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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . referring to fig1 - 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 ( fig1 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 fig3 , 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 . 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 . fig3 is a cross section through the elevation shown in fig2 along the plane a - a . in fig1 and 3 , like elements are given like numbers . 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 . referring to fig4 - 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 . fig5 is a perspective view of the assembled device . the view in fig5 is from the opposite side of that in fig4 , so that clockwise motions in fig4 are counterclockwise in fig5 . like elements from fig4 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 . fig6 presents a plane view of the assembled device , and fig7 presents a cross section along plane b - b . like elements from fig4 are given like numbers . as can be seen in fig7 , 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 . fig8 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 fig1 - 3 . the pawls are held in place and biased outwardly by pawl spring 804 . fig9 is an end - on plane view of the components of fig8 . like elements from fig8 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 . fig1 is an end - on plane view of the assembled device incorporating the components of fig8 - 9 , showing the installation of drive ring 801 into a freewheel sprocket 805 . fig1 presents a cross section of the assembly of fig1 along plane c - c . like elements from fig8 - 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 . 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 . referring to fig1 - 15 , yet another embodiment of the invention is shown . fig1 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 ( fig1 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 . fig1 a 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 ( fig1 a ) and fully counterclockwise ( fig1 b ) positions . the change in the axial position of the mode selection ring is evident . fig1 a and 14b are side views of the assembled device , with the mode selection ring 1210 omitted , showing pin 1208 in fully clockwise ( fig1 a ) and counterclockwise ( fig1 b ) positions . when the mode selection ring 1210 is rotated counterclockwise until pin 1208 reaches the end of slot 1211 , as shown in fig1 b , pin 1208 engages with drive ring 1201 , as shown in fig1 a 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 . fig1 a and 15b are side views of the interior of the device , corresponding to fig1 a and 14b with the freewheel sprocket , pawls , and pawl spring omitted for clarity . when retracted , as in fig1 a , 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 fig1 b . 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 . fig1 is an end - on plane view of the assembled device incorporating the components of fig1 - 15 , showing the installation of drive ring 1201 into a freewheel sprocket 1205 . fig1 presents a cross section of the assembly of fig1 along plane a - a . like elements from fig1 - 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 fig1 - 3 and are not detailed here . 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 .