Longitudinally located semielliptic springs

(Fig. 10.28(a)) When semi-elliptic leaf springs support the body, the pivoting point or body roll centre will be roughly at spring-eye level but this will become lower as the spring camber (leaves bow) changes from positive upward bowed leaves when unloaded to negative downward bowed leaves with increased payload.

Transverse located Panhard rod (Fig. 10.28(b)) The use of coil springs to support the body requires some form of lateral body to axle restraint if a torque tube type axle is to be utilized. This may be provided by a diagonally positioned Panhard rod attached at its ends to both the axle and body. When the body tilts it tends to move sideways and either lifts or dips depending which way the side force is applied. Simultaneously the body will roll about the mid-position of the Panhard rod.

Diagonally located tie rods (Fig. 10.28(c)) To provide both driving thrust and lateral support for

(c) Four link and coil spring

Fig. 10.28(a-d) Body roll centres for rigid beam axle suspensions

(c) Four link and coil spring

Fig. 10.28(a-d) Body roll centres for rigid beam axle suspensions a helical coil spring live axle layout, a trailing four link suspension may be adopted which has a pair of long lower trailing arms which absorb both the driving and braking torque reactions and a pair of short upper diagonally located tie rods to control any lateral movement. Any disturbing side forces which attempt to make the body tilt sideways will cause it to roll about a centre roughly in line with the upper tie rod height.

Transverse Watt linkage (Fig. 10.28(d)) An alternative arrangement for controlling the sideways movement for a coil spring suspension when used in conjunction with either a live axle or a DeDion tube is the Watt linkage. Suspension linkages of this type consist of a pair of horizontal tie rods which have their outer ends anchored to the body and their inner ends coupled to a central balance lever which has its pivot attachment to the axle beam. If the body is subjected to an overturning moment it will result in a body roll about the Watt linkage balance lever pivot point. This instantaneous centre is therefore the body roll centre.

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