Trailing arm with progressive quarterelliptic spring

(Fig. 10.101) Each axle is carried on a trailing arm; the arms on one side are interconnected by a spring in such a way that the upward reaction at one wheel increases the downward load on the other (Fig. 10.101). The inverted quarter-elliptic spring is clamped to the rear trailing arm.

Trailing arms

Fig. 10.101 Trailing arm with progressive quarter-elliptic spring

Its leading end is shackled to a bracket on the front trailing arm. Both trailing arms are welded fabricated steel members of box-section. The attachment of the quarter-elliptic springs to the rear trailing arms is so arranged that as the spring deflects on bump a greater length of spring comes into contact with the curved surface of the arm, thereby reducing the effective spring length with a corresponding increase in stiffness. On rebound, the keeper plate beneath the spring is extended forward and curved downward so that there is some progressive stiffening of the spring also on rebound. With this effective spring length control, the trailer will ride softly and easily when unladen and yet the suspension will be able to give adequate upward support when the trailer is fully laden.

Tri-axle semi-trailer suspension (Fig. 10.102(a and b)) Tri-axle bogies are used exclusively on trailers. Therefore all these axles are dead and only laden weight distribution and braking torque reaction need to be considered.

The reactive balance beam interlinking between springs is arranged in such a way that an upward reaction at one wheel increases the downward load on the other, so that each of the three axles supports one third of the laden load (Fig. 10.102(a)).

The load distribution between axles is not quite so simple when the vehicle is being braked, owing to torque reaction making the axle casings rotate in the opposite sense to that of the road wheels. Consequently the foremost end of each spring tends to pull downwards while the rearmost spring ends push upwards. Accordingly the balance beams will react and therefore tilt clockwise. The net change in axle height relative to the chassis is that as the first axle is raised slightly so that tyre to road contact is reduced, the second axle experiences very little height change since the spring front end is made to dip while the rear end is lifted, and the third axle is forced downwards which increases the axle load and the tyre to road contact grip. This uneven axle load distribution under braking conditions is however acceptable since it does not appear to greatly affect the braking efficiency or to cause excessive tyre wear.

One problem with tri-axle trailers is that it is difficult and even impossible to achieve true rolling for all wheels when moving on a curved track due to the large wheel span of the three parallel axles, thus these layouts can suffer from excessive tyre scrub. This difficulty can be partially remedied by using only single wheels on the foremost and rearmost axles with the conventional twin wheels on

lb} Plan view

Fig. 10.102(a and b) Tri-axle semi-trailer with self-steer axle lb} Plan view

Fig. 10.102(a and b) Tri-axle semi-trailer with self-steer axle the middle axle (Fig. 10.102(b)). An alternative and more effective method is to convert the third axle into a self-steer one. Self-steer axles, when incorporated as part of the rearmost axle, not only considerably reduce tyre scrub but also minimize trailer cut-in because of the extent that the rear end is kicked out when cornering. Not only do self-steer axles improve tri-axle wheel tracking but they are also justified for tandem axle use.

Self-steer axle (Fig. 10.102(b)) The self-steer axle has a conventional axle beam with kingpin bosses swept forward to that of the stub axle centre line to provide the offset positive castor trail (Fig. 10.102(b)). Consequently the cornering side thrust on the tyre walls causes the wheels to turn the offset kingpins into line with the vehicle's directional steered path being followed. Excessive movement of either wheel about its kingpin is counteracted by the opposite wheel through the interconnecting track rod, while the trail distance between the kingpin and stub axle provides an automatic self-righting action when the vehicle comes out of a turn.

Possible oscillation on the stub axles is absorbed by a pair of heavy-duty dampers which become very effective at speed, particularly if the wheels are out of balance or misaligned.

Since the positive castor trail is only suitable for moving in the forward direction, when the vehicle reverses the wheels would tend to twitch and swing in all directions. Therefore, when the vehicle is being reversed, the stub axles are locked by a pin in the straight ahead position, this operation being controlled by the driver in the cab. The vehicle therefore behaves as if all the rear wheels are attached to rigid axles.

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