Swing arm rear wheel drive independent suspension

(Fig. 10.51) This suspension normally takes the form of a pair of triangular transverse ('A' arm) swing arm members hinging on inboard pivot joints situated on either side of the final drive with their axes parallel to the car's centre line (Fig. 10.51). Coil springs are mounted vertically on top of the swing arm members near the outer ends. The wheels are supported on drive hubs mounted on ball or tapered roller bearings located within the swing arm frame.

Each drive shaft has only one universal joint mounted inboard with its centre aligned with that of the swing arm pivot axes. If the universal joints and swing arm pivot axes are slightly offset (above and below in diagram), the universal joints must permit a certain amount of sliding action to take place to compensate for any changes in drive shaft length as the spring deflects. Usually the outer end of the drive shaft forms part of the stub axle wheel hub.

Any increase in static vehicle weight causes the swing arms to dip so that the wheels which were initially perpendicular to the road now become negatively cambered, that is, both wheels lean towards the body at the top. Consequently, when the body rolls during cornering conditions, the inner and outer wheels relative to the turn become cambered negatively and positively respectively; they both lean towards the centre of rotation. With a change in static vehicle weight both swing arms pivot and dip an equal amount which reduces the wheel track width. Similarly, if the body rolls the inner swing arm pivot centre rises and the outer swing arm pivot drops, so in fact both the swing arm pivots tend to rotate about their roll centres thus reducing the width of the wheel track again. Both wheels at all times will remain parallel as there is no change in wheel toe-in or -out.

Low pivot split axle coil spring rear wheel drive independent suspension (Fig. 10.52) The conventional transverse swing arm suspension suffered from three major limitations:

Transverse Swing Arm Design

Fig. 10.52 Low pivot split axle coil spring rear wheel drive suspension

1 The swing arms were comparatively short because the pivot had to be mounted on either side of the final drive housing; it therefore caused a relatively large change in wheel camber as the car's laden weight increased or when wheel bounce occurred.

2 Due to the projection lines extending from the tyre to ground centre contact to and beyond the swing arm pivot centres, the body roll centre with this type of suspension was high.

3 There was a tendency when cornering for the short swing arms to become jacked up and with the load concentrated on the outside, the highly positively cambered wheel reduced its ability to hold the road so that the rear end of the car was subjected to lateral breakaway.

To overcome the shortcomings of the relatively large change in wheel camber and the very high roll centre height, the low pivot split axle suspension was developed.

With this modified swing axle arrangement the axle is split into two, with the adjacent half-axles hinged on a common pivot axis below the final drive housing (Fig. 10.52). A vertical strut supports the final drive assembly; at its upper end it is mounted on rubber discs which bear against the rear cross-member and at its lower end it is anchored to a pin joint situated on the hinged side of the final drive pinion housing. The left hand half-axle casing houses a drive shaft, crownwheel and differential unit. A single universal joint is positioned inside the casing so that it aligns with the pivot axis of the axles. The right hand half-axle houses its own drive shaft and a rubber boot protects the final drive assembly from outside contamination, such as dirt and water. A horizontal arm forms a link between the pivot axis and body structure and controls any lateral movement of the body relative to the axles. Fore and aft support for each half-axle is given by trailing radius arms which also carry the vertically positioned coil springs. The body roll centre thus becomes the pivot axis for the two half-axles which is considerably lower than for the conventional double pivot short swing arm suspension.

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