柴油動(dòng)力微型客車設(shè)計(jì)（懸掛設(shè)計(jì)）（含CAD圖紙）

柴油動(dòng)力微型客車設(shè)計(jì)（懸掛設(shè)計(jì)）（含CAD圖紙）,柴油,動(dòng)力,微型,客車,設(shè)計(jì),懸掛,CAD,圖紙 46123Suspension systemssimple pendulum, the physical relationship between the sus-pension frequency and deflection may be expressed as follows:where N is the number of cycles per second (hertz, Hz), g isthe acceleration due to gravity (9810mm/s2) and D is thestatic deflection (mm). In modern passenger car practice,suspension frequencies generally fall within the range of 1 to1.2Hz (60 to 72 cycles/min) with corresponding staticdeflections of 225 to 175mm (8.8 to 6.9in).The suspension frequency therefore varies in inverse pro-portion to the spring load and the amount of static deflectionproduced by that load. This explains why the ride comfort ina heavily laden car is often significantly better than when itis lightly laden, albeit within the limitations imposed by thecurtailed bump travel of the suspension due to the greaterstatic deflection. Whereas this limitation may be removed byincluding a self-levelling facility in the suspension system,the same comfortable ride in both the lightly and the heavilyladen conditions can only be achieved by a suspension systemthat automatically varies its resistance to deflection or stiff-ness in accordance with load, so that the suspension frequencyremains constant. Since there is a fractional time lapse betweenthe front and the rear wheels passing over an obstacle, thesuspension frequency at the rear end of the car is usuallyraised above that chosen for the front end, so that the quickerrestoring effect of the rear suspension tends to synchronizethe front and rear ride motions and thus minimize pitching.23.2TYPES OF SUSPENSIONMotor vehicle suspension systems may be classified as follows:DependentBoth wheels of a pair are mounted on a common axle thatacts as a rigid beam, which is then connected by springs tothe vehicle structure.NgD? 12?23.1BASIC RIDE CONSIDERATIONSIn the design of motor vehicles in general and passenger cars inparticular, the comfort of the occupants is clearly of primeimportance. The basic function of the suspension system istherefore to provide a flexible support for the vehicle, so that itsoccupants ride comfortably isolated from the imperfections ofthe road surface. An additional and no less important require-ment of the suspension system is that of stabilizing the vehicleunder all conditions of driver handling, involving as it doescornering, braking and accelerating manoeuvres. These twobasic requirements in respect of vehicle ride and handling tendto conflict in actual practice, since softspringing is indicatedon the one hand and relatively hardspringing on the other. Asuccessful design of suspension system is therefore one thatachieves an acceptable compromise between these two areas ofconflict.The basic factors that govern the ride comfort of a vehiclein practice may be defined as vertical flexibility, horizontalflexibility, shock damping and linkage friction. It will laterbecome evident how these factors are related to the designand construction of modern suspension systems, but first it isnecessary to understand a little more about vertical flexibilitysince this is at the heart of any vehicle suspension system.When the wheels rise and fall over road surface irregular-ities, the springs momentarily act as energy storage devices andthereby reduce greatly the magnitude of impact loading trans-mitted by the suspension system to the car structure. Theenergy of impact loading is related to the product of the dis-turbing force and the distance over which it is constrained toact by the spring medium. It thus follows that a soft or low-ratespring, which permits a generous deflection for a moderateloading, will reduce to a minimum the impact or disturbingforces transmitted to the vehicle occupants. The rate of thespring is equal to the load per unit of deflection (Figure 23.1a).To lend real meaning to the commonly used expressionssoft and hard springing, the suspension engineer prefersto define vertical flexibility in terms of static deflection(Figure 23.1b). A value for static deflection is obtained fromthe normal spring load acting at the wheel divided by theeffective spring rate measured at the wheel. Here the springload is less than the actual load at the wheel by an amountequal to the unsprung weight of the suspension mechanism,and the effective spring rate is measured at the road wheel totake into account any leverage exerted on the spring by thesuspension linkage.Static deflection is in turn directly related to the natural fre-quencyof the suspension system: that is, the number of oscil-lations or cycles per second that the sprung mass wouldperform if allowed to vibrate freely on its suspension. Sincethis motion of the sprung mass is comparable with that of aSlope = spring rateDeflection(a)(b)LoadStaticdeflectionFigure 23.1Spring rate and static deflectionCh23-H8037.qxd 8/21/06 2:41 PM Page 461Semi-dependentBoth wheels of a pair are again mounted on a sprung commonaxle, but in this case it acts as a beam with limited flexibility.IndependentEach wheel of a pair is separately linked and sprung from thevehicle structure, so that its movement relative to the latter isthe same in roll as it is for ride.InterdependentBoth wheels on each side of the vehicle have their springscoupled together, either mechanically or hydraulically, so thatsingle wheel bumps are shared between front and rear springsfor a softer ride, together with reduced pitching except duringacceleration and braking.Dependent front suspensionIn dependent front wheel suspension systems a beam-type axleis used to support the two stub axle pivots at a fixed distanceapart. Although beam axles have long since been obsolete forcars, for reasons that will be explained later, this form of frontwheel suspension continues to be used on almost all commer-cial vehicles on the basis of the following considerations:1 Front wheel alignment is better maintained with a beamaxle because, unlike independent linkages, it does notdemand great rigidity at the front end structure of the chas-sis frame. This is an important consideration in the designof a commercial vehicle chassis frame, which is intendedto accommodate a certain amount of flexing in response toroad shocks transmitted through the suspension springs.2 Tyre wear is minimized because, first, the front wheelsremain perpendicular to the road in the presence of bodyroll, and second, the track width remains constant with sus-pension movements of the wheels (Figure 23.2). It shouldbe appreciated that the attainment of satisfactory tyre life isan important factor in the economic operation of a largefleet of commercial vehicles.3 By virtue of its rugged construction and its mechanical sim-plicity with a minimum number of wearing parts, the beamaxle is also favoured on the grounds of low initial and serv-icing costs.4 Finally, the use of a beam axle ensures that the groundclearance of a commercial vehicle remains constant in bothits unladen and laden conditions.Dependent and semi-dependent rear suspensionBoth live and dead rear axles are, of course, forms of dependentbeam axle suspension and have previously been described inconnection with final drive arrangements (Section 20.4). A fur-ther development of the type of dead axle beam that is used tomount the trailing rear wheels on some front-wheel drive cars isthe semi-dependent H-beam axle system, which was originallyintroduced by Volkswagen for their 1980 Passat. In this ingeni-ous system each wheel is mounted on a trailing arm that pivotsfrom rubber bushes carried by the body structure, and botharms are connected together by a flexible axle beam that liesbetween the axes of the pivot bushes and wheel bearings. Theaxle beam is made flexible in torsion and stiff in bending, suchthat on the one hand it allows the trailing arms and wheels tomove up and down almost independently of each other, whileon the other hand it prevents the wheels from tilting to the sameextent as if they were attached directly to the ends of the axlebeam. That is, both wheels can move in unison about a purelytrailing axis AA (Figure 23.3) while a single wheel can moveabout a semi-trailing axis BB.Lateral location of rear axlesIn the study of mechanics a parallel motion mechanism is anarrangement for constraining a point to move in a straight line.Two such mechanisms are known as the Watt and the Scott-Russell linkages, so-named after James Watt and F. Scott-Russell, who invented them in the 18th and 19th centuriesrespectively for use on steam engines. Our interest in thesehistoric mechanisms is concerned with their application tolive and dead axle rear suspension systems, and their abilityto provide accurate sideways control of vehicle body andaxle motions.The Watt linkage basically comprises two equal or nearlyequal length transverse links, staggered in height, so thattheir outer ends are pivoted from each side of the car bodystructure, while their inner ends are pivoted to a third verti-cal link that is centrally pivoted on the axle (Figures 23.4aand 19.36). It will therefore be evident that any upward ordownward movements of the axle, relative to the body, mustcause the transverse links to describe opposing arcs of motion.Since this has the effect of increasing the lateral separationof their inner ends, the vertical link to which they are con-nected on the axle turns slightly in sympathy with their motions.Hence, its central pivot has no choice but to continue in astrictly vertical path, which therefore imposes the requiredsideways control for the axle relative to the body. Notableexamples of Watt linkage applications to the rear suspensionof high-performance cars are those used to control the DeDion and the live rear axles on Aston Martin and Bristol carsrespectively.462SUSPENSION SYSTEMSBoth wheels tilt when either passes over an obstacleBoth wheels remain upright during body rollFigure 23.2The action of beam axle front suspensionFigure 23.3Plan view of H-beam axle systemCh23-H8037.qxd 8/21/06 2:41 PM Page 462Perhaps an even subtler, but albeit rarer, approach to obtain-ing a parallel motion for the axle is that conferred by the Scott-Russell linkage. This essentially comprises a transverse link,one end of which pivots from near the center of the bodystructure, while the other end is allowed a limited lateral free-dom where it connects close to the appropriate end of theaxle beam. To complete the linkage a shorter link is pivotedboth near to the center of the axle and towards the mid-pointof the longer link (Figure 23.4b). The parallel action of thislinkage is most easily visualized by imagining a pair of scis-sors resting edgewise on a table and the point of the lowerblade held from moving. If then the scissors are opened andclosed while still in contact with the table, the point of theupper blade will travel in a vertical path above the point ofthe lower one. In other words, the upper and lower points ofthe scissor blades correspond to the upper pivot of the longerlink and the lower pivot of the shorter link respectively. A potential difficulty in applying this type of linkage in prac-tice concerns the need to provide lateral freedom of move-ment where the longer link connects to the axle end, becausea metal-to-metal sliding joint would clearly be unacceptablein modern design. This problem has been neatly solved inthe sole application of the Scott-Russell linkage by Nissanfor their high-grade QX model (Figure 23.5), and involvesthe use of a special rubber bush at this point in the linkage.The bush is partially voided so as to offer the lowest resistanceto lateral and the highest resistance to vertical deflections,hence the former characteristic accommodates the requiredlateral freedom where the longer link connects to the torsionbeam type of dead rear axle.For lateral location of rear axles, both the Watt and theScott-Russell linkages offer a superior geometry as comparedto the more commonly used and less complex Panhard rodlinkage (Sections 19.4 and 20.4). Two shortcomings of the lat-ter are scuffing and jacking effects, which result from theswinging movement of the transverse rod and its installedangularity. The former hinders the straight-line stability of thecar (Figure 23.6a), while the latter imposes either a jacking-upor -down effect according to the direction of lateral or corner-ing force (Figure 23.6b) and therefore produces inconsistenthandling characteristcs.Independent front suspension (IFS)A significant step towards reducing the earlier mentionedconflict between vehicle ride and handling was the properSUSPENSION SYSTEMS463Figure 23.4Rear axle lateral location linkages: (a) Watt (b) Scott-Russell5Front674321Figure 23.5Arrangement of Scott-Russell axle linkage on Nissan QX car (Nissan Motor GB)1 Shock absorber cap5 Torsion beam2 Shock absorber mounting seal6 Lateral link3 Coil spring7 Control rod4 Shock absorberCh23-H8037.qxd 8/21/06 2:41 PM Page 463application of independent front wheel suspension whichfollowed chiefly from the research work done in the early1930s by Maurice Olley, an ex-Rolls-Royce engineer thenworking for the Cadillac Motor Car Company in America.With an independent front wheel suspension system thesteered wheels are located by entirely separate linkagesrather than being united by a common axle beam. The use ofsome form of independent front suspension (IFS) has longbeen established practice for all conventional motor cars forthe following reasons.Improved ride comfortThe more precisely controlled location of the front wheelsafforded by using an independent linkage system in con-junction with a rigid vehicle structure permits them to havea greater range of suspension movement. This in turn allowsthe use of much softer springs, which reduce the magnitudeof impact loads transmitted by the front suspension to the carstructure. Furthermore, the springs themselves are generallyno longer required to play any part in locating the wheels, sothat leaf springs can be discarded in favour of other types ofsprings possessing very little internal friction and therebyprevent harshness of ride.Better roadholdingTo some extent the springs can be made softer with an IFS sys-tem without reducing the roll resistance at the front end of thecar, which otherwise could lead to over-steer on corners as aresult of the rear suspension then offering too much resistanceto roll. With beam axle suspension the lateral separation of itspair of semi-elliptic leaf springs is restricted to about one-halfthe wheel track dimension so as to leave sufficient clearancefor the wheels to be steered. This narrow spring base comparesunfavourably with that of an independent system where it isalways equal to the wheel track irrespective of the lateral sep-aration of the springs.More accurate steeringAn independent linkage is better able to ensure that eachfront wheel follows its prescribed geometrical path relativeto the car structure and hence those parts of the steering link-age carried thereon. This can be difficult to achieve with abeam axle which is located solely by semi-elliptic leaf springs.For example, early attempts to increase their flexibility usu-ally required the addition of an axle control linkage to preventthe axle from winding up on its springs and causing instabil-ity during braking.Reducing steering jogglesAs compared with a beam axle system, an independent link-age can be arranged to reduce by about one-half the amounteither front wheel tilts inwards when passing over an obstacle.This serves to lessen the gyroscopic forces acting on the roadwheels, because in tilting inwards they also attempt to steerthemselves inwards and this produces an unwanted reaction orjoggle at the steering wheel. Furthermore, both wheels of abeam axle system are tilted in unison when either of thempasses over an obstacle, a state of affairs that at worst can leadto a wobble or shimmy of the steered wheels.Increased passenger spaceLast, but by no means least, the introduction of IFS made adirect contribution to improved passenger accommodation byhaving the power unit mounted further forward in the car, anarrangement which removed the need to provide front endclearance for the moving center portion of the axle beam. It thus became practicable to reposition the rear seats fromabove the rear axle to a lower level within the wheel base.Similarly, the rear-mounted fuel tank could then be movedforward, thereby increasing the capacity of the luggage boot.The linkages used in modern IFS systems generally fall intotwo basic categories: the unequal transverse links, or wishbonesystem; and the transverse link and strut, or MacPhersonsystem.Unequal transverse links IFSThis system, pioneered by General Motors of America in themid 1930s, is sometimes referred to as a wishbone system,because in plan view the front suspension links of their Buickmodels were originally of this form. With this type of IFS,each wheel is guided over obstacles by a short upper and 464SUSPENSION SYSTEMSFigure 23.6Shortcomings of Panhard rod linkage (Nissan Motor GB) (a) scuffing effects (b) jacking effectsCh23-H8037.qxd 8/21/06 2:41 PM Page 464a long lower link, the inner ends of these links being pivotedfrom the car structure and their outer ends now ball jointed toa stub axle carrier or yoke (Figure 23.7).As viewed from the front, the relative lengths and anglesof these links are chosen so as to offer the following basiccompromises:1 To reduce by about one-half the tilting or camber change aseither wheel rises or falls (Figure 23.8), which otherwisewould be imposed on both wheels of a beam axle system.2 To minimize changes in wheel track and thereby reduce anytyre scrub accompanying the rise and fall of the wheels,changes which would be greater if similar links were madeequal in length and arranged to lie parallel. If the lengths ofthe links are made to vary inversely as their height above the ground, then the tyre contact will move up and downvertically without lateral scrub (Figure 23.9).This explains,of course, why the shorter link is always placed above thelonger one. However, a small amount of scrub is usuallyaccepted so as to raise the roll-centre above ground level(Section 23.3).3 To allow the wheels to remain more nearly upright as thecar rolls (Figure 23.8) as compared with the use of equallength and parallel links. In recent years, the advent oflow-profile tyres of greater tread width has made it evenmore desirable to maintain the wheels upright so that thetreads are kept flat on the road during cornering.4 To realize certain installation advantages, namely that theshort upper links protrude less into the engine compartmentand the longer lower links reduce bowing of the suspensioncoil springs acting against them.Various forms of springing medium may be used in conjunc-tion with unequal transverse links IFS systems, includingcoil, torsion bar and rubber cone springs, all of which will beconsidered in more detail at a later stage, together with adescription of shock damper action.The spring and shock dampers may act against either thelower (Figure 23.7) or, less commonly, the upper link of eachwheel linkage (Figure 23.32). In modern practice the transverselinks almost invariably pivot from rubber bushings, with thetwo fold objective of isolating the car from road noise and elim-inating the need for lubrication. The widespread use of rubberbushings for the suspension linkage pivots has largely obviatedthe static friction that could exist with the previously usedmetal-to-metal ones. These were particularly prone to exces-sive friction if any misalignment errors were present during ini-tial build and, of course, in the absence of proper lubrication inSUSPENSION SYSTEMS465Figure 23.7A traditional arrangement of unequal length transverse links IFS (Alfa-Romeo)Only affected wheel tilts when passing over an obstacleBoth wheels tilt during body rollFigure 23.8The action of unequal transverse links IFSFigure 23.9Constant wheel track IFS with unequal transverselinkswhere:DC?Ch23-H8037.qxd 8/