An explanation of the Double Wishbone Suspension And the Four-Wheel Dynamic Geometry Control

The traditional description of the New RX-7's front and rear wheel suspension is double wishbone. This type of layout offers optimum lateral rigidity and camber control, both of which are important in maximizing handling capability as well as the pleasure of cornering. However, we did not simply adopt the classic double wishbone layout for the new RX-7- to reduce weight, while eschewing complication, many innovative ideas to this basic suspension were necessary. The wheel's axis of rotation is fixed by the suspension upright, which is, in turn, located by three joints - the upper arm, the lower

This force slightly deforms point C, and as a result, point A' moves rearward as it draws an arc centered around axis Z, which is located vertically above point B. Furthermore, since the car's nose dives underbraking, point A actually moves to points A'' and a'', rather than points A' and a'. In other words, we must view point A as dynamically moving within the area defined by the sector surrounded by points A', A'', a' and a'' aligned on the surface of the sphere which has point B as its center. Similarly, the joints of the upper arm and control link move over the spheres that are described by the radii defined by their respective arm or link. Consequently, theupright

arm and the control link. The relative positions of these three joints control the wheel's axis of rotation. Therefore, the first critical decision in the design process was the positioning of these arms and links to optimize their location and movement. The positions of the three joints however, do not trace the geometric locus as defined in the engineering drawings. The mounting points of the arms seem immovable in the drawing, but unless it is a pure-bred race car with a double wishbone set-up, a reasonable amount of compliance is necessary for better NVH. The compliance on the locus varies according to the bushings characteristics as well as the magnitude and direction of suspension input. The second critical decision in the design process was to determine

contacts the surfaces of the three spheres seemingly glide over them. When i) the radii of the spheres, ii) the relative distance between the centers of the spheres, or iii) the area defined by the sector, changes, the movement of the axle, which is fixed by the upright, also changes. In fact, this compliance causes changes to these factors in varying degrees, depending on driving conditions. In response to the aforementioned considerations, the arms and links of the RX-7 suspension are out in optimum positions. To withstand lateral forces in a straight line, the front lower arms are L-shaped. The rear lower arms, unlike in a conventional double wishbone layout, are constructed of separated I-arms and trailing links.

the amount of compliance of the pivot and joints. If we examine the movement of the joint between the front wheel upright and lower arm, point A is the joint, and points B and C are respectively the front and rear body mounting

position of the lower arms. As the suspension operates under normal driving conditions, A' a' describes the locus traced by point A, which draws an arc around the B - C axis with a radius of B - A. During braking, point A is subject to a rearward retardation force, and this because of the relative locations of points A, B and C, becomes an inward force at point C.

The changes in geometry of the arms and links are utilized to control the axis of rotation of the wheels. The bushes and joint are made to satisfy certain specific characteristics, in order to contain the negative effects of compliance. The basic approach was to increase the rigidity of the arms and links so that they follows the locus described in the engineering drawings. To accomplish this, all of the joints for the front and rear uprights are of the pillow-ball type, and the rear suspension employs bushings impregnated with pillow balls for the body mounting points of the lower arm. New bushings with special characteristics have also been developed. For the front suspension, sliding rubber bushings are used at the front and rear pivots of the upper arms and the front pivot of the lower arms. The same type are employed at the front and rear bushings are collars that slide in the axial direction, thus creating fore and aft compliance. In the lateral or torsional directions, the bushings provide minimum compliance and maintain a high level of rigidity. Liquid-filled bushings are used for the rear pivots of the front suspension's lower arms, to limit their fore and aft movement between the uprights and lower arms. In contrast to the front suspension, compliance of the rubber bushings is fully utilized at the rear pivots between the body and the rear suspension's lower arms. The bushings are stiff if moved in a forward direction and supple if moved rearward. The geometric movements caused by this compliance are used to control toe changes during acceleration and deceleration. Lateral forces generated during cornering, acceleration and braking change the tyres' contact with the road surface, which, when not adequately controlled, upsets the handling and braking characteristics. The suspension of the new Rx-7 controls the wheels and tyres to maintain ideal angles and locations under any driving conditions, and is designed to maintain ideal sports car handling characteristics at all times. That is why we called our new system Dynamic Geometry Control Suspension.