Trailing arm rear wheel drive independent suspension

(Fig. 10.53) The independent trailing arm suspension has both left and right hand arms hinged on an axis at right angles to the vehicle centre line (Fig. 10.53). Each arm, which is generally semi-triangular shaped, is attached to two widely spaced pivot points mounted on the car's rear subframe. Thus the trailing arms are able to transfer the drive thrust from the wheel and axle to the body structure, absorb both drive and braking torque reactions and to restrain transverse body movement when the vehicle is subjected to lateral forces. The

Plan view

Fig. 10.53 Trailing arm coil spring rear wheel drive independent suspension

Plan view

Fig. 10.53 Trailing arm coil spring rear wheel drive independent suspension rear ends of each arm support a live wheel hub, the drive being transmitted from the final drive to each wheel via drive shafts and inner and outer universal joints to accommodate the angular deflection of the trailing arms. The inner joints also incorporate a sliding joint to permit the effective length of the drive shafts to vary as the trailing arms articulate between bump and rebound.

When the springs deflect due to a change in laden weight, both wheels remain perpendicular to the ground. When the body rolls on a bend, the inner wheel becomes negatively cambered and the outside wheel positively cambered; both wheels lean away from the turn. Spring deflection, caused by either an increase in laden weight or wheel impact, does not alter the wheel track toe-in or -out or the wheel track width, but body roll will cause the wheel track to widen slightly.

Semi-trailing arm rear wheel drive independent suspension (Fig. 10.54) With the semi-trailing arm suspension each arm pivots on an axis which is inclined (skewed) to something like 50 to 70 degrees to the car's centre line axis (Fig. 10.54). The pivot axes of these arms are neither transverse nor longitudinally located but they do lie on an axis which is nearer the trailing arm pivot axis (which is at right angles to the car's centre line axis). Consequently the arms are classified as semi-trailing.

Swivelling of these semi-trailing arms is therefore neither true transverse or true trailing but is a combination of both. The proportion of each movement of the semi-trailing arm will therefore depend upon its pivot axis inclination relative to the car's centre line. With body roll the transverse swing arm produces positive camber on the inside wheel and negative camber on the outer one (both wheels lean inwards when the body rolls), whereas with a trailing arm negative camber is produced on the inside wheel and positive camber on the outer one (both wheels lean outwards with body roll).

Skewing the pivot axis of the semi-trailing arm suspension partially neutralizes the inherent ten dencies when cornering for the transverse swing arm wheels to lean towards the turn and for the trailing arm wheels to lean away from the turn. Therefore the wheels remain approximately perpendicular to the ground when the car is subjected to body roll.

Because of the relatively long effective swing arm length of the semi-trailing arm, only a negligible change to negative camber on bump and positive camber on rebound occurs when both arms deflect together. However, there is a small amount of wheel toe-in produced on both inner and outer wheels for both bump and rebound arm movement, due to the trailing arm swing action pulling the wheel forward as it deflects and at the same time the transverse arm swing action tilting the wheel laterally.

By selecting an appropriate semi-trailing arm pivot axis inclination, an effective swing arm length can be produced to give a roll centre height somewhere between the ground and the pivot axis of the arms. By this method the slip angles generated by the rear tyres can be adjusted to match the under-steer cornering characteristics required.

Transverse double link arm rear wheel drive independent suspension (Figs 10.55 and 10.56) This class of suspension may take the form of an upper and lower wishbone arm linking the wheel hub carrier to the body structure via pivot joints provided at either end of the arms. Drive shafts transfer torque from the sprung final drive unit to the wheel hub through universal joints located at the inner and outer ends of the shafts. Driving and braking thrust and torque reaction is transferred through the wide set wishbone pivot joints. One form of transverse double link rear wheel drive independent suspension uses an inverted semi-elliptic spring for its upper arm (Fig. 10.55).

A double wishbone layout has an important advantage over the swing axle and trailing arm arrangements in that the desired changes of wheel camber, relative to motions of the suspension, can

Arm Suspension Independent
Fig. 10.54 Semi-trailing arm coil spring rear wheel drive independent suspension
Fig. 10.55 Transverse swing arm and inverted semi-elliptic spring rear wheel drive independent suspension
Fig. 10.56 Transverse swing arm and double universal joint load bearing drive shaft rear independent suspension

be obtained more readily. With swing axles, camber changes tend to be too great, and the roll centre too high. Wheels located by trailing arms assume the inclination of the body when it rolls, thereby reducing the cornering forces that the tyres produce. Generally, transverse double link arm suspensions are designed to ensure that, when cornering, the outer wheel should remain as close to the vertical as possible.

A modified version (Fig. 10.56) of the transverse double link suspension comprises a lower transverse forked tubular arm which serves mainly to locate the wheel transversely; longitudinal location is provided by a trailing radius arm which is a steel pressing connecting the outer end of the tubular arm to the body structure. With this design the upper transverse link arm has been dispensed with, and a fixed length drive shaft with Hooke's universal joints at each end now performs the task of controlling the wheel hub carrier alignment as the spring deflects. Compact twin helical coil springs are anchored on both sides of the lower tubular forked arms with telescopic dampers positioned in the middle of each spring.

DeDion axle rear wheel drive suspension (Figs 10.57 and 10.58) The DeDion axle is a tube (sometimes rectangular) sectioned axle beam with cranked (bent) ends which are rigidly attached on either side to each wheel hub. This permits the beam to clear the final drive assembly which does not form part of the axle beam but is mounted independently on the underside of the body structure (Figs 10.57 and 10.58).

To attain good ride characteristics the usual sliding couplings at the drive shaft to the wheels are dispensed with in this design since when transmitting drive or braking torque, such couplings generate considerable frictional resistance which opposes the sliding action. A sliding joint is provided in the axle tube to permit wheel track variation during suspension movement (Fig. 10.57). Axle lateral location is therefore controlled by the drive shafts which are permitted to swing about the universal joint centres but are prevented from extending or contracting in length. Fore and aft axle location is effected by two Watt linkages. These comprise two lower trailing fabricated pressed steel arms, which also serve as the lower seats for the coil springs. Their rear ends are carried on pivots below the hub carriers. The other parts of the Watt linkage consist of two rearward extending tubular arms, each attached to a pivot above the hub carrier. The upper and lower unequal length link arm pivot centres on the body structure are arranged in such a way that the axle has a true vertical movement as the spring deflects so that there are no roll steer effects. When the body rolls

Plan view

End view

DeDion tube

Fig. 10.58 DeDion tube with diagonal radius arms and Watt transverse linkage rear suspension one hub carrier tends to rotate relative to the other, which is permitted by the sliding joint in the axle tube. The inner and outer sliding joints of the axle tube are supported on two widely spaced bronze bushes. The internal space between the inner and outer axle tube is filled about two thirds full of oil and lip seals placed on the outboard end of each bearing bush prevents seepage of oil. A rubber boot positioned over the axle sliding joint prevents dirt and water entering between the inner and outer tube members.

A DeDion axle layout reduces the unsprung suspension weight for a rear wheel drive car, particularly if the brakes are situated inboard. It keeps both road wheels parallel to each other under all driving conditions and transfers the driving and braking torque reactions directly to the body structure instead of by the conventional live axle route by way of the axle casing and semi-elliptic springs or torque rods to the body. The wheels do not remain perpendicular to the ground when only one wheel lifts as it passes over a hump or dip in the road. The body roll centre is somewhere near the mid-height position of the wheel hub carrier upper and lower link arm pivot points; a typical roll centre height from the ground would be 316 mm.

An alternative DeDion axle layout forms a triangle with the two diagonal radius arms which are rigidly attached to it (Fig. 10.58). The apex where the two radius arms meet is ahead of the axle and is pivoted by a ball joint to the body cross-member so that the driving and braking thrust is transferred from the axle to the body structure via the diagonal arms and single pivot. A transverse Watt linkage mounted parallel and to the rear of the axle beam controls lateral body movement relative to the axle. Therefore the body is constrained to roll on an axis which passes between the front pivot supporting the radius arms and the central Watt linkage pivot to the rear of the axle.

The sprung final drive which is mounted on the underside of the rear axle arch transmits torque to the unsprung wheels by way of the drive shaft and their inner and outer universal joints. The effective length of the drive shaft is permitted to vary as the suspension deflects by adopting splined couplings or pot type joints for both inner universal joints.

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