The Motor Vehicle 2010 Part 17 pps

1146 The Motor Vehicle will also be equal to P plus half the weight W of the axle and wheels. When a side force F acts on the body it sets up forces f 1 and f 2 at the points of connection of the body and springs. The relative magnitudes of these forces and the exact positions at which they act will always be somewhat uncertain. However, assuming them to be equal, and to act as shown, their resultant is a force F′ equal and opposite to F. These two forces F and F′ thus constitute a couple of magnitude Fh, h being the perpendicular distance between them which, unless the tilt is very large, may be assumed to be equal to OG, where O is the roll centre and, if F is the centrifugal force, G is the centre of gravity. For equilibrium, there must be an equal and opposite couple to balance the couple Fh. This balancing couple is supplied by an increase q in the left- hand vertical reaction and a decrease of the same magnitude in the right- hand one. Consequently, the left-hand spring will sink a little lower than before while the right-hand one will rise by the same amount. The body will thus tilt about the point O, on the roll axis. Similarly there will be a corresponding centre O′ at the rear, so the line OO′ is the roll axis. The change q in the spring forces on each side will be equal to Fh/t, t being the spring base. The vertical forces between the wheels and the ground will change by amounts p, where p = FH/T, T being the wheel track and H the height of the line of action of the force F above the ground. If, under the action of the side forces f 1 and f 2 or, more accurately, under the reactions to those forces, the springs deflect slightly sideways, the point O would move sideways and therefore the centre of tilt would be slightly lower. Considering now the car with independent suspension as shown in Fig. 43.2, the side force F again sets up a tilting couple and this has to be balanced by an increase p in the force exerted by the left-hand spring and an equal decrease in that exerted by the right-hand spring. If the tilt is not excessive, these changes in spring force will be equal, one increasing from Q to Q + s and the other decreasing to Q – s. The compression of the left- hand spring will be increased by some amount and that of right-hand spring will be decreased by the same amount. Therefore the suspension will assume the position shown and the body will have tilted about the point O. Thus O is now a point on the roll axis and the line joining it to the similar centre of tilt O′ at the rear will be the roll axis. For a car having rigid axles at front and back, the roll axis will be some distance above ground level while for a car having independent suspension at front and back, the roll axis will be at or near ground level. The roll axis G F h P+q f 1 F 1 t T P–q W 2 +p W 2 –p f 2 Q+s f 1 F 1 O f 2 Q–s G F H W 2 +p W 2 – p T p = FH T q = Fh t O Fig. 43.1 Fig. 43.2 1147Suspension systems of a car having independent suspension at the front and a rigid axle at the rear will be inclined, from approximately ground level at the front rising to about axle level at the rear. With the MacPherson-type independent front suspension, a roll centre height just above ground level is usually chosen because it offers minimum lateral scrub, coupled with relatively little change in roll centre height with deflection of the suspension. On the other hand, a higher roll centre is obtained with the angled single link now popular for rear suspension, Section 43.19, but because the axis along which the pivots are set is fixed, Fig. 43.24, its height does not vary much with deflection. If the roll axis lies at ground level, the overturning couple FH will be greater than Fh obtaining when the roll axis is above ground level. Although this will tend to make the tilt of the car with independent suspension greater than that of the car with rigid axles front and rear, this tendency is offset because the effective spring base of the independent suspension is the wheel track T, which is considerably wider than the spring base t. It can be shown that, for a vehicle with rigid axles, the angle of tilt is approximately proportional to 2q/t 2 = 2Fh/t 2 while, for a car with independent suspension, it is proportional to 2p/T 2 = 2Fh/T 2 . In most cases, the latter will be the smaller because of the greater effective spring base, which will outweigh the effect of the increase in the arm H of the overturning couple. The greater the roll stiffness – resistance to roll – at one end of the car, the larger will be the proportion of the tilting couple that will be reacted at that end. Indeed, if the roll stiffness were infinite at the front, the whole of the tilting couple would be reacted there, unless the vehicle were flexible enough to deflect torsionally and thus throw some of the load on to the rear suspension. Cars are very rigid torsionally relative to the loads they carry. This is because the whole body can be regarded as a torque tube. Heavy commercial vehicles, however, because of the great weights that they must carry – relative to which their chassis frames are shallow – are flexible torsionally, which is one reason why independent suspension is not normally used on them. In practice, however, ‘roll centre’ is not such a simple concept as it might at first appear. Although it is generally assumed that the vehicle rolls about an axis represented by a line passing through the roll centres of the front and rear suspension, this could be true only if the wheel and tyre assemblies were rigid and did not move sideways on the road. The roll centre, as determined from the kinematics of the suspension links, moves as the suspension deflects and does so increasingly towards the extremes of this deflection. Obviously the actual motions of the carriage unit, taking into account not only the movements of the roll centre due to variations of the suspension geometry with wheel deflection but also both vertical and lateral deflections of the tyre calls for the use of a computer. Even so, a first approximation accurate enough for practical purposes can be obtained from consideration of the suspension link geometry. The method of finding the roll centre is illustrated in Fig. 43.3. It involves extending the axes of the suspension links until they meet at O, and then joining O and the centre of contact T between the tyre and the road. Point O is then the instantaneous centre about which rotate all parts of the suspension and its pivots on the carriage unit on that side of the vehicle. If we then do the same for the suspension on the other side, we find that the two lines OT 1148 The Motor Vehicle O C O T C T O T C at infinity Fig. 43.3 Note: If the suspension on the other side of the vehicle were drawn, the diagram for obtaining the instantaneous centre would be a mirror image of that shown here, which is why C is always on the vertical centre line of the vehicle, except when the wheels on each side deflect in opposite directions intersect at C, which is the instantaneous centre about which the two points T rotate and is therefore the roll centre of the vehicle as a whole. By re- drawing this diagram with one suspension deflected up and the other down – the situation when the body rolls – it can be shown that the points O move down and up respectively on the two sides which of course move the roll centre C not only up or down, according to the geometry, but also laterally, off the vertical centre line of the vehicle. Some typical linkage systems and their roll centres, with the suspension in its static position, are shown in Fig. 43.4. 43.3 Double transverse-link suspension A double transverse-link suspension, with a torsion-bar spring, is illustrated diagrammatically in Fig. 43.5. To eliminate wheel tilt with deflection, that is change in the camber angle φ , the two arms would have to be of equal lengths, which would place the roll centre at ground level. This, however, would have the effect of varying the track with roll, having undesirable steering effects and could also adversely affect tyre wear. By shortening the upper link it is possible to keep the track almost constant without introducing too much variation in camber angle. Moreover, the slight change in the camber angle is negative on the outer wheel when the vehicle is turning, which increases the cornering power. In the early independent suspension systems, these links were generally both parallel to the ground. After the Second World War, however, the practice of inclining them, to move the roll centre, became widespread. Exactly what these effects are can be seen easily by drawing sketches on the lines indicated in Fig. 43.3, but with the arms at various angles. It will be seen that sloping the upper link down towards the wheel raises the roll centre, and vice versa. Where a coil spring is to be used, instead of a torsion bar, it is usually installed coaxially with the telescopic damper S in Fig. 43.5. Incidentally, it is more common for the upper ends of the spring and shock absorber to be on the vehicle structure instead of on the upper transverse arm. On some vehicles, the springs and dampers have been mounted separately, so that 1149Suspension systems access could be gained more easily for servicing the shock absorber. An alternative is to arrange for removal of the shock absorber through a hole in the spring seating pan. With front-wheel-drive cars, the coil spring and shock absorber are in most instances interposed between the upper transverse link and the vehicle structure, to leave space for the driveshaft to the wheel. With unequal-length wishbones, the end B of the steering arm, Fig. 43.5, moves in a curve that is not an arc of a circle, so vertical deflection of the suspension inevitably has some steering effect if the centre of the other end – that of the joint at its connection to the remainder of the steering linkage – is in line with the axis X 1 X 1 of the pivot for the lower transverse link. This problem is generally overcome in one of two ways: either a position for the centre of the connection to the remainder of the steering linkage can be chosen so that it coincides with the centre of an arc approximating to the curve through which B moves, in which case the undesirable steering effect may become negligible; or the centre of B is placed in line with the axis x 1 of the lower arm A 1 , in which case, with the other connection in line with the axis X 1 , the motion would be truly circular and there would be no steering effect. Similarly, the two ends could be in line with the axes X 2 and x 2 of the upper arm A 2 , though this is more difficult to arrange. Two variants of this type of suspension linkage are shown in Figs 43.6 and 43.7. In the first, a laminated spring serves as the upper arm, while in the second, two pairs of laminated springs, one at the top and the other at the bottom, replace both transverse arms. The first can of course be inverted, as Spring eyes C Semi-elliptic spring Leading or trailing arm Infinity C De Dion with Panhard rod C Transverse leaf spring Swing axle with low pivot (Mercedes) C Sliding pillar suspension C Macpherson strut C C C Fig. 43.4 De Dion with trailing links 1150 The Motor Vehicle in the case of the Fiat 600, which had a wishbone-type upper link and no drag link. When the lever arm type shock absorber was widely used, its arm was sometimes made to serve as the upper transverse link, a notable example having been in the Morris Minor. Another variant is one in which the lower arm is replaced by the driveshaft. One example is the Triumph Herald, in which the upper transverse link is formed by the ends of a transverse leaf spring, and the lower by what is sometimes termed a swinging halfshaft. This shaft swings up and down about a universal joint at its inner end, where it is connected to the final drive differential gear. Provision for the necessary articulation at its outer end is made in an unusual manner. The wheel is keyed directly on to the tapered outer end of the halfshaft, and the wheel bearings are immediately inboard of it. These bearings, with the outer end of the halfshaft rotating in them, are carried in a housing that is pivot-mounted on the lower end of the vertical link to the upper end of which the eye of the transverse leaf spring is connected. The axes of these two pivot connections are of course parallel, so that the two links can articulate together, carrying the vertical link and thus the wheel assembly up and down with them. To provide fore-and-aft location, a drag link, pivoted at both ends, serves as a radius rod between the wheel bearing housing and a transverse member further forward on the chassis frame. A similar independent rear-suspension layout, but with coil springs, Fig. 43.8, is used on several Jaguar cars. Here the swinging halfshaft forms the upper transverse link, but it has at its outer end a universal joint instead of the pivoted bearing housing arrangement of the Triumph Herald, to give it the required freedom to articulate. The lower link comprises a single transverse arm and a drag link. The disc brakes are mounted inboard, on the final-drive assembly, thus considerably reducing the unsprung weight and relieving the suspension linkage of the brake torque except in so far as it is transmitted X 2 A 2 X 2 S C X 1 X 1 X 1 A 1 D X 2 A 1 A 2 X 2 X 1 B φ A B C B C C C A B Fig. 43.5 Fig. 43.7 Fig. 43.6 1151Suspension systems Fig. 43.8 Jaguar Mk X independent rear suspension assembly 1152 The Motor Vehicle back through the swinging halfshaft to the brakes. Since the stub axle carrier pivots about the outer end of the lower link, the vertical loads applied at the wheel put the halfshaft in compression while, during cornering, the side load on the outer wheel places it in tension, the two tending to cancel each other out. On the inner wheel, although the two loads are additive, they are in any case lighter and, during extreme cornering, fall to zero. There are some of the variants of the double transverse link system, in which each of the two links can be a single arm used in conjunction with a drag link, as in the Jaguar system, or it may be triangulated to form what is termed a wishbone link. In the latter, the apex of the link is adjacent to the stub-axle carrier, and the base is secured by a pivot bearing at each corner to the vehicle structure. The more widely spaced are these bearings, the lighter becomes their loading due to brake drag and torque. There are examples in which the axis of the pivot bearings are not parallel to the longitudinal axis of the vehicle. This arrangement is usually adopted, as in the Humber Super Snipe chassis, simply to enable the front transverse member of the chassis frame to clear the engine sump or, in other words, to enable the engine to be installed as far forward as possible. The frame of this vehicle is illustrated in Fig. 38.2, and the wishbone links (not shown) trail at the same angle as the outer ends of the front transverse member. 43.4 MacPherson strut type The principle of the strut-type suspension invented in the nineteen-forties by Earle S. MacPherson, a Ford engineer in the USA, is illustrated in Fig. 43.9 and details in Figs 43.10 and 41.8. This type is common because, with its widely spaced attachments to the carriage unit, it fits in well with the basic concepts of chassisless construction. Moreover, with transverse engines there may be no room for upper transverse links and, even if there is, the strut type leaves more space around the engine, so access for maintenance is easier. The only significant disadvantage is the radial loading on the piston, due to lateral forces during cornering and to brake torque. Two links, one taking the lateral and the other the drag loading, and one nearly vertical telescopic strut, make up the complete mechanism, Fig. 43.9. A single transverse arm is pivoted on the structure at B and connected by a ball-and-socket joint D to the base of the strut C. Sliding in C is the member E, the upper end of which is secured to the body by the equivalent of a ball- B D C E F A Fig. 43.9 1153Suspension systems and-socket joint – in practice, since the articulation at F is small, a rubber joint is generally used. The spring is compressed between two flanges, one on the member C and the other on E. This telescopic strut also serves as the hydraulic damper. For steering, member C rotates about the axis DF. In the mid-nineteen-eighties Ford introduced, for their Transit vans and Escort models, a variant with its coil springs interposed between the transverse links and the frame, so that it would not intrude into the space where the clutch and brake pedals had to be placed. The fore-and-aft forces acting on the road wheel are taken by a tie or drag link, the rear end of which is pivoted at H, near the outer end of the link A; at its other end, this drag link is pivoted to the body structure. Both pivots are usually rubber bushes. In plan view, it is usually set approximately 45° relative to the longitudinal axis of the car and, since it is in front of the link A, it is in tension. In many instances, it is formed simply by bending the ends of a transversely-installed anti-roll bar, so that they will perform the dual functions of drag links and lever arms for actuation of the anti-roll bar, which is usually carried in rubber bushes adjacent to its cranked ends. To clear the engine sump, the anti-roll bar is usually mounted at the front, but if it can be mounted behind the engine, the application of brake torque to its forward extending lever arms tends to lift their pivot bearings and thus to counteract brake drive. In Fig. 43.10, the strut and stub-axle forging are made as separate components, for ease of replacement of a damper. The top end of the piston rod E is free to rotate in a bush in the centre of a large rubber mounting by means of which the unit is secured to the body. This bush serves as a swivel bearing for the steering, while the rubber mounting insulates the body from noise, vibration and strains due to deflection of the suspension unit. The lower transverse link A is connected by a rubber bush H to the drag link, which has another rubber bush for its connection to the vehicle structure. 43.5 Single transverse link What might be described as a single transverse-line system was used for the front suspension of the Allard vehicles in the nineteen-fifties. In effect, the front axle beam was divided in the centre, where each half was pivoted to the chassis frame. Coil springs were interposed between the outer ends of these half-axles and the frame. Drag loads were taken by two radius rods, both pivoted beneath the centre of the front transverse member of the frame and extending rearwards and outwards to a point beneath the spring seats, where they were rigidly attached to the axle. The axis of the pivots of the radius rods were in line with those of the divided axle. Thus, each half-axle and drag rod together formed a single transverse link. This type of system has been more commonly used in rear suspensions, as the swing axle arrangement described in Section 43.18, and it suffers the same disadvantages. Figure 43.11 illustrates diagrammatically a front axle arrangement, the dotted line showing how a wishbone-type link can be used, instead of a drag link, to react brake drag. Both the track T and the camber angle φ change with suspension deflection. The steering rods have to pivot on ball joints the centres of which are in line with the axes X, so that the rods rise and fall parallel with the transverse link. If the forward arm of the wishbone is shorter than the rear one, and thus sweeps the link forward, the application 1154 The Motor Vehicle of brake torque tends to lift the pivot bearings and therefore to counteract brake dive, though the magnitude of this effect depends on the angle the axis XX makes with the longitudinal axis of the vehicle, and therefore is not normally of great significanee. 43.6 Single leading or trailing link This type, shown diagrammatically, in Fig. 43.12, is the simplest but has several disadvantages, which include variations in angle of inclination θ of the kingpin axis, the difficulty of obtaining adequate stiffness to resist satisfactorily the couple due to lateral loading at ground level during cornering, and the fact that the wheels tilt to the same angle as the sprung mass when the vehicle rolls. Such a system has been used also with torsion-bar springing, Fig. 43.10 E C H A 1155Suspension systems φ T X X Fig. 43.11 Fig. 43.12 Fig. 43.13 including with laminated torsion bars. Normally the two torsion bars, one for the left-hand and the other for the right-hand arms, are attached one at each of the pivots X of the arms A, and their other ends are anchored to the frame or vehicle structure, usually on the side remote from the pivots, so that they overlap instead of being coaxial, and therefore can be longer. When the wheel is steered straight ahead, the joint B on the end of the steering arm moves through arc x 1 while the stub axle moves through arc x as the suspension deflects. Therefore, if unwanted steering effects are to be avoided, the other end of the steering rod should pivot somewhere along axis XX, which can be difficult with torsion-bar springing. Because of this and the variation of castor angle of trail, this type of suspension has been used more for rear than for front suspension systems. 43.7 Double leading or trailing link With two parallel links, A 1 and A 2 , of equal lengths and pivoting about axes X 1 and X 2 , Fig. 43.13, the vertical member C, which carries the stub axle and X X X A X θ X 1 B X 2 A 2 X 2 C X 1 B y θ Y A 1 X X Y B X 1 [...]... transfer the vertical load of the wheel to the spring G and thus to the body 1160 The Motor Vehicle At the lower end of the member A, the ball of the joint is fixed in the composite wishbone member H and J, whose two parts are pivoted in rubber bushes on the vehicle structure at their inner ends and are pivoted to each other at their outer ends, at K, by a ball-and-socket joint at an angle to each other... result of the centrifugal force on the vehicle times the height of the roll centre above the ground is greater than the righting couple due to the weight of the vehicle acting vertically downwards multiplied by the horizontal distance from the centre of the outer wheel contact patch on the ground to the projected centre of gravity of the vehicle The latter effect leads to instability because, as the jacking-up... by the steering mechanism, transfers hydraulic fluid from one side to the other of the vehicle through the dampers in the links between the suspension arms and the levers actuating the spool valve It does this in a direction such as to lift the lever on the outer and lower that on the inner side of the turn  1176 The Motor Vehicle To opposite front valve Steering input Displacement ram Fig 43.32 The. .. is used, the reaction point, at its forward end, can be offset to the right of the vehicle so that the lift due to the reaction to the torque in the halfshafts counteracts the downward pressure due to the reaction to the torque in the propeller shaft Otherwise, the only remedy apart from the use of complex torque-reaction rods is a de Dion or independent suspension system – in other words, either a dead... the axle can be reduced by using the arrangement (a), but with the rear spring connected to the front end of the lever and the front spring connected to rear of the lever; as the ends of the springs then overlap one has to be placed above the other, and one is usually placed on the top of the axle casing, the other being underslung In (d) a single spring is used and the rear wheels are carried on the. .. will then have a greater influence than the inner wheel on the behaviour of the vehicle The larger the angle between the pivot axis and the transverse plane, the greater will be the movement of the outer wheel towards negative camber as the roll progresses Since negative camber tends to cause the tyre to steer inwards, the result will be a tendency to understeer (see Section 40.8) Furthermore, the lower... in the ends of the two parts A1 and A2 of the dead axle These two parts of the dead axle are free both to slide and rotate relative to each other on the axis XX The outer ends of the driveshafts G1 and G2 are coupled by universal joints to the short flanged shafts I in the wheel hubs, while their inner ends are similarly connected to the differential gear F There are no variations in track, since their... generally, half the weight of these is unsprung The greater the unsprung mass the larger will be the amplitudes of the hop and tramp motions of the axle and the more difficult it becomes to obtain good ride, roadholding and stability Another disadvantage of the live rear axle is the tendency for the propeller shaft torque to press the wheel on one side down and to lift that on the other side In the early... offset of the roadwheel on the Fig 43.26 Lancia Delta rear suspension 1170 The Motor Vehicle Fig 43.27 Ford Escort rear suspension strut This relieves the piston and rod assembly of some side load and therefore the friction between it and its cylinder The piston rods are Teflon coated to reduce the sliding friction between them and the glands at the top ends of the cylinders As can be seen from the illustration,... on the movement of the suspension strut and linkage In the front suspension of this vehicle, Fig 40.8, the method of avoiding such constraint has been simply to place the rubber mountings in line with the axes of the pivots of the wishbone links Yet another method – the use of shackles on the ends of the anti-roll bar – has been adopted by Alfa Romeo for the rear suspension of the Alfa 6, Fig 43.28 The . side of the vehicle. If we then do the same for the suspension on the other side, we find that the two lines OT 1148 The Motor Vehicle O C O T C T O T C at infinity Fig. 43.3 Note: If the suspension. assembly 1152 The Motor Vehicle back through the swinging halfshaft to the brakes. Since the stub axle carrier pivots about the outer end of the lower link, the vertical loads applied at the wheel put the. Normally the two torsion bars, one for the left-hand and the other for the right-hand arms, are attached one at each of the pivots X of the arms A, and their other ends are anchored to the frame or vehicle