Abstract:
A vehicle suspension comprising a pair of leaf springs arranged to extend longitudinally of the frame of a vehicle on opposed sides thereof with each leaf spring having one of its ends pivotally connected to the vehicle frame at a fixed location and which have an axle secured thereto between the ends thereof. A first bracket rigidly attaches each leaf spring at or adjacent the fixed location where the end of the leaf spring is connected to the vehicle frame. A radius arm extends longitudinally on each side of the vehicle frame and has one end thereof pivotally connected to the first bracket in a spaced-apart relationship to the fixed location. A second bracket is rigidly attached to the axle and is connected to the other end of the corresponding radius arm in a spaced relationship to the axle. Modified versions of the suspension are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from PCT application Serial No. PCT/US2005/016269 filed May 10, 2005, and published as International Publication No. WO 2006/121438 on Nov. 16, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a vehicle leaf spring suspension with radius arms. 
       BACKGROUND OF THE INVENTION 
       [0003]    In vehicle suspensions where the axles are mostly located and controlled by leaf springs, many compromises have to be made. Such compromises can adversely reflect on the suspension performance under various loading conditions to which it is subjected during the operation of the vehicle. 
         [0004]    When a vehicle axle is loaded and located with “symmetrical” and “conventional” leaf springs, the axle will deflect, on the suspension, without any change in angle, if any effect from the rear location is ignored. 
         [0005]    Throughout this specification, the terms “symmetrical” and “conventional” are as used generally in the suspension industry and defined in the SAE Leaf Spring Design Manual SAE J788. 
         [0006]    Conventionally, an axle is located by a fixed front eye and an effective link provided by a forward section of the leaf spring between the front eye and axle, with the centre section of the leaf spring moving up and down on the two effective links of equal length fore and aft of the axle. As the position of the front eye is fixed, any point around the axle moves on an effective parallelogram linkage. As the axle angle does not change the effective instantaneous centre of the axle assembly is at infinity. 
         [0007]    There are many advantages or characteristics associated with this type of installation, one being that linkages or drivelines connected to the axle, such as propeller shafts and steering rods, can maintain constant or equal angles during the suspension deflections, namely, the axle does not revolve as the suspension deflects. 
         [0008]    The braking and traction forces acting on the vehicle effectively act at the tyre ground contact point and create a torque around the spring front location eye. This is resisted by the axle to vehicle suspension system. 
         [0009]    In this context, another important advantage with a symmetrical leaf spring suspension system is that this torque will not create lifting or lowering forces or motions, which means that the suspension height stays as a linear function of the vertical imposed weight. The suspension will not create any dive or squat under braking or traction forces with the effective instantaneous centre point at infinity. Therefore, any braking, traction or load sensing control devices, working from the suspension height or pressure, will stay more accurate and effective under these braking and traction loads. On softly sprung vehicles, spring wind-up can cause problems. As a result of the above features, symmetrical leaf spring suspensions can absorb this wind-up more readily. 
         [0010]    A further advantage with symmetrical leaf springs is that the vertical stiffness of the suspension, under direct vertical loads, is a linear summation of the rates of the two leaf spring cantilevers, this being the most efficient use of the spring material. 
         [0011]    A characteristic, and sometimes a disadvantage of symmetrical springs, is that the anti-roll stiffness of the suspension is just a function of the spring vertical and torsional stiffnesses and the spring spacing width across the vehicle. 
         [0012]    As leaf springs become less symmetrical, with one cantilever being stiffer than the other, the axle has to change its angle as the suspension deflects. The effective links for and aft of the axle are now of unequal lengths and each cantilever deflects by a different amount. In moderate amounts, this lack of symmetry can usually be accommodated without any major compromises. As the instantaneous centre of the axle area now becomes finite and the effective swing arm length becomes shorter, there is a degree of axle lift or squat under braking or traction forces. This can normally be accommodated in full leaf spring suspension applications. 
         [0013]    The vertical stiffness of the suspension for ride quality is now stiffer than the sum of the stiffnesses of the two spring cantilevers and is therefore less efficient. Also, softly sprung vehicles are often more sensitive to wind-up under braking or traction. 
         [0014]    During vehicle roll, the leaf springs on each side of the vehicle deflect in different directions. A characteristic with asymmetrical springs that sometimes is an advantage is that, during roll, the axles try to change their angles in different directions. As the axles are normally relatively torsionally stiff, the axle will not allow any change in the angle, and this distorts the springs, making them effectively stiffer. This creates extra anti-roll stiffness when using asymmetrical leaf springs. 
         [0015]    This characteristic is taken to extremes when leaf springs are used to locate axles with offset air springs. This type of suspension effectively creates a very asymmetrical spring installation, with the rear, air spring cantilever being much softer than the front, locating leaf spring cantilever. 
         [0016]    As air springs have virtually no internal friction, air spring suspensions require very high extra anti-roll stiffnesses. This high roll stiffness is required for both cornering, roll control and straight ahead vehicle stability. The required high anti-roll stiffness is usually created by using a very stiff, front leaf spring cantilever, with the previously discussed axle twisting effect. 
         [0017]    In many cases, the front leaf spring cantilever effectively just becomes a radius arm. This means that the instantaneous centre of the axle area is virtually at the front eye. This has several disadvantages, one being that the system&#39;s deflection stiffness becomes high. If the leaf spring cantilever is effectively rigid, the system&#39;s stiffness is the air spring stiffness increased by the square of the distance (La) which the air spring is spaced from the front eye divided by the front cantilever length (Lc). This means that the air spring has to be very soft to create only a moderate, suspension softness. This is very inefficient and expensive. 
         [0018]    Thus as the instantaneous centre of the system is at the front eye:— 
         [0000]      Axle vertical stiffness=air spring stiffness×( La/Lc ) 2    
         [0019]    Also, under braking and traction forces, there tends to be a very high axle lifting or squatting effect due to the very large difference in deflection between the cantilevers and short effective axle instantaneous centre arm. This leads to adhesion instability and makes the electronic or other accurate, load height sensing for braking and traction very difficult. It can also lead to adverse propeller shaft, universal joint angles, which increases vehicle vibrations and creates poor drive system durability. 
         [0020]    To reduce these braking and traction height and axle angle changes, many of these air suspension applications have lowered front eyes. This reduces the moment distance from the ground contact point and, thus, the moment which the suspension has to absorb. 
         [0021]    However, this creates structural problems in the frame suspension attachment area. The braking force acts through the front eye and as this is now at an increased distance from the vehicle frame, the frame, the front eye and bracket fastenings to the frame, are subjected to much higher loads. This requires stronger and wider spaced frame fasteners and mountings, with the frame and cross members usually requiring extra reinforcements. 
         [0022]    Many air suspension applications are options to standard steel, leaf spring suspensions. The need to lower the front eye for these designs requires special frame assemblies for the optional air suspension, leading to production line complications which often make air suspension options very expensive. 
         [0023]    If the suspension&#39;s anti-roll stiffness could be provided by means other than the above “axle twisting” effect, the front leaf spring cantilever could be softened. This would immediately improve the suspension&#39;s performance. The effective instantaneous centre line would increase in length and be well forward of the front eye. This would soften the suspension by both the softer front cantilever deflection and by making the air spring rate closer to the axle suspension rate. Adding an extra, conventional anti-roll bar system to an air suspension, to retain the required high anti-roll stiffness, makes these air suspension systems very heavy and expensive. 
         [0024]    There is also another problem which often prevents the above improvement to the suspension from being adopted. As previously discussed, on a leaf spring suspension, the spring and axle wind-up under braking and traction Is a function of the moment created by the force at the ground contact and the vertical distance to the front eye. This moment is absorbed and reacted to over the length of the overall leaf spring. As stated above, most leaf spring applications can absorb this moment, although the wind-up can limit the degree to which the springs can be softened, especially with short spring installations or high spring installations, such as with solid front drive axles. 
         [0025]    Considering some air spring applications, the effective length of the leaf springs is severely reduced from a full leaf spring suspension to one with an air suspension. This effectively increases the forces on each leaf spring cantilever, whereby another design pressure contributes to the expensive lowering of the front eye. 
         [0026]    An attempted solution to this wind-up problem which is sometimes currently employed, is to build in an anti-wind-up linkage. However, this type of linkage creates more high frame loading problems than the attempted “lowering of the front eye” improvement. 
         [0027]    A braking force at the ground contact point would require a reaction of at least twice the braking force along the link and a force at least equal to the braking force at the spring eye. The offset frame bracketry required to support and control the link would need to be more substantial than that required for just lowering the front eye. 
         [0028]    Another well known air suspension arrangement has the normal full length leaf spring replaced by a lighter leaf spring which locates the axle and often just carries the unladen vehicle suspension loads. The air spring which is usually located close to the axle centre line, carries the remainder of the load applied to the suspension when the vehicle is loaded. Thus, this suspension has a leaf spring and air spring operating in parallel. 
         [0029]    With these arrangements, the friction from the leaf spring is again very limited and the high extra anti-roll force is again required. Also, the leaf spring still provides solely the only resistance to braking and traction wind-up. As this leaf spring carries only part of the load, it now is so much lighter than the original, solo leaf spring. Therefore, axle wind-up control is now a major problem. 
         [0030]    Alternatively, the leaf spring can be stiffened up to create adequate control but as the air spring is now in parallel with the leaf spring, the combined suspension would now have a very stiff deflection rate. This defeats one of the main reasons for specifying this expensive suspension. These factors can severely restrict the performance of these known suspension arrangements. 
         [0031]    To improve partially these wind-up and roll stiffness restrictions, the leaf spring is often designed to hare high asymmetrical stiffness which then moves the suspension in to the problem areas discussed with the earlier air suspensions. It is also a very expensive and inefficient use of spring materials. Sometimes the rear cantilever of the leaf spring is removed altogether and replaced with a transverse linkage to laterally locate the axle. 
       OBJECT OF THE INVENTION 
       [0032]    It is an object of the present invention to provide a vehicle leaf spring suspension which overcomes, or at least substantially reduces, the disadvantages associated with the prior art suspensions discussed above. 
       SUMMARY OF THE INVENTION 
       [0033]    A first aspect of the invention resides in a vehicle suspension comprising:
       a pair of leaf springs arranged to extend longitudinally of a frame of an associated vehicle on respective opposed sides thereof, each leaf spring having one of its ends connectable pivotally to the vehicle frame at a fixed location with respect thereto and arranged to have mounted thereto intermediate its ends an axle extending transversely of the vehicle frame;   a first bracket attached rigidly to each leaf spring at or adjacent the fixed location at which the one end of each leaf spring is connectable pivotally to the vehicle frame;   a radius arm arranged to extend longitudinally on each side of the vehicle frame and having an end thereof connected pivotally to the first bracket in spaced relationship to said fixed location; and   a second bracket which is attachable rigidly to the axle and to which is connected the other end of the corresponding radius arm in spaced relationship to the axle.       
 
         [0038]    A second aspect of the invention provides a vehicle suspension assembly comprising a vehicle suspension and an axle, the vehicle suspension comprising:
       a pair of leaf springs arranged to extend longitudinally of a frame of an associated vehicle on respective opposed sides thereof, each leaf spring having one of its ends connectable pivotally to the vehicle frame at a fixed location with respect thereto and having mounted thereto intermediate its ends the axle which is arranged to extend transversely of the vehicle frame;   a first bracket attached rigidly to each leaf spring at or adjacent the fixed location at which the one end of each leaf spring is connectable pivotally to the vehicle frame;   a radius arm arranged to extend longitudinally on each side of the vehicle frame and having an end thereof connected pivotally to the first bracket in spaced relationship to said fixed location; and   a second bracket which is attached rigidly to the axle and to which is connected the other end of the corresponding radius arm in spaced relationship to the axle.       
 
         [0043]    A third aspect of the invention resides in a vehicle comprising a frame, a suspension and an axle, the suspension comprising:
       a pair of leaf springs extending longitudinally of the frame of the vehicle on respective opposed sides thereof, each leaf spring having one of its ends connected pivotally to the vehicle frame at a fixed location with respect thereto and having mounted thereto intermediate its ends the axle which extends transversely of the vehicle frame;   a first bracket attached rigidly to each leaf spring at or adjacent the fixed location at which the one end of each leaf spring is connected pivotally to the vehicle frame;   a radius arm extending longitudinally on each side of the vehicle frame and having an end thereof connected pivotally to each first bracket in spaced relationship to said fixed location; and   a second bracket which is attached rigidly to the axle and to which is connected the other end of the corresponding radius arm in spaced relationship to the axle.       
 
         [0048]    In each of the three aspects of the invention defined above, the one end of each leaf spring may include an eye, preferably a front eye, which can be or is connected pivotally to the vehicle frame at the fixed location with respect thereto by means of a bush which may be mountable or mounted in a hanger bracket fixable or fixed rigidly to the vehicle frame. 
         [0049]    The other end of each leaf spring may be connectable or connected to the vehicle frame in any conventional manner for linear movement with respect thereto, to account for length changes of the spring under load conditions. For example, a bush, bush and shackle, slider bracket or air spring could be used for this purpose. 
         [0050]    Either or both ends of each radius arm may be connected pivotally or rigidly to its respective bracket by means of a bush or a clamp, as the case may be. Also, each second bracket may be attached rigidly to an axle assembly of which the corresponding transverse end of the axle forms part. Each second bracket may be attached rigidly to the respective axle assembly by any suitable means, such as at least one U-bolt. 
         [0051]    In practice, each leaf spring, and preferably the front cantilever thereof, the first and second brackets and the radius arm spaced from the leaf spring, form a linkage which may be generally parallelogram-shaped. This linkage may be tuned to the operative requirements of the suspension. 
         [0052]    The suspension may include symmetrical or asymmetrical leaf springs. 
         [0053]    With leaf spring-controlled suspensions of this type, there may be included further suspension means which may be in parallel or series with each leaf spring. The further suspension means may be mountable or mounted to the axle and/or each leaf spring, as the case may be. Such further suspension means may be of any suitable type, for example, an air spring, elastomeric spring or coil spring or any combination thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]    In order that the invention may be more fully understood, embodiments of vehicle suspensions in accordance therewith will now be described by way of example and with reference to the accompanying drawings in which: 
           [0055]      FIG. 1  is a diagrammatic side view of a prior art, symmetrical deflection, cantilevered leaf spring suspension, illustrating the leaf spring deflection geometry; 
           [0056]      FIG. 2  is a diagrammatic side view of the prior art suspension shown in  FIG. 1 , illustrating axle wind-up when the associated vehicle is subjected to a braking force; 
           [0057]      FIG. 3  is a diagrammatic side view of a prior art, unsymmetrical deflection, cantilevered leaf spring suspension, illustrating the associated spring deflection geometry; 
           [0058]      FIG. 4  is a diagrammatic side view of a prior art air spring suspension incorporating a leaf spring radius arm mounted to the associated vehicle frame by a front eye bracket; 
           [0059]      FIG. 5  is a diagrammatic side view of another prior art air spring suspension similar to that shown in  FIG. 4  but with the leaf spring radius arm mounted to the vehicle frame by a lowered front eye bracket; 
           [0060]      FIG. 6  is a diagrammatic side view of a modified version of the prior art suspension shown in  FIG. 4  but with the addition of an anti-wind-up lower linkage and brackets; 
           [0061]      FIG. 7  is a side view of another prior art air suspension which shares the load between a lighter leaf spring and an air spring acting in parallel; 
           [0062]      FIG. 8  is a side view of a modified version of the prior art air suspension shown in  FIG. 7 ; 
           [0063]      FIG. 9  is a side view of a first embodiment of air suspension incorporating a leaf spring radius arm in accordance with the invention; 
           [0064]      FIG. 10  is a perspective view of the first embodiment of suspension shown in  FIG. 9 ; 
           [0065]      FIG. 11  is a diagrammatic side view of a second embodiment of air suspension incorporating a radius arm in accordance with the invention; and 
           [0066]      FIG. 12  is a diagrammatic side view of a third embodiment in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PRIOR ART 
       [0067]    In the following examples of prior art suspensions, as shown in  FIGS. 1 to 8 , and embodiments of inventive suspensions, as shown in  FIGS. 9 to 12 , it is to be understood that even though each suspension comprises, inter alia, a pair of leaf springs extending longitudinally in the fore and aft direction of the frame of an associated vehicle on respective opposed transverse sides thereof, only one side of each suspension is illustrated, with the exception of  FIG. 10  which is a perspective view of a first embodiment of inventive suspension. 
         [0068]    Also, it is to be appreciated that in each prior art example and inventive embodiment of suspension, the fore or front end of each leaf spring is connected directly or indirectly to the frame of the associated vehicle at a fixed location with respect thereto by means of, say, a bush. Thus, the front end of each leaf spring can undergo pivotal movement with respect to the vehicle frame but not linear movement with respect thereto. 
         [0069]    Referring firstly, therefore, to  FIG. 1  of the accompanying drawings, a prior art, symmetrical deflection, cantilevered leaf spring suspension, indicated generally at  1 , comprises a pair of leaf springs  2  (only one shown) extending longitudinally on respective opposed sides of a vehicle frame  4 . Each leaf spring  2  is mounted generally centrally to an end of a transverse axle  3 . The front end eye  5  of the leaf spring  2  is connected to the frame  4  of the associated vehicle at a fixed location point A by means of a bush  6  mounted within a front frame hanger bracket  7 . 
         [0070]    The aft or rear end of the leaf spring  2  is connected to the vehicle frame  4  for pivotal and linear movement with respect thereto, the latter movement taking into account the change of the length of the leaf spring  2  as its curvature changes under load. Such connection between the rear end eye  10  of the leaf spring  2  is indicated generally at  9  and may comprise a shackle or cam slider. 
         [0071]    The added complexity of the movement of the leaf spring  2  is simplified by assuming that its rear eye location D moves horizontally during the change of length of the spring as it changes its curvature and deflects under differing loads and other operating conditions. In practice, movement of this location point D would be controlled by the manner in which it is mounted to the frame  4 , for example, the shackle or cam slider  9  referenced above. Any change of height of that mounting would distort the instantaneous centre length of the leaf spring  2  which, in the prior art symmetrical suspension of  FIG. 1  or  FIG. 2 , is at infinity. 
         [0072]    Also, the prior art suspension of  FIG. 1  is based upon the layout construction arrangements specified in the SAE Manual of Leaf Springs (SAE HS788) and, in particular, is shown at  FIG. 4.2  on page 29 of that Manual. 
         [0073]    Thus, when the axle  3  is loaded and as it is located by symmetrical and conventional leaf springs  2 , it will deflect on the suspension  1  without any change in angle, ignoring any effect from the location of the rear eye  10  of the leaf spring  2 , as discussed above. The axle  3  is located with respect to the vehicle frame  4  by the fixed location of the front eye of the leaf spring  2  and the effective link section A-B of the leaf spring  2 . The centre section B-C of the leaf spring  2  moves up and down on the two equal length link sections A-B and C-D. As the front eye  5  of the leaf spring  2  is fixed at A, any point around the axle  2  moves on an effective, generally parallelogram linkage ABFE, for example, point F around the virtual link section E-F as the angle of the axle  3  does not change, the effective instantaneous centre of the axle assembly is at infinity. 
         [0074]    Thus, the deflection geometry of the suspension  1  is created by assuming that the centre section B-C of the leaf spring  2  in the region around the axle  3  is effectively dead or inoperative. The axle  3  is then located by the two link sections A-B and C-fl of the leaf spring  2 , with the front eye fixed. These link sections A-B and C-D are typically about three quarters of the working length of each of the fore and aft cantilevers of the leaf spring  2 . With this geometry, points on the axle will move in parallel arcs, for example, F will rotate about E, with the effective link section E-F parallel and equal in length to the link section A-B. 
         [0075]    Under load, the axle  3  is located by the two link sections A-B 1  and C 1 -D, with the virtual link section E-F 1 . 
         [0076]      FIG. 2  shows the suspension  1  having the same spring geometry as that shown in  FIG. 1  but under a braking force B F  at the ground contact point P G  of the running wheel  11  of the associated vehicle. 
         [0077]    As discussed above, the braking and traction forces acting on the vehicle effectively act at the wheel ground contact point P G  and create a torque around the fixed location point A of the front end eye of the leaf spring  2 . This torque is resisted by the axle  3  and suspension  1 . 
         [0078]    When the suspension  1  is static, the vertical force exerted on the ground due to the sprung weight W of the associated vehicle, is distributed evenly to the location points A, D at which the eyes  5  and  8  of the respective front and rear ends of the leaf spring  2  are connected to the vehicle frame  4 . 
         [0079]    Thus, each distributed vertical force is equal to W/2. 
         [0080]    The braking force B F  creates a moment B F ×X around the front eye  5  of the leaf spring  2 , which is resisted by extra vertical forces at the front and rear leaf spring location points A, D. 
         [0081]    Thus, the static vertical force at the front eye  5  of the leaf spring  2  changes from W/2 to (W/2−B F X/L) where X is the height of the location point A about ground level and L is the distance between the front and rear location points A, D of the leaf spring  2 . As a consequence, the static rear vertical force changes from W/2 to (W/2+B F X/L). The braking force B F  is also reacted by an equal horizontal force in the opposite direction at the front location point A of the leaf spring  2 . A similar, but reversed, loading situation can occur under traction reaction torque, which reverses B F  for normal forward motion. 
         [0082]    As the front and rear cantilevers of the leaf spring  2  now have lighter and heavier vertical loads, they deflect in different directions, thus causing the axle  3  to rotate. With the consequential deflection rates of these symmetrical cantilever suspensions, there is the same deflection change in each cantilever and, therefore, there is no change in the vertical height of the suspension. 
         [0083]    In  FIG. 3 , there is shown the vertical loading deflection geometry of a suspension  21  similar to that described above with reference to  FIGS. 1 and 2  but with asymmetrical deflection, cantilevered leaf springs  22 . Thus, the different stiffnesses of the front and rear cantilevers of the leaf spring  22  and the different reactive loads at each end thereof, create a rotation of the axle  23 , as the spring  22  deflects under load. This deflection creates a finite, instantaneous centre, virtual arm length around which the axle  23  rotates as it deflects, as indicated at I. In a similar manner and with this asymmetrical deflection leaf spring suspension  21 , the vertical height of the axle  23  changes as it rotates under braking and traction forces, due to the differing cantilever deflections discussed above. 
         [0084]    The geometry of this suspension  21  provides that the axle  23  twists as the leaf spring  22  deflects under load changes. During roll of the associated vehicle, when the leaf springs  22  on respective opposed ends of the axle  23  attempts to deflect in different directions, the axle  23  rotates in different directions on respective opposed sides of the vehicle frame. 
         [0085]    As the axle  23  is normally torsionally stiff, the leaf springs  22  cannot deflect naturally, which causes them to distort. As a result, this distortion causes the springs  22  to deflect by a smaller amount, thus stiffening or increasing the rate of the leaf springs  22 , thereby increasing the anti-roll stiffness of the suspension  21 . 
         [0086]    The prior art air suspension  31  shown diagrammatically in  FIG. 4  comprises a pair of air spring bags  40  (only one shown) mounted adjacent the rear axle  33  of the associated vehicle. The lateral location of the axle  33  with respect to the vehicle is by means of a lateral or Panhard rod (also not shown), as in the cases of the prior art suspensions to be described hereinbelow in conjunction with  FIGS. 5 and 6 . 
         [0087]    Under braking or traction, the reactive vertical load changes of the mountings, such as the front eye  36  of the leaf spring  32  and the associated bush  35  and bracket  37  connecting the front end of the leaf spring  32  to the frame  34  of the associated vehicle, which resist the braking and traction force moments, have to be higher than in the prior art suspensions discussed above with respect to  FIGS. 1 to 3 , because they are spaced over a shorter span L a , instead of L for the equivalent length of leaf spring described above in relation to the prior art suspensions  1 ,  21  of  FIGS. 1 and 2  and  FIG. 3 . 
         [0088]    In order to obtain the higher roll stiffness required by air suspensions  31  of this type, the front cantilever of the leaf spring  32  has to be very stiff, thereby creating a very high asymmetrical deflecting cantilever effect with very high torsional loads in the axle  33 . 
         [0089]    Because this front leaf spring cantilever is very stiff, the instantaneous centre of the axle  33  is very close to the front eye  36  of the leaf spring  32 , thereby creating very high axle vertical movements under braking and traction forces which, in turn, produce many adverse performance and attitude effects on the associated vehicle and difficulties associated with propeller drive shaft angle changes. 
         [0090]    This short instantaneous centre, virtual cantilever length also creates a very stiff ride, as discussed above. This means that in order to create an improved ride, the air spring  40  has to be made even more flexible, or softer, which compounds the previously-discussed problems. 
         [0091]      FIG. 5  shows an improved version of the suspension  31  discussed above in relation to  FIG. 4 , the improvement being the lowering of the front eye  46  of the front cantilever of the leaf spring  42  to reduce the height X above ground level of the location point of that eye  46 . Thus, the braking and traction moments B F X which the suspension and associated components, such as the axle  43  and hanger bracket  47  and associated bush  45 , have to absorb, are reduced by lowering the connection of the front eye  46  of the leaf spring  42  to the vehicle frame  44 , using a dropper arm  48  of the hanger bracket  47 , with the bush and the leaf spring front eye  46  mounted at the lower end thereof. 
         [0092]    As discussed above, the main problem created by this type of suspension  41  is that it requires a specially designed and heavier front frame bracket  47 . The horizontal forces through the now-lowered front eye  46  of the front cantilever of the leaf spring  42  produce a high moment on to the frame  44 , requiring a wider spread of mountings and heavier bracket, frame and cross members. This arrangement also requires a special frame assembly which increases greatly the expense and production disruption created when specifying an air suspension option against a standard leaf spring suspension. 
         [0093]    In  FIG. 6 , there is shown another improved version of air suspension  51  which has been devised to reduce the problems and adverse features associated with the previously-described suspensions. 
         [0094]    In that suspension  51 , a leaf spring  52  has its front eye  56  connected to the frame  54  via a bush  55  and hanger bracket  57 , with an air spring  40  mounted to its other end, similar to the suspension  31  described above in relation to  FIG. 4 . 
         [0095]    However, in this suspension  51 , a secondary link  61  is added to create a double link geometry which urges the instantaneous centre forwards with respect to the suspension  51  and frame  54 . 
         [0096]    This secondary link  61  has a front eye  66  connected, via a bush  65 , to an extension  58  of the hanger plate  57 . A rear eye  68  of the secondary link  61  is connected pivotally to the axle  53  of the associated vehicle via a bush  67  and a rigid axle bracket  69 . 
         [0097]    In this prior art example of suspension  51  and for illustration purposes only, the distance Y between the front eye  56  of the leaf spring  52  and the front eye  67  of the secondary link  61  has been made the same as the distance Y between the front eye  67  of the secondary link  61  and ground level G. In this case, the horizontal force (2B F ) exerted on the front eye  66  of the secondary link  61  is twice the braking or traction force B F  at ground level G. In practice, however, the secondary link front eye  66  is often closer to the front eye  56  of the leaf spring  52 , thereby increasing the force exerted upon the eye  66  by an even larger amount, as well as increasing the force on the front eye  56  of the leaf spring  52 . Such adversity also creates higher frame loading problems than just lowering the front eye of the leaf spring, as well as the bracketry  57 ,  58 ,  69  required to support and control the secondary link  61 . 
         [0098]    In  FIG. 7 , there is shown a well known type of air suspension  71  which is often applied to vehicle front end suspensions. 
         [0099]    In this particular air suspension  71 , the front eye (not shown) of the front cantilever of the leaf spring  72  is connected to the vehicle frame  74  via a bush  76  mounted on a hanger bracket  77  bolted or riveted at  78  to the vehicle frame  74 . 
         [0100]    The rear eye (also not shown) of the rear cantilever of the leaf spring  72  is connected to the frame  74  via a shackle  75  and an associated hanger bracket  73  which is also connected to the frame  74  by bolts or rivets  78 . 
         [0101]    The leaf spring  72  is mounted centrally to an axle  79  and has an air spring  80  mounted thereon. 
         [0102]    In this prior art example of suspension  71 , the regular full weight leaf spring is replaced by a lighter leaf spring  72  which locates the axle  79  and often carries just the unladen vehicle suspension loads. The air spring  80  which, as in this case, is usually located adjacent the axle  79 , carries the remainder of the load which is applied to the suspension  71  when the vehicle is loaded. 
         [0103]    Thus, this suspension  71  includes a leaf spring  72  and an air spring  80  operating in parallel with each other. 
         [0104]    The friction from the leaf spring  72  is again very limited and a high additional anti-roll force is again required. 
         [0105]    Basically, the leaf spring  72  still solely provides the only resistance to braking and traction wind-up and as this replacement leaf spring  72  carries only part of the load, it is much lighter than the original arrangement where the leaf spring is used alone. 
         [0106]    Thus, control of wind-up of the axle  79  is now a major problem. 
         [0107]    Alternatively, the leaf spring  72  can be stiffened-up to create adequate control but as the air spring  80  is now in parallel with the leaf spring  72 , the whole suspension would have a very stiff deflection rate which defeats one of the main reasons for specifying such a suspension  71 . Such factors can restrict severely the performance of such a suspension  71 . 
         [0108]    In an attempt to improve at least partially these wind-up and roll stiffness restrictions associated with the suspension  71 , the leaf spring can be designed to comparatively high asymmetrical stiffnesses. Such an example is shown in  FIG. 8  wherein the suspension  81  comprises substantially the same components as those of the suspension  71  described above in relation to  FIG. 7 , except that the front cantilever of the leaf spring  72  is stiffened-up and the rear cantilever is softened. 
         [0109]    The reduced section at  82  provides softening of the rear cantilever of the leaf spring  72 , whilst an additional leaf  83  is provided between the front eye (not shown) of the front cantilever of the leaf spring  72  and the axle  79 . This front cantilever stiffening could also be created by just thickening the front cantilever of the main leaf, without the need for the additional leaf  83 . 
         [0110]    This arrangement of air suspension  81  employs the previously-discussed high asymmetric roll stiffness and comparatively high stiffness of the front cantilever of the leaf spring  72 , to prevent axle wind-up, to control the axle  79 . The effect is to reduce heavily the ride quality of the vehicle, whilst also creating a very inefficient use of spring steel. As a consequence, this particular arrangement of suspension  81  has all the adverse characteristics of the previously-discussed suspension  31  of  FIG. 4 . 
         [0111]    In some cases, the rear cantilever of the leaf spring  72  can be removed completely, with transverse location of the axle  79  being provided by a transverse or Panhard rod linkage. The presence of the rod linkage can create installation problems with vehicle components, such as engines and gearboxes. 
         [0112]    An improvement over the prior art examples discussed above is taught in our European Patent No. 1185428, wherein the leaf springs are stiffened only during vehicle roll to control the stability and handling of the vehicle at acceptable cost and weight increases. However, to gain the maximum advantage from this improvement, the leaf springs still need extra wind-up control when subjected to braking and traction forces. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0113]    Referring now to  FIGS. 9 and 10  of the accompanying drawings, here is shown a first embodiment of vehicle suspension  91  in accordance with the invention, which comprises a pair of double leaf springs  92 . 
         [0114]    The front eye  96  of the front cantilever of each upper leaf spring  92 ′ is connected to the associated vehicle frame (not shown) via a bush  95  mounted with respect to a front hanger bracket  97 . 
         [0115]    The rear eye  105  of the rear cantilever of each upper leaf spring  92 ′ is connected to the vehicle frame by means of a shackle  106  mounted pivotally at  108  to a rear hanger bracket  107 . 
         [0116]    Mounted intermediate each pair of front and rear hanger brackets  97 ,  107 , each double leaf spring  92  is clamped rigidly to the transverse axle  93  of the associated vehicle. In turn, further suspension means in the form of an air spring  111  is mounted to each double leaf spring  92  above and adjacent the corresponding end of the axle  93 , with the upper end of each air spring  111  being connected rigidly to a frame bracket  112 . 
         [0117]    In accordance with the invention, a radius arm in the form of a leaf spring  114  has a front eye  113  connected pivotally at bush  115  to the lower end of an extension hanger bracket  116  whose upper end is connected rigidly to the double leaf spring  92  at or adjacent the front eye  96  of the upper leaf spring  92  which is connected by a bush  95  to the front hanger bracket. 
         [0118]    At the rear end of each radius leaf spring arm  114 , there is provided a rear eye  117  which is pivotally attached by a bush  118  to the lower end of an axle bracket  119  whose upper end is attached rigidly to the axle  93 . 
         [0119]    Optionally, a shock absorber mounting eye for the suspension  91 , such as that shown at  120 , may be provided, possibly along with an anti-roll unit indicated generally at  130 . Such unit  130  may comprise a stabilizer bar or tube  131  having respective opposed ends clamped at  132  to the front cantilever of the double leaf spring  92 ′. 
         [0120]    As this first embodiment of suspension  91  in accordance with the invention is mounted to the double leaf spring  92  and assembly for the axle  93 , the whole can be mounted to the associated vehicle frame without changing the mountings for a conventional leaf spring suspension. 
         [0121]    The geometry of the radius leaf spring arm  114 , the front cantilever of the double leaf spring  92  and the front and axle brackets  116 ,  119  can be designed to match exactly the geometry of deflection of the leaf springs  92 , although this can be complicated by the movement of the front eye  113  of the radius arm  114  during spring deflection. However, it has been found that with accurate analysis, these geometries can be matched. 
         [0122]    During spring wind-up, however, when the suspension  91  is subjected to braking and traction forces, the geometries tend to mis-match, thus preventing the spring  92  deflecting adversely, thereby limiting axle wind-up problems. 
         [0123]    During braking and traction, the resistance to wind-up creates tension or compression along the radius leaf spring arm  114 . Therefore, the degree of wind-up stiffness can be varied by adjusting the compliance in the bushes  115 ,  118  at the front and rear ends of the arm  114 , about which the front and rear eyes  113 ,  117  of the arm  114  are pivotable. Alternatively, the arm  114  can be allowed to flex which can be achieved by making the arm  114  form a flat or curved leaf spring section, whereby peak shock loads can be removed substantially from the suspension  91 . 
         [0124]    An alternative might be to create a degree of mis-match between the geometries of the double leaf spring  92  and the radius leaf spring arm  114  and associated components, during straight deflection. This effect can be employed to create different suspension characteristics during operation of the suspension  91  and, again, can be modified using the compliance in the front and rear bushes  115 ,  118 , of the arm  114 , as discussed above. 
         [0125]    An example of this suspension characteristic change could be to obtain a better match to the steering or the axle drive propeller shaft. 
         [0126]    The second embodiment of suspension  201  shown in  FIG. 11  comprises a leaf spring  202  whose front eye  205  is connected to the associated vehicle frame  204  via a bush  203  and frame bracket  206 . 
         [0127]    Mounted to the rear end of the leaf spring  202  is further suspension means in the form of an air spring  207 , with an axle  208  and associated running wheel  209  provided. 
         [0128]    As in the case of the prior art suspensions of  FIGS. 4 to 6 , the transverse location of the axle  208  can be provided by any suitable means. In the embodiment shown in  FIG. 11 , such means is shown diagrammatically at  221  as a transverse location or Panhard rod fastened to an axle bracket  222  which is mounted rigidly to the axle  208 . The other end of the rod  221  is connected pivotally to a frame bracket (not shown). 
         [0129]    In accordance with the invention, a radius leaf spring arm  214  has a front eye  217  pivotally connected to the lower end of an extension bracket  216  via a bush  215 . The upper end of the bracket  216  is connected rigidly to the leaf spring  202  adjacent the front eye  205  thereof. 
         [0130]    The rear eye  219  of the radius leaf spring arm  214  is connected pivotally by a bush  218  to an axle bracket  220  which, in turn, has its upper end fixed rigidly to the axle  208  or an associated axle assembly. 
         [0131]    The leaf spring of each radius arm  214  may be replaced with a rod connected pivotally to the axle bracket  220 . 
         [0132]    This innovative arrangement could also be applied to a soft leaf spring suspension or any other combined leaf and other spring medium suspension, such as those of the prior art suspensions discussed above. 
         [0133]    A third embodiment of suspension  301  shown in  FIG. 12  comprises a leaf spring  302  whose front eye  305  is connected to the associated vehicle frame  304  via a bush  303  and frame bracket  306 , in a similar manner to the corresponding components of the second embodiment of suspension  201  of  FIG. 11 . 
         [0134]    Mounted to the rear end of the leaf spring  302  is further suspension means in the form of an air spring  307 , with an axle  308  and associated running wheel  309  provided, again in a similar manner to the second embodiment of suspension  201  of  FIG. 11 . 
         [0135]    As in the case of the prior art suspensions of  FIGS. 4 to 6  and the second embodiment of  FIG. 11 , the transverse location of the axle  208  can be provided by any suitable means. In this third embodiment shown in  FIG. 12 , such means is again shown diagrammatically at  321  as a transverse location or Panhard rod fastened to an axle bracket  322  which is mounted rigidly to the axle  308 . The other end of the rod  321  is connected pivotally to a frame bracket (not shown). 
         [0136]    In accordance with the invention, a radius leaf spring arm  314  has a front eye  317  pivotally connected to the lower end of an extension bracket  316  via a bush  315 . 
         [0137]    The upper end of the bracket  316  is connected rigidly to the leaf spring  302  adjacent the front eye  305  thereof. 
         [0138]    The rear end  319  of the radius leaf spring arm  314  is connected rigidly to an axle bracket  320  which, in turn, has its upper end fixed rigidly to the axle  308  or an associated axle assembly. 
         [0139]    It is to be appreciated that the air springs  111 ,  207 ,  307  of the three embodiments described above with reference to  FIGS. 9 and 10 ,  11  and  12  could be replaced or used in combination with other forms of further suspension means, for example, an elastomeric spring or a coil spring. 
         [0140]    The suspensions discussed above in accordance with the invention provide an effective linkage to control wind-up without the need for any extra frame structure and can be assembled to the vehicle frame as a direct replacement for a conventional leaf spring suspension, without requiring any extra frame brackets and frame strengthening which the current air suspension applications require. 
         [0141]    Also, the inventive suspensions allow for an effectively controlled, high quality air suspension using soft-rated leaf springs and this application can employ symmetrical springs and could be used in the prior art suspension disclosed in our European Patent No. 1185428, as discussed above. The novel linkage provided by the radius arm of suspensions discussed above in accordance with the invention would be able to control spring wind-up under braking and traction loading and forces and the suspension and axle assembly could be installed to the vehicle frame in the same way as that of an equivalent conventional leaf spring suspension. Using this arrangement, the suspension could also use softer springs and thus improve ride quality. 
         [0142]    Further, suspensions in accordance with the invention could also be used with high ride, quality leaf spring only suspensions and it could also be used with suspensions which are controlled by leaf springs using further suspension means, such as air, elastomeric and/or coil springs, either in parallel or series with the leaf springs. 
         [0143]    The linkage afforded by the radius arm may also be used to alter the leaf spring deflection characteristics in other ways, for example, by selecting the linkage geometry to match the associated leaf cantilever geometry or by choosing a required mis-match. Any such mis-match could then be modified by the linkage&#39;s compression or tension stiffness. This degree or rate of compression or tension stiffness can also be used to reduce the peak stresses in the attachments and other components of the suspension. 
         [0144]    In its simplest geometric form, the linkage can match the spring cantilever geometry during leaf spring deflection. That is to say, it can allow the spring cantilever to deflect under increasing and decreasing loads without any resistance from the linkage. 
         [0145]    Thus it can be seen that the invention accomplishes at least all of its objectives.