Abstract:
A bicycle freewheel assembly is provided which is capable of being readily and reversibly switched from fixed-gear to freewheel operation, without the necessity of removing the wheel from the bicycle. The freewheel assembly is mountable on pre-existing freewheel hubs.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional patent application No. 62/237,642 filed on Oct. 6, 2015, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to rear-wheel hubs for bicycles, and in particular to hubs that are convertible between freewheel and fixed-gear modes of operation. 
       BACKGROUND 
       [0003]    Almost all consumer-grade bicycles sold today are multi-speed bikes, in which a sprocket package, also referred to as a cogset, is connected via a freewheel mechanism to the rear wheel hub. A derailleur is used to move the drive chain from one sprocket to another, enabling the rider to select the gearing. Some bicycles employ gear hubs, wherein a planetary epicyclic gearset is contained within the hub. These usually feature a single sprocket, but a freewheel mechanism is still needed. 
         [0004]    One design employs an assembly of a cogset and a drive ring, the assembly having built within it a ratchet mechanism. The cogset, drive ring, and internal ratchet are collectively referred to as a freewheel assembly, which in older designs is threaded onto a freewheel hub. More modern designs employ splines to couple the freewheel hub to the drive ring. 
         [0005]    An alternative design employ a cassette, which is a bolted-together cluster of sprockets having no internal ratchet, and a “freehub” which contains the ratchet mechanism. A set of interior straight splines around the open axis of the cassette align with matching splines on the freehub axle, enabling the cassete to be rotationally locked to the freehub axle when mounted thereon. The freehub body and axle are in a freewheeling relationship. 
         [0006]    There are two common ratchet mechanisms in use today. One design employs movable pawls distributed around (or within) the circumference of an internal ratchet. The pawls ride on the ratchet, sliding up the gently sloped side of each tooth when the ratchet and pawls are rotated in one relative direction, but rigidly engaging the other, steeply-sloped side of the ratchet teeth when the ratchet and pawls rotate in the opposite relative direction. The other design is the “star ratchet”, in which two disks with spur or face gearing face each other and are biased against each other. The biasing force is provided by a spring, or in some cases by magnets. Relative rotation in one direction engages opposing, steeply-faced ratchet teeth, rotationally locking the disks together and enabling pedaling force to be transmitted from one disk (coupled to the sprocket or cogset) to the other (coupled to a drive ring or to the hub). Relative rotation in the opposite direction, as when the rider stops pedaling, results in opposed, sloped surfaces of the ratchet teeth sliding past one another. The resulting axial displacement of the disks against the biasing force results in decoupling and free relative rotation of the disks. Representative examples of the star ratchet may be found in U.S. Pat. Nos. 6,588,564, 7,562,755 and 8,757,341, the disclosures of which are incorporated herein by reference in their entireties. 
         [0007]    Whether it is of the pawl-and-ratchet or star-ratchet design, the freewheel mechanism ensures that during a forward motion of the pedals the drive force, transmitted via the chain, is transmitted to the rear wheel, and that during a motion in the opposite direction, i.e., when the crank is moving backwards or is at a standstill, the sprocket or cogset is decoupled from the wheel, which therefore rotates freely. 
         [0008]    In contrast to the above-described multi-speed bicycles with freewheel mechanisms, there is the simple fixed-gear bicycle, or “fixie”, featuring a single sprocket rigidly attached to the wheel hub. The rider of a fixed-gear bicycle cannot stop pedaling while the bike is in motion, due to the absence of a freewheel mechanism between the pedal crank and the rear wheel, although some riders learn to brake by kicking back and locking the pedals and rear wheel, causing the bike to skid to a stop. The fixed-gear design, commonly found on track racing bicycles, has become popular with urban riders, particularly bicycle messengers, and a fixie sub-culture has emerged among riders in many U.S. and European cities. The fixed-gear bicycle has certain advantages, chiefly light weight, simplicity, and reliability, and the fixed connection between rider and wheel enables stunts that are difficult or impossible with a freewheel. A fixed-gear bicycle generally offers a low gearing ratio, which is well-suited to stop-and-go urban riding, although some riders prefer the high ratio of a track bicycle, which affords greater speed. 
         [0009]    Riders of fixed-gear bicycles find that the design, while suited for short trips in a city, is not convenient for long rides or touring, where high gearing ratios are desirable for high speeds, and where the ability to coast on downhill runs conserves a rider&#39;s energy. High speed downhill runs are particularly problematic for the rider of a fixed-gear bike, because there is a limit to how fast the legs can be moved. Riders of freewheel, multispeed bicycles find that their machines are not suited for stunt riding, and that—because a derailleur cannot function when the bike is stopped—shifting gears in stop-and-go city riding can become a chore. In both cases, an expensive solution is to own two bicycles, one of each design, but this still requires that the rider select one bicycle or the other prior to setting out for the day. 
         [0010]    A common solution to this problem is the “flip-flop hub”, a rear bicycle hub that features a fixed cog on one side and a freewheel on the other. The rider can switch between freewheel and fixie modes by removing and flipping the rear wheel. While this is, in principle, a simple solution, the flip-flop hub has some disadvantages. Although long dropouts permit some variation, the gearing on the freewheel side of a flip-flop hub cannot much differ from the gearing on the fixed side, unless one also alters the chain length. Given that higher gearing is one of the principal reasons one would want to switch out of the fixed-gear mode, this is a major shortcoming. The need to not only remove the rear wheel, but to add or remove links from the chain, or to substitute a chain of different length, makes the switch between modes an inconvenient, messy operation if altered gearing is desired. Also, given the different axial dimensions of fixed sprockets and freewheel assemblies, dishing of the rear wheel may not be symmetrical, and the brakes may require re-alignment when the wheel is flipped. Brakes may also need re-positioning if a long dropout is relied on to enable altered gearing. 
         [0011]    A complicated friction clutch mechanism, that reversibly fixes a freewheel hub via the engagement and disengagement of male and female conical surfaces, is described in U.S. Pat. No. 1,278,648 (issued to G. Henry in 1918.) The design, intended for a stationary exercise bike, suffers from weak frictional coupling of the surfaces, which cannot hold against the forces applied by a rider engaged in braking or in performing stunts. 
         [0012]    A design for a hub in which pawls are reversibly engaged or disengaged from a ratchet to effect a switch between fixed-gear and freewheel modes is disclosed in U.S. patent application publication No. 2013/0299293 (Nov. 14, 2013). In this design, in order to switch modes, it is necessary to disassemble the hub, remove and relocate springs that bias the pawls into or out of engagement with the ratchet, and then re-assemble the hub. This is an inconvenient process for the user, and is impractical for use when on the road. 
         [0013]    Further designs are disclosed in GB Patent Nos. 446,136 (to J. MacKay et al., 1936) and 451,299 (to W. Woodcock, 1936). In these designs, pawls are mechanically displaced to disengage them from their ratchets. These designs are mechanically complex, and cannot be retroactively installed on an existing hub assembly. 
         [0014]    A more recent design for a hub in which pawls are reversibly engaged or disengaged from a ratchet to effect a switch between fixed-gear and freewheel modes is disclosed in U.S. Pat. No. 8,100,242 (to M. Schneider and K.-J. Kiihne, Jan. 24, 2012). A commercial embodiment of this design is marketed by SRAM Deutschland GmbH of Schweinfurt, Germany under the trade name “Torpedo Singlespeed”. This hub weighs 480 g, which would be considered heavy by riders of track and fixie bicycles. The hub is switched between fixed and freewheel modes by driving an axial screw inwards or outwards, thereby sliding a set of pawl-blocking blades into or out of place between the pawls and the ratchet. This requires components specifically manufactured with and installed within the hub, and thus the device cannot be installed on an existing bicycle hub. A would-be user must purchase the entire specialized hub, and then build (or have built) a wheel around it. Furthermore, the construction of this custom wheel is limited to the number of spokes supported by the hub. 
         [0015]    There remains a need for a bicycle that can be quickly, conveniently, and easily switched between freewheel and fixed-gear modes. There is a need for a device which provides positive engagement between the hub and sprocket, and which can be installed by bike shops or consumers on their current wheels and hubs. 
       SUMMARY OF THE INVENTION 
       [0016]    The invention provides a freewheel assembly which can be set into a locked mode, wherein the sprocket is positively engaged with a drive ring that is mountable on existing freewheel hubs. The locked mode is achieved by various positive locking mechanisms, as described more fully below. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0017]      FIG. 1  is an exploded, perspective view of a first embodiment of the invention. 
           [0018]      FIG. 2  is a side view of the fully-assembled embodiment of  FIG. 1 . 
           [0019]      FIG. 3  is a sectional view taken along line A-A in  FIG. 2 . 
           [0020]      FIG. 4  is an exploded, perspective view of a second embodiment of the invention. 
           [0021]      FIG. 5  is a perspective view of the assembled device of  FIG. 4 . 
           [0022]      FIG. 6  is a side view of assembled device of  FIG. 4   
           [0023]      FIG. 7  is a cross-sectional view taken along line B-B of  FIG. 6 . 
           [0024]      FIG. 8  is a side view of an embodiment employing solenoids and pawl pins. 
           [0025]      FIG. 9  is a cutaway end-on view of the embodiment of  FIG. 8 . 
           [0026]      FIG. 10  is an end-on view of the embodiment of  FIG. 8 . 
           [0027]      FIG. 11  is a cross-sectional view taken along line C-C of  FIG. 10 . 
           [0028]      FIG. 12  is an exploded view of an embodiment employing a locking pin. 
           [0029]      FIG. 13A-13B  are external views of the embodiment of  FIG. 12 . 
           [0030]      FIGS. 14A-14B  are internal views of the embodiment of  FIG. 12 . 
           [0031]      FIGS. 15A-15B  are cutaway views of the embodiment of  FIG. 12 . 
           [0032]      FIG. 16  is an end-on plane view of the embodiment of  FIG. 12 . 
           [0033]      FIG. 17  is a cross-sectional view taken along line A-A of  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The present invention provides bicycle freewheel assemblies which are readily and reversibly converted from freewheel to fixed gear operation. In the various embodiments disclosed herein, a freewheeling sprocket is mechanically linked to a drive ring via a non-frictional positive locking mechanism. As used herein, a non-frictional positive locking mechanism is any mechanism that mechanically locks the freewheeling sprocket to the drive ring, without any possibility of slippage. Such locking is referred to as “positive engagement.” The physical strength of the resulting positively locked linkage is limited only by the mechanical strength of the components. By employing a non-frictional positive locking mechanism, the invention avoids the use of a friction clutch, and thereby provides a rigid, reliable non-slipping linkage when the user selects the fixed-gear mode. 
         [0035]    Suitable non-frictional positive locking mechanisms include, but are not limited to, set screws and pins that physically interfere with the rotation of a freewheeling sprocket relative the drive ring to which it is mounted, and set screws or pins that physically interfere with the motion of pawls in a pawl-and-ratchet freewheel assembly. Pins may be moved into a physically interfering position by hand, by a mechanical linkage such as a cable, by activation of a solenoid, or by any other convenient method of mechanical displacement known in the art. Another suitable non-frictional positive locking mechanism is a circumferential band or spring wrapped around the pawls. The band or spring defines a notched circle, which varies in diameter as the distance between the spring ends is varied. The band or spring is configured to physically interfere with the motion of the pawls in a pawl-and-ratchet freewheel assembly when the diameter is adjusted into the locking configuration, and to permit motion of the pawls when the diameter is adjusted into the freewheeling configuration. The spring may be a pawl spring, as described in greater detail below. 
         [0036]    In one embodiment, one or more pins or set screws are set into the sprocket (or the drive ring), and the user moves these pins or set screws into holes or slots in the drive ring (or the sprocket) so as to prevent relative rotation of the sprocket and drive ring. The pin(s) or set screw(s) mechanically lock the drive ring and sprocket, preventing relative rotation and creating a rigid, non-slipping mechanical linkage. The drive ring, when mounted on a freewheel hub, thus locks the sprocket to the hub and wheel, which enables performance as a fixed-gear bicycle. 
         [0037]    In another embodiment, the pawls of a conventional pawl-and-ratchet freewheel assembly are locked into place, preventing them from sliding up and over the teeth of the ratchet. The pawls may be locked by the motion of pins into a position that impedes their normal motion, or by a pawl spring that locks the pawls in an extended configuration, holding them firmly against the ratchet teeth and thereby preventing displacement of the pawls by the ratchet. 
         [0038]    In yet another embodiment, a pair of star ratchets are rotationally fixed to the sprocket and to the drive ring, in a mutually facing co-axial arrangement. At least one of the ratchets is movable along the axis of the device. When the star ratchets are moved into engagement, the sprocket and the drive ring are locked against relative rotation in the freewheeling direction. 
         [0039]    In certain embodiments, the motions of pins or ratchets are effected by one or more solenoids. The solenoids may be activated by a manual switch, or alternatively by an electrical circuit that is in turn activated via a short-range radio signal, preferably employing a digital wireless standard such as Bluetooth. The controller in these embodiments can be a dedicated Bluetooth transmitter, or it can be the user&#39;s portable wireless device, such as a smart phone, running an application designed to control the solenoids via the smart phone&#39;s Bluetooth transmitter. 
         [0040]    Solenoids can be powered by batteries installed within the hub of the wheel, and/or by a dynamo or generator interior to the hub or attached to the bicycle frame and powered by the rotation of the wheels. In preferred embodiments the batteries are rechargeable cells and/or capacitors, which are maintained in a charged state by either a hub or “bottle” style magneto. In preferred embodiments, the mechanism is bi-stable, having stable locked and unlocked modes, and the solenoid is energized only when needed to transition between the two modes. The two states may be stabilized against undesired motion by mechanical detents, by biasing springs, or by magnets holding the movable parts in each of the two modes. 
         [0041]    In other embodiments, the movement of pins or star ratchets are effected by electromechanical means, as is known in the art. Such means are typically DC motors drawing power from batteries, as used for electrical gear shifting systems. See, for example, U.S. Pat. Nos. 8,886,417 and 8,909,424, the contents of both of which are incorporated herein by reference in their entirety. As with solenoids, operation of a motor may be activated by a manual switch, or alternatively by an electrical circuit that is in turn activated via a short-range radio signal, preferably using a digital transmission standard such as Bluetooth. 
         [0042]    Referring to  FIGS. 1-3 , a drive ring  1  rotates within ball bearing sets  2   a  and  2   b , and is provided with recesses  3   a  which accommodate pawls  3 . Pawls  3  are retained in the recesses  3   a  by pawl spring  4 . Pawls  3  are also biased by pawl spring  4  against ratchet  6 , which is formed on the inner surface of freewheel sprocket  5 . Pawls  3  engage the steep faces of ratchet surface  6 , and provide positive engagement between freewheel  5  and the drive ring  1 , when the sprocket is driven the forward direction ( FIG. 1  is a view from the wheel side of the sprocket, from which perspective “forward” is counter-clockwise.) When the sprocket is not being driven (when the rider stops pedaling), sprocket  5  stops rotating while the drive ring  1  continues to rotate counter-clockwise. In this mode, pawls  3  slide up and over the shallow sides of ratchet surface  6 . Sealing ring  7  threads into freewheel  5  and retains the ball bearing set  2   b . Set screw  8  is set within complimentarily-threaded hole  9   a . Drive ring  1  is provided with hole  9   b , which is located so that when the drive ring and freewheel are rotated to bring the holes  9   a  and  9   b  into register, the holes are coaxial. In the embodiment shown in  FIG. 3 , hole  9   b  is threaded with female threads complimentary to set screw  8 . In alternative embodiments, hole  9   b  may be smoothly bored, and set screw  8  may optionally feature a complimentary unthreaded region. When the holes  9   a  and  9   b  are aligned in a coaxial relationship, set screw  8  can be spun down into hole  9   b , thereby rotationally locking drive ring  1  to sprocket  5 . In this locked mode, freewheeling is prevented, and the assembly, when mounted on a freehub, acts as a fixed-gear hub. 
         [0043]    The holes  9   a  and  9   b  are preferably located so that they are coaxial when pawls  3  are in contact with the steep sides of ratchet  6 , i.e., in position to transfer motive force from freewheel  5  to drive ring  1 . This avoids the application of force from freewheel  5  to the set screw  8  which would occur if the pawls were not engaged.  FIG. 3  is a cross section through the elevation shown in  FIG. 2  along the plane A-A. In  FIGS. 1 and 3 , like elements are given like numbers. 
         [0044]    In all embodiments shown in the drawings, the drive ring as shown is adaptated for mounting on a splined hub. The invention is not limited to this mounting structure, and the drive ring may be outfitted with any hub mounting structure known in the art. 
         [0045]    Referring to  FIGS. 4-6 , an alternative embodiment of the invention is shown, wherein drive ring  401  rotates within ball bearing sets  402   a  and  402   b . Ratchet surface  406  is formed on the outer circumference of drive ring  401 . The inner circumference of freewheel sprocket  405  is provided with recesses  403   a , which accommodate pawls  403 . Pawls  403  are held in place in the recesses  403   a  by pawl springs  404   a  and  404   b . Pawls  403  are biased by pawl spring  404   a  and  404   b  against ratchet surface  406 . Sealing ring  407  threads into freewheel  405  and retains the ball bearing set  402   b . Pawl spring  404   b  is provided with a radially-extending extension ending in knob  408 . In operation, motive force applied to freewheel sprocket  405  causes pawls  403  to engage the steep-sided faces of ratchet  406 , transferring motive force to drive ring  401 . Drive ring  401  is rigidly engaged with the hub of the wheel (not shown), and the motive force is thus transferred to the wheel. When the rider stops pedaling, freewheel sprocket  405  stops rotating, and as the drive ring  401  continues to rotate, pawls  403  slide over the shallow faces of ratchet  406 . This is ordinary freewheeling operation. 
         [0046]      FIG. 5  is a perspective view of the assembled device. The view in  FIG. 5  is from the opposite side of that in  FIG. 4 , so that clockwise motions in  FIG. 4  are counterclockwise in  FIG. 5 . Like elements from  FIG. 4  are given like numbers. The extended end of pawl spring  404   b  traverses slot  409  which is cut into the body of freewheel sprocket  405 , and knob  408  rests outside the body of sprocket  405 . Slot  409  has concave detents at either end, into which knob  408  drops and is retained by the biasing force of pawl spring  404   b . When the user of the device moves knob  408  into the position shown, pawl spring  404   b  is constricted into a smaller diameter. This locks pawls  403  against ratchet  406 , preventing them from riding up and over ratchet  406  and thereby preventing freewheeling. In this mode the device operates as a fixed gear system. Movement of knob  408  to the other detent frees the pawls  403  and restores freewheeling operation. 
         [0047]      FIG. 6  presents a plane view of the assembled device, and  FIG. 7  presents a cross section along plane B-B. Like elements from  FIG. 4  are given like numbers. As can be seen in  FIG. 7 , pawl springs  404   a  and  404   b  occupy slots cut into the inner circumference of freewheel sprocket  405 . Ball bearing sets  402   a  and  402   b  ride on races cut into drive ring  401  and sealing ring  407 , respectively. 
         [0048]      FIG. 8  shows a plane view of the internal components of an embodiment in which three permanent magnet solenoid cores  820  are configured to move three pawl locking pins  821  into (or out of) drive ring  801  when the solenoids are energized. Movement into the drive ring places the pawl locking pins in positions that interfere with the movement of pawls  803 , while movement of the pawl locking pins out of the drive ring frees the pawls, resulting in operation identical to that of the embodiment shown in  FIGS. 1-3 . The pawls are held in place and biased outwardly by pawl spring  804 . 
         [0049]      FIG. 9  is an end-on plane view of the components of  FIG. 8 . Like elements from  FIG. 8  are given like numbers. The ends of pawl locking pins  821  are shown just behind pawl spring  804 . When extended into the drive ring, as shown in the drawings, the pawl locking pins  821  prevent the displacement of pawls  803  into their recesses in drive ring  801 . With pawls  803  locked into their extended positions by pawl locking pins  821 , freewheeling of the ratchet (not shown) is prevented, and the full assembly, in combination with a freewheel hub, will function as a fixed gear hub. 
         [0050]      FIG. 10  is an end-on plane view of the assembled device incorporating the components of  FIGS. 8-9 , showing the installation of drive ring  801  into a freewheel sprocket  805 . 
         [0051]      FIG. 11  presents a cross section of the assembly of  FIG. 10  along plane C-C. Like elements from  FIGS. 8-10  are given like numbers. Drive ring  801  rotates within ball bearing sets  802   a  and  802   b . Pawl  803  is held in place by pawl spring  804 . Pawl  803  is biased by pawl spring  804  against a ratchet surface (not shown) on the inner circumference of freewheel sprocket  805 . Sealing ring  807  threads into drive ring  801  and retains the ball bearing set  802   a . Solenoid core  820  is shown in the locked position, with pawl locking pin  821  inserted into drive ring  801 . In this embodiment, pawl locking pin  821  travels as far as pawl spring  804 . In other embodiments, e.g. where each pawl has its own independent spring, the pawl locking pins may traverse the full width of the pawls. By physically preventing pawl  803  from retreating into its recess, pawl locking pin  821  locks pawl  803  into its radially extended position. 
         [0052]    The solenoid core moves in response to the energizing of solenoid coil  830 , which is affixed to the interior surface of a cavity  850  in hub body  840 . In the embodiment shown, there are six radially distributed cavities  850 , three of which are occupied by solenoid coils and cores. In an alternative embodiment, there are six pawls and six solenoids. In yet other embodiments, there may be pawls without corresponding solenoids. The number of pawls, cavities and solenoids may be varied as desired by the practitioner, consistent with maintaining a symmetrical distribution of mass around the axis of the hub. Greater mechanical strength may obtained with more pawls and more solenoids, but with a trade-off in the mass of the device. Preferred embodiments employ two or three pawls, and two or three solenoids. The solenoid coils  830  may be powered by one or more batteries (not shown), which may be installed within the hub (e.g. in cavities  850 ) or externally, and/or by an external power source such as a wheel-driven magneto. External power may be delivered to the coils  830  by a brush and ring, with the circuit being closed via the axle and frame of the bicycle. Alternatively, a magneto may be incorporated into the hub, as is known in the art; see e.g. U.S. Pat. No. 9,093,887 and references cited therein. As an alternative to, or supplement for, batteries, capacitors may be incorporated into the circuitry for storage and delivery of brief pulses of electrical current to the solenoids. 
         [0053]    Referring to  FIGS. 12-15 , yet another embodiment of the invention is shown.  FIG. 12  is an exploded view of the components. Drive ring  1201  rotates within ball bearing sets  1202   a  and  1202   b , and is provided with recesses  1203   a  which accommodate pawls  1203 . Pawls  1203  are held in place in the recesses  1203   a  by pawl spring  1204 . Pawls  1203  are also biased by pawl spring  1204  against ratchet  1206 , which is formed on the inner surface of freewheel sprocket  1205 . Pawls  1203  engage the steep faces of ratchet surface  1206 , and provide positive engagement between freewheel  1205  and the drive ring  1201 , when the sprocket is driven in the forward direction ( FIG. 1  is a view from the outer side of the sprocket, from which perspective forward is clockwise.) When the sprocket is not being driven (when the rider stops pedaling), freewheel  1205  stops rotating while the drive ring  1201  (and the hub and wheel) continue to rotate clockwise. In this freewheeling mode, pawls  1203  slide up and over the shallow sides of ratchet surface  1206 . Sealing ring  1207  threads into freewheel  1205  and retains the ball bearing set  1202   a . Pin  1208  is fixed to mode selection ring  1210  and extends through slot  1211  in freewheel sprocket  1205 . Slot  1211  defines a spiral path along the axis of the device, so that when mode selection ring  1210  is rotated, and pin  1208  is impelled along this spiral path, mode selection ring  1210  and pin  1208  undergo an axial translation. In the interest of clarity, only a single pin is illustrated, but in preferred embodiments, there are two to four symmetrically disposed pins and two to four corresponding slots. 
         [0054]      FIGS. 13A and 13B  are side views of the assembled device, with the mode selection ring illustrated, showing the mode selection ring  1210  in its fully clockwise ( FIG. 13A ) and fully counterclockwise ( FIG. 13B ) positions. The change in the axial position of the mode selection ring is evident. 
         [0055]      FIGS. 14A and 14B  are side views of the assembled device, with the mode selection ring  1210  omitted, showing pin  1208  in fully clockwise ( FIG. 14A ) and counterclockwise ( FIG. 14B ) positions. When the mode selection ring  1210  is rotated counterclockwise until pin  1208  reaches the end of slot  1211 , as shown in  FIG. 14B , pin  1208  engages with drive ring  1201 , as shown in  FIGS. 15A and 15B , thereby locking drive ring  1201  to freewheel  1205 . In this locked mode, freewheeling is prevented, and the entire assembly acts as a fixed-gear hub. 
         [0056]      FIGS. 15A and 15B  are side views of the interior of the device, corresponding to  FIGS. 14A and 14B  with the freewheel sprocket, pawls, and pawl spring omitted for clarity. When retracted, as in  FIG. 15A , the pin  1208  encounters no obstacles as the freewheel sprocket (not shown) rotates relative to drive ring  1201 , and the device operates as a normal freewheel assembly. The spiral path of the pin  1208 , as mode selection ring  1210  (not shown) is rotated counter-clockwise, brings it into the confines of a recessed pin receiving cavity, in the form of a notch  1212  cut into drive ring  1201 , as illustrated in  FIG. 15B . When pin  1208  is confined to the notch  1212 , as the freewheel rotates the pin  1208  cannot proceed beyond the end of the notch  1211 . A mechanical connection between the freewheel and the drive ring is thereby established, and the freewheel sprocket and drive ring  1201  are rotationally locked, i.e., forced to rotate together. In this configuration the device enables a freewheel hub to operate as a fixed-gear hub. 
         [0057]      FIG. 16  is an end-on plane view of the assembled device incorporating the components of  FIGS. 12-15 , showing the installation of drive ring  1201  into a freewheel sprocket  1205 . 
         [0058]      FIG. 17  presents a cross section of the assembly of  FIG. 16  along plane A-A. Like elements from  FIGS. 12-15  are given like numbers. Drive ring  1201  rotates within ball bearing sets  1202   a  and  1202   b . Sealing ring  1207  threads into freewheel  1205  and retains the ball bearing set  1202   a . Pin  1208  is shown fixed to mode selection ring  1210 , and is shown in the retracted position, where it clears notch  1211  and permits freewheeling between the drive ring  1210  and the freewheel sprocket  1205 . The pawls, pawl spring, and ratchet operate as shown in  FIGS. 1-3  and are not detailed here. 
         [0059]    The above descriptions and the referenced drawings are presented for illustrative purposes, and are not meant to be limiting. Modifications and alterations of these exemplary non-frictional positive locking mechanisms will be apparent to those of skill in the art, and are contemplated to be within the scope and spirit of the invention.