Abstract:
A motorized toy vehicle or Wall Racer that is capable of operating on vertical and inverted horizontal surfaces such as walls and ceilings, while being manufacturable at reasonable cost and operable on batteries having sufficient lifetime as to be enjoyable. One or more battery-powered fans draw air from around all or defined portions of the periphery of the chassis of the Wall Racer through a carefully-shaped duct, so that the air in the portion of the duct immediately adjacent the surface flows at high velocity and low pressure; the relatively greater pressure of the surrounding air urges the vehicle against the surface, allowing it to operate on vertical surfaces, such as walls, or inverted on horizontal surfaces, such as ceilings.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of Ser. No. 11/177,428, filed Jul. 11, 2005 now U.S. Pat. No. 7,753,755, which claimed priority from U.S. provisional application 60/640,041, filed Dec. 30, 2004. This application also claims priority from U.S. design patent application Ser. No. 29/312,447, filed Oct. 21, 2008. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to radio-controlled motorized toy vehicles capable of operation on surfaces of all orientations, e.g., walls and ceilings as well as floors. 
     BACKGROUND OF THE INVENTION 
     Radio-controlled motorized toy vehicles, that is, vehicles driven by motors and steered responsive to commands transmitted remotely, are of course well-known. Toy vehicles that are very sophisticated in terms of their suspension and steering systems are available and are very popular. A toy vehicle that operated other than on essentially horizontal surfaces, e.g., which could operate on a vertical wall, or inverted on a ceiling, and which could be made and sold at a competitive price, would be very desirable. 
     U.S. Pat. No. 5,014,803 to Urakami shows a device for “suction-adhering” to a wall and moving along the wall, e.g. for cleaning the interiors of tanks and the like. The Urakami device relies on a relative vacuum; that is, air is drawn by a vacuum pump out from a sealed volume formed between the interior of the device and the wall, so that air pressure on the outer surface of the device forces it against the wall. This necessitates that an essentially air-tight seal be formed around the periphery of the device. Forming an air-tight seal between a moving device and a fixed surface is not a simple problem, and the Urakami patent is directed primarily to such seals. The obvious problems to be overcome are friction between the sealing member and the wall, which impedes motion of the device and causes wear of the sealing members, loss of vacuum at irregularities in the surface, and the large amount of power required to form an effective vacuum. This approach is not satisfactory for a toy vehicle that must be durable when operated by children and be able to be operated for a sufficiently long time with a limited amount of battery capacity to not frustrate the user. 
     SUMMARY OF THE INVENTION 
     The present invention provides a motorized toy vehicle that is capable of operating on vertical and inverted horizontal surfaces such as walls and ceilings, while being manufacturable at reasonable cost and operable on batteries having sufficient lifetime as to be enjoyable. The vehicle of the invention, referred to hereinafter as the Wall Racer, employs a freely-flowing stream of air between the surface-abutting periphery of the interior volume of the vehicle to create a pressure differential with respect to the surrounding air, so that the pressure of the surrounding air urges the Wall Racer against the surface. 
     More specifically, one or more battery-powered fans draw air from around all or defined portions of the periphery of the chassis of the Wall Racer through a carefully-shaped duct formed between the undersurface of the chassis and a juxtaposed surface, so that the air in the portion of the duct immediately adjacent the surface flows at high velocity. According to Bernoulli&#39;s Principle, this high-velocity air stream is of low pressure; the differential between this low-pressure air stream and the relatively greater pressure of the surrounding air urges the vehicle against the surface, allowing it to adhere to vertical surfaces, such as walls, or be operated inverted on horizontal surfaces, such as ceilings. The differential pressure thus urging the vehicle against the surface is referred to hereinafter, as in the automotive industry, as “downforce”. Because the air stream must be freely flowing to attain high velocity, seals such as required for wall-climbing vehicles employing a vacuum (and which make it very difficult to provide workable vehicles, as above) are unnecessary. Indeed, entry of the air into the duct formed between the undersurface of the chassis and the juxtaposed surface is essential, and is controlled carefully to ensure stable, and insofar as possible non-turbulent flow. 
     It would be of self-evident amusement interest, or “toy value”, to provide a radio-controlled vehicle capable of making the transition between operation on a floor to climbing a wall, and the Wall Racer in certain embodiments can do so. In order that the vehicle can make the transition, the fan(s) driving the air stream are actuated only when the vehicle begins to climb the wall. 
     Other inventive aspects of the Wall Racer will appear as the discussion below proceeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood if reference is made to the accompanying drawings, in which: 
         FIG. 1  and  FIG. 2  show respectively a perspective view and an elevation in partial cross-section of a first embodiment of the Wall Racer; 
         FIG. 3  and  FIG. 4  show respectively a perspective view and an elevation in partial cross-section of a second embodiment of the Wall Racer; 
         FIGS. 5 ,  6 , and  7  show views of a gear train employed in the embodiment of  FIGS. 3 and 4 ; 
         FIG. 8  and  FIG. 9  show respectively a perspective view and an elevation in partial cross-section of a third embodiment of the Wall Racer; 
         FIG. 10  and  FIG. 11  show respectively a perspective view and an elevation in partial cross-section of a fourth embodiment of the Wall Racer; 
         FIG. 12  shows a detailed diagram of one successful shape for the duct employed to form the high-velocity air stream, e.g., as employed in the second embodiment of  FIGS. 3 and 4 ; 
         FIG. 13  shows a cross-sectional view of a switch employed to actuate the fans when the Wall Racer transitions from floor to wall operation, and which prevents its operation inverted on a ceiling, for safety reasons, while  FIG. 13A  shows a typical circuit in which it may be used; 
         FIGS. 14 ,  15 , and  16  show respectively a perspective view, an elevation in partial cross-section, and an enlarged cross-section of a caster used in several of the embodiments of the Wall Racer, while  FIG. 14A  shows a partial view corresponding to  FIG. 14 , illustrating a optional variation; and 
         FIGS. 17 ,  18 , and  19  show a further embodiment of the invention, wherein  FIG. 17  is a schematic plan view,  FIG. 18  a partial cross-section taken along the line  18 - 18  in  FIG. 17 , with certain components shown in dotted lines, and  FIG. 19  is a partial cross-section taken along the line  19 - 19  in  FIG. 17 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be apparent that one type of Wall Racer toy vehicle that would be desirably offered is one resembling an automobile, for example a race car, while other sorts of vehicles, such as trucks or military vehicles, e.g., armored tanks, might also be of interest. The first, second, fourth and fifth embodiments of the Wall Racer discussed herein are of generally elongated shape, so as to be fitted with model automobile bodies not otherwise contributing to the operation of the Wall Racer, while the third embodiment is circular and might be made to resemble a “flying saucer” type of space vehicle. All of these embodiments operate similarly, with differences as occasioned by the differing body shapes. 
     For example,  FIGS. 1 and 2  show respectively a perspective and an elevation in partial cross-section of a first embodiment of the Wall Racer, which as noted is generally elongated and could readily be fitted with a model race car or other vehicle body (not shown). As mentioned above, in order that downforce urging the Wall Racer against an abutting surface W (hereinafter simply “the wall”) can be developed, a high velocity stream of air is induced to flow in an underbody venturi duct formed between the undersurface of the chassis of the Wall Racer and the wall W. According to Bernoulli&#39;s Principle, as above, such a high velocity stream of air will be of reduced pressure with respect to the ambient air. The differential between this reduced pressure and the surrounding atmospheric pressure generates a resultant force D, termed “downforce” where, as here, its direction is such as to urge the vehicle “downwardly” toward the wall W. The amount of downforce D developed is proportional to the area over which the low pressure is created, and to the differential in pressure per unit area, so this area and the differential pressure must be adequate for the purpose. 
     Thus, as illustrated in  FIGS. 1 and 2 , a fan  10  is mounted in a fan duct extending through the chassis  12 , and is driven by a battery-powered motor  11  so as to draw a high-velocity stream of air in from around at least a portion of the periphery of chassis  12 . The stream of air flows through an underbody venturi duct  15  formed between the underside of chassis  12  and the juxtaposed surface of wall W, and is exhausted on the “upper” side of chassis  12 , that is, on the side away from the abutting wall W. Downforce D is created as noted due to the differential in pressure between the low pressure of the high-velocity air stream in the underbody venturi duct and the ambient air; as noted, the total amount of downforce is proportional to the area over which the low pressure is developed, and to the differential in pressure at each point. 
     To maximize the area of low pressure by avoiding air being drawn in along the edges  12   a  of the chassis  12 , that is, to ensure that the air stream is principally drawn in at the ends  12   b  of the chassis  12 , flexible “skirts”  14  extend from the chassis  12  toward wall W and form a partial seal therebetween, limiting “short-cutting” of air from the sides of the chassis juxtaposed to the fan duct. The skirts thus define one or more, in this case two, sections of the periphery of the underbody of the chassis at which air is drawn into an entry portion of the underbody venturi duct, which directs airflow into the fan duct. Accordingly, air is drawn in primarily at the ends  12   b , which are provided with a broad radius to ensure smooth and insofar as possible non-turbulent airflow; for similar reasons, the undersurface  12   c  of the chassis  12  is smooth. Thus the high-velocity air stream extends for a substantial portion of the overall length of the chassis, ensuring that adequate downforce is developed. In the absence of the skirts  14 , air would tend to be drawn in along the sides of the chassis, limiting the area over which the reduced pressure is developed, and thus limiting downforce; there would likely also be considerable turbulence, further interfering airflow and reducing downforce. 
     In some circumstances, a further increase in downforce can be realized by limiting the clearance between the ends of the undersurface of the chassis and the wall surface, e.g., by providing downwardly extending baffles, akin to the side skirts  14  but extending only to the wall surface, that is, not intended to be drawn against the wall surface as are the side skirts  14 . The reduction in intake area causes a further acceleration of the air flowing under these baffles, further reducing the pressure and increasing the downforce. 
     By comparison, in the generally circular third embodiment of the Wall Racer shown in  FIGS. 8 and 9  (discussed further below) a substantial distance exists between all points on the outer periphery of the undersurface of its chassis and the centrally-located exhaust duct, so that the airflow in this embodiment is radially inwardly from all directions, the downforce is developed uniformly around the chassis, and no skirts need to be fitted. 
     As noted, the differential in pressure and thus the downforce developed is a function of the air velocity, which up to a point can be increased by reducing the cross-sectional area of the duct formed between the underside of the chassis and the wall W, that is, by reducing the ground clearance of the Wall Racer. However, if the cross-sectional area is reduced too much, turbulence will impede flow and reduce the desired effect; reducing the ground clearance would also increase the Wall Racer&#39;s sensitivity to surface irregularities and the like. No detailed theoretical calculations have as yet been carried out which would allow optimization of the effect sought. For example, by optimizing the duct design the current draw of the motor powering the fan inducing the flow could perhaps be reduced, increasing operating time per battery charge. Detailed specifications of the duct and other components employed in a successfully-tested embodiment of the Wall Racer are provided below. 
     Returning to discussion of the first embodiment of  FIGS. 1 and 2 , as illustrated the chassis  12  is supported by two opposed drive wheels  16  and  18 , spaced transversely from one another on either side of the chassis near the midpoint thereof, and by opposed casters  20  (that is, devices comprising freely-rotating wheels mounted for pivoting about an axis perpendicular to their axis of rotation) at either end of the chassis  12 . As indicated schematically by belt drives  22 , the opposed drive wheels  16  and  18  are separately powered by motors  24  that are supplied with current by a battery pack  28  in response to control signals provided by radio-controlled receiver  26 . The overall construction and operation of these components is conventional except as noted and will not be discussed in detail herein. Thus, if both motors are controlled to drive wheels  16  and  18  in the same direction, the Wall Racer moves in that direction, while turning is accomplished by driving the wheels  16  and  18  in differing directions or at differing rates. Casters  20  are unpowered, mounted on the longitudinal centerline of chassis  12 , and simply serve to maintain the correct spacing between undersurface  12   c  of chassis  12  and wall W; preferred locations and design of casters  20  are discussed below. 
     The “differential” drive scheme shown is preferred over, for example, a conventional four-wheel chassis, with one pair of wheels powered and one pair steering, for the following reasons. In order that a vehicle can climb a vertical wall, sufficient downforce must be exerted, urging the vehicle toward the wall, not only to support the vehicle against the force of gravity but also to provide sufficient traction to propel the vehicle vertically against gravity. (By comparison, providing a vehicle that operates inverted on a ceiling is simplified, since it need only support itself and need not also climb vertically.) Ensuring good traction thus becomes paramount. So as to maximize the traction provided by the downforce available, the drive wheels are located centrally, at the center of the pressure exerted by the downforce, so that essentially all of the downforce is transmitted directly to the drive wheels, maximizing traction. 
     The casters  20  are preferably mounted so that both do not simultaneously touch a flat surface, so that a three-point support is always available, with the drive wheels  16  and  18  forming two of the three contact points. The motion thus provided, whereby the vehicle can rock slightly back and forth about the axis of the drive wheels  16  and  18 , as one or the other of casters  20  touches the wall W, is referred to as “teeter” herein. Thus the downforce is balanced over the central drive axle, which maximizes traction, while allowing the vehicle to be steered by differential driving of the opposed drive wheels  16  and  18 . 
       FIGS. 3 and 4  show a second embodiment of the Wall Racer; this embodiment appears likely to correspond to the earliest production version of the Wall Racer.  FIG. 12  provides detailed dimensional information concerning this embodiment, and preproduction specifications are provided below as well. 
     As shown by  FIG. 3 , in this embodiment two exhaust fans  38  are provided, spaced laterally from another on the transverse centerline of the chassis  40 , and each fan being driven by a motor  39  with the fan mounted directly on the motor shaft. Six drive wheels  42  are provided, three on either side of the chassis  40 , with the three wheels  42  on either side of the chassis being geared (or belt-driven) to one another so as to be driven in common by two separately radio-controlled motors. The radio control receiver and battery are not shown, as being generally within the skill of the art.  FIGS. 5 ,  6 , and  7  (discussed below) show a preferred gear train and motor arrangement. Thus, as in the  FIG. 1  embodiment, steering is accomplished by differentially driving the wheels on either side of the chassis  40 . As shown, skirts  44  are again provided, so as to ensure that the airflow is primarily from the ends of the chassis to the fan exhaust duct  46 , in turn to ensure that an adequate area of high-velocity, low-pressure air flow is provided to generate adequate downforce. As illustrated by  FIG. 4 , the center pair of wheels are slightly lower in the chassis than the end pairs, so as to provide “teeter” and ensure that the center pair of drive wheels are always in good contact with the wall W. 
     The pairs of wheels  42  at each end of the chassis are slightly proud of (i.e., extend slightly beyond) the respective ends of the chassis, so that as the vehicle approaches a wall while operating on a floor, the wheels contact the wall first. Thus providing the six-wheel arrangement of this embodiment allows the Wall Racer to make the transition from floor to wall in either direction. So that downforce urging the Wall Racer toward the floor does not prevent the Wall Racer from initially climbing the wall, the fans  38  are only energized when the chassis  40  reaches a predetermined inclination with respect to the horizontal.  FIG. 13  shows a preferred switch, and  FIG. 13A  a circuit, for controlling the fans accordingly. 
     As indicated above,  FIGS. 5 ,  6 , and  7  show a preferred arrangement of the two drive motors and corresponding gear trains for differentially driving the six wheels of the Wall Racer in its  FIGS. 3 and 4  embodiment.  FIG. 5  shows a plan view, and  FIGS. 6 and 7  cross-sectional views along lines  6 - 6  and  7 - 7  respectively. Thus, assuming the Wall Racer is traveling toward the right in  FIG. 5 , so that the upper side of the drawing is the “left”, and the lower the “right”, there are provided left-side and right-side drive motors  150  and  152  respectively. Motors  150  and  152  each drive reduction gear trains,  154  and  156  respectively; the gears of each are idlers, that is, spin freely on shafts  158 , so that gears from both trains can be supported on the same shafts  158  while turning independently of one another. The output gears of train  154  and  156  drive gears  160 ,  162  respectively, which are fixed with respect to sleeve axles  164 ,  166  respectively, riding on a fixed axle  168 , and thence to gears  170 ,  172  respectively. Gears  170 ,  172  are fixed to corresponding drive wheels  174 ,  176 , and also drive further gear trains  178 ,  180 , which drive central drive gears  182 ,  184 , which are fixed to central drive wheels  186 ,  188 . Central drive gears  182 ,  184  also drive further gear trains  190 ,  192 ; these in turn drive gears  194 ,  196 , to which are fixed wheels  198 ,  200 . Implementation of this drive arrangement is within the skill of the art; while the gear trains and axles are shown as mounted on a metallic frame  202 , in production this chassis will typically comprise molded components. 
     It is also within the scope of the invention to employ a generally comparable arrangement to provide a four-wheel drive version of the vehicle of the invention, with differential steering as above. In this case one of the wheels might be mounted so as to spaced very slightly away from a plane contacted by the other three wheels; consequently the vehicle would “teeter” about an axis connecting the contact patches of the two of the wheels not diagonally opposite the wheel so spaced from the plane, so that either that wheel or the one diagonally opposite it would contact the plane. For example, if the left front (“LF”) wheel were slightly spaced from a plane contacted by the RF, LR, and RR wheels, the vehicle would teeter about an axis connecting the points at which the RF and LR wheels contact the plane, and the teeter would be limited by contact of either the LF or RR wheels with the plane. By comparison, if the wheels were located so as to simultaneously contact a flat plane, the vehicle would tend to be much more sensitive to any irregularities in the surface. 
     Implementation of differential steering of a four-wheel drive vehicle would not be unduly complex. By comparison, if steering were to be accomplished by pivoting of one or both pairs of wheels, this would involve additional complexity. 
     It is to be noted that a differential steering arrangement in a four-wheel drive vehicle with all four wheels in good contact with the surface would involve substantial resistance to steering due to “tire scrub”, that is, frictional resistance caused by the different effective turning radii of the “contact patch” of the tires on opposite sides of the vehicle. In general, to limit tire scrub within a given tire, relatively narrow tires are fitted to the drive wheels of the vehicles of the invention. Tire scrub becomes less significant as the overall size of the vehicle is reduced. To improve appearance, and to allow operation on thick carpets and the like, wider supplemental tires of slightly lesser diameter and formed of a lightweight foam or the like (not shown) can be assembled to the outer surfaces of the drive wheels. 
     As mentioned,  FIG. 12  shows a detailed view of the underbody venturi duct  50  formed between the undersurface of chassis  40  and a juxtaposed surface, such as a wall W. This embodiment of the underbody was employed in one successfully-tested version of the second embodiment of the Wall Racer of the invention, as shown in  FIGS. 3 and 4 .  FIG. 12  further provides reference to dimensional details of the chassis  40 . In this version, the overall chassis length H is 11.828″, with six wheels of 2.524″ diameter; the wheelbase F of the outer pairs of wheels is 9.50″, so that the wheels are proud of the chassis, i.e., extend slightly beyond the end of the chassis  40 , in order to engage a vertical surface and thus enable the Wall Racer to climb a wall from the floor. The center axle is 0.050″ closer to the wall W than the end pairs of wheels, so that the desired “teeter” is provided. 
     The underbody venturi duct  50  is longitudinally symmetric about a centerline J, with one end only shown in detail by  FIG. 12 . As shown in detail by  FIG. 12 , each “half” of the underbody duct  50  formed between the undersurface of the chassis  40  and the wall W comprises an entry portion  50   a , a transition portion  50   b , and an exit portion  50   c , which makes a smooth transition into a fan duct  46 , also of venturi shape, in which the fan(s) are located. Air enters each half of the underbody venturi duct at an inlet opening at the periphery of the chassis  40 , defined by the entry portion  50   a  of underbody venturi duct  50 . Entry portion  50   a  is defined by a radius R formed on the end of the chassis; in the version shown, this radius is 1.164″. The axles of the front and rear pairs of wheels lie on the center of this radius, and are slightly larger in radius, so that each tire&#39;s rolling surface is somewhat proud of the chassis end, as noted. Entry portion  50   a  is faired into and connects smoothly with an extended flat transition portion  50   b  formed by a flat surface on the underside of the chassis; since the duct  50  formed between the underside of chassis  40  and the wall is of minimum cross-sectional area in this region, the maximum air flow velocity, and accordingly the maximum downforce per unit area, are developed here. 
     The goal in designing the underbody venturi duct  50  is to maximize the extent of the region of minimum cross-sectional area, while optimizing its cross-sectional dimension, so as to provide smooth, preferably non-turbulent flow into and out of this region, all in order to maximize flow velocity. To ensure smooth flow, the section of the undersurface of chassis  40  defining the upper bound of entry portion  50   a  is radiused, and the corresponding section defining the upper bound of exit portion  50   c  describes a portion of an ellipse. In the successfully-tested version depicted, this elliptical contour was drawn using a 2″×4″ ellipse as found on a draftsman&#39;s “30-degree” template; that is, dimensions D and C are 1″ and 2″, respectively. As illustrated, then, the extent E of flat portion  50   b  is 2.25″ long, forming a “tunnel flat”. With the vehicle balanced on the center pair of wheels, so that the flat portion  50   b  is parallel to the wall, the ground clearance G therebetween is 0.098″. Flat portion  50   b  makes a smooth transition to exit portion  50   c , which as noted is 2.00″ long and elliptical in longitudinal cross-section. Exit portion  50   c  in turn makes a smooth transition to a central venturi section  46   a  of fan duct  46 , in which the fan(s) are located. In the two-fan embodiment of  FIGS. 3 and 4  and detailed in  FIG. 12 , the longitudinal dimension B of the narrowest portion of this venturi section  46   a  is 1.00″; section  46   a  extends across the chassis  50  so as to form a transverse “mail slot”. As it extends away from the wall, the mail slot section  46   a  then broadens out gradually in the longitudinal direction and is divided along the longitudinal centerline to form two circular-section ducts  46   b  in which the fans  38  are located; their diameter, dimension A, is 1.625″. 
     The following are the principal specifications of a successfully-tested version of the Wall Racer, as shown in  FIGS. 3 and 4  and dimensioned as in  FIG. 12 : 
     Wheelbase (dimension F) 9.5″ (front to rear axle) 
     Track width 5.8″ (centerline to centerline, at contact points) 
     Underbody duct width 4.9″ (between skirts) 
     Chassis weight 584 g. 
     Body weight 29 g. 
     Total weight 613 g. 
     Weight distribution (without body, center axle unsupported):
         Front axle 260 g (44.5%)   Rear axle 324 g (55.5%)       

     Ground clearance (dimension G) 0.098″ 
     Motor voltage 6 v. nominal (five 1.2 v. 1000 mah NiMH cells) 
     Downforce fans current draw 4 amperes total 
     Ducted fans (two)—1.625″ diameter, 3 blades 
     Total net downforce 1280 g. 
     Teeter (center axle offset) 0.050″ 
     Fan RPM 30,000 
     The chassis itself can be molded of a lightweight foam material, having its undersurface shaped to define the venturi duct  50  in cooperation with the surface of the wall W. It is convenient to mount the components, such as bearings for the axles carrying the wheels, drive motors and gear or belt drive components, radio control receiver, batteries, and motor and fan assemblies, in recesses molded into the foam of the chassis. In particular, the fan assemblies may alternatively comprise hard plastic molded ducts within which the fan and drive motor are retained; the exit portion of the underbody venturi duct is then shaped to mate smoothly therewith. 
     In a successfully-tested prototype, the skirts  44  ( FIG. 3 ) were formed of “Tyvek” spunbonded nonwoven olefin envelope material sized and located so as to curve outwardly at a nominal 45 degrees when in contact with the wall; a stiffening strip of plastic glued to the lower edge of the skirts, but spaced slightly therefrom, may be desirable to prevent local buckling. 
     Given the above detailed disclosure of the invention, those of skill in the art would have no difficulty in its practice. In particular, it will be appreciated that batteries (exemplary specifications being provided above) must be provided to power the fans and the drive wheels, that the drive wheels, three on each side in the embodiment of  FIGS. 3 and 4 , must be linked to one another and to the respective drive motor by gears, as illustrated in  FIGS. 5 ,  6 , and  7 , or by belts or other means, and that the motors must be individually controllable by signals provided by an operator by way of a radio or infrared transmitter and receiver pair. These aspects of the implementation of the invention are within the skill of the art. It is also within the scope of the invention to drive each of the six wheels individually, that is, to eliminate the gear or belt arrangement in favor of separate motors for each wheel, but this alternative is considered undesirable as it would involve a weight penalty. 
       FIGS. 8 and 9  show as mentioned a third version of the Wall Racer, in this case with a circular chassis  60  to provide a “flying saucer” appearance. In this version, two drive wheels  62  and  64  are provided on diametrically opposed points on the chassis  60 , with casters  66  on opposite sides, along a line perpendicular to the axis of the drive wheels  62  and  64 . The casters may be raised slightly from a plane including both drive wheels and the casters, to provide “teeter” as above. (It will be apparent that this version of the Wall Racer cannot negotiate the transition between floor and wall.) Downforce is provided by a centrally-located fan  68  disposed in a venturi duct  70  and driven by a motor  72 . Drive wheels  62  and  64  are individually driven by motors  74  and  76  responsive to control signals from a radio-control receiver  78  and powered by battery  80 . 
     In this version, as mentioned above, the exhaust duct  70  is equidistant from all points on the periphery of chassis  60 , so that the inward air flow path is of equal length at all points around the chassis  60 . Hence there is no need for skirts, and the air flow is radially inward all around the periphery. Again, a radius is provided around the periphery of the lower edge of chassis  60 , as illustrated at  60   b , so that the inlet opening of the underbody venturi duct extends circumferentially around the chassis, and a large-radius or elliptical transition portion  60   c  is provided where the underbody duct  82  meets the exhaust duct  70 , to ensure smooth and substantially non-turbulent airflow. The transition portion of the underbody duct  82  formed between the underside  60   a  of chassis  60  and the wall is preferably shallow and substantially conical in section, as illustrated, so that the cross-sectional area of the duct  82  stays constant as its radius from the center of exhaust duct  70  varies; in this way the velocity of the inward-flowing air stream and the differential pressure exerted thereby are both substantially constant, so that the downforce is exerted evenly at substantially all points on the chassis  60 , that is, outside of duct  70 . 
       FIGS. 10 and 11  show a further version of the Wall Racer, again having an elongated chassis  90  suitable for mounting of a model race car body or the like. In this embodiment, a single fan  92  is located centrally on the chassis, is driven by a motor  94 , and is disposed within an exhaust duct  96  communicating with an underbody venturi duct  98  formed between the underside of chassis  90  and the wall W. The underbody duct  98  is designed generally as discussed above with respect to  FIG. 12 . 
     In this embodiment, a single drive wheel  100  driven by a motor powered by a battery and responsive to control signals provided by a radio control receiver (none of the unnumbered components being shown) is located on the vehicle&#39;s longitudinal centerline, near the center of effort of the downforce, but disposed toward one end of the chassis so as not to interfere with the exhaust duct  96 . Two casters  102  and  104  are mounted at the opposite end of the chassis  90 . Caster  102  is free to pivot about an axis perpendicular to wall W, while caster  104  is also pivoted about a similarly perpendicular axis, but only between angular limits (see  FIG. 14A , below). 
     Thus, chassis  90  rests on a tripod comprising drive wheel  100  and casters  102  and  104 . If drive wheel  100  is driven so as to propel the vehicle toward the direction of the end of the chassis on which drive wheel  100  is disposed, that is, rightwardly in  FIG. 11 , the casters trail behind, and the vehicle travels in a straight line; if drive wheel  100  is driven in the opposite direction (counterclockwise in  FIG. 11 ), the caster  104  provided with angular stops rotates about the axis perpendicular to wall W until its pivoting is stopped at one or the other of its angular limits, so the vehicle turns in one direction until the direction of travel is reversed. Hence directional control of the Wall Racer in this embodiment is substantially constrained; being greatly simplified, this embodiment might be best suited to a low-cost version of the invention. 
     As mentioned,  FIGS. 14-16  show respectively a perspective view, a cross-section, and an enlarged partial cross-section of a caster  102  used in several of the embodiments of the Wall Racer, while  FIG. 14A  shows a partial view corresponding to  FIG. 14 , illustrating a optional variation. In these views, the caster  102  is shown inverted, that is, with its face which would be juxtaposed to wall W oriented “up” in the drawings. A roller  110 , which contacts wall W, is carried by an axle  112  that is mounted for rotation in a rotating plate  114 ; plate  114  rotates about an axis A perpendicular to but offset from that defined by axle  112 . In the embodiment shown, rotating plate  114  in turn rides on a number of balls  116 , which bear against a closure ring  118 ; closure ring  118  is secured to a frame  120 , which can be mounted to the chassis. Thus, roller  110  engages the wall, and rotates about axle  112  as the vehicle is maneuvered; the assembly of roller  110 , axle  122  and plate  114  can rotate with respect to frame  120  and thus with respect to the vehicle chassis, as the latter is steered. The axle  112  is offset with respect to the axis A about which plate  114  rotates, so that as the vehicle is steered, plate  114  rotates and roller  110  trails the axis A of rotation of plate  114 . 
     If it is desired to restrict the rotation of plate  114 , e.g., as discussed above with respect to the version of the Wall Racer shown in  FIGS. 7 and 8 , so as to provide some turning, albeit not precisely controlled, this can be accomplished as shown, for example, in  FIG. 14A . As illustrated, a pin  122  extends through and is retained in the upper flange of frame  120  and fits within an angular recess  114   a  formed in the upper surface of rotating plate  114 , limiting the degree of rotation about axis A that is permitted to plate  114 . 
     As mentioned, in the embodiments of the Wall Racer in which it is capable of operation on a floor and climbing onto a wall (that is, the embodiment of  FIGS. 3-7 ), it is desired to provide a switch that actuates the exhaust fan(s) only when the Wall Racer reaches a desired angle, typically between 30 and 60 degrees with respect to the horizontal, so that downforce does not prevent the vehicle from beginning to climb the wall as the wheels engage the wall&#39;s surface.  FIG. 13  shows a switch  128  for so doing, and which also de-energizes the fan if the Wall Racer is placed upside-down, against a ceiling; this may be preferred for safety reasons, so that the Wall Racer cannot fall on anyone.  FIG. 13A  shows a typical circuit in which switch  128  may be used. 
     Switch  128  comprises an electrically conductive metal ball  130  disposed within a hollow nonconductive switch body  132 . Body  132  is symmetrical about a vertical axis, and defines a generally frusto-conical lower portion  132   a , in which ball  130  rests when the vehicle is on the floor, as shown in full, a disc-shaped central portion  132   b , into which the ball falls, as indicated in dotted lines, when the vehicle begins to be oriented vertically, as when it begins to climb a wall, and a generally frusto-conical upper portion  132   c , in which ball  130  falls if the Wall Racer is placed inverted against a ceiling. Conductive contacts  134  are disposed on the inner surfaces of lower portion  132   a  and upper portion  132   c , so that when ball  130  is disposed in either the upper or the lower portions, it connects the contacts  134 . 
     As shown in  FIG. 13A , contacts  134  (two of which are connected in common) are wired in series with a normally-open relay  140  and battery  28 , which controls a circuit including battery  28  and fan motor  39 . Thus, with switch  128  closed, that is, with the Wall Racer essentially horizontal, and ball  130  making the connection between contacts  134 , relay  140  is closed, as shown; when the Wall Racer leaves the horizontal sufficiently that ball  130  falls out of lower section  132   a , into upper section  132   b , relay  140  opens, closing the motor circuit and energizing motor  39 , so as to drive fan  38 . In this embodiment, if the Wall Racer is placed inverted against a ceiling, ball  130  falls into upper portion  132   c , similarly connecting contacts  134 , and preventing operation of fan motor  39 . 
     As mentioned,  FIGS. 17-19  show a further embodiment of the invention. The principal improvements provided by this embodiment with respect to those discussed above are the provision of a radial-flow fan rather than the axial-flow fan(s) shown in the previous embodiments, provision of two drive wheels offset longitudinally from one another, principally for reasons of packaging convenience, and elimination of the casters or other wheels in favor of allowing the undersurface of the chassis to touch the wall. 
     Thus, as shown in  FIGS. 17-19 , a fan motor  150  drives a radial-flow fan  152 , that is, comprising a circular end plate  152   b  and vanes  152   a  that are generally perpendicular to the end plate and angled with respect to the axis of rotation. Air is drawn in along the axis, that is, flowing upwardly around motor  150 , and is exhausted radially outwardly. The radially outward ends of vanes  152   a  are curved so as to be closely juxtaposed to a diffuser or fan duct  160  defining a generally bell-shaped interior surface, for efficiency in use. Motor  150  is received in a recess  154  in a transverse member  156 . Member  156  extends transversely across chassis  158 , filling the central portion of a transverse “mail slot”  158   d  in chassis  158 . 
     Generally as discussed above in connection with  FIG. 12 , and as shown by  FIG. 18 , chassis  158  is radiused at  158   a  to define entry portions of the underbody venturi duct, is flat at  158   b  to provide the transition portions thereof, and defines a smooth duct at  158   c  to define the exit portions thereof. Skirts  159  are again provided to prevent air entry along the long sides of the chassis  158 . On either side of the motor-receiving recess  154 , member  156  is shaped as indicated by dashed lines  156   a , in order to provide a fair flow path for air drawn in at the ends of chassis  158 . The exit portions of the venturi duct as formed by chassis  158  at  158   c  mate with diffuser duct  160 , the inside surface of which is generally bell-shaped so as to be closely juxtaposed to vanes  152   a  of fan  152 , as noted above. As also shown, assembly is simplified by formation of transversely-extending ears  160   a  on diffuser duct  160 . Ears  160   a  mate with posts  156   b  formed on transverse member  156 , as shown in  FIG. 19 ; fasteners passing therethrough also secure gearboxes  162 , which are discussed further below. 
     Propulsion for the vehicle is provided by two motors  164 , which drive two drive wheels  166  through reduction gearboxes  162 , as mentioned above. As previously, motors  164  are controlled responsive to radio, or preferably, infrared signals transmitted by a remote transmitter (not shown) and received by a receiver  168 . Power for motors  164  as well as for fan motor  150  is provided by a battery  170 . Electrical connection between these components, provision for battery charging, on-off switching, mechanical details such as the construction of gearboxes  162 , selection and operation of receiver  168 , and the control of motors  164  responsive to the received signals are within the skill of the art and need not be detailed here. 
     As illustrated, drive wheels  166  are offset longitudinally with respect to one another, and no casters are provided. The axes of drive wheels  166  are located with respect to the bottom surface of chassis  158  such that the flat central portion  158   b  of the chassis is spaced on the order of 0.020″ from the wall surface W. Consequently, the chassis  158  “teeters”, that is, pivots very slightly about a diagonal axis extending between the points at which drive wheels  166  contact the wall surface W, such that in use the teeter or pivoting is limited by undersurface of the chassis  158  contacting the wall surface W at one or the other diagonal corner. The undersurface of chassis  158  is made smooth to reduce friction between it and the wall surface W as the vehicle is propelled. Slight “bumps” might also be formed at the diagonal corners of the chassis, to localize the contact between the chassis and wall surface W. It is found that the friction experienced in use of the toy of the invention with walls and other surfaces of typical smoothness—e.g., conventionally painted interior walls—is sufficiently small as to present no difficulty, and likewise that the slight asymmetry in the airflow path under the chassis presents no difficulty. 
     Thus, in use, the fan  152  is energized and the vehicle is placed against a surface W. Air drawn by fan  152  passes inwardly from the ends of the chassis  158 , up through the venturi tunnel collectively formed by the mail slot  158   d  in the chassis  158 , transverse member  158 , and diffuser duct  160 , and exits fan  152  in the radially-outward direction. Downforce is thereby created, pulling the vehicle toward the wall surface W. Motors  164  can then be differentially activated to propel the vehicle in any desired direction. 
     While several preferred embodiments of the invention have been disclosed herein in detail, the invention is not to be limited by the disclosed embodiments, which are exemplary only.