Abstract:
A compact tailgate accessory for assisting in the one-handed safe raising and lowering of the tailgate and to permit easy removal of the tailgate includes a counterbalance module tucked in-between the tailgate and the truckbed sides in a pocket so as to be hidden from view. The module counterbalances the tailgate so that raising and lowering the tailgate can be controlled easily by one hand, with the linkage sharing one or more common attachment points with the detachable tailgate holding straps to facilitate installation. Sharing the common attachment points with the detachable tailgate holding straps also facilitates removal of the tailgate. The accessory provides the ability to lower or raise a tailgate by a small person or non-muscular individual such as a woman with one hand so that the bed of the truck can be easily and safely loaded or unloaded, the tailgate being maintained in the down position by its own weight. The tailgate accessory permits one handed control of the tailgate with minimal danger of wrist injury, knee injury or injury to any object or person below the tailgate when the tailgate is unlocked. In one embodiment, a specialized spiral wound spring is used which is compact enough to fit in the cavity between the tailgate and the truckbed, yet strong enough to provide the desired counterbalance force. In another embodiment a pair of scissor links are spring loaded at the joint where the scissors are pivoted to counterbalance the weight of the tailgate. In a further embodiment, a lost motion device permits partial opening of the tailgate to a secure intermediate position so that the tailgate is prevented from crashing down. In a still further embodiment, a gas spring is used to counterbalance the tailgate.  
     A tailgate counterbalance employs lost motion and pretension of a torsion bar to achieve both a safety stop position that arrests the tailgate at about 15 degrees as well as reducing the effort required to lift and lower the tailgate beyond that point.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates the controlled opening and closing of tailgates and more particularly to a compact counterbalancing module for controlling the opening and closing of the tailgate.  
         BACKGROUND  
         [0002]    As will be appreciated, tailgates for pickup trucks typically range in weight from 40 to 70 pounds and are hinged at the rear of the truckbed such that when the tailgate is unlocked by the pulling of a lever or handle, the tailgate opens to a point level with the truckbed where it is restrained usually by detachable folding straps. Typically, the tailgates can be removed from the truck body when the tailgate is pivoted to a partially open tailgate removal position.  
           [0003]    The weight of the tailgate precludes one handed operation for any but the strongest individual. People of normal build have to use their second hand, or shoulder or knee to support the tailgate in an effort to kept it from dropping too quickly.  
           [0004]    The single-handed supporting of the tailgate often times can result in wrist injury as the weight of the tailgate overpowers the ability of the individual to control its descent and it comes crashing down in an uncontrolled fashion. Normally a person drops the gate or sometimes raises his or her knee in order to arrest the fall of the tailgate.  
           [0005]    The raising of one&#39;s knee to prevent the tailgate from dropping has reportedly resulted in crushed knee caps or other damage to the knee, whereas supporting the relatively heavy tailgate through one handed operation has resulted in sprained wrists due to the inability of the individual to hold the heavy tailgate with one hand.  
           [0006]    It will be appreciated that a free falling tailgate can, in fact, crush anything that is underneath it and can, for instance, hurt children who are standing at the back of the tailgate when it falls. Moreover, it is hard to close a heavy tailgate.  
           [0007]    Note that pickup trucks and vehicles with tailgates are now more popular in suburban settings in which the tailgate is to be lowered, for instance, to load groceries into the back of the pickup. Pickups are also utilized routinely in camping and the provision of camper tops for the pickups has increased their popularity for carrying lighter loads than a pickup is normally designed to carry. This popularity has resulted in the use of these trucks by non-professionals such as family members and indeed anybody associated with the family. Thus, it is important that the tailgate be counterbalanced in such a way that it not cause damage to one trying to lower it and must afford the ability to close the tailgate in a single handed operation easily even by a small person, yet permit rapid removal of the tailgate and the counterbalance mechanism from the truck body.  
           [0008]    In the past, tailgates have been counterbalanced by internal torsion bars which are factory installed such as illustrated by U.S. Pat. No. 5,358,301. Since these counterbalancing mechanisms are only installed in a limited number of vehicles, there is a necessity for providing an after-market device to be able to counterbalance the tailgate.  
           [0009]    As illustrated by U.S. Pat. No. 5,988,724, issued to Tiete O. Wolda, a torsion bar or torquing device called a torque rod is utilized in the tailgate hinge mechanism such that the tailgate has a torque rod which assists in the opening or closing of the tailgate. While the Wolda patent indicates that his device is suitable for after-market installation, it requires the removal of the hinge bracket, which previously was bolted to the truck&#39;s tailgate. However, presently, the hinge brackets are often spot welded to the tailgate, making after-market utilization impossible.  
           [0010]    In an effort to address the after-market, there are those who have tried to use coil springs to counterbalance tailgates. However, coil springs are much too cumbersome and unsightly to be attached to tailgates. One such attempt to use coil springs is illustrated by U.S. Pat. No. 5,954,383, in which a spring-loaded pulley arrangement along with a tether is utilized to assist in the lowering and raising of a tailgate. However, such a lift kit involves the mounting of a channel that houses the pulleys and a sizable spring which obstructs the loading area and is so big that it is difficult to merchandise, for instance on a rack display. Moreover, the device is cumbersome to install and is unsightly.  
           [0011]    There is thus a need for an extremely compact easy to attach a counterbalance device that can be hidden from view and yet has the strength to provide the required counterbalance force.  
           [0012]    By way of further background, there are a number of devices, which limit the positions of a tailgate, one of which is shown in U.S. Pat. No. 4,585,265. However, there is no counterbalancing action in such a positioning device.  
           [0013]    Additionally, there are a number of devices, which are utilized to reduce the aerodynamic drag caused by a tailgate by partially opening it. One such device is illustrated in U.S. Pat. No. 5,645,310, in which a strap keeps the tailgate at a predetermined angle so as to minimize the aerodynamic load. Note that in this device there is no spring loading or any attempt to counterbalance the tailgate.  
           [0014]    A further air drag reducing tailgate device is shown in U.S. Pat. No. 5,234,249 issued to Jay I. Durrell in which the tailgate is positioned responsive to the force of the wind or air pressure on the tailgate panel. This is done through the utilization of a dashpot type device in which a conventional door closure device is utilized to position the tailgate.  
           [0015]    It will be appreciated in the above-mentioned patent that there is no attempt to counterbalance the tailgate during opening or, in fact, facilitate opening and closing of the tailgate. In this patent the spring tension is set to be responsive not to the weight of the tailgate but rather to the aerodynamic pressure on the tailgate which pressure exceeds counterbalancing forces. The reason is that not only must the weight of the tailgate be accommodated through the tensioning mechanism; it must counteract the force on the tailgate presented as a cross-section to the wind.  
           [0016]    Wind pressures on a tailgate of 60″×20″ at 75 miles an hour exceed 150 pounds. On the other hand, the tailgate itself weighs, for instance, 50 pounds so that the tension set by the Durrell device would have to be three times that necessary for counterbalancing. The result is that with this device installed it would be virtually impossible to open the tailgate at all. As a result, the Durrell device must be disconnected in order to lower the tailgate. To this end, Durrell describes that its links can be quickly disassembled so that the tailgate can be lowered for loading of the truck. In short, although the Durrell reference utilizes a door closure apparatus for providing that the tailgate be partially opened in the presence of wind loading it does not aid in lowering and raising the tailgate to load or unload the truck.  
         SUMMARY OF THE INVENTION  
         [0017]    In contradistinction to the above noted systems for controlling the lowering and raising of a tailgate, in the subject system a compact easily hidden and easily installed accessory provides a counterbalancing force for the tailgate that does not interfere with removal of the tailgate. The counterbalance force is set to be less than the weight of the tailgate at its lowest point so that once lowered, the tailgate remains down.  
           [0018]    In one embodiment the counterbalance force is provided by a specially constructed spiral wound spring which although compact and hide able provides an unusual amount of counterbalance force. The spring is contained within an easily mounted module in a compartment to either side of the tailgate, between the tailgate and the sides of the truck bed. Although in spiral wound form, the spring pressure is sufficient to permit the counterbalancing of at least a portion of the weight of the tailgate. The spring permits controlled lowering of the tailgate at a safe rate as well as assisting lifting the tailgate by smaller, weak individuals, with the spring permitting a one-handed operation.  
           [0019]    The spring in one embodiment is a 2″ diameter coil made of 4½ wraps of 0.098″ thick×0.500″ wide Type 1095 Carbon Steel which although compact provides 35 foot pounds of torque. The use of the spiral wound spring makes possible a small module that can be tucked away between the tailgate and the side of the truckbed where it is out of sight and unobtrusive.  
           [0020]    In another embodiment, a concealed gas spring is used which when the tailgate is up is hidden between the tailgate and the truck bed side. The gas spring is set up so as to provide a counterbalance force that is insufficient to raise a lowered tailgate when fully lowered. The use of the gas spring has an additional advantage. Because of the damping action associated with the gas spring, its use for controlling tailgates means that even with handsoff, the tailgate will descend slowly. Thus once the tailgate is unlocked, even unattended, the tailgate will not come crashing down. Moreover, the top of the gas spring is offset for clearance for the usual safety strap that keeps the tailgate level when lowered. As an added advantage, concealment of the gas spring when the tailgate is up prevents contamination from loads hauled by the truck and general road debris.  
           [0021]    The subject module is designed as an after-market package and can be bolted into place quite easily by non-professionals without drilling using the existing attachment bolts provided for the detachable folding straps and/or the tailgate hinge bracket on the tailgate side and the existing quick connect bolt on the truck side. The tailgate and the subject module can be easily lifted off the truck body without the use of tools by simply unclipping the detachable folding straps and the subject module from the quick connect bolts on the truck side. Its use is therefore attractive to suburban family members and non-professionals who are utilizing the pickup truck not only to haul light loads but also as a convenient family vehicle.  
           [0022]    The subject system thus permits the lowering of the tailgate in a single handed operation due to the fact that the majority of the weight of the tailgate is counterbalanced by the restoring force of the spring. This precludes the necessity of utilizing either a person&#39;s other hand to hold the tailgate or another portion of the person&#39;s body.  
           [0023]    Since tailgates are usually exposed to dust and dirt as well as mud and other grime, single-handed operation is useful so as to not soil one&#39;s clothes when operating the tailgate. Additionally, the single-handed operation of the tailgate made possible through the subject invention eliminates the problem of strain on wrist ligaments and spraining of one&#39;s wrist when unlocking the tailgate. Due to the tendency of one to use one&#39;s knee to prevent the tailgate from falling, injury to one&#39;s kneecap is avoided when using the subject invention.  
           [0024]    Finally, injury to small children standing under the tailgate is eliminated due to the controlled lowering of the tailgate through the single-handed operation described above.  
           [0025]    In another embodiment, a lost motion device is employed so that the tailgate can be cracked open freely; but is then maintained in a safe slightly open position by the use of a lost motion linkage in combination with a sufficient degree of pretension of the counterbalancing spring. One reason for the need for the lost motion device is that often the tailgate handle gets stuck due to road debris, mud or other contaminants which cause the tailgate handle to jam. In the process of pulling at the handle to free it, sometimes the tailgate is released suddenly and comes crashing down. The lost motion device permits an individual to yank at the handle to get it unstuck and yet keep the tailgate up until it is desired to lower it. In short, the lost motion device prevents free fall crash and provides a safety stop position no matter how the tailgate is opened.  
           [0026]    The subject accessory thus provides safety to protect wrists, knees and small children. Its ease of use permits one-handed operation. The module is unobtrusive and does not protrude into the bed of the truck. Because of its design it promotes simple aftermarket installation with no drilling required and permits tailgate removal without the need to demount the module or use tools. Moreover when a lost motion linkage is employed it takes the worry out of opening the tailgate and any possible crash. Once the tailgate handle is lifted, the tailgate opens only partially and stays there even if the handle is let go. Thereafter the individual is free to lower the tailgate in a counterbalanced controlled fashion.  
           [0027]    In summary, a comnpact one-handed tailgate accessory for assisting in the safe raising and lowering of tailgates includes a module tucked in-between the tailgate and the truck bed sides in a pocket so as to be hidden from view, in one embodiment having a spring loaded linkage which counterbalances the tailgate so that raising and lowering the tailgate can be accomplished with one hand, with the linkage sharing one or more common attachment points with the detachable holding straps of the tailgate to simplify installation of the module and facilitate removal of the tailgate together with the module and the holding straps from the truck body. The accessory provides the ability to lower or raise a tailgate by a small person or non-muscular individual such as a woman so that the bed of the truck can be easily and safely loaded or unloaded, the tailgate being maintained in the down position by its own weight. The tailgate attachment prevents wrist injury, knee injury and injury to any object or person below the tailgate when the tailgate is unlocked. Raising the tailgate is assisted by the spring loading. In one embodiment, a specialized spiral wound spring is used which is compact enough to fit in the cavity between the tailgate and the truckbed, yet strong enough to provide the counterbalance force. In another embodiment a pair of scissor links are utilized with spring loading at the joint where the scissors are pivoted to counterbalance the weight of the tailgate. In a further embodiment, a lost motion device permits free opening of the tailgate but then arrests it in a safe partially opened position to prevent free fall crash no matter how the tailgate is opened. In a still further embodiment, a gas spring is used to counterbalance the tailgate, with the end of the gas spring cylinder attached to the truck bed side offset to provide clearance in one embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings, of which:  
         [0029]    [0029]FIG. 1 is a diagrammatic illustration of an individual seeking to lower a tailgate in a one handed operation in which the other hand of the individual is otherwise occupied;  
         [0030]    [0030]FIG. 2 is a diagrammatic illustration of the individual of FIG. 1 in which the individual has released the tailgate by pulling on the release handle in the center of the tailgate;  
         [0031]    [0031]FIG. 3 is a diagrammatic illustration of the individual of FIGS. 1 and 2 illustrating the downward motion of the tailgate putting pressure on the individual&#39;s hand;  
         [0032]    [0032]FIG. 4 is a diagrammatic illustration of the individual of FIGS.  1 - 3  in which the entire weight of the tailgate is born by the wrist of the individual, thereby causing the individual wrist pain;  
         [0033]    [0033]FIG. 5 is a diagrammatic illustration of the individual of FIGS.  1 - 4  indicating the use of the individual&#39;s knee to support the tailgate when the individual cannot support its weight single-handedly, thereby potentially causing injury to the individual&#39;s knee;  
         [0034]    [0034]FIG. 6 is an exploded view of one embodiment of the subject invention in which scissored linkage arms are spring loaded at the pivot point of the scissored linkages through the use of a spiral wound spring having one end anchored at the pivot point of one of the linkages and its other end anchored to a point offset from the rotational center of the linkage;  
         [0035]    [0035]FIG. 7 is a diagrammatic illustration of the module of FIG. 6 installed in a pocket between a side of the tailgate and a side of the truckbed, illustrating the position of the scissored linkages and the position of the spiral wound spring;  
         [0036]    [0036]FIG. 8 is an exploded view of a second embodiment of the subject invention in which one of the linkages is secured to an end of the tailgate, with the spiral wound spring located adjacent to the pivot point of the tailgate to the truckbed and (the other linkage is attached to the truck with) a lost motion device which allows the tailgate to open slightly and then come to a stop  
         [0037]    [0037]FIG. 9 is an exploded view of another embodiment of the subject invention in which the customary support cables are replaced with a set of support brackets and a lost motion device is utilized at one end of a scissor linkage system which allows the cracking or slight opening of the tailgate until it reaches a predetermined stop;  
         [0038]    [0038]FIG. 10A is a diagrammatic illustration of the lost motion scissor linkage system of FIG. 9 folded completely in the fully up position of the tailgate;  
         [0039]    [0039]FIG. 10B is a diagrammatic illustration of the lost motion device of FIG. 10A at the end of its travel allowing the tailgate to be freely opened 12 degrees initially;  
         [0040]    [0040]FIG. 10C is a diagrammatic illustration of the tailgates of FIGS. 10A and 10B illustrating the utilization of the spring loaded module to counterbalance the weight of the tailgate once the tailgate is moved past 12 degrees;  
         [0041]    [0041]FIG. 10D is a diagrammatic illustration of the tailgate assembly of FIGS. 10A, 10B and  10 C illustrating the position of the tailgate when it is down in a horizontal position;  
         [0042]    [0042]FIG. 11A is a diagrammatic illustration of the utilization of a gas spring in which the gas spring cylinder is contained within a pocket between the tailgate and the trunk bed side when the tailgate is in the up position;  
         [0043]    [0043]FIG. 11B is a diagrammatic illustration of the gas spring embodiment of FIG. 11A in which the gas spring provides the counterbalancing forces to permit the lowering of the tailgate;  
         [0044]    [0044]FIG. 12 is an isometric exploded view of a tailgate with the outside panel partially cut away to show the layout of the working parts of the invention;  
         [0045]    [0045]FIGS. 13, 14,  15  and  16  are enlarged isometric views to show the construction of the various working parts in detail; and,  
         [0046]    [0046]FIG. 17 is a graphical representation of the forces needed to raise and lower the tailgate with and without the invention as a function of the opening angle of the tailgate. 
     
    
     DETAILED DESCRIPTION  
       [0047]    Referring now to FIG. 1 the subject device to be described is utilized to assist in an individual  10  to lower a tailgate  12  which is pivotally attached at its base to a truck bed  14  of truck  16 . Here it can be seen that a handle  20  is grasped by an individual&#39;s had  22 , with the upward lifting of the handle  20  causing the tailgate to become unlatched and be free of the sides of the truck bed. It will be appreciated that in the individual&#39;s other hand, here shown at  24  a pail  26  of material  28  is being held while the individual is seeking to lower the tailgate.  
         [0048]    Referring to FIG. 2 in which like reference characters are applied between the Figures, tailgate  12  is illustrated in position  12  prime such that the tailgate, may be as much as 60 pounds, moves in the direction of arrow  30  once it has been released by handle  20 .  
         [0049]    Referring to FIG. 3 as the tailgate  12 ″ descends further as illustrated by  12 ′″, the wrist  32  of individual  10  begins to bear the weight of the tailgate as the tailgate moves in the direction of arrow  30 , with the position of hand  22 ′″ showing a backwards and downwards slant thereby putting pressure on the tendons of wrist  32 . The reason for the pressure on wrist  32  is the weight of the tailgate that is now in free fall.  
         [0050]    As the tailgate descends to a nearly horizontal position as illustrated  12 ″″, hand  22  is bent backwardly causing severe pain at wrist  32  due to the full weight of the tailgate being born by the individual&#39;s hand. So intense may be the pain that the individual simply lets go of the tailgate which causes the tailgate to come crashing down.  
         [0051]    As seen in FIG. 5, individual  10  may seek to limit the free fall of tailgate  12  by raising his knee, here shown at  34  to brace a portion of the tailgate. By so doing, the individual has a chance of being injured if the tailgate falls on the knee during this process.  
         [0052]    What will be appreciated in connection with FIGS.  1 - 5  is that single-handed operation of the tailgate is difficult, if not impossible, for anyone other than the strongest individual and may cause injury to a normal individual seeking to open the tailgate.  
         [0053]    Also because of the weight of the tailgate once it is down, it is difficult to raise the tailgate single-handedly assuming the individual has something else in his other hand.  
         [0054]    There is therefore a need, as mentioned above, to be able to counterbalance the tailgate during the lowering thereof and to also assist in the raising of the tailgate. As we will be seen, the purpose of the spring loading is to permit greater control by the individual utilizing one hand so that the descent of the tailgate can be controlled quite easily in a one handed operation.  
         [0055]    Not only is it important to be able to control the lowering of the tailgate in a controlled fashion, it is obviously important to be able to raise the tailgate easily, since the tailgate is quite heavy. Moreover there is a need to provide a counterbalancing unit which is easily mountable to the truck and tailgate so that the unit may be offered as an after-market device which while it is easy to install, also does not preclude the removal of the tailgate from the truck bed. Usually the tailgates are lowered to a pre-determined angle and a pinion which has a flat side permits the socket into which the pinion protrudes to be slid off of the pinion. The upper end of the retaining strap is unclipped from the attachment to the side wall of the truck bed and the tailgate can then be lifted off the truck. It will be appreciated that by virtue of this arrangement the entire operation can be performed without tools. It is therefore important to be able to provide a device which stays with the tailgate and is easily removed in terms of attachment from the side wall of the truck bed so that when the tailgate is removed the counterbalancing device is likewise removed.  
         [0056]    Referring now to FIG. 6, in one embodiment, a counterbalancing unit or module  40  includes for each side of the tailgate two spring loaded scissor linkages  42  and  44 , each installed alongside a flexible brace  46  that customarily holds the tailgate in a horizontal position when the tailgate is down. The sidewalls of the tailgate are customarily spaced away from the inside faces of the truck sides and the gaps are covered up by the lips of the tailgate shown in FIG. 7 at  48 . Brace  46  and scissor linkages  42  and  44  mount inside these gaps and are hidden behind the lips  48  when the tailgate is closed (not shown in this figure). The arrangement of braces  46  and scissor linkages  42  and  44  is entirely symmetrical with respect to the left and right side of the tailgate. For clarity, the following description will refer only to one side, it being understood that all features pertain to both sides.  
         [0057]    Brace  46  is generally made out of a flexible cable. At one end, brace  46  attaches to the inside face  50  of the truck side wall  52  with a mounting bolt  54  that secures to the inside face  50  and allows that the end of brace  46  pivot freely. At the other end, brace  46  attaches to the outside wall  60  of the tailgate with a shoulder bolt  62  that secures to outside wall  60  and allows this end of brace  46  to pivot freely. Scissor linkages  42  and  44  attach to the same mounting bolt  54  as the brace  46  and, using washer  66 , to the same shoulder bolt  62  as brace  46 .  
         [0058]    The conventional construction of brace  46  as supplied by the manufacturer consists of a flexible cable  70  that is terminated with a round eyelet  72  swaged to one end and a slotted eyelet  74  swaged to the other end. Slotted eyelet  74  has an elongated slot  76  with an enlarged circular opening  78 . Spring clip  80  has legs  82  that fit around shank  84  and position it so that a flexible tab  86  covers circular opening  78 . Mounting bolt  50  is of conventional construction as supplied by the manufacturer and has a head  90  that can fit through circular opening  78  and a groove  90  sized to receive elongated slot  76  adjacent to a boss  94 . Shoulder bolt  62  is also of a conventional construction as supplied by the manufacturer and has a shoulder  98  sized to fit into round eyelet  72  and an enlarged flange  96  sized to retain round eyelet  72 .  
         [0059]    Washer  66  is supplied to pivotally attach scissor linkage  42  with shoulder bolt  62  and has an internal diameter to fit over the threaded portion of shoulder bolt  62 , an outside diameter equal to that of its shoulder  98  and is wide enough to accommodate the added thickness of the lower scissor link  42 .  
         [0060]    Thus, scissor linkage consists of an upper scissor link  44  and a lower scissor link  42 . They are pivotally joined by shoulder rivet  100  and are spring loaded by spiral spring  102 . Shoulder rivet  100  has a round end  104  with flats  106 , shoulder portion  108  and a head portion  110  with a slot  112 . Lower scissor link  42  has a circular hole  114  sized to fit shoulder  98  of shoulder bolt  62  at one end and another hole  116  with flats  118  sized to firmly hold shoulder rivet  100  at the other end. Upper scissor link  44  has a hole  120  sized to fit over the boss  94  mounting bolt  50  at one end, a hole  122  to receive the post rivet  124  and a hole  126  sized to pivot freely on shoulder portion  104  of shoulder rivet  100  at the other end. Lower scissor link  42  may be longer than upper scissor link  44  since the shoulder bolt  62  is usually located above the mounting bolt  50  when the tailgate is closed.  
         [0061]    The combined length of scissor links  42  and  44  must be greater than the length of the brace  46  so that the included angle between the two scissor links is less than 180 degrees in the open position of the tailgate.  
         [0062]    The spiral spring  102  fits over the head portion  108  of shoulder rivet  100  and has a flat  130  at the inside end that fits into the slot  112  of shoulder rivet  100 . The outside end of the spiral spring  102  has a hook  132  that loops over a post rivet  124  of upper scissor link  44 .  
         [0063]    The number of turns of the spiral spring  102  must be chosen such as to permit the required scissor opening plus any desired preload without exceeding the elastic stress limit of the spring material to avoid a permanent set or a premature stress failure of the spring. As will be seen hereinafter, preloading is important when using the lost motion device of FIGS. 9 and 10.  
         [0064]    The installation of the scissor linkages  42  and  44  is simple and straightforward. The truck owner lowers the tailgate, unclips slotted eyelet  74  from mounting bolt  50  and unscrews the shoulder bolt  62 . He inserts the shoulder bolt  62  through hole  114  of lower scissor link  42 , adds washer  66 , eyelet  72  and re-installs shoulder bolt  62  into the tailgate, then slips hole  120  of the upper scissor link over the boss  94  of mounting bolt  50  and clips the slotted eyelet  74  over it. Once installed, the scissor linkage functions without any need for attention. It folds and opens in tandem with brace  46  as the tailgate is closed and opened. As lower scissor link  42  opens, it turns the shoulder rivet  100  relative to upper scissor link  44  transferring this rotation to the flat  130  of spring  102  and winding up the spring.  
         [0065]    The torque exerted by the spring  102  is thus used to counterbalance the weight of the tailgate. The stiffness and preload of the spring can be chosen to achieve the degree of counterbalance desired.  
         [0066]    In order to remove the tailgate from the truck, the linkage may be unclipped from the mounting bolt  50  in the conventional manner by lifting the tab  86  of the spring clip  80  and sliding the brace  46  and the upper link  44  over the head  90 .  
         [0067]    In an alternative embodiment shown in FIG. 8 the scissor linkage has a link  200  fixedly attached to tailgate  12  via bolts  202  which go through holes  204  in the linkage and holes  206  in a socket bracket  208 , which has a cup at its lower end which is notched as illustrated at  212 . Note cup  210  is adapted to receive arbor  214 , which in a preferred embodiment is oblong as illustrated.  
         [0068]    It will be appreciated at the upper part of linkage  200  provides a lever arm vis a vis the point of attachment of this linkage to the tailgate, with the upper portion having a hole  116  having flat sides  118  adapted to receive the flats  106  of slotted shoulder rivet  100 .  
         [0069]    An offsetting link  220  having holes  222  and  224  therein is pivotally attached to link  219  via a rivet or bolt through hole  120 . A link  230  is provided which is pivotally attached to link  220  via a bolt or rivet through hole  232 . Link  230  is provided with a large bore  234  sized to slip over the head  90  and pivot on shoulder  94  of bolt  50  which is attached to the truck bed side through a hole  238 . Link  230  is retained in this position by the slotted eyelet  242  of the restraining strap  240  which clips into the groove  92  of bolt  90  in the conventional manner, with enlarged hole  246  slipping over head  90  and with slot  248  residing in groove  92 . Restraining strap  240  can be easily detached from the truckbed side by removal of eyelet  242  through the use of hole  246 . Note that the other end of strap  240  is secured to tailgate  12  eyelet  244  and bolt  245 . Links  220  and  230  allow for a lost motion between the scissor linkage  40  and the truck bed side  254  to permit the tailgate to be cracked freely before the scissor linkage exerts any force. As an added feature, the lost motion facilitates the attachment of the module to bolt  50  during installation as it offers a range of attachment positions without having to overcome the tension of the scissor links.  
         [0070]    It will be appreciated that in this embodiment spring  102  is carried close to the base of the hinged tailgate. Note that in this embodiment one of the scissor arms is fixed to the tailgate and has a lever arm which extends up the side  252  of tailgate  12  thus to provide a relatively compact unit in which both the spring and one of the scissor arms lies between side  252  and side  254  of the truck bed shown in a dotted outline.  
         [0071]    Thus in this embodiment the subject module is tucked into the space between the tailgate and the truck bed side in a pocket defined by lip  48  of the tailgate and side  254  of the truck bed.  
         [0072]    [0072]FIG. 9 shows an embodiment, which has a lost motion device consisting of a single attachment link  360  with an elongated slot  76 . This embodiment shares all the features of the embodiment of FIG. 6 with the following modifications:  
         [0073]    Brace  46  supplied as standard equipment from the factory, is omitted.  
         [0074]    Two links  340  and  342  are added to the linkage. As seen from top to bottom, link  342  attaches pivotally to link  362  with shoulder rivet  344  through holes  346  and  348 . Link  340  attaches pivotally to link  342  with shoulder rivet  350  through holes  352  and  354  at its upper end, and attaches to bolt  62  with washer  358  at its lower end.  
         [0075]    A third link  360  is attached pivotally to the end of the upper scissor link  362  with a shoulder rivet  364  through holes  336  and  368 . At its free end, link  360  is fashioned in the same manner as the slotted eyelet  76  of the brace  46  with an elongated slot and an enlarged circular opening  78 .  
         [0076]    All links may be of different lengths to accommodate the truck geometry; the length of the elongated slot  76  may be extended to provide added degree of lost motion to slide over the mounting bolt  50  without spreading the scissor links  42  and  44  when the tailgate is partially opened as shown in FIGS.  10 A- 10 D. The combined length of links  340 ,  342 , segment  348 - 126  of link  362  and link  361  equals that of former brace  46 . The combined length of links  362  and  361  must be greater than the combined length of links  340  and  342  so that the included angle between the two scissor Inks  362  and  361  is less than 180 degrees in the open position of the tailgate.  
         [0077]    The principle of the linkage of the FIG. 9 embodiment is the same as that of the FIG. 6 embodiment. The differences arising from substituting pivotal links  340  and  342  flexible brace  46  are self-evident. There is a minor simplification in the installation process since the spiral spring  102  biases the link  360  away from the circular opening  78  and the spring clip  80  therefore becomes redundant.  
         [0078]    The addition of link  360  serves as a lost motion device. By pivoting and sliding slot  76  over mounting bolt  50  as shown in FIGS.  10 A- 10 D, the tailgate can be opened partially without spreading the scissor links  361  and  362  at all. This feature may be desirable as it allows some play to actuate the tailgate handle and crack the tailgate open without having to overcome any preload exerted by the scissor linkage. However, in order to maintain the tailgate cracked open, spring  102  is preloaded to prevent the tailgate from lowering past the point dictated by slot  76  until the tailgate is further urged downward to open it. The restoral force of the spring is such as to maintain the tailgate at for instance 12 degrees, given the particular weight of the tailgate.  
         [0079]    In general and referring now to FIGS.  10 A- 10 D, it will be appreciated that what is described is a lost motion device  400  which permits cracking tailgate  12  as illustrated in FIG. 10B by an amount, for instance, 12 degrees, prior to the time that the counterbalancing apparatus takes effect. In this case, as described hereinbefore, a slotted linkage  402  is provided with a slot  404  that accommodates an attachment bolt  406  therethrough.  
         [0080]    It will be appreciated that the lost motion is between, bolt  406  and the end of slot  404 , here illustrated at  408 , in which a travel length L is that which provides for the lost motion.  
         [0081]    As can be seen in FIG. 10B, the tailgate moves from its dotted position at  410  to the position illustrated at  410 ′ in which bolt  406  is now at the end of its travel at the end  408  of slot  404 . It is only at this time that the counterbalancing provided by the subject module comes into play. Because of spring preloading, when at the end  408  of slot  404 , module  40  provides enough restoring force to balance the weight of the tailgate at the pre-selected angle. This means that with hands off the gate simply stays at 12 degrees.  
         [0082]    Referring to FIG. 10C as the tailgate  12  is manually lowered as illustrated at  410 ″. For an opening of 55 degrees there is a 44 degree angle between link  361  and link  362  as illustrated. Here the counterbalancing of module  40  assists in the controlled one-handed lowering of the tailgate.  
         [0083]    Finally as illustrated in  10 D tailgate  12  is fully lowered to a horizontal position as illustrated at  410 ′″, a full 90 degrees from its upright position at  410 , with links  340  and  342  fully extended to limit the downward travel of the tailgate. The angle between link  361  and link  362  is 88 degrees as illustrated.  
         [0084]    It is noted that the lost motion device of FIG. 9 works equally well for the FIGS. 6 and 8 embodiments to provide for a convenient cracking of the tailgate, with the tailgate being maintained in a safe, partially open position by preloading of the spring. To lower the tailgate, the operator pulls the tailgate down gently with one hand until the weight of the tailgate exceeds the counterbalancing moment of the spring and the tailgate comes to rest in the fully open position.  
         [0085]    Referring now to FIGS. 11A and 11B, what is illustrated is a gas spring counter balance in which a gas spring  500  is attached at one end  502  to the side of the truck bed and at another end  504  to tailgate  12 . A safety retaining strap  506  restrains the tailgate in its down position as illustrated in FIG. 11B, with the strap  506  being attached to the truck bed side at  502  and also to the tailgate at  508 . The attachmnent hole  502  may be offset by a distance D to the axis of the gas cylinder so as no to interfere with the safety retaining strap  506 .  
         [0086]    As can been seen in FIG. 11A, the entire apparatus including the gas spring and the strap is contained within the lip  510  of tailgate  12  with strap  506  being shown in its folded condition in the pocket provided between the lip  510  of the tailgate and the top surface  512  of tailgate  12 .  
         [0087]    As will be appreciated, the spring constant of the counterbalance mechanism is such as to be insufficient to return the tailgate to its upright position, with the counterbalance only balancing a portion of the weight of the tailgate. This assures that the tailgate when lowered remains in its down position.  
         [0088]    The attachment hole for ends  502  or  504  may be fashioned as an elongated slot so as to permit several degrees of lost motion before the gas spring is tensioned. Here the attachment hole associated with end  504  is shown elongated by distance L. In combination with the right amount of pre-tensioning of the gas spring, this feature will arrest the tailgate in a partially open position. As the tailgate is lowered by hand beyond this position, the weight of the tailgate overcomes the counterbalance force of the spring so that the tailgate remains lowered in its down position. As an added feature, the elongated slot facilitates attachment of the gas spring during installation, as it offers a range of attachment positions.  
         [0089]    The advantage of the gas spring is that when the tailgate is unlocked it will not crash down due to the fluid damping in the gas spring. Rather the tailgate descends at a controlled rate regardless of the individual touching the tailgate. Its advantages are the same as the one-handed control of the spiral spring embodiment. However, the operation of the lowering of the tailgate can be accomplished in a completely hands off mode due to the damping action of the gas spring.  
         [0090]    Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.  
         [0091]    The layout of the working parts of the invention is shown in FIG. 12 in an exploded view.  
         [0092]    A torsion bar  60  attaches rigidly to the driver&#39;s side hinge bracket  22  with a fastener  66 . Hinge bracket  22  attaches to the side panel  12  of the tailgate. Side panel  12  has an aperture  16  through which the torsion bar  60  passes into the tailgate  10  running along the whole width of the tailgate  10  and protrudes through aperture  18  on the other side panel  14  and through an aperture  34  of the passenger&#39;s side mounting bracket  32 .  
         [0093]    The tailgate hinges on support brackets  42  and  52  which attach rigidly to the sides of the truck (not shown). On the driver&#39;s side the support bracket (see FIG. 15) has a cylindrical boss  44  which mates with the hollow cup  25  on the outside face of the hinge bracket  22  (see FIG. 13). On the passenger&#39;s side the support bracket  52  has a slot  54  which mates with the protruding end  64  of the torsion bar  60 .  
         [0094]    [0094]FIG. 13 also show stud  23  with a hole  24  for rigidly mounting the driver&#39;s side end of the torsion bar  60  to the hinge bracket  22 . Stud  23  is inclined by angle “a” measured from the vertical. The other end of the torsion bar  60  passes through the aperture  34  of the passenger&#39;s side hinge bracket  32 . Aperture  34  has a pair of stops  36  and  38  which restrain the rotation of the torsion bar  60  in the vertical position when rotated counterclockwise (as seen from the passenger&#39;s side), but permits clockwise rotation by the angle “b”.  
         [0095]    [0095]FIG. 16 shows the slot  54  in the passenger&#39;s side support bracket  52 . The slot  54  has two flat sides  56  and  58  and is closed at the bottom to carry the torsion bar end  64  protruding from the hinge bracket  32 . Flat sides  56  and  58  are spread at an angle “c” of usually 15 degrees to afford lost motion travel of the torsion bar within that angle.  
         [0096]    The tailgate  10  installs in the conventional fashion by first sliding the cup  25  over the stud  42  on the driver&#39;s side and then lowering the protruding end  66  of the torsion bar  60  into the slot  54  on the passenger&#39;s side while holding the tailgate at an inclination of approx. 15 degrees. Once installed in the 15 degree position the tailgate can be closed freely due to the lost motion afforded by the sloping sides  56  and  58  of the slot  54 . The torsion bar  60  is mounted at angle “a” on the diver&#39;s side but restrained in the vertical position by stops  36  and  38  so that the torsion bar  60  is twisted, or pre-tensioned by the angle “a”. When the tailgate is opened it can travel freely by amount of angle “c”. To open the tailgate beyond that point requires that the torsion bar is twisted or tensioned. The initial pretension if the torsion bar due to the twist of angle “a” can be sized such the turning moment of the torsion bar exceeds the opening moment of the tailgate due to its weight at the angle “a” position and the tailgate is thus balanced in that “safety” position. To open the tailgate further then requires a gentle pull to overcome the tension of the torsion bar. The opening moment of the tailgate  10  increases in a sinusoidal manner as a function of the opening angle while the retaining moment of the torsion bar  60  increases only linearly. A point can thus be reached as the tailgate  10  is opened further at which the weight of the tailgate  10  overcomes the restrain of the torsion bar  60  and the tailgate falls of its own accord as long as the torsion bar  60  is sized correctly.  
         [0097]    This relationship is illustrated in FIG. 17 in a graphical fashion.. The opening moment due to the weight of the tailgate is shown by a dotted line. For a full sized pickup truck with a tailgate of 50 lbs the opening moment at 15 degrees is approx. 12 ft-lbs and at 90 degrees, or the fully open position, it is about 50 ft-lbs. To achieve the desired behavior the angle “a” should be approx 60 degrees to pretension the torsion bar by that amount, and the ultimate stiffness of the torsion bar should be approx 40 ft-lbs when the tailgate is fully opened and the torsion bar twisted an additional 75 degrees. The restoring spring force of the torsion bar is shown by a negative straight line that starts at −60/(60+75)×40=−18 ft-lbs at 15 degrees and reaches −40 ft-lbs at 90 degrees. The net resulting moment acting on the tailgate as assisted by the torsion bar is shown by a heavy solid line.  
         [0098]    The torsion bar assisted tailgate moves freely up to 15 degrees with a maximum closing force of approx. 8 ft-lbs. At 15 degrees the pretension of the torsion bar exceeds the weight of the tailgate as shown by the force reversal from +8 ft-lbs to −5 ft-lbs at that point. To open the tailgate beyond that point requires a pull of approx. 5 ft-lbs. The required pull diminishes as the tailgate is opened further and reaches 0 at approx 25 degrees. Beyond this point the opening moment of the tailgate exceeds the restraining moment of the torsion bar and the tailgate falls of its own accord.  
         [0099]    The impact of the tailgate as it opens fully is however greatly reduced as it falls through an arc of only 65 degrees (vs. 90 degrees of the unassisted tailgate) and the gravitational pull is absorbed by twisting the torsion bar.  
         [0100]    [0100]FIG. 17 also shows the reduction in effort to close the tailgate. Comparing the dotted line that represents the unassisted tailgate and the heavy solid line that represents the net resulting moment of the torsion bar assisted tailgate, it can be seen that the pull required is reduced from 50 to approx. 10 ft-lbs, a reduction of over 75%.