Patent Publication Number: US-2004040769-A1

Title: All wheel steering scooter

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] This application claims the benefit of U.S. Provisional Application Serial No. 60/386,639, filed on Jun. 5, 2002. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates generally to conveyances and, more particularly, to motorized conveyances such as scooters and the like having mid-wheel drives with rearward stability and scooters having all wheel steering systems.  
       BACKGROUND OF THE INVENTION  
       [0003] Scooters are an important means of transportation for a significant portion of society. They provide an important degree of independence for those they assist. However, this degree of independence can be limited if scooters are required to navigate small hallways or make turns in tight places such as, for example, when turning into a doorway of a narrow hallway. This is because most scooters have a three-wheel configuration that creates a less than ideal minimum turning radius for the scooter. Such three wheel configuration typically has a front steering wheel and two rear drive wheels. As such, the two rear drive wheels propel the scooter forward or rearward, while the front steering wheel steers the scooter by rotating through a plurality of steering angles. Alternative configurations include a front drive and steering wheel and two rear wheels. Because the steering wheel is typically located in the front portion of the scooter and the other wheels are typically located in the rear portion of the scooter, the scooter&#39;s turning radius is directly dependent on the physical dimensions that separate these components. As such, the minimum turning radius formed by such a three wheel configuration, while adequate for most purposes, is too large for simple navigation of the scooter in tight spaces such as in narrow doorways and hallways. Hence, a need exists for a scooter that does not suffer from the aforementioned drawbacks.  
       SUMMARY OF THE INVENTION  
       [0004] According to one embodiment of the present invention, a scooter having at least one front wheel, a plurality of rear wheels and a steering column linked to the front and rear wheels is provided. An angular change in the steering column is translated to angular change in the front and rear wheels.  
       [0005] According to another embodiment of the present invention, a scooter having a steering mechanism is provided. The steering mechanism includes a steering column which is linked to front and rear wheels of the scooter. A plurality of linkages providing physical communication between the rear wheels is optionally provided. The steering mechanism further optionally includes additional linkages, pulleys, a torque tube and a crank for facilitating translation of angular change in the steering column to the wheels.  
       [0006] According to yet another embodiment of the present invention, a scooter having a front wheel drive and a steering mechanism is provided. According to still another embodiment of the present invention, a scooter having a rear wheel drive and a steering mechanism is provided.  
       [0007] An advantage of the present invention is to provide a more maneuverable personal assist vehicle such as a scooter and the like having an all-wheel steering configuration. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008] In the accompanying drawings which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which together with a general description of the invention given above and the detailed description given below, serve to example the principles of this invention.  
     [0009]FIG. 1 is an exemplary perspective view of an all-wheel steering scooter in accordance with one embodiment of the present invention.  
     [0010]FIG. 2 is an exemplary side elevational view of an all-wheel steering scooter in accordance with one embodiment of the present invention.  
     [0011]FIGS. 3A and 3B are exemplary schematic diagrams of a steering mechanism in accordance with one embodiment of the present invention. FIG. 3C is an exemplary diagram of a scooter in accordance with one embodiment of the present invention. FIG. 3D is an exemplary schematic diagram of a steering mechanism for a scooter in accordance with one embodiment of the present invention.  
     [0012]FIGS. 4A and 4B are exemplary schematic diagrams of a steering mechanism for a scooter in accordance with one embodiment of the present invention.  
     [0013]FIGS. 5A and 5B are exemplary schematic diagrams of a steering mechanism for a scooter in accordance with one embodiment of the present invention.  
     [0014]FIG. 5C is an exemplary diagram of a scooter in accordance with one embodiment of the present invention.  
     [0015]FIGS. 6A, 6B,  6 C and  10 A,  10 B,  10 C,  10 D,  10 E and  10 F are exemplary perspective and partial views of a mid-wheel drive vehicle in accordance with one embodiment of the present invention.  
     [0016]FIGS. 6D, 6E, and  6 F are exemplary partial views of a drive mechanism of a mid-wheel drive vehicle in accordance with one embodiment of the present invention.  
     [0017]FIGS. 7A, 7B, and  7 C are exemplary partial views of a mid-wheel drive vehicle in accordance with one embodiment of the present invention.  
     [0018]FIG. 8 is an exemplary schematic illustration of a mid-wheel drive vehicle in accordance with one embodiment of the present invention.  
     [0019]FIG. 9 is an exemplary schematic drawing of a comparison between a rear-wheel scooter and a mid-wheel drive vehicle in accordance with one embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT  
     [0020] Generally, a scooter is a vehicle used to assist those having an impaired ability to transport themselves. In an embodiment, a scooter of the present invention has one or more wheels including at least one front wheel and two rear wheels. The front or rear wheels can be drive wheels. At least one motor (also called a drive mechanism) or combination motor/gear box is provided to drive the drive wheels. The motor is typically controlled by an electronic controller connected to one or more user control devices. The user control devices generally provide selection of forward and reverse movement of the vehicle, as well as controlling the velocity or speed. A battery typically supplies the controller and drive motors with an energy supply. Dynamic braking and an automatic park brake are also incorporated into the scooter. The dynamic brake allows the operator to proceed safely, even down a slope. Further, the park brake automatically engages to hold the vehicle in place when the vehicle is standing still.  
     [0021] The present invention provides multiple embodiments of scooters. One embodiment is an all-wheel steering scooter and another embodiment is a mid-wheel drive scooter. In an embodiment relating to all-wheel steering, a scooter has a forward steering wheel and two drive wheels located rearward of the steering wheel and, most preferably, near the rear portion of the scooter. The steering wheel is in physical communication with a steering column that can be rotated by a user of the scooter to change the angular direction of travel of the scooter. The drive wheels are in physical communication with each other via a plurality of linkages that are linked with the steering column so that any angular or rotation changes in steering column are translated to the drive wheels. When translated, the drive wheels themselves undergo angular displacement in a direction opposite to the steering wheel&#39;s angular displacement. In this manner, all of the scooter&#39;s wheels undergo angular displacement to assist in the steering function of the scooter.  
     [0022] Referring now to FIGS. 1 and 2, an embodiment of an all-wheel steering scooter  100  is illustrated. The scooter  100  has body or frame  102  that is typically covered by a decorative shroud  104 . The scooter  100  also includes a seat  106 , drive wheels  108  and  109  (FIG. 2), and forward steering wheel  110 . The drive wheels can be linked to one or more electric motors (not shown) or electric motor/gear box combinations. Forward steering wheel  110  is physically linked to steering column  112 . Steering column  112  further has steering handles, an instrumentation display, and a user input control device such as, for example, a throttle or the like.  
     [0023] Illustrated in FIGS. 3A and 3B are schematic diagrams illustrating one embodiment of an all-wheel steering mechanism  300  suitable for scooter  100 . In this regard, steering mechanism  300  has pulleys  302  and  304  interconnected together by a flex cable  306 . A sheath  308  is provided to protect the flex cable  308 . Pulley  302  is connected to steering column  112  such that any rotation or angular movement of steering column  112  causes pulley  302  to also undergo rotation or angular movement.  
     [0024] Pulley  304  is connected to a pin or bearing assembly  312  and a plurality of Ackermann linkages generally indicated at  310 . Pin or bearing assembly  312  is secured to the body  102  of the scooter  100  and allows pulley  304  to freely rotate. Pulley  304  is further connected to linkages  310  via rod  324 .  
     [0025] Linkages  310  include rod  324 , first angular linkage  316 , second angular linkage  318 , and tie linkage  314 . Rod  324  has a first pivotal attachment  326  a radial distance away from the center of pulley  304  and a second pivotal attachment  328  to first angular linkage  316 . First and second angular linkages  316  and  318  are each attached to tie linkage  314  via pivotal attachments  320  and  322 , respectively. First and second angular linkages  316  and  318  each include a pivotal connection  334  and  336  to the frame or body  102  of the scooter and an angled extension portions  330  and  332 , respectively. Angled extension portions  330  and  332  are coupled to the drive wheels. Being fixed to the frame or body  102 , pivotal connections  334  and  336  do not physically move but allow first and second angular linkages  316  and  318  to rotate or pivot there around. The pivotal connections as used herein can range from a simple hinge joint, such as pin or bolt extending through apertures formed in the relative rotational bodies or linkages, or a bearing assembly provided between and connected to the rotating bodies or linkages. Other joints allowing for rotation movement can also be applied.  
     [0026] In operation, rotation of steering column  112  causes pulley  302  to rotate. Rotation of pulley  302  causes flex cable  306  to cause rotation of pulley  304 . Rotation of pulley  304  causes rod  324  to undergo lateral displacement. Lateral displacement of rod  324  causes first angular linkage  316  to pivot about pivot connection  334 . This causes drive wheel  108  to undergo angular displacement. Because first angular linkage  316  is also connected to second angular linkage  318  by tie linkage  314 , second angular linkage  318  also rotates or pivots around its pivotal connection  336 . This in turn causes drive wheel  109  to undergo angular displacement. When turning, the scooter of the present invention is configured to allow a speed differential to develop between the two drive wheels. This speed differential is necessary because each drive wheel is a different distance from the turning point of the scooter, the turning point being the center of the curvature of the scooter&#39;s turn. This speed differential can be provided by mechanically such as, for example, by a transaxle, or electrically such as, for example, by a parallel or series wiring of the power drive signal to the drive motors or by control directly within the electronic controller controlling the power distribution to the scooter&#39;s drive motors.  
     [0027] As shown in FIG. 3C, the angular displacement of steering wheel  110  causes drive wheels  108  and  109  to undergo a corresponding change in angular position. This change in angular position is configured to be opposite in direction from the steering wheel&#39;s change in angular position. Additionally, since drive wheels  108  and  109  are different distances from a turning point C of the scooter, each drive wheel&#39;s angular displacement is preferably configured to be 90 degrees from a line running through the turning point C and the drive wheel&#39;s point of contact with the drive surface. Hence, for a particular turning point C, the angular displacement of each drive wheel  108  and  109  will be different. This difference is primarily provided by appropriately configuring the angular configuration of first and second angular linkages  316  and  318 .  
     [0028]FIG. 3D illustrates another embodiment that employs a push-pull cable  342 . Push-pull cable  342  is any suitable mechanical push-pull cable or wire rope such as manufactured by, for example, Cable Manufacturing and Assembly Co., Inc. of Bolivar, Ohio. The push-pull cable  342  preferably comprises an outer conduit having a multi-strand wound cable or solid core. The cable or core can move within the conduit and thereby translate linear motion input at one end of the cable or core to the other. In this regard, the cable or core of push-pull cable  342  has a first end preferably connected to steering column  112  via linkage  338 . Linkage  338  is rigidly affixed to steering column  112  so as to rotate therewith. The connection of push-pull cable  342  to linkage  338  is accomplished by any suitable joint, including but not limited to, a pivot joint such as, for example, by a bolt, screw or rivet extending through an “eye” fitting attached to one end of the cable or core of push-pull cable  342  and an corresponding aperture in linkage  338 . Since push-pull cable  342  is flexible, it can be curved or bent to translate the reciprocating movement experienced by its connection to steering column  112  to linkages  314 ,  316 , and  318 , as illustrated. In this regard, a second end of push-pull cable  342  is connected to linkage  316  via connection  344 . Connection  344  can also be via a bolt, screw or rivet extending through an “eye” fitting on the second end of cable or core of push-pull cable  342  and a corresponding aperture in linkage  316 . Other suitable connections are also possible.  
     [0029] In operation, the rotational movement of steering column  112  causes linkage  338  to undergo rotation movement thereabout. This causes the first end of the cable or core of push-pull cable  342  to undergo linear movement that is translated to linkage  316 . Because push-pull cable  342  is flexible, it can be arranged so as to cause pivotal movement of linkage  316  about its pivotal connection  334 . This motion is translated by linkage  314  to linkage  318  as described earlier and results in wheels  108  and  109  pivoting to prescribed steering angles.  
     [0030]FIGS. 4A and 4B illustrate another embodiment  400  having a torque tube  402  and a bell crank  404 . More specifically, embodiment  400  has steering column  112  linked to torque tube  402  via linkages  406 ,  410 , and  412 . Linkage  406  has a fist end attached to steering column  112  and a second end attached to linkage  410  via a pivotal connection  408 . Linkage  410  is further connected to linkage  412  via pivotal connection  414 . Linkage  412  is connected to a first distal portion of torque tube  402 . Torque tube  402  includes a second distal portion that is attached to a projecting linkage  416 . Torque tube  402  is fixedly attached to the frame or body  102  of the scooter so as to not undergo any lateral or longitudinal displacement, but to allow pivotal movement of linkages  412  and  416 . Linkage  416  is connected to bell crank  404  via tie linkage  420  and pivotal connections  418  and  422 . Bell crank  404  has a pivotal connection  424  to the frame or body  102  of the scooter. This keeps bell crank  404  in place while also allowing it to rotate around pivotal connection  424 . Bell crank  404  further has a pivotal connection  426  to rod  428 . Rod  428  connects bell crank  404  to linkages  310 . In this embodiment, first angular linkage  432  is configured slightly different from first angular linkage  316  of FIG. 3B. More specifically, first angular linkage  432  has a pivotal connection  430  to rod  428  and pivotal connection  320  to tie linkage  314 . In this regard, pivotal connection  320  to tie linkage  314  is shown in a middle portion of first angular linkage  432  between the pivotal connections  430  and  334 . However, it is also possible to configure first angular linkage  432  to be the same as first angular linkage  314  (not shown). The remaining linkages and their pivotal connections are essentially the same as described in the embodiment of FIG. 3B.  
     [0031] In operation, rotation of steering column  112  causes linkage  406  to rotate. Rotation of linkage  406  causes longitudinal movement on linkage  410 , which causes angular displacement of linkage  412  about torque tube  402 . Torque tube  402  translates along a vertical height dimension the angular displacement of linkage  412  to a corresponding angular displacement of linkage  416 . This angular displacement of linkage  416  translates to a longitudinal movement of tie linkage  420 . The longitudinal movement of tie linkage  420  causes bell crank  404  to undergo pivotal movement about pivotal connection  424 . This pivotal movement causes rod  428  to undergo lateral displacement that causes first angular linkage  432  to pivot about pivot connection  334 . This causes drive wheel  108  to undergo angular displacement. Because first angular linkage  432  is also connected to second angular linkage  318  by tie linkage  314 , second angular linkage  318  correspondingly rotates or pivots around its pivotal connection  336 . This in turn causes drive wheel  109  to undergo angular displacement. The torque tube  402  allows the rotational movement of steering column  112  to be input above the vehicle&#39;s frame and to translate this motion to linkages under the frame.  
     [0032] Illustrated in FIGS. 5A and 5B is another embodiment  500  that eliminates the torque tube  402 , linkages  410 ,  412 ,  416 ,  420  and their associated pivotal connections of FIGS. 4A and 4B. In this regard, a single tie linkage  502  is provided between linkage  406  and bell crank  404 . Tie linkage  502  has a pivotal connection  408  to linkage  406  and a pivotal connection  422  to bell crank  404 . In operation, the pivotal movement of linkage  406  translates to longitudinal movement of tie linkage  502 . The longitudinal movement of tie linkage  502  translates to rotational or pivotal movement of bell crank  404 . The rotational or pivotal movement of bell crank  404  is translated to rotation or angular displacement of drive wheels  108  and  109 , as already described above. The embodiment of FIGS. 5A and 5B allow for all of the linkages to be placed beneath the vehicle frame.  
     [0033] Illustrated in FIG. 5C is an embodiment illustrating drive mechanisms of a scooter of the present invention. As illustrated, a drive mechanism  520  may be connected to front wheel  110  to facilitate front wheel drive of the scooter. Alternatively and/or additionally, drive mechanisms  535  and  540  may be connected to rear wheels  108  and  109  to provide either rear-wheel drive or all-wheel drive of the scooter. Drive mechanisms may be connected to a corresponding drive wheel in any suitable manner. For example, drive mechanisms  535  and  540  may be rigidly connected to rear wheels  108  and  109  or may be pivotally connected by, for example, a universal joint. Alternatively, rear-wheel drive can be effectuated by using a single drive mechanism for the rear wheels, as illustrated with respect to FIGS. 6E and 6F herein.  
     [0034] Referring now to FIGS. 6A, 6B, and  6 C, the second general embodiment of the present invention will now be discussed. In particular, FIG. 6A illustrates a mid-wheel drive scooter  600  having a body  602 , frame  604 , front steering wheel  606 , steering column  608 , mid-wheel drive wheels  610  and  612 , motor or a motor/gearbox  622  for each drive wheel, walking beams or pivot arms  614  and  616 , and casters  618  and  620 . As further illustrated in FIG. 6B, scooter  600  has a chair  624  mounted to a post  626 . The post  626  is further mounted to the frame  604 . Also, as further illustrated in FIG. 6B, walking beam or pivot arm  614  is connected to frame  604  at a pivotal connection P. Walking beam or pivot arm  616  is similarly connected to frame  604  via a similar pivotal connection.  
     [0035] Pivotal connection P may be laterally offset on frame  604  behind the seat post  626 . The pivotal connection P between walking beam or pivot arm  614  and scooter frame  604  can be formed by any appropriate means including a pivot bolt or pin extending between brackets mounted on the frame  604  and apertures located in the walking beam or pivot arm  614 . Other suitable pivotal joints can also be formed at pivotal connection P.  
     [0036] Walking beams or pivot arms  614  and  616  preferably have a caster wheel (e.g.,  618 ,  620 ) located proximate a first distal end and a motor/drive wheel assembly (e.g.,  610  and  622 ) mounted proximate a second opposite distal end. In between the first and second distal ends, apertures are provided in the walking beams or pivot arms that facilitate connection to the frame  604  to form pivotal connection P. The precise location of the apertures and pivotal connection P defines the weight distribution between the caster and drive wheel on the walking beam or pivot arm.  
     [0037] Referring now to FIG. 6C, a planar top view of the relative positioning of drive wheels  610  and  612 , walking beams or pivot arms  614  and  616 , casters  618  and  620 , and seat post  626  are illustrated. In this regard, it can be seen that walking beams or pivot arms  614  and  616  are located adjacent to the lateral sides of frame  604 . Line PL represents a line drawn through the pivotal connection P of each walking beam or pivot arm to frame  604 . Line CL represents a line drawn through the connection of casters  618  and  622  to walking beams or pivot arms  614  and  616 . Line DL′ represents a line drawn through the connection of drive wheels  610  and  612  to walking beams or pivot arms  614  and  616 . In this embodiment, it can be seen that seat post  626  is located between drive wheel reference line DL and pivot point reference line PL. Most preferably, seat post  626  is located on frame  604  such that a user&#39;s head and shoulders are located approximately along drive wheel reference line DL when the user is seated in seat  624 . It should be understood that relative positioning the drive wheels, pivotal connection P, rear casters and seat post can be adjusted on frame  604  to obtain optimum results according to the above user position requirement.  
     [0038] In summary, the walking beam or pivot arm distributes the scooter&#39;s and user&#39;s weight between the rear caster and the drive wheel. The walking beam or pivot arm supports the scooter frame behind the seat providing stability so the scooter doesn&#39;t tip rearward. As shown in FIG. 6B, an optional spring  630  may be placed between the frame  604  and the walking beams or pivotal arms to further increase rearward stability. In addition to providing rearward stability, the walking beam or pivot arm positions the drive wheel forward of the rear portion of the scooter&#39;s frame for improved maneuverability.  
     [0039] Illustrated in FIG. 6D is a scooter embodiment similar to FIGS.  6 A- 6 C, except that the drive wheels  610  and  612  are driven by a single motor  622  and a transaxle  628 . An axle joint  630  is provided for connecting transaxle  628  to drive wheel  610 . In this regard, motor  622  is connected to transaxle  628  and the combination thereof is used to impart rotational motion to drive wheels  610  and  612 . As described earlier, a gear box can also be present between motors  622  and transaxle  628 . In this regard, transaxle  628  is configured to drive both drive wheels  610  and  612  at the same speed, as well as allowing a speed differential for each drive wheel when the vehicle is driving through a turn. Such transaxle assemblies can also include integrated motor and brake combinations as well.  
     [0040]FIG. 6E illustrates a partial elevational view illustrating the motor  622 , transaxle  628 , walking beams or pivot arms  614  and  616 , axle joint  630 , and drive wheels  610  and  612 . FIG. 6F illustrates a partial elevational view of a transaxle system that incorporates universal joints and drive axles having a suspension systems. More specifically, transaxle  628  and motor  622  are rigidly mounted to frame  604  via bracket  638 . A universal joint  634  connects drive axle  632  to transaxle  628 . Drive wheel  610  is similarly connected to transaxle  628 . Hence, an independent suspension for the drive wheels is provided. FIGS.  10 A- 10 F illustrate further aspects of the embodiment shown in FIGS.  6 A- 6 C.  
     [0041] Referring now to FIGS. 7A, 7B, and  7 C, a scooter embodiment  700  having spring-loaded rear casters is shown. The spring-loaded casters prevent the scooter from tipping rearward and flex to allow the scooter to go over bumps and up ramps such as, for example, ramp  706 . In particular, scooter  700  is similar to scooter  600  of FIGS.  6 A- 6 D, except that drive wheels  610  and  612  and their associated motors  622  are mounted directly to frame  604  and rear casters  618  and  620  are mounted to composite leaf springs  702  and  704  instead of walking beams or pivot arms. The composite leaf springs  702  and  704  are preferably made from a flexible composite material such as, for example, fiberglass and resin or other suitable composite materials or plastics. Alternatively, composite leaf springs  702  and  704  can be made from a material such as, for example, stainless steel, spring steel or other suitable metals or metal alloys.  
     [0042] As such, composite leaf springs  702  and  704  have first and second distal ends. The first distal end is preferably connected to a wheel or a caster such as, for example, castor  618 . The second distal end is preferably connected to the frame  604 . The second distal end&#39;s connection to frame  604  is preferably to a rear portion thereof that may or may not be the rearward most portion of frame  604 . The connection may be by any suitable means including bolting, bracketing or clamping. The remaining aspects of the embodiment shown in FIGS.  7 A- 7 C are similar to the embodiment illustrated and described in connection with FIGS.  6 A- 6 D.  
     [0043] Illustrated in FIG. 8 is a scooter embodiment  800  having one or more weight-loaded casters, such as caster  820 . In this embodiment, seat  624  and the rear caster or casters  820  are mounted to the frame  604  on separate four-bar link systems. When a user sits on the seat  624 , a portion of the user&#39;s weight is applied to the casters through a laterally projecting tab  806  and caster spring  818 . The amount of weight transferred to the caster(s) is dependent upon the strength of the spring  818 . A strong spring will transfer more weight than a weak spring.  
     [0044] As described above, seat  624  is linked to frame  604  by seat post  804  and a four-bar link system having two upper links  814  and two lower links  816 . Since FIG. 8 is a side elevational view of the scooter, only one upper link  814  and one lower link  816  are visible. An opposite side elevational view of the scooter would reveal a second pair of identical upper and lower links. In this regard, upper and lower links  814  and  816  each have first and second distal ends. The first distal ends of the upper and lower links have a first pivotal connection to seat post  804 . The second distal ends of the upper and lower links have a second pivotal connection to frame post  802 . The pivotal connections can be as described earlier for the walking beams or pivot arms.  
     [0045] Rear caster(s)  820  are connected to frame  604  via a caster post  808  and a second four-bar link system having upper and lower links  810  and  812 . As described earlier, only one upper and one lower link  810  and  812  are shown in this side elevational view, with an identical second pair visible in an opposite side elevation view of the scooter (not shown). As such, upper and lower links  810  and  812  each have first and second distal ends. The first distal ends of the upper and lower links have a first pivotal connection to caster post  808 . The second distal ends of the upper and lower links have a second pivotal connection to frame post  802 . As described above, these pivotal connections can be according to any of the aforementioned pivotal structures.  
     [0046] Castor spring  818  also has first and second distal ends. At least one of the first and second distal ends is in physical communication with either tab  806  or link  810  when no user is seated in seat  624 . Alternatively, the first distal end can be in physical communication with tab  806  and of the second distal end can be in a physical communication with link  810  when no user is seated in seat  644 .  
     [0047] In operation, a user sits in seat  624  thereby causing a downward force to be applied to seat  624 . This downward force is translated through tab  806 , caster spring  818 , and upper link  810  to caster post  808 . Configured as such, tab  806 , caster spring  818  and upper link  810  maintain a downward force on caster(s)  820 . Since caster spring  818  is somewhat resilient, caster(s)  820  are allowed limited upward movement such as, for example, when traversing a bump or obstacle or when scooter  800  is climbing up a ramp (see FIG. 7C). An option seat spring  822  can be provided to cushion seat post  804  against frame  604 .  
     [0048] The four-bar linkages associated with the seat post  804  and caster post  808  are advantageous because they always maintain seat post  804  and caster post  808  in a relatively vertical orientation while seat post  804  and caster post  808  undergo vertical movement. This configuration is especially advantageous because it selectively engages the caster spring  818  only when a force is applied to seat  624 . Once the force has been removed from seat  624 , caster  820  is no longer urged downwards. This configuration prevents the force of spring castor  818 , if too strongly constituted, from lifting wheels  610  and  612  from the driving surface when there is no force applied to seat  624 . Such a configuration also provides a mid-wheel drive scooter with variable rearward stability.  
     [0049] Referring now to FIG. 9, a diagram illustrating the increased side stability of a mid-wheel drive scooter compared to a conventional rear wheel drive scooter is shown. More specifically, steering wheel  606 , mid-wheel drive wheels  610  and  612 , and user center of gravity  910  are illustrated in their respective relative positions. Also illustrated are the relative positions of conventional rear wheel drive wheels  610   a  and  612   a . Using the center of gravity  910  and riding surface contact points  904 ,  906 , and  908  of the steering and drive wheels, respectively, a mid-wheel tilt line  902  and rear wheel tilt line  900  can be generated. As can be seen, mid-wheel tilt line  902  has a center of gravity tilt reference  914  that is further from the scooter&#39;s center line  916  than rear wheel tilt line  900  center of gravity tilt reference  912 . The further the center of gravity reference is from scooter center line  916 , the more the stable the scooter is with respect to side tilt. For example, when the scooter of FIG. 9 makes a left-hand turn, as the turning speed increases, the rear wheel drive configuration scooter will tend to tilt to the right at a lesser speed than the mid-wheel drive scooter of the present invention. This is important because tipping or tilting of a scooter can cause serious injury both to the user and bystanders.  
     [0050] While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, pivotal connections can be made of any number of structures including bearing assemblies, pins, nuts and bolts, and frictionless sleeve assemblies. Additionally, springs or shock absorbers can be added between pivoting and non-pivoting components to limit, dampen, or somewhat resist the pivotal motions of these components. Still additionally, skids or any suitable device with a curvilinear surface may be used in the place of wheels or casters. Moreover, the present invention may driven with via a front-wheel drive configuration wherein the front wheel is driven by a motor or motor and gearbox combination. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures can be made from such details without departing from the spirit or scope of the applicant&#39;s general inventive concept.