108 Suspension design consideration

10.8.1 Suspension compliance steer

Rubber bush type joints act as the intermediates between pivoting suspension members and the body to reduce the transmission of road noise from the tyres to the body. The size, shape and rubber hardness are selected to minimize noise vibration and ride hardness by operating in a state of com-pressive or torsional distortion.

If the rubber joints are subjected to any abnormal loads, particularly when the suspension pivots are being articulated, the theoretical geometry of the swing members may be altered so that wheel track misalignment may occur.

The centrifugal force when cornering can produce lateral accelerations of 0.7 to 8.0 g which is sufficient to compress and distort the rubber and move the central pin off-centre to the outer hole which supports the rubber bush.

With transverse or semi-trailing arms suspension (Fig. 10.59(a)) the application of the brakes retards the rotation of the wheels so that they lag behind the inertia of the body mass which is still trying to

Fig. 10.59(a and b) Semi-trailing suspension compliance steer

thrust itself forward. Consequently the opposing forces between the body and suspension arms will distort the rubber joint, causing the suspension arms to swing backwards and therefore make the wheel track toe outwards.

The change in the wheel track alignment caused by the elastic deflection of the suspension rubber pivot joints is known as suspension compliance steer since it introduces an element of self-steer to vehicle.

Compliance steer is particularly noticeable on cornering if the brakes are being applied since the heavily loaded outside rear wheel and suspension is then subjected to both lateral forces and fore and aft force which cause an abnormally large amount of rubber joint distortion and wheel toe-out (Fig. 10.59(a)), with the result that the steering will develop an unstable oversteer tendency.

A unique approach to compliance steer is obtained with the Weissuch axle used on some Porsche cars (Fig. 10.59(b)). This rear transverse upper and lower double arm suspension has an additional lower two piece link arm which takes the reaction for both the accelerating and decelerating forces of the car. The lower suspension links consist of a trailing tubular steel member which carries the wheel stub axle and the transverse steel plate arm. The trailing member has its front end pivoted to a short torque arm which is anchored to the body by a rubber bush and pin joint pivoted at about 30° to the longitudinal car axis. When the car decelerates the drag force pulls on the rubber bush pin joint (Fig. 10.59(b)) so that the short torque arm is deflected backward. As a result, the transverse steel plate arm distorts towards the rear and the front end of the trailing tubular member supporting the wheel is drawn towards the body, thus causing the wheel to toe-in. Conversely, when the car is accelerated the wheel tends to toe-out, but this is compensated by the static (initial) toe-in which is enough to prevent them toeing-out under driven conditions. The general outcome of the lower transverse and trailing link arm deflection is that when cornering the more heavily loaded outside wheel will toe-in and therefore counteract some of the front wheel steer, thus producing a degree of understeer.

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