Four link coil spring live axle rear suspension

(Fig. 10.44) Substituting coil springs for semi-elliptic springs requires a separate means of locating and maintaining body and axle alignment when

Fig. 10.44 Four link coil spring live axle rear suspension

subjected to longitudinal and transverse forces caused by spring deflection, body roll or driving and braking thrust loads.

The locating links are comprised of a pair of long trailing lower arms and a pair of short diagonally positioned upper torque arms (Fig. 10.44). Rubber pin joints secure the forward ends of the arms to the body structure but the lower rear ends are attached underneath the axle tubes as far apart as possible and the upper short torque arms attached much closer together onto the final drive housing. The coil springs are mounted between the upper body structure and the lower pressed steel trailing arms. These springs only provide vertical support and cannot restrain any horizontal movement on their own. Spring deflection due to a change in laden weight causes both sets of arms to swivel together, thereby preventing the axle assembly rotating and possibly making the universal joints operate with very large angles. Both driving and braking thrust are transmitted through the lower trailing arms which usually are of a length equal to roughly half the wheel track so that when the arms swing the change in wheelbase is small. The upper arms are normally inclined at 45° to the car's centre line axis so that they can absorb any axle reaction torque tending to rotate the axle, and at the same time prevent relative lateral movement between the body and axle. Body roll or axle tilt are permitted due to the compliance of the rubber pin joints.

A relatively high roll centre is obtained which will be roughly at the upper torque arm height.

Torque tube rear wheel drive suspension (Fig. 10.45) One of the major problems with the Hotchkiss drive layout is that the axle torque reaction tends to spin the axle casing when transmitting drive torque in the opposite direction to the rotating wheels and when braking to twist the axle casing in the same direction as the revolving wheels. The result is a considerably distorted semi-elliptic spring and body to axle misalignment. To overcome this difficulty, a rigid tube may be bolted to the front of the final drive pinion housing which extends to the universal joint at the rear of the gearbox or a much shorter tube can be used which is supported at its front end by a rubber pin or ball joint attached to a reinforcing cross-member

Plan view Rear view

Fig. 10.45 Torque tube with trailing arm and transverse Watt linkage live axle rear suspension

Plan view Rear view

Fig. 10.45 Torque tube with trailing arm and transverse Watt linkage live axle rear suspension

(Fig. 10.45). On either side of the torque tube is a trailing arm which locates the axle and also transmits the driving and braking thrust between the wheels and body. Coil springs are mounted vertically between the axle and body structure, their only function being to give elastic support to the vehicle's laden weight. Lateral body to axle alignment is controlled by a transverse Watt linkage. The linkage consists of an equalizing arm pivoting centrally on the axle casing with upper and lower horizontal link arms anchored at their outer ends by rubber pin joints to the body structure. Thus when the springs deflect or the body rolls, the link arms will swing about their outer body location centres causing the equalizing arm to tilt and so restrain any relative lateral body to axle movement without hindering body vertical displacement.

With the transversely located Watt linkage, the body roll centre will be in the same position as the equalizing arm pivot centre. The inherent disadvantages of this layout are still the high amount of unsprung weight and the additional linkage required for axle location.

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