Priority is claimed to provisional application Ser. No. 60/860,570, filed Nov. 21, 2006, entitled: A Two-Wheeled Human-Powered Vehicle Propelled by an Elliptical Drive Train, which is referred to and incorporated herein in its entirety by this reference. 


   FIELD OF THE INVENTION 
   The present invention generally relates to bicycles. More particularly, the invention concerns a bicycle having an elliptical drive train. 
   BACKGROUND OF THE INVENTION 
   The most common human-powered vehicle is the bicycle. Use of the bicycle for exercise, recreation, and transportation is well-known. Operators of conventional bicycles are in a seated position and pedal in an essentially circular motion to perform the mechanical work necessary to propel the vehicle. During operation, the operator&#39;s upper body is typically bent forward at the waist and held in place by the muscles of the arms, shoulders, abdomen, and lower back. This most common riding position is relatively stressful. Bicycle riders often experience pain, discomfort, and/or numbness in the pelvic region from sitting on the bicycle seat or “saddle”, and discomfort in the lower back, arms, and shoulders from the bent-over riding position. 
   To alleviate the discomfort associated with prolonged use of conventional bicycles, recumbent bicycles in which the operator propels the bicycle from a reclined position are known. Although recumbent bicycles alleviate much of the discomfort associated with conventional bicycles, the reclined riding position makes these vehicles less stable and more difficult to ride. The recumbent bicycle is also limited as a commuter vehicle because the low-to-the-ground configuration allows obstacles to easily obstruct the operator&#39;s line of sight and makes him or her less visible to other vehicles, cyclists, and pedestrians. In addition, because operators of conventional and recumbent bicycles are seated, they do not receive the musculoskeletal benefits of weight-bearing exercise when operating these vehicles. 
   Therefore, there remains a need to overcome one or more of the limitations in the above-described, existing art. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective view of one embodiment of the bicycle; 
       FIG. 2  shows a side elevation view of the bicycle of  FIG. 1 , depicting schematically the elliptical pedaling profile; 
       FIG. 3  shows a side elevation view of the bicycle of  FIG. 1 , depicting schematically an operator&#39;s zone; 
       FIG. 4  shows a perspective view of another embodiment of the bicycle; 
       FIG. 5  shows a perspective view of yet another embodiment of the bicycle; 
       FIG. 6  shows a perspective view of yet another embodiment of the bicycle; 
       FIG. 7  shows a perspective view of yet another embodiment of the bicycle that includes an adjustable guide track; 
       FIG. 8  shows a close-up perspective view of adjustable crank arms that may be coupled to the bicycle; 
       FIG. 9A  shows a perspective view of one embodiment of an adjustable foot platform that may be coupled to the bicycle; 
       FIG. 9B  shows a perspective view of another embodiment of an adjustable foot platform that may be coupled to the bicycle; 
       FIGS. 10A-L  show side elevation views of different embodiments of load wheel retention devices that may be coupled to the bicycle; 
       FIG. 11  shows a perspective view of one embodiment of an adjustable steering tube that may be coupled to the bicycle; 
       FIG. 12  shows a perspective view of one embodiment of a direct drive system that may be coupled to the bicycle; 
       FIG. 13A  shows a perspective view of another embodiment of the bicycle that includes a foldable frame; 
       FIG. 13B  shows a perspective view of the bicycle depicted in  FIG. 13A  after it has been folded; and 
       FIG. 14  shows a perspective view of yet another embodiment of the bicycle. 
   


   It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. The Figures are provided for the purpose of illustrating one or more embodiments of the bicycle with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the bicycle of the present invention. It will be apparent, however, to one skilled in the art that the bicycle may be practiced without some of these specific details. For example, a variety of load wheel retention devices may be employed. Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than as limitations on the bicycle. That is, the following description provides examples, and the accompanying drawings show various examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are merely intended to provide examples of the bicycle rather than to provide an exhaustive list of all possible implementations of the bicycle. 
   The present disclosure relates generally to human-powered transportation, and more specifically to transport, exercise, and recreational vehicles powered by an elliptical pedaling motion that generally mimics the kinematics of walking or running. The apparatus, of which one embodiment is a bicycle, described herein provides an improved means of human-powered transportation that has advantages over conventional bicycles, scooters, upright step-cycles, and other human-powered vehicles. 
   As defined herein, a “bicycle” is every vehicle propelled, at least in part, by human power in the form of feet, or hands, acting upon pedals, having at least two wheels, except scooters and similar devices (which are defined as vehicles operated by a foot contacting the ground). The term “bicycle” also includes three and four-wheeled human-powered vehicles. 
   More specifically, disclosed herein is a low cross-over height bicycle powered by an elliptical pedaling motion that generally mimics the kinematics of running or walking and provides a means of human-powered transportation that has advantages over conventional upright step-cycles, bicycles, and scooters. Also disclosed herein are methods for enabling the operator to adjust the pedaling profile of such a vehicle. 
   An upright step-cycle is known in the art, but has several drawbacks. For example, the length of the wheelbase of several upright step-cycles described limits the means by which they can be transported in another vehicle, such as a passenger car, and prevents them from being turned around on narrow bike paths or streets without the operator dismounting the vehicles. Conventional upright step-cycles also have sprockets and a chain positioned near the operator. If contacted, these moving parts can damage the operator&#39;s clothing and/or injure the operator. Furthermore, conventional upright step-cycles have frames that position support structures in the operator&#39;s zone, discussed in detail below. Frame members located in the operator&#39;s zone are likely to injure the operator if he or she contacts them while riding or during a fall. Frame members located in the operator&#39;s zone also make mounting and dismounting the vehicle more difficult. 
   In addition, conventional upright step-cycles lack several features that could enable the operator to easily modify the vehicle&#39;s pedaling profile and thereby allow a single vehicle to be adjusted to accommodate a wide range of different sized riders. As discussed above, the path of motion through which the operator&#39;s foot travels while pedaling these vehicles is generally elliptical. Some people prefer this generally elliptical motion to the generally circular motion used to propel a conventional or recumbent bicycle because the generally elliptical motion more closely mimics walking, running, or climbing and has been shown to be a more effective means for strengthening the leg muscles than cycling while avoiding much of the stress and impact generated by running. However, because the pedaling motion mimics human movement, operators with different anatomical dimensions will generally require different pedaling profiles. Specifically, a taller operator would likely require a pedaling profile with a longer stride length than a shorter operator would. In addition, a more aggressive operator might prefer a steeper foot platform take-off angle so that he or she could generate more low-end torque, while a less aggressive rider might prefer a flatter pedaling profile to reduce foot and knee flexion during the pedal stroke. 
   As discussed below, the shape of the pedal stroke is generally determined by the length of the crank arms, the length of the foot links, the location of the foot platforms on the foot links, and the angle of the foot link guide tracks. Conventional upright step-cycles lack easy methods for adjusting the length of the crank arms and the location of the foot platforms. Enabling operators to easily optimize the pedaling profile by adjusting these aspects of the propulsion system would enhance the functionality of an upright step-cycle. 
   Another form of human-powered transportation is the scooter. Conventional scooters are operated in a standing position. The operator propels a scooter forward by pushing one leg against the ground in a rearward direction. Scooters have the advantage of being more comfortable to ride than conventional bicycles without many of the drawbacks of recumbent bicycles. Because the operator of a scooter rides in an upright position, he or she does not experience the numbness and pain caused by sitting on a seat or saddle. In addition, the operator is less susceptible to shoulder and lower back pain because he or she is not hunched over the handlebars. As compared to a recumbent bicycle, the operator&#39;s standing position reduces the likelihood that his or her line of sight will be obstructed and makes him or her more visible to other vehicles and pedestrians. A scooter is also more stable and easier to ride than a recumbent bicycle, thereby reducing the frequency of falling for unskilled operators. Moreover, riding a scooter is a weight-bearing exercise that provides the operator with a means of strengthening the leg muscles and bones that is not available to operators of conventional and recumbent bicycles. 
   However, the scooter does have disadvantages. Although an operator can travel longer distances at higher speeds on a scooter than he or she generally could by walking or running, a scooter&#39;s propulsion mechanism is not very efficient, especially when compared to that of a conventional bicycle. As a result, scooters are generally not used for business commuting, sustained exercise, or for other applications that require long-distance or high-speed travel. 
   Mechanical devices that improve the efficiency of conventional scooters are known. A typical pedal-driven scooter is propelled forward by the operator pumping one or two platforms up and down. Although this mechanism can be a more efficient means of propulsion than pushing backwards against the ground, it is not ideal because it must be translated into rotational motion to propel the vehicle forward. These mechanisms can also cause knee injuries because of the operator&#39;s need to reverse his or her leg&#39;s direction of motion at the top and bottom of each pedal stroke. Therefore, the introduction of a more efficient and lower impact scooter propulsion system would enhance the utility of pedal driven scooters. 
   With reference now to the Figures, disclosed herein is an operator-propelled vehicle in which rotation of left and right crank arms causes the respective left and right foot platforms to move along an elliptical path. The term “elliptical” with regard to “elliptical pedaling motion” or “elliptical pedaling profile” or “elliptical path” or “elliptical motion” is intended in a broad sense to describe a closed path of motion having a relatively longer first axis and a relatively shorter second axis (which extends perpendicular to the first axis as in an ellipse). 
   The embodiments shown and described herein are generally symmetrical about a vertical plane extending lengthwise. Reference numerals are generally used to designate both the “right-hand” and “left-hand” parts, and when reference is made to one or more parts on only one side of the apparatus, it is to be understood that corresponding part(s) may be disposed on the opposite side of the apparatus. The portions of the frame that are intersected by the plane of symmetry exist individually and thus, may not have any “opposite side” counterparts. Also, to the extent that reference is made to forward or rearward portions of the apparatus, it is to be understood that the drive arm assembly is movable in either of two opposite directions. 
     FIG. 1  shows a general embodiment of the apparatus, or bicycle  100 . The apparatus  100  generally includes a foot link assembly  105  movably mounted on a frame, or frame structure  110 , on which a pair of wheels (front wheel  115 , rear wheel  117 ) are mounted. Generally, each foot link assembly  105  is moveably mounted to the frame  110  at its forward end where it is slideably coupled to a foot link guide track  255  and at its rearward end where it is rotatably coupled to the crank assembly  215 . 
   Generally, each foot link assembly  105  includes a foot link  205 , each with a foot platform  210 , and a load wheel  250 . The foot platforms  210  on which the operator stands are mounted on an upper surface of each foot link  205  near a forward end of each foot link  205 . Below each foot platform  210  near the frontal section of each foot link is a load wheel  250  that contacts a sloping foot link guide track  255 . In the embodiment depicted in  FIG. 1 , two foot link guide tracks  255  run parallel to each other on either side of the longitudinal axis of the apparatus  100  and are integral with the frame  110 . The load wheel  250  and bearings are mounted to a fixed axle to allow nearly frictionless linear motion of the foot links  205  along the foot link guide tracks  255  and provide rotational freedom of the foot links  205  with respect to the foot link guide tracks  255 . 
   As shown in  FIG. 2 , during pedaling, the operator (not shown) uses his mass in a generally downward and rearward motion as in walking or jogging to exert a force on the foot platforms  210  and thereby, the foot links  205 . This force causes the load wheel to roll down the slope of the foot link guide track  255  towards the rear of the apparatus  100  and rotate the crank arms  235  about the crank arm bearing  245 , turning the drive sprocket  240 . As with conventional bicycles, rotating the drive sprocket  240  causes the rear wheel sprocket  135  to rotate because they are linked by a chain or belt  130 . It will be appreciated that the chain or belt  130  may also comprise a rotating shaft or other drive means. Rotating the rear wheel sprocket  135  causes the rear wheel  117  to rotate because the rear wheel sprocket is attached to the rear wheel hub  145 . Rotating the rear wheel  117  provides motive force that enables the apparatus  100  to move along a surface. The apparatus  100  can employ a “fixed” or “free” rear wheel, as is known in the art. The apparatus  100  can also employ a planetary gear hub having different gear ratios, as manufactured by Shimano, Sturmey-Archer and others. 
   One feature of the apparatus, or bicycle  100  is that the pedaling motion described above results in the operator&#39;s foot traveling in a shape that can be described as generally elliptical. Propulsion using an elliptical pedaling motion, as opposed to an up and down pedaling motion or a circular pedaling motion, has the advantage of substantially emulating a natural human running or walking motion. Further, an elliptical pedaling motion is a simpler and more efficient means to rotate the rear wheel  117  than is, for example, a vertical pumping motion. Moreover, the major axis of the ellipse in an elliptical pedaling motion can be much longer than the stroke length of a circular or vertical pumping pedaling motion, allowing the operator to employ a larger number of muscle groups over a longer range of motion during the pedal stroke than he or she could employ in a circular or up and down pedaling motion. 
   As shown in  FIG. 2 , dashed line E depicts the generally elliptical path that the ball of the operator&#39;s foot would take throughout the pedaling motion. The region where the ball of the operator&#39;s foot contacts the foot platform  210  is labeled as item F. The power stroke during forward motion is from front-to-back and follows the lower half of the elliptical path E. As the operator&#39;s foot moves rearward through the power stroke of the described elliptical pedaling motion, the heel portion falls more quickly than does the toe portion. The return stroke during forward motion is from back-to-front and follows the upper half of the elliptical path E. As the operator&#39;s foot moves forward through the return stroke of the described elliptical pedaling motion, the heel portion of the foot rises more quickly than does the toe portion. 
   As illustrated in  FIG. 2 , the shape of the elliptical path E is generally defined by the following parameters: (1) the length of the major axis A; (2) the length of the minor axis B; and (3) the major axis angle γ. The length of the major axis A is generally equal to the stride length of the pedaling motion. The length of the minor axis B relative to the length of major axis A generally determines the vertical lift of the operator&#39;s foot and angular foot plantar-flexion throughout the pedaling motion. Decreasing the ratio of A to B increases the vertical lift of the operator&#39;s foot and increases the angular foot plantar-flexion. Conversely, increasing the ratio of A to B reduces the vertical lift of the operator&#39;s foot and decreases the angular foot plantar-flexion. As the ratio of A to B approaches infinity, the elliptical path E collapses into a straight line of length A and eliminates the vertical lift altogether. 
   The major axis angle γ of the ellipse reflects the incline angle of the pedaling motion. A major axis angle γ of zero degrees emulates natural walking or running motion on flat ground. Increasing the major axis angle γ emulates natural walking or running motion on an incline. Foot link guide track angle θ is the angle of the foot link guide track  255  from horizontal and is generally parallel with the major axis angle γ. 
   The three parameters that govern the shape of the generally elliptical pedaling path E (major axis A, minor axis B, and major axis angle γ, discussed above) are generally a function of the following frame and drive mechanism dimensions: crank arm length C, foot link length D, crank pivot offset P, operator foot offset J, and foot link guide track angle θ. Crank arm length C is the distance between the center of the crank arm bearing  245  to the foot link bearing  220 . Foot link length D is the distance between the center of the load wheel  250  and the foot link bearing  220 . Operator foot offset J is the distance from the center of the load wheel  250  to the region where the ball of the operator&#39;s foot contacts the foot platform, point F. Foot link guide track angle θ is the angle of the foot link guide track  255  from horizontal and is generally parallel with the major axis angle γ. As discussed below, modifying these parameters will change the elliptical pedaling profile experienced by the operator. 
   As illustrated in  FIGS. 1-7 , the frame, or frame structure  110  of the apparatus  100  can be comprised of a variety of materials.  FIG. 1  depicts one embodiment of the apparatus  100  in which the frame  110  is comprised of a rigid tubular metal, such as aluminum, steel, or titanium. As illustrated in  FIG. 1 , the frame structure  110  includes a lower frame member and two foot link guide tracks  255  that, in this embodiment, also act as structural frame members.  FIG. 5  depicts an embodiment of the apparatus  100  in which the frame  110  is comprised of sheet metal, and in this embodiment, one frame member may be the lower portion of the frame  110  (nearest the ground) and a second frame member may be the foot link guide track  255  that comprises an upper portion of the frame  110 .  FIG. 6  depicts an embodiment of the apparatus  100  in which the frame  110  is comprised of a graphite composite, and in this embodiment, similar to the embodiment illustrated in  FIG. 5 , one frame member may be the lower portion of the frame  110  (nearest the ground) and a second frame member may be the foot link guide track  255  that comprises an upper portion of the frame  110 , even though the two frame members may be formed together. 
   Other materials may also be used to construct the frame for the apparatus, such as plastics, alloys, other metals, etc. The frame  110  provides the structural rigidity necessary to support the rider while he or she is operating the apparatus  100 . The frame  110  also connects the movable portions of the apparatus  100  together into a complete system. 
   One of the features common to all of the proposed embodiments of apparatus  100  is a low cross-over height frame. As defined herein, a frame has a low cross-over height if there are no structural frame members positioned in the operator&#39;s zone. Generally, the operator&#39;s zone is the area of space occupied by the operator when riding the apparatus. For example, one embodiment of the operator&#39;s zone is illustrated in  FIG. 3 , and comprises an area defined by points K and L, and line N. Point K is the aft-most position of the load wheel  250 , point L is the mid-point of travel of the load wheel  250 , and line N is formed by a line that extends between the tops of the front wheel  115  and rear wheel  117 . During operation, the load wheel  250  travels from a forward-most position  103  to a rear-most, or aft-most position K. As shown in  FIG. 3 , the mid-point of travel for the load wheel  250  is point L, which is half the distance of the load wheel total distance of travel  107 . Put differently, the load wheel total distance of travel  107  is the maximum stride length that an operator would be able to achieve, and may range from about 14 inches to about 26 inches. As shown in  FIG. 3 , the operator&#39;s zone extends from line N upwards, and is bounded by two substantially vertical lines that extend from points K and L. 
   It will be appreciated that the operator&#39;s zone may extend further forward or backward depending on the amount of forward and rearward movement the operator must undertake when operating a specific embodiment of the apparatus. For example, for some embodiments, point L may be defined as the location of the ball of the operator&#39;s foot when it is located at the forward extreme of the pedal stroke (point  103 ) on the foot platform  210 , and point K may be defined as the heel of the operator&#39;s foot when it is on the foot platform  210  and the load wheel  250  is at its aft-most position. Similarly, it may be appreciated that for embodiments where the front wheel  115  and the rear wheel  117  are small (have diameters less than 20 inches), line N may be set at a given distance off of the ground (approximately 26 inches) rather than formed by a line that extends between the tops of the front wheel  115  and rear wheel  117 . 
     FIGS. 1 ,  4 ,  5 ,  6 ,  7 ,  12 ,  13 , and  14  depict several different proposed embodiments, all of which have low cross-over height frames  110 . As shown in  FIG. 3 , generally, the frame  110  includes truss members  112  and two foot link guide tracks  255 . However, some frame  110  embodiments, like those shown in  FIGS. 5 and 6 , do not include truss members  112 . Moreover, the foot link guide tracks  255  may be an integral component of the frame, as shown, for example, in  FIGS. 4-6 . The individual guide tracks may also be integrated together to form a single guide track, as depicted in  FIG. 14 . 
   Low cross-over height frames  110  are safer and more convenient to use than conventional upright step-cycle or bicycle frames. The low cross-over height design is safer because there are no support structures in the operator&#39;s zone that could cause injury during a fall or during riding. These frames are also more stable to ride because they have a lower center of gravity. The low cross-over height design also makes the apparatus  100  easier and safer to mount and dismount because there are no support structures in the operator&#39;s zone to step over or around when mounting or dismounting. In addition, the low cross-over height makes the apparatus  100  easier to maneuver in tight spaces because it enables the operator to easily step across the apparatus  100 , which facilitates moving the apparatus  100  into and out of storage areas, trains, buildings, and the like. 
   One consideration when designing low crossover-height frames  110  is stiffness in bending. Unlike conventional frames, a low cross-over height frame  110  does not include a structural member above the plane of the top of the wheels to provide stiffness in bending. Because the frame  110  must support the dynamic weight of the operator during riding, stiffness in bending is important not only to prevent frame member failure, but also to improve pedaling efficiency and handling. 
   The proposed embodiments have been designed to provide sufficient frame  110  stiffness in bending. For example, the frame  100  design in  FIG. 4  has a stiffness of approximately 2500 lbf/in. When the embodiment depicted in  FIG. 4  is subjected to a 200 pound load in the center of the foot link guide tracks  255 , the frame  110  will deflect no more than about 0.08 inches, thereby minimizing the negative effects of frame flexing discussed above. This improved stiffness in bending is achieved by several features contained in the low cross-over height frame  110 , including incorporation of the foot link guide tracks  255  into the frame  110  as frame members, and the use of truss members  112  to enhance stiffness. 
   As shown in several of the Figures, embodiments of the apparatus  100  include a steering mechanism  120  that may comprise handlebars  119 , a steering wheel (not shown), or other steering means. The steering mechanism  120  can be mounted upon a fixed or adjustable steering extender  125  that extends upward from the frame  110 . The steering mechanism  120  can be telescopically adjustable, as well as adjustable forward and backward, and can incorporate a pivot to provide rotational adjustability. One feature is that an adjustable steering mechanism will permit easy and safe use by a variety of operators having different heights and arm dimensions. 
     FIG. 11  depicts a detailed view of one embodiment of a telescoping steering mechanism. In this embodiment, the steering extender  125  is held by a steering extender sleeve  126 . The inside diameter of the steering extender sleeve  126  is larger than the outside diameter of both the front fork steer tube  127  and the steering extender  125 . In this embodiment, the front fork steer tube  127  has been inserted into the bottom of the steering extender sleeve  126  and is clamped to it by means of one or more fasteners  128 , such as a bolt and nut, pin, clip or other means. In addition, the steering extender  125  is inserted into the top of the steering extender sleeve  126  and is clamped to it by means of another fastener  128 . The height of the steering mechanism  120  can be adjusted by varying the position where the steering extender sleeve  126  clamps to the steering extender  125 .  FIGS. 13A-B  depict a steering tube assembly with both translational and rotational adjustability. 
   As shown in  FIGS. 1 ,  4 , and  6 , embodiments of the apparatus  100  can also incorporate a rear wheel cover  190 . The purpose of the rear wheel cover  190  is to prevent the operator&#39;s legs, feet, clothing, and other objects from contacting the rear wheel  117 . The cover  190  can be made from metal, plastic, graphite composite, fiberglass, or other materials. It can be attached to the frame by bolts, welds, brazes, or other methods, or it can be an integrated part of the frame  110  as shown in  FIG. 6 . To facilitate transporting and maneuvering the apparatus  100  while walking, a handle  191  can be attached to, or incorporated into, the rear wheel cover  190 , or a handle  191  can be attached to, or incorporated into, the frame  110  and protrude through an opening in the rear wheel cover  190 . 
     FIG. 1  depicts a rear wheel cover  190  with a handle  191 . The handle  191  is integrated into the rear wheel cover  190  and the rear wheel cover  190  is bolted to the frame  110 .  FIG. 4  depicts a rear wheel cover  190  without a handle that is bolted to the frame  110 .  FIG. 6  depicts a rear wheel cover  190  without a handle that is integrated into a carbon fiber frame  110 . 
   Referring now to  FIGS. 10A-B , each foot link  205  can be laterally constrained onto its respective foot link guide track  255  in a variety of ways.  FIGS. 10A and 10B , which is a sectional view about section M-M shown in  FIG. 10A , and  FIGS. 10C and 10F  depict one method of laterally constraining the foot link  205 . In this method, the load wheel  250  has a V-groove  305  that mates to the counterpart geometry of a substantially diamond-shaped foot link guide track  255 . The top of the foot link guide track  255  fits into the center of the groove of the load wheel  305 , thereby laterally constraining the foot link  205 . 
     FIGS. 10D ,  10   i ,  10 J, and  10 K depict a similar mechanism for laterally constraining a foot link  205  onto a round or tubular-shaped foot link guide track  255 . In these embodiments, the contact surface of the load wheel  250  has a concave shape that mates with the counterpart geometry of the round foot link guide track  255 . The top of the foot link guide track  255  aligns with the center of the load wheel  250  and the foot link  205  is laterally constrained onto the foot link guide track  255  by the interface of the concave load wheel  205  and the round tube comprising the foot link guide track  255 . 
     FIG. 10E  depicts another method of laterally constraining a foot link  205  onto a foot link guide track  255 . This embodiment uses a load wheel carrier  271  that is attached to each foot link  205 . In the depicted embodiment, the load wheel carrier  271  holds two load wheels  250 . The load wheels  250  are set into the load wheel carrier  271  at opposing angles. The interaction of each load wheel  250  with the foot link guide track  255  results in the lateral constraint of the attached foot link  205 . Although a diamond shaped foot link guide track  255  is depicted in  FIG. 10E , this method could also be used with round, tubular, or similarly shaped foot link guide tracks  255 . 
   The lateral constraining methods discussed above are intended to prevent the foot link assembly from laterally disengaging with, or “falling off” of, the foot link guide track  255 . The list is not intended to be exhaustive. Its purpose is only to illustrate a few of the many methods of restraining the foot links  205  in the lateral direction. 
   In addition to lateral constraint, each foot link  205  may also be retained in the normal direction (i.e., a direction generally perpendicular to the foot link guide track  255 ). That is, each foot link  205  may be restrained from “jumping off” the foot link guide track  255 . The foot links  205  could be subject to disengaging in the normal direction whenever, for instance, the apparatus  100  travels over sharply undulating or rough terrain, or strikes an obstacle. The retention methods discussed below are intended to prevent the foot link assembly from disengaging with the foot link guide track  255  in the normal direction during operation of the apparatus  100 . The list is not intended to be exhaustive. Its purpose is only to illustrate a few of the many methods of restraining the foot links  205  in the normal direction. 
     FIGS. 10A ,  10 B,  10 C,  10 D,  10 E,  10 F, and  10   i  depict a method of normal retention in which one or more retaining links  605  holds a retaining member  610  underneath a feature of the foot link guide track  255 , or the foot link guide track  255  itself. The interaction of the retaining member  610  with the foot link guide track  255  or a feature on the foot link guide track  255  prevents the load wheel  250  from disconnecting with the foot link guide track  255 . 
   There are many ways to vary this method of retention, including changing the shape, size, number or other characteristic of the retaining links  605 , changing the shape, size, number, or other characteristic of the retaining member  610 , changing the shape, size, or number of foot link guide tracks  255 , or changing the shape, size, number, or other characteristics of features connected to the foot link guide track  255  or frame  110 . For example,  FIGS. 10F and 10   i  depict just two kinds of the many features that could be attached to the foot link guide tracks  255  to facilitate retention.  FIG. 10F  depicts an eave-like structure, and  FIG. 10   i  depicts a rail-like structure. A variety of other features could be employed for this purpose. Similarly,  FIGS. 10A and 10D  depict different shapes of the retaining member  610 .  FIG. 10A  shows a round member and  FIG. 10D  shows a cylindrical member. In fact, the retaining member  610  could be any manner of bar, pin, wheel, cable, or other mechanism that could serve to prevent the load wheel  250  from disengaging with the guide track  255 . 
   Moreover, the retaining link  605  or links can be designed to either hold the retaining member  610  at a fixed distance from the load wheel, or to allow for adjustment of that distance.  FIGS. 10C ,  10 D and  10 E depict retaining links  605  that hold the retaining member  610  at a fixed distance.  FIG. 10B  depicts one embodiment of a retaining link  605  in which a preloaded spring mechanism  630  holds the retaining member  610  in contact with the guide track  255  throughout the pedal stroke. The preloaded force can be adjusted by rotating the set screws  615  and  620 . This same system could also be used to establish and then adjust a gap between the retaining member  610  and the guide track  255 , thereby preventing the retaining member from contacting the guide track  255  except when needed to prevent the load wheel from disengaging with the guide track  255 . Such a gap can be set by rotating the set screws  615  and  620 . The gap can then be adjusted over time in the same manner to compensate for wear of the load wheel  250 . Again, the depictions described herein are meant to be illustrative only, and the apparatus  100  may include any number of variations and embodiments relating to retention of the load wheel  250  to the foot link guide track  255 . 
     FIGS. 10G ,  10 H,  10 J and  10 K illustrate several embodiments of an axle bar retention system. In this system, a retention member is passed through the axle of the load wheel such that it protrudes from one or both ends of the axle. The load wheel is constrained in the normal direction by the interface of this retention member and a structure attached to the frame  110 . The retention member can be any manner of pin, bolt, bar, or the like. 
   For example, as shown in  FIGS. 10G and 10H , the axle retention member  1010  protrudes from both sides of the load wheel  250 . In  FIG. 10G , each end of the axle retention member  1010  passes through a slot  285  formed in the foot link guide track  255 . The interaction between the axle retention member  1010  and the slot  285  prevents the foot link  205  from disengaging with the foot link guide track  255  in the normal direction. Similarly, in  FIG. 10H , each end of the axle retention member  1010  is positioned below a ledge  280  included in the foot link guide track  255 . The interaction between the axle retention member  1010  and the ledge  280  prevents the foot link  205  from disengaging with the foot link guide track  255  in the normal direction. 
   In  FIGS. 10J and 10K , only one end of the axle retention member  1010  protrudes from the load wheel  250 . In  FIG. 10J , the protruding end of the axle retention member  1010  has a hole drilled through it. The hole captures a securing member  287  that is connected to the foot link guide tracks  255  or another part of the frame  110 . The securing member  287  can be any manner of rod, cable, bar, or similar item. Similarly, in  FIG. 10K , the axle retention member  1010  is slotted to capture a securing member  287  that is connected to the foot link guide tracks  255  or another part of the frame  110 . In both embodiments, the interaction between the retention member  1010  and the securing member  287  prevents the foot link  205  from disengaging with the foot link guide track  255  in the normal direction. 
     FIG. 10L  depicts an alternative embodiment that provides both lateral and normal constraint. In this embodiment, the load wheel  250  has been replaced by a linear bearing  1030 . The linear bearing  1030  is free to slide along the foot link guide track  255 , however the lower portion  1020  of the linear bearing  1030  captures the foot link guide track  255 , thereby preventing the foot link  205  from disengaging from the foot link guide track  255  in the lateral or normal direction. 
   As discussed above, there are several ways to modify the elliptical pedaling profile of the apparatus  100 . One method is to change the location of the ball of the operator&#39;s foot (identified as location F in  FIGS. 2 , and  9 A-B) with respect to the load wheel  250  or the first end of each foot link  205 . Referring now to  FIGS. 9A-B , the first end of the foot link  205  is the end of the foot link  205  that is directly adjacent to the load wheel  250 , and the second end of the foot link  205  is the end of the foot link  205  that is directly adjacent to the foot link bearing  220  (shown in  FIG. 1 ). Modifying the location of the operator&#39;s foot  121  relative to the load wheel  250  or the first end of the foot link  205  changes the operator foot offset (identified as distance J in FIGS.  2  and  9 A-B). To achieve a flatter and more eccentric pedaling profile, the operator can position his or her foot closer to the first end of each foot link  205 . Alternatively, by positioning his or her foot further away from the first end of the foot link  205 , the operator can create a more circular and less eccentric pedaling profile. Because the distance between the operator&#39;s foot  121  and the load wheel  250  or first end of each foot link  205  influences the pedaling profile, the repeatability of adjustments to this distance ensures that the operator can experience the desired pedaling profile. 
   There are a variety of ways to enable the operator to repeatably modify the position of his or her foot relative to the first end of the foot link  205 .  FIG. 9A  depicts one method. In this embodiment, each foot platform designates a single position for an operator&#39;s foot  121 . The interface between each foot platform  210  and its respective foot link  205  is adjustable such that the foot platforms  210  can be attached onto the foot links  205  at different distances from the first ends of the foot links  205 . The attachment method in this embodiment is a pair of releasable clamps  905  that connect each foot platform  210  to its respective foot link  205 . This mechanism enables the operator to adjust each foot platform to achieve a repeatable placement of his or her foot relative to the first end of each foot link. In addition, each foot platform  210  could also include one or more securing elements  910  such as ridges or straps to prevent the operator&#39;s foot  121  from unintentionally disengaging from the foot platform  210 . It will be appreciated that the securing elements  910  can take many equivalent forms, such as baskets, clips, bumps, cleats, or the like. In addition, index lines (not shown) could be incorporated into the foot link  205  to facilitate more accurate and repeatable positioning of the foot platforms  210  relative to the first end of the foot link  205 . 
   There are additional ways to create a repeatable adjustable interface between the foot platforms  210  and foot links  205 . For example, a repeatable interface could also be created by a series of mounting holes in the foot links  205  and/or the foot platforms  210  that allow for different mounting positions of the foot platform  210  along the foot link  205 . 
     FIG. 9B  depicts an alternative method for enabling the operator to repeatably change the position of his or her foot relative to the first end of each foot link  205 . In this embodiment, the foot platform  210  is large enough to permit the operator to change the position of his or her foot relative to the first end of the foot link  205  without moving the foot platform  210 . The foot platform  210  includes one or more foot locators  920  to enable the repeatable use of the various foot positions on the foot platform  210 . The foot locators  920  could include features such as cleats, bumps, ridges, or the like. Each foot platform  210  could also include securing elements  910  as discussed in connection with  FIG. 9A . 
   As discussed above, another method for adjusting the pedaling profile is modifying the length of the crank arms  235 . As shown in  FIGS. 1 and 8 , the crank assembly  215  includes a crank extender  230  rotatably connected to the second end of the foot link  205  at the foot link bearing  220 . The crank assembly  215  also includes a crank drive arm  235  rotatably connected at the crank arm bearing  245  to a drive sprocket  240 . As shown in  FIG. 2 , Circle R, shown as a dashed line, is generated by rotating the crank assembly  215  around the crank arm bearing  245 . The distance between the center of the crank arm bearing  245  and the center of the foot link bearing  220  is crank arm length C. Shortening crank arm length C will shorten the stride length A. Correspondingly, increasing crank arm length C will increase stride length A. Therefore, adjustments in crank arm length C can be made to modify stride length A to allow operators of different stature to adjust the apparatus  100  to suit their individual dimensions. 
   There are many ways to modify the length of the crank arms  235 .  FIG. 8  depicts one method for making the crank assembly  215  adjustable. This method employs a slot-bolt assembly  810  where the crank extender  230  includes a slot  270  and the crank drive arm  235  includes apertures configured to receive crank fasteners  275  that can locate the crank drive arm  235  at any position along slot  270 . The crank extender  230  can thereby telescope, or adjust its length with respect to the crank drive arm  235 . 
   There are additional ways to make the crank assembly  215  adjustable. For example, the slot-bolt assembly  810  discussed above can be replaced by a clamp with pins that can clamp the crank extender  230  to the crank drive arm  235  at various positions. Another embodiment may make the crank assembly  215  adjustable by incorporating a series of holes in the crank extender  230  or the crank drive arm  235 , or both. In such an embodiment, the length of the crank drive  235  arms may be modified by changing which holes are used to fasten the crank extender  230  to the crank drive arm  235 . 
   As discussed above, and again with reference to  FIG. 2 , crank arm length C is a significant factor that determines major axis length A, which approximately equals the stride length of a given pedaling profile. For a rider of average height and body dimensions, as the stride length shrinks below approximately 17 inches, the rider&#39;s ability to transfer power to the apparatus  100  for purposes of acceleration and climbing becomes reduced. As a result, while embodiments with stride lengths generally less than about 17 inches may be appropriate for a small percentage of operators, the vast majority of riders will desire stride lengths longer than 17 inches to achieve sufficient pedaling efficiency. Embodiments of the apparatus  100  presented herein can accommodate stride lengths in excess of 23 inches. 
   As the stride length increases, it may be desirable to increase the wheelbase W as shown in  FIG. 2 . For a low cross-over height frame  110 , the longer the wheelbase W, the more difficult it is to maintain an appropriate level of bending stiffness, yet a wheelbase W that is significantly longer than conventional bicycles is desirable. For example, a conventional bicycle may have a wheelbase of about 40 inches, but embodiments of the present invention may have a wheelbase W that may range from about 55 inches to about 65 inches. As discussed above, embodiments of the apparatus  100  include a frame  110  having a sufficient bending stiffness to accommodate a stride length beyond 23 inches. 
   Alternative embodiments of the apparatus  100  can incorporate additional features, such as a direct drive propulsion mechanism, adjustable guide tracks, and/or foldability.  FIG. 12  depicts an embodiment of the apparatus that employs a direct drive propulsion system. In this embodiment, the crank arms  235  are connected directly to the hub  1210  by means of bearings (not shown) mounted in the frame  110 , through which passes a linkage from each crank arm  235  to the hub  1210 . This alternative embodiment alleviates the need for a chain and sprockets. This embodiment could incorporate a gearing system in the crank-to-hub-wheel linkage that could allow the rear wheel to rotate more quickly than the crank arms. Such a gearing system could provide a fixed input-output ratio, or could allow for one of a series of gears to be selected by the operator. In addition, the rear wheel  117  could be enlarged to allow the operator to achieve a greater rate of speed for each completed pedal stroke. 
     FIG. 7  depicts a low cross-over height frame  110  with adjustable foot link guide tracks  255 . The forward end of each foot link guide track  255  is attached to a foot link guide track support  705  by the use of a rotary bearing  710 , so that the forward end of the foot link guide track  255  can rotate about the foot link guide track support  705 . Each foot link guide track support  705  is attached to its respective side of a collar  715  by the use of a bolt and low friction washers, or other suitable means. The collar  715  can be clamped to the downtube  725  at various locations by means of bolts or other fasteners. The low friction washers allow each foot link guide track support  705  to rotate about the bolt. The rearward or second end of each foot link guide track  255  is attached to the frame by the use of a rotary bearing  720 . Unclamping the collar  715  allows the operator to slide the collar  715  along the downtube  725 , thereby adjusting the angle of the foot link guide tracks  255 . As discussed below, changing the angle of the foot link guide tracks  255  modifies the elliptical pedaling profile experienced by the operator. 
     FIGS. 13A and 13B  depict an embodiment of the apparatus  100  that can be folded to facilitate transport or storage in small spaces. In this embodiment of a foldable apparatus  100 , the apparatus  100  is folded according to following procedure. First, the foot link retainer  610  on each foot link assembly  105  is released from the guide track  255  by removing a pin (not shown). Next, each foot link assembly  105  is rotated about its respective foot link bearing  220  towards the rear wheel  117  approximately one hundred eighty (180) degrees. Next, the coupling  1340  on each side of the apparatus is released. Each foot link guide track  255  is then rotated downwards about guide track pivot  1330 . Next, the crank assembly  215  is rotated forward about pivot  1320  until the rear wheel  117  passes through the frame  110 . Next, each guide track  255  is rotated upwards about guide track pivot  1330 . Then the crank assembly  215  is rotated until the right crank arm  235  points to the rear, as depicted in  FIG. 13B . At that point, each foot link  205  may be strapped to the adjacent crank arm extender  230 . The right foot link assembly  105  is then positioned on top of the front fork  127  and the left foot link assembly  105  is positioned on top of the axle of the rear wheel  117 . Next, the steering assembly pivot  1310  is released and the steering extender sleeve  126  is rotated rearward. The steering extender sleeve  126  is then locked in place at steering assembly pivot  1310  as depicted in  FIG. 13B . Once locked, the steering extender sleeve  126  may be used as a handle to carry or help direct the path of travel for the folded apparatus  100 .  FIG. 13B  depicts the results of following the folding procedure described above. 
   In addition, the apparatus  100  can include gearing. Gearing can be implemented through techniques known in the art, including a series of different sized sprockets attached to the rear wheel  117  and selected by a derailleur, or a single rear sprocket connected to a hub that contains a series of gears inside of it which enable the hub to produce a variety of input-to-output ratios. This embodiment could incorporate techniques known in the art to permit the operator to select gears. This could include mounting a shift lever on the steering mechanism  120  as is known in the art. The apparatus  100  can also include a fixed gear system with no freewheel on the rear wheel  117 . 
   The apparatus can also include mechanisms to retard motion, such as rim or disc braking systems known in the art. These mechanisms can be located on the front and/or rear wheels. The braking mechanisms can be actuated by, for example, a hinged handle or other structure mounted on the handlebars to which the brake cables or some other mechanism are connected, as is known in the art. In addition, the apparatus can include other attributes that are commonly incorporated onto other human powered vehicles, such as reflectors, lights, bottle cages, etc. 
   It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Moreover, the components A and B should not be limited to a specific relationship or form; instead, they can be integrated together into a single structure or can operate independently. 
   Similarly, it is to be noticed that the term “coupled”, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression “a device A coupled to a device B” should not be limited to devices or systems wherein device A is directly connected to device B. It means that there exists a path between A and B which may be a path including other devices or means. In addition, “coupled” does not necessarily mean “in a fixed position or relationship” as “coupled” may include a moveable, rotatable or other type of connection that allows relative movement between A and B. Finally, “coupled” may also include “integral” where device A and device B are fabricated as an integral component or single structure. 
   Thus, it is seen that a bicycle is provided. One skilled in the art will appreciate that the bicycle of the present invention can be practiced by other than the above-described embodiments, which are presented in this description for purposes of illustration and not of limitation. The specification and drawings are not intended to limit the exclusionary scope of this patent document. It is noted that various equivalents for the particular embodiments discussed in this description may practice the invention as well. That is, while the bicycle has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description. Accordingly, it is intended that the bicycle embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.
A bicycle that includes an elliptical pedaling motion is provided. The bicycle includes a frame, having a front wheel and a rear wheel, and a first and a second foot link guide track. A first and a second foot link, each including a foot platform and a guide track coupler that slideably couples each foot link to a respective foot link guide track and a crank assembly that is coupled to the first and second foot links and to a sprocket that is rotatably coupled to the frame. This Abstract is provided for the sole purpose of complying with the Abstract requirement rules that allow a reader to quickly ascertain the subject matter of the disclosure contained herein. This Abstract is submitted with the explicit understanding that it will not be used to interpret or to limit the scope or the meaning of the claims.