Abstract:
A combined tramp rod and anti-roll bar is secured between a rear axle beam and the chassis of a vehicle. The tramp rod and anti-roll bar has an intermediate portion, a right arm and a left arm that are connected by bushings to either the rear axle beam or the frame of the vehicle. The combined tramp rod and anti-roll bar provides roll stiffness control and also resists wind-up of the rear axle when high torque loads are applied to the rear axle. D-bushings that resist sliding movement of the intermediate portion in combination with the splayed arms provide improved compliance understeer performance when lateral loads are applied to the rear suspension.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to mechanical linkages for reducing lateral roll and axle beam rotation. 
     2. Background Art 
     Suspension systems function to isolate the sprung mass of the vehicle from the unsprung masses comprising wheel and axle assemblies. Shock loading and vibration are reduced by springs and dampers. Mechanical linkages may be provided to improve the stability, steering and general handling performance of the vehicle. 
     Anti-roll bars, or stabiliser bars, are used to reduce the tendency of the vehicle body to roll or rotate about a longitudinal axis in a turn. During cornering, a typical passenger car may roll from 3-8° per g toward the outside of the turn. Too much roll is undesirable because it places difficulties for the driver to make accurate observations on the road conditions and the motion of the vehicle. Excessive roll may also interfere with steering performance and/or the stability of the vehicle. 
     An anti-roll bar is connected between left and right side of a suspension in such a manner that it is only articulated during opposite wheel motion i.e. suspension/vehicle roll, i.e. one wheel moving into bump (closer to vehicle body) and another into rebound (away from the vehicle body). The stiffness of the anti-roll bar reduces the amount of roll for a given change in vertical forces, or increases the load transfer for a given roll angle. Its stiffness is tuned to achieve desired levels of roll, and change the distribution of roll stiffness between front and rear axle. 
     Several ways to construct an anti-roll bar, or an anti-roll stiffness for a suspension are known from prior art. The most common set-up consists of a bar or an essentially wide U-shaped bar or beam mounted in front or behind a suspension. The arms of the U are connected to the opposite sides of the suspension, and the centre section to vehicle frame, sub-frame, or chassis. In a typical arrangement, the arms of the anti-roll bar are connected to a suspension member (for example, control arm, suspension strut, a knuckle, or axle) using a link with either bushings or ball joints at each end. The centre section is usually connected to the chassis using purpose-built bushings, sometimes referred to as D-blocks due to the characteristic shape of their cross-section. The shape is dictated by the fact that it first must be possible to mount the bush on the bar, and secondly because it is normally affixed on a flat surface by means of a clamp. 
     In the typical installation described above, the arms of the anti-roll bar are essentially loaded in bending, and the centre section essentially in torsion. Due to package constraints the centre section will sometimes be subjected to a combination of torsion and bending loads. On a live (or a dead beam) axle, the installation is often reversed, such that the centre section is connected to the axle, and the arms to the chassis. 
     Anti-roll bars are generally attached to suspension members using links of varying length, commonly referred to as drop links. The purpose of the drop link is to allow the anti-roll bar tip and suspension member to follow different trajectories during wheel travel, such that roll resistance is provided without introducing side loads on either the anti-roll bar or the suspension member. 
     Leaf spring live axle, also referred to as Hotchkiss suspension, is rarely used in modern passenger car applications, but it is still a common solution on light trucks and commercial vehicles. Its popularity is due to its cost-effectiveness and low overall system weight, resulting in a higher payload for a given gross vehicle weight than many other systems. Further, leaf spring divides the load to two attachment points on a chassis, which is favourable from durability point of view. 
     However, leaf spring suspensions suffer from the inherent disadvantage of relatively low wind-up stiffness, i.e. angular articulation of the axle (in side view) under driving/braking forces. The driving or braking torque is reacted on the leaf springs through axle housing. Under severe braking or acceleration, the leaf springs will presume an s-shape deformation. This articulation is undesirable, because it deteriorates road holding and traction through uncontrolled axle motion. Further, it has adverse NVH (noise, vibration and harshness) effects and poses additional stress on the driveshaft, which in extreme cases may lead to axle breakage if the joints are over-articulated. Thus it is desirable to minimise wind-up. 
     For a given vertical spring rate, it is possible to improve wind-up stiffness by making springs longer. However, this requires package space and adds mass to the vehicle. Also, this option will generally not be available on an existing vehicle platform where modification is desired, such as increased engine torque output. Tramp rods, also referred to as torque arms or radius arms, have historically been applied in vehicles equipped with a leaf-sprung live axle (normally rear) that produce high torque, particularly in high performance and racing car application. Tramp rod or rods—usually a single one or a pair per axle—are most commonly installed such that they extend parallel or nearly parallel to the leaf spring in forward direction. The tramp rod counteracts wind-up of the axle and differential caused by high torque loads. The length and position of the tramp rods have to be selected carefully, such that the installation will not be ‘fighting’ the motion of axle housing under normal wheel travel. 
     Another conventional means to improve wind-up performance is staggered installation of the dampers. In such arrangement, one of the dampers is located in front of the axle and inclined forward of the axle, while the other is behind the axle inclined rearward. Such arrangement is often used on overslung suspension. It does not increase wind-up stiffness, but generally results in better wind-up performance, because the articulation during transients is reduced. However, the asymmetric installation produces asymmetric motion ratios, leading to asymmetric vehicle response. Especially in laden condition the rearward damper will be less effective. Except for wind-up control, staggered installation is thus undesirable. 
     A further desired characteristic of a suspension system is compliance understeer. On a rear suspension, compliance understeer is achieved when a cornering (lateral) force applied on outside corner wheel causes the axle to toe-in. In practice, this is difficult to achieve on a conventional leaf spring suspension. Panhard rod or Watt&#39;s linkage have previously been applied, but it is often difficult to find either the package or, on a vehicle with ladder frame, a sufficiently stiff mounting point. 
     These problems, and possibly other problems, are addressed by applicants as summarized below. 
     SUMMARY 
     According to one aspect of the applicants&#39; disclosure, a combined anti-roll bar and tramp rod are provided for a vehicle. A stabilizer bar having right and left arms and an intermediate portion provides the combined function of an anti-roll bar and a pair of tramp rods. The bar includes an intermediate portion and right and left arms that extend generally in the fore-and-aft vehicle direction. The arms of the bar are directly secured by bushings or ball joints to the chassis of the vehicle without any intervening drop links. 
     Combining the functions of an anti-roll bar and a pair of tramp rods is achieved by the deletion of anti-roll bar links (drop links) and carefully selected location of the attachment points, while limiting the complexity of the system. Prior art anti-roll bars have not been used to control wind-up. Increased wind-up stiffness will thus allow the use of parallel damper installation on a suspension. 
     By eliminating the drop links and directly connecting the arms of the bar to the chassis, axle beam rotation, or wind-up, can be impeded without providing a separate set of tramp rods. 
     The combined anti-roll bar and tramp rod may be provided with D-blocks or bushings that secure the bar to an axle beam and may also provide compliance understeer resulting in the outside tire steering into the cornering radius (toe-in). 
     With the anti-roll bar installation described below, compliance understeer, or as a minimum, a reasonable reduction of compliance oversteer, can be realised. This can be achieved by splaying the arms of the anti-roll bar out and forward in plan view. Under lateral force, the lateral displacement of the axle induces a toe-correcting effect on the axle. 
     These and other features of the combined tramp rod and anti-roll bar disclosed in this application will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front/side fragmentary perspective view of a rear suspension including the combined tramp rod and anti-roll bar; 
         FIG. 2  is a side elevation view of the combined tramp rod and anti-roll bar shown in  FIG. 1 ; 
         FIG. 3  is a bottom plan view of the combined tramp rod and anti-roll bar shown in  FIG. 1 ; 
         FIG. 4  is a bottom plan view of an alternative embodiment of the combined tramp rod and anti-roll bar for use with a rear axle that is secured to the vehicle above the axle and differential and is intended to be used with the suspension having leaf springs mounted above the rear axle beam; 
         FIG. 5  is another alternative embodiment in which the combined tramp rod and anti-roll bar is secured with the intermediate or transverse portion secured to the chassis of the vehicle with the right and left arms of the combined tramp rod and anti-roll bar attached to the rear axle beam; and 
         FIG. 6  is a side front perspective view of an alternative embodiment of the combined tramp rod and anti-roll bar having right and left arms attached to bushings in the bracket blocks that are attached between the vehicle frame and the rear axle beam. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a rear suspension system generally indicated by reference numeral  10  is shown with the vehicle and several component parts not being illustrated to provide better visibility of the components of the suspension system  10  that are material to the illustrated combined tramp rod and anti-roll bar  12 . 
     The tramp rod and anti-roll bar  12  is assembled to a rear axle beam  16 . Dampers  18 , or shock absorbers, are secured between the rear axle beam  16  and the body of the vehicle (not shown). A differential  20  is provided on the axle beam  16 . Torque is delivered through a driveshaft (not shown) to the differential  20 . Leaf springs  22  are provided on opposite sides of the rear suspension system  10 . The rear axle beam  16  in  FIG. 1  is shown in an overslung arrangement wherein the rear axle  16  is below the leaf springs  22 . A frame rail  24  is partially shown in  FIG. 1 . It should be understood that the frame rail  24  is part of the vehicle chassis that is not otherwise shown. The frame rail  24  may be an integral frame rail that is part of a unibody vehicle. Leaf spring brackets  26  (shackles) are provided to secure the rear ends of the leaf springs  22  to the vehicle chassis or frame. 
     A wheel  28  is shown on both sides of the rear axle beam  16  with a tire  30  being illustrated on the right side wheel  28 . The tire on the left side of the vehicle is not shown so that the other parts of the suspension system  10  may be more easily observed. 
     The tramp rod and anti-roll bar  12  includes an intermediate portion  32 , or transverse portion, that extends between a right arm  34  and a left arm  36 . A bushing  40 , referred to as D-block, is used to secure the intermediate portion  32  of the tramp rod and anti-roll bar  12  to the rear axle beam  16 . A pair of bushings  40  is provided at or near the juncture of the arms  34 ,  36  and the intermediate portion  32 . Bushings  42  are provided at the ends of the arms  34  and  36  and are spaced from the intermediate portion  32 . The arms  34  and  36  are connected by the bushings  42  to the frame rails  24  directly. It is important to note that the ends of the arms  34  and  36  are not connected to drop links which would be the conventional way of connecting an anti-roll bar to a vehicle chassis. By eliminating the drop links of conventional anti-roll bars, the combined anti-roll bar function and tramp rod function may be provided by this unique mounting arrangement for the tramp rod and anti-roll bar  12 . 
     Referring to  FIG. 2 , the rear suspension system shown in  FIG. 1  including the tramp rod and anti-roll bar  12  is shown in a diagrammatic side elevation view. In  FIG. 2 , the rear axle beam  16  is not depicted so that the rear axle  44  may be shown more clearly as it extends from the differential  20 . The leaf springs are supported by the brackets (shackles)  26 . The bushing  42  is shown connecting the right arm  34  to the frame rail  24 . The intermediate portion  32  of the tramp rod and anti-roll bar  12  is shown extending below the differential  20 . The bushing  40  connects the intermediate portion  32  to the rear axle beam  16  (not shown in  FIG. 2 ). 
     Referring to  FIG. 3 , the tramp rod and anti-roll bar  12  is shown with the arms  34  and  36  being connected to a frame rail  24  that extends in a transverse orientation relative to the vehicle. The ends of the arms  34  and  36  are secured by bushings  42  to the frame rail  24 . The tramp rod and anti-roll bar  12  is secured to the rear axle beam  16  by the bushings  40 . The bushings  40  are located at the intersection of the intermediate portion  32  and the right and left arms  34  and  36 . The intermediate portion  32  extends below the differential  20 . This embodiment is intended to be used with an overslung rear axle i.e. where axle  16  located below the leaf springs (not shown in  FIG. 3 ). 
     Referring to  FIG. 4 , an alternative embodiment is shown wherein the tramp rod and anti-roll bar  12  is secured to the rear axle beam  16  by bushings  40  with the intermediate portion  32  extending above the differential. This embodiment is intended to be used with an underslung rear axle where the axle  16  is located over the leaf springs (not shown in  FIG. 4 ). The right and left arms  34  and  36  are secured to ball joints  46  that secure the arms  34  and  36  to the frame member  24 . 
     Referring to  FIG. 5 , an alternative embodiment of a tramp rod and anti-roll bar  48  is illustrated in which the intermediate portion  50 , or transverse portion, is secured to a transversely extending frame beam  52 . The transversely extending frame beam  52  is secured between parallel frame rails  54  in the illustrated embodiment. The tramp rod and anti-roll bar  48  includes a right arm  58  and a left arm  60 . The right arm  58  and left arm  60  are attached to a rear axle beam  62 . A pair of bushings  64  is provided on opposite sides of a differential  66 . The bushings  64  secure the right arm  58  and the left arm  60  to the rear axle beam  62  between the differential  66  and the tires  70 . In this embodiment, the anti-roll function and the anti-rotation function of a tramp rod may be performed by the tramp rod and anti-roll bar  48 . 
     Referring to  FIG. 6 , yet another embodiment of a tramp rod and anti-roll bar  72  is illustrated. A tramp rod system  74  is illustrated in  FIG. 6 . The tramp rod and the anti-roll bar  72  include a transverse portion  76 , a right arm  78  and a left arm  80 . Cast bracket blocks  82  are provided that receive the right arm  78  and the left arm  80  on bushings (not shown). The D-bushings  84  are secured to the transverse portion  76  of the tramp rod and anti-roll bar  72 . The D-bushings  84  include plates  86  that are used to secure the transverse portion to a rear axle beam (not shown in  FIG. 6 ). Leaf springs  88  extend from the bracket blocks  82  in a rearward direction. Spring hangers  90  are provided to support helper leaf spring. A rear spring mounting bracket  92  is provided to support the rear ends of the leaf springs  88 . An axle bracket  94  is provided to support the axle (not shown) below the leaf springs  88 . 
     As shown in  FIGS. 3-5 , the arms  34 ,  36  and  58 ,  60  may be splayed laterally outward from the respective intermediate portion  32 ,  50 . Splaying the arms improves compliance understeer as compared to conventional anti-roll bar designs, where anti-roll bar is connected to the chassis (or axle, as the case may be) through drop links. When the arms are not splayed outward, the amount of compliance understeer is reduced. 
     The bushings  40 ,  56 , and  84  may include gripping features such as cooperating ribs that grip the tramp rod and anti-roll bars  12 ,  48 , and  72  to a greater extent to provide a compliance understeer. Compliance understeer is created when lateral forces are applied to a suspension in a turn. For example, a lateral force applied on a right rear wheel while making a left cornering turn, handling is improved by causing the outside tire to steer into the corner. Compliance understeer results in a stabilization of the vehicle in the turn. The D-bushings used to secure the intermediate portions of the bars to the rear axle beam may be smooth. If smooth or sliding type D-bushings are used, the compliance understeer advantage of the invention is not expected to be realized. 
     By not providing drop links between the ends of the arms and the chassis, the tramp rod and anti-roll bars  12 ,  48  and  72  also serve to provide the tramp rod function. Drop links are conventionally used to secure the ends of an anti-roll bar to a chassis tend to compensate for longitudinal motion between the frame and the two arms, which prevents the arms from functioning as tramp rods.