1082 Suspension roll steer

When a vehicle is cornering the body tilts and therefore produces a change in its ground height between the inside and outside wheels. By careful design, the suspension geometry can be made to alter the tracking direction of the vehicle. This self-steer effect is not usually adopted on the front suspension as this may interfere with steering geometry but it is commonly used for the rear suspension to increase or decrease the vehicle's turning ability in proportion to the centrifugal side force caused by cornering. Because it affects the steering handling characteristics when corner ing it is known as roll oversteer and roll understeer respectively.

Roll steer can be designed to cancel out large changes in tyre slip angles when cornering, particularly for the more heavily loaded outer rear wheel since the slip angle also increases roughly in proportion to the magnitude of the side force.

The amount of side force created on the front or rear wheels is in proportion to the load distribution on the front and rear wheels. If the car is lightly laden at the front the rear wheels generate a greater slip angle than at the front, thus producing an oversteer tendency. When the front is heavily loaded, the front end has a greater slip angle and so promotes an understeer response.

The object of roll steer on the rear wheels is for the suspension geometry to alter in such a way that

Rolle Lenkbar

the rear wheels steer the back end of the vehicle either outwards or inwards to compensate for the deviation in directional steer caused by changes in tyre slip angle.

A good example which illustrates suspension roll steer is with the semi-trailing arm steer rear suspension (Fig. 10.60(a, b and c)). If the body tilts when the vehicle corners the arms swing about their pivots so that the wheel axle attached to their free ends scribes circular arcs as they deflect up or down.

When the body rolls with the trailing arms set horizontally in their static position (Fig. 10.60(a)), the outer wheel and arm swings upwards towards the body whereas the inner wheel and arm rotates downwards and away from the body.

The consequence of the movement of the arms is that both axles move forward a distance x but because the axles of both trailing arms pivot at an inclined angle to the central axis of the vehicle the axis of end wheel axle will be slightly skewed inward so that both wheels now toe-in.

If the static position of the trailing arms were now set upwards an angle from the horizontal (Fig. 10.60(b)), when the body rolls the outer wheel and arm swing further upwards, whereas the inner wheel swings in the opposite direction (downwards towards the horizontal position). The outcome is that the outer wheel axle moves forwards whereas the inner wheel axle moves slightly to the rear. As a result, both the outer and inner stub axles skew the wheels towards the turn so that the outer wheel track toes-in and the inner wheel toes-out. Thus the change in tracking would tend to counteract any increase in slip angle due to cornering and so cause more understeer.

Setting the trailing arm static position so that both arms are inclined downwards an angle ^ from the horizontal (Fig. 10.60(c)) produces the opposite effect to having an upward tilt to the trailing arms. With body roll the outer wheel and arm now swings towards the horizontal and moves backwards slightly whereas the inner wheel and arm pivots further downward and moves forwards. Consequently both wheels are skewed outward from the turn, that is, the inner wheel toes-in and the outer wheel toes-out. The tracking in this situation compounds the increase in slip angle which is experienced while cornering and therefore produces an oversteer tendency.

10.8.3 Anti-dive and squat suspension control (Fig. 10.61)

All vehicles because of their suspended mass suffer from weight transfer when they are either acceler-

(b| Braking dive

Fig. 10.61 (a and b) Vehicle squat and dive

(b| Braking dive

Fig. 10.61 (a and b) Vehicle squat and dive ated, as when pulling away from a standstill, or when retarding while being braked.

A vehicle driven from a standstill (Fig. 10.61(a)) experiences a rapid change in speed in a short time interval so that a large horizontal accelerating force Fa is delivered at axle level to overcome the opposing body's inertia force FI which acts in the opposite sense through the centre of gravity but which is generally situated well above axle height somewhere between the two axles. Due to the vertical offset distance between the accelerating force FA and the inertia reaction force FI, a pitch moment will be produced which transfers weight from the front to the rear wheels as the front of the car lifts and the rear sinks, thereby making the car body squat at the rear.

Likewise weight transfer occurs from the rear to the front wheels when the vehicle is braked (Fig. 10.61(b)) which causes the body to pitch forward so that the rear rises and the front suspension dips, which gives a front nose dive appearance to the vehicle. The forces involved when braking are the ground level retarding frictional force FB and the inertia reaction force FI at the centre of gravity height. Therefore there is a larger offset between the two opposing forces when braking than when accelerating because with the latter the driving force acts at axle level. Consequently when the brakes are applied, the offset opposing retarding frictional force and the inertia reaction force produce a couple which attempts to make the body pitch and dive towards the front.

A leading and trailing arm suspension layout can be designed to counteract both squat (Figs 10.62 and 10.63) and dive (Fig. 10.64) tendencies.

When the vehicle accelerates forwards, the reaction to the driving torque pivots the suspension arm about the axle in the opposite direction to the input driving torque. Thus in the case of a front wheel drive vehicle (Fig. 10.62) the arm swings downwards and opposes the front upward lift caused by the reluctant inertia couple. Likewise with a rear wheel drive vehicle (Fig. 10.63) the reaction to the driving torque swivels the suspension arm upward and so resists the rearward pitch caused by the weight transference from the front to the rear axle.

For both drive acceleration and braking the amount of squat and dive is controlled by the length of the leading and trailing arms. The shorter they are, the greater their resistance to weight transference will be, and from that point of view alone, the better the quality of ride will be.

A large number of modern suspensions are based on trailing or semi-trailing arm designs which can build in anti-squat and -dive control but leading arm front suspension has inherent undesirable features and therefore is rarely used. However, antisquat and -dive control can be achieved by producing a virtual lead arm front suspension, that is, by arranging the swing axis of a double wishbone arm suspension to converge longitudinally along the wheelbase at some point.

The double transverse wishbone arm suspension geometry (Fig. 10.65) is laid out so that the top wishbone arm axis tilts downwards and the lower slightly upward towards the rear so that lines drawn through these pivot axes intersect somewhere towards the rear.

When the brakes are applied, the body will tend to pitch downward at the front but the clamped disc caliper or back plate will attempt to rotate with the road wheel. The result is that the upper and lower wishbone pivot axis converging projections form in effect an imaginary leading arm of length R which tends to swing upwards to the rear about the wheel axle. It therefore imparts an upthrust which opposes and cancels the downward pitch of the body.

Similar results can be obtained with the MacPher-son strut suspension (Fig. 10.66) where the strut is made to tilt backward from the top and the lower transverse wishbone arm pivot axis tilts upwards to the rear. A line drawn perpendicular to the strut through the top pivot will then intersect a line projecting from the wishbone pivot axis. The distance between the strut to wishbone ball pivot and the meeting point of the two rearward projected lines therefore provides the effective trailing arm length or swing radius R. Thus an anti-dive torque T is produced of magnitude FR which opposes the forward transfer of weight when braking.

Griggs Torque Arm Length Pivot
Fig. 10.63 Leading and trailing arm rear wheel drive anti-squat suspension action
Fig. 10.65 Transverse double wishbone suspension with longitudinal converging axis geometry
Fig. 10.66 MacPherson strut suspension with longitudinal converging axis geometry

Unfortunately the amount of anti-dive control must be limited since the upward swing of the imaginary trailing arm rotates the steering swivel joints so that the castor angle changes from positive to negative, thus destabilizing the steering firmness and so producing steering reaction and wander. Normally front suspension design restricts the anti-dive control to within 50 to 70% and the rear suspension may provide a 100% cancellation of brake dive.

10.8.4 Front wheel drive independent suspension wheel bearing arrangements (Figs 10.67 and 10.68)

With a front wheel drive independent suspension two major functions must be fulfilled:

1 The wheels must be able to turn about their swivel pins simultaneously as the suspension members deflect between bump and rebound.

2 The transmission of drive torque from the final drive to the wheels must be uninterrupted as the suspension members move between their extremes.

The majority of steered independent suspensions incorporate a wheel hub carrier supported between either;

a) an upper and lower ball and socket joint mounted between a pair of transverse arms (Fig. 10.67), b) a leg strut mounted on a swivel bearing and a lower ball and socket joint located at the free end of a transverse arm (Fig. 10.68).

In both suspension arrangements the hub carrier has a central bore which may directly or indirectly house the wheel hub bearings. For light and medium loads, roller ball bearings are preferred but for heavy duty applications the taper roller bearing is more suitable.

Traditional wheel bearing assemblies employ two separate bearings; either ball or taper roller

Aircraft Steering Design
Fig. 10.67 Front wheel drive MacPherson strut suspension with single double row ball or roller wheel bearings

types. The present trend is the use of a single bearing with double row rolling elements, be they ball or taper rollers which are sealed, pre-set and lubricated for life. The preference is because they provide a more compact and cheaper assembly.

These double row rolling element single bearings can be of the following classes:

1 Detachable double row angular contact ball or taper roller bearing type (Fig. 10.67). There are two separate inner track rings and one wide outer track ring. The contact angle for the balls is 32° to give the greatest distance between pressure centres of the bearing, thus reducing the reactions caused by the tilting action of the wheels. This angle is so chosen that the bearing

Fig. 10.68 Front wheel drive transverse wishbone suspension with fully integrated double low wheel bearings

has sufficient radial load capacity to withstand the weight imposed on the wheel and also to provide adequate axial load carrying capacity under cornering conditions. The cage that separates the balls is made from Nylon and does not, if damaged, affect the bearing performance. Preloading of the ball or taper roller bearings is set at the factory, therefore no adjustment is required after the bearing is assembled to its hub.

When assembled, the inner track rings are a force fit over the hub sleeve which is internally splined to the constant velocity joint's output stub shaft and the outer track ring is a press fit inside the hub carrier bore. 2 Fully integrated double row angular contact ball bearing type (Fig. 10.68). With this arrangement the inner track ring is extended on the outside with a flange to locate and support the wheel while its middle is bored and splined to accommodate the constant velocity joint splined output shaft. Thus the inner bearing member (track ring) takes over the whole function of the normal drive wheel hub. The outer track ring also supports both rows of balls and it is enlarged in the centre to provide a flange which aligns accurately within the wheel hub carrier's bore. Thus the inner and outer bearing members are integral parts of the wheel hub and bearing housing attached to the hub carrier respectively.

In both bearing arrangements the stub shaft nut is fully tightened to prevent axial movement between the hub and stub shaft and also, in the case of the detachable double row bearing, to secure its position.

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  • Grimalda Boffin
    How to rol stearing a car video?
    2 years ago

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