10143 Willetts velvet ride leading and trailing arm torsional rubber spring suspension Fig 10105a and b

The tandem suspension consists of leading and trailing swing arms. These arms are mounted back to back with their outer ends attached to the first and second drive axles whereas the swivel ends are supported on central trunnion pivot tubes which are mounted on a frame cross-member on either side of the chassis (Fig. 10.105(a)).

Torque arms attached to the suspension cross-member and to brackets in the centres of each axle casing assist the swing arms to transfer driving and braking torque reaction back to the chassis. These stabilizing torque arms also maintain the axles at the correct angular position. Good drive shaft geometry during articulation is obtained by the torque arms maintaining the axles at their correct angular position. Panhard rods (transverse tracking arms) between the frame side-members and the axle casings provide positive axle control and wheel tracking alignment laterally.

The spring consists of inner and outer annular shaped rubber members which are subjected to both torsional and vertical static deflection (Fig. 10.105(b)). The inner rubber member is bonded on the inside to the pivot tube which is supported by the suspension cross-member and on the outside to a steel half shell.

The outer rubber member is bonded on the inside to a median ring and on the outside to two half shells. The inside of the median ring is profiled

Fig. 10.104(a-c) Rubber springs mounted leading and trailing arms interlinked by rocking beam

(a) Exploded view

Fig. 10.105(a and b) Willetts (velvet) leading and trailing arm torsional rubber spring suspension to the same shape as the inner rubber member and half shell thus preventing inter rotation between the inner and outer rubber members. Key abutments are formed on the circumference of each outer half shell. These keys are used to locate (index) the outer rubber spring members relative to the trailing pressed (keyed) swing arm (Fig. 10.105(b)).

When assembled, the outer rubber member fits over the inner rubber member and half shell, whereas the trailing swing arm spring aperture is a press fit over the outer pair of half shells which are bonded to the outer rubber member. The leading swing arm side plates fit on either side of the median ring and aligned bolt holes enable the two members to be bolted together (Fig. 10.105(a and b)).

Load equalization between axles is achieved by torsional wind-up of the rubber spring members. Thus any vertical deflection of one or other swing arm as the wheels roll over any bumps on the road causes a torque to be applied to the rubber members. Accordingly an equal torque reaction will be transferred through the media of the rubber to the other swing arm and axle. As a result, each axle will support an equal share of the laden weight. Therefore contact and grip between wheels of both axles will be maintained at all times.

The characteristic of this springing is a very low stiffness in the unladen state which therefore provides a soft ride. A progressive spring stiffening and hardness of ride occurs as the swing arms are made to deflect against an increase in laden weight. An overall cushioned and smoothness of ride results.

An additional feature of this suspension geometry is that when weight is transferred during cornering from the inside to the outside of the vehicle, the deflection of the swing arms spreads the outer pair of wheels and draws the inner pair of wheels closer together. As a result smaller turning circles can be achieved without excessive tyre scrub.

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