A ladder-like access device is suited for securing to a tire of a vehicle by way of a strap, rope, chain, cable or similar structure. The device may include extendable legs to adapt its length to a range of wheel and tire sizes. A variety of structures, materials and construction methods are described and claimed.

CONTINUITY AND CLAIM OF PRIORITY 
     This U.S. patent application claims the benefit of provisional application No. 61/518,229 filed 2 May 2011 and provisional application No. 61/505,275 filed 7 Jul. 2011. 
    
    
     FIELD 
     The invention relates to access or work platforms that may be removably mounted to a vehicle, to provide improved access to the engine, windows or roof of such vehicles. 
     BACKGROUND 
     The prior art includes several different tire mount platform devices. Examples include those disclosed in U.S. Pat. Nos. 2,575,503; 2,848,150; 2,851,312; 2,973,052; 4,947,961; 5,133,429; 6,044,928; and 6,550,578; the disclosures of which are incorporated by reference. Clearly, the general idea of mounting a work or access platform to a vehicle tire is known. 
     Problems with prior-art wheel-mounting platform devices, however, include that many offer only a single step or platform, thus limiting or restricting access (e.g., ascending to the engine compartment/hood area) and usability on different-sized vehicles. In the prior art embodiments that include more than one step, only two steps are provided and they are arranged substantially vertically. It is more difficult to climb vertically-disposed steps than steps mounted diagonally, and vertical steps offer less flexible access when loading, securing or unloading cargo from a roof of a vehicle. 
     Furthermore, when climbing vertically-arranged steps, one may be forced to use one hand to hold on to the vehicle if the vehicle body bulges out beyond the tire. A different step orientation may alleviate this requirement, freeing both the user's hands to carry tools, supplies, a light, beverage or the like as he mounts the steps. 
     Another shortcoming of prior art tire mounting steps and/or platforms is that their height is of limited adjustability. Many such platforms simply hang from the top of the tire, so it may be difficult to step onto the platform when attached to very large tires, and it may be difficult to raise or lower heavy cargo when there is a tall step. 
     Yet another disadvantage of prior-art tire-mount platform devices is that many attach to the top of the wheel and simply dangle clown. If the vehicle does not have the parking brake set and for automatics, the transmission placed in “park,” then the vehicle may accidentally roll. This results in the step(s) falling off the wheel, with a high chance of injury to the user. 
     An adjustable tire-mount access device that addresses these shortcomings may be of significant value in this field. 
     SUMMARY 
     Embodiments of the invention provide height-adjustable tire-mount platforms that offer improved access to upper portions of vehicles, such as engine compartments, windows, roofs, roof racks, and so on. Embodiments may be particularly useful on pickup trucks, vans, SUVs and service vehicles. 
     An extendable tire-mount device in accordance with the present invention may include one or more of extendable legs, diagonal positioning when mounted, a flexible and adjustable mounting harness, and a tire-side face on the platform that is concave or recessed to better fit the tire. The extendable legs may be rungless, merely extending the legs of the runged section; or may provide additional rungs to facilitate climbing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
         FIGS. 1A and 1B  show views of an embodiment of the invention in collapsed and extended configurations. 
         FIGS. 2A and 2B  show views of another embodiment of the invention in collapsed and extended configurations. 
         FIG. 3  shows a third embodiment of the invention. 
         FIG. 4  shows a simplified structure to explain how the top step of an embodiment may be shaped to fit more securely to a vehicle tire. 
         FIG. 5  shows a non-extendable embodiment secured to a tire. 
         FIG. 6  shows another non-extendable embodiment with an alternate method of securement. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are generally similar to, and give the visual impression of, fixed or extendable stepladders, but all include a structure or component for securing the embodiment to a tire, so that a user can more easily access out-of-reach areas on a taller vehicle for cleaning, repair or storage manipulation. 
       FIGS. 1A and 1B  show two views of a first representative embodiment of the invention. In  FIG. 1A , the “stepladder” portion of the embodiment is collapsed: a front portion  100  comprising three steps  105 ,  110 ,  115  and a top portion  120 ; is secured to a rear, extendable portion  130  by side-mounted rail guides  140  and  145 . A locking pin  150  is shown near a hole in the frame  155 ; to lock the extendable portion at a desired position, the pin is inserted into the hole (a similar locking pin is provided on the other side of the embodiment, but it is not visible in this view). A web  160  is attached at several points to the ladder frame, and a buckle  170  allows adjustment of a portion of the web. The steps and top portion may be provided with anti-slip features such as the grit tape depicted as black stripes on each step. 
       FIG. 1B  shows the same embodiment in an extended configuration: rear portion  130  has been released by withdrawing the locking pins, extended or slid down, and secured by re-inserting the pins. In this view, the right side rails  101  and  131  of front and rear portions  100  and  130  are identified. Either or both pairs of side rails may have a protective, anti-slip foot attached, as shown at  102 . The web structure identified generally as  160  in  FIG. 1A  can be seen to comprise a number of straps  162 ,  164 ,  166  and  168 , which may be sewn, riveted, or woven together, or connected using buckles or other conventional means. Buckle  170  permits adjustment of the size of circumferential loop  162 . 
       FIGS. 2A and 2B  show another embodiment in collapsed and extended configurations. In this embodiment, a lower portion  200  surrounds an upper extendable portion  240 , allowing the upper portion to slide up and clown (as suggested by arrow  250 ) when the locking mechanism at  280  is disengaged. 
     Lower portion  200  comprises side rails  210 ,  215 ; rungs  220  and a rear brace  230 . The lower ends of rails  210  and  215  may be capped by anti-scratch, anti-slip feet  212  and  217 . Upper portion  240  comprises side rails (not identified), rungs (one of which is identified as  260 ), and a top  270 . This embodiment has a single securing strap identified as  290 . 
       FIG. 3  shows a third extendable embodiment  300 , in the extended position only. Like the previous embodiments, this one is generally similar to a single-sided stepladder. The side rails of the upper portion  310  are hollow, square tubes, within which smaller tubes  320 ,  330  can slide. To raise or lower upper portion  310 , the user can withdraw a locking pin connected to handle  340 , adjust the sliding tube to the desired position, and reinsert the locking pin. In this embodiment, the securing web  350  is made from round, elastic cord (“bungee” or “shock cord”). The size and elasticity of the cord may be chosen so that more-complex locking or adjusting mechanisms are not necessary. 
     Embodiments of the invention are to be placed against and secured to the tire of a vehicle, to allow the user to climb the ladder safely and reach higher points on the vehicle. For example, an embodiment may be secured to the front tire of a front-engine truck to allow a mechanic to reach the engine bay more easily, or an embodiment may be secured to the rear tire of a van to allow the user to reach storage bins or luggage placed on the top of the vehicle. 
       FIG. 4  shows a simplified three-dimensional representation of a rectangular block  410  (representing the ladder structure of an embodiment) near a toroidal object  420  representing a tire. Block  410  and tire  420  have been separated slightly to expose a scalloped shaped area  450 . The top step of an embodiment of the invention may be formed with a similar scallop-shaped profile, to improve the stability of the ladder when it is secured to the tire. The scallop-shaped depression helps keep the ladder from rocking back and forth against the tire, even if the lower ends are placed on an uneven or unstable surface. 
     Although the profile of the scallop-shaped indentation is difficult to describe in words, it is easy to compute using standard engineering computer-aided design (“CAD”) tools. Furthermore, since embodiments may be used with a variety of tire sizes and at differing angles of lean against the tire, the precise size and shape of the scalloped indentation (including top profile  430  and side profile  440 ) is not critical. An embodiment may comprise scores, protrusions, lugs or the like (indicated generally at  460 ) to help keep the top step from sliding from side to side against the tire. 
     The complex shape of the top step of an embodiment can be manufactured easily by molding it of plastic, or with somewhat greater difficulty by forming it from sheet metal or cutting it from another material. In a preferred embodiment, the top step is molded from thermoplastic resin. 
     The ladder structure of an embodiment may be constructed of any conventional materials, using any conventional techniques. For example, side rails of the upper and/or lower parts may be made from fiberglass channels, aluminum channels, wood or another material. Rungs and braces may be fiberglass, metal or wood. Side rails and rungs may be joined by screws, rivets, glue, welding, or other techniques. Protective feet at the bottom of side rails may be plastic, rubber, or metal. Extension-locking mechanisms may be shear pins, as discussed above, or compression/friction fittings, ratchets or other structures. 
     An embodiment may sized to fit a range of tires, but generally, the width of an embodiment is preferably within about ±20% of the diameter of the applicable tire, and the length such that the embodiment can be leaned against the tire at an angle from about 45° to about 80° from the horizontal. (This implies a ladder length from slightly longer than the tire diameter to almost 1.5 times the tire diameter. Of course, an extendable embodiment may be suitable for use with a wider range of tire sizes.) Narrower ladders may provide inferior access for the user, while vehicle fenders or wheel wells may interfere with wider ladders. Very short ladders are not much use, whereas overly long ladders may be inconvenient to transport, or may be more difficult to secure to the tire. 
       FIG. 5  shows a non-extendable embodiment  500  secured to a tire  410  using a multi-strap web, with a circumferential strap  520 , a coronal strap  530 , and an auxiliary positioning strap  540 . (This embodiment comprises at least one more auxiliary positioning strap, but it is not visible in this view.) An embodiment may use buckles, friction fittings, or other rope/cord/strap-length adjustment mechanisms to set the securing-web size appropriately for the tire to which the embodiment is secured. 
     The strap or web to secure an embodiment to a tire may be made of nylon webbing, rope, chain, metal cable, elastic bands or cords, or combinations of such materials. It is preferred that the main horizontal securement strap pass around the tire above the horizontal centerline of the wheel (to avoid the axle, brake mechanisms, etc.) and below about 75% of the diameter of the tire. If the horizontal strap is too high, it may tend to slide further up to the top of the tire, thus impairing the secure attachment. However, it is appreciated that tire-tread features (e.g., side traction lugs) may help keep the strap in place. Other parts of the securing web (e.g., coronal and auxiliary positioning straps) may be placed to provide easy access to length adjusters and/or suit single or double-tire wheels. In some embodiments, the securing web may be constructed of fabric in a half-wheel-cover configuration. The lower edge of such a half wheel cover is similar in positioning and purpose to the main horizontal strap of other embodiments. 
       FIG. 6  shows another view of a non-extendable embodiment  600  which uses a single strap  620  to secure it to tire  610 . Strap  620  may have a length adjustment mechanism (not shown), but for final attachment, the bottom end of the embodiment may simply be pulled away from the tire as shown by arrow  630 . This will tighten strap  620  and lever the top of the embodiment against the top of the tire at  640 . In this Figure, the scalloped shape of the top of the embodiment and its relationship with the top of the tire are clearly visible. 
     Several specific embodiments of the present invention have been described with reference to corresponding Figures. However, those of skill in the art will recognize that changes in materials and configurations may be made without departing from the general principles of the invention. Such changes and alternate implementations are understood to be captured according to the following claims.