Technical information

Chassis

Volvo 850 GLT

IntroductionThe demands on the car of the 90's arevery high in every respect. As vehiclemanufacturer one can no longer con-centrate only on a subset of all prop-erties. To gain success, excellence is re-quired in all areas.

Furthermore, the customer of the 90'swill not accept having to adapt himselfor herself to the vehicle. The vehiclemust instead be adapted to man. This

makes it necessary to study man-machine interactions when developing anew car.

These demands, coupled with themany properties affected by the chassis,create the need for a property orienteddevelopment process where the com-bination of all properties are taken intoconsideration from the very beginning.

Driver functionDriving a car is a complex task. How-ever, by learning and practice mostdrivers achieve a competence thatmakes the driving appear and feelquite simple under normal conditions.During the learning process, as a be-ginner or under an adaptation period inthe case of a new car, the driver grad-ually learns an automatic behaviour inthe same way as when learning towalk. An analysis of factors affectingthe driving task becomes very com-plicated. Figure 1 is one way of illus-trating this for the example of steering.

Much effort is put into research ofthe human being as a car driver. Criter-ias for handling properties developedby Volvo and others, are used as a basefor more or less comprehensive com-puter simulations in early stages of de-velopment.

It can be stated that feed back of carreactions, according to figure l, is ofthe utmost importance for the driver'sability to control the vehicle. The inputto the driver come through vision,sound and balance organs, as well asthrough forces acting on the wholebody supported by the seat. Alsothrough the hands gripping the steer-i ng wheel. It is found from researchthat these signals have to be very closein time and that they have to be withincertain values and have certain re-lationships. The more demanding theroad and traffic environment, themore stringent these demands become.

Using clear feedback signals im-proves the driver's ability to act as anelement in the control system. Thedriver will also feel clearly the im-.portant role he or she plays in this sys-tem and hence get a positive stimu-lance, maybe even satisfaction -driving pleasure.

The task of driving must be learnt.This means that the driver gets used toa certain response from his or her ac-tions and will also be able to predictthem. The leaning of the car's be-haviour is effective under the mostcommon circumstances. This meansthat the driver has good knowledge ofhow the car reacts when driving atmoderate speed, at moderate manoeu-vering and with small loads.

A surprisingly tight motorway exitduring a holiday journey, with the carfilled with people and luggage, may re-

Figure 2

Figure 1suit in a very difficult situation if theresponse has changed too drastically.The tendency in such a situation isthat the car is less stable and body rollwill be more pronounced because ofthe heavier load. It will also be moresensitive to steering due to the higherspeed. Put together, this can result inan unexpected and different responsethat may be very difficult for the driv-er to control.

Figure 2 illustrates the combinationof circumstances in which differentdrivers normally have experiencecompared to the car's performancelimits.

The conclusion of this driver orien-ted approach gives the following

guide lines for the development of thechassis:- the control properties of the car

must be adapted to the driver- the control properties must be as

consistent as possible- the effects of load, speed and fric-

tion etc must be minimized- the driver shall get a distinct feed-

back of the car's reactions in differ-ent situations and manoeuvres

- the magnitude of disturbances mustbe small

- the ride comfort and noise level aswell as seating position must begood in order to avoid unnecessarydriver fatigue.

Chassis demandsThe challenges that the chassis de-signer is confronted with when de-signing a new car do not only consistof requirements for certain chassis re-lated properties, like in our case driv-ing pleasure, driving safety, ride com-fort and noise comfort, but also to agreat extent of functional requirementswithin other areas.

For the Volvo 850, the main targetshave been total efficiency and versatil-ity. These properties have directly andindirectly influenced the choice ofchassis concepts.

Versatility means the ability totransport a varying number of pas-sengers and many different kinds ofluggage in a safe and comfortable way,and in different environments: duringhigh speed motorway driving, on slip-pery winter roads or in dense city traf-fic.

For the chassis, this means demandsfor loading capacity, consistent drivingproperties even on low friction sur-faces, a small turning circle and lowsteering forces when parking.

The total efficiency can be dividedin three main parts:- space efficiency- fuel efficiency- cost efficiency

For the customer cost efficiency isnot only decided by the purchasingprice of the car but also by the run-ning costs. These consist among otherthings of service cost and second handvalue which in turn depend of the re-liability of the car. Reliable and easilyserviceable designs are thereforeneeded.

To meet the demand for space ef-ficiency, the choice of front wheeldrive is natural for a car of the Volvo850's size. The space in the passengercompartment will be free of space oth-erwise occupied by prop shaft and fi-nal drive. In addition, by mounting theengine transversely, the longitudinalspace of the car can be utilized op-ti mally. A conflict arise here with thedemands for a small turning circle andthereby wide wheel angles. When de-signing the suspension, attention mustalso be given to how much space willbe required in the passenger and lug-gage compartments. Low positionedattachment points for the suspension isimportant and the axle geometry

should be chosen in such a way thatthe need for wheel housing space isminimal.

The front wheel drive in itself hasgreat influence on certain drivingproperties. Among the positive can bementioned:- improved traction during light loadconditions due to the weight bias( more weight on the driven wheels)- stable behaviour due to increasedundersteering during acceleration

Of course, special attention mustbe given to the problems that occurwhen the steered wheels are alsodriven. In order not to create un-wanted steering wheel torque or in-fluenced course stability of the car,the front axle and steering geometryand elasticities must be designed withthis in mind.

Throttle-off effects when nego-tiating corners are also generally moreemphasized in a front wheel drive car.A limitation of these effects is neces-sary in order to fully achieve our driv-ing safety philosophy.

In order to combine driving pleasurein the form of engine performancewith fuel efficiency, the drag must bekept low. By optimizing the bodyshape for low drag, the sensitivity forsidewind disturbances is generally in-creased. The choice of front wheeldrive, with accordingly the centre-of-gravity position moved forwards,counteracts this drawback.

The overruling requirements forversatility and total efficiency therebyplay an important role for the chassisdesign. The choice of concept pro-vides good possibilities to reach highcourse and sidewind stability andgood traction. However, the space forchassis components is very limitedand much attention must be paid tominimize torque steer and throttle-offeffects.

In the preceding part, "soft" termslike driving pleasure and driving safe-ty have been used. To make an analy-tical development, these terms mustbe interpreted into objectively meas-ureable entities, which then will formthe requirements for the complete car.

Driving pleasureThis is an often used expression al-though it is seldom defined. It must beclear that it focus upon the driver andvehicle interaction, in the way de-scribed earlier.Important examples are:- ti me lags and gains regarding steer-

i ng response- degree of under and oversteer- low control efforts

Driving safetyThe most important ingredient of driv-ing safety is the consistent behaviour

of the

vehicle in all driving situations.Other examples are:- li mited addition of yaw motion at

throttle-off- high lateral movement capabilities

combined with stability- short braking distances combined

with steerability

Ride comfortGood ride comfort means absence ofvibrations and car motions when driv-ing on different types of roads. Ex-tremities and body are sensitive to cer-tain frequencies. The car bodymotions should be kept small as theyaffect the senses of balance and sight.

Noise comfortLow noise levels are generally re- quired in order to minimize negative

noise experiences. Also a good bal-ance between high and low frequencynoise as well as between differentnoise sources (power train etc) are im-portant.

Driving comfortBy this we mean the vehicle behaviourin common driving situations in orderto achieve relaxed driving.Examples:- good course stability also in side-

winds- sm all turning circle and small steer-

ing forces when parking- small torque-steer effects

Chassis layout

Front suspension, figure 3The spring struts have "wet" dampersand inclined coil springs. The strut-mounts to the body have separate iso-lators for damper and spring. The low-er arm is connected to a rubberisolated subframe to which an anti-rollbar is also connected.

Rear suspension, figure 4The Delta-link rear axle is of a uniqueVolvo design, combining the ad-vantages of an independent semi-trailing suspension and a beam axle.

The two axle halves are connectedto each other by two rubber bushings.The body attachments consist of twoangled links with rubber elements thatcontrol the side force steer and lateralas well as longitudinal compliances.All anti-roll bar is mounted betweenthe two axle halves. Coil springs withconcentric hollow buffers are fittedclose to the wheels. Dampers of pres-surized type are mounted between thearms and the underbody structure.

Steering, figure 3The rack-and-pinion power assistedsteering gear is solidly fixed to thesubframe and positioned behind thewheel centre. The steering wheel isadjustable both axially as well as ver-tically. For crash safety reasons boththe steering column and the lowershaft are collapsible .

Brake system 5The dual circuit ABS brake systemhas disc brakes on all four wheels.The front discs are ventilated and havesliding calipers. The parking brake isa separate duo-servo drum brake. TheABS system is a three-channel systemwith individually controlled frontwheels, "select low" at the rear andyaw torque control for stable braking

on my-split surfaces. The brake forcedistribution between the front and rearwheels is controlled by a pressure-sensitive reduction valve.

Figure 5

Combining propertiesSeveral requirements on a car are con-tradictory. An important task, beforeh ardware solutions are chosen andfixed, is to identify the factors that acti n "common" or "opposite" directions.Some of the more important and well-known conflicts are:- handling - ride and noise comfort- steering properties - course stability- lateral stiffness - longitudinal com-

pliance of a suspension- steering properties at low lateral ac

celerations - at high lateral accelera-tions

The combination of the most im-portant factors which make the carsafe and enjoyable to drive was one ofthe Volvo 850 objectives and chal-lenges. Without the use of more ex-clusive design solutions like fourwheel steering and active suspension,the boundaries are set by on one sidethe human perception, on the otherside by the laws of nature.

To achieve these goals, the front andrear axles must be designed to interactoptimally with each other and with thetyres. The properties of the tyres werestudied using the Magic Formula TyreModel, developed by Volvo Car Cor-poration in collaboration with theTechnical University of Delft in theNetherlands.

The results of the i nitial analysisshowed that all the demands couldonly be met using a new rear axle de-sign, the Delta-link, described later.For example, to add more drivingpleasure it must be possible to aim thecar quickly in a new direction. This isachieved by giving the car an oversteerat the beginning of the steering ma-noeuvre. To fulfill the apparent con-flicting driving safety demands, thesteering links of the Delta-link thencounteracts the oversteering whenthe car has entered a corner and sta-bilizes it.

Another example of how to copewith conflicting demands is the steer-i ng response. It depends on the designof the suspensions and the tyres, butalso on the steering system ratio. Thisratio is also one of the significant fac-tors for the steering wheel forces andsteering wheel angles. A power steer-ing serves to decouple these factors,i.e. gives the possibility to have a rel-atively low ratio for good steering re-sponse combined with low parkingsteer forces. Power steering is obvi-

ously a standard item on the Volvo850. The power steering is also ef-fective at higher speeds where rea-sonably high steer forces in relation tolateral acceleration are desired. Thepower steering characteristics aretherefore accordingly carefully op-timized.The behaviour of the car during pow-er-off in a corner is of utmost im-portance, especially considering theprevious discussion regarding con-sistent properties. Although manydrivers - especially those with frontwheel drive cars - consider a certainamount of power-off effect as de-sirable, it is from a safety point of viewnecessary to limit the amount in orderto support the untrained driver in anem ergency situation, figure 2. This is avery plausible situation, for instanceFigure z

when a driver enters a corner too fastand instinctively releases the throttle.In such a situation it is important thatthe characteristics of the car, i.e, thepath curvature and yaw velocity,change little and slowly. The desiredamount of power-cuff is achieved us-ing:- appropriate front suspension anti-

dive- adequate steer angle changes at

wheel movements- small camber angle increase at the

rear- high roll stiffness- tuned power steering characteristicsWith these measures the sensitivity forthrottle-off oversteer is kept on a levelwhich allows full driver control with-out being too high for the less ex-perienced driver.

Figure 6

Figure 3

The strut mounting, figure 7, has a ballbearing in the upper end of the shockabsorber piston rod to allow for freesteering rotation of the strut, even un-der large bending forces acting on thepiston rod. Otherwise, the reactionforces that occur under accelerationacting on the rod could make the steer-i ng wheel "freeze".The positioning of the king pin axiswith negative steering radius in theground contact patch, and a small leverto the wheel centre, gives low sensibi-lity for unbalanced wheels and the pos-sibility to bring about the desired elas-tokinematics for longitudinal forces.

Figure 7

The Delta-link rearsuspension, figure 4The combination of demands on cor-nering characteristics for handling,longitudinal compliance for isolation,low weight, low cost and little spacerequired a special design.To achieve the goals, two different ap-proaches can be used:- use an existing multi-link suspen-

sion, like the 960 suspension, thatcan give the required cornering andlongitudinal compliances as a baseand then reduce the space demandand cost.

- use a low weight, compact suspen-sion as a base and then develop it,adding functions to give the re-quired "high-level" cornering andlongitudinal compliances.

The first alternative was excludedsince the existing multi-link suspen-sions could not be made space ef-ficient enough; also the reduction ofcost would require an immense effortin detail design and production en-gineering. This in itself would result ina paradox of increased costs.

The second alternative could be re-alized with a relatively small effortand cost addition, and was thus cho-sen. It can be characterized as a trail-ing twist beam suspension. In its basicform it can fulfill either longitudinalcompliance or cornering demands.However, the last is only true if thejoints to the body and the axle struc-ture itself are ideally rigid which is un-realistic.

Figure 4

The axle is equipped with two linkscalled Delta links because their lines ofaction form the Greek letter Delta.During cornering the rear wheels aresubjected to side forces. Thanks to thesteering links, the rear axle is sub-mitted to a motion pattern that com-pensates for elastic deformations of theaxle and its bushings. The result is thatthe wheels move parallel in in the later-al direction without any steering angle,figure 8. The Delta links also give anincreased longitudinal flexibilitywhich contributes to the good ridecomfort, figure 9.

Camber variation (figure 10) androll steer characteristics are essentialfor the utilization of the cornering ca-pacity of the tyres. They are designedby the basic kinematics of the suspen-sion, i.e. the position of the body at-tachment points and the position of thetransverse arm bushes, figure 11. TheDelta-link suspension reduces the cam-ber angle to the road and gives possi-bilities to select a roll steer that op-ti mizes the handling properties.

Brake systemThe brake system is of the ABS-type,chosen as standard equipment since itcombines stability, steerability andshort braking distances on all types ofroad surfaces. The brake force is dis-tributed through a front/rear brake cir-cuit split. The rear circuit contains a re-duction valve that is automaticallydecoupled in case of a pressure loss inthe front circuit. The brakes are dimen-sioned to give high braking per-formance/retardation and low pedalforces in all situations and high ther-mal resistance.The system consists of;- vacuum booster- ABS tandem master cylinder- hydraulic control unit- four wheel speed sensors- electronic controller- pedal travel sensorDuring ABS braking, excessive wheelbrake pressure is released back to thereservoir when a wheel shows a ten-dency to lock. The front wheels arecontrolled individually. For stabilityreasons, both rear wheels are con-trolled by the rear wheel that has thethe most locking tendency ("select-low") by one inlet/outlet valve pair.

The vacuum booster is equipped with apedal travel sensor. The informationfrom the sensor is used to ensure suf-ficient brake fluid volume in the mas-ter cylinder and to provide good brakepedal comfort during ABS braking,

One situation that needs special at-tention is braking on different frictionon the left and right wheel pairs, sincethe unequal braking forces on the leftand right sides of the car creates a yaw-ing moment. To ensure that the driverhas maximum possibilities to copewith this, the yaw motions of the carshould be slow. This is achieved partlyby giving the front suspension ad-equate longitudinal compliance steer-ing, partly by slowing down the pres-sure build-up in the front wheelcylinder on the high friction side usingthe ABS electronic controller.

For safety reasons, the ABS systemis designed using a fail-safe concept,utilizing demands for continous sig-nals and doubled electronic functions,including the comparison of sensorsignals. Whenever a possible malfunc-tion occurs, the system switches backto conventional brake function and awarning lamp is lit in the instrumentpanel.

Steering columnApart from the main function of steer-ing, the steering column has to meettwo main requirements: to isolate thesteering wheel from road and enginevibrations, and to ensure the driver agood road feel and steering precision.Other demands are for easy productionassembly and for crash collapse.

Since the rubber-mounted subframewith the steering gear is subject tolarge motions in certain driving situa-tions, the steering column is also af-fected by this motions. The isolation oflarge motions is done by a splinescoupling between the lower and upperparts of the steering column. To elim-inate the interplay, the splines has aspecial plastic coating. The design alsopermits the driver to adjust the steeringwheel position vertically and axiallyaccording to individual requirements.

Smaller amplitudes, higher fre-quency vibrations and noise stemmingfrom e.g. the power steering valvecould however still be transmittedthrough the hysteresis in the splinescoupling. These vibrations are there-fore isolated by a disc-shaped rubbercoupling which is soft axially and also

initially soft in torsion. The torsionalcharacteristics are strongly pro-gressive in order to create a high rigid-ity whenever steering torque is ap-plied. This has been carefully tuned inorder to create the desired steering pre-cision.

In production, the steering column isassembled after the marriage point(thejoining together of the complete chas-sis with the body) by connecting thelower shaft between the universaljoints. This ensures an effective andergonomic correct assembly.

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