IN THE SP OTLIGHT

The Weissach Effect The Evolution of a Revolution

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© Porsche

When the Porsche 928 was launched 40 years ago, it was the first completely new design from Porsche. Some of the technical innovations introduced in what was then an ultra-modern grand tourer are still applicable today, such as the Weissach axle, which combined the cornering ability of the 911 with a more comfortable ride. The basic principle of this rear axle, which involves using lateral loads to produce toe-in in the wheel on the outside of the corner and using load shifts to produce the same result in the event of changes in the longitudinal forces, is still applied in almost all modern multi-link rear suspension systems.

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FROM RIGID TO MULTI-LINK SUSPENSION SYSTEMS

Since the early days of the automobile, developments in car rear axles have resulted in a wide variety of different suspension designs. These range from rigid and swing suspension, trailing arm and semi-trailing arm suspension, and torsion beam suspension through to a number of different variants of the multi-link suspension system. Axles with swing suspension were widely used in rear-wheel-drive cars from the 1920s to the 1970s. The disadvantages of these systems were the jacking effect on rebound and an increasingly positive camber change in the outside wheel when cornering. This led to a reduction in the tire contact patch that could ultimately result in the rear-end of the car starting to drift. For this reason, swing systems have increasingly been replaced by semi-trailing arm suspension. In the 1970s the requirements became even more difficult to meet, with more powerful engines and sportier driving styles. This resulted in an increasing tendency for cars to oversteer in corners. Significant advances were made in tire technology and the lateral and circumferential forces that could be achieved were considerably greater. However, the threshold range became narrower and the handling more unpredictable. In rear-wheel drive cars, the lateral forces needed for cornering and the drag torque of the engine are both applied to the rear axle. The combination of these forces can lead to an unstable rear-end, unless the driver countersteers correctly in order to compensate. With the 928, Porsche set itself the goal of developing a car with lateral dynamics and agility that were as good as the classic 911, but with lighter and more comfortable handling. According to Hans-Hermann Braess, who at that time was investigating new axle concepts at Porsche together with Gebhard Ruf, it was no longer possible to meet the requirements for sports cars using conventional means. The results of the two men’s research were incorporated into the Weissach axle that Wolfhelm Gorissen, Manfred Bantle, and Helmut Flegl developed to the stage of production readiness.

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IN THE SP OTLIGHT

The compact design of the Weissach axle takes into consideration the limited space available in the Porsche 928 (© Porsche)

THE PORSCHE 928 – FULL OF INNOVATIONS

The main driving forces behind the development of the 928 were tougher exhaust and noise emission standards and more stringent crash regulations on the important US market. The then CEO of Porsche, Ernst Fuhrmann, gave the go-ahead for the development process in October 1971. Production development began under the leadership of Wolfgang Eyb and Wolfhelm Gorissen. When it was first launched on the market in 1977, the 928 had a number of revolutionary features. According to Gorissen, it was “packed” with innovations, including plastic bumpers that perfectly matched the paintwork, a mixture of steel and aluminum components, a blow-molded plastic tank made from low-pressure polyethylene, a front axle manufactured entirely from aluminum, and, of course, the Weissach axle. Unlike the 911, the engine of the 928 was at the front of the car and the gearbox was located forward of the rear axle. The cylinder block and heads of the water-cooled V8 gasoline engine, which initially produced 176 kW (240 hp) and later up to 257 kW (350 hp), were manufactured from aluminum alloy. A longitudinal shaft in a rigid central tube was used to connect the engine to the gearbox.

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“We rather boldly called it the transaxle system,” says Gorissen, “and no one contradicted us.” The even distribution of the two masses at the front and rear of the car not only improved its crash behavior, but also helped with the weight distribution between the front and rear axles. The new rear axle resolved one fundamental problem of the suspension systems at the time. As Flegl explains: “Elastic forces generally traveled in the

wrong direction.” The Weissach axle functioned quite differently. It allowed for toe-in correction of the dominant rear wheel on the outside of the corner to minimize the car’s load change behavior. In this context, Weissach does not refer to the town where the Porsche development center is based. It stands for “Winkel einstellende, selbst stabilisierende Ausgleichs-Charakteristik” or “toe-setting, self-stabilizing compensation characteristics.”

The experimental axle from 1976: The elasto-kinematic axis is angled; it is outside the wheel and has been moved backward (© H.-H. Braess)

THE ROUTE TO THE WEISSACH EFFECT

Extensive research and testing work in the early 1970s preceded the development of the Weissach axle. In 1973 Braess described an elasto-kinematic suspension system [1] that allowed for specific elastic movements when it was exposed to certain forces. These movements would not be possible using a purely kinematic system. One key disadvantage of semi-trailing arm suspension systems is the fact that the elasto-kinematic center of rotation in the case of lateral forces is in front of the wheel hub (in the direction of travel) and in the case of longitudinal forces is on the vehicle-facing side of the wheel. As a result, lateral forces cause toe-out and oversteer. The drag torque produced by decelerating in corners can result in abrupt oversteer, in particular in the case of powerful engines in low gears. However, in their research project Braess and Ruf were aiming to achieve precisely the opposite. In this type of driving situation, the wheel on the outside of the

corner that is under load would automatically be adjusted to a toe-in position to encourage an understeering behavior, thus reduce the tendency to oversteer while cornering, and prevent the rearend of the car from stepping out. The semi-trailing arm suspension system described here was designed so that the vertical center of rotation moved behind the wheel and to its outer side. The axis is not real, but virtual and is the result of the elasto-kinematic design. An additional wheel carrier was used which was attached to the subframe with two rubber components of different levels of stiffness. The front rubber bearing was softer than the rear one, which put the rear wheels in a toe-in position around the virtual center of rotation under lateral and deceleration forces. This design would not be mass-produced, but paved the way for future developments. THE “BROKEN DOWN” MULTI-LINK SUSPENSION SYSTEM

While the Porsche 928 was under development, Braess and Ruf took another

step forward and designed a “broken down” double wishbone suspension system [2]. The two wishbones were replaced by three “struts” and a V-shaped link to create a kind of fivelink suspension system of the kind commonly used today. In this experimental axle, the toe-in adjustment was also achieved by using bearings with different degrees of elasticity on the links. The virtual center of rotation was outside the wheel and had been moved further back, but the key difference was that the vertical axis was angled. At the point where it met the road surface, it was further out and back than at the level of the hub. Why is this important? Because when the car is cornering, the lever effect of the lateral forces is applied at road level, while the braking and drag torque have an impact at hub height. The braking torque is compensated for in part by the braking torque around the wheel. In contrast the drag torque is applied in full. The angled center of rotation of the multi-link suspension system allows the effective lever to be adjusted to the different effects of the forces [3, 4].

© Christoph Bauer | Porsche

2 QUESTIONS FOR …

Dr. Manfred Harrer Vice President Chassis Development, Dr. Ing. h. c. F. Porsche AG, Weissach, Germany

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ATZ _ How have the development methods used to design elasto-kinematic systems changed since 1977?

What new technologies do you expect to see in suspension systems and chassis?

HARRER _ Simulation

We already have a variety of mechatronic chassis systems with intelligent control functions in cars, including rear axle steering and electromechanical roll stabilization. In future these systems will be networked more effectively. The additional information that this will generate and the integration of other sensors will allow the chassis to respond more quickly and effectively to the current driving situation. However, it will not only use external data, but will also supply important safety information to other vehicle systems and other vehicles. As well as introducing further developments in the field of mechatronics, we will also be continuing to develop high-quality mechanical axle systems with very lightweight components.

methods were already available in 1977 for the design of elasto-kinematic systems, but because of the limited computing capacity the systems had to be significantly simplified. Today we have a much wider range of possibilities. We can produce sophisticated, dynamic simulations of almost any kind very quickly using highly complex CAE models which also show dynamic component and bearing stiffnesses. This also includes active chassis systems with the accompanying control functions like those in real control units. We can also produce mathematical optimization algorithms for every conceivable driving maneuver to identify the ideal elasto-kinematic design, taking into consideration the interaction of active systems.

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IN THE SP OTLIGHT

However, at the time there were two disadvantages to the multi-link suspension system. Its structure was rather delicate and it was not easy to dimension the right design for the elastic components. THE WEISSACH A XLE IN THE 928

In the 928, Porsche opted initially for a functional synthesis of the multi-link and double wishbone suspension systems in the form of the Weissach axle [5]. There were several reasons for this decision. Porsche drivers often take their cars out on race circuits. They want to be able to steer the car via the rear axle using the throttle to produce controlled drifting, as Bantle explains. In this case a smaller oversteer angle is ideal in order to “position” the rear-end. Too much toe-in would also make the changeover from cornering to driving in a straight line unpleasantly noticeable. Finally, the limited space in the 928 called for a more compact solution. A multi-link suspension system would have required more width, which was not available because it was needed for the rear seat pans. The Weissach axle with larger gaps between the bearings and therefore longer levers allowed for a relatively small toe-in correction to suit a sporty driving style. The lower level of the suspension system

Steady-state cornering and drag torque losses caused by the engine produce a slight toe-in correction (© H.-H. Braess)

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The Weissach axle is a functional synthesis of the multi-link and double wishbone suspension systems (© Porsche)

consisted of a longitudinal control rocker with elastic bearings at the front. At the back it was linked to a thin strut also fitted with elastic bearings which were slightly harder in order to move the center of stiffness behind the axle. A single stiffener was used on the upper level. Gorissen explains that the longitudinal elastic strut “was highly sophisticated.” It guaranteed stable tracking of the wheel, but provided protection against hard impacts. The test vehicle based on an Opel Admiral was remarkable as well. Bantle sat at the steering wheel in the front of the car, while Walter Näher, who later

became well-known as a race engineer, was in the back. He had a specially installed steering wheel to steer the rear axle, which allowed the effect of the toe-in changes to be tested. The results of the test showed that 20 angular minutes were enough to counteract the oversteer. However, the adjustments had to be made quickly, within 0.2 s of the driver backing off the throttle. Bantle explains that, of course, the axle design is not the only important factor and it must always be considered in relation to the tire characteristics. However, the figures are impressive: In 1954

The Opel Admiral test car had a second steering wheel fitted in the back for the rear axle; this allowed the effects of active toe-in correction to be tested (© Porsche)

Porsche sports cars had a lateral acceleration of 0.7 g, which corresponds to around 6.87 m/s². In 1971 the specifications issued by the ESV conference (Enhanced Safety of Vehicles – part of the US National Highway Traffic Safety Administration) required only 0.65 g. The Porsche 928 achieved a lateral acceleration figure of 8.8 m/s², which put it on an equal footing with the 911 from the same era. THE BREAKTHROUGH OF THE MULTI-LINK SUSPENSION SYSTEM

In the 1970s and 1980s other manufacturers adopted multi-link suspension systems. Many people will certainly remember the Mercedes-Benz “Raumlenkerachse” (multi-link independent suspension system) which was fitted to the 190 in 1982. At Porsche the full Weissach effect with the angled virtual center of rotation was finally replaced in 1993 by the LSA axle in the 911 Carrera [6]. LSA stands for light, stable and agile. In the steady-state circular test the Carrera achieved a maximum lateral acceleration of 9.3 m/s². By this time it was easier for manufacturers to move to multi-link suspension systems. New simulation processes such as the FE method allowed them to design each individual link and the overall elastokinematic behavior of the system more precisely, even in the case of smaller gaps and the resulting heavier loads. Multi-link suspension systems based on the Weissach principle are used today in front-wheel-drive compact cars. They function in fundamentally the same way. Under the lateral forces produced by steady-state and non-steady-state cornering, the wheel on the outside of the corner is subjected to toe-in correction. However, in front-wheel-drive cars there are no longitudinal forces on the rear wheels caused by drag torque from the engine which makes the task of designing the system easier. Nowadays a great deal can be achieved with ESP/ ESC, torque vectoring, active kinematics, and active rear axle steering. During the era of the Porsche 928 only kinematic and elasto-kinematic solutions were available. However, even the best chassis electronics systems cannot fully compensate for the failings of the suspension system. The developers all agree on one thing: the Weissach axle was the inspiration for the development of new rear suspension systems based on principles that are still applicable today.

WHAT DO WE THINK? “Since the Porsche 928 appeared in 1977 a great deal has changed. Today complex simulations of driving dynamics processes can be created and integrated into vehicle models in a way which developers 40 years ago could hardly have conceived of. And the developments are continuing. Software is an increasingly important component of electronic vehicle systems and intelligent cars, which in turn are becoming part of communication networks. This could lead to the temptation to neglect mechanical solutions, because the electronics systems will handle everything. Of course an active steering system can achieve much more, for example specific toe-out adjustment for better maneuverability, and networking with other driving dynamics systems. This makes the Weissach effect even more impressive because it used only elasto-kinematic components. It still plays a role today in making low-cost compact models both more dynamic and safer.”

Gernot Goppelt REFERENCES [1] Braess, H.-H.: Examples of the development of ESV subsystems. 4 th ESV Conference, Kyoto, March 13-16, 1973, pp. 103-108 [2] Braess, H.-H.; Ruf, G.: Influence of tire properties and rear axle compliance steer-on power-off effect in cornering. 6 th ESV Conference, Washington, October 12-15, 1976, pp. 656-664 [3] Otto, H.: Lastwechselreaktion von Pkw bei Kurvenfahrt. Braunschweig, Technische Universität, dissertation, 1986 [4] Pischinger, S.; Seiffert, U. (ed.): Vieweg-Handbuch Kraftfahrzeugtechnik, pp. 853-856. Wiesbaden: Vieweg, 2016 [5] Bantle, M.; Braess, H.-H.: Fahrwerksauslegung und Fahrver halten des Porsche 928. In: ATZ 79 (1977), No. 9, pp. 369-378 [6] Berkefeld, V.; Görich, H. J.; Schote, N.; Wöhler, H. J.: Die LSA-Hinterradaufhängung des neuen Porsche 911 Carrera. In: ATZ 96 (1994), No. 6, pp. 340-351

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Gernot Goppelt is Freelance Journalist.

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