assembly  104  while the concentric rotor assembly  102  is engaged around the first bicycle sprocket and the chassis assembly  104  is secured to the bicycle frame element; comprising a drive subassembly configured to engage the concentric rotor assembly  102  via the circular outer drive surface  132 ; and comprising a motor  162  configured to rotate the concentric rotor assembly  102  about a center axis of the circular outer drive surface  132  via the drive subassembly, the motor  162  causing rotation of the first bicycle sprocket while the concentric rotor assembly  102  is engaged around the first bicycle sprocket. Thus, the chassis assembly  104  houses and locates the motor  162 , the drive subassembly, and the retention subassembly such that the motor  162  transfers torque to the concentric rotor assembly  102  via the drive belt  164 . The concentric rotor assembly  102  then transfers this torque to the sprocket via the set of sprocket brackets  150 , thereby assisting the cyclist in applying torque to the sprocket of the bicycle. 
     The chassis assembly  104  includes a chassis that houses the retention subassembly, the drive subassembly, the motor  162 , and the electronics subsystem  180 . The chassis assembly  104  can include a chassis configured to house the abovementioned subassemblies and subsystems within a form factor that fits within the chain stay and/or seat stay of most bicycles. 
     In one implementation, as shown in  FIGS. 3, 4, 6, 6, and 7 , the chassis includes: an outboard frame  114 ; an outboard frame  114  parallel to the inboard frame  116 ; an electronics housing; and a motor cowling  113 . In this implementation, the outboard frame  114  and the inboard frame  116  are separated by a set of standoffs  118  fastened to the outboard frame  114  and the inboard frame  116  via a set of threaded bores in the outboard frame  114  and the inboard frame  116 . Thus, the outboard frame  114  and the inboard frame  116  contain the retention subassembly and the drive subassembly between them. In this implementation, the electronics subsystem i 8 o and motor  162  are attached outboard of the outboard frame  114  and are housed within the electronics housing and motor cowling  113  respectively. Thus, the chassis assembly  104  can define distinct regions for the mechanical and electronic components of the bicycle propulsion system  100 . 
     The chassis assembly  104  can include an outboard frame  114  and an inboard frame  116  stamped from aluminum, steel or any other rigid material in order to support the retention subassembly and the drive subassembly. The chassis assembly  104  can also include an outboard frame  114  and an inboard frame  116  that define attachment points for the axles of rollers and gears (from the retention subassembly and the drive subassembly) and the motor  162  axle, thereby locating each of these components relative to each other. The chassis assembly  104  can also include an outboard frame  114  that further defines attachment points for the motor  162 , the electronics subsystem  180 , the electronics housing and the motor cowling  113 . In one implementation, the chassis assembly  104  can include an outboard frame  114  that includes an attachment point for a sensor arm  171 . In another implementation, the chassis assembly  104  can include an outboard frame  114  defining a derailleur stop  115 , configured to extend into the path of a derailleur of the bicycle, shown in  FIGS. 3, 4, and 5 , in order to prevent the derailleur of the bicycle from shifting the bicycle chain into the sprocket with which the concentric rotor  130  is engaged, thereby preventing physical interference between the derailleur of the bicycle and/or the chain of the bicycle with the bicycle propulsion system  100 . Thus, the chassis assembly  104  includes comprises a derailleur stop  115  configured to prevent a derailleur of the bicycle from shifting into the first bicycle sprocket. 
     The chassis assembly  104  can include an electronics housing manufactured from a hard plastic or other rigid, non-conductive material in order to prevent dirt and/or water ingress to the electronics subsystem  180  housed by the electronics housing, while also enabling wireless communication between the electronics subsystem  180  and a personal computing device of a user. The chassis assembly  104  can include an electronics housing manufactured via molding (e.g., injection molding) or additive manufacturing processes. 
     The chassis assembly  104  can also include a motor cowling  113  configured to surround the motor  162  and prevent physical damage to the motor  162  upon incidental impact. The motor  162  itself can include an additional waterproof housing separate from the motor cowling  113 . In one implementation, the chassis assembly  104  includes a single plastic member that functions as both the electronics housing and the motor cowling  113 . 
     The chassis assembly  104  also includes an attachment mechanism configured to transiently secure the chassis assembly  104  to a frame element of the bicycle in order to prevent rotation of the chassis assembly  104  about the concentric rotor assembly  102 , upon application of torque to the concentric rotor assembly  102  by the chassis assembly  104 . In one implementation, the chassis assembly  104  includes an attachment mechanism configured to attach the chassis assembly  104  to the drive-side chain stay of the bicycle. In this implementation, the motor  162  and motor cowling  113  can be positioned below the attachment mechanism such that, while the bicycle propulsion system  100  is engaged with the bicycle, the motor  162  and motor cowling  113  can extend outboard from the outboard frame  114  beneath the drive side chain stay of the bicycle. In this implementation, the chassis assembly  104  can include a flexible rubber or fabric strap configured to wrap around the chain stay of the bicycle and connect to the outboard face of the chassis assembly  104 . However, the chassis assembly  104  can include other types of attachment mechanisms such as a clamp- or latch-based attachment mechanism. 
     4.1 Retention Subassembly 
     Generally, as shown in  FIG. 6 , the chassis assembly  104  includes a retention subassembly that further includes a set of inner retaining rollers  122  and a set of outer retaining rollers  124  configured to locate the concentric rotor assembly  102  within the chassis assembly  104  such that the drive belt  164  engages the outer drive surface  132  of the concentric rotor assembly  102  while also enabling the concentric rotor  130  to rotate about its center axis (e.g., as a hub-less wheel) when torque is applied to the concentric rotor via the drive belt  164 . More specifically, the chassis assembly  104  includes a retention subassembly further including a set of retaining rollers configured to translationally constrain the concentric rotor  130  subsystem as a hub-less wheel via contact with the inner retention surface  133  and the circular outer drive surface  132 . Additionally, the chassis assembly  104  can include a retention subassembly that does not interfere with the teeth on the outer drive surface  132  of the concentric rotor assembly  102 , thereby reducing wear on and excess noise produced by the retention subassembly during operation of the bicycle propulsion system  100 . Furthermore, the chassis assembly  104  can include a retention subassembly that enables removal of the concentric rotor assembly  102  from the chassis assembly  104  such that a user may perform maintenance on the mechanical components of the bicycle propulsion system  100 . 
     The retention subassembly includes a set of inner retention rollers configured to ride along the inner retention surface  133  of the concentric rotor assembly  102  without interfering with the set of sprocket brackets  150  arranged about the inner retention surface  133  of the concentric rotor assembly  102 . In one implementation, the retention subassembly includes two inner retention rollers to constrain (in combination with the set of outer retention rollers) the concentric rotor assembly  102  in two dimensions coplanar with the rotational plane of the concentric rotor assembly  102 . In another implementation, the retention subassembly includes inner retention rollers defining a slotted outer surface and defining a chamfer on either side of the slotted surface such that the inner retention rollers fit across the corresponding inner retention surface  133  of the concentric rotor assembly  102 , thereby laterally constraining the concentric rotor assembly  102  within the slotted surfaces of the retention rollers. In this implementation, the retention subassembly can include a set of retention rollers defining asymmetrical slots such that the inboard side of the retention rollers in the set of retention rollers can clear the sprocket brackets  150  attached on the inboard side of the concentric rotor assembly  102 . 
     The retention subassembly includes a set of outer retention rollers configured to ride along a chamfered edge of the outer drive surface  132  of the concentric rotor assembly  102 . Thus, the retention subassembly contains the concentric rotor assembly  102  between the set of outer retention rollers and the set of inner retention rollers. In one implementation, the retention subassembly includes a set of outer retention rollers including two outer retention rollers. In another implementation, the retention subassembly can include a set of outer retention rollers can define a slotted outer surface such that the teeth of the outer drive surface  132  do not come into contact with the outer retention rollers and instead the outer retention rollers contact the chamfered surface of the concentric rotor assembly  102 . 
     In one implementation, the retention subassembly includes rollers manufactured from polyoxymethylene, molybdenum-disulfide-filled nylon, or any other hardwearing plastic. 
     4.2 Drive Subassembly 
     Generally, as shown in  FIG. 6 , the chassis assembly  104  includes a drive subassembly, in order to transfer torque and power from the motor  162  to the concentric rotor assembly  102 . More specifically, the chassis assembly  104  can include a drive subassembly that further includes: a drive gear  166  coupled to the motor  162 ; a drive belt  164  configured to engage the drive gear  166  and the circular outer drive surface  132  of the concentric rotor assembly  102 ; and a set of drive belt rollers  168  configured to maintain engagement of the drive belt  164  with the drive gear  166  and with the outer drive surface  132  of the concentric rotor assembly  102 . Thus, the drive subassembly, by including the drive belt  164  as the primary wear component of the bicycle propulsion system, can operate with no grease, thereby reducing maintenance overhead, while producing less noise when compared to a chain or meshed gear transmission system. Additionally, the drive belt  164  can be easily removed from the drive gear  166  and drive belt rollers  168  and replaced further improving the serviceability of the bicycle propulsion system  100 . 
     The drive subassembly can include a drive gear  166  that shares an axle with the motor  162  and functions to transfer power to the drive belt  164 . The drive belt  164  is then directed within the confines of the inboard frame  116  and the outboard frame  114 , via the set of drive belt rollers  168 , to conform with an arc of the outer drive surface  132  of the concentric rotor assembly  102 , while the concentric rotor assembly  102  is engaged with the chassis assembly  104 . In one implementation a first pair of drive belt rollers  168  located proximal to the drive gear  166  maintain tension in the drive belt  164  around the drive gear  166  while a third drive belt roller  168  extends the drive belt  164  toward an upper side of the chassis assembly  104  such that the drive belt  164  meshes with the outer drive surface  132  of the concentric rotor assembly  102  over a large arc, thereby distributing torque transfer across a longer length of the drive belt  164  in order to further reduce maintenance frequency of the bicycle propulsion system. In another implementation, the drive subassembly can include a set of drive rollers  168  defining a smooth outer surface and configured to engage the smooth side of the drive belt  164  in order to direct the drive belt  164  around the drive gear  166  and around the outer drive surface  132  of the concentric rotor assembly  102 . 
     The drive subassembly can include a geared jockey (or idler) pulley configured to redirect and tension a section of the drive belt  164  between the pair of drive rollers  168  proximal to the drive gear  166  and the drive roller located on the upper end of the chassis assembly  104 . In implementations where the chassis assembly  104  defines a different form factor than the form factor shown in  FIGS. 3, 4, 5, 6, and 7 , the drive subassembly can include different and/or additional drive rollers  168  and/or jockey pulleys  169  in order to position the drive belt  164  around the drive gear  166  and around a portion of the outer drive surface  132  of the concentric rotor assembly  102 . 
     In one implementation, the drive belt  164  includes a timing belt. Alternatively, the drive subassembly can include a friction belt. In this implementation, the drive gear  166  is replaced with a drive wheel, and the drive rollers  168  and jockey wheel are configured to increase the tension in the friction belt when compared to the timing belt. 
     In another implementation, the drive subassembly can include a planetary gearbox arranged between the motor and the drive gear  166  and configured to transfer torque between the drive gear  166  and the motor  162 , thereby reducing backlash between the drive gear  166  and the motor  162 . In this implementation, the planetary gearbox can be configured with the drive gear  166  as the sun gear in the planetary gearbox. Alternatively, the planetary gearbox can be configured with the drive gear as the ring gear in the planetary gearbox. 
     In yet another implementation, the drive subassembly can include a gearbox (e.g., a planetary gearbox) in replacement of the drive-belt-based system described above. In this implementation, the drive subassembly can include a gearbox arranged, within the chassis assembly  104 , between the drive gear  166  and the outer drive surface  132 , when the bicycle propulsion system  100  is in the engaged configuration. In one example, the drive subassembly can include a planetary gearbox (e.g., a single stage planetary gearbox), where the drive gear  166  is configured as a sun gear in the planetary gearbox and the carrier of the planetary gearbox is configured to transfer torque to the outer drive surface  132  (e.g., via a toothed concentric surface). In another example, the drive subassembly can include a planetary gearbox, where the drive gear  166  is configured as a sun gear in the planetary gearbox and the ring gear of the planetary gearbox is configured to transfer torque to the outer drive surface  132  of the concentric rotor assembly  102 . 
     However, the drive subassembly can include additional components configured to transfer torque between the motor  162  and the concentric rotor assembly  102  via the outer drive surface  132 . 
     4.3 Motor 
     Generally, as shown in  FIGS. 3, 4, and 7 , the chassis assembly  104  includes a motor  162  configured to transfer torque to the drive subassembly via the drive gear  166 , which then transfers torque to the concentric rotor assembly  102 , causing rotation of the concentric rotor assembly  102  and, therefore, the sprocket to which the concentric rotor assembly  102  is engaged. In one implementation, the motor  162  includes a compact electric motor  162 , such as a radial flux or axial flux motor  162 . 
     The motor  162  can be coupled to the outboard frame  114  of the chassis assembly  104 , thereby preventing interference between the motor  162  and the wheel of the bicycle. The motor  162  also includes an output shaft extending through the outboard frame  114  into the internal volume of the chassis assembly  104 . This output shaft is coupled to the drive gear  166  of the drive subassembly and transfers power to the drive belt  164 . 
     In one example, the motor  162  is characterized by a peak power output of greater than 1000 watts and characterized by a sustained power output of 350 watts in order to sufficiently augment the power of the cyclist over a sustained period of time. Additionally or alternatively, the chassis assembly  104  can include a motor  162  that is electronically limited (e.g., to an output of 350 watts) in order to comply with regional government regulations for motorized vehicles. 
     In one implementation, the chassis assembly  104  includes a clutch interposed between and configured to selectively engage the output shaft and the drive gear  166  of the drive subassembly. In this implementation, the bicycle propulsion system  100  can engage the clutch upon activation of the motor  162  and can disengage the clutch upon deactivation of the motor  162 , or while coasting, in order to reduce friction on the drive train in these circumstances due to internal resistance of the motor  162  to free rotation of the output shaft. The cutch can also be configured to disengage the output shaft and the drive gear  166  by default, thereby limiting motor drag on the rear wheel when the bicycle propulsion system  100  is off or when the battery assembly  106  is discharged. 
     4.4 Sensor Subassembly 
     Generally, as shown in  FIGS. 3, 4, 12A and 12B , the chassis assembly  104  can include a sensor subassembly  170  configured to detect power applied to the bicycle by the cyclist during operation of the bicycle propulsion system  100 , thereby enabling the electronics subsystem  180  to execute closed-loop control of the motor  162  in order to assist the cyclist in propelling the bicycle based on the current effort of the cyclist. In one implementation, the sensor subassembly  170  includes a sensor arm  171  attached to a chain roller  176  configured to measure tension in the chain of the bicycle. In another implementation, the sensor subassembly  170  is integrated into the motor  162  housing and configured to measure the pressure of the motor  162  housing against a chain stay of the bicycle. 
     In addition to the implementations described below, the sensor subassembly  170  can estimate the power input to the bicycle by the cyclist in any other way (such as by utilizing a separate power communicating with the bicycle propulsion system  100 ). 
     4.4.1 Sensor Arm 
     In one implementation shown in  FIGS. 12A and 12B , the sensor subassembly  170  can include a sensor arm  171  configured to: extend from the chassis assembly  104  to the chain of the bicycle; include a chain roller  176  configured to engaged with the chain of the bicycle; and configured to deflect based on the tension in the chain of the bicycle. More specifically, the sensor subassembly  170  includes a sensor arm  171  configured to engage with a bicycle chain via a chain roller  176  biased against the chain of the bicycle while the concentric rotor assembly  102  is engaged around the first bicycle sprocket and the chassis assembly  104  is secured to the bicycle frame element; and an electronics subsystem  180  configured to detect deflection of the sensor arm  171  caused by tension in the bicycle chain and activate the motor  162  to rotate the concentric rotor  130  based on the deflection of the sensor arm  171 . Thus, the sensor subassembly  170  can detect the tension in the chain of the bicycle such that the electronics subsystem  180  can estimate an applied power by the cyclist based on this detected tension, and execute closed-loop control of the motor  162  based on this estimated power. 
     In one implementation, the sensor subassembly  170  includes a sensor arm  171  that is biased against the chain of the bicycle by a spring at one end and engages with the chain with a chain roller  176  at the opposite end. More specifically, the sensor subassembly  170  includes: a chain roller  176  coupled to the sensor arm  171  at a first end; and a biasing spring coupled to a second end of the sensor arm  171  and the chassis assembly  104  and configured to bias the chain roller  176  against the chain of the bicycle. Thus, the sensor assembly includes a sensor arm  171  configured as a lever with a sensor arm  171  axle as a fulcrum with a spring attached at one end of the sensor arm  171  biasing the opposite end toward the chain of the bicycle. 
     The sensor subassembly  170  further includes a chain roller  176  in order to engage with the chain and ensure that deflection of the sensor arm  171  is not due to the shape of the chain and is instead caused by the tension in the chain. Thus, the chain roller  176  can define a pitched surface configured engage the links of the chain to reduce periodic deflection of the chain roller  176  as the chain roller  176  rolls along the chain. As shown in  FIG. 13 , the chain roller  176  can define a pitched surface and is constructed from: a set of pitched shells  185  installed around the roller axle, the pitched shells defining a series of valleys  187  and peaks  189 , where the distance between consecutive valleys and consecutive peaks is equal to the pitch of the bicycle chain. Additionally, the chain roller  176  can include a rubber sleeve  191  configured to be tensioned around the outside surface of the installed pitched shells  185 . 
     As shown in  FIG. 2 , the sensor subassembly  170  can include a chain roller  176  that is biased downward toward the chain such that the angle a is less than  180  degrees. Additionally, the sensor subassembly  170  can include a chain roller  176  that extends across the full length of the cogset of the bicycle to ensure contact with the chain irrespective of the current gear selection of the cyclist. Due to the changes in the angle of the chain of the bicycle dependent on the current gear selection, the sensor assembly can be configured to remain biased against the chain for the full range of possible chain angles corresponding to the full range of possible gear selections for a typical bicycle (e.g., an 8-speed, 9-speed, 10-speed, 11-speed, 12 speed and/or a 13-speed system). 
     In one implementation, shown in  FIGS. 12A and 12B , the sensor assembly includes a sensor arm  171  further including a pivot  178  attached to an axle of the chain roller  176 , where the pivot  178  is configured to bias the chain roller  176  against the chain of the bicycle and position the roller axle  177  perpendicular to the chain of the bicycle in a first position (shown in  FIG. 12A ); and configured to remove the chain roller  176  from the chain of the bicycle in a second position (shown in  FIG. 12B ). Thus, during installation of the bicycle propulsion system  100  by a user, the user may fold the chain roller  176  such that the roller axle  177  is coplanar with the outboard frame  114  of the chassis assembly  104 , thereby facilitating installation by preventing the chain roller  176  from being caught on the chain while the bicycle propulsion system  100  is moved into position at the chain stay of the bicycle. 
     In another implementation, the sensor assembly can include a sensor arm  171  further including: a chain roller  176  coupled to the sensor arm  171  at a first end; and a magnet  175  coupled to a second end. In this implementation, the electronics subsystem  180  (further described below) includes a Hall effect sensor proximal to the second end of the sensor arm  171  and is configured to detect deflection of the sensor arm  171  via the Hall effect sensor based on displacement of the magnet  175 . Thus, by the inclusion of a magnet  175  at one end of the sensor arm  171 , the bicycle propulsion system  100  can measure the deflection of sensor arm  171  due to tension in the chain of the bicycle via one or more Hall effect sensors arranged within the electronics subsystem  180  proximal to the second end of the sensor arm  171 . 
     4.4.2 Pressure Sensor 
     In one implementation, the sensor assembly includes a pressure sensor integrated into the top side of the motor cowling  113  or electronics housing and configured to measure the pressure applied by the bicycle propulsion system  100  on the chain stay of the bicycle. Due to the arrangement of the motor cowling  113  below the chain stay of the bicycle in this implementation, an increase in torque applied by the motor  162  compared to torque applied by the cyclist increases the pressure exerted by the chassis assembly  104  on the chain stay. Therefore, by measuring the pressure in this location, the bicycle propulsion system  100  can correlate this pressure with the power input to the bicycle by the cyclist and adjust the power of the motor  162  accordingly. 
     4.5 Electronics Subsystem 
     Generally, as shown in  FIG. 7 , the chassis assembly  104  includes an electronics subsystem  180  that can further include a controller, a 6-axis inertial measurement unit (or a 3-axis accelerometer and a 3-axis gyroscope), and/or a set of Hall effect sensors. Thus, the electronics subsystem  180  can regulate power from the battery assembly  106  to the motor  162  in order to selectively apply torque to the sprocket of the bicycle in response to riding conditions detectable by the inertial measurement unit and the set of Hall effect sensors in cooperation with the sensor subassembly  170 . Additionally, the electronics subsystem  180  can measure the orientation of the chassis assembly  104  relative to the ground and estimate the speed of the bicycle in order to identify whether the bicycle propulsion system  100  is operating with its safe operational envelope. Furthermore, the electronics subsystem  180  can wirelessly communicate with a mobile computation device—such as smartphone, tablet, or smartwatch worn or carried by the cyclist—in order to report ride-related data such as the current battery charge, the current level of pedal assistance, and/or the current operating power of the motor  162 . 
     Generally, the controller can include a processor configured to execute operational envelope detection and pedal assistance algorithms of the bicycle propulsion system  100 . Thus, the controller can access data from the various sensors included in the electronics subsystem  180  and from the controller and can wirelessly communicate (e.g., via an integrated wireless chip) with other I/O devices in order to execute various processes further described below. 
     4.5.1 Operational Envelope Detection 
     In one implementation, the electronics subsystem  180  is configured to detect whether the bicycle propulsion system  100  is within its operational envelope in order to ensure that the bicycle propulsion system  100  only applies power to the sprocket of the bicycle while the concentric rotor assembly  102  is engaged with the sprocket of the bicycle, while the chassis assembly is secured to a frame element of the bicycle, and while the bicycle itself is in a safely operable state (e.g., not exceeding a maximum speed or in an inoperable orientation). More specifically, the electronics subsystem  180  is configured to, in response to detecting the position of the chassis assembly  104  outside of a predefined operational envelope, halting the motor  162 . Thus, the bicycle propulsion system  100  can ensure that power is cut from the motor  162  in the case of a crash or dislodgement of the bicycle propulsion system  100  from its nominal position relative to the bicycle. 
     In one implementation, the electronics subsystem  180  can store a set of parameters indicating the operational envelope for the bicycle propulsion system  100 , such as a maximum and minimum lateral angle (i.e. inboard/outboard tilt), a maximum and minimum transverse angle (i.e. forward and backward tilt), a maximum and minimum speed, and the state of engagement of the sensor subassembly  170 . In this implementation, the electronics subsystem  180  can measure the orientation of chassis assembly prior to and/or during operation of the bicycle propulsion system  100  and in response to detecting that the orientation of the chassis assembly  104  exceeds the maximum lateral angle and/or the maximum transverse angle and/or is less than the minimum lateral angle or the minimum transverse angle, the electronics subsystem  180  halts and/or cuts power to the motor  162 . In one example, the electronics subsystem  180  can halt the motor  162  in response to detecting a lateral angle greater than 30 degrees from vertical. Likewise, the electronics subsystem  180  can estimate the speed of the chassis assembly  104  by executing an inertial algorithm on data recorded via the inertial measurement unit and, in response to detecting a speed exceeding the maximum speed or a speed less than the minimum speed, the electronics subsystem  180  can halt the motor  162 . 
     Additionally, the electronics subsystem  180  can measure velocity of the chassis assembly in multiple dimensions and can store multiple maximum and minimum velocities, each corresponding to velocity measured in a different dimension. Thus, the electronics subsystem  180  can detect lateral movement (e.g., skidding) and halt the motor  162  to enable the cyclist to more easily regain traction between the rear wheel and the ground. 
     In another implementation, the electronics subsystem  180  can detect whether the sensor arm  171  is engaged with the chain by detecting whether the sensor arm  171  is deflected by less than a threshold deflection caused by a tensionless chain. For example, the electronics subsystem  180  can include a predefined deflection corresponding to a state where the sensor arm  171  is not engaged with the chain and is fully biased (e.g., by the biasing spring) against a hard stop integrated within the chassis assembly  104 . Therefore, in response to detecting that the chain is disengaged with the chain roller  176  of the sensor arm  171  and the tension of the chain is no longer detected by the electronics subsystem  180 , the electronics subsystem  180  can halt the motor  162 . 
     4.5.2 Adaptive Pedal Assistance 
     Generally, the electronics subsystem  180  can execute an adaptive pedal assistance algorithm based on the estimated power output by the cyclist (e.g., via measurement of chain tension by the sensor subassembly  170 , via integration with a power meter, or via a pressure sensor detecting the force exerted by the chassis assembly  104  on the chain stay of the bicycle), the current gear selection of the cyclist, the cadence of the cyclist, the estimated inclination of the bicycle, and/or the estimated speed of the bicycle in order to selectively apply additional power to the sprocket of the bicycle without substantially altering the handling of the bicycle or the operational experience of the bicycle when compared to manual operation of the same bicycle. More specifically, the electronics subsystem  180  is configured to estimate the power output by the cyclist based on deflection of the sensor arm  171  caused by tension in the bicycle chain and modify the output power of the motor  162  based on this measured deflection; estimate the gear selection of the bicycle based on step changes in the deflection of the sensor arm  171 ; estimate the inclination of the bicycle relative to the ground plane based on data from the inertial measurement unit; and estimate the speed of the bicycle based on an estimated cadence of the cyclist and the gear selection of the bicycle. 
     In one implementation, the electronics subsystem  180  can store a predefined lookup table (based on empirical data) or a predefined function correlating deflection of the sensor arm  171  to the power output by the cyclist. In this implementation, the electronics subsystem  180  can receive (e.g., via an associated application running on a smartphone) the gear configuration (e.g., brand cassette and chainring selection) of the bicycle. The electronics subsystem  180  can then select a function or lookup table corresponding to the gear configuration of the bicycle. 
     Alternatively, the electronics subsystem  180  can initiate a calibration procedure based on input from a mobile computation device (e.g., via an associated application running on a smartphone) in order to associate the power output by the cyclist with deflection of the chain. During the calibration procedure, the electronics subsystem  180  can measure the deflection of the chain of the bicycle as the cyclist is instructed to perform a series of hard and easy efforts. Based on these data, the electronics subsystem  180  can then correlate the deflection of the chain of the bicycle with maximum and minimum efforts of the cyclist. 
     In another implementation, the electronics subsystem  180  can: store a model, map, or lookup table that links predefined deflection ranges of the sensor arm to a particular sprocket selection in the rear cogset; measure deflection of the sensor arm  171 ; and predict the gear selection of the bicycle based on this model and the measured deflection. Alternatively, the electronics subsystem  180  can execute a calibration procedure by: for a first sprocket in the cogset of the bicycle, prompting the cyclist to shift into the first sprocket and pedal (e.g., at variable effort levels); recording deflection of the sensor arm  171  for a first duration; and repeating this procedure for successive sprockets of the cogset of the bicycle. 
     In yet another implementation, the electronics subsystem  180  can: execute frequency analysis on the measured deflection of the sensor arm  171  over time to estimate the cadence of the cyclist; and modify the power output by the motor  162  based on the cadence of the cyclist. For example, in response to estimating a low cadence of the cyclist (e.g., less than 70 rotations-per-minute), the electronics subsystem  180  can increase the power output of the motor  162 . Alternatively, in response to estimating a high cadence of the cyclist (e.g., greater than 100 rotations-per-minute), the electronics subsystem  180  can decrease the power output of the motor  162 . Thus, the electronics subsystem  180  leverages the cyclic nature of the torque applied by the cyclist during each pedal stroke to estimate the cadence of the cyclist and can modify the power output of the motor  162  based on this estimated cadence. 
     In another implementation, the electronics subsystem  180  can estimate the inclination of the bicycle by calculating, via the inertial measurement unit, the transverse orientation of the chassis assembly  104 . Based on a known orientation of the chassis assembly  104  while the bicycle is on flat ground, the electronics subsystem  180  can calculate the inclination of the bicycle and modify the power output of the motor  162  based on this inclination. 
     In another implementation, the electronics subsystem  180  can implement dead reckoning techniques to estimate the speed of the speed of the bicycle based on inertial data output by the inertial measurement unit. Additionally or alternatively, the electronics subsystem  180  can calculate the speed of the bicycle directly based on the estimated cadence of the cyclist, the estimated gear selection of the bicycle, and a known wheel diameter of the bicycle. In yet another implementation, the electronics subsystem  180  can: measure a rotational speed of the motor  162  (e.g., via a rotational encoder, via Hall effect sensors proximal to the motor, or via measurement of the counter-electromotive force of the motor  162 ); and estimate the speed of the bicycle based on the measured rotational speed of the motor  162 , the gear ratio between the motor  162  and the wheel of the bicycle, and the known wheel diameter of the bicycle. 
     Upon calculating and/or estimating each of the above values, the electronics subsystem  180  can input these values into a tuned function in order to calculate an output power for the motor  162 . The electronic subsystem  180  then communicates this output power to the motor  162 ; and draws power from the battery assembly  106  sufficient to operate the motor  162  at this output power. In one implementation, the electronics subsystem  180  is configured to calculate an output power of zero upon detecting a speed of the bicycle greater than a threshold speed in order to comply with regulations on electric bicycles. 
     Thus, the electronics subsystem  180  can be configured to: calculate a cadence of the bicycle based on periodic deflection of the sensor arm  171 ; calculate a speed of the bicycle via the six-axis inertial measurement unit; identify a current gear ratio of the bicycle based on the speed of the bicycle and the cadence of the bicycle; and drive the motor  162  based on the current gear ratio of the bicycle. 
     4.5.3 Automatic Backpedaling Assistance 
     In one implementation, the electronics subsystem  180  can execute automatic backpedaling assistance to enable the bicycle equipped with the bicycle propulsion system  100  to mimic the pedaling dynamics of a standard bicycle. Because, the concentric rotor assembly  102 , the drive subassembly, and the motor  162  all impose additional resistance (e.g., in the form of friction, additional rotational weight) onto the sprocket when the motor  162  is not powered, without automatic backpedaling assistance, the cyclist may be unable to backpedal the bicycle. Thus, upon detecting that the cyclist is no longer pedaling (e.g., based on the chain tension estimated via the sensor arm  171 ), the electronics subsystem  180  can cause the motor  162  to reverse direction at a predetermined speed, thereby enabling the user to pedal backward up to a threshold cadence corresponding to the predetermined backpedaling speed. 
     5. Battery Assembly 
     Generally, as shown in  FIG. 1 , the bicycle propulsion system  100  can include a battery assembly  106  connected to the chassis assembly  104  by a power cable  182  (or integrated directly with the chassis assembly  104 ) in order to supply power to the electronics subsystem  180  and the motor  162 ). In one implementation, the bicycle propulsion system  100  is configured to: fit within a standard bicycle bottle holder; supply power to the motor  162 ; and supply power to the electronics subsystem  180 . In this implementation, the battery assembly  106  also includes a power cable  182  electrically coupling the battery assembly  106  to the electronics subsystem  180  and the motor  162 . Thus, by including a battery assembly  106  that fits within a standard bicycle bottle holder, the bicycle propulsion system  100  can be more easily installed on any bicycle already including a standard bottle holder. 
     In one implementation, the bicycle propulsion system  100  includes a battery assembly  106  further including a set of modular battery packs configured to engage with each other and configured to fit within a standard bicycle bottle holder. This modular battery assembly  106  enables the user to bring only the battery capacity needed for a planned trip and reduce the total weight of the bicycle propulsion system  100  in accordance with the needed capacity. In one example, the battery assembly  106  can include a set of cylindrical modular batteries configured to connect at the top and bottom of the cylinder and configured to slide into a standard bicycle bottle holder. Additionally, in this example the battery assembly  106  can include a topmost cylindrical battery configured to engage the power cable  182  and a bottommost cylindrical battery defining a flat bottom such that the bottommost cylindrical battery rests evenly at the bottom of a standard bicycle bottle holder. In another example, the battery assembly  106  can include an exterior battery shell (e.g., in the form of a hollow cylinder) and configured to support a set of modular batteries within the exterior battery shell. In this example, the exterior battery shell can include an integrated electronic battery management unit connected to each modular battery in the set of modular batteries in order to modulate power drawn from each modular battery in the set of modular batteries. The exterior battery shell can be configured to secure the set of modular batteries within the exterior battery shell via friction or via a set of mechanical locks or latches. Each modular batter in the set of modular batteries can include a female surface and a male surface on the top and bottom of the modular battery respectively (or vice versa) in order to aid in engaging each modular battery with other modular batteries in the set of modular batteries. Additionally, in this example, the topmost modular battery in the set of modular batteries an include a connector or adapter configured to electrically couple the battery assembly io 6  to the power cable  182 . 
     In another implementation, the bicycle propulsion system  100  can include a battery assembly  106  integrated with the chassis assembly  104  or configured to attach to the chain stay, seat stay, seat tube, downtube, or top tube of the bicycle. In each implementation, the bicycle propulsion system  100  can include a power cable  182  of an appropriate length to connect the battery assembly  106  the chassis assembly  104 . Alternatively, the bicycle propulsion system  100  can include a battery assembly  106  that directly connects directly the chassis assembly  104  without a power cable  182 . 
     6. Throttle Assembly 
     In one variation shown in  FIG. 1 , the bicycle propulsion system  100  includes a throttle assembly  108 . For example, the throttle assembly  108  can include a set of buttons and can transmit button selections to the controller. The controller can then: adjust a relationship between chain tension (or cyclist output power) and torque or power output of the motor; or switch the bicycle propulsion system  100  on and off based on selections of these buttons. The throttle assembly io 8  can additionally or alternatively display system data received from the controller, such as battery level, assistance level, and/or ride statistics. 
     7. Chainring-Mounted Variation 
     In one variation, the bicycle propulsion system  100  is configured to engage with one or more front sprockets (i.e. chainrings) of the bicycle (as opposed to the rear cogset) in order to convert bicycles without sufficient clearance in proximal to the rear triangle of the bicycle or mountain bikes with cogset sprockets above a threshold diameter to an electrically assisted bicycle. In this variation, the bicycle propulsion system  100  can include a concentric rotor assembly  102  configured to engage with an innermost set of chainrings of the bicycle and a chassis assembly  104  configured to rest between the seat tube and the downtube of the bicycle or configured to attach below the downtube of the bicycle. Alternatively, in this variation, the bicycle propulsion system  100  can include a concentric rotor assembly  102  configured to engage with the outermost chainring of the bicycle. This variation of the bicycle propulsion system  100  can include the same set of components described above with respect to the rear cogset variation that instead defines a form factor configured to fit within the bottom bracket region of the bicycle. 
     8. Disk-Brake-Mounted Variation 
     In another variation, the bicycle propulsion system  100  is configured to mount to and/or replace the rear disk brake rotor and the rear disk brake caliper of a disk brake bicycle in order to vacate the innermost sprocket, thereby enabling use of the entire cogset of the bicycle. More specifically, in this variation, the bicycle propulsion system  100  can include: a concentric rotor assembly  102  configured to replace the rear disk brake rotor of the disk brake bicycle and including a braking surface; and a chassis assembly  104  configured to mount to the left side rear chain stay and/or the left side rear seat stay and including a braking caliper. Thus, in this variation, the bicycle propulsion system  100  can drive the rear wheel of the bicycle via a concentric rotor assembly  102  configured to replace the disk brake rotor of a disk brake bicycle and can replace the functionality of the replaced disk brake rotor via the inclusion of a braking surface on the concentric rotor assembly  102 . Additionally, in this variation, the concentric rotor assembly  102  can include a center axle that replaces the through-axle of the disk brake assembly and can, therefore, be driven via a direct power transmission between the motor  162  and this through-axle. Alternatively, in this variation of the bicycle propulsion system  100 , the concentric rotor assembly  102  can include an outer drive surface  132  and the bicycle propulsion system  100  can apply torque to this outer drive surface  132  via the drive subassembly as described above. 
     The systems and methods described herein can be embodied and/or implemented at least in part as a machine configured to receive a computer-readable medium storing computer-readable instructions. The instructions can be executed by computer-executable components integrated with the application, applet, host, server, network, website, communication service, communication interface, hardware/firmware/software elements of a user computer or mobile device, wristband, smartphone, or any suitable combination thereof. Other systems and methods of the embodiment can be embodied and/or implemented at least in part as a machine configured to receive a computer-readable medium storing computer-readable instructions. The instructions can be executed by computer-executable components integrated by computer-executable components integrated with apparatuses and networks of the type described above. The computer-readable medium can be stored on any suitable computer readable media such as RAMs, ROMs, flash memory, EEPROMs, optical devices (CD or DVD), hard drives, floppy drives, or any suitable device. The computer-executable component can be a processor but any suitable dedicated hardware device can (alternatively or additionally) execute the instructions. 
     As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the embodiments of the invention without departing from the scope of this invention as defined in the following claims.