1TheEffectofaViscousCouplingUsedasaFront-WheelDriveLimited-SlipDifferentialonVehicleTractionandHandling1ABCTRACTTheviscouscouplingisknownmainlyasadrivelinecomponentinfourwheeldrivevehicles.Developmentsinrecentyears,however,pointtowardtheprobabilitythatthisdevicewillbecomeamajorplayerinmainstreamfront-wheeldriveapplication.ProductionapplicationinEuropeanandJapanesefront-wheeldrivecarshavedemonstratedthatviscouscouplingsprovidesubstantialimprovementsnotonlyintractiononslipperysurfacesbutalsoinhandingandstabilityevenundernormaldrivingconditions.Thispaperpresentsaseriousofprovinggroundtestswhichinvestigatetheeffectsofaviscouscouplinginafront-wheeldrivevehicleontractionandhanding.Testingdemonstratessubstantialtractionimprovementswhileonlyslightlyinfluencingsteeringtorque.Factorsaffectingthissteeringtorqueinfront-wheeldrivevehiclesduringstraightlinedrivingaredescribed.Keyvehicledesignparametersareidentifiedwhichgreatlyinfluencethecompatibilityoflimited-slipdifferentialsinfront-wheeldrivevehicles.Corneringtestsshowtheinfluenceoftheviscouscouplingontheselfsteeringbehaviorofafront-wheeldrivevehicle.Furthertestingdemonstratesthatavehiclewithaviscouslimited-slipdifferentialexhibitsanimprovedstabilityunderaccelerationandthrottle-offmaneuversduringcornering.2THEVISCOUSCOUPLINGTheviscouscouplingisawellknowncomponentindrivetrains.Inthispaperonlyashortsummaryofitsbasicfunctionandprincipleshallbegiven.Theviscouscouplingoperatesaccordingtotheprincipleoffluidfriction,andisthusdependentonspeeddifference.AsshowninFigure1theviscouscouplinghasslipcontrollingpropertiesincontrasttotorquesensingsystems.Thismeansthatthedrivetorquewhichistransmittedtothefrontwheelsisautomaticallycontrolledinthesenseofanoptimizedtorquedistribution.Inafront-wheeldrivevehicletheviscouscouplingcanbeinstalledinsidethedifferentialorexternallyonanintermediateshaft.TheexternalsolutionisshowninFigure2.Thislayouthassomesignificantadvantagesovertheinternalsolution.First,2thereisusuallyenoughspaceavailableintheareaoftheintermediateshafttoprovidetherequiredviscouscharacteristic.Thisisincontrasttothelimitedspaceleftintodaysfront-axledifferentials.Further,onlyminimalmodificationtothedifferentialcarrierandtransmissioncaseisrequired.In-houseproductionofdifferentialsisthusonlyslightlyaffected.Introductionasanoptioncanbemadeeasilyespeciallywhentheshaftandtheviscousunitissuppliedasacompleteunit.Finally,theintermediateshaftmakesitpossibletoprovideforsideshaftsofequallengthwithtransverselyinstalledengineswhichisimportanttoreducetorquesteer(shownlaterinsection4).Thisspecialdesignalsogivesagoodpossibilityforsignificantweightandcostreductionsoftheviscousunit.GKNViscodriveisdevelopingalowweightandcostviscouscoupling.Byusingonlytwostandardizedouterdiameters,standardizedplates,plastichubsandextrudedmaterialforthehousingwhichcaneasilybecuttodifferentlengths,itispossibletoutilizeawiderangeofviscouscharacteristics.AnexampleofthisdevelopmentisshowninFigure3.3TRACTIONEFFECTSAsatorquebalancingdevice,anopendifferentialprovidesequaltractiveefforttobothdrivingwheels.Itallowseachwheeltorotateatdifferentspeedsduringcorneringwithouttorsionalwind-up.Thesecharacteristics,however,canbedisadvantageouswhenadhesionvariationsbetweentheleftandrightsidesoftheroadsurface(split-)limitsthetorquetransmittedforbothwheelstothatwhichcanbesupportedbythelow-wheel.Withaviscouslimited-slipdifferential,itispossibletoutilizethehigheradhesionpotentialofthewheelonthehigh-surface.ThisisschematicallyshowninFigure4.Whenforexample,themaximumtransmittabletorqueforonewheelisexceededonasplit-surfaceorduringcorneringwithhighlateralacceleration,aspeeddifferencebetweenthetwodrivingwheelsoccurs.Theresultingself-lockingtorqueintheviscouscouplingresistsanyfurtherincreaseinspeeddifferenceandtransmitstheappropriatetorquetothewheelwiththebettertractionpotential.ItcanbeseeninFigure4thatthedifferenceinthetractiveforcesresultsinayawingmomentwhichtriestoturnthevehicleintothelow-side,Tokeepthevehicleinastraightlinethedriverhastocompensatethiswithoppositesteeringinput.Thoughthefluid-frictionprincipleoftheviscouscouplingandtheresultingsoft3transitionfromopentolockingaction,thisiseasilypossible,TheappropriateresultsobtainedfromvehicletestsareshowninFigure5.Reportedaretheaveragesteering-wheeltorqueTsandtheaveragecorrectiveoppositesteeringinputrequiredtomaintainastraightcourseduringaccelerationonasplit-trackwithanopenandaviscousdifferential.Thedifferencesbetweenthevalueswiththeopendifferentialandthosewiththeviscouscouplingarerelativelylargeincomparisontoeachother.However,theyaresmallinabsoluteterms.Subjectively,thesteeringinfluenceisnearlyunnoticeable.Thetorquesteerisalsoinfluencedbyseveralkinematicparameterswhichwillbeexplainedinthenextsectionofthispaper.4FACTORSAFFECTINGSTEERINGTORQUEAsshowninFigure6thetractiveforcesleadtoanincreaseinthetoe-inresponseperwheel.Fordifferingtractiveforces,Whichappearwhenacceleratingonsplit-withlimited-slipdifferentials,thetoe-inresponsechangesperwheelarealsodifferent.Unfortunately,thiseffectleadstoanundesirableturn-inresponsetothelow-side,i.e.thesameyawdirectionascausedbythedifferenceinthetractiveforces.Reducedtoe-inelasticityisthusanessentialrequirementforthesuccessfulfront-axleapplicationofaviscouslimited-slipdifferentialaswellasanyothertypeoflimited-slipdifferential.GenerallythefollowingequationsapplytothedrivingforcesonawheelVTFFWithTFTractiveForceVFVerticalWheelLoadUtilizedAdhesionCoefficientThesedrivingforcesresultinsteeringtorqueateachwheelviathewheeldisturbancelevelarm“e”andasteeringtorquedifferencebetweenthewheelsgivenbytheequation:eT=loHhiHFFecosWhereeTSteeringTorqueDifferencee=WheelDisturbanceLevelArmKingPinAngle4hi=high-sidesubscriptlo=low-sidesubscriptInthecaseoffront-wheeldrivevehicleswithopendifferentials,Tsisalmostunnoticeable,sincethetorquebias(loHhiTFF/)isnomorethan1.35.Forapplicationswithlimited-slipdifferentials,however,theinfluenceissignificant.Thusthewheeldisturbanceleverarmeshouldbeassmallaspossible.DifferingwheelloadsalsoleadtoanincreaseinTesothedifferenceshouldalsobeassmallaspossible.WhentorqueistransmittedbyanarticulatedCV-Joint,onthedriveside(subscript1)andthedrivenside