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1.3 The independent suspension mechanical linkages 21 FIGURE 1.12. This table is evidence of the alternatives that were examined by Falchetto while designing the front suspensions of the Lancia Lambda. The architec- tures considered are almost all found on present cars. FIGURE 1.13. The front view of the Lancia Lambda shows the front suspension and the gate structure of connection to the body (Automobile Museum of Turin). 22 1. HISTORICAL EVOLUTION FIGURE 1.14. This front suspension was presented by Cottin-Deguttes in 1927. No other examples of this kind were developed. to the problem of efficient lubrication of the sliding tubes by integrating, in a sealed element, guide, spring and hydraulic shock absorber. The elastic element is now a coil spring; this is probably the first application in which the car weight is entirely sustained by a spring of this kind. An interesting variation of this suspension family was presented by Cottin- Degouttes on a car shown in Fig. 1.14 and introduced in 1927; the vertical tubes suspension was modified, but we do not know the reason for this decision. The tubes are slightly inclined with the top closer to the center of the car, while the elastic element is again a cross leaf spring, as in the Stephens. The leaf spring is articulated to the sliding strut; because the path of the tip of the spring is almost circular and the strut is coupled to the tube with spherical element, we can assume that the wheel can partially recover the camber angle variations caused by body roll. Although the kinematic properties of this suspension can be similar to a double wishbone suspension, this kind of architecture had no industrial follow up. A different scheme of front independent wheel suspension is attributed to Dubonnet, an important independent car designer, and was exploited by many car manufacturers, among them Fiat which applied this solution to production starting in 1935. The suspension includes a sealed bearing element (cartridge) that integrates the helical spring in the same oil as the shock absorber, as shown in the detail on 1.3 The independent suspension mechanical linkages 23 FIGURE 1.15. Two versions of independent front suspensions designed by Dubonnet and produced by Fiat, beginning in 1935. The suspension includes a bearing sealed cartridge, integrating the elastic element and the shock absorber. the far right of Fig. 1.15; spring and damper work on a finger crossing through the cartridge. This finger is connected to one of the arms of a double wishbone mechanism. The cartridge can be integrated into the suspension in two different ways. As we can see on the left of the same figure, the cartridge can be mounted on a non-suspended rigid axle through a king-pin; in this case the entire suspension is steering with the wheel and the steering mechanism is the same as for a rigid steering axle, making the motion of wheels completely independent. The kinematic behavior is almost the same as a vertical tube suspension, but the spring and shock absorber unit is easier to manufacture. The architecture is similar to that of a double trailing arms suspension. The second alternative, shown on the right of the same figure, offers a car- tridge flanged to a front cross member of the chassis structure. The suspension arms can swing but not steer; the wheel strut is mounted with knuckles to the oscillating arms. As a double wishbone suspension the steering mechanism is different and the track rod is replaced by an articulated system. A suspension similar to the first scheme of double trailing arms is attributed to Porsche and is shown in Fig. 1.16; it was developed in 1931 and until the 1970s on the Volkswagen Beetle and other cars from this company. The two articulated parallelograms are mounted at the end of a double tube structure that also build up the front cross member of the platform. The upper arms are flanged to the same torsion bar, contained in the upper tube; this bar limits the roll angle. The lower arms are flanged to two different torsion bars, each of them flanged at the other end to the tubular structure; these act as elastic elements. The dimensions of this suspension are therefore constrained in the three directions. While these suspensions were effective at reducing shimmy, they later demonstrated an undesired feature: They caused the wheel to have, during turns, a camber angle equal to the roll angle of the body. 24 1. HISTORICAL EVOLUTION FIGURE 1.16. The Porsche suspension is made with two double trailing arms elements; it was developed in 1931 and produced by Volkwagen till the 1970s. Bearing in mind that the roll angle, due to the centrifugal force, tilts the body to the outside of the curve, the wheels assume a camber angle so as to red- uce the cornering stiffness of the tire. It would be better if the camber variation were opposite, but the ideal behavior is to have no variation under any condition. To improve this undesirable situation double wishbone suspensions were developed with arms of unequal length; the shorter upper arm increases the camber angle when the suspension compresses and decreases when the suspension is extends. The value of the camber angle is such as to compensate partially for the effect of the roll angle. One of the first examples of this concept was introduced by Studebaker in 1939 and followed by many car manufacturers until the present. A drawing of this suspension is shown in Fig. 1.17. We would like to point out that the elastic element is integrated with the lower arm and is still made with a transversal leaf spring; as a matter of fact many manufacturers were not keen to abandon leaf springs. The reasons were many, chief among them reliability, but we can also see in this application that the leaf spring integrates the function of two elements: the arms and the spring. The reliability of the production process was likewise important and, last but not least, the existing investments for a high volume of production. An interesting Fiat patent exist on this subject, applied to cars with rear engines, beginning in 1955; it fit the leaf spring to the body through two sym- metric bearing points. With a suitable distance between the bearing points it is possible to obtain different elastic characteristics on symmetric and asymmetric suspension strokes; in this way, it is possible with a single spring element to obtain the function of the main elastic element and the anti-roll bar. We can also observe in Fig. 1.17 that the Jeantaud mechanism present on previous steering systems has disappeared; the track rod can no longer be ap- plied, because the distance between the articulation points of the track rod arms changes with the suspension stroke. 1.3 The independent suspension mechanical linkages 25 FIGURE 1.17. Front suspension made by Studebaker in 1939. The mechanism is the double wishbone type and features an upper arm of reduced length. The lower arm integrates the elastic function, being made with a leaf spring. Before the advent of the rack steering system, applied beginning in 1960s, a small articulated parallelogram was employed whose rods were articulated to the body and rotated by the steering box; to suitable points on these rods were connected the two steering linkages. This mechanism also approximates, with some error, the ideal Ackermann law; the presence of ever increasing sideslip angles, because of increasing speed makes this error less important. The theoretical model of the behavior of tires under applied side forces and the derivative concept of sideslip angle were also developed during these years. Double wishbone suspensions with different length arms spread quite rapidly in the following years and became almost universal in front axles during the 1960s. Figure 1.18 represents a suspension with stamped steel low thickness steel arms and coil springs launched by Fiat in 1950 on the 1400 and applied with minor modifications on smaller models in the following years. McPherson, a design engineer of Ford in the U.S., introduced the front suspension in 1947 that was named after him; it can be considered a double wishbone suspension with different length arms, where the length of the upper arm is infinite (see Fig. 1.19). 26 1. HISTORICAL EVOLUTION FIGURE 1.18. Double wishbone suspension adopted on Fiat cars in 1950; this solu- tion was almost universal in the following years because of fairly good elastokinematic behavior. This suspension should not be confused with a basic simplification of the double wishbone suspension for cost reduction; it later contributed to the diffu- sion of modern front wheel driven cars, because the lack of the upper arm left the necessary space for a transversal engine installation. This suspension spread rapidly beginning at the end of the 1960s, not only on front wheel driven cars built in those years but on numerous cars with longitudinal engines. The second advantage of this suspension is that it simplifies the body struc- ture thanks to the more rational distribution of connection points. The reduction of kinematic performance as compared with the double wish- bone solution is not very relevant and this solution is also applied to contempo- rary sport cars. Honda in the 1980s introduced a final innovation in the front suspension ar- chitecture. It conceived a swan-necked wheel strut (see Fig. 1.20), which allowed the double wishbone suspension to also be installed on transversal front wheel driven cars. The descendants of the McPherson and Honda suspension today share the market, with the introduction of many improved details that we will omit for the sake of simplicity. Rear suspension history should be much more complicated; the attempt to identify evolutionary trends risks oversimplifying the explanation. It should be pointed out that the rigid axle, receiving an elastic member more sophisticated than leaf springs and additional linkages, had long life both on front and rear driven cars. On front wheel driven cars the weight of the axle was not relevant; in fact the simplified function, because of the absence of the differential and final drive allowed the use of tubular structures that were quite light. 1.3 The independent suspension mechanical linkages 27 FIGURE 1.19. McPherson introduced on American Ford cars this kind of suspension beginning in 1947. The kinematic equivalent of this suspension is a double wishbone suspension where the upper arm has infinite length. On rear wheel driven cars, particularly on luxury and sport cars adopting the rigid axle, the weight was reduced with suspended differentials, good kinematic behavior was obtained with coil spring and more complicated linkages. A particularly elegant example of rigid rear axle is provided by Alfa Romeo. This scheme was adopted on different cars starting in the 1970s. It can be con- sidered an improvement of the De Dion-Bouton suspension. Figure 1.21 shows this design. 28 1. HISTORICAL EVOLUTION FIGURE 1.20. Honda front suspension with high double wishbone; high refers to the position of the upper arm compared to the conventional case. FIGURE 1.21. A particularly elegant example of a rigid rear axle is provided by Alfa Romeo. This scheme was adopted on different cars starting from the 1970s. It can be considered an improvement of the De Dion-Bouton suspension. 1.3 The independent suspension mechanical linkages 29 FIGURE 1.22. One of the first applications of an independent rear suspension with trailed arms appeared on the Lancia Aprilia in 1937. A triangular structure building up the rigid axle is linked to a spherical joint in the front, which determines a precise position for the roll axis; suspension stroke and body roll do not affect axle steering thanks to the guidance given by a Watt mechanism in the back. The need to obtain a well shaped and spacious trunk imposed the develop- ment of rear suspensions different from the rigid axle. One of the first applications of an independent rear suspension with trailing arms appeared on the Lancia Aprilia in 1937. This suspension, presented in Fig. 1.22, is adapted for rear wheel drive and consists of two longitudinal arms with the articulation point in front of the wheel; the elastic element remains a transversal leaf spring with the addition of a transversal torsion bar. Being an independent suspension, the differential and final drive is sus- pended on the body. This kind of suspension, in connection with the application of coil springs, had considerable diffusion up until the present, primarily on cars with front wheel drive. Such suspensions enjoy the advantage of reduced space for linkages and elastic system; the disadvantage is the modest performance and the weight of the trailing arms. Rear wheel driven cars with rear engine had a significant diffusion between the 1940s and 1960s; today they are no longer found, with the exception of some niche sport cars. The reason for the diffusion of this architecture was the excellent interior roominess with contained exterior dimensions, thanks to the absence of the pro- peller shaft, at a time when economical and reliable components for the front wheel drive were not yet available. 30 1. HISTORICAL EVOLUTION FIGURE 1.23. The rigid axle could not be applied because of compatibility with the rear power train. On the Fiat 600 of 1955 the semi-trailing arm suspension was chosen because it allowed the use of simple constant velocity joints. The rigid axle could not be applied because of the short distance between power train and axle; so called semi-trailing arms were usually selected, because of their compatibility with inexpensive constant velocity joints. In Fig. 1.23 is shown a scheme of the Fiat 600 of 1955. The traced dotted lines show how it was possible to have the suspension roll axis cross the differen- tial; this condition is mandatory for avoiding the application of sliding constant velocity joints. The camber angle relative to the ground does not change because of the roll of the body, but varies remarkably because of payload variations; the look of the unloaded car is characterized by a noticeable positive camber (wheel mean planes cross below the ground). In this condition rod holding is quite approx- imate. In addition, on bumpy roads the track changes continuously because of the suspension stroke, causing premature tire wear. This kind of suspension was also adopted in many rear wheel driven cars with front engine because of its limited height; today it has been abandoned. In 1969 Volkswagen conceived a new rear suspension quite suitable to front wheel driven cars: The so called semirigid axle or twist axle; in a short time it became one of the most widely diffused architectures. . leaf spring, as in the Stephens. The leaf spring is articulated to the sliding strut; because the path of the tip of the spring is almost circular and the strut is coupled to the tube with spherical. them flanged at the other end to the tubular structure; these act as elastic elements. The dimensions of this suspension are therefore constrained in the three directions. While these suspensions. Volkwagen till the 1970s. Bearing in mind that the roll angle, due to the centrifugal force, tilts the body to the outside of the curve, the wheels assume a camber angle so as to red- uce the cornering