Abstract:
An aerodynamic underbody fairing or “skirt” that is attached to the rear wheels or “bogeys” of a truck cargo body which typically can be slidably adjusted forward and aft along the cargo body. The various embodiments allow for a skirt that attaches to the bogeys of a truck cargo body and extends rearward to the rear plane of the trailer, and is of a retractable nature to allow for slidable adjustment of bogeys forward and aft along the cargo body for the purpose of continuing streamlined flow along the side of a truck cargo body already equipped with a side skirt forward of the bogeys. In the various embodiments of the invention, the retractable nature of the invention allows for a continuous skirt surface in-plane with the side of the truck cargo body extending from the bogeys to the rear plane of the truck cargo body, regardless of bogey position.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/623,858, filed Sep. 20, 2012, entitled AERODYNAMIC STRUCTURES SECURED TO THE UNDERBODY OF CARGO BODIES, now abandoned, which claims the benefit of U.S. Provisional Application Ser. No. 61/626,112, filed Sep. 20, 2011, entitled AERODYNAMIC STRUCTURES SECURED TO THE UNDERBODY OF CARGO BODIES, and also claims the benefit of U.S. Provisional Application Ser. No. 61/544,783, filed Oct. 7, 2011, entitled AERODYNAMIC STRUCTURES SECURED TO THE UNDERBODY OF CARGO BODIES, the entire disclosure of each of which applications is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to aerodynamic structures that are secured to the underbody of cargo bodies, for example, on trucks or trailers, and other aerodynamic structures to improve airflow around and under the cargo body. 
     BACKGROUND OF THE INVENTION 
     Trucking is the primary mode of long-distance and short-haul transport for goods and materials in the United States, and many other countries. Trucks typically include a motorized cab in which the driver sits and operates the vehicle. The cab is attached to a box-like cargo section. Smaller trucks typically include an integral cargo section that sits on a unified frame which extends from the front wheels to the rear wheel assembly. Larger trucks often include a detachable cab unit, with multiple driven axles, and a separate trailer with a long box-like cargo unit seated atop two or more sets of wheel assemblies. These truck assemblages are commonly referred to as “semi-trailers” or “tractor trailers.” Most modern trucks&#39; cabs, particularly those of tractor trailers, have been fitted with aerodynamic fairings on their roof, sides and front. These fairings assist in directing air over the exposed top of the box-like cargo body, which typically extends higher (by several feet) than the average cab roof. The flat, projecting front face of a cargo body is a substantial source of drag, above the cab roof. The use of such front-mounted aerodynamic fairings in recent years has served to significantly lower drag and, therefore, raise fuel economy for trucks, especially those traveling at high speed on open highways. 
     However, the rear end of the truck&#39;s cargo body has remained the same throughout its history. This is mainly because most trucks include large swinging or rolling doors on their rear face. Trucks may also include a lift gate or a lip that is suited particularly to backing the truck into a loading dock area so that goods can be unloaded from the cargo body. It is well-known that the provision of appropriate aerodynamic fairings (typically consisting of an inwardly tapered set of walls) would further reduce the aerodynamic profile of the truck by reducing drag at the rear face. The reduction of drag, in turn, increases fuel economy. 
     To improve the aerodynamics of a truck or trailer cargo body by reducing drag, several solutions (i.e. rear-drag-reduction devices) have been provided that focus on trucks having swinging doors, which are not always readily applicable to cargo bodies having rolling doors. For useful background information on aerodynamic structures for swinging cargo doors, refer to commonly assigned U.S. patent application Ser. No. 12/122,645, entitled REAR-MOUNTED AERODYNAMIC STRUCTURE FOR TRUCK CARGO BODIES, by Smith et al., and U.S. patent application Ser. No. 12/903,770, entitled REAR-MOUNTED AERODYNAMIC STRUCTURE FOR TRUCK CARGO BODIES, by Smith et al., which are both incorporated herein by reference. 
     Only recently has an effort been made to improve the aerodynamic efficiency of these vehicles with the user of underbody fairings or “skirt” devices to reduce the aerodynamic drag of the trailer. However, there is currently no device that extends to cover the region behind the bogeys through to the rear plane of the trailer. This is primarily due to the fact that the position of the bogeys are frequently adjusted based on loading scenario for a given trip as well as in accordance with some state laws that mandate specific locations of the bogeys. 
     Still a further challenge is to improve the airflow passing under the cargo body as this space is generally configured to optimize mechanical and clearance considerations rather than aerodynamic considerations. One particular problem affecting the solutions to this is the movability of the wheel bogeys (clusters of two-three-axle wheel sets at the rear of the cargo body/trailer), thus rendering it challenging to provide a stationary skirt that covers the underbody from bogeys rearwardly to the rear of the cargo body, so as to form a continuous bottom aerodynamic envelope for the body. More particularly, it is desirable to provide a mechanism that increases the aerodynamic effectiveness of the cargo body. It is also desirable to affect the airflow, for example, in relation to a rearwardly located pocket of the cargo body provided by a rear-mounted aerodynamic structure. To this end, it is desirable to direct air away from the particular pocket created by exemplary rear aerodynamic structures such as those generated by, for example, the above-incorporated U.S. patent application Ser. Nos. 12/122,645 and 12/903,770 to Smith. 
     It is further desirable to provide an aerodynamic skirt structure for the underbody that is adjustable and improves aerodynamic efficiency. It is also desirable to provide a structure that maintains improved airflow underneath the trailer at the rear of the trailer, thereby reducing drag. 
     SUMMARY OF THE INVENTION 
     This invention overcomes the disadvantages of the prior art by providing a continuous skirt that extends from the bogeys to the rear plane of the truck cargo body, thereby allowing for slidable adjustment of the bogeys forward and aft (rearward). The various embodiments of the invention allow for a retractable skirt to be attached permanently or semi-permanently to the bogeys and the rear plane of the truck cargo body, such that it is adjustable, yet can be moved if desired. 
     In an embodiment of the invention, the skirt is of a woven fabric material, and is wound around a drum inside of a canister. The drum is spring-loaded or otherwise biased in such a way to apply tension to the fabric at all times, and in all positions. The canister that contains said drum is attached rigidly to a subframe that mounts to the aft-most portion of the bogey chassis. The skirt is extended from the canister and is rigidly mounted to the rearmost portion of the truck cargo body. In the functionality of the invention, when the bogey is slidably adjusted from its aftmost position to its foremost position, the spring-loaded drum unwinds skirt fabric while maintaining a near-constant tension on the fabric during the entire range of motion. In the functionality of the invention, when the bogey is slidably adjusted from its foremost position to its aftmost position, the spring-loaded drum winds skirt fabric while maintaining a near-constant tension on the fabric during the entire range of motion. 
     In an embodiment of the invention, the skirt is composed of overlapping sections of rigid panel material hanging from one or more sliding tracks mounted to the underbody of the truck cargo body. The foremost panel section is mounted rigidly to a subframe that is mounted to the aftmost portion of the bogey chassis. The aftmost panel section is mounted rigidly to the rearmost portion of the truck cargo body. In the functionality of the invention, when the bogey is slidably adjusted from its aftmost position to its foremost position, the overlapping panels become exposed one after another to keep a continuous rigid skirt panel surface for the entire range of motion. In the functionality of the invention, when the bogey is slidably adjusted from its foremost position to its aftmost position, the overlapping panels become covered by one another to keep a continuous rigid skirt panel surface for the entire range of motion. 
     For several variations on this embodiment of the invention, in the regions forward of the canister but aft of the aftmost bogey tire, the invention provides for a rigid panel to be mounted to allow for access to the tire and wheel for repair or removal while improving aerodynamic benefic of the invention. In the region aft of the aft skirt mount but forward of the aft plane of the truck cargo body, which might contain a control box, such as for a lift-gate-style trailer, the embodiment of the invention contains provision for a rigid panel to be mounted to allow for full access to the control panel, while improving the aerodynamic benefit of the invention. 
     An aerodynamic structure for securing to a cargo body underbody comprises a skirt on each of opposing sides of the cargo body, each skirt being operatively connected on one end to wheel bogeys of the cargo body and on another end to a location adjacent to a rear frame of the cargo body, wherein the skirt is variably extendible and retractable to account for a change in positioning of the bogeys. In an illustrative embodiment, the aerodynamic structure further comprises a front fill panel operatively connected to the skirt that includes a biased element for biasing the skirt toward a front of the cargo body. The skirt is movable based upon a rolling action. The skirt is further flexible and is operatively connected to guide tracks along the cargo body that maintain a shape of the skirt. The aerodynamic structure as set forth in claim  4  wherein the tracks are attached to an underside of the cargo body. Guide rollers can further be employed to urge the skirt into a curved orientation. Each skirt can curve inwardly toward the centerline of the cargo body in a rearward direction and each skirt overlaps the other at a rearmost position. 
     Illustratively, the skirt can comprise a plurality of overlapping panels that slide with respect to each other along a track. The skirt can also comprise an expanding and contracting material. The expanding and contracting material is one of (a) an elastic fabric, (b) a network of panels having thick sections and flexible interposed thin sections therebetween, and (c) a segmented panel with living hinge material constructed in the manner similar to popular window blinds. 
     The skirt can comprise a fabric that is selectively taken up by and paid out (i.e. some of the fabric is let out) from a roller. The roller includes at least one of a manually operated and motorized tensioning mechanism that rotates the roller. Each skirt is one of (a) approximately parallel with a side of the cargo body, and (b) inwardly tapered in a rearward direction with respect to a centerline of the cargo body. 
     A rear fill panel can also be provided, located between an rear attachment location of the skirt and a location adjacent to an underride guard of the cargo body. 
     In an illustrative embodiment, the skirt tapers upwardly toward a bottom of the cargo body and away from a road surface in a rearward direction. A structure having an airfoil cross-section located along at least a portion of a horizontal bar of an underride guard of the cargo body can be provided. 
     In an illustrative embodiment, the skirt panel or panels can be folded up and in so that they rest parallel and directly beneath the trailer floor, allowing the rear bogeys to travel underneath them when the bogey is slidably adjusted from its aftmost position to its foremost position or vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention description below refers to the accompanying drawings, of which: 
         FIG. 1  is a perspective side view of an aerodynamic skirt structure secured to the underbody of a cargo body, according to an illustrative embodiment; 
         FIG. 2  is a side view of the aerodynamic skirt structure secured to the underbody of the cargo body, with the bogeys in the forward position, according to the illustrative embodiment; 
         FIG. 3  is a side view of the aerodynamic skirt structure secured to the underbody of the cargo body, with the bogeys in the rearward position, according to the illustrative embodiment; 
         FIG. 4  is a top view of the aerodynamic skirt structure as mounted to the frame of the cargo body proximate the tires, according to the illustrative embodiment; 
         FIG. 5  is a rear view of a subframe for securing the aerodynamic skirt structure to the cargo body, according to the illustrative embodiment; 
         FIG. 6  is a side view of the fabric aerodynamic skirt structure mounted between the bogeys and the rear mount of the cargo body, in accordance with the illustrative embodiment; 
         FIG. 6A  is a cross-section taken through  FIG. 6 , showing the mount plate for the rear fill panel, according to the illustrative embodiment; 
         FIG. 6B  is a cross-section taken through  FIG. 6 , showing the bogey chassis and fabric drum and associated subframe, according to the illustrative embodiment; 
         FIG. 7  is a side view of the inner side of the aerodynamic skirt structure, including a front fill panel, collector mechanism, and a rear fill panel, according to the illustrative embodiment; 
         FIG. 8  is a side view of an aerodynamic sliding skirt structure comprising a plurality of panels that slide along a track, with the bogeys in the forward position, in accordance with an illustrative embodiment; 
         FIG. 9  is a side view of the aerodynamic sliding skirt structure employing sliding panels, shown with the bogeys in the rearward position, in accordance with the illustrative embodiment; 
         FIG. 10  is a top view of the aerodynamic sliding skirt structure mounted between the mud flaps and the rear mounting bracket, according to the illustrative embodiment; 
         FIG. 11  is a side view of a canister-type aerodynamic sliding skirt structure employing a roll-up set of panels, according to an illustrative embodiment; 
         FIG. 12  is a bottom view of the aerodynamic sliding skirt structure of  FIG. 12  employing a roll-up set of panels, according to the illustrative embodiment; 
         FIG. 13  is a side view of the aerodynamic sliding skirt structure employing parallel tracks for the panels, according to an illustrative embodiment; 
         FIG. 13A  is a cross-section taken through  FIG. 13 , showing the parallel tracks to allow for overlapping sliding; 
         FIG. 13B  is a detailed view of the track and associated panel, according to the illustrative embodiment; 
         FIG. 14  is a front perspective view of a rear underride drag structure for mounting on an underride guard, in accordance with another illustrative embodiment; 
         FIG. 14A  is a perspective view of a first alternate clamping mechanism for the rear underride drag structure, in accordance with the illustrative embodiment; 
         FIG. 14B  is a perspective view of a second alternate clamping mechanism for the underride drag structure, in accordance with the illustrative embodiment; 
         FIG. 15  is a front perspective view of a foam rear underride drag structure for sliding onto an underride guard, in accordance with the illustrative embodiments; 
         FIG. 16  is a bottom view of a fabric spun skirt having a tapered line of extension so as to attach to the underride guard inward of the ends of the trailer, according to an illustrative embodiment; 
         FIG. 17  is a side perspective view of an aerodynamic skirt structure secured to the underbody of a cargo body, defining an inwardly angled panel with the bottom curved up, according to an illustrative embodiment; 
         FIG. 18  is a rear view of the flow extender as extended rearwardly, the panels/fabric makes an angle inward and upward to terminate at or before the underride guard, according to the illustrative embodiment; 
         FIG. 19  is a perspective bottom rear view of a fabric skit having a tensioning mechanism and a plurality of secure points where the fabric skirt is secured to the trailer box, in accordance with an illustrative embodiment; 
         FIG. 20  is a perspective view detailing an exemplary tensioning mechanism for applying tension to a fabric skirt, in accordance with an illustrative embodiment employing a gear-driven tensioner; 
         FIG. 21  is a side view of an aerodynamic skirt structure mounted between the bogeys and the rear mount of the cargo body and including reinforced fabric, in accordance with the illustrative embodiment; 
         FIG. 22  is a perspective view of a reinforced fabric skirt, shown partially rolled up, in accordance with an illustrative embodiment; 
         FIG. 23  is a perspective bottom view of a skirt mounted between the bogeys and the rear underride guard, in accordance with an illustrative embodiment; 
         FIG. 24  is a perspective view of a fully extended skirt employing a plurality of panels having thin and thick sections, in accordance with an illustrative embodiment; 
         FIG. 25  is a perspective view of overlapping sliding panels secured to the underbody of the cargo body, in accordance with an illustrative embodiment; 
         FIG. 26  is a perspective view of overlapping, stacked panels shown in the stacked position as secured to the underbody of the cargo body, in accordance with the illustrative embodiments; 
         FIG. 27  is a perspective view of a multiple-panel system where the panels are slidable and employ a roller and rolling track, in accordance with the illustrative embodiments; 
         FIG. 28  is a bottom perspective view of a flow extender arrangement employing over-lapping panels, in accordance with the illustrative embodiments; 
         FIG. 29  is a cross-sectional diagram as taken through the over-lapping figures and showing the panel and track arrangement in greater detail, in accordance with the illustrative embodiments; 
         FIG. 30  is a perspective bottom view of flexible panels having guides and/or channels to control panel movement as bogey chassis is moved into various positions, in accordance with the illustrative embodiments; 
         FIG. 31  is a perspective bottom view of a flexible panel and track arrangement in which the bogey is in the forward bogey chassis position, according to the illustrative embodiments; 
         FIG. 32  is a perspective bottom view of a flexible panel and track arrangement in which the bogey is in the rear bogey chassis position, in accordance with the illustrative embodiments; 
         FIG. 33  is a perspective bottom view of a panel and track arrangement in which the bogey is in a rear bogey chassis position and there is a track for receiving each end of the fabric material, in accordance with an illustrative embodiment; 
         FIG. 34  is a perspective side view of a fixed panel that moves with the bogey chassis, in accordance with an illustrative embodiment; 
         FIG. 35  is a bottom perspective view of a panel supported underneath a cargo body so as to move with the bogey chassis in accordance with an illustrative embodiment; 
         FIG. 36  is a perspective view of a fixed panel having moving supports in accordance with an illustrative embodiment; 
         FIG. 37  is a perspective side view of a rigid panel that is hingedly secured proximate an outer edge of the underside of the trailer, in accordance with an illustrative embodiment; 
         FIG. 38  is a perspective rear view of the rigid panel of  FIG. 37  as it is folded inwardly toward the underside of the trailer, in accordance with the illustrative embodiment; and 
         FIG. 39  is a perspective side view of a rigid panel assembly including a plurality of separate rigid panels that can be individually folded inward toward the underside of the trailer, to provide the desired length of the rigid panel assembly, in accordance with the illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An underbody fairing structure, flow extender or “skirt” extends from just rearward of the bogeys to the rear plane of a truck body to improve airflow surrounding, underneath and emanating from the cargo body. The aerodynamic skirt structure allows for slidable adjustment of the bogeys forward and aft/rearward without the need to remove the structure. The various embodiments of the invention allow for a retractable skirt attached permanently or semi-permanently between the bogeys and the rear plane of the truck cargo body, yet remains adjustable for varying bogey positioning. The skirt further prevents air from disadvantageously curling under the cargo body. In particular, the skirt assists in directing air out of the pocket created by the rear aerodynamic structures, for example the aerodynamic structures described in the commonly assigned &#39;645 and &#39;770 Smith applications incorporated by reference hereinabove. 
     Referring to  FIG. 1 , a cargo body  110  is shown having a conventional rear-mounted side aerodynamic panel  112 , for example as described in the commonly assigned &#39;645 and &#39;770 Smith applications incorporated by reference hereinabove. According to an illustrative embodiment, mounted rearward of the bogeys  114  is an aerodynamic skirt structure  120 . The skirt  120  includes a front fill panel  122  with a spring-loaded spindle enclosure or compartment  124  for receiving the skirt  120 . The skirt  120  is secured at its rearward end to post  130  or other supporting bracket of the cargo body. The front fill panel  122  is secured to the mud flap support or a subframe that replaces the mudflap frame, for example as shown in  FIG. 5 . Angle iron or other appropriate brackets can be employed to secure the axle piece to the frame or mudflap structure. With reference to incorporated &#39;645 and &#39;770 Smith patent applications, where such structures may omit lower panels, the skirt structure  120  provides an effective mechanism for improving the aerodynamics, by directing air away from the pocket created by the aerodynamic structure described in those applications. 
     The materials employed for the skirt  120  can be highly variable within ordinary skill and can include nylon, a tarp, a rubberized or plasticized canvas, a plastic sheeting, a tarp with a spring-loaded spindle, among other materials. The skirt  120  can also comprise vertical slats, instead of fabric, that roll up around the spindle. The thickness of the fabric skirt  120  can vary and desirably has at least some amount of elasticity. The fabric is durable to withstand rough terrain and to endure rough surfaces. Any material can be employed that provides the sufficient elasticity to allow the material to stretch while wrapped within the spring-loaded spindle. 
     Reference is now made to  FIGS. 2 and 3  showing the bogeys  114  in the forward and rearward position, respectively.  FIG. 2  shows a side view of the skirt  120  with the bogeys  114  in the forward position. Given that the rear axles move forward and backward to accommodate for load leveling or state/local laws, this structure allows the skirt to be readily adjustable. In operation, a driver disengages the rear axles, releases the wheels, and pulls the trailer forward to be able to re-engage the wheels to place the bogeys at the desired location. Once the skirt  120  is mounted on the cargo body it is semi-permanent in that it is removable, but does not need to be modified to accommodate for varying position of the bogeys. The skirt  120  is shown with the bogeys  114  a distance D 2  from the rear of cargo body  110  to the front fill panel  122 , which provides clearance for the tire. Also included as part of the skirt  120  in this embodiment is a rear fill panel  230 . The rear fill panel  230  is described in greater detail hereinbelow with reference to  FIG. 6 . As shown in  FIG. 3 , with the bogeys  114  in the rearward position, the skirt  120  is significantly shorter than D 2  or  FIG. 2 . 
       FIG. 4  is a bottom view of the skirt  120  including the front fill panel that improves (i.e. reduces) the gap between the bogeys  114  and the skirt  120 . The skirt  120  is spring-loaded and biased closed within a container or other compartment  124  via a spring  410  or other appropriate biasing structure. The container  124  and corresponding front fill panel are secured via a vertical post or bracket  420  to the mud flap bar  430 . Note the skirt  120  includes a hook  440  or other appropriate securing mechanism such as a hook or strap to secure to the underride guard. The securing member depends upon the structure of the cargo body, and can also be a more robust structure as desired for alternate uses. A rear fill panel, although not shown in  FIG. 4 , can also be employed, as described in greater detail hereinbelow. 
       FIG. 5  shows a rear cut-away view of a subframe  510  that can be installed (or provided direct from the manufacturer) which replaces a conventional mudflap structure. There is also shown an optional bracket/frame  520  disposed behind the bogeys for additional support if needed. The rolled fabric skirt  120  is disposed in  124  and shown on opposing sides of the cargo body, with the subframe  510  disposed rearwardly behind the bogeys. 
     Reference is now made to  FIGS. 6, 6A and 6B , showing, respectively, a side view of the fabric skirt  120  secured to a cargo body, a cross-sectional view of the rear mount and a cross-sectional view of the front mount. As shown in  FIG. 6 , the skirt  120  includes a front fill panel  122  which includes a canister  124  or other appropriate mechanism for retracting the skirt  120 . The skirt structure also includes a rear fill panel  230  secured via mount  240  to the cargo body. The rear fill panel  230  allows the skirt  120  to terminate before the back-most stop structure of the cargo body. The rear fill panel  230  is desirably a solid panel that covers a control panel to provide improved protection therefore. The rear fill panel  230  also allows the skirt  120  to fully retract rearwardly such that it can extend to the rear underride guard. Trailers with underhanging structure at the rear end can be secured via hooks, welded angle iron or a clamp to the crossmembers on the underside of the trailer. 
       FIG. 7  shows a more detailed view of the skirt  120  with front fill panel  122  and rear fill panel  230  as taken from an inside view of the overall skirt structure. The skirt  120  is constructed and arranged to be retracted into the container  124 , which is secured just rearward of the bogeys and associated front fill panel, such that the skirt  120  is adjustable as the bogeys adjust. Note the mounting bracket  240  for securing the rear fill panel  230  to a rearward portion of the cargo body (although not shown). 
     Reference is now made to  FIGS. 8-10  detailing a sliding skirt structure comprising a plurality of panels that slide along a track. As shown in  FIG. 8 , the bogeys  114  are in the forward position, disposed a distance D 8  from the rear of the cargo body  110 . The sliding skirt structure  810  includes a plurality of panels  811 ,  812 ,  813 ,  814  and  815  which are altogether secured rearward of the bogeys, for example to a subframe or mudflap structure, and to the rear underride guard  820 . As shown in  FIG. 9 , the sliding skirt structure  810  has been retracted to account for the rearward positioning of the bogeys  114 . The sliding skirt  810  is now a distance of D 9  from the rear of the cargo body  110 , which is less than the distance D 8  of the bogeys  114  in the forward position. The adjustable skirt structure is thus highly variable to accommodate bogey location. 
       FIG. 10  is a bottom view of the sliding skirt structure  810  as secured rearward of the bogeys  114  to a rear mudflap guard  1010  on one end and at the rear underride guard  820  on an opposing end. Although not shown, the panels according to the sliding skirt embodiments can slide along a track at the top edge of the panels (i.e. the bottom edge of the cargo body) or there can be a pair of tracks, one at the top edge of the panel and another at the bottom edge of the panel, upon which the panels slide. 
     Reference is now made to  FIGS. 11 and 12  showing another skirt arrangement in which a plurality of panels are slidable and rolled within a canister in a “riot gate” or “fish scale” embodiment. The rolling skirt arrangement  1100  includes a plurality of panels  1112 - 1116  which are rolled within a canister  1110  that also comprises a front fill flap. The panels roll up into the canister and are rigid to form a continuous surface from the rear wheels to the rear panel of the truck when deployed. The rolling skirt arrangement  1100  is secured at its rearward end to the rear underride guard  1120 . The rolling skirt arrangement  1100  can comprise a polymer sheet, a composite plastic, stretched fabric over frameworks, fabric connecting fishtail, an umbrella or accordion-like folding arrangement, or other arrangement readily apparent to those having ordinary skill. 
       FIG. 13  shows still another sliding skirt embodiment in which parallel tracks are used to allow, for example, for overlapping sliding of panels. As shown in  FIG. 13 , a sliding skirt  1300  is secured to the underbody of a cargo body  101 . The sliding skirt  1300  includes a plurality of sliding or segmented panels  1301 ,  1302 ,  1303  and  1304  which are each disposed on parallel tracks to allow for overlapping sliding of the panels. As shown in  FIG. 13A , each panel  1301  and  1302  resides within a track. In even greater detail in  FIG. 13B , the panel  1301  is shown as disposed within its track  1310  via a slide ball-bearing assembly  1320 . These sliding panels likewise provide improved aerodynamic efficiency of the cargo body. 
     Reference is now made to  FIGS. 14, 14A, 14B and 15 , showing a “rear drop” aerodynamic structure for mounting to the trailer rear underride guard to further improve aerodynamic structures. In particular, these rear drop aerodynamic structures improve aerodynamic efficiency by directing airflow surrounding the rear of the vehicle and are particularly useful when combined with the illustrative sliding skirt embodiments described herein.  FIG. 14  shows a rear drop structure  1400  emulating a “clamshell” design having an upper half  1410  and a lower half  1412  which are secured to the trailer rear underride guard  1420  via appropriate holes  1430  through which bolts  1435  are driven and secured by corresponding nuts  1440 .  FIG. 14A  shows an alternate clamping mechanism which includes an upper protrusion  1442  for engaging a hole  1445  and secured in place by hook  1446 .  FIG. 14B  shows yet another clamping mechanism which includes the top half  1450  that extends through the bottom half  1455 , and the top half includes a protrusion with a hook  1457  for securing the top half and bottom half together.  FIG. 15  shows a foam slip-on design for the rear drop structure for securing to the rear underride guard  1420 . The foam slip-on structure  1500  includes a hole  1510  constructed and arranged to engage the guard  1420 . The slip-on can be secured via appropriate bolts, rivets, high-tension adhesive, or other appropriate securing mechanism. The rear drop structures are particularly useful to replace a bottom panel or in addition to other pre-existing aerodynamic structures. The rear drop structures significantly improve airflow at the rear of a cargo body. 
       FIG. 16  shows a bottom view of a cargo body having bogeys  114  with container  1600 ,  1602  for respective skirts  1610  and  1612 . As shown, the skirts  1610  and  1612  are tapered in slightly to secure to the underride guard at an inward width (WI) from the approximate outer edge of the trailer body/override guard of approximately two (2) feet. This arrangement further enhances the aerodynamic efficiency by reducing drag and improving the flow of airflow. This is particularly useful when combined with the rear teardrop structures, for example as shown in  FIGS. 14 and 15  and described herein. 
     In a further embodiment as shown in  FIG. 17 , the skirt assembly  1700  can also angle inwardly and the bottom edge curves up slightly. The angled skirt assembly, when illustratively combined with the above-described rear teardrop structures on the outer legs of the override assembly, has been shown in wind tunnel tests to improve fuel efficiency by at least 2%, and thus overall provides a highly desirable enhancement to the aerodynamic efficiency. It should be noted that the slides or other framework structures can provide a curvilinear shape to the inwardly tapered structure contemplated herein. In various embodiments, this arrangement can further enhance aerodynamic efficiency. Likewise, the inwardly tapered and/or tapered-curvilinear structure can be formed using the above-described fish-scale construction. 
     With reference further to  FIG. 18 , the inwardly tapered (or other) rear aerodynamic skirt, can be inwardly directed along the vertical direction, as well as the horizontal direction described above. This effects a somewhat frustoconical shape to the structure. The inward vertical taper can be defined linear (i.e. planar as shown) or curvilinear. 
     Fabric Trailer Skirt Panel with Tensioner Assembly 
     Referring now to  FIGS. 19 and 20 , a fabric skirt panel  1900  is shown that is fixed to the underside of the trailer body  1901  at secured points  1910 ,  1912  and the tensioner box  1916 . The fabric can be rolled up and tensioned by a tensioner assembly  1916  that applies tension to the fabric  1914 . With reference to  FIG. 20 , the tensioner assembly  1916  allows the driver or other individual to either manually (via crank  2010 , for example) or automatically (non-manually) utilize the tensioner assembly to ensure rigidity of the trailer skirts. Additionally, the tensioner assembly assists in ensuring adjustability during installation of the fabric skirt  1900 . The tensioner assembly  1916 , as shown in greater detail in  FIG. 20 , includes a crank  2010  that is typically located proximate the bogeys, and drives a gear  2020  which in turn rotates gear  2030 . The gear  2030  includes a one-way freewheel  2032  and a torsional spring  2034  that is biased closed. The rotation of gear  2030  causes the shaft  2040  to rotate. The fabric skirt is mounted to the shaft  2040  such that the skirt winds around the shaft  2040  to apply tension to the skirt  1900 . Although depicted as a gear-driven tensioner comprising two gears, the tensioner assembly can comprise any appropriate gears and/or pulleys and associated cabling to achieve the desired tensioning on the fabric skirt panel  1900 , thereby achieving the desired length of the fabric skirt in accordance with movement of the bogey position via the crank  2010 . 
     Rigid Elements in Fabric Skirt Panels 
     Reference is now made to  FIGS. 21 and 22  showing illustrative embodiments of an aerodynamic skirt structure mounted between the bogeys and the rear mount of the cargo body and including reinforced fabric. The reinforced fabric  2100  is secured to the underside of the trailer body  2101  in accordance with any of the embodiments shown and described herein. The fabric skirt, as shown in greater detail as partially rolled-up in  FIG. 22 , is reinforced by adhering, sewing, or otherwise affixing segments, ribs, inserts, or other rigid elements  2200  to the material. The segments are small enough in width that the fabric can be rolled up into a compact package  2210 . 
     Sectioned Skirt Panel 
     Referring to  FIG. 23  is a perspective bottom view of a skirt mounted between the bogeys and the rear underride guard, in accordance with an illustrative embodiment. The skirt can comprise flexible panels, fabric, or other materials known in the art. A network of panels  2300  is secured to the underside of a trailer cargo body  2301  by securing the panel to the rear underride guard at  2310  and at  2320  proximate the rear bogeys. As shown in greater detail in  FIG. 24 , the semi-rigid panel comprises multiple segments  2402 ,  2404 ,  2406 ,  2406  of larger cross sectional area which are interconnected by multiple segments  2422 ,  2422 ,  2424 ,  2426 . The multiple smaller segments  2422 ,  2422 ,  2424 ,  2426  allow the panel  2300 , as a whole, to bend and roll up (in the direction  2430  show in  FIG. 24 ). The rolled up panel shortens the deployed panel length to clear the bogey as its position is moved rearward toward the back of the trailer body. In an illustrative embodiment, the segmented panel has a living hinge material, constructed in the manner similar to popular window blinds. In an illustrative embodiment, the skirt fabric can be selectively taken up by and paid out (i.e. some of the fabric is let out) from a roller. 
     Further Structures for Panel Assemblies 
     Reference is now made to  FIGS. 25-29  showing various arrangements for a panel assembly in which the panels can slide, telescope, or stack together when the rear wheels (and associated bogeys) as slid backwards.  FIG. 25  is a perspective view of overlapping sliding panels  2500  secured to the underbody of the cargo body  2501 , in accordance with an illustrative embodiment. The sliding panels  2502 ,  2504 ,  2506  are secured to the underside of the cargo body  2501  and include a top seal  2510  and an intermediate seal  2512 , disposed between the panels to provide interconnectivity, sealing and allow for over-lapping sliding of the panels. As shown in  FIG. 26 , as the bogeys are slid rearward (in the direction of arrow  2600 ), the panels  2502 ,  2504 ,  2506  overlap and slide together such that one panel, such as  2506 , becomes the outermost panel, and the remaining panels are behind the panel  2506 . 
       FIG. 27  is a perspective view of a multiple-panel system where the panels are slidable and employ a roller and rolling track, in accordance with the illustrative embodiments. The panel assembly  2700  includes panels  2702 ,  2704 ,  2706  sealed therebetween by a seal  2708 , and which are each slightly over-lapping in at least a portion of the panel to maintain an air-tight seal through the use of a deformable elastic material, or other appropriate material known in the art. The panels are each supported by and slidable within a rolling track  2720  that the roller assemblies  2722 ,  2724 ,  2726  reside, respectively, for the panels  2702 ,  2704 ,  2706 . When the bogeys slide forward, panels slide forwardly as well (in the direction of arrow  2730  into an extended array position (or a plurality of different positions) in order to lengthen the covered length when the bogey position changes. The panels can be locked in place, or held in place by springs (not shown) at the desired position. 
       FIG. 28  is a bottom perspective view of a flow extender arrangement employing over-lapping panels, in accordance with the illustrative embodiments. As shown, the panel assembly  2800  is mounted to the underside of the cargo body  2801 . The panel assembly  2800  includes rolling panels  2802 ,  2804 ,  2806  and a fixed panel  2808 . The rolling panels  2802 ,  2804 ,  2806  are secured by rollers within the rolling track  2810 , as shown in greater detail with reference to  FIG. 29 . Referring back to  FIG. 28 , the panel assembly  2800  is secured rearward of the rear bogey chassis  2820  and the rear bogey support  2822 . Referring now to  FIG. 29 , a cross-sectional diagram as taken through the over-lapping figures and showing the panel and track arrangement is shown in greater detail, in accordance with the illustrative embodiments. The overlapping panel assembly  2900  includes a panel  2910  and an overlapping panel  2912  (the support and roller for the over-lapping panel  2912  are not shown). The panel  2910  is supported by a hooked or curved bracket  2914 . A support  2920  is secured to the panel  2910  and to a roller  2930  that resides within a rolling track  2940 , in accordance with the illustrative embodiments shown and described herein. 
     Flexible Panels with Rollers 
     Reference is now made to  FIGS. 30-33  detailing various embodiments of flexible panels that employ rollers to force bends in the panels at predetermined positions as the rear wheels of the trailer are slid backwards. Referring to  FIG. 30 , a perspective bottom view of flexible panels with rollers that engage guides and/or channels to control panel movement as bogey chassis is moved into various positions, in accordance with the illustrative embodiments, is shown. The panel assembly  3000  is secured to an underside of the cargo body  3001  and includes a flexible panel  3010  that is guided by rollers  3012 . The panel  3010  is secured on one end to the bogey chassis  3020  and more particularly the bogey chassis frame  3022 , and on the other end is guided into the channels  3024 . As the bogey chassis frame  3022  is slid rearward (in the direction of arrow  3030 ), the panel  3010  is correspondingly slid into the channel  3024  (as shown by arrow  3034 ) and into the channel  3026  (as shown by arrow  3036 ). Accordingly, the panel assembly  3000  is moved along with the bogey chassis movement. The guide rollers and channels are used to control the panel movement as the bogey chassis is moved into various positions. The guides are also used to maintain a flat and stiff outboard surface for maximum aerodynamic benefit. 
       FIG. 31  is a perspective bottom view of a flexible panel and track arrangement in which the bogey is in the forward bogey chassis position, according to the illustrative embodiments. As shown, the panel assembly  3100  is mounted to an underside of the cargo body  3101 . The flexible panel  3110  is secured at both ends to a roller  3114  that rolls within a rolling track  3116 . The panel assembly  3100  includes a pair of rollers  3112  that are used to guide the flexible panel  3110  into the track  3116 . The flexible panel  3110  is secured to the bogey chassis  3120  along the frame  3122 . 
       FIG. 32  is a perspective bottom view of a flexible panel and track arrangement in which the bogey is in the rear bogey chassis position, in accordance with the illustrative embodiments. As shown, the bogey chassis has moved (in the direction of arrow  3220 ) and accordingly, the rollers  3114  have rolled into the track  3216 . 
       FIG. 33  is a perspective bottom view of a panel and track arrangement in which the bogey is in a rear bogey chassis position and there is a track for receiving each end of the fabric material, in accordance with an illustrative embodiment. As shown, the panel assembly is secured to an underside of the cargo body  3301 . The panel  3310  is secured at one end to the bogey chassis bar  3322  and to the rollers  3312  which reside within tracks  3316 ,  3317 . As the bogey chassis is in the rear position shown in  FIG. 33  by the arrow  3320 , the panel  3310  has retracted into the tracks  3316 ,  3317  so as to provide the desired position of the bogey chassis. Illustratively, the tracks  3316 ,  3317  are fixed to the trailer  3301 , and the flexible panels are fixed to the bogey chassis. The panel guide rollers are placed in positions that prevent binding, contact, rubbing or other undesired consequences of the panels, while ensuring proper panel movement. The rollers are also placed in positions which ensure proper panel positions for maximum aerodynamic drag reduction. 
     Fixed Panel that Moves with Bogey Chassis 
       FIG. 34  is a perspective side view of a fixed panel that moves with the bogey chassis, in accordance with an illustrative embodiment. As shown, the fixed panel assembly  3400  covers the rear wheels but does not extend more than approximately 24 inches rearward. As shown in  FIG. 35 , the panel  3510  tapers inward at the rear  3512 . It is expressly contemplated that a flat panel or other structured panel within ordinary skill can be employed. The fixed panel assembly  3400  is secured to the vehicle body at a plurality of panel fixing points  3410 . The panel moves with the bogey chassis and is fixed to it in various places. The rear of the panel tapers inward ( 3512 , toward the trailer centerline) to reduce rear drag. The panel covers the outside faces of the wheels/tires. The pane is easily removed or includes removable sections for tire replacement. The panel is supported so that it does not contact any wheels/tires, but is flexible in other areas. Referring to  FIG. 35 , the panel is secured to the truck  3530  and more particularly the bogey chassis  3520  by use of panel supports  3513  that move with the bogey chassis. To provide maximum clearance with the wheels and tires, the panel could be located flush with or outboard of the trailers sides, as long as full regulatory compliance is met. 
     Fixed Panel with Moving Supports 
     Reference is now made to  FIG. 36  showing a perspective view of a fixed panel having moving supports in accordance with an illustrative embodiment. As shown, the fixed panel assembly  3600  is secured to the underside or sidewall of a trailer cargo body  3601 . The fixed panel assembly  3600  includes a fixed panel  3610  (represented by the dotted line) and defines a plurality of panel fixing positions  3620  for adjustable bogey chassis positions. The panel  3610  is illustratively substantially flush with the side of the trailer  3601 . The panel is fixed in place at the rear and along its upper seam at points  3620  and at fixed support  3622 . Illustratively, although not shown, additional rigidity members can be fixed to the underside of the trailer or the rear wheel chassis but which have sliding mounts on the fixed panel (with respect to the bogey chassis  3640 ). In accordance with the illustrative embodiment, the panel is fixed at the front to the fixed support  3622  and at the rear to the trailer frame  3630 . It is also fixed along the top edge at fixed points  3620  which can protrude from the side of the trailer based on the wheel/tire clearance. The panel has holes, slots, or fastening clip points along its length to secure the bottom edge. Supports which are fixed to the bogey chassis are fastened to these points when they line up together as the bogey is repositioned along its track. These supports maintain panel rigidity along the bogey chassis so that the panel does not contact the wheels/tires. Additional supports (not shown) for the panel bottom edge can slide into a fixing position or be added to support larger unsupported lengths. 
     Rigid Panel Hingedly Secured to Trailer Underside 
     Reference is now made to  FIGS. 37-39  showing various illustrative embodiments of a rigid panel (or multiple sections of rigid panels) hingedly secured to the underside of a trailer. The rigid panels allow for manual or automatic (non-manual) folding inwards and upwards of the skirt panel, so that it lies parallel or just below the trailer floor when in the folded position, thereby allowing the rear wheels to be moved backward underneath the panel when so desired.  FIG. 37  is a perspective side view of a rigid panel that is hingedly secured proximate an outer edge of the underside of the trailer, in accordance with an illustrative embodiment. The panel supporting brackets are also hinged and spring-loaded, such that they fold up on themselves when the panel is folded into a collapsed position. In an illustrative embodiment, the panel has a height of no more than approximately 25 to 30 inches, however the size is highly variable depending upon the truck construction and size. As shown in  FIG. 37 , a rigid panel assembly  3700  is secured to an underside of the trailer  3701  by an appropriate hinge  3720  at its attachment seam to the underside of the trailer. When a driver or other person needs to move the bogeys back, he or she can manually or automatically fold the panel inwards and up so that it sits parallel to, and just below, the trailer floor. This allows the rear bogeys to slide under the panel. With reference to  FIG. 38 , a user can stow the rigid panel  3700  by moving the panel in the direction of arrow  3801  and  3802  such that the bogey wheels can be moved as desired. 
     Referring now to  FIG. 39 , a perspective side view of a rigid panel assembly includes a plurality of separate rigid panels that can be individually folded inward toward the underside of the trailer, to provide the desired length of the rigid panel assembly, in accordance with the illustrative embodiment. The rigid panel assembly  3900  is secured to the trailer body  3900  and includes a plurality of rigid panels  3902  each having its own respective hinge  3904  for hingedly securing the panel to the trailer body  3901 . Each panel  3902  is thus separately foldable so that the user can manually or automatically fold only one or a certain number of panels, depending on how far back it is desired that the rear wheels slide. This allows some panels to stay down/deployed in their fuel saving configuration. 
     While not shown, in further embodiments the aerodynamic skirt herein can be part of a continuous fairing that partially covers the wheels and/or that is also located ahead of the bogies. 
     The teachings herein should be readily applicable to those having ordinary skill to a variety of aerodynamic structure designs and geometries secured to the underbody of a cargo body, to thereby improve the aerodynamic efficiency of the cargo body. It should be further clear that the various aerodynamic structures provided according to the above embodiments effectively direct air with respect to the bottom of a trailer body or other vehicle cargo structure. This enhances the effectiveness of any rear aerodynamic structure employed on the vehicle, particularly where such structures contain only a top panel and side panels, and are generally free of a bottom panel. 
     The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Each of the various embodiments described above may be combined with other described embodiments in order to provide multiple features. Furthermore, while the foregoing describes a number of separate embodiments of the apparatus and method of the present invention, what has been described herein is merely illustrative of the application of the principles of the present invention. For example, additional attachments and improvements can be made to the rear of the vehicle to further enhance the security and capabilities of the aerodynamic structure of this invention. Such enhancements can include extended bumper assemblies that project rearward beyond the folded aerodynamic assemblies, or special reflectors and/or lighting on the edges of the structure and/or spacer frame. Similarly, while not shown, any of the embodiments described herein can include flexible or rigid gaskets or other seal members that extend between the aerodynamic assembly and the trailer body to further streamline the junction therebetween. The panels can be constructed from a variety of durable materials or a combination of materials. For example, the panels can include rigid or semi-rigid frames covered in a flexible fabric or similar sheet material. In further embodiments, a series of fabric or flexible wells of a predetermined shape (for example a bowl or dish shape) can be defined within the central cavity of each aerodynamic structure when deployed. Such a well shape may enhance the aerodynamic effect. In addition, it is expressly contemplated that any of the mechanisms and features shown and described herein can be combined with other mechanisms and features as appropriate. Moreover, it is expressly contemplated that the skirt structures described herein can be employed in other types of vehicle bodies, such as dump bodies, tankers and auto carriers, with or without a corresponding rear-drag-reduction device. Additionally, all embodiments could alternatively be constructed as panels parallel with the trailer side or inwardly tapered. The panels could also have any number of complex curves to shape airflow inwards and upwards in this region. Finally, it should be clear that directional terms such as “up”, “down”, “left”, “right”, “top”, “bottom”, “front”, “rear”, “vertical”, “horizontal”, and the like are relative conventions and not necessarily absolute orientations with respect to a fixed coordinate system such as the operating direction of gravitational force. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.