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2009 | Buch

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The Automotive Chassis

Vol. 2: System Design

herausgegeben von: Giancarlo Genta, Lorenzo Morello

Verlag: Springer Netherlands

Buchreihe : Mechanical Engineering Series

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik" , Springer Professional "Wirtschaft"

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Über dieses Buch

This work serves as a reference concerning the automotive chassis, i.e. everything that is inside a vehicle except the engine and the body. It is the result of a decade of work mostly done by the FIAT group, who supplied material, together with other automotive companies, and sponsored the work. The first volume deals with the design of automotive components and the second volume treats the various aspects of the design of a vehicle as a system.

Inhaltsverzeichnis

Frontmatter

Functions and Specifications

17. Transportation Statistics

Data reported in this chapter were extracted from institutional documents of ANFIA, ACEA, ISTAT and Eurostat.

ANFIA (Associazione Nazionale Fra le Industrie Automobilistiche), the Italian national association of automotive manufacturers1, was established in 1912 and is spokesman for its associates, on all issues (from technical, economic, fiscal and legislative to qualitative and statistical) regarding the mobility of people and goods.

Among several objectives, ANFIA has the task of gathering data and information, providing official statistical data for this segment of industry.

ANFIA publishes every year a report called Autoincifre (Figures of the Automobile), which is one of the fundamental references for statistical data on motoring in Italy and Europe. Much of the data collected in this report comes also from PRA (Pubblico Registro Automobilistico), the public vehicle register managed by ACI, the Association of Italian Motorists.

18. Vehicle Functions

The goal of a system approach to vehicle design is to define the technical specifi-cations of each component, in such a way that the vehicle, as a whole, performs its functions according to assigned procedures and objectives.

By technical specifications, we mean a set of physical measurements that define each part, completely without the use of detailed drawings.

The system approach to design allows project, even the most elaborate one, to be carried out by assigning activities to teams working in parallel, each with comprehensible objectives that can be checked autonomously, and finalized to the overall performance of the vehicle. The system approach also allows a project to be developed, using standard components produced by suppliers, these components being developed for the purpose or chosen from a catalogue.

19. Regulations

A vehicle cannot be sold and obtain the necessary registration for driving on public roads, unless it is built according to legal specifications. In Europe the agreement of these specifications with existing laws is demonstrated, by two official documents: the certificate of homologation and the certificate of conformity.

The first document proves that the vehicle is designed according to legal requirements. It is issued by a public authority in charge of this function; in Italy, for instance, that authority is the Department of Transportation. The homologation certificate is issued on the basis of a technical report of the manufacturer and the completion of given tests, performed on prototypes of that vehicle.

The Chassis as a Part of the Vehicle System

20. General Characteristics

Motor vehicles, like most machines, have a general bilateral symmetry. Only hypotheses can be advanced to explain why this occurs. Certainly to have a symmetry plane simplifies the study of the dynamic behavior of the system, for it can be modelled, within certain limits, using uncoupled equations. However, the reason is likely to be above all an aesthetic one: symmetry is considered an essential feature in most definitions of beauty.

All complex animals that evolved on our planet, including humans, have a symmetry plane defined by a vertical axis and an axis running in the longitudinal direction; symmetry is, however, not complete since some internal organs are positioned in an unsymmetrical way and some small deviations from symmetry are always present even in exterior appearance. When such lack of symmetry is too evident, it is felt to be incompatible with the aesthetic canons developed by all human civilizations.

21. An Overview On Motor Vehicle Aerodynamics

The forces and moments the vehicle receives from the surrounding air depend more on the shape of the body than on the characteristics of the chassis. A detailed study of motor vehicle aerodynamics is thus beyond the scope of a book dealing with the automotive chassis.

However, aerodynamic forces and moments have a large influence on the longitudinal performance of the vehicle, its handling and even its comfort, so it is not possible to neglect them altogether.

22. Prime Movers For Motor Vehicles

The motion of all vehicles requires the expenditure of a certain quantity of mechanical energy, and in motor vehicles the system that supplies such energy (in most cases an internal combustion engine) is on board. The lack of an adequate prime mover is the main reason that mechanical vehicles could be built only at the end of the industrial revolution, and enter mass production only in the Twentieth Century, in spite of attempts dating back to ancient times.

For a mechanical vehicle to be built, a prime mover able to move not only itself, but the vehicle structure and payload as well, was needed. Remembering that the power needed to move the mass m at the speed V on a level surface with coefficient of friction (sliding or rolling) f is equal to P = mgfV , it is easy to conclude that the minimum value of the power/mass ratio of a prime mover able to move itself is

(22.1)

where α is the ratio between the mass of the engine and the total mass of the vehicle and η is the total efficiency of the mechanism which transfers the power and propels the vehicle.

23. Driving Dynamic Performance

When computing the performance of a vehicle in longitudinal motion (maximum speed, gradeability, fuel consumption, braking, etc.), the vehicle is modelled as a rigid body, or in an even simpler way, as a point mass.

The presence of suspensions and the compliance of tires are then neglected and motion is described by a single equation, the equilibrium equation in the longitudinal direction. If the x-axis is assumed to be parallel to the ground, the longitudinal equilibrium equation reduces to

(23.1)

where Fxi are the various forces acting on the vehicle in the longitudinal direction (aerodynamic drag, rolling resistance, traction, braking forces, etc.).

24. Braking Dynamic Performance

The study of braking on straight road is performed using mathematical models similar to those seen in Chapter 23 for longitudinal dynamics. But in this case, the presence of suspensions and the compliance of tires are neglected and the motion is described by the longitudinal equilibrium equation (23.1) alone

(24.1)

Apart from cases in which the vehicle is slowed by the braking effect of the engine, which can dissipate a non-negligible power (lower part of the graph of Fig. 22.2), and by regenerative braking in electric and hybrid vehicles, braking is performed in all modern vehicles on all wheels. Subscript i thus extends to all wheels or, when thinking in terms of axles, as is usual for motion in symmetrical conditions, on all axles.

25. HANDLING PERFORMANCE

Low speed or kinematic steering is, as already stated, defined as the motion of a wheeled vehicle determined by pure rolling1 of the wheels. The velocities of the centres of all the wheels lie in their midplane, that is the sideslip angles ai are vanishingly small. In these conditions, the wheels cannot exert any cornering force to balance the centrifugal force due to the curvature of the path. Kinematic steering is possible only if the velocity is vanishingly small.

26. Comfort Performance

The definition of comfort in a motor vehicle is at once complex and subjective, changing not only with time (cars considered comfortable just twenty years ago are nowadays considered unsatisfactory) but also from user to user. The same user may change his appraisal depending on circumstances and his psychophysical state. But comfort remains an increasingly important parameter in customer choice and strongly competitive factor among manufacturers.

This chapter will deal primarily with vibrational comfort, although it is difficult to separate it from acoustic comfort without entering into details linked more with the driveability and handling of the vehicle. Not just driving comfort, but vibrational and acoustic comfort as well (the latter deeply affects the conditions in which the driver operates), all have a strong impact on vehicle safety.

27. Control of the Chassis and ‘by Wire’ Systems

As already stated, a road vehicle on pneumatic tires cannot maintain a given trajectory under the effect of external perturbations unless managed by some control device, which is usually a human driver. Its stability solely involves such state variables as the sideslip angle β and the yaw velocity r.

In the case of two-wheeled vehicles the capsize motion is intrinsically unstable forcing the driver not only to control the trajectory but stabilize the vehicle.

Mathematical Modelling

28. Mathematical Models for the Vehicle

An increasingly competitive automotive market offers its products to increasingly demanding customers. Numerous standards and rules, primarily regarding safety and environmental impact, are issued by regulating bodies and governments, making today's vehicles more and more complex.

The specifications vehicles must comply with often contrast with each other. The time between the conception of a vehicle and its entering the market, the so called time to market, is an essential factor for its commercial success. The traditional approach, based on the construction of prototypes, subsequent experimentation and modification, is no longer adequate.

29. Multibody Modelling

A vehicle on elastic suspensions may be modelled as a system made by a certain number of rigid bodies connected with each other by mechanisms of various kinds and by a set of massless springs and dampers simulating the suspensions. A vehicle with four wheels can be modelled as a system with 10 degrees of freedom, six for the body and one for each wheel. This holds for any type of suspension, if the motion of the wheels due to the compliance of the system constraining the motion of the suspensions (longitudinal and transversal compliance of the suspensions) is neglected. The wheels of each axle may be suspended separately (independent suspensions) or together (solid axle suspensions), but the total number of degrees of freedom is the same (Fig. 29.1). Additional degrees of freedom, such as the rotation of the wheels about their axis or about the kingpin, can be inserted into the model to allow the longitudinal slip or the compliance of the steering system to be taken into account.

The multibody approach can be pushed much further, by modelling, for instance, each of the links of the suspensions as a rigid body. To model a shortlong arms (SLA) suspension it is possible to resort to three rigid bodies, simulating the lower and upper triangles and the strut, plus a further rigid body simulating the steering bar. While modelling the system in greater detail, the number of rigid bodies included in the model increases. However, if the compliance of the various elements is neglected (i.e. if these bodies are rigid bodies), the number of degrees of freedom does not increase along with the number of bodies: an SLA suspension always has a single degree of freedom, even if it is made up of a number of rigid bodies simulating its various elements.

30. Transmission Models

The take-off manoeuvre of a vehicle was studied in Section 23.9 using a simple model where the inertia of both engine and vehicle were modelled as two flywheels connected to each other by a rigid shaft and a friction clutch. This model can be made more realistic by adding the torsional compliance of the shaft, of the joints and possibly the gear wheels, as well as the rotational inertia of the various elements of the driveline. A model of the whole driveline is thus obtained, with the engine and vehicle modelled as two flywheels located at its ends.

However, the engine shaft is itself a compliant system. Moreover, its pistonconnecting rod-crank systems should be modelled as systems with variable inertia in time. At the other end of the driveline, the dynamics of the transmission and the longitudinal dynamics of the vehicle are coupled by the tires, which are themselves compliant in torsion. The longitudinal compliance of the suspensions may affect the dynamics of the driveline and couples with the dynamics of the vehicle, which is in turn coupled with comfort dynamics.

31. Models for Tilting Body Vehicles

The models seen in the previous chapters dealt with vehicles that maintain their symmetry plane more or less perpendicular to the ground; i.e. they move with a roll angle that is usually small. Moreover, the pitch angle was also assumed to be small, with the z axis remaining close to perpendicular to the ground. Since pitch and roll angles are small, stability in the small can be studied by linearizing the equations of motion in a position where θ = ø = 0.

Two-wheeled vehicles are an important exception. Their roll angle is defined by equilibrium considerations and, particularly at high speed, may be very large. To study the stability in the small, it is still possible to resort to linearization of the equations of motion, but now about a position with θ = 0, ø = ø₀, where ø₀ is the roll angle in the equilibrium condition. An example of this method is shown in Appendix B, where the equation of motion of motorcycles is discussed.

Backmatter

Titel: The Automotive Chassis
herausgegeben von: Giancarlo Genta
Lorenzo Morello
Verlag: Springer Netherlands
Electronic ISBN: 978-1-4020-8675-5
Print ISBN: 978-1-4020-8673-1
DOI: https://doi.org/10.1007/978-1-4020-8675-5

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