Fig. 13.3. Elements of the front suspension of the Fiesta: 1 – McPherson shock absorber strut; 2 – anti-roll bar; 3 – steering knuckle with hub assembly; 4 – subframe; 5 – L-shaped wishbones
The front suspension of the Fiesta has a high "elasticity" when cornering. Excellent suspension behavior is provided by shock absorber struts 1 (Fig. 13.3) McPherson, stabilizer 2 of transverse stability, improving the behavior of the car when cornering, and L-shaped transverse levers 5 of independent suspension, guiding the front wheels in the longitudinal and lateral directions.
Independent wheel suspension.
The McPherson type front wheel suspension (patented in 1949) consists of a shock absorber, a spring and a steering knuckle with a wheel hub.
Shock absorber strut.
Combines the functions of a guide device and a damping element and consists of a coiled cylindrical spring and a telescopic shock absorber that operates inside the spring coils. The internal stops of the rack limit the downward movement of the wheel, for example, during a hard impact or driving over a pothole, while preventing the spring from locking and the shock absorber from suddenly collapsing.
Shock absorber strut dome (in the wheel wing).
The upper part of the shock absorber strut is attached to the dome. On top, it is made in the form of a rubber support with a thrust bearing, which rests on the upper and lower disc disks with a centering ring. The shock absorber strut rod is attached to the thrust bearing.
Swivel support (ball joint).
It is attached to the lower part of the shock absorber strut with two bolts and connects the steering knuckle to the lower wishbone of the independent suspension. The wishbone of the independent suspension is fixed in the axle beam and takes up lateral forces.
Anti-roll bar.
The transverse control arms of the independent suspension are attached to the anti-roll bar, which is a suitably bent rod made of spring steel. It works as follows: if, when turning, the wheel located on the inner radius of the turn moves away from the car, the anti-roll bar twists. This resulting elastic force supports the shock absorber strut, which is located on the outer radius of the turn, which, together with the springs, increases the rigidity of the strut. The result of the anti-roll bar is a significant reduction in the roll of the car when turning.
Rack and pinion steering.
It is fixed to the axle beam behind the engine. The two-piece steering shaft acts directly on the toothed rack, to the ends of which the left and right steering rods are respectively attached. The turning movements are transmitted through the steering rod ends to the steering knuckle and wheels.
TECHNICAL DICTIONARY
Front wheel alignment angles
Fig. 13.4. Front wheel alignment
Wheel alignment angles have a significant impact on vehicle stability, tire wear and fuel consumption (Fig. 13.4, 13.5).
Fig. 13.5. Front wheel alignment angles: A — longitudinal tilt angle of the steering axis; B - collapse; C — angle of transverse inclination of the wheel rotation axis
Convergence —the difference in distance between the side flanges of the rims, measured at the rear and front of the wheels at the level of their centers. Positive toe-in means that the front wheels are closer to each other than the rear wheels (at the level of their centers). Toe-in affects the straightness of the vehicle's movement and its controllability. Due to toe-in, the front wheels rotate parallel. When turning the vehicle, due to the trapezoidal arrangement of the steering rods, the wheel located on the inner radius of the turn turns at a greater angle than the wheel located on the outer radius of the turn. This is due to the fact that when turning, the inner wheels must move in a circle of a smaller radius than the outer ones. This automatically maintains the forces in the turn and the trajectory of the vehicle. In addition, toe-in prevents wheel vibration and tire wear.
Collapse — is the angle between the plane of rotation and the vertical of the wheel. It can be positive, as in the Fiesta, if the wheels are tilted outward, or negative, if the wheels are tilted inward. Correctly set camber reduces the impact of road surface bumps on the steering, reduces turning forces and affects the uniformity of wear of the front tires. If the camber angle of one wheel is positive and the other is negative, the car will pull to one side when driving straight.
Transverse angle of inclination of the wheel pivot axis— is the angle between the axis of inclination of the steering knuckle and the vertical drawn through the wheel attachment point in the longitudinal plane of the car. The distance from the center line through the wheel (the average point of contact of the tire with the surface) is the turning radius. To reduce the impact of negative forces in the steering, the turning radius should be as small as possible. Due to the camber angles and the transverse inclination of the steering axis, the points of contact of the wheels with the road are located closer to the axis of the steering knuckle. This maintains the so-called rolling shoulder. The smaller the rolling shoulder, the easier it is to steer the car. This leads to the fact that with the wheels turned out, the car rises slightly. When the steering wheel is released, the wheels spontaneously return to the middle position (restoring moment). In addition, impacts from uneven surfaces are transmitted less to the steering.
Angle of longitudinal inclination of the axis of rotation — is the angle between the axis of the steering knuckle tilt and the vertical drawn through the wheel attachment point in a plane perpendicular to the longitudinal axis of the vehicle. The axis around which the wheel turns is positioned in space so that its lower part is tilted forward. Such an angle of longitudinal tilt is called positive. A positive angle ensures better stability and stabilization of the steered wheels during straight-line movement.