Patent Publication Number: US-2018050562-A1

Title: Wheel with selective suspension system

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/114,843, filed on Oct. 30, 2013, entitled “Selective Wheel Suspension System”, which is a National Phase of PCT Patent Application No. PCT/IB2012/000530 having international filing date of Mar. 20, 2012, which claims the benefit of priority under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/485,086 filed on May 11, 2011 and U.S. Provisional Patent Application No. 61/552,505 filed on Oct. 28, 2011. The contents of all of the above applications are incorporated by reference as if fully set forth herein in their entireties. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to suspension systems for vehicles, and in particular to suspension systems having selective activation according to parameters of the impact or perturbation. 
     Rotating masses tend to accommodate vibrations and shocks due to internal and/or external forces and impacts from surfaces in contact. One example is the vibratory motion of a wheel when it travels a distance on a non-purely smooth surface. Motorized and other vehicles commonly include cumbersome suspension systems in order to protect their chassis or other affiliated parts from early failure as well as to avoid unpleasant conditions to passengers. 
     Suspension systems, mostly including springs and damping elements, are commonly connected to static parts of the machine or vehicle, on one end, and in direct contact with the axle or other elements that provide a stable axis of rotation to the rotating mass or rotator. For example, a wheel that travels over a rough surface will transfer axial, vertical and other forces (e.g., impacts and/or vibratory) to the axle, which will be partially absorbed and diminished using suspension means that can be located between the axle and the chassis. Several attempts are known for implementing suspension mechanisms inside the wheels. 
     In recent years there is a growing trend towards more efficient self-propelled vehicles where the invested human power is transferred to movement of the vehicle with minimal energy loss. Modern wheelchairs and bicycles incorporate lightweight structural parts, wheels structures with improved strength-weight ratio, tires designed for minimized resistance to rolling, etc. There is also a preference of most riders to feel a rigid or responsive ride, rather than a soft one, especially when driving over substantially smooth surfaces and/or when riding upward inclines, and also when accelerating, decelerating or maneuvering. When suspension is implemented the manufacturers usually make some accepted tradeoffs between the physiological and improved comfort needs with the dynamic preferences of the users. 
     SUMMARY OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to a wheel with an in-wheel suspension system, and a suspension member thereof, the wheel being connectable to a vehicle. The wheel includes: a stationary member including an axis, a rotary member rotatable about the axis, and a selective suspension system. The selective suspension system has at least one suspension member positioned between the stationary member and the rotary member, thereby providing a fixed distance therebetween, and retains the fixed distance when stressed up to a threshold value, and recoverably alters the fixed distance when stressed over the threshold value. The suspension member includes: a cylindrical housing, a piston slidably movable in the cylindrical housing, and a compression spring whose first end is connected to the piston and second end is connected to the housing. The compression spring, when fully relaxed or compressed under the threshold value, maintains the housing and piston at a normally fixed distance. Applicable to wheelchairs and bicycles, among other self-propelled and powered vehicles. 
     According to an aspect of some embodiments of the present invention, there is provided a wheel connectable to a vehicle, the wheel includes a stationary member which includes an axis and a rotary member rotatable about the axis. In some embodiments of the invention, at least one support member is positioned between the stationary member and the rotary member thereby providing a normally fixed distance therebetween. In some embodiments of the invention, the support member is adapted to retain the distance when stressed up to a threshold value and to recoverably alter the distance when stressed over the  30   o  threshold value. 
     In some embodiments of the invention, the vehicle is a self-propelled vehicle, for example a wheelchair or a bicycle. In some embodiments, the wheel includes a castor which is fixedly rotatable about at least two axes projecting from the vehicle. In some embodiments of the invention, the stationary member includes at least one of: an axle, a caster housing, and a bearing inner ring. In some embodiments, the rotary member includes at least one of: a tire, a wheel rim, a hub shell, a fork, and a bearing outer ring. 
     In some embodiments of the invention, the support member includes an elastic component adapted to change in size and/or shape only at the support member stressing over the threshold value. In a first exemplary embodiment, the elastic component includes a spring, optionally a coil spring or a gas spring, optionally the spring is provided preloaded to compress only over the threshold value. In some embodiments, the threshold allows spring compression at compressive forces which are greater than a total weight of the vehicle and passengers, divided by the number of wheels and/or number of preloaded springs. In a second exemplary embodiment, the elastic component includes a compressible block polymer. In a third exemplary embodiment, the elastic component includes a buckling member adapted to buckle only at the support member stressing over the threshold value. In some embodiments of the invention, the elastic component is held in a fixed size, shape and/or position until the support member is stressed over the threshold value. In some embodiments of the invention, the wheel includes a plurality of the support member evenly distributed around the axis. 
     In some embodiments of the invention, the wheel includes a selective suspension member adapted to absorb and/or dissipate kinetic energy only while and/or after the support member recoverably alters the distance. In some embodiments of the invention, the wheel includes a non-threshold suspension member adapted to absorb and/or dissipate kinetic energy at the fixed distance. 
     In some embodiments of the invention, the threshold value relates to a minimal shock magnitude absorbed by the wheel. Alternatively or additionally, the threshold value relates to a minimal vibration amplitude absorbed by the wheel. Alternatively or additionally, the threshold value reciprocally relates to a maximal vibration amplitude absorbed by the wheel. Alternatively or additionally, the threshold value reciprocally relates to a maximal vibration frequency absorbed by the wheel. 
     In some embodiments of the invention, the support member is controllable using a controller linked thereto. Optionally, the controller is set to determine and/or monitor at least one of the normally fixed distance, the threshold value, and a maximally allowed alteration of the distance by the support member. In some embodiments of the invention, the controller includes at least one of a fuse, a burst valve, a movable barrier, and a buckling member. In some embodiments, the controller is programmable. 
     In some embodiments of the invention, the wheel includes a sensor in direct communication with the controller adapted to sense a change in a measured variable. In some embodiments, the measurable variable is indicative of the support member stressing. Alternatively or additionally, the measurable variable is indicative of an upcoming stressing. 
     In some embodiments of the invention, at least one of: a selective suspension member, a non-threshold suspension member, and at least one support member, is rotatable with the wheel. 
     According to an aspect of some embodiments of the present invention there is provided a suspension system of a self-propelled vehicle, including suspension means and a controller for controlling the suspension means. 
     In some embodiments, the suspension means are configured for absorbing and/or dissipating energy originating from an impact between a wheel, optionally a rear wheel of a wheelchair, and a surface. In some embodiments, the suspension means include at least one of a spring, a damper, an elastic implement, an elastomeric implement, a buckling element, and a plastically deformable implement. 
     In some embodiments, the controller is configured to allow or withhold operation or activity of the suspension means according to the impact parameters. In some embodiments, the controller is configured for selectively operating the suspension means when the impact includes a magnitude equal or greater than a chosen value. Alternatively or additionally, the controller is set and/or configured to withhold operation of the suspension means if the impact is lower in magnitude than the chosen value (for example: Low pass, High pass, or Band pass). In some embodiments, the controller includes at least one of a thresholding mechanism, a fuse, a burst valve, a movable barrier, and a buckling implement. 
     Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. 
       In the drawings: 
         FIGS. 1A-B  schematically illustrate side views of a wheelchair and a wheel anticipating to different obstacles during motion, in accordance with embodiments of the present invention; 
         FIGS. 2A-B  schematically illustrate a wheel portion comprising an exemplary selectively formable support member before and after formation, in accordance with embodiments of the present invention; 
         FIGS. 3A-C  illustrate schematic diagrams of exemplary selective suspension units, in accordance with embodiments of the present invention; 
         FIGS. 4A-B  provide schematic illustration of a wheel connected with a chassis part and a block diagram of an exemplary suspension system and possible implementations thereof, in accordance with some embodiments of the present invention; 
         FIGS. 5A-B  schematically illustrate a wheel comprising an exemplary selective wheel suspension unit, comprising an exemplary buckling member, before and after impact with an obstacle, in accordance with some embodiments of the present invention; 
         FIGS. 6A-B  schematically illustrate a wheel comprising an exemplary selectively deflatable bladder, in accordance with embodiments of the present invention; 
         FIGS. 7A-B  schematically illustrate side and back views of an exemplary caster-type wheel unit comprising selective suspension members for discrete degrees-of-freedom motions, in accordance with embodiments of the present invention; 
         FIGS. 8A-B  illustrate an exemplary wheel comprising selectively formable hub unit, in accordance with embodiments of the present invention; 
         FIGS. 9A-B  illustrate an exemplary wheel comprising a in-hub leaf-spring type selective suspension unit, in accordance with embodiments of the present invention; 
         FIGS. 10A-B  illustrate an exemplary wheel comprising an in-hub spring-loaded cam-type selective suspension unit, in accordance with embodiments of the present invention; 
         FIGS. 11A-C  illustrate an exemplary wheel comprising a plurality of spokes type selective suspension members, in accordance with embodiments of the present invention; 
         FIGS. 12A-B  illustrate an exemplary wheel comprising selectively slidable hub unit, in accordance with embodiments of the present invention; and 
         FIGS. 13A-B  illustrate an exemplary wheel and a leaf-spring type selective suspension designed as a chassis-to-wheel adaptor, in accordance with embodiments of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The present invention, in some embodiments thereof, relates to a wheel with an in-wheel suspension system, and a suspension member thereof, the wheel being connectable to a vehicle. The wheel includes: a stationary member including an axis, a rotary member rotatable about the axis, and a selective suspension system. The selective suspension system has at least one suspension member positioned between the stationary member and the rotary member, thereby providing a fixed distance therebetween, and retains the fixed distance when stressed up to a threshold value, and recoverably alters the fixed distance when stressed over the threshold value. The suspension member includes: a cylindrical housing, a piston slidably movable in the cylindrical housing, and a compression spring whose first end is connected to the piston and second end is connected to the housing. The compression spring, when fully relaxed or compressed under the threshold value, maintains the housing and piston at a normally fixed distance. Applicable to wheelchairs and bicycles, among other self-propelled and powered vehicles. 
     The following preferred embodiments may be described in the context of exemplary suspension mechanisms for wheelchairs, or other types of self-propelled vehicles, for ease of description and understanding. However, the invention is not limited to the specifically described devices, and may be adapted to various applications without departing from the overall scope of the invention. For example, devices including concepts described herein may be used for suspension of any rotatable mass including wheels of motorized or otherwise powered vehicles. 
     Common suspension systems are built to absorb interruptions and obstacles which cause deceleration and/or undesired vibration to the vehicle and/or aid the wheel in following the terrain and avoiding loss of contact with it, or grip. In doing so, the suspension systems are built to absorb and/or dissipate energy, including such that can be translated to effective kinetic energy. Furthermore, the common suspension systems (which include, for example, parts like metal springs, cushioning materials and elastomers) cause a feeling of plushness, or softness, which may cause a sense of instability, which are undesirable by many users. 
     In order to answer these and other considerations, the present invention provides or includes means for selective responsiveness (or irresponsiveness) according to types and/or magnitudes of absorbed interferences or perturbations. 
     Referring now to the drawings,  FIGS. 1A-B  schematically illustrate side views of a wheelchair  10  and a rear wheel  11  anticipating different obstacles during motion, in accordance with embodiments of the present invention. Besides combining two rear wheels such as wheel  11 , wheelchair  10  further includes a seat  12  and a pair of casters  13 . In  FIG. 1A , wheelchair  10  moves along path  20  which includes a step or a curb descent  21  with height x, as well as a plurality of recesses  22 , such as tile gaps or chamfers on paved surfaces. Height x may be about 10 cm or more in case of a sidewalk curb, or 15 cm or more in case of a standard stairway step. Recesses  22 , on the other hand, are of heights of less than 3 cm, usually around 1 cm. In some embodiments, the suspension system of the present invention includes means for selective differentiation between drops from different heights, for example drops from up to 1 cm, optionally up to 3 cm and those which are equal or higher than 1 cm, optionally 3 cm, optionally 5 cm, or higher, or lower or intermediate. Also, a selective differentiation may be applicable for a range of drops or perturbations, such as over 3 cm and under 10 cm, for example. 
     Wheelchair  10  is shown in motion adjacent a forward-facing step  21  with its front end is tilted upwardly (commonly known as performing a “wheelie”), pivoting around rear wheel(s)  11 —a common practice when riding over steps, performed either by an attendant or by the wheelchair user himself. The tilting angle α may be between 0 to 40°, and optionally higher. Such tilting maneuver changes the impact angle of the wheelchair with the ground and should be considered when designing an effective suspension mechanism. In some embodiments, the suspension system of the present invention is configured for effective suspension of falls at different impact angles, optionally in angles range of at least −10° to 10°, optionally −30° to 30°, optionally −60° to 60°. Also, in some cases the drop can be taken in reverse, meaning that the rear wheels go first, while the front casters are still on the top platform, generating a mild “nose up” angle of generally less than 20°, but optionally higher. 
       FIG. 1B  shows a second demonstrative scenario in which wheel  11  (shown independently for ease of demonstration only) travels along path  30  that includes a bump  31  of a significant height followed by a substantially shallow coarse road  32 . In some embodiments, the suspension system of the present invention includes means for selectively differentiating between bumps of different heights, and for example may allow suspension of bumps of 0.5 cm or higher, optionally 1 cm or higher, optionally 3 cm or higher. Alternatively or additionally, such or other means may allow suspension of bumps shorter in height than wheel radius, optionally shorter than ¾ its radius, optionally shorter than ½ its radius. Alternatively or additionally, such or other means may differentiate between road types (such as between coarse roads like road  32 ) which cause vehicle&#39;s and/or wheel&#39;s vibrations differentiated by acceleration impact amplitude and/or frequency, optionally depending also on vehicle&#39;s velocity. 
     In an aspect of some embodiments, there is provided a wheel connectable to a vehicle, the wheel includes a stationary member having an axis, optionally a long axis and/or a symmetry axis, and a rotary member rotatable about the stationary member axis. The vehicle may be a powered vehicle, including but not limited to a motorized vehicle or a powered-assisted vehicle (such as intended for assisting pedal propelled bicycles or for assisting hand propelled wheelchairs), or a self-propelled vehicle such as unpowered bicycles and wheelchairs. The wheel may be a separate wheel intended for releasable connection to a vehicle body or a wheel unit such as a castor type wheel which is provided with wheel-to-vehicle connection means having at least two pivoting axes, thereby allowing fixed rotations about at least two axes projecting from the vehicle. In some embodiments, the stationary wheel member includes at least one of: an axle (including but not limited to a quick release axle), a castor housing, and a bearing inner ring, whereas the rotary wheel member includes at least one of: a tire (pneumatic or solid), a wheel rim, a hub shell, a fork, and a bearing outer ring. 
     According to some embodiments of the present invention, there is provided at least one support member positioned between the stationary member and the rotary member thereby providing a normally fixed distance therebetween. In some embodiments, the normally fixed distance corresponds or correlates with concentricity between the stationary and rotary members and optionally even whole wheel symmetry, whereas a change of that distance will break such geometrical balances. Optionally, alternatively or additionally, the normally fixed distance corresponds or correlates with symmetry or axisymmetry of the stationary member and/or the rotary member, whereas a change of that distance will recoverably deform stationary member and/or rotary member to a non-symmetric or a non-axisymmetric shape. A support member may be a spoke member or any other wheel body part or member extending between the rim and the hub radial. Optionally, alternatively or additionally, a support member may be located inside a wheel hub, for example between the bearing and the axle, or between the bearing and the hub shell. Optionally, alternatively or additionally, the support member is positioned inside a wheel bearing (e.g., bushing) between the inner ring and the outer ring. Optionally, the wheel includes a plurality of support members evenly distributed around the stationary member axis. 
     In some embodiments, the support member is selectively formable and/or operable in the sense that it is adapted to retain the normally fixed distance when stressed up to a threshold value and to recoverably alter this distance when stressed over the threshold value. In one exemplary embodiment, the support member includes an elastic component, such as but not limited to a spring, a block polymer (e.g., elastomer), and/or a buckling member, adapted to change in size and/or shape only when stressed over the threshold value. Optionally, the elastic component is held in a fixed size, shape and/or position until the support member is stressed over the threshold value. In case of a support member comprising a buckling member, the latter is adapted to buckle only when the support member is stressed over the threshold value. In case of a support member comprising a spring, for example a coil spring, a leaf spring or a gas spring, it may be preloaded so it thereby able to further compress only after the threshold value has been surpassed. Unless otherwise defined in specific embodiments of the invention, a “preload” or “preloading” shall mean a fixed compression force applied to a spring which generate or increase internal stresses tending to force the spring to recover to its relaxed form. A preloaded spring shall be fixed at a nominal compression form so that compressive forces acting thereupon shall be able to further compress it only at magnitudes that exceed the nominally preloading force at a same axis. 
     Present art suspension systems may incorporate some extent of preloading in the spring element, in order to ensure that no free movement of parts occurs when the suspension is fully unloaded (as during a jump) and/or in order to have more suspension travel in certain situations. In automotive applications, for example, the self-weight of the vehicle is commonly designed to surpass the force value of the preload as soon as the vehicle is provided freely on the ground. In case of common self-propelled vehicles (such as bicycles and wheelchairs), where the user/rider own-weight is the substantial part of the overall weight compressing the spring, the force value of the preload is sometimes designed to be higher than that of the weight of the vehicle itself, but substantially lower than the aggregate weight of the rider with the vehicle, therefore, as soon as the vehicle is mounted by the rider and loaded with its weight, the suspension is put into its operational regime. In the present invention the preload force value is referred to as being substantially higher than the aggregate weight of the vehicle and the rider together; therefore, the suspension is not in its operational range while the rider is on the vehicle, and optionally also under normal or predefined conditions. Optionally, the preload is designed or set according to a predetermined threshold value. 
     A threshold value, according to embodiments of the present invention, may include at least one of a minimal shock magnitude absorbed by the wheel (such as a 5 cm free fall onto an infinitely stiff platform), a minimal vibration amplitude applied to the wheel (such as 5 g or 50 m/s 2 ), a threshold value reciprocally related to a maximal vibration amplitude applied to the wheel and a threshold value reciprocally related to a maximal vibration frequency absorbed by the wheel (such as 3 Hz). The threshold value may be predetermined according to parameters of the vehicle and/or user or it may be inherent by the wheel and/or support member design. The threshold value may be programmable, mechanically and/or electronically, by the user, by a professional having special threshold value setting means, or by the manufacturer during production and/or assembly. A threshold value may be, alternatively or additionally, subject to continuous changes or adjustment by continuously correlating sensed or otherwise measured parameters with a pre-set memory. For example, a change of overall weight may be measured and be translated to adjustment in the threshold value, so that an identical impact absorbed by the wheel will be similarly suspended using suspension means while neglecting the added weight that would have otherwise may be translated as a magnified impact. 
     At some instances, at a fall of a rider riding a self-propelled vehicle, the rider may begin absorbing an impact instantly after the vehicle does, as they may act at a fall as separate bodies having different weights, momenta and/or and inertia. Therefore, a wheel contacting the ground at the beginning of vehicle&#39;s impact, or a structural member thereof, may encounter a first smaller load derived from vehicle own weight/momentum followed by a second larger load derived from rider and vehicle combined weight and/or rider momentum. In some embodiments of the invention, a selectively formable support member in the wheel is configured to deform only at impacts of the combined rider and vehicle weights at falls of minimal heights. Alternatively, the selectively deformable support member may be configured to deform also at impacts under vehicle&#39;s weight only, optionally at higher falls, provided that it effectively recovers to encounter the following impact of the combined rider-vehicle weight. 
     In some embodiments, the wheel of the present invention includes a selective suspension member, unit and/or system adapted to absorb and/or dissipate kinetic energy only while and/or after support member recoverably altering the normally fixed distance as previously described. 
     According to an aspect of some embodiments of the present invention there is provided a suspension system of a vehicle, optionally a self-propelled vehicle such as a manual wheelchair, which includes suspension means and a controller for controlling the suspension means, thereby providing a differentiation and/or selectivity properties. In some embodiments, the suspension means and/or controller are positioned in the wheel, provided attached to a wheel, and/or are rotatable with the wheel. In some embodiments, the suspension means and/or controller are positioned between an axle and a hub, optionally in a hub, optionally between a hub and a wheel body, optionally in the wheel body, optionally between a wheel body and a tire. 
     In some embodiments, the suspension means are configured for absorbing and/or dissipating energy originating from an impact between a wheel, optionally a rear wheel of a wheelchair, and a surface. In some embodiments, the suspension means include at least one of a spring, a damper, an elastic implement, an elastomeric implement, and a plastically deformable implement. 
     In some embodiments, the suspension means are adapted to generate response frequencies to the impact that are essentially over 12 Hz, optionally over 15 Hz, optionally over 20 Hz. In some embodiments, the suspension means are adapted to transmit a force to a seated user of a magnitude smaller than 90%, optionally smaller than 70%, optionally smaller than 50%, of a force magnitude originated in the impact. In some embodiments, suspension means and/or controller are adapted to effectively confront impacts at a certain range of impact angles. 
     In some embodiments, the controller is linked to the at least one support member and/or is configured to control its operation, size and/or form. In some embodiments, the controller is set to determine and/or monitor at least one of the normally fixed distance, the threshold value, and a maximally allowed alteration of said distance by said support member. Optionally the controller is programmable, for example by the user and/or by the manufacturer. In some embodiments, the controller is configured to allow or withhold an operation or activity of the suspension means according to the impact parameters. In some embodiments, the controller is configured for selectively operating the suspension means when the impact includes a magnitude equal or greater than a chosen value and/or a threshold value. Alternatively or additionally, the controller is set and/or configured to withhold operation of the suspension means if the impact is lower in magnitude than the chosen value. In some embodiments, the controller includes at least one of a fuse, a burst valve, a movable barrier, and a buckling implement. In some embodiments, the controller is adapted to selectively operate the suspension means before and/or during the impact, thereby substantially not reducing suspension efficiency and/or performances compared with identical non-controlled suspension means. 
     In some embodiments, the chosen value may be unchangeable, predetermined, and/or irreversibly programmable by a manufacturer or alternatively may be adjustable, pre-settable, and/or reversibly programmable by a user. In some embodiments, the threshold value relates to any of height, distance, velocity, acceleration, impulse, impact, temperature, pressure, and momentum. In some embodiments, the chosen value is a minimal value, a maximal value or a range, having minimal and maximal limiters. 
     In some embodiments, the suspension system of the present invention includes additional suspension means, e.g., a “non-threshold” suspension member, unit and/or system, which is uncontrollable by the controller and/or is adapted to absorb and/or dissipate kinetic energy at the normally fixed distance between the stationary member and the rotary member of the wheel. Such additional suspension means may be configured for handling undesired motion of different types and/or magnitude. For example, the controlled suspension means may be configured for handling impacts of magnitudes greater than a chosen value, whereas the additional uncontrolled suspension means may be configured for handling vibrations, optionally in an effective manner at a specific frequency range, optionally 0.5-80 Hz, optionally 1-30 Hz, optionally 4-12 Hz. The two suspension members types may be located at same or different parts of the vehicle. For example, in a wheelchair, the controlled/threshold suspension member may be located at the rear wheels whereas the uncontrolled/non-threshold suspension member may be located at or above the front castors and/or forks. In a second example, the two suspension members types may be located at different areas of a rear wheel, for example adjacent the hub and adjacent the tire, respectively. In some embodiments, the at least one of the selective suspension member, the non-threshold suspension member, and/or the at least one support member is rotatable with the wheel. 
     In some embodiments, the suspension system includes a sensor in direct communication with the controller adapted to sense a change in a measured variable indicative of a parameter related to the support member stressing and/or to a parameter related to an upcoming stressing of the support member. In some embodiments, the sensor is operable as an arming and/or a safety mechanism of the controller and/or the suspension means. In some embodiments, the sensor is adapted to transmit information to the controller before the impact. 
     In some embodiments, the suspension system includes a user interface in direct communication with any of the controller, the suspension means, the sensor and any communication link therebetween. In some embodiments, the user interface includes bypassing means allowing shutoff of the controller, the suspension means and/or the sensor. 
     Referring back to the drawings,  FIGS. 2A-B  schematically illustrate a wheel portion  36  comprising an exemplary selectively formable support member  35  before ( FIG. 2A ) and after ( FIG. 2B ) formation, in accordance with embodiments of the present invention. Wheel portion  36  includes an inner member  34  and an outer member  33  that are preferably, though not necessarily, maintain geometric similarity (e.g., both are circular, as illustrated). In some embodiments, when the wheel revolves there is also relative rotation between inner member  34  and outer member  33 . In some embodiments, inner member  34  is fixed during wheel rotation, so that outer member  33  rotates in a velocity which is equal, less or more than wheel velocity. Alternatively, inner member  34  is rotationally free whereas outer member  33  is rotationally fixed. Inner member  34  may be any of an axle connecting between the wheel and a vehicle (e.g., a wheelchair) and an inner ring of a wheel bearing or bushing which is rotationally fixated to a vehicle at connection. Outer member  33  may accordingly be any of an outer ring of a wheel bearing or bushing or any other outwardly fixed thereto, including but not limited to a hub shell, a wheel body portion, an assembly of supporting members and/or a rim connected thereto (with or without a tire attached thereto). 
     Support member  35  is positioned in-between inner member  34  and outer member  33  whereas its form directly influences the distance therebetween. In case that inner member  34  and outer member  33  are circular, a distance may be considered the distance between their axes. In some embodiments, support member  35  is substantially elongated along an axis and capable to compress along this axis, or otherwise bend or buckle, and thereby decreasing the distance between inner member  34  and outer member  33 . Alternatively, support member  35  may be at least partially circular (not shown), covering an angular path between inner member  34  and outer member  33 , and is radially compressible, discretely or continuously, along the angular path. 
     In some embodiments, support member maintains a substantially or fully constant form and/or size, and optionally is stiff and/or rigid, under external forces aiming for reshaping thereof, such as compressive forces or any other compressive stresses facilitators, until such stresses or forces goes beyond a predetermined threshold value and/or once it meets a preset rule allowing it to take a different form and/or to yield. As shown in  FIG. 2A , support member  35  maintains a primary form and/or size while being affected by compressive force F 1  acting thereon, therefore maintains concentricity and a normally fixed distance between inner member  34  and outer member  33 . In some embodiments, support member  35  maintains this primary form and/or size during variable forces, including force F 1 , at least until reaching or exceeding a threshold compressive force F Th . Threshold force F Th  may derive from a certain design or any mechanical, chemical or physical property of support member  35  or any component or portion thereof, or it may be programmable (as by using processor or other controlling means) or otherwise settable by a user and/or in production. As shown in  FIG. 2B , once being compressed by a force F 2  that exceeds threshold force F Th , support member then reshapes and/or changes in size (in this example: shortens in length) thereby decreasing the distance between inner member  34  and outer member  33 . In some embodiments, support member  35  is configured to regain its primary shape once compressive stresses decreases totally or below threshold force F Th . Support member  35  may be programmed to regain primary form and/or size and/or otherwise include an elastic component or a portion acting against any change in form and/or size and to regain it once stresses decreases. 
     In some embodiments, support member  35  includes suspension (not shown) used for absorbing and/or dissipating kinetic energy associated with shocks, vibrations and/or other interfering signals acting thereto. In some embodiments, the suspension includes selective operable means which thereby determine overall selectivity of support member  35  to a change in form and/or size according to a preset or predetermined threshold force F Th  and in view of actual stresses acting onto, as described above. 
     Reference is now made to  FIGS. 3A-C  which illustrate schematic diagrams of exemplary selective suspension units, in accordance with embodiments of the present invention.  FIG. 3A  relates to a selective suspension unit  40  which comprises a spring member  41 , a damper  42  and an on/off switch  43 . Spring member  41  may be any spring type, including but not limited to a coil spring, a leaf spring and a gas spring, configured to absorb incoming shocks. Damper  42  may be any damping type mechanism or component, including but not limited to a pneumatic damper, hydraulic damper, elastomeric damper, friction members or others, configured to dissipate kinetic energy, at least partly, to other forms of energy, including heat. The combined utilization of spring member  41  and damper  42  aims to change and transfers incoming shocks and vibration to usually having extended cycle time and/or decreased magnitude. In some embodiments, when switch  43  is opened (as shown), suspension unit  40  is not activated and will not act upon absorbed kinetic energy. In some embodiments, switch  43  is configured to remain open during stresses and/or forces of up to a predetermined threshold value and to close once the predetermined threshold value is crossed. Switch  43  may be connected (not shown) to a controller and/or a sensor for operation thereof in view of concurrent and/or upcoming events. 
       FIG. 3B  relates to selective suspension unit  44  which comprises a spring member  45 , preloaded with a preloading PL, and a damper  46  which may be similar to as described above with respect to damper  42 . In some embodiments, preloading PL determines a threshold compressive force value (a “compression threshold”) until which the spring will not compress and effectively absorb incoming shocks. Spring  45  may be preloaded by using means to fixedly compressing it to a chosen degree according to need. In some embodiments, preload PL is significant with respect to condition (e.g., form and/or size) of spring  45  at relaxed state, for example, spring  45  is initially compressed by at least 20% its relaxed length, optionally to 50% its relaxed length, or more. In some embodiments, the maximal travel of preloaded spring  45  upon receiving an over-the-threshold shock is substantially lessened with respect to its max travel if not preloaded (e.g., its relaxed length), optionally up to 10% its relaxed length, optionally up to 30% its relaxed length, optionally up to 50% its relaxed length, or higher or lower or intermediate. 
       FIG. 3C  relates to selective suspension unit  47  which comprises a spring  48  (which may be similar to spring  41 ), a damper  49  (which may similar to damper  42 ), and a self-releasing locking member, herein refer to as snap S. In some embodiments, snap S is configured to refrain spring  48  and/or damper  49  from acting until opening. In some embodiments, snap S will open only at stresses over a threshold value. In some embodiments, snap S recloses/relocks suspension unit  47  once acting stresses decreases below the threshold. 
     Reference is now made to  FIGS. 4A-B  which provide a schematic illustration of a rear wheel  60  of a wheelchair connected with a chassis part  70  via an axle  72 , including a block diagram of an exemplary suspension system  50  and possible implementations thereof, in accordance with some embodiments of the present invention. In some embodiments, wheel  60  comprises a bearing(s)  68  rotatable around axle  72 , a hub  66 , wheel body  64  which may or may not include a plurality of spokes, and optionally a tire  62  (pneumatic or otherwise). In some embodiments, suspension system  50  includes suspension means  56 , an optional controller  54  and an optional sensor  52 , which are characterized as described above. As shown in  FIG. 4A , suspension system  50  is positioned in or adjacent wheel  60 , and may optionally be part of wheel  60 . In different configurations suspension system  50  may be positioned between tire  62  and body  64 , between body  64  and hub  66 , or optionally in hub  66 , as for example between bearing  68  and axle  72 . Suspension system  50  may be provided as an adapter between wheel  60  and axle  72  (not shown). Sensor  52  may be distant to rest of suspension system  50  parts, in order to optionally sense relevant incoming events in advance. In an exemplary embodiment, sensor  52  is directly linked to front castors (not shown) of the wheelchair while transmitting measured signals to controller  54  which then controls action of suspension means  56  optionally provided in wheel  60 . Sensor  52  may be used to measure parameters including but not limited to any of: a shock magnitude, a vibration average, minimal or maximal magnitude, a frequency, a temperature, a pressure, a visual content, a sound bit, a sitter&#39;s added weight and/or others. Controller  54  may be mechanical and/or electrical, optionally programmable. Alternatively or additionally, controller  54  is provided as auxiliary device, optionally a personal computer, a tablet computer, a Bluetooth device or controller, a PDA or a Smartphone. 
     Reference is now made to  FIGS. 5A-B  which schematically illustrate a wheel  100  that comprises at least one exemplary selective wheel suspension unit  150  before ( FIG. 5A ) and after ( FIG. 5B ) impact with an obstacle OBS, in accordance with some embodiments of the present invention. Wheel  100  includes a tire  110 , a hub  120  and a body  130  which laterally extends between the hub and the tire. Hub  120  may include a bearing or other means which facilitate free rotation possibility with an axle (not shown). Tire  110  may be pneumatic, elastic, semi-rigid or rigid. In some embodiments, selective suspension unit  150  includes a selector member  152  which in this example is an elastic elongated member that is configured to maintain a substantially straight form under axial forces that are less than a chosen threshold buckling force, and buckle to a bent form when encountering axial forces exceed the threshold buckling force. In some embodiments, the elastic elongated member regains its original straighten form when the axial forces decreases or reaches a smaller predetermined value, either the same or different than the minimal buckling force. 
     In some embodiments, selective suspension unit  150  includes a damping member  154  which may or may not be connected to, adjacent and/or encapsulating selector member  152 , and is configured to deform when selector member  152  bends (as shown in  FIG. 5B ). In some embodiments, damping member  154  includes an elastomeric material, a sponge, a hydraulic or pneumatic damper and/or a metal or composite material that is plastically deformable at the suggested situation; therefore it is characterized by energy absorption and/or dissipation when deformed. 
     In some embodiments, selective suspension unit  150  includes a spring member  156  which may include any type of spring element, including but not limited to coil spring (as illustrated in  FIG. 5B ), gas spring, leaf spring or others. In some embodiments, spring member  156  is configured to exert axial force when compressed or otherwise deformed when selector member  152  buckles, thereby resists overall compressive deformation and assists in regaining the original form. Optionally, spring member  156  includes damping features, in which case there may not be a need for damping member  154 . In some embodiments, damping member  154  and/or spring member  156  permits a limited allowed bending and/or compressing, which also limits the bending of the buckled selector member  152 . In some embodiments, selective suspension system includes means for avoiding deflection of selector member  152  under non axial stresses (not shown). 
     As shown in  FIG. 5B , wheel  100  is shown during impact with an obstacle OBS causing it to partially deform. A wheel segment  115 , comprising of a segment of a tire  110  and a corresponding rim of wheel body  130 , is centrally supported with selective suspension unit  150  and extending in between two other adjacent suspension units. In some embodiments, wheel segment  115  ultimately deforms during impact when selector member  152  buckles (as shown in  FIG. 5B ) but remains intact if the impact is too small to buckle selector member  152 . In some embodiments, each wheel segment will react independently during impact and, for example, a deformation of wheel segment  115  will not reflect to other segments of wheel  100  and they will remain intact as if there is no impact. In some embodiments, the action-reaction characteristics of selective suspension unit  150 , when deformed and regain its nominal shape, causes wheel  100  to improve its traversability over obstacles such as OBS, as wheel segment  115  in contact, substantially increases in curvature diameter and/or even bent over OBS. In some embodiments, such a design may allow travel over extremely rough and uneven terrain, or even climbing stairs up or down, depending on preset threshold magnitude. In some embodiments, additional non-threshold and/or uncontrolled suspension means are positioned in wheel  100  (not shown), for example around or in hub  120 . In some embodiments, such uncontrolled suspension means are configured to absorb and/or dissipate energy at any encounter. In some embodiments, the uncontrolled suspension means are adapted to work and/or be especially effective at low-amplitude and/or low-frequencies (or, alternatively, high-frequency) encounters, such as in cases of riding over shallow coarse surfaces and patterned paved surfaces; whereas, optionally, selective suspension unit  150  is adapted to work and/or be especially effective at high-amplitude encounters, such as in cases of falls from stairs and curbs or impacts with non negligible bumps. Exemplary uncontrolled suspension means may include elastomeric or hydraulic bushing(s), and/or annular hydraulic or pneumatic rotatable dampers, such as exemplary dampers disclosed in International Patent Application number PCT/IB2012/000100 to Rogozinski et al, the disclosure of which is fully incorporated herein by reference. 
     Reference is now made to  FIGS. 6A-B  which schematically illustrate a wheel  200  which comprises an exemplary selectively deflatable bladder  250 , in accordance with embodiments of the present invention. Wheel  200  is similar to wheel  100  in that it contains independent portions each having “regional” selective suspension capabilities. Wheel  200  includes a wheel body  220  wearing a tire  210 . In some designs, tire  210  may be excluded. Wheel body  220  includes a plurality of compartments such as compartment  230  that houses bladder  250 . Compartment  230  is at least partly elastic or plastic, at least at its outer surface in contact with tire  210 . Bladder  250 , serving as a sectional selective suspension unit, includes an elastic shell  252  which is normally filled with air to a predetermined degree, optionally to maximal capacity. A spring element  254  is positioned inside shell  252  thereby forcing it to an expanded size depending on total forces acting thereupon. Bladder  250  further includes at least two unidirectional air-flow valves—deflation valve  256  which allows air to only flow out of bladder  250  and inflation valve  258  which allows air only to flow in bladder  250 . Preloading (not shown) provided to spring  254  and/or to deflation valve  256  will determine a threshold impact magnitude by which bladder  250  deflates and therefore reshapes and reduces in volume. Deflation valve  256  may include a minute opening to thereby resist air flow passing therethrough and by doing so act as a damping agent (e.g., the damped energy is turned into acceleration of the jet of air exiting the bladder). Inflation valve  258  may be provided with a large enough opening so that once compression stress decreases from the threshold value, optionally as the wheel rotates, reinflation will be prompt and make bladder  250  ready to possible future interactions. 
     Reference is now made to  FIGS. 7A-B  which schematically illustrate side and back views of an exemplary caster type wheel unit  300  comprising selective suspension members  340  and  350  for discrete degrees-of-freedom motions, in accordance with embodiments of the  30   o  present invention. Caster wheel unit  300  includes a wheel  310  fixedly rotatable to a fork  322  with an axle  312 . Fork  322  is pivotally connected to a stem  324  with a pin  326  and further supported with a fork support member  340  resisting pitching motion of stem  324  and/or fork  322  about pin  326 . In some embodiments, fork support member  340  is rigid and/or maintains a fixed height up to a predetermined threshold, and thereafter recoverably decreases in size. Alternatively, fork support member  340  behaves reciprocally to magnitude in the sense that it can elastically change in height and/or shape when being stressed with magnitudes that are less than a predetermined threshold, but becomes stiff and maintains a height and/or a shape when stresses exceed threshold. In some embodiments, viscoelastic or non-Newtonian materials (e.g., Silly-Putty™ by Crayola LLC, PA, USA) may be provided in fork support member  340  thereby providing viscous flow characteristics (e.g., providing plushness) at low-magnitude shocks, and a stiff and/or elastic behavior at high-magnitude shocks. Stem  324  is pivotally connectable to a fork or stem housing  330  that is provided as a component in a wheelchair (not shown) using a bolt connection  332 . Such pivoting allows swiveling or yawing motion of wheel unit  300  in fork housing  330 , thereby improving wheelchair maneuverability. In some embodiments, a stem support member  350  is provided in-between stem  324  and fork housing  330  that is variably or selectively resistive to swiveling motion. In some embodiments, stem support member  350  is adapted to change in form and/or size only if stressed over a threshold value, or alternatively it is subject to change in form and/or size only while it is stressed under a threshold value. In some embodiments, at least one of fork support member  340  and stem support member  350  includes suspension capabilities so that when motion is selected to occur, the kinetic energy is at least partially absorbed and/or dissipated. 
     Reference is now made to  FIGS. 8A-B  which illustrate an exemplary wheel  400 , optionally a bicycle wheel or a wheelchair rear-wheel, comprising selectively formable hub unit  450 , in accordance with embodiments of the present invention. Wheel  400  includes a rim  410  wearing a tire  420 , a plurality of compression spokes, such as spoke  415 , supporting rim  410  and connecting it to outer boundaries of a substantially rigid hub shell  440  that houses formable hub unit  450 . An optional pushrim  430  is connected laterally to rim  410  for manually propelling a wheelchair. Formable hub unit  450  includes an elastic outer sleeve  452  covering an axisymmetric truss  455  comprising a plurality of support members  454  which are outwardly radially projected from an inner sleeve  456  and connected at their radially farther end to outer sleeve  452 . Inner sleeve  456  is provided over a bearing  458  that is mountable on a quick release axle (not shown) connectable to a wheelchair. In some embodiments, truss  455  is substantially stiff, at least with respect to outer sleeve  452 , although support members  454  maintain shape and/or size recoverable (e.g., elastic) properties. In some embodiments, truss  455  is configured to support axisymmetric shape of outer sleeve  452  and/or concentricity thereof with bearing  458 , thereby allowing constant radii when wheel  400  rotates, up to a compressive stressing which exceeds a predetermined threshold value. Optionally, alternatively or additionally, outer sleeve  452  is a springy tubular member configured to recoverably alter its tubular or axisymmetric form at some ranges of compressive forces, whereas truss  455  facilitates a minimal threshold value which below that no actual or substantial compression will take place. In the latter case, the outer sleeve  452 , with or without an added effect made by truss  455 , will force the entire hub unit  450  to recover back into its axisymmetric form once a compressive force is removed or falls under a value, optionally the threshold value. In some embodiments, when compression threshold is crossed, at least one of support members  454  is forced to recoverably buckle, thereby altering the distance and/or concentricity between hub sell  440  and bearing  458 . In some embodiments, elastic characteristics of outer sleeve  452  assists or facilitate recoverability of truss  455  or any of support members  454  once compressive stresses decreases to under the compressive threshold. In some embodiments, formable hub unit  450  acts as a selective suspension unit once allowed to shape under compressive stresses. In some embodiments, outer sleeve  452  and/or truss  455  includes elastomeric members or portions which act as damping means, whereas the combination of support members  454  and outer sleeve  452  acts as axisymmetric radial spring. 
     Reference is now made to  FIGS. 9A-B  which illustrate an exemplary wheel  500 , optionally a bicycle wheel or a wheelchair rear-wheel, comprising an in-hub leaf-spring type selective suspension unit  550 , in accordance with embodiments of the present invention. Wheel  500  includes a rim  510  wearing a tire  520 , a large diameter bearing  540 , and a plurality of tension spokes  530  connecting therebetween and supporting rim  510  to maintain a circular form under variable compressive stresses. Selective suspension unit  550  is connected to the inner surface of bearing  540  inner ring, and includes a rigid circular frame  552  and a two-members leaf spring  554  connected at two points to its inner surface. Leaf spring  554  includes two ports  556  for rotatably fixating it to a wheelchair using pins provided therethrough. A hydraulic damper  558  is provided across leaf spring  554  and fixated with its piston member  560  to frame  552 . In some embodiments, at least one of leaf spring  554  and damper  558  are provided preloaded to thereby act as a selective suspension unit as previously described. Only above a predetermined threshold value, a shock absorbed by wheel  500  will cause leaf spring  554  to bend, thereby recoverably altering the normally fixed distance between fixation ports  556  and frame  552 . 
     Reference is now made to  FIGS. 10A-B  which illustrate an exemplary wheel  600  comprising an in-hub spring-loaded cam-type selective suspension unit  650 , in accordance with embodiments of the present invention. Similarly to wheel  500 , wheel  600  includes a rim  610  wearing a tire  620 , a large diameter bearing  640 , and a plurality of tension spokes  630  connecting therebetween and supporting rim  610  to maintain a circular form under variable compressive stresses. Selective suspension unit  650  is connected to the inner surface of bearing  640  inner ring, and includes a rigid circular frame  652  that is fixedly rotatable to a wheelchair chassis portion  670  with a pin  672 . A cam-like rounded slot  656  is provided in frame  652  and allows a traveling pin  674  projecting from chassis portion  670  to travel along its rounded inner periphery while being supported with a spring  656  continuously pressing it against the rounded periphery. A damper  660  is also provided and connected at a first end  662  to traveling pin  674  and at a second end  664  to frame  652 . In some embodiments, at least one of spring  654  and damper  660  are provided preloaded thereby facilitating selective suspension capabilities. Only above a predetermined, possibly adjustable, threshold value, a shock absorbed by wheel  600  will cause frame  652  to rotate around pin  672  while inner periphery of slot  656  moves relatively to traveling pin  674 , whereas the shock is absorbed with spring  656  and damper  660 . 
     Reference is now made to  FIGS. 11A-C  which illustrate an exemplary wheel  700  comprising a plurality of spokes type selective suspension members  740  (or  760 ), in accordance with embodiments of the present invention. Wheel  700  includes a rim  710  wearing a tire  720 , a hub  730  and the plurality of members  740  that are provided symmetrically and evenly distributed and connecting between rim  710  and hub  730 . In some embodiments, members  740  support a fixed distance, under a compressive forces of less than a threshold magnitude, between hub  730  and contact regions (e.g., flanges  715 ) at rim  710 . Optionally, members  740  do not maintain or only partially support circularity of rim  710 , and therefore the latter is optionally provided strengthen with respect to previously shown rims. In some embodiments, hub  730  includes a center rounded portion  736  having a bore  738  passing therethrough and housing a bearing (not shown) mountable to a chassis (e.g., of a wheelchair) using an axle. Three outwardly radial extensions  734  originate from hub center  736 ; each radial extension  734  ends with an angularly extended head  732 ; each angularly extended head  732  includes two lateral sides; wherein each lateral side is hingedly connected to an inward connection portion  742  of a member  740 . Member  740  includes an outward connection portion  746  which is hingedly connected to rim  710  at flange  715 . Each member  740  includes a piston  741  slidably movable in a cylindrical housing  745 . Both piston  741  and housing  745  includes linear slots ( 744  and  748 , respectively) provided along and in parallel to their longitudinal axes, and each include a movable pin ( 743  and  747 , respectively) that is slidably movable in a corresponding slot (pin  743  in slot  748  and pin  747  in slot  744 ). A preloaded compression spring  750  is provided connected in-between pin  743  and  747 . Spring  750 , when fully relaxed or compressed under a predetermined threshold value (according to preloading), maintains pins  743  and  747  at a normally fixed distance. When piston  741  and housing  745  are subject to compression or extension stresses that are over the predetermined threshold value, the pins ultimately move one towards the other thereby compressing spring  750 . A damping member (not shown) may also be provided and configured to act in parallel to contraction motions of spring  750 . Member  760  is an alternative design that can replace member  740 , and while preserving similar qualities, it is based on gas spring  770  instead of coil spring  750 . Similarly, when member  760  elongates or shortens at stresses exceeding the threshold value, gas spring  770  will be forced to compress. In some embodiments, gas spring  770  includes damping capabilities, as known in the art. 
     Reference is now made to  FIGS. 12A-B  which illustrate an exemplary wheel  800  comprising selectively slidable hub unit  840 , in accordance with embodiments of the present invention. Wheel  800  includes a rim  810  wearing a tire  820 , the slidable hub unit  840  and a plurality of tension spokes  830  connecting between rim  810  and slidable hub unit  840 . Slidable hub unit  840  includes a rigid anterior plate  841  fixedly connected to a rigid posterior plate  842  with a rigid channel  843  encapsulating a bearing (not shown) mountable to a chassis (e.g., of a wheelchair) using an axle (not shown). An annular elastomer  844  is provided around channel  843  and in-between plates  841  and  842 . Four double-sided fasteners  845  are evenly distributed around outer periphery of elastomer  844 , each fastener  845  is compressedly maintained by its two sides in opposite corresponding recesses (not shown) on inner surfaces of plates  841  and  842 . In some embodiments, fasteners  845  includes a ball-and-spring plunger mechanism, with a ball popping out at each end and being compressed against a corresponding recess by the spring. In some embodiments, all springs are preloaded so that the fasteners may migrate out of the recesses only over a predetermined threshold value related to an impact magnitude acting thereto in a certain direction. A ring (not shown) is provided over the four fasteners  845  and elastomer  844 , to which the spokes  830  inwardly projecting ends are connected. Therefore, an impact absorbed by the tire  820  and rim  810  is projected via spokes  830  and their connecting ring to fasteners  845 . Under stresses that are less than the threshold value the fasteners  845  will remain in-place hence wheel  800  will remain rigid, whereas under stresses higher than the threshold value, fasteners  845  will be forced to pop out of their corresponding recesses. A relative motion is therefore made between the wheel body (comprising of tire  820 , rim  810 , spokes  830  and fasteners  845 ) with respect to hub unit  840 , and elastomer  844  provided in-between is forced to deform while resisting the motion and acting both as a spring and as a damper. Once stresses decrease, hub unit  840  regains its normal shape and fasteners  845  slip back into place. 
     Reference is now made to  FIGS. 13A-B  which illustrate an exemplary wheel  900  connected to a leaf-spring type selective suspension  960  designed as a chassis-to-wheel adaptor, in accordance with embodiments of the present invention. Wheel  900  is optionally a commercially available rear wheel releasably connectable to a wheelchair via a hub  940 . Suspension  960  is connectable to a wheelchair via plate  961  and comprising a leaf spring  963  fixated to plate  961  with a support  962  and having a free end  964  having a lumen and connectable to hub  940  using an axle  950 . Leaf spring  963  is preloaded by repositioning it from a primary position ( 966 ) and fixating it to its preloaded position using a pin  965 . A damper  967  is configured to act if spring  964  is forced to further bend when stresses are over a threshold value determined by its preloading. 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. 
     All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.