10142 Rubber spring mounted on leading and trailing arms interlinked by balance beam

This tandem axle suspension is comprised of leading and trailing swing arms pivoting at their inner ends on the downward extending chassis frame with their outer ends clamped to the axle casings (Fig. 10.104(a)). The front and rear rubber springs are sandwiched between swing arm rigid mounting plates and a centrally pivoting balance beam. When in position these springs are at an inclined angle and are therefore subjected to a combination of compression and shear force.

When the swing arms articulate the spring mounting plate faces swivel and move in arcs. Thus the nature of the spring loading changes from a mainly shear action with very little com-pressive loading when the axles are unladen (Fig. 10.104(b)) to much greater compressive loading and very little shear as the axles become fully laden (Fig. 10.104(c)). Since the rubber springs are about 14 times stiffer in compression than shear, the springs become progressively harder as the swing arms deflect with increasing laden weight. If the first axle is deflected upward as it moves over a bump, the increased compressive load acting on the spring will tilt the balance beam so that an equal increase in load will be transferred to the second axle.

Because the axles are mounted at the ends of the swing arms and the springs are positioned nearer to the pivot centres, axle movement will be greater than spring deflection. Therefore the overall suspension spring stiffness is considerably reduced for the ratio of axle and spring plate distance from the swing arm pivot centre which accordingly lowers the bounce frequency by 30%. Both leading and trailing swing arms absorb the braking torque reaction so that load distribution between axles will be approximately equal.

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