Patent Publication Number: US-2016220431-A1

Title: Low floor chassis conversion method and apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/899,144, filed May 21, 2013, and claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/649,707, filed May 21, 2012, both of which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Various embodiments of the present invention pertain to methods and apparatus for modifying an OEM vehicle chassis so as to lower the floor of the payload compartment, and in some embodiments to lower the floor of the payload compartment and further support it with one or more air springs in the suspension. 
     BACKGROUND OF THE INVENTION 
     There is an increased need for vehicles that provide easier accessibility transportation to all persons, especially for passengers with disabilities, such as persons requiring the use of wheelchairs. However, it is relatively expensive to design and fabricate wheelchair-ready transit buses, especially considering that the commercial market for wheelchair-accessible transports is still relatively small. Therefore, it is increasingly important to be able to modify existing vehicles in a cost-effective manner so that wheelchair accessibility can be achieved within the financial constraints of the commercial market. However, existing vehicles (OEM vehicles) including bus and truck chassis often have frames and wheel suspensions that are adapted and configured for higher volume markets such as standard “high floor” school buses, transit shuttle buses and short haul trucks. For these existing vehicles to be commercially viable, they must have very high strength and high stiffness ladder frames that can easily accommodate a variety of different payloads. As such, these frames are typically fabricated from a high strength steel of substantial thickness and substantial cross sectional moment of inertia. Further, these ladder OEM ladder frames tend to be flat, such that the suspensions are attached beneath it, and the payload mounted on top of it. Further, the frames of such vehicles tend to be relatively high relative to the road surface. 
     In order to add wheelchair accessibility to such OEM chassis, some manufacturers resort to the use of expensive, heavy wheelchair lifts to provide wheelchair accessibility, as often seen with school and standard “high floor” transit shuttle buses. School buses, transit shuttle buses, and short haul trucks typically have simple ladder frames that include a pair of opposing channel members (fabricated by processes such as extrusion, forming by press, or stamping) that extend the entire length of the vehicle, and located above the rotational axes of the supporting wheels. In those applications in which the modified vehicle is intended to be used for everyday transport of persons such as at airports (where passengers must contend with luggage) and senior citizen homes (carrying persons of reduced mobility) the high floor of the payload section is often one to three tall steps upward from the road surface, even if the OEM suspension is brought down to its lowest possible height and resting on the suspension travel stops (such as the jounce stops). A significant barrier to the modification of such heavy duty, high profile, ladder frame chassis lies in the challenge that transforming such chassis to have low floors for easier passenger accessibility can result in significant compromises to vehicle handling, stiffness and strength, especially if the modifications are to be made cost effectively. 
     What is needed are conversion kits and methods that can economically and safely reduce the floor height of the passenger compartment so that it is readily accessible to all persons, especially those persons with reduced mobility. Various embodiments of the present invention provide this in novel and unobvious ways. 
     SUMMARY OF THE INVENTION 
     Various aspects of the present invention pertain to methods and apparatus for modifying an OEM ladder frame-based chassis to accommodate easier passenger vehicle accessibility, with or without a wheelchair access device, and further to be compliant with ADA requirements. 
     One aspect of some embodiments pertains to methods and apparatus for modifying the main longitudinally-extending rails of a vehicle to include a drop-down midsection adapted and configured to accommodate the fore to aft length needed to internally locate a back-to-back pair of passengers that use wheelchairs. 
     Still further embodiments include the aspect of modifying the OEM stiffness of the front and rear suspensions, and still further the amount the suspensions can be compressed, to permit the vehicle frame to be temporarily lowered to a greater extent than what is available in the OEM chassis. In this more extreme lowered state the payload section of the chassis can be made available to passengers with only modest steps by the passengers, and to passengers needing wheelchair access with an ADA compliant 1:6 ramp ratio. Such suspension modifications can include the replacement of OEM suspension springs with longer travel and/or higher internal volume replacement air springs, the relocation of air springs to an outboard position permitting a higher range of travel, and/or the replacement of OEM mechanical springs (such as leaf, coil, or torsional varieties) with replacement mechanical springs that are less stiff. 
     One aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. Some embodiments include providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each having a forward end adapted and configured to suspend corresponding right and left front wheels from respective right and left OEM springs each spring having an OEM spring stiffness. Other embodiments include replacing each OEM spring with a corresponding replacement spring having a replacement spring stiffness less than the OEM spring stiffness. Yet other embodiments include lowering the front jounce limit for each front suspended wheel. Still other embodiments include suspending the right and left wheels with corresponding right and left air springs, each air spring acting in parallel with the corresponding right or left replacement spring. 
     Another aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. Some embodiments include providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each having a forward end adapted and configured to suspend corresponding right and left front wheels below the channel members and an aft end adapted and configured to suspend corresponding right and left rear wheels below the channel members. Other embodiments include removing the OEM midsection of each channel member and thereafter inserting into each channel member corresponding right or left dropped height midsections, each dropped height midsection having a top surface lower than the top surface of the corresponding OEM channel member, each dropped height midsection having a bottom surface lower than the bottom surface of the corresponding OEM channel member. Yet other embodiments include lowering the front jounce limit for each front suspended wheel. Still other embodiments include lowering the rear jounce limit for each rear suspended wheel. 
     Yet another aspect of the present invention pertains to a chassis for a road vehicle. Some embodiments include a rear axle for rotatably supporting a pair of right and left rear wheels about a centerline. Yet other embodiments include a ladder frame including a pair of substantially straight longitudinal members each extending above said rear axle and from in front of the rear axle to behind the rear axle and each on opposite right or left sides of the frame. Still other embodiments include a pair of axle trailing arms or support members each located on opposite right or left sides of the frame, each support member being located outboard of the corresponding right or left longitudinal member and extending above the rear axle, the rear end of each the support member including an air spring support located aft of the centerline and behind a respective rear wheel. Yet other embodiments include a pair of air springs each having a top and a bottom, each air spring being located outboard of the corresponding longitudinal member and behind a respective rear wheel and each reacting loads between the ladder frame and the bottom of the corresponding the air spring support. 
     Still another aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. Some embodiments include providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each extending aft from the cab of the vehicle to an aft end adapted and configured to suspend corresponding right and left rear wheels below the channel members, each rear wheel being biased to a position by a corresponding OEM rear air spring located underneath the corresponding channel member, each OEM rear air spring providing a predetermined biasing force at an OEM air pressure. Other embodiments include removing the OEM midsection of each channel member behind the cab and thereafter inserting into each channel member corresponding right or left dropped height midsections, each dropped height midsection having a top surface lower than the top surface of the corresponding OEM channel member, each dropped height midsection having a bottom surface lower than the bottom surface of the corresponding OEM channel member. Still other embodiments include removing the OEM rear air springs. Yet other embodiments include modifying the OEM right and left rear suspension to accept an air spring located outboard of the corresponding channel member. Still other embodiments include installing right and left replacement rear air springs in the respective right and left positions of the modified rear suspension, each replacement air spring providing the predetermined biasing force at an air pressure less than the OEM air pressure. 
     It will be appreciated that the various apparatus and methods described in this summary section, as well as elsewhere in this application, can be expressed as a large number of different combinations and subcombinations. All such useful, novel, and inventive combinations and subcombinations are contemplated herein, it being recognized that the explicit expression of each of these combinations is unnecessary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further, some of the figures shown herein may have been created from scaled drawings or from photographs that are scalable. It is understood that such dimensions, or the relative scaling within a figure, are by way of example, and not to be construed as limiting. 
         FIG. 1  is a rear, right side, perspective photographic representation of a modified chassis according to one embodiment of the present invention, looking forward. 
         FIG. 2  is a front, right side perspective photographic representation of the apparatus of  FIG. 1 , looking aft. 
         FIG. 3  is a left side, top perspective view looking aft of a central portion of the apparatus of  FIG. 1 . 
         FIG. 4  is a right side perspective photographic representation looking aft of the apparatus of  FIG. 1 . 
         FIG. 5  is a top, right side perspective photographic representation of a central portion of the apparatus of  FIG. 1 , looking forward and left. 
         FIG. 6  is an enlarged photographic representation of a portion of the hardware shown in  FIG. 5 . 
         FIG. 7  is a top, right side view looking aft of a photographic representation of the rear suspension of the apparatus of  FIG. 1 . 
         FIG. 8  is an enlargement of a photographic representation of a portion of the apparatus of  FIG. 7 . 
         FIG. 9  is a photographic representation of a portion of the rear suspension of the apparatus of  FIG. 1 . 
         FIG. 10  is a right side perspective photographic representation looking left and forward of a portion of the rear suspension of the apparatus of  FIG. 1 . 
         FIG. 11  is a photographic representation of a view looking forward of the suspension of  FIG. 10 . 
         FIG. 12  is a right side, front perspective photographic representation looking left and aft of a portion of the front suspension of the vehicle of  FIG. 1 . 
         FIG. 13  is an enlarged photographic representation of a portion of the apparatus of  FIG. 12 . 
         FIG. 14A  is a largely side view of a photographic representation of an OEM leaf spring for the front suspension of a vehicle. 
         FIG. 14B  is a side schematic representation of the apparatus of  FIG. 14A  as installed on a vehicle. 
         FIG. 14C  is a right side schematic representation looking left of a portion of the front suspension of a vehicle according to one embodiment of the present invention. 
         FIG. 15  is a right side, rear perspective photographic representation looking forward of a vehicle to be modified according to another embodiment of the present invention. 
         FIG. 16  is a top perspective photographic representation looking forward of the apparatus of  FIG. 15 . 
         FIG. 17  is a right side, front perspective photographic representation of the rear suspension of the apparatus of  FIG. 15 . 
         FIG. 18  is a photographic representation looking downward at the left side suspension of the vehicle of  FIG. 15 . 
         FIG. 19  is a side photographic representation looking forward and right at the rear suspension of  FIG. 18 . 
     
    
    
     ELEMENT NUMBERING 
     The following is a list of element numbers and at least one noun used to describe that element. It is understood that none of the embodiments disclosed herein are limited to these nouns, and these element numbers can further include other words that would be understood by a person of ordinary skill reading and reviewing this disclosure in its entirety. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 20 
                 Vehicle 
               
               
                   
                 22 
                 cab 
               
               
                   
                 23 
                 Front wheels 
               
               
                   
                 24 
                 Payload section 
               
               
                   
                 25 
                 Rear wheels 
               
               
                   
                 26 
                 Wheelchair ramp 
               
               
                   
                 28 
                 Fuel tank 
               
               
                   
                 30 
                 Frame 
               
               
                   
                 32 
                 Longitudinal channeled support 
               
               
                   
                 34 
                 Midsection 
               
               
                   
                 34(b) 
                 Lower elevation 
               
               
                   
                 35 
                 Gusset 
               
               
                   
                 36 
                 Aft section 
               
               
                   
                 36(b) 
                 Higher elevation 
               
               
                   
                 37 
                 Fuel tank 
               
               
                   
                 38 
                 Driveshaft 
               
               
                   
                 39 
                 Exhaust system 
               
               
                   
                 40 
                 Rear suspension 
               
               
                   
                 42 
                 axle 
               
               
                   
                 43 
                 Clamps 
               
               
                   
                 44 
                 Leaf spring 
               
               
                   
                 46 
                 Height sensor 
               
               
                   
                 48 
                 Lateral frame member 
               
               
                   
                 50 
                 Trailing arm (or support 
               
               
                   
                 51 
                 Pivot 
               
               
                   
                 52 
                 Front section 
               
               
                   
                 54 
                 Rear section 
               
               
                   
                 56 
                 Air spring 
               
               
                   
                 57 
                 Bottom support 
               
               
                   
                 58 
                 Top support 
               
               
                   
                 60 
                 Air system 
               
               
                   
                 62 
                 Air compressor 
               
               
                   
                 64 
                 Air tank 
               
               
                   
                 65 
                 Purge tank 
               
               
                   
                 66 
                 Heat exchanger 
               
               
                   
                 68 
                 Dryer and dump valve 
               
               
                   
                 70 
                 Front suspension 
               
               
                   
                 71 
                 Shock absorber 
               
               
                   
                 72 
                 Wheel support 
               
               
                   
                 73 
                 Anti roll bar 
               
               
                   
                 74 
                 Leaf spring assembly 
               
               
                   
                 74(b) 
                 Front attachment 
               
               
                   
                 74(c) 
                 Rear attachment 
               
               
                   
                 74(d) 
                 Clamp 
               
               
                   
                 74(e) 
                 Central attachment 
               
               
                   
                 74(f) 
                 Top leaf 
               
               
                   
                 74(g) 
                 Bottom leaf 
               
               
                   
                 75 
                 Front axle 
               
               
                   
                 76 
                 Air spring 
               
               
                   
                 77 
                 Bottom support 
               
               
                   
                 78 
                 Top support 
               
               
                   
                   
               
            
           
         
       
     
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. At least one embodiment of the present invention will be described and shown, and this application may show and/or describe other embodiments of the present invention. It is understood that any reference to “the invention” is a reference to an embodiment of a family of inventions, with no single embodiment including an apparatus, process, or composition that should be included in all embodiments, unless otherwise stated. Further, although there may be discussion with regards to “advantages” provided by some embodiments of the present invention, it is understood that yet other embodiments may not include those same advantages, or may include yet different advantages. Any advantages described herein are not to be construed as limiting to any of the claims. The usage of words indicating preference, such as “preferably,” refers to features and aspects that are present in at least one embodiment, but which are optional for some embodiments. 
     The use of an N-series prefix for an element number (NXX.XX) refers to an element that is the same as the non-prefixed element (XX.XX), except as shown and described. As an example, an element  1020 . 1  would be the same as element  20 . 1 , except for those different features of element  1020 . 1  shown and described. Further, common elements and common features of related elements may be drawn in the same manner in different figures, and/or use the same symbology in different figures. As such, it is not necessary to describe the features of  1020 . 1  and  20 . 1  that are the same, since these common features are apparent to a person of ordinary skill in the related field of technology. Further, it is understood that the features  1020 . 1  and  20 . 1  may be backward compatible, such that a feature (NXX.XX) may include features compatible with other various embodiments (MXX.XX), as would be understood by those of ordinary skill in the art. This description convention also applies to the use of prime 0, double prime (″), and triple prime (″) suffixed element numbers. Therefore, it is not necessary to describe the features of  20 . 1 ,  20 . 1 ′,  20 . 1 ″, and  20 . 1 ′″ that are the same, since these common features are apparent to persons of ordinary skill in the related field of technology. 
     Although various specific quantities (spatial dimensions, temperatures, pressures, times, force, resistance, current, voltage, concentrations, wavelengths, frequencies, heat transfer coefficients, dimensionless parameters, etc.) may be stated herein, such specific quantities are presented as examples only, and further, unless otherwise explicitly noted, are approximate values, and should be considered as if the word “about” prefaced each quantity. Further, with discussion pertaining to a specific composition of matter, that description is by example only, and does not limit the applicability of other species of that composition, nor does it limit the applicability of other compositions unrelated to the cited composition. 
     One embodiment of the present invention pertains to a kit for modifying a truck chassis. The kit includes a pair of frame midsections, a wheelchair ramp, and a pair of suspension arms. The frame sections are adapted and configured to be placed in the middle of the vehicle&#39;s existing frame rails. Preferably, this replacement midsection provides a dropped configuration to the OEM longitudinal channels, such that the resulting integrated structure extends at an OEM height along a forward section (such as under the cab), then drops down to a lower height for easier vehicle passenger (ambulatory/non-ambulatory) access, and then jogs back up to the OEM height in front of, over, and aft of the rear axle. The passenger (ambulatory/non-ambulatory) ramp is adapted and configured to be attached to one of the replacement frame midsections. The suspension trailing arms (or support arms) are adapted and configured to support the chassis with corresponding suspension air springs. 
     In yet another embodiment, the present invention pertains to a replacement trailing arm for the rear suspension of an existing chassis. Each trailing arm includes a forward portion that is adapted and configured to pivotally connect to a portion of the frame. This forward portion is located between the outboard side of a frame rail and the inboard side of a rear tire. This forward portion extends aft over the top, or aft under the rear axle to an aft portion. The aft portion of the trailing arm extends past the rear of the rear tires, and in some embodiments past the rear of the rear wheels, and further jogs outboard. The aft end of the trailing arm includes a mounting location for a spring located aft of the rear wheel or aft of the rear tire. In one embodiment, the aft mount supports an air spring. 
     Yet another embodiment of the present invention pertains to the addition of an air supply system dedicated to a vehicle&#39;s suspension. In one embodiment, the air system includes a compressor, a heat exchanger, a dryer, a filter, valves, a reservoir, and a plurality of air springs. In some embodiments, the size of the air springs is selected such that their typical inflation pressure is a moderate or low pressure in terms of the capability of the air compressor, and in some embodiments the range of typical operation is less than about 70 psig. 
     Preferably, the air springs that replace any OEM suspension springs are selected to provide equivalent spring force at a lower pressure (or, a greater spring force than the OEM spring force at the same predetermined pressure). By operating with a larger spring and/or a lower pressure, it has been found that the time to fill the air spring (such as from a completely deflated position) can be substantially reduced, especially if the selected air compressor provides the required range of air spring operating pressures at a point on the compressor map where the relatively lower pressure permits relatively higher flow rate out of the compressor. 
     It has been found that some current OEM air spring suspensions utilize a relatively small, higher pressure compressor that can be mounted in the engine compartment and using higher pressure air from this underhood compressor with relatively smaller OEM air springs. Although such an OEM system can provide a smaller packaging for both the suspension and the compressor, the OEM systems nonetheless require relatively long fill times, especially if the air springs have been completely deflated so as to bring the frame down to its lowest OEM level. 
     Various embodiments of the present invention provide modification kits to an OEM chassis that include an air system adapted and configured to be mounted aft of the cab section, and not require any underhood components. Still further, some embodiments of the present invention include the use of a replacement air compressor having substantially higher volumetric flow rate at the levels of pressure needed by the replacement air springs to provide OEM-type air spring biasing forces between the frame and the suspension. Still further, the replacement air springs are selected to require a lower air pressure level to generate the OEM-type biasing forces than the OEM air springs would. In combination, the higher flowing replacement air compressor and lower pressure air springs result in a system in which the time to reinflate the replacement air springs of the modified vehicle to restore the modified vehicle back to a vehicle height suitable for driving is substantially less than the time required by fully deflated OEM air compressor. This reinflation time can be useful in those applications where the vehicle is routinely expected to lower (deflate) and rise (back to operating ride height) during trips such as the frequent stopping/deflating/reinflating actions of shuttle buses at airports. 
     Still further, the various air handling components of the kit should be adapted and configured to be compatible with the other low profile aspects of the kit. In some embodiments, the air system components are adapted and configured to be placed entirely aft of the cab (with the exception of any front air springs), and more preferably in alignment with the profile of the drop-down midsection. In still further consideration that the drop-down midsection will be temporarily brought very close to the road surface (such as when the vehicle is in a kneeling configuration with deflated air springs), the air handling components need to be of narrow enough cross section so that they can be protected by the bottom surface of the drop-down midsections. Still further, in some embodiments the heat exchanger includes a plurality of fins that span the length of a tubular section, thus providing a heat exchanger with a low profile that can be protected by the drop-down midsection channels. In some embodiments, this heat exchanger is placed proximate to, and inboard, of a vehicle frame rail. 
     Yet other embodiments of the present invention pertain to a kit for modifying the front suspension of a vehicle. In one embodiment, the kit includes a replacement spring for the vehicle&#39;s OEM front spring. The replacement spring is adapted and configured to have a lower spring constant than the OEM spring. The kit can include replacement coil springs or replacement leaf springs dependent upon the configuration of the OEM spring. When the leaf spring of the kit is installed, the overall stiffness of the modified vehicle front leaf spring is reduced from the OEM stiffness in some embodiments by using a replacement leaf spring that is a modification of an OEM spring and having at least one leaf with reduced span. 
     The kit preferably further includes an air spring for additionally supporting the front of the vehicle from the OEM front spindle or front axle, the combination of the kit air spring and the reduced-stiffness kit mechanical spring combining to provide an overall spring rate from the replacement kit that is substantially the same as the OEM spring rate. However, the kit replacement springs can sustain a greater amount of suspension compression to permit a lower kneeled height when the air springs are completely deflated. This greater amount of compression is not available in the OEM suspension, in which the OEM jounce stop limits the maximum compression, and further because the OEM spring can be so stiff as to not permit the total amount of compression. Preferably, the replacement spring and the air spring support the front wheel relative to the frame in parallel. 
     Yet other embodiments of the present invention pertain to a kit for modifying the front and rear suspensions of a vehicle, especially a vehicle such as an ambulance. The vehicle includes the addition of air springs at the front and rear, and further includes modifications of the OEM front and/or rear springs to have a reduced spring stiffness. In yet other embodiments the OEM suspensions are modified with a replacement suspension jounce stop that permits additional compression of the suspension. By doing this, the vehicle can be lowered (by a reduction in air pressure) to a height that is lower than what would otherwise be achievable with the OEM standard, higher stiffness springs. In still further embodiments, the vehicle can have the rear section lowered (by removal of air pressure) and the front section lifted (by introduction of higher pressure) such that the payload section tilts aft at a higher angle than what would be otherwise achievable with an unmodified OEM suspension. With such a higher degree of tilt, a patient on a stretcher can be more easily placed in the payload section by a medical professional. 
     Referring to  FIG. 1 , a vehicle  20  according to one embodiment of the present invention is shown. In one embodiment, one example of a vehicle (the prime ′ superscript is used herein to refer to an OEM configuration) is a Ford® F550® cab and chassis. However, this identification of a particular manufacturer and model is by way of example only, and is not limiting on any embodiment of the present invention. As used herein, the suffixes “R” and “L” pertain to the right and left sides of the vehicle. Further, as is convention in this art, the terms fore and forward refer to a direction from the rear wheels toward the front wheels, and the terms inboard and outboard refer to the location of a component that is spaced either closer to the centerline or further from the centerline, respectively, of another feature. 
     Vehicle  20  includes a cab section  22  with provisions for a driver, an engine, and steerable front wheels  23 . A payload section  24  extends aft from cab section  22 , including a frame  30  supported by a pair of rear wheels  25 . The OEM frame includes right and left substantially straight longitudinal members that extend from the front wheels and under the cab to a location aft of the rear wheels. In some embodiments, the OEM channel members have a “C” cross section, although any configuration of OEM channel member is contemplated in various embodiments, including open-C channels, closed cross sectional channels, I cross sections, and other extruded and formed high stiffness configurations. In some embodiments, the OEM channel sections are typically of a “C” shape with the open side facing inboard. The height of the C channel is typically more than about six inches tall, and the material is typically more than about three-sixteenths inches thick. A typical material for the OEM rails is ASTM A36 steel. Various embodiments of the present invention pertain to the modification of an OEM chassis that is capable of operating with a gross vehicle weight requirement (GVWR) of more than ten-thousand pounds. Preferably, the right and left longitudinal OEM channels are substantially straight, and extend from aft to rear at locations over both the front rotational axis and the rear rotational axis. 
       FIG. 2  shows the wheelchair ramp  26  fully deployed from the right side of payload section  24 . Preferably, wheelchair ramp  26  is of the type that unfolds, although other embodiments contemplate the use of telescopic and/or elevator-type wheelchair assist mechanisms. Ramp  26  is coupled to a longitudinal support member  32 R that extends along the right side of frame  30 . Frame  30  includes a mirror image frame rail  32 L extending aft from cab section  22  toward the rear of the vehicle. Although reference may be made to certain features in terms of the right or left sides of frame  30 , it is understood that there is substantial symmetry between the right and left frame rails, and further that any of the other components placed relative to a frame rail could likewise be placed relative to the other frame rail. 
       FIG. 2  shows that longitudinal channel member  32 R includes a midsection  34 R and an aft section  36 R. In one embodiment, frame section  36 R is a remnant of the OEM channel frame member that extended substantially straight aft from cab  22 . Frame midsection  34 R is inserted into and replaces a section of OEM frame rail. It can be seen in  FIG. 2  that frame section  34 R jogs downwardly from a forward section  36 R (as best seen in  FIG. 1 ), extends aft in a low height midsection, and jogs upwardly to meet the rear remnant  36 R of the frame rail. The lowered midsection  34  permits the use of a payload section  24  that has a floor lower and closer to the surface of the roadway than would be otherwise available in the OEM vehicle. As can be seen in  FIG. 2 , the top surface of the midsection is at a lower elevation than the top surface of the aft OEM channel, and the bottom surface of the replacement midsection is lower than the height of the lower surface of the aft OEM section. 
       FIG. 2  further shows that the fore to aft length of the dropped height midsection is substantially longer than the width of the wheelchair ramp  26 . In some embodiments, the length of the replacement midsections are adapted and configured to support a payload section for passengers (not shown) that has sufficient length to internally support a pair of passengers and wheelchairs, one in front of the other. As shown in  FIG. 2 , the fore to aft span of the replacement midsection is more than about twice the width of the wheelchair ramp  26 . 
       FIG. 3  shows the aft section of the chassis of vehicle  20 . A pair of longitudinal support members extend aft. The midsection  34   b  of these frame members are at a lower height than the height  36   b  of the aft portion. The exhaust system  39  and drive shaft  38  are located in substantial part inboard of each longitudinal member  34 . In some embodiments, the position of the drive shaft is lowered relative to the OEM position. A pair of rear wheels  25  are supported along a rear axle  42  and are located outboard support members  32 . Each support member  32  includes a midsection  34  at a lowered height  34   b , that jogs upward to an aft section  36  located at a higher elevation  36   b.    
     In some embodiments, this placement of the wheel chair ramp permits a payload section to be adapted and configured for improved access by wheelchairs. As one example, the low height midsection  34  is adapted and configured such that the payload section on the side of the vehicle opposite the wheel chair ramp can accommodate two wheel chairs, and the side with the wheel chair ramp can accommodate a third wheel chair. These wheel chair locations of the payload section can further be adapted to include hinged seats when the spaces are not occupied with wheel chairs. 
       FIG. 4  shows additional details of vehicle  20 . A strengthening gusset  35  couples the midsection  34  to the frame aft section  36 . Each pair of rear wheels  25  is supported by a trailing arm  50  that includes a front section  44  including a leaf spring. Preferably, the forward portion  44  of trailing arm  50  is coupled to a longitudinal support member by a pivot joint  51 . In some embodiments, pivot joint  51  is preferably maintained at the same configuration as with the OEM vehicle, although other embodiments contemplate the use of other locations for coupling of the trailing arm pivot joint to the frame. 
       FIG. 5  shows a portion of the air system  60  of the vehicle  20 . An air compressor  62  powered by an electric motor provides compressed ambient air to one or more air reservoirs  64 . Some embodiments of the present invention place this compressor aft of the cab  22 , especially when the cab  22  has insufficient under hood space for the addition of an engine-driven air compressor. Prior to storage in reservoirs  64 , the compressed air is cooled in a heat exchanger  66 . Compressed air from tanks  64  is provided through a dryer  68  (best seen in  FIG. 3 ). A purge tank  65  provides a source of air to blow out dryer  68  after each usage of air system  60 . 
     Referring to  FIG. 6 , one embodiment of heat exchanger  66  can be seen having a generally longitudinal configuration, with a plurality of radially extending fins to exchange heat with the ambient. In one embodiment, heat exchanger  66  is of the type that flows internally in a single direction (as seen in  FIG. 6 , from fore to aft). Heat exchanger  66  in some embodiments is particularly suited to packaging and placement under the payload section, and inboard of the frame midsections  34 . 
       FIGS. 7-11  photographically show different views of the rear suspension  40  of vehicle  20  according to one embodiment of the present invention. Wheels  25  are driven by an axle  42  that is powered by the engine through driveshaft  38 . However, the present invention also contemplates those embodiments in which the rear wheels are not powered, and including those embodiments in which the vehicle front wheels are powered. The wheels  25  are coupled to frame  30  by right and left trailing arms  50 . As best seen in  FIG. 7 , vehicle  20  preferably includes a height sensor  46 , the signal of which is utilized by an onboard controller (not shown) to maintain a predetermined height of vehicle  20  by control of air pressure in the air springs. 
     Trailing arm  50  includes a forward section  52  that couples to the frame at a pivot joint  51 . In some embodiments, the front section  52  includes one or more leaf springs for resilient support of the rear of vehicle  20 . However, various other embodiments of the present invention include trailing arms  50  that have generally rigid front sections  52 , but which pivotally couple to longitudinal member  36 R. Still further embodiments contemplate trailing arms  50  that are coupled to longitudinal member  36 R by way of one or more brackets, and using a resilient, elastomeric bushing as an interface member between the front end of the trailing arm and the bracket or channel member of the frame. 
     As best seen in  FIGS. 4, 7, and 8 , trailing arm  50  includes a lower front section  52  that, especially in those embodiments including a rear axle, extends aft and rises upward to pass over the axle. Referring to  FIGS. 7 and 8 , it can be seen that the midsection of trailing arm  50  is clamped to axle  42  by a pair of U-bolt clamps  43 . Further, as best seen in  FIG. 8 , the aft end of leaf springs  44  are coupled to axle  42  by these same U-bolts. 
     A rear section  54  of trailing arm  50  extends aft from the axle coupling in a substantially rigid section. As best seen in  FIGS. 7, 8, and 11 , this aft section extends downward, aft of the axle, and outboard, to a lower spring support  57 . As best seen in  FIG. 8 , a central part of the aft section  54  extends both rearward and outboard such that at least a portion of spring support  57  is located behind one of the rear wheels  25 . As best seen in  FIGS. 9, 10, and 11 , the spring support  57  of trailing arm  50  is further located lower than the central section of trailing arm  50  shown in  FIG. 8 . However, such geometry is by way of example only, and is not limiting on any embodiment of the present invention. As yet another example, in those embodiments in which the rear wheels are not driven, the central portion of the trailing arm may extend aft from the front portion of the trailing arm and connect in a more geometrically direct manner with the lower spring support. 
     The trailing arm  50  is adapted and configured in some embodiments to support an end of an air spring  56 , the air spring itself supporting part of the vehicle weight from the suspension arm  50 . Preferably, air spring  56  is of a lower pressure, larger diameter design, adapted and configured in accordance with the output characteristics of the air compressor so as to use an inflation pressure that is preferably toward the middle or lower region of the air compressor&#39;s pressure versus flow characteristics. In this manner, the compressor is able to provide substantially more flow at the lower inflation pressure than would be the case for an air spring of smaller diameter that requires higher pressure to support the vehicle. With such utilization of the higher flow characteristics of the compressor, it is possible to size the air springs and reservoirs for a quick refill after the vehicle has been lowered. In this manner, the quick refill permits a relatively quick overall cycle time for the vehicle (from the time the vehicle stops, lowers itself, raises itself, and continues traveling) this provides the unexpected benefit of more productive usage of the vehicle by lowering the typical air pressure within the air springs. 
     Vehicle  20  preferably includes a lateral frame member  48  that extends across the rear of the frame  30 , as best seen in  FIGS. 9 and 10 . In some embodiments, this lateral member  48  extends across, and is coupled to, each aft section  36 R and  36 L of frame  30 , such as by welding (although the present invention contemplates any manner of attachment). Preferably, lateral member  48  has a C-shaped or similar cross section for a combination of weight, stiffness, and drainage of water. As shown in  FIG. 9 , lateral member  48  is preferably unitary and coupled to both right and left side longitudinal channel members of the frame. In this manner, top spring supports  58 R and  58 L are provided with sufficient bending stiffness relative to the reaction loads imposed by air springs  56 .  FIG. 11  shows the bottom side of lateral member  48  welded to the top surface of C-channeled longitudinal member  36 R. Further,  FIG. 11  shows the location of air spring  56 R behind the tire attached to rear wheel  25 R. 
     Lateral member  48  supports at each end a top spring support  58 . Top support  58 L supports the top of air spring  56 L. The top spring support  58 R supports and provides mounting for the top air spring  56 R. With placement of air springs  56  aft and behind rear wheels  25 , and with the outboard spring support points provided by lateral frame member  48 , the stability of vehicle  20  (especially in roll) is improved from that of the OEM vehicle. In some embodiments, a portion of the rear axle is coupled by a panhard rod to one of the longitudinal support members  32 R or  32 L for lateral stability of axle  42  relative to frame  30 . In still further embodiments the modifications include the attachment of a sway bar to the differential of rear axle  42 . 
       FIGS. 12, 13, and 14  show and describe various aspects of a front suspension  70  of vehicle  20 . Vehicle  20  includes right and left wheels  23 R and  23 L, respectively, that support vehicle  20  from the roadway. Each wheel is coupled to a wheel support  72  attached by clamps  74 ( d ) to a leaf spring assembly  74 . A pair of shock absorbers  71  couple each wheel support  72  to the vehicle frame and dampen the movement of wheels  23 . A roll bar  73  interconnects the right and left suspensions of vehicle  20  to improve the roll stability of the vehicle. 
       FIGS. 12, 13, 14A and 14B  depict the leaf spring  74 ′ of the OEM vehicle. Leaf spring  74 ′ includes a top leaf spring  74 ′ f  and bottom leaf spring  74 ′ g  that extend from a foreword pivot joint  74 ′ b  be to an aft pivot joint  74 ′ c . These top and bottom OEM leaf springs are coupled together by an aft clamp  74 ′ d  which is best seen in  FIGS. 14A and 14B . Bottom OEM leaf spring  74 ′ g  is coupled to the front pivot joint  74 ′ b  and extends aft and is located underneath aft pivot joint  74 ′ c . Referring to  FIG. 14B , leaf spring assembly  74 ′ is coupled to front wheel support  72  by a central attachment  74 ′ e . In one embodiment, this central attachment includes a pair of U-clamps and a centrally located fastener, as best seen in  FIGS. 13 and 14B . 
     In one embodiment, the front suspension of vehicle  20  is modified to include an air spring  76 , and further to reduce the stiffness of the leaf spring  74 ′.  FIG. 14C  shows a right side front suspension according to one embodiment of the present invention. As shown in  FIG. 14C , the bottom spring  74   g  in one embodiment of the present invention has a reduced length, and extends from the front pivot  74   b  to a point just aft of wheel support  72 . Bottom leaf  74   g  is coupled to support  72  by the central attachment  74   e . The aft section of OEM bottom leaf spring  74 ′ has been removed, which provides an overall reduced stiffness to leaf spring  74 . However, in yet other embodiments a similar reduction in stiffness can be accomplished by using, as examples, a reduced thickness bottom leaf spring that extends from the front pivot to the aft pivot, or a bottom leaf of reduced width and commensurate reduced stiffness, or by eliminating the bottom spring altogether. In the latter case, the top leaf may be the OEM leaf, as one example, or could be a top leaf of increased stiffness, but yet in other embodiments could be a top leaf of reduced stiffness (as compared to the OEM top leaf). In those embodiments in which the springs of the front suspension are of the coil type, the OEM coils can be replaced with coils having reduced stiffness, such as by a reduction in wire diameter, change in the number of coils, change in the overall diameter of the spring, or other methods known for the reduction of coil spring stiffness. 
     Referring again to  FIG. 14C , in some embodiments the front suspension of vehicle  20  includes a pair of air springs  76 , one each for support of the right and left front suspension. The bottom of air spring  76  is preferably attached by a bottom support  77 , which in some embodiments can also be the central attachment  74   e  which couples to suspension arm  72 . However, in yet other embodiments, the air spring bottom and top supports  78 , respectively, and air spring  76 , are located outboard of the OEM attachment positions  74   e , especially in those vehicles in which packaging constraints are best met with outboard placement of the air spring. However, the present invention contemplates any mounting of air spring  76 . 
     Preferably, vehicle  20  includes a front section in which the OEM spring supports have reduced stiffness, and in which that stiffness is compensated by the introduction of the air support. In such embodiments, by reducing the internal pressure of the air support the vehicle can be brought to a lower position temporarily for ingress and egress of passengers from the payload section. This lower position is permitted by the reduced stiffness of the Front suspension springs  74 . The continued use of modified front springs  74  in vehicle  20  allows for OEM-levels of reliability during operation. 
       FIGS. 15, 16, 17, 18, and 19  show various aspects of a vehicle  120  according to another embodiment of the present invention. It is understood that the vehicle  120  depicted in these figures has not been modified to include the front suspension, rear suspension, air system, or midsection longitudinal support members as shown in the previous figures with regards to vehicle  20 . However, these similar features, components, and aspects can also be incorporated into a modified vehicle  120 . In one embodiment, vehicle  120  is based on a cab and chassis fabricated by manufacturer International Harvester®.  FIG. 16  shows the C-shaped cross sectional shape of the OEM longitudinal channel members, and their substantially straight and level top surfaces that extend fore and aft.  FIG. 16  further shows the fuel tank  137 ′ between the OEM longitudinal members. 
     Vehicle  120  includes a pair of longitudinally-extending frame rails  132 ′L and  132 ′R that extend from  122  aft to the end of the vehicle. Rear wheels  125  are supported by trailing arms  150 ′ from corresponding frame rails. Referring to  FIGS. 17, 18, 19 , the rear suspension of vehicle  120  includes in its OEM state rear air springs  156 ′L and  156 ′R that are coupled to the corresponding bottom spring supports  157 ′ of suspension arms  150 ′. Further, the forward sections  152 ′ incorporate leaf springs  144 ′ that are pivotally attached to a corresponding frame rail  132 . 
     As modified, vehicle  120  includes a suspension trailing arm  150  with an aft portion  154  that extends laterally outboard from its OEM position. The corresponding air springs  156  are located aft and preferably behind rear wheels  125 . A top spring support  158  (not shown) supports the top of air spring  156 , and is further supported by frame  130  by a lateral frame member  148  (not shown). As previously discussed, the air springs  156  that are selected to replace the OEM air springs  156 ′ preferably provide a lower spring force at a predetermined pressure than the OEM springs provide at that same predetermined pressure. Still further, the replacement air springs  156  preferably provide a greater range of overall suspension travel than the OEM springs. 
     Comparing  FIG. 16  to  FIG. 10 , it can be seen that the OEM spring in some embodiments is generally of a smaller diameter and smaller overall height than the replacement air spring, such that the modified rear suspension is capable of greater travel (from bump stop to bump stop) than the OEM suspension. Comparing  FIGS. 19 and 11 , it can be seen that the top spring support is located at a higher position than the OEM top spring support, whereas the lower spring support  57 R is at generally the same location (in some embodiments) as the lower spring support  157 ′L. By so changing the spring characteristics and further changing the location of the top spring mount, and especially in those embodiments combined with modified jounce stops, it is possible to compress the inventively modified rear suspension more than the OEM suspension, thus providing easier wheelchair access by having a lower overall modified vehicle. Still further, frame  130  is preferably modified in its midsection to include a lower elevation midsection  134  (not shown) that provides accommodation for a payload section  124  having a floor that is lower than what would be otherwise permitted by the OEM frame. 
     Various aspects of different embodiments of the present invention are expressed in paragraphs X1, X2, and X3 as follows: 
     X1. One aspect of the present invention pertains to a chassis for a road vehicle. The chassis preferably includes a rear axle for rotatably supporting a pair of right and left rear wheels about a centerline. The chassis preferably includes a ladder frame including a pair of substantially straight longitudinal members each extending above said rear axle and from in front of the rear axle to behind the rear axle and each on opposite right or left sides of said frame. The chassis preferably includes a pair of axle support members each located on opposite right or left sides of said frame, each said support member being located outboard of the corresponding said right or left longitudinal member and extending above said rear axle, the forward end of each said support member being coupled to said corresponding longitudinal member to permit vertical movement of said rear axle relative to said ladder frame, the rear end of each said support member including an air spring support located aft of the centerline and behind a respective said rear wheel. The chassis preferably includes a pair of air springs each having a top and a bottom, each said air spring being located outboard of said corresponding longitudinal member and behind a respective rear wheel and each reacting loads between said ladder frame and the bottom of the corresponding said air spring support. 
     X2. Another aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. The method preferably includes providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each having a forward end adapted and configured to suspend corresponding right and left front wheels from respective right and left OEM springs each spring having an OEM spring stiffness. The method preferably includes replacing each OEM spring with a corresponding replacement spring having a replacement spring stiffness less than the OEM spring stiffness. The method preferably includes lowering the front rebound limit for each front suspended wheel. The method preferably includes suspending the right and left wheels with corresponding right and left air springs, each air spring acting in parallel with the corresponding right or left replacement spring. 
     X3. Yet another aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. The method preferably includes providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each having a forward end adapted and configured to suspend corresponding right and left front wheels below the channel members and an aft end adapted and configured to suspend corresponding right and left rear wheels below the channel members, each rear wheel being driven by a driveshaft. The method preferably includes removing the OEM midsection of each channel member and thereafter inserting into each channel member corresponding right or left dropped height midsections, each dropped height midsection having a top surface lower than the top surface of the corresponding OEM channel member, each dropped height midsection having a bottom surface lower than the bottom surface of the corresponding OEM channel member. The method preferably includes lowering the driveshaft. The method preferably includes lowering the front rebound limit for each front suspended wheel. The method preferably includes lowering the rear rebound limit for each rear suspended wheel. 
     X4. Still another aspect of the present invention pertains to a method of modifying a chassis for a road vehicle. The method preferably includes providing an OEM ladder frame chassis having a pair of right and left substantially straight longitudinal channel members each extending aft from the cab of the vehicle to an aft end adapted and configured to suspend corresponding right and left rear wheels below the channel members, each rear wheel being biased to a position by a corresponding OEM rear air spring located underneath the corresponding channel member, each OEM rear air spring providing a predetermined biasing force at an OEM air pressure. The method preferably includes removing the OEM midsection of each channel member behind the cab and thereafter inserting into each channel member corresponding right or left dropped height midsections, each dropped height midsection having a top surface lower than the top surface of the corresponding OEM channel member, each dropped height midsection having a bottom surface lower than the bottom surface of the corresponding OEM channel member. The method preferably includes removing the OEM rear air springs. The method preferably includes modifying the OEM right and left rear suspension to accept an air spring located outboard of the corresponding channel member. The method preferably includes installing right and left replacement rear air springs in the respective right and left positions of the modified rear suspension, each replacement air spring providing the predetermined biasing force at an air pressure less than the OEM air pressure. 
     Yet other embodiments pertain to any of the previous statements X1, X2, X3, or X4 which are combined with one or more of the following other aspects: 
     Wherein each said support member includes a leaf spring having a forward end clamped with a bushing to said corresponding longitudinal member. 
     Wherein each said leaf spring having an aft end clamped to said corresponding air spring support, or wherein each said support member includes a leaf spring having an aft end clamped to said corresponding air spring support. 
     Wherein the forward end of each said support member is pivotally coupled to said corresponding longitudinal member, or herein each said support member includes a leaf spring having a forward end pivotally coupled to said corresponding longitudinal member. 
     Which further comprises a lateral member extending across the width of said ladder frame, said lateral member including right and left air spring platforms, each said platform providing a load path from the top of the corresponding said air spring to said ladder frame, said lateral member being located aft of the centerline. 
     Wherein said lateral member is attached to the top of each said longitudinal member. 
     Wherein said air spring supports are bottom air spring supports, and which further comprises a pair of top air spring supports, each said top air spring support extending laterally outboard of a corresponding said longitudinal member and providing a load path from the top of the corresponding said air spring to said longitudinal member, each said top air spring support being located aft of the centerline. 
     Wherein said rear axle provides motive power to each said right and left wheels. 
     Which further comprises a wheel chair access platform, and wherein said ladder frame includes a midsection located forward of said right longitudinal member and adapted and configured for attachment to said platform. 
     Wherein said platform is a folding platform, or includes a laterally extending ramp, or includes a wheelchair lift. 
     Wherein said ladder frame includes a pair of midsections each located forward of said corresponding longitudinal member, each said longitudinal member having a top surface that is coplanar with the top surface of the other said longitudinal member, each said midsection having a top surface that is coplanar with the top surface of the other said midsection, and the top surface of said midsection is lower than the top surface of said longitudinal member. 
     Wherein each OEM spring is a leaf spring and each replacement spring is a leaf spring, or wherein each OEM spring is a leaf spring and each replacement spring is a modified OEM leaf spring, or wherein the OEM leaf spring has predetermined number of leaves, and the replacement leaf spring has at least one-half less leaf than the predetermined number. 
     Wherein each OEM spring is a coil spring and each replacement spring is a coil spring. 
     Wherein the OEM coil spring has predetermined number of coils and a predetermined wire diameter, and the replacement coil spring has at least one of fewer coils or a smaller wire diameter. 
     Wherein said providing includes OEM right and left front leaf springs suspending corresponding front wheels from the ladder frame, wherein said lowering the front jounce limit includes replacing each OEM leaf spring with a leaf spring having a reduced spring stiffness. 
     Which further comprises adding right and left front air springs each suspending the corresponding front wheel and acting in parallel with the corresponding reduced stiffness leaf spring. 
     Wherein said providing includes OEM right and left rear air spring mounts, wherein said lowering the rear jounce limit includes moving each rear air spring mount to a position outboard of the corresponding channel member. 
     Wherein said providing includes right and left OEM rear air springs each suspending a corresponding rear wheel from the ladder frame and each having an OEM spring force at a predetermined pressure, and which further comprises replacing each OEM rear air spring with corresponding replacement rear air springs each having a replacement spring force greater than the OEM spring force at the predetermined pressure. 
     Wherein said providing includes an OEM air compressor providing an OEM volumetric flowrate of compressed air at the OEM air pressure, and which further comprises installing a replacement air compressor providing a replacement volumetric flowrate at the OEM air pressure that is greater than the OEM volumetric flowrate. 
     Wherein said installing a replacement air compressor is behind the cab. 
     Which further comprises installing a compressed air heat exchanger proximate to a dropped height midsection at a location between the top and bottom surfaces. 
     Wherein the heat exchanger is a tube with a plurality of longitudinally arranged external fins. 
     Which further comprises lowering the rear jounce limit for each rear suspended wheel. 
     Wherein the OEM frame permits an OEM range of travel of the OEM air springs from typical operation to full compression when deflated, and which further comprises modifying the OEM ladder frame to permit a replacement range of travel of there placement air springs from typical operation to full compression when deflated that is greater than the OEM range of travel. 
     Wherein said modifying the OEM ladder frame includes moving up and outboard the top of the rear air spring support. 
     While the inventions have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.