Abstract:
A magnetic braking, governing, or speed retarding system for use with a wheeled conveyance may take advantage of eddy currents induced when a magnet moves past a non-magnetic conductor. A plurality of magnets may be disposed within a rotor that rotates as a wheel axle rotates. The magnets rotate past one or more relatively stationary stators to generate eddy currents that create a resistance on the rotor, thereby acting to retard or slow the rotational speed of the rotor and the axle. The system may be particularly well-suited with a wheeled conveyance such as a sled that is gravity driven and travels downhill along a track. The speed governing system may apply lesser force in relatively flat sections of the track, due to slower wheel rotational speeds, and greater force as the conveyance attempts to pick up speed, e.g., in steeper sections.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is directed generally to the fields of magnetic speed retarding or braking systems using eddy currents and conveyances incorporating such systems. 
         [0003]    2. Description of the Related Art 
         [0004]    Wheeled conveyances such as sleds can attain high speeds when traveling downhill. Oftentimes, these conveyances travel along a course defined by a track that makes a series of turns as the track winds from the start towards a finish line. As the conveyance moves along the track, it may increase or decrease in speed due to factors such as decline angle, the presence and frequency of turns or bends, the user&#39;s weight, etc. 
         [0005]    The conveyances typically include brakes in order to stop the conveyance at the bottom of the track and to slow the conveyance at other points along the track. Traditionally, these speed retarding systems used on these conveyances have been mechanical or hydraulic in nature. Some systems apply braking forces automatically, while others require that the user applies the force manually in order to slow down the conveyance. These systems also may be affected negatively by moisture and dirt by reducing their effectiveness, increasing wear, and decreasing their useful lives. 
         [0006]    With manual systems, if the brakes are applied too late, the user may reach a high rate of speed that prevents him from fully controlling the conveyance. Conversely, too large of a braking force may be applied when it is not necessary, such as during flatter portions of the track. If this happens, the conveyance may decelerate to such a slow speed that it may be difficult to get the conveyance moving again. Additionally, mechanical brakes may lock, requiring the user to repair or replace the brakes in order to use the sled again. Mechanical brakes also may include elements such as brake pads that wear down with repeated braking and require routine maintenance and replacement, which increases down-time during which the conveyance cannot be used and also increases the cost to own and operate the conveyance. 
         [0007]    Some hydraulic brakes may lose braking effect at increased speeds. Thus, in those hydraulic braking systems, the user may experience undesirable braking effects at both high and low speeds. For example, the conveyance may not brake sufficiently at high speeds when the user desires more braking force, while it may brake too much at slower speeds, stopping or unacceptably slowing the user nearing the start of the run or during relatively flat portions of the run. In addition, regardless of whether braking increases or decreases with these hydraulic systems, hydraulic brakes also suffer from potential leaks of hydraulic fluid, which require additional downtime and increased cost for maintenance and repair, as well as clean-up of the conveyance, the track, and the user. 
         [0008]    Other conveyances may rely on centrifugal braking systems to help retard speed. These systems often result in the wheels locking up and skidding when braking pressure is applied, which may cause deformation such as flattening of the portion of the wheels in contact with the ground. As the deformation grows, the wheels may have a tendency to rest in the worn out portions as more skidding occurs, further wearing out the wheels. This may result in a bumpier ride for the user and the need to replace the wheels more frequently. 
         [0009]    What is needed is a speed retarding system that overcomes these drawbacks. 
       SUMMARY OF THE INVENTION 
       [0010]    A system for retarding the speed of a conveyance by applying a decelerating torque, i.e., a torque opposite in direction to the rotating direction. Depending on the speed and accelerating forces applied on the conveyance, the system may slow the conveyance or lessen the increase in speed of the conveyance. 
         [0011]    In one aspect, a magnetic speed governing device may include an axle coupled to at least one wheel, a rotor coupled to and configured to rotate with the axle, a stator coaxial with the axle and configured to not rotate with the axle, and a gap between the rotor and stator. The device also may include a second stator coaxial with the axle on the opposite side of the rotor as the first stator. At least one of the rotor and stator may include a magnetic material, and the other may be a non-magnetic, highly conductive material. Preferably, the magnetic material may be in the rotor and may comprise a plurality of magnets substantially similarly radially spaced from an axis of the axle. The magnets may have alternating polarities and may extend substantially from one side of the rotor to the other. 
         [0012]    The governing device also may include a second rotor coupled to a second axle, the second axle coupled to a second wheel, and a second stator coaxial with the second axle and configured to not rotate with the second axle. At least one of the second rotor and second stator may include a magnetic material, and the other may include a non-magnetic, highly conductive material (although not necessarily in the same configuration as the first rotor and stator). Here, the first and second axles may be different from one another, even though the first wheel may be on one side of a conveyance and the second wheel may be on an opposite side of said conveyance, e.g., it could be possible for two rear wheels to have separate axles. 
         [0013]    In another aspect, a magnetic speed retarding system may include a rotor rotatable on an a shaft, the rotor having a first face and a second face that may be generally perpendicular to an axis of rotation, the rotor including a plurality of magnets. The system also may include a first stationary element generally perpendicular to and spaced a first distance from the first face and a second stationary element generally perpendicular to and spaced a second distance from the second face, the first and second distances being substantially equal. The stationary elements may comprise non-magnetic, highly conductive materials, and each may have a radial extent measured from the axis of rotation that is greater than a radial extent of the rotor. 
         [0014]    The magnets may be substantially equally circumferentially spaced about the rotor&#39;s axis of rotation, and each magnet may have a diameter of about ½″. The magnets also may have alternating polarities, i.e., if one magnet has a positive polarity in one direction, an adjacent magnet has a negative polarity in that direction. Magnets preferably are permanent magnets such as rare earth magnets, e.g., neodymium magnets. 
         [0015]    In still another aspect, a wheeled conveyance including a magnetic retarding system may comprise a frame, a plurality of wheels coupled to at least one axle, a bracket coupled to the frame for operably coupling the axle to the frame, a rotor configured to rotate as one of the axles rotates, a stator operably coupled to the frame and configured to not rotate as the axles rotates, and a plurality of magnets disposed within said rotor and spaced generally equally circumferentially about an axis of rotation of said rotor. The stator may be a non-magnetic, highly conductive material, and the rotor and stator may be separated by an air-filled gap, which may be generally planar and may have a generally constant thickness. Additionally, the conveyance may include a mechanical brake. 
         [0016]    The conveyance also may include a second stator operably coupled to the frame, and the first and second stators may be on opposite sides of the rotor. Here, the magnets may be disposed substantially completely through the rotor, so that a single magnet may act on both stators. 
         [0017]    The bracket that operably couples the axle to the frame also may include a spanning portion extending width-wise across at least a portion of the frame, an arm extending generally perpendicular to the spanning portion and generally length-wise across at least a portion of the frame, and an axle mount extending generally parallel to and away from the arm, said axle mount including an opening through which said one of said at least one axles is disposed. In addition, the stator also may be fixedly coupled to the bracket. 
         [0018]    These and other features and advantages are evident from the following description, with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]      FIG. 1  is a perspective view of a conveyance that may include a magnetic speed retarding or governing system. 
           [0020]      FIG. 2  is a rear view of a conveyance including a magnetic speed retarding or governing system. 
           [0021]      FIG. 3  is a detail view of the magnetic speed retarding or governing system of  FIG. 1 . 
           [0022]      FIG. 4  is a side, detail view of a rotor and stator used with the speed retarding or governing system of  FIG. 1 . 
           [0023]      FIG. 5  is a rear view of a conveyance including a second embodiment of a magnetic speed retarding or governing system. 
           [0024]      FIG. 6  is a side view of another configuration of magnets disposed in a speed retarding system. 
           [0025]      FIG. 7  is a side view of still another configuration of magnets disposed in a speed retarding system. 
           [0026]      FIG. 8  is a rear, section view of another embodiment of a magnetic speed retarding or governing system. 
           [0027]      FIG. 9  is a side view of another embodiment of a reaction plate, which may be a rotor or stator and may be used in combination with a magnetic disc. 
           [0028]      FIG. 10  is a section view of a combination of a pair of the reaction plates of  FIG. 9  and a magnetic disc, taken through section  10 - 10  in  FIG. 9 . 
           [0029]      FIG. 11  is a side view of another embodiment of a magnetic disc, which may be a rotor or stator and may be used in combination with a reaction plate. 
           [0030]      FIG. 12  is a side view of yet another embodiment of a magnetic disc, which may be a rotor or stator and may be used in combination with a reaction plate. 
           [0031]      FIG. 13  is a side, section view of an alternative embodiment of a magnetic speed retarding or governing system. 
           [0032]      FIG. 14  is a rear, section view of another alternative embodiment of a magnetic speed retarding or governing system. 
           [0033]      FIG. 15  is a rear, section view of still another alternative embodiment of a magnetic speed retarding or governing system. 
           [0034]      FIG. 16  is a graph showing the relationship between radial distance for the magnets and retarding torque as a function of rotational rate. 
           [0035]      FIG. 17  is a graph showing the relationship between the number of magnets and retarding torque as a function of rotational rate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    As described herein, a magnetic speed retarding system  12  uses an induced eddy current to govern and/or brake a conveyance  10 . As seen in  FIG. 1 , conveyance  10  may be of a type that rolls downhill within a track  2 . Here, conveyance  10  also may be called a sled. 
         [0037]    Conveyance  10  may include a body or frame  14  having a top  16  and an underside  18 . A rider may be disposed on the top  16  during use, and system  12  may be disposed substantially between underside  18  and track  2 . 
         [0038]    Conveyance may include a plurality of wheels, including front wheels and rear wheels  22 . Rear wheels  22  may be disposed proximate a rear end of frame  14 . Front wheels may be disposed proximate a front end of frame, but preferably, front wheels may be more centrally disposed between front and rear ends. 
         [0039]    In this embodiment, speed retarding system  12  preferably acts on rear wheels  22 , but a similar system may apply additionally or alternatively to front wheels. System  12  is described here with respect to one of rear wheels  22 , and similar components may be used with respect to the other wheels. 
         [0040]    Wheel  22  may be coupled to axle  24  via hub  26 . Hub  26  may be formed integrally with axle or may be coupled to axle  24  in any manner that prevents hub  26  from moving relative to axle  24  during use. For example, hub  26  may include an opening  28 , which may be threaded and sized to receive a fastener  30 . Fastener  30  may pass through hub  26  and either into a matching opening in axle  24  or, alternatively, may engage outer surface of axle  24 , creating an interference fit between axle  24  and hub  26 . 
         [0041]    Preferably, conveyance  10  includes separate axles  24  for each of rear wheels  22 , which may allow each wheel to rotate independently of the other and at a different speed from the other, e.g., when conveyance  10  moves through a curve. In another embodiment, as seen in  FIG. 5 , conveyance  110  may include a single axle  124  for each pair of wheels, such as rear wheels  122 . In this figure, a single speed retarding system  112  is shown, e.g., substantially centrally disposed between wheels  122 . Alternatively, a plurality of speed retarding systems may be disposed along axle  124 , e.g., generally symmetrically about a center of axle  124 . 
         [0042]    Returning to  FIGS. 2-4 , speed retarding system  12  may couple to conveyance  10  via mounting bracket  32 , preferably at a plurality of locations on conveyance. Mounting bracket  32  may include various components, which may be separate and spaced from each other, coupled together, or formed integrally. Mounting bracket  32  may include a spanning portion  34 , which may extend substantially across a width of conveyance  10 . Spanning portion  34  may anchor parts of system  12  that do not rotate with wheels  22 . In addition, spanning portion  34  may be common to elements of system  12  for each of rear wheels  22 , i.e., conveyance may include a single spanning portion  34  for both wheels  22 . Spanning portion  34  may be a relatively shallow plate, which may maximize clearance between conveyance  10  and track  2 . For example, spanning portion may have a thickness between about ⅛″ and about 1″, preferably between about ¼″ and about ⅝″. 
         [0043]    Mounting bracket  32  further may include one or more arms  36 . Preferably, mounting bracket  32  includes a plurality of arms  36  for each axle  24 , still more preferably, mounting bracket  32  includes two arms  36  for each axle  24 . 
         [0044]    Arms  36  may be coupled to, may extend rearwardly from, and may be generally perpendicular to, spanning portion  34 . A first arm may be disposed proximate the end of axle  24  opposite wheel  22 . A second arm may be disposed between first arm and wheel  22 , e.g., approximately as equally spaced from rotor  50  (discussed below) as the first arm is. 
         [0045]    Staying with  FIG. 2 , mounting bracket  32  also may include one or more flanges or axle mounts  38 . Mounting bracket  32  may include a substantially similar number of arms  36  and axle mounts  38 , and each axle mount  38  may extend generally perpendicular to a respective arm  36 , e.g., generally upward toward underside  18  of conveyance  10  when in use. 
         [0046]    Each axle mount  38  may include an opening  40  disposed laterally through mount  38 , i.e., in the same direction as the length of axle  24 . Axle  24  may pass through and be supported by opening  40 . Bearing  46  may be disposed in opening  40  to facilitate rotation and reduce friction of axle  24 . Axle  24  may include a channel  48  at a location aligned with bearing  46 , which may cause bearing  46  to overlap radially with an outer diameter of the remainder of axle  24 , constraining axle  24  and preventing lateral movement of axle  24  during use. Axle  24  then may be restrained proximate axle mount  38  via one or more retaining rings  49 , e.g., one on each side of axle mount  38 . 
         [0047]    Mounting bracket  32  also may include or be operatively coupled to one or more braces  37 . Braces may couple to axle mounts  38  and extend upward toward underside  18  of frame  14 . Preferably, a distal end of one or more braces  37  contacts and/or is coupled to frame  14 , which may provide added rigidity to mounting bracket  32 . Distal end of brace  37  may include a opening  42  configured to receive a fastener, and fastener may be sized and inserted either to bring brace  37  and frame  14  together or to allow for a gap between brace  37  and frame  14 , while still keeping those elements coupled. 
         [0048]    In one embodiment, mounting bracket  32  may be cast as a single piece. In another embodiment, as shown in the figures, elements of mounting bracket  32  may be coupled together via a plurality of fasteners configured to be received in a plurality of openings. For example, fasteners  44  may be received in openings  42  to couple arms  36  to spanning portion  34 . Similarly, fasteners  44  may be received in openings  42  to couple axle mounts  38  to arms  36 . 
         [0049]    Turning now to  FIG. 3 , speed retarding system  12  may comprise rotor  50  coupled to, and rotating with, axle  24 . Rotor  50  may be substantially cylindrical or disc-shaped, i.e., generally free from eccentricities, to provide for more consistent rotation of axle  24 . Rotor  50  may be integrally formed with axle or, alternatively, may include an opening  60  through which axle  24  passes. In the latter case, rotor  50  may be coupled to axle  24  in at least one of a variety of manners, including, e.g., via a key-type connection, through the use of one or more set screws, or via press-fit onto axle  24 , i.e., via an interference fit that is loose enough to allow for assembly but tight enough to prevent rotor  50  from moving with respect to axle  24  during use. 
         [0050]    Rotor  50  may be between about 2″ and about 6″ in diameter, preferably between about 3″ and about 5″, still more preferably about 4″ in diameter. Rotor  50  may have a thickness between about ¼″ and about 1″, preferably about ½″, although, as with rotor diameter, rotor thickness may be adjusted to allow for a different number, size, and/or thickness of magnets. 
         [0051]    Rotor  50  may include a magnetic material, preferably a permanent magnetic material, still more preferably a rare earth magnetic material, e.g., a neodymium magnetic material such as neodymium iron boron (NdFeB) or samarium cobalt (SmCo). Alternatively, other magnets such as aluminum nickel cobalt (AlNiCo) may be used. In one embodiment, substantially all of rotor may comprise the magnetic material. 
         [0052]    Preferably, however, rotor  50  may comprise magnetic material  52  interspersed between portions of non-magnetic material  54 . Rotor  50  may include a plurality of openings  62 , which may be substantially equally circumferentially spaced and which also may be spaced radially substantially the same distance from a center of rotor. In another embodiment, openings may be radially staggered around rotor, or rotor may include a first set of openings at a first radial distance and at least a second set of openings at at least a second radial distance. 
         [0053]    Openings  62  may extend inward from one or both of first face  56  and second face  58 . Preferably, openings  62  extend completely through rotor  50 , from first face  56  to second face  58 , although other configurations are possible. For example, openings may extend only partially through rotor on first face  56 , and additional openings may extend only partially through rotor on second face  58 . In that embodiment, first face openings may overlap or, alternatively, may be circumferentially offset from second face openings. 
         [0054]    Returning to the embodiment shown in  FIG. 3 , in which openings  62  extend from first face  56  to second face  58 , magnets  64  may be disposed within openings  62 . Each magnet  64  may be substantially similar to the other magnets, i.e., they may be similarly sized and have similar magnetic strength values (i.e., remanence, in the case of permanent magnets). One example of magnets usable with speed retarding system  12  may be Bunting Magnetics N35p500500 magnets, which may be generally cylindrical, with a diameter of about ½″ and a length also of about ½″, as well as a holding power of about 11½ lbs. 
         [0055]    Magnets  64  also may be sized to fit substantially flush against first face  56  and second face  58 , i.e., magnets  64  may have a thickness substantially equal to rotor thickness. Alternatively, magnets  64  may be embedded slightly with respect to first face  56  and/or second face  58 , e.g., up to about 1/16″. Still further, although less preferably, magnets  64  may protrude slightly from first face  56  and/or second face  58 , but magnets  64  should not protrude so far as to eliminate gap  76  between rotor  50  and one or more of stators  70  (discussed below). 
         [0056]    Circumferentially consecutive magnets  64  may be aligned to have the same polarities. Preferably, however, magnets may be configured to have alternating polarities. For example, as seen in  FIG. 4 , a magnet having a “north” polarity  66  proximate first face  56  may be surrounded by a plurality of magnets having “south” polarities  68 , and vice versa. As such, the system preferably includes an even number of magnets  64  disposed within rotor  50  so that magnets with the same polarities are not adjacent to one another. 
         [0057]    All other things being equal, a greater number of magnets has an increased braking effect. Speed retarding system  12  may include between about 2 and about 20 magnets (although more magnets are possible, depending on the size of the magnets and rotor, the strength of the magnets, and the desired amount of braking force), preferably between about 6 and about 16 magnets, and in one embodiment, about 12 magnets. Other alternative arrangements, having 10 and 8 magnets are shown in  FIGS. 6 and 7 , respectively. 
         [0058]    Returning to  FIG. 3 , speed retarding system may include at least one stator  70  disposed proximate rotor  50 . Preferably, system  12  includes a pair of stators for each rotor; one proximate first face  56  and the other proximate second face  58 , and stators may be substantially similar to each other. Stator  70  is configured to remain substantially stationary with respect to rotor  50 . In addition, stator  70  may include a face  78  proximate rotor  50  that is substantially planar and substantially parallel to first and/or second face of rotor  50 . 
         [0059]    Stator  70  may be fixedly coupled to frame  14 , either directly or indirectly, e.g., via coupling to mounting bracket  32 . In one embodiment, bracket  71  may couple to both mounting bracket  32  and stator  70 . Bracket  71  may be generally L-shaped to sit flush against generally perpendicular surfaces of mounting bracket  32  and stator  70 . Each of mounting bracket  32 , stator  70 , and bracket  71  may include openings configured to receive fasteners to couple these elements together. Alternatively, bracket  71  may be integrally formed with one or both of mounting bracket  32  and stator  70   
         [0060]    Like axle mounts  38 , each stator  70  may include an opening  72  through which axle  24  is disposed. Opening  72  also may include bearing  74  to reduce frictional forces between axle  24  and stator  70 . 
         [0061]    Stator  70  may extend outward from axle axis at least as much as radial extent of rotor  50  or a furthest radial extent of magnets  64  disposed within rotor  50 . Additionally, each stator  70  may be about as thick or, preferably, thicker than rotor  50 . 
         [0062]    In one embodiment, e.g., rotor may be about ½″ thick and stator may be at least about twice as thick, or about 0.6″. 
         [0063]    Additionally, stator  70  may have a radius or height extending downward from axis that is sized to maintain clearance between braking system  12  and track  2 . As seen in  FIG. 3 , stator  70  may not extend about as low as bottoms of arms  36 . Clearance in other directions may be as or more important, e.g., stator  70  preferably does not extend rearward beyond a rear end of frame  14 . In a forward direction, stator  70  may be sized so as to not interfere with spanning portion  34 , although a portion of spanning portion  34  may be removed to provide additional clearance, much like a portion of stator mounting bracket  71  has been removed to provide clearance for stator  70 , as seen in  FIG. 4 . There also preferably is sufficient clearance above stator  70 , i.e., between stator  70  and underside  18  of frame  14  to allow for flexion of frame during use. 
         [0064]    Stator  70  preferably is a highly conductive, non-magnetic material. In addition, both stator  70  and non-magnetic portions of rotor  50  preferably comprise materials that help dissipate heat, particularly when magnets  64  are rare earth magnets, which tend to have low Curie temperatures, i.e., they can lose their magnetic properties at high temperatures. As such, several choices for rotor  50  and/or stator  70  materials include aluminum, aluminum alloys (including aircraft aluminum), copper, gold, etc. In one embodiment, 6061-T6 aluminum may be used. 
         [0065]    In another embodiment, as seen in  FIG. 8 , instead of rotor  50  containing magnetic material, rotor  250  may comprise a substantially uniform non-magnetic, highly conductive material. Similarly, one or both of stators  270  may contain magnetic material, e.g., each stator  270  may be a magnet. Preferably, stator  270  may be a substantially stationary plate, e.g., having a ferritic backing plate in which magnets  264  are embedded within the stator  270 . In this embodiment, if stator comprises one large magnet, that magnet&#39;s polarity may be the same or the reverse of the opposing stator. If stator  270  comprises a plurality of magnets  264 , adjacent magnets  264  on the same stator  270  may have the same or reversed polarities. Additionally, magnets  264  on opposing sides of rotor  250  may be aligned with or offset from one another, radially and/or circumferentially. In any of these embodiments, magnets may remain substantially stationary with respect to conveyance  10 , while rotor  250  rotating past magnets  264  may generate eddy currents and a braking effect. 
         [0066]    In order to assist with heat dissipation, one or both of rotor and stators may include fins or other heat dissipating elements, as seen in the various embodiments of  FIGS. 9-12 .  FIG. 9  illustrates an alternative reaction plate, i.e., non-magnetic element, which may be either a rotor or stator. Fins  380  preferably are generally evenly and uniformly distributed over side of reaction plate facing away from magnetic disc. Additionally, fins  380  may extend outward a substantially uniform distance, which may allow for substantially uniform cooling, although fins  380  may extend outward different amounts. For example, radially outward fins may extend a smaller distance than radially inward fins, reducing inertial drag. 
         [0067]      FIG. 10  illustrates the alternative reaction plate of  FIG. 9  in cross-section with a magnetic disc and a similar, complementary reaction plate opposite the magnetic disc. Fins  380  on the reaction plate may extend around at least one side of the disc, which may provide for convective cooling while minimizing or eliminating disturbances to magnetic fields extending from the disc. Additionally, were magnetic disc not disposed between a plurality of reaction plates, magnetic disc may include fins to extend outward from a side of magnetic disc facing away from reaction plate, i.e., on a side generally opposite magnets  364 . In the previous case or in the event the magnetic disc is sandwiched between a plurality of reaction plates, magnetic disc also may include fins  382  such as those shown in  FIG. 11 , the fins extending generally perpendicular to the axis of rotation. 
         [0068]    Still another version of a magnetic disc may be seen in  FIG. 12 . In this embodiment, the disc may include a plurality of channels  484 . Magnets  464  may be disposed alongside channels  484 , which may act as impellers to move more air around magnetic disc, aiding in cooling. Alternatively, channels  484  may surround respective magnets  464 , e.g., channels may include a plurality of generally linear sidewalls with a curved portion between them. One or both sidewalls may be angled relative to a radius, and the angle of each sidewall may be controlled to direct airflow. Essentially, some of one or more faces of the magnetic disc may be channeled in comparison to another part or parts of the face, which may be comparatively protruding. In the embodiment seen in  FIG. 12 , magnetic disc may be configured to rotate counter-clockwise, so a leading sidewall may be angled more steeply than a trailing sidewall. 
         [0069]    Fins preferably add mass to non-fin versions of rotor  50  and stators  70 , as reducing mass may lead to increased heat generation or retention instead of heat dissipation. 
         [0070]    Turning now to  FIG. 13 , yet another embodiment of a speed retarding system  512  is shown. In this embodiment, instead of orienting magnets to have poles generally parallel to axis of rotation, magnets  564  may be disposed with poles generally perpendicular to axis of rotation, e.g., in a generally radial direction. For the sake of convenience, magnetic disc may be considered rotor  550  and reaction plate may be stator  570 , but as in the other embodiments, magnetic disc alternatively may be a stator and reaction plate may be a rotor. Here, rotor  550  may include one or more extensions  557  extending away from first face  556  and towards stator  570 . Similarly, stator  570  may include one or more channels  577  extending inward from a stator face  578  that are configured to receive extensions  557 . Each magnet  564  may extend from its own extension  557 , or extension  557  may include a ring having generally constant inner and outer diameters. In either case, channel  577  preferably is a continuous substantially circular groove disposed entirely around stator  570 , where channel  577  provides clearance for each extension  557  as rotor  550  rotates about an axis. 
         [0071]    Sled may include one or more axles for rear wheels, and one or more speed retarding systems per axle. Speed retarding systems may be substantially similar to one another, or they may differ, e.g., one axle may use the system shown in  FIG. 3  while a second axle may use the system shown in  FIG. 11 . 
         [0072]    Still other embodiments of a magnetic braking or retarding system may be seen in  FIGS. 14 and 15 . In  FIG. 14 , system  612  may be disposed proximate to or formed integral with wheel  622 . Hub  626  may comprise and/or be coupled to rotor  650 , rotating as wheel  622  rotates. Axle  624  may be rotatably coupled to hub  626 , e.g., via bearings  646 . In addition, stator  670  may be fixedly coupled to axle  624  so as to not rotate as wheel  622  rotates. Magnetic material  664  dispersal in stator  670  may be similar to the various possible arrangements described above with respect to the embodiment of  FIGS. 2-4 . Moreover, a second rotor  650  may fixedly couple to hub  622 , and also may be rotatably coupled to axle  624 , again, e.g., via bearings  646 . Additionally, while not shown, one or more of rotors  650  and stator  670  may include fins for heat dissipation. 
         [0073]    In the speed retarding system  712  of  FIG. 15 , magnets  764  may be disposed in hub  722 , which may be rotatably coupled to axle  724 , e.g., via bearings  746 . Like the embodiment of  FIG. 13 , magnets  764  may be disposed with their poles generally perpendicular to axis of rotation, whereas poles in  FIG. 14  may be disposed generally parallel to axis. Stator  770  may be fixedly coupled to axle  724  and may include a groove through which magnets  764  pass. Stator  770  also may include fins  780  to assist with heat dissipation. 
         [0074]    Rotation of rotor causes magnets  64 , and their corresponding magnetic fields, to move circumferentially past stators  70 , which generate induced eddy currents, resisting rotation of rotor  50 , leading to a braking or governing action for conveyance  10 . 
         [0075]    Resistive torques may be affected by several factors, including the size, number, and strength of magnets  64  and the distance between rotor  50  and stators  70 . Stronger magnets create larger eddy currents and increased governing. Similarly, larger eddy currents are created when rotor  50  is closer to stators. Preferably, gap is between about 10/1000″ and about ⅛″, still more preferably between about 20/1000″ and about 1/16″, even more preferably between about 25/1000″ and about 40/1000″, and in one embodiment, about 1/32″. 
         [0076]    At lower speeds, eddy currents may be significantly lower than at increased speeds, so resistance caused by system  12  may be minimal. Rotational speed of rotor  50  affects resistive torques and forces, i.e., those forces increase with speed increases, as seen in the graphs of  FIGS. 16 and 17 . This may depart from the principle of operations of some sled hydraulic braking systems, which tend to apply greater forces at lower speeds that diminish as speed increases. Users, however, may not wish to experience significant braking at lower speeds, whereas more braking at increased speeds may be desirable to assist in controlling conveyance  10 . 
         [0077]    As  FIG. 16  shows, retarding torque increases as magnets are moved further away from an axis of rotation. In addition,  FIG. 17  illustrates that increasing the number of magnets used increases the retarding torque. In both instances, after subtracting out the retarding torque attributed to bearing friction, it can be seen that retarding torque at high RPMs may be between about 2 times and about 4 times greater than at low RPMs. 
         [0078]    Without speed retarding system, wheels  22  on conveyance  10  may reach, e.g., between about 4,000 and about 5,000 rpm. In contrast, a conveyance  10  such as the one seen in  FIG. 2 , with a system having about twelve neodymium magnets may have a maximum speed restricted to between about 2,500 and about 3,000 rpm, even on straightaway decline portions of track  2 . Thus, system  12  also may act as a governor to limit maximum speed of conveyance. 
         [0079]    Speed retarding system  12  may overcome many of the drawbacks of mechanical and hydraulic braking systems. For example, with no parts to interface or rub together, system  12  may be substantially wear-free. Additionally, performance of system  12  may be substantially unaffected by water, dirt, and other debris, as eddy currents and resistive magnetic fields may be able to travel through these media and create drag on conveyance. 
         [0080]    In addition to eddy current speed retarding system  12 , conveyance also may include a mechanical braking system. For example, front wheels may be pivotable and/or translatable with respect to frame  14 . A joystick or other control may be coupled to front wheels to change height of front wheels relative to underside  18  of frame  14 . At rest, front wheels may be raised so that underside  18  or pads/rails  19  proximate underside  18  contact surface of track  2 . The user may release the manual braking mechanism, lowering front wheels so that they contact track  2 , and causing separation between underside or pads  19  and track  2 . From here, conveyance may progress forward, with speed retarding mechanism taking over, as described above. 
         [0081]    While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific exemplary embodiment and method herein. The invention should therefore not be limited by the above described embodiment and method, but by all embodiments and methods within the scope and spirit of the invention as claimed.